Depletion of calcium from the sarcoplasmic reticulum during calcium release in frog skeletal muscle

Voltage sensor current, SR Ca2+ release, and Ca2+ channel current during trains of action potential-like depolarizations of skeletal muscle fibers

In skeletal muscle, CaV 1.1 serves as the voltage sensor for both excitation-contraction coupling (ECC) and L-type Ca2+ channel activation. We have recently adapted the technique of action potential (AP) voltage clamp (APVC) to monitor the current generated by the movement of intramembrane voltage sensors (IQ ) during single imposed transverse tubular AP-like depolarization waveforms (IQAP ). We now extend this procedure to monitoring IQAP , and Ca2+ currents during trains of tubular AP-like waveforms in adult murine skeletal muscle fibers, and compare them with the trajectories of APs and AP-induced Ca2+ release measured in other fibers using field stimulation and optical probes. The AP waveform remains relatively constant during brief trains (<1 sec) for propagating APs in non-V clamped fibers. Trains of 10 AP-like depolarizations at 10 Hz (900 ms), 50 Hz (180 ms), or 100 Hz (90 ms) did not alter IQAP amplitude or kinetics, consistent with previous findings in isolated muscle fibers where negligible charge immobilization occurred during 100 ms step depolarizations. Using field stimulation, Ca2+ release did exhibit a considerable decline from pulse to pulse during the train, also consistent with previous findings, indicating that the decline of Ca2+ release during a short train of APs is not correlated to modification of charge movement. Ca2+ currents during single or 10 Hz trains of AP-like depolarizations were hardly detectable, were minimal during 50 Hz trains, and became more evident during 100 Hz trains in some fibers. Our results verify predictions on the behavior of the ECC machinery in response to AP-like depolarizations and provide a direct demonstration that Ca2+ currents elicited by single AP-like waveforms are negligible, but can become more prominent in some fibers during short high-frequency train stimulation that elicits maximal isometric force.

Rapid subcellular calcium responses and dynamics by calcium sensor G-CatchER. iScience 2021;24:102129. [PMID: 33665552 PMCID: PMC7900224 DOI: 10.1016/j.isci.2021.102129]

The precise spatiotemporal characteristics of subcellular calcium (Ca²⁺) transients are critical for the physiological processes. Here we report a green Ca²⁺ sensor called "G-CatchER⁺" using a protein design to report rapid local ER Ca²⁺ dynamics with significantly improved folding properties. G-CatchER⁺ exhibits a superior Ca²⁺ on rate to G-CEPIA1er and has a Ca²⁺-induced fluorescence lifetimes increase. G-CatchER⁺ also reports agonist/antagonist triggered Ca²⁺ dynamics in several cell types including primary neurons that are orchestrated by IP₃Rs, RyRs, and SERCAs with an ability to differentiate expression. Upon localization to the lumen of the RyR channel (G-CatchER⁺-JP45), we report a rapid local Ca²⁺ release that is likely due to calsequestrin. Transgenic expression of G-CatchER⁺ in Drosophila muscle demonstrates its utility as an in vivo reporter of stimulus-evoked SR local Ca²⁺ dynamics. G-CatchER⁺ will be an invaluable tool to examine local ER/SR Ca²⁺ dynamics and facilitate drug development associated with ER dysfunction.

SOCE Is Important for Maintaining Sarcoplasmic Calcium Content and Release in Skeletal Muscle Fibers

Store-operated Ca²⁺ entry (SOCE) is a Ca²⁺-entry process activated by the depletion of intracellular stores and has an important role in many cell types. In skeletal muscle, however, its role during physiological muscle activation has been controversial. To address this question, sarcoplasmic reticulum (SR) calcium release in a mouse strain with a naturally occurring mutation in the myostatin gene (Compact (Cmpt)) leading to a hypermuscular yet reduced muscle-force phenotype was compared to that in wild-type mice. To elicit Ca²⁺ release from the SR of flexor digitorum brevis (FDB) fibers, either a ryanodine receptor agonist (4-chloro-meta-cresol) or depolarizing pulses were used. In muscles from Cmpt mice, endogenous protein levels of STIM1 and Orai1 were reduced, and consequently, SOCE after 4-chloro-meta-cresol-induced store depletion was suppressed. Although the voltage dependence of SR calcium release was not statistically different between wild-type and Cmpt fibers, the amount of releasable calcium was significantly reduced in the latter, indicating a smaller SR content. To assess the immediate role of SOCE in replenishing the SR calcium store, the evolution of intracellular calcium concentration during a train of long-lasting depolarizations to a maximally activating voltage was monitored. Cmpt mice exhibited a faster decline in calcium release, suggesting a compromised ability to refill the SR. A simple model that incorporates a reduced SOCE as an important partner in regulating immediate calcium influx through the surface membrane readily accounts for the steady-state reduction in SR calcium content and its more pronounced decline after calcium release.

Impaired calcium signaling in muscle fibers from intercostal and foot skeletal muscle in a cigarette smoke-induced mouse model of COPD

INTRODUCTION

Respiratory and locomotor skeletal muscle dysfunction are common findings in chronic obstructive pulmonary disease (COPD); however, the mechanisms that cause muscle impairment in COPD are unclear. Because Ca2+ signaling in excitation-contraction (E-C) coupling is important for muscle activity, we hypothesized that Ca2+ dysregulation could contribute to muscle dysfunction in COPD.

METHODS

Intercostal and flexor digitorum brevis muscles from control and cigarette smoke-exposed mice were investigated. We used single cell Ca2+ imaging and Western blot assays to assess Ca2+ signals and E-C coupling proteins.

RESULTS

We found impaired Ca2+ signals in muscle fibers from both muscle types, without significant changes in releasable Ca2+ or in the expression levels of E-C coupling proteins.

CONCLUSIONS

Ca2+ dysregulation may contribute or accompany respiratory and locomotor muscle dysfunction in COPD. These findings are of significance to the understanding of the pathophysiological course of COPD in respiratory and locomotor muscles. Muscle Nerve 56: 282-291, 2017.

Altered Ca(2+) signaling in skeletal muscle fibers of the R6/2 mouse, a model of Huntington's disease

Huntington's disease (HD) is caused by an expanded CAG trinucleotide repeat within the gene encoding the protein huntingtin. The resulting elongated glutamine (poly-Q) sequence of mutant huntingtin (mhtt) affects both central neurons and skeletal muscle. Recent reports suggest that ryanodine receptor-based Ca(2+) signaling, which is crucial for skeletal muscle excitation-contraction coupling (ECC), is changed by mhtt in HD neurons. Consequently, we searched for alterations of ECC in muscle fibers of the R6/2 mouse, a mouse model of HD. We performed fluorometric recordings of action potentials (APs) and cellular Ca(2+) transients on intact isolated toe muscle fibers (musculi interossei), and measured L-type Ca(2+) inward currents on internally dialyzed fibers under voltage-clamp conditions. Both APs and AP-triggered Ca(2+) transients showed slower kinetics in R6/2 fibers than in fibers from wild-type mice. Ca(2+) removal from the myoplasm and Ca(2+) release flux from the sarcoplasmic reticulum were characterized using a Ca(2+) binding and transport model, which indicated a significant reduction in slow Ca(2+) removal activity and Ca(2+) release flux both after APs and under voltage-clamp conditions. In addition, the voltage-clamp experiments showed a highly significant decrease in L-type Ca(2+) channel conductance. These results indicate profound changes of Ca(2+) turnover in skeletal muscle of R6/2 mice and suggest that these changes may be associated with muscle pathology in HD.

Intracellular calcium movements during relaxation and recovery of superfast muscle fibers of the toadfish swimbladder

Ca²⁺ movements during intercall intervals in superfast toadfish swimbladder muscle fibers depend on the accumulation of Ca²⁺ on parvalbumin and a consequent slow rate of Ca²⁺ pumping.

The mating call of the Atlantic toadfish is generated by bursts of high-frequency twitches of the superfast twitch fibers that surround the swimbladder. At 16°C, a calling period can last several hours, with individual 80–100-Hz calls lasting ∼500 ms interleaved with silent periods (intercall intervals) lasting ∼10 s. To understand the intracellular movements of Ca²⁺ during the intercall intervals, superfast fibers were microinjected with fluo-4, a high-affinity fluorescent Ca²⁺ indicator, and stimulated by trains of 40 action potentials at 83 Hz, which mimics fiber activity during calling. The fluo-4 fluorescence signal was measured during and after the stimulus trains; the signal was also simulated with a kinetic model of the underlying myoplasmic Ca²⁺ movements, including the binding and transport of Ca²⁺ by the sarcoplasmic reticulum (SR) Ca²⁺ pumps. The estimated total amount of Ca²⁺ released from the SR during a first stimulus train is ∼6.5 mM (concentration referred to the myoplasmic water volume). At 40 ms after cessation of stimulation, the myoplasmic free Ca²⁺ concentration ([Ca²⁺]) is below the threshold for force generation (∼3 µM), yet the estimated concentration of released Ca²⁺ remaining in the myoplasm (Δ[Ca_M]) is large, ∼5 mM, with ∼80% bound to parvalbumin. At 10 s after stimulation, [Ca²⁺] is ∼90 nM (three times the assumed resting level) and Δ[Ca_M] is ∼1.3 mM, with 97% bound to parvalbumin. Ca²⁺ movements during the intercall interval thus appear to be strongly influenced by (a) the accumulation of Ca²⁺ on parvalbumin and (b) the slow rate of Ca²⁺ pumping that ensues when parvalbumin lowers [Ca²⁺] near the resting level. With repetitive stimulus trains initiated at 10-s intervals, Ca²⁺ release and pumping come quickly into balance as a result of the stability (negative feedback) supplied by the increased rate of Ca²⁺ pumping at higher [Ca²⁺].

Major contribution of sarcoplasmic reticulum Ca(2+) depletion during long-lasting activation of skeletal muscle

Depolarization of skeletal muscle fibers induces sarcoplasmic reticulum (SR) Ca²⁺ release and contraction that progressively decline while depolarization is maintained. Voltage-dependent inactivation of SR Ca²⁺ release channels and SR Ca²⁺ depletion are the two processes proposed to explain the decline of SR Ca²⁺ release during long-lasting depolarizations. However, the relative contribution of these processes, especially under physiological conditions of activation, is not clearly established. Using Fura-2 and Fluo-5N to monitor cytosolic and SR Ca²⁺ changes, respectively, in voltage-controlled mouse muscle fibers, we show that 2-min conditioning depolarizations reduce voltage-activated cytosolic Ca²⁺ signals with a V_1/2 of −53 mV but also induce SR Ca²⁺ depletion that decreased the releasable pool of Ca²⁺ with the same voltage sensitivity. In contrast, measurement of SR Ca²⁺ changes indicated that SR Ca²⁺ release channels were inactivated after SR had been depleted and in response to much higher depolarizations with a V_1/2 of −13 mV. In response to trains of action potentials, cytosolic Ca²⁺ signals decayed with time, whereas SR Ca²⁺ changes remained stable over 1-min stimulation, demonstrating that SR Ca²⁺ depletion is exclusively responsible for the decline of SR Ca²⁺ release under physiological conditions of excitation. These results suggest that previous studies using steady-state inactivation protocols to investigate the voltage dependence of Ca²⁺ release inactivation in fact probed the voltage dependence of SR Ca²⁺ depletion, and that SR Ca²⁺ depletion is the only process that leads to Ca²⁺ release decline during continuous stimulation of skeletal muscle.

Calcium buffering properties of sarcoplasmic reticulum and calcium-induced Ca(2+) release during the quasi-steady level of release in twitch fibers from frog skeletal muscle

Experiments were performed to characterize the properties of the intrinsic Ca²⁺ buffers in the sarcoplasmic reticulum (SR) of cut fibers from frog twitch muscle. The concentrations of total and free calcium ions within the SR ([Ca_T]_SR and [Ca²⁺]_SR) were measured, respectively, with the EGTA/phenol red method and tetramethylmurexide (a low affinity Ca²⁺ indicator). Results indicate SR Ca²⁺ buffering was consistent with a single cooperative-binding component or a combination of a cooperative-binding component and a linear binding component accounting for 20% or less of the bound Ca²⁺. Under the assumption of a single cooperative-binding component, the most likely resting values of [Ca²⁺]_SR and [Ca_T]_SR are 0.67 and 17.1 mM, respectively, and the dissociation constant, Hill coefficient, and concentration of the Ca-binding sites are 0.78 mM, 3.0, and 44 mM, respectively. This information can be used to calculate a variable proportional to the Ca²⁺ permeability of the SR, namely d[Ca_T]_SR/dt ÷ [Ca²⁺]_SR (denoted release permeability), in experiments in which only [Ca_T]_SR or [Ca²⁺]_SR is measured. In response to a voltage-clamp step to −20 mV at 15°C, the release permeability reaches an early peak followed by a rapid decline to a quasi-steady level that lasts ∼50 ms, followed by a slower decline during which the release permeability decreases by at least threefold. During the quasi-steady level of release, the release amplitude is 3.3-fold greater than expected from voltage activation alone, a result consistent with the recruitment by Ca-induced Ca²⁺ release of 2.3 SR Ca²⁺ release channels neighboring each channel activated by its associated voltage sensor. Release permeability at −60 mV increases as [Ca_T]_SR decreases from its resting physiological level to ∼0.1 of this level. This result argues against a release termination mechanism proposed in mammalian muscle fibers in which a luminal sensor of [Ca²⁺]_SR inhibits release when [Ca_T]_SR declines to a low level.

Voltage clamp methods for the study of membrane currents and SR Ca(2+) release in adult skeletal muscle fibres

Skeletal muscle excitation-contraction (E-C)(1) coupling is a process composed of multiple sequential stages, by which an action potential triggers sarcoplasmic reticulum (SR)(2) Ca(2+) release and subsequent contractile activation. The various steps in the E-C coupling process in skeletal muscle can be studied using different techniques. The simultaneous recordings of sarcolemmal electrical signals and the accompanying elevation in myoplasmic Ca(2+), due to depolarization-initiated SR Ca(2+) release in skeletal muscle fibres, have been useful to obtain a better understanding of muscle function. In studying the origin and mechanism of voltage dependency of E-C coupling a variety of different techniques have been used to control the voltage in adult skeletal fibres. Pioneering work in muscles isolated from amphibians or crustaceans used microelectrodes or 'high resistance gap' techniques to manipulate the voltage in the muscle fibres. The development of the patch clamp technique and its variant, the whole-cell clamp configuration that facilitates the manipulation of the intracellular environment, allowed the use of the voltage clamp techniques in different cell types, including skeletal muscle fibres. The aim of this article is to present an historical perspective of the voltage clamp methods used to study skeletal muscle E-C coupling as well as to describe the current status of using the whole-cell patch clamp technique in studies in which the electrical and Ca(2+) signalling properties of mouse skeletal muscle membranes are being investigated.

Measurement of RyR permeability reveals a role of calsequestrin in termination of SR Ca(2+) release in skeletal muscle

The mechanisms that terminate Ca²⁺ release from the sarcoplasmic reticulum are not fully understood. D4cpv-Casq1 (Sztretye et al. 2011. J. Gen. Physiol. doi:10.1085/jgp.201010591) was used in mouse skeletal muscle cells under voltage clamp to measure free Ca²⁺ concentration inside the sarcoplasmic reticulum (SR), [Ca²⁺]_SR, simultaneously with that in the cytosol, [Ca²⁺]_c, during the response to long-lasting depolarization of the plasma membrane. The ratio of Ca²⁺ release flux (derived from [Ca²⁺]_c(t)) over the gradient that drives it (essentially equal to [Ca²⁺]_SR) provided directly, for the first time, a dynamic measure of the permeability to Ca²⁺ of the releasing SR membrane. During maximal depolarization, flux rapidly rises to a peak and then decays. Before 0.5 s, [Ca²⁺]_SR stabilized at ∼35% of its resting level; depletion was therefore incomplete. By 0.4 s of depolarization, the measured permeability decayed to ∼10% of maximum, indicating ryanodine receptor channel closure. Inactivation of the t tubule voltage sensor was immeasurably small by this time and thus not a significant factor in channel closure. In cells of mice null for Casq1, permeability did not decrease in the same way, indicating that calsequestrin (Casq) is essential in the mechanism of channel closure and termination of Ca²⁺ release. The absence of this mechanism explains why the total amount of calcium releasable by depolarization is not greatly reduced in Casq-null muscle (Royer et al. 2010. J. Gen. Physiol. doi:10.1085/jgp.201010454). When the fast buffer BAPTA was introduced in the cytosol, release flux became more intense, and the SR emptied earlier. The consequent reduction in permeability accelerated as well, reaching comparable decay at earlier times but comparable levels of depletion. This observation indicates that [Ca²⁺]_SR, sensed by Casq and transmitted to the channels presumably via connecting proteins, is determinant to cause the closure that terminates Ca²⁺ release.

Mice null for calsequestrin 1 exhibit deficits in functional performance and sarcoplasmic reticulum calcium handling

In skeletal muscle, the release of calcium (Ca²⁺) by ryanodine sensitive sarcoplasmic reticulum (SR) Ca²⁺ release channels (i.e., ryanodine receptors; RyR1s) is the primary determinant of contractile filament activation. Much attention has been focused on calsequestrin (CASQ1) and its role in SR Ca²⁺ buffering as well as its potential for modulating RyR1, the L-type Ca²⁺ channel (dihydropyridine receptor, DHPR) and other sarcolemmal channels through sensing luminal [Ca²⁺]. The genetic ablation of CASQ1 expression results in significant alterations in SR Ca²⁺ content and SR Ca²⁺ release especially during prolonged activation. While these findings predict a significant loss-of-function phenotype in vivo, little information on functional status of CASQ1 null mice is available. We examined fast muscle in vivo and in vitro and identified significant deficits in functional performance that indicate an inability to sustain contractile activation. In single CASQ1 null skeletal myofibers we demonstrate a decrease in voltage dependent RyR Ca²⁺ release with single action potentials and a collapse of the Ca²⁺ release with repetitive trains. Under voltage clamp, SR Ca²⁺ release flux and total SR Ca²⁺ release are significantly reduced in CASQ1 null myofibers. The decrease in peak Ca²⁺ release flux appears to be solely due to elimination of the slowly decaying component of SR Ca²⁺ release, whereas the rapidly decaying component of SR Ca²⁺ release is not altered in either amplitude or time course in CASQ1 null fibers. Finally, intra-SR [Ca²⁺] during ligand and voltage activation of RyR1 revealed a significant decrease in the SR[Ca²⁺]_free in intact CASQ1 null fibers and a increase in the release and uptake kinetics consistent with a depletion of intra-SR Ca²⁺ buffering capacity. Taken together we have revealed that the genetic ablation of CASQ1 expression results in significant functional deficits consistent with a decrease in the slowly decaying component of SR Ca²⁺ release.

Effects of conformational peptide probe DP4 on bidirectional signaling between DHPR and RyR1 calcium channels in voltage-clamped skeletal muscle fibers

In skeletal muscle, excitation-contraction coupling involves the activation of dihydropyridine receptors (DHPR) and type-1 ryanodine receptors (RyR1) to produce depolarization-dependent sarcoplasmic reticulum Ca²⁺ release via orthograde signaling. Another form of DHPR-RyR1 communication is retrograde signaling, in which RyRs modulate the gating of DHPR. DP4 (domain peptide 4), is a peptide corresponding to residues Leu²⁴⁴²-Pro²⁴⁷⁷ of the central domain of the RyR1 that produces RyR1 channel destabilization. Here we explore the effects of DP4 on orthograde excitation-contraction coupling and retrograde RyR1-DHPR signaling in isolated murine muscle fibers. Intracellular dialysis of DP4 increased the peak amplitude of Ca²⁺ release during step depolarizations by 64% without affecting its voltage-dependence or kinetics, and also caused a similar increase in Ca²⁺ release during an action potential waveform. DP4 did not modify either the amplitude or the voltage-dependence of the intramembrane charge movement. However, DP4 augmented DHPR Ca²⁺ current density without affecting its voltage-dependence. Our results demonstrate that the conformational changes induced by DP4 regulate both orthograde E-C coupling and retrograde RyR1-DHPR signaling.

D4cpv-calsequestrin: a sensitive ratiometric biosensor accurately targeted to the calcium store of skeletal muscle

Current fluorescent monitors of free [Ca(2+)] in the sarcoplasmic reticulum (SR) of skeletal muscle cells are of limited quantitative value. They provide either a nonratio signal that is difficult to calibrate and is not specific or, in the case of Forster resonant energy transfer (FRET) biosensors, a signal of small dynamic range, which may be degraded further by imperfect targeting and interference from endogenous ligands of calsequestrin. We describe a novel tool that uses the cameleon D4cpv, which has a greater dynamic range and lower susceptibility to endogenous ligands than earlier cameleons. D4cpv was targeted to the SR by fusion with the cDNA of calsequestrin 1 or a variant that binds less Ca(2+). "D4cpv-Casq1," expressed in adult mouse at concentrations up to 22 µmole/liter of muscle cell, displayed the accurate targeting of calsequestrin and stayed inside cells after permeabilization of surface and t system membranes, which confirmed its strict targeting. FRET ratio changes of D4cpv-Casq1 were calibrated inside cells, with an effective K(D) of 222 µM and a dynamic range [(R(max) - R(min))/R(min)] of 2.5, which are improvements over comparable sensors. Both the maximal ratio, R(max), and its resting value were slightly lower in areas of high expression, a variation that was inversely correlated to distance from the sites of protein synthesis. The average [Ca(2+)](SR) in 74 viable cells at rest was 416 µM. The distribution of individual ratio values was Gaussian, but that of the calculated [Ca(2+)](SR) was skewed, with a tail of very large values, up to 6 mM. Model calculations reproduce this skewness as the consequence of quantifiably small variations in biosensor performance. Local variability, a perceived weakness of biosensors, thus becomes quantifiable. It is demonstrably small in D4cpv. D4cpv-Casq1 therefore provides substantial improvements in sensitivity, specificity, and reproducibility over existing monitors of SR free Ca(2+) concentration.

Store-operated calcium entry remains fully functional in aged mouse skeletal muscle despite a decline in STIM1 protein expression

Store-operated Ca(2+) entry (SOCE) is a robust mechanism in skeletal muscle, supported by abundant STIM1 and Orai1 in the junctional membranes. The precise role of SOCE in skeletal muscle Ca(2+) homeostasis and excitation-contraction coupling remains to be defined. Regardless, it remains important to determine whether the function and capacity of SOCE changes in aged skeletal muscle. We identified an approximate 40% decline in the expression of the integral SOCE protein, stromal interacting molecule 1 (STIM1), but no such decline in its coupling partner, Orai1, in muscle fibers from aged mice. To determine whether this changed aspects of SOCE functionality in skeletal muscle in aged mice, Ca(2+) in the cytoplasm and t-system were continuously and simultaneously imaged on a confocal microscope during sarcoplasmic reticulum Ca(2+) release and compared to experiments under identical conditions using muscle fibers from young mice. Normal activation, deactivation, Ca(2+) influx, and spatiotemporal characteristics of SOCE were found to persist in skeletal muscle from aged mice. Thus, SOCE remains a robust mechanism in aged skeletal muscle despite the decline in STIM1 protein expression, suggesting STIM1 is in excess in young skeletal muscle.

Paradoxical buffering of calcium by calsequestrin demonstrated for the calcium store of skeletal muscle

Contractile activation in striated muscles requires a Ca(2+) reservoir of large capacity inside the sarcoplasmic reticulum (SR), presumably the protein calsequestrin. The buffering power of calsequestrin in vitro has a paradoxical dependence on [Ca(2+)] that should be valuable for function. Here, we demonstrate that this dependence is present in living cells. Ca(2+) signals elicited by membrane depolarization under voltage clamp were compared in single skeletal fibers of wild-type (WT) and double (d) Casq-null mice, which lack both calsequestrin isoforms. In nulls, Ca(2+) release started normally, but the store depleted much more rapidly than in the WT. This deficit was reflected in the evolution of SR evacuability, E, which is directly proportional to SR Ca(2+) permeability and inversely to its Ca(2+) buffering power, B. In WT mice E starts low and increases progressively as the SR is depleted. In dCasq-nulls, E started high and decreased upon Ca(2+) depletion. An elevated E in nulls is consistent with the decrease in B expected upon deletion of calsequestrin. The different value and time course of E in cells without calsequestrin indicate that the normal evolution of E reflects loss of B upon SR Ca(2+) depletion. Decrement of B upon SR depletion was supported further. When SR calcium was reduced by exposure to low extracellular [Ca(2+)], release kinetics in the WT became similar to that in the dCasq-null. E became much higher, similar to that of null cells. These results indicate that calsequestrin not only stores Ca(2+), but also varies its affinity in ways that progressively increase the ability of the store to deliver Ca(2+) as it becomes depleted, a novel feedback mechanism of potentially valuable functional implications. The study revealed a surprisingly modest loss of Ca(2+) storage capacity in null cells, which may reflect concurrent changes, rather than detract from the physiological importance of calsequestrin.

A reappraisal of the Ca2+ dependence of fast inactivation of Ca2+ release in frog skeletal muscle

Two procedures to inhibit Ca(2+) release designed to differentiate between local and common pool mechanisms for the Ca(2+) dependent, fast inactivation of Ca(2+) release in skeletal muscle of the frog were compared. Inhibition by voltage dependent inactivation of Ca(2+) release, without modification of the single channel current of the Ryanodine Receptor (RyR) and the [Ca(2+)] close to the open pore, produced a reduction in the rate of inactivation linearly related to the reduction in the peak of Ca(2+) release flux. Linear fits in the individual fibers were performed, giving average values (+/-SEM, N = 8) of the best fit parameters of 5.75 x 10(-3) +/- 7.35 x 10(-4 )microM(-1) for the slope and 0.07 +/- 0.015 ms(-1) for the ordinate intercept. Inhibition of Ca(2+) release by reducing the Ca content of the sarcoplasmic reticulum (SR) involves reduction of the Ca(2+) current through the single RyR. The reduction in rate of inactivation also followed linearly the reduction in Ca(2+) peak release flux. The average values (+/-SEM) of the best fit parameters of linear fits were 14 x 10(-3) +/- 3.76 x 10(-3 )microM(-1) and 0.019 +/- 0.006 ms(-1) (N = 7) for slope and ordinate intercept respectively. The differences between both parameters were statistically significant (by t test, at P = 0.05). The extent of inactivation, measured by the peak/final Ca(2+) release flux ratio, was differentially affected by the two procedures. Inhibition by voltage dependent inactivation, despite slowing down the fast inactivation, increased the peak/final Ca(2+) release flux ratio. In contrast, depletion of the SR reticulum reduced it. If the fast inactivation is driven by the high [Ca(2+)] attained locally, close to the open pore of the RyR, the inhibition of Ca(2+) release due to voltage dependent inactivation should not modify the rate of inactivation while inhibition by SR Ca(2+) depletion should reduce it. A process driven by [Ca(2+)] in a common pool should depend on the overall Ca(2+) release independently of how it was modified. In this case both inhibitory procedures should reduce the inactivation rate similarly. Our findings are generally consistent with a common pool process. The differences between the two protocols could be understood if the organization of RyR in junctional and parajunctional release units is considered.

Deconstructing calsequestrin. Complex buffering in the calcium store of skeletal muscle

Since its discovery in 1971, calsequestrin has been recognized as the main Ca(2+) binding protein inside the sarcoplasmic reticulum (SR), the organelle that stores and upon demand mobilizes Ca(2+) for contractile activation of muscle. This article reviews the potential roles of calsequestrin in excitation-contraction coupling of skeletal muscle. It first considers the quantitative demands for a structure that binds Ca(2+) inside the SR in view of the amounts of the ion that must be mobilized to elicit muscle contraction. It briefly discusses existing evidence, largely gathered in cardiac muscle, of two roles for calsequestrin: as Ca(2+) reservoir and as modulator of the activity of Ca(2+) release channels, and then considers the results of an incipient body of work that manipulates the cellular endowment of calsequestrin. The observations include evidence that both the Ca(2+) buffering capacity of calsequestrin in solution and that of the SR in intact cells decay as the free Ca(2+) concentration is lowered. Together with puzzling observations of increase of Ca(2+) inside the SR, in cells or vesicular fractions, upon activation of Ca(2+) release, this is interpreted as evidence that the Ca(2+) buffering in the SR is non-linear, and is optimized for support of Ca(2+) release at the physiological levels of SR Ca(2+) concentration. Such non-linearity of buffering is qualitatively explained by a speculation that puts together ideas first proposed by others. The speculation pictures calsequestrin polymers as 'wires' that both bind Ca(2+) and efficiently deliver it near the release channels. In spite of the kinetic changes, the functional studies reveal that cells devoid of calsequestrin are still capable of releasing large amounts of Ca(2+) into the myoplasm, consistent with the long term viability and apparent good health of mice engineered for calsequestrin ablation. The experiments therefore suggest that other molecules are capable of providing sites for reversible binding of large amounts of Ca(2+) inside the sarcoplasmic reticulum.

A retrograde signal from RyR1 alters DHP receptor inactivation and limits window Ca2+ release in muscle fibers of Y522S RyR1 knock-in mice

Malignant hyperthermia (MH) is a life-threatening hypermetabolic condition caused by dysfunctional Ca(2+) homeostasis in skeletal muscle, which primarily originates from genetic alterations in the Ca(2+) release channel (ryanodine receptor, RyR1) of the sarcoplasmic reticulum (SR). Owing to its physical interaction with the dihydropyridine receptor (DHPR), RyR1 is controlled by the electrical potential across the transverse tubular (TT) membrane. The DHPR exhibits both voltage-dependent activation and inactivation. Here we determined the impact of an MH mutation in RyR1 (Y522S) on these processes in adult muscle fibers isolated from heterozygous RyR1(Y522S)-knock-in mice. The voltage dependence of DHPR-triggered Ca(2+) release flux was left-shifted by approximately 8 mV. As a consequence, the voltage window for steady-state Ca(2+) release extended to more negative holding potentials in muscle fibers of the RyR1(Y522S)-mice. A rise in temperature from 20 degrees to 30 degrees C caused a further shift to more negative potentials of this window (by approximately 20 mV). The activation of the DHPR-mediated Ca(2+) current was minimally changed by the mutation. However, surprisingly, the voltage dependence of steady-state inactivation of DHPR-mediated calcium conductance and release were also shifted by approximately 10 mV to more negative potentials, indicating a retrograde action of the RyR1 mutation on DHPR inactivation that limits window Ca(2+) release. This effect serves as a compensatory response to the lowered voltage threshold for Ca(2+) release caused by the Y522S mutation and represents a novel mechanism to counteract excessive Ca(2+) leak and store depletion in MH-susceptible muscle.

Evolution and modulation of intracellular calcium release during long-lasting, depleting depolarization in mouse muscle

Intracellular calcium signals regulate multiple cellular functions. They depend on release of Ca(2+) from cellular stores into the cytosol, a process that in many types of cells appears to be tightly controlled by changes in [Ca(2+)] within the store. In contrast with cardiac muscle, where depletion of Ca(2+) in the sarcoplasmic reticulum is a crucial determinant of termination of Ca(2+) release, in skeletal muscle there is no agreement regarding the sign, or even the existence of an effect of SR Ca(2+) level on Ca(2+) release. To address this issue we measured Ca(2+) transients in mouse flexor digitorum brevis (FDB) skeletal muscle fibres under voltage clamp, using confocal microscopy and the Ca(2+) monitor rhod-2. The evolution of Ca(2+) release flux was quantified during long-lasting depolarizations that reduced severely the Ca(2+) content of the SR. As in all previous determinations in mammals and non-mammals, release flux consisted of an early peak, relaxing to a lower level from which it continued to decay more slowly. Decay of flux in this second stage, which has been attributed largely to depletion of SR Ca(2+), was studied in detail. A simple depletion mechanism without change in release permeability predicts an exponential decay with time. In contrast, flux decreased non-exponentially, to a finite, measurable level that could be maintained for the longest pulses applied (1.8 s). An algorithm on the flux record allowed us to define a quantitative index, the normalized flux rate of change (NFRC), which was shown to be proportional to the ratio of release permeability P and inversely proportional to Ca(2+) buffering power B of the SR, thus quantifying the 'evacuability' or ability of the SR to empty its content. When P and B were constant, flux then decayed exponentially, and NFRC was equal to the exponential rate constant. Instead, in most cases NFRC increased during the pulse, from a minimum reached immediately after the early peak in flux, to a time between 200 and 250 ms, when the index was no longer defined. NFRC increased by 111% on average (in 27 images from 18 cells), reaching 300% in some cases. The increase may reflect an increase in P, a decrease in B, or both. On experimental and theoretical grounds, both changes are to be expected upon SR depletion. A variable evacuability helps maintain a constant Ca(2+) output under conditions of diminishing store Ca(2+) load.

Sarcoplasmic reticulum Ca2+ release declines in muscle fibers from aging mice

This study hypothesized that decline in sarcoplasmic reticulum (SR) Ca(2+) release and maximal SR-releasable Ca(2+) contributes to decreased specific force with aging. To test it, we recorded electrically evoked maximal isometric specific force followed by 4-chloro-m-cresol (4-CmC)-evoked maximal contracture force in single intact fibers from the mouse flexor digitorum brevis muscle. Significant differences in tetanic, but not in 4-CmC-evoked, contracture forces were recorded in fibers from aging mice as compared to younger mice. Peak intracellular Ca(2+) in response to 4-CmC did not differ significantly. SR Ca(2+) release was recorded in whole-cell patch-clamped fibers in the linescan mode of confocal microscopy using a low-affinity Ca(2+) indicator (Oregon green bapta-5N) with high-intracellular ethylene glycol-bis(alpha-aminoethyl ether)-N,N,N'N'-tetraacetic acid (20 mM). Maximal SR Ca(2+) release, but not voltage dependence, was significantly changed in fibers from old compared to young mice. Increasing the duration of fiber depolarization did not increase the maximal rate of SR Ca(2+) release in fibers from old compared to young mice. Voltage-dependent inactivation of SR Ca(2+) release did not differ significantly between fibers from young and old mice. These findings indicate that alterations in excitation-contraction coupling, but not in maximal SR-releasable Ca(2+), account for the age-dependent decline in intracellular Ca(2+) mobilization and specific force.

Action of perchlorate on the voltage dependent inactivation of excitation-contraction coupling in frog skeletal muscle fibres

Perchlorate is an agonist of excitation-contraction coupling (ECC) in skeletal muscle displacing charge movement and release activation towards more negative voltages. Contradictory effects of this compound on the voltage dependent inactivation (VDI) of ECC ranging from no effect to a negative shift have been previously reported. In this study we report the effect of the extracellular application of 8 mM perchlorate to cut frog fibres on: (1) the charge movement that activates release (Q(1)), (2) the charge movement measured in fibres inactivated by depolarization (Q(2)) and (3) on the steady state VDI of Q(1) and Ca(2+) release. Our findings were: (1) The central voltage of Q(1) was negatively displaced by perchlorate from -29.0 +/- 1.6 to -38.4 +/- 1.7 mV (n = 4). The maximum Q(1) was not significantly affected while the slope of the Q(1) vs. V was increased by perchlorate. (2) The central voltage of Q(2) was shifted from -91.6 +/- 1.4 to -102.3 +/- 1.5 mV (n = 4). (3) The central voltage of the steady state inactivation curve of Q(1) went from -39.3 +/- 1.8 to -48.6 +/- 1.2 mV (mean +/- SEM, n = 6). Perchlorate had a paradoxical effect on Ca(2+) release, while potentiated the release flux in fibres held at -90 mV (peak release flux increased from 3.9 +/- 1.1 to 6.8 +/- 1.9 microM/ms, n = 5) it had an inhibitory effect when applied to fibres at a depolarized holding potential (peak release flux decreased from 3.9 +/- 0.9 to 2.0 +/- 0.5 microM/ms, n = 9). The above findings suggest that the effect on the steady state inactivation is a direct consequence of the negative shift in Q(1) activation. The negative shift in the steady state inactivation of Q(1) correlated well with the effect on Ca(2+) release.

The auxiliary subunit gamma 1 of the skeletal muscle L-type Ca2+ channel is an endogenous Ca2+ antagonist

Ca2+ channels play crucial roles in cellular signal transduction and are important targets of pharmacological agents. They are also associated with auxiliary subunits exhibiting functions that are still incompletely resolved. Skeletal muscle L-type Ca2+ channels (dihydropyridine receptors, DHPRs) are specialized for the remote voltage control of type 1 ryanodine receptors (RyR1) to release stored Ca2+. The skeletal muscle-specific gamma subunit of the DHPR (gamma 1) down-modulates availability by altering its steady state voltage dependence. The effect resembles the action of certain Ca2+ antagonistic drugs that are thought to stabilize inactivated states of the DHPR. In the present study we investigated the cross influence of gamma 1 and Ca2+ antagonists by using wild-type (gamma+/+) and gamma 1 knockout (gamma-/-) mice. We studied voltage-dependent gating of both L-type Ca2+ current and Ca2+ release and the allosteric modulation of drug binding. We found that 10 microM diltiazem, a benzothiazepine drug, more than compensated for the reduction in high-affinity binding of the dihydropyridine agent isradipine caused by gamma 1 elimination; 5 muM devapamil [(-)D888], a phenylalkylamine Ca2+ antagonist, approximately reversed the right-shifted voltage dependence of availability and the accelerated recovery kinetics of Ca2+ current and Ca2+ release. Moreover, the presence of gamma 1 altered the effect of D888 on availability and strongly enhanced its impact on recovery kinetics demonstrating that gamma 1 and the drug do not act independently of each other. We propose that the gamma 1 subunit of the DHPR functions as an endogenous Ca2+ antagonist whose task may be to minimize Ca2+ entry and Ca2+ release under stress-induced conditions favoring plasmalemma depolarization.

The elusive role of store depletion in the control of intracellular calcium release

The contractile cycle of striated muscles, skeletal and cardiac, is controlled by a cytosolic [Ca2+] transient that requires rapid movements of the ion through channels in the sarcoplasmic reticulum (SR). A functional signature of these channels is their closure after a stereotyped time lapse of Ca2+ release. In cardiac muscle there is abundant evidence that termination of release is mediated by depletion of the Ca2+ store, even if the linkage mechanism remains unknown. By contrast, in skeletal muscle the mechanisms of release termination are not understood. This article reviews measurements of store depletion, the experimental evidence for dependence of Ca2+ release on the [Ca2+] level inside the SR, as well as tests of the molecular nature of putative intra-store Ca2+ sensors. Because Ca2+ sparks exhibit the basic release termination mechanism, much attention is dedicated to the studies of store depletion caused by sparks and its relationship with termination of sparks. The review notes the striking differences in volume, content and buffering power of the stores in cardiac vs. skeletal muscle, differences that explain why functional depletion is much greater for cardiac than skeletal muscle stores. Because in skeletal muscle store depletion is minimal and reduction in store [Ca2+] does not appear to greatly inhibit Ca2+ release, it is concluded that decrease in free SR [Ca2+] does not mediate physiological termination of Ca2+ release in this type of muscle. In spite of the apparent absence of store depletion and its putative channel closing effect, termination of Ca2+ sparks is faster and more robust in skeletal than cardiac muscle. A gating role of a hypothetical "proximate store" constituted by polymers of calsequestrin and associated proteins is invoked in an attempt to preserve a role for store depletion and unify mechanisms in both types of striated muscle.

Differential sensitivity to perchlorate and caffeine of tetracaine-resistant Ca2+ release in frog skeletal muscle

In voltage clamped frog skeletal muscle fibres 0.2 mM tetracaine strongly suppresses Ca(2+) release. After this treatment Ca(2+) release flux lacks its characteristic initial peak and the remaining steady component is strongly reduced when compared with the control condition. We studied the effect of two agonists of Ca(2+) release on these tetracaine treated fibres. 8 mM ClO(4)(-) added after tetracaine potentiated release flux from 0.11 +/- 0.03 mM s(-1) to 0.34 +/- 0.07 mM s(-1) (n = 6) although without recovery of the peak at any test voltage. The voltage dependence of the increased release was shifted towards more negative potentials (approximately -10 mV). The effects of ClO(4)(-) on charge movement under these conditions showed the previously described characteristic changes consisting in a left shift of its voltage dependence (approximately -9 mV) together with a slower kinetics, both at the ON and OFF transients. Caffeine at 0.5 mM in the presence of the same concentration of tetracaine failed to potentiate release flux independently of the test voltage applied. When the cut ends of the fibre were exposed to a 10 mM BAPTA intracellular solution, in the absence of tetracaine, the peak was progressively abolished. Under these conditions caffeine potentiated release restoring the peak (from 0.63 +/- 0.12 mM s(-1) to 1.82 +/- 0.23 mM s(-1)) with no effect on charge movement. Taken together the present results suggest that tetracaine is blocking a Ca(2+) sensitive component of release flux. It is speculated that the suppressed release includes a component that is dependent on Ca(2+) and mainly mediated by the activation of the beta ryanodine receptors (the RyR3 equivalent isoform). These receptors are located parajunctionally in the frog and are not interacting with the dihydropyridine receptor.

Depletion "skraps" and dynamic buffering inside the cellular calcium store

Ca2+ signals, produced by Ca2+ release from cellular stores, switch metabolic responses inside cells. In muscle, Ca2+ sparks locally exhibit the rapid start and termination of the cell-wide signal. By imaging Ca2+ inside the store using shifted excitation and emission ratioing of fluorescence, a surprising observation was made: Depletion during sparks or voltage-induced cell-wide release occurs too late, continuing to progress even after the Ca2+ release channels have closed. This finding indicates that Ca2+ is released from a "proximate" compartment functionally in between store lumen and cytosol. The presence of a proximate compartment also explains a paradoxical surge in intrastore Ca2+, which was recorded upon stimulation of prolonged, cell-wide Ca2+ release. An intrastore surge upon induction of Ca2+ release was first reported in subcellular store fractions, where its source was traced to the store buffer, calsequestrin. The present results update the evolving concept, largely due to N. Ikemoto and C. Kang, of calsequestrin as a dynamic store. Given the strategic location and reduction of dimensionality of Ca2+-adsorbing linear polymers of calsequestrin, they could deliver Ca2+ to the open release channels more efficiently than the luminal store solution, thus constituting the proximate compartment. When store depletion becomes widespread, the polymers would collapse to increase store [Ca2+] and sustain the concentration gradient that drives release flux.

A possible role of the junctional face protein JP-45 in modulating Ca2+ release in skeletal muscle

We investigated the functional role of JP-45, a recently discovered protein of the junctional face membrane (JFM) of skeletal muscle. For this purpose, we expressed JP-45 C-terminally tagged with the fluorescent protein DsRed2 by nuclear microinjection in myotubes derived from the C2C12 skeletal muscle cell line and performed whole-cell voltage-clamp experiments. We recorded in parallel cell membrane currents and Ca(2+) signals using fura-2 during step depolarization. It was found that properties of the voltage-activated Ca(2+) current were not significantly changed in JP-45-DsRed2-expressing C2C12 myotubes whereas the amplitude of depolarization-induced Ca(2+) transient was decreased compared to control myotubes expressing only DsRed2. Converting Ca(2+) transients to Ca(2+) input flux using a model fit approach to quantify Ca(2+) removal, the change could be attributed to an alteration in voltage-activated Ca(2+) permeability rather than to altered removal properties or a lower Ca(2+) content of the sarcoplasmic reticulum (SR). Determining non-linear capacitive currents revealed a reduction of Ca(2+) permeability per voltage-sensor charge. The results may be explained by a modulatory effect of JP-45 related to its reported in vitro interaction with the dihydropyridine receptor and the SR Ca(2+) binding protein calsequestrin (CSQ).

Inactivation of Ca2+ transients in amphibian and mammalian muscle fibres

MagFluo-4 fluorescence (Ca2+) transients associated with action potentials were measured in intact muscle fibres, manually dissected from toads ( Leptodactylus insularis ) or enzymatically dissociated from mice. In toads, the decay phase of the Ca2+ transients is described by a single exponential with a time constant ( tau ) of about 7 ms. In mice, a double exponential function with tau 's of 1.5 and 15.5 ms, respectively gives a better fit. In both species the amplitude of Ca2+ transients diminished during repetitive stimulation: in amphibian muscle fibres, the decrease was about 20% with 1 Hz stimulation and 55% at 10 Hz. In mammalian fibres, repetitive stimulation causes a less conspicuous decrease of the transient amplitude: 10% at 1 Hz and 15% at 10 Hz. During tetanic stimulation at 100 Hz the transient amplitude decays to 20% in toad fibres and 40% in mouse fibres. This decrease could be associated with the phenomenon of inactivation of Ca2+ release, described by other authors. Recovery from inactivation, studied by a double stimuli protocol also indicates that in toad fibres the ability to release Ca2+ is abolished to a greater extent than in mouse fibres. In fact the ratio between the amplitudes of the second and first transient, when they are separated by a 10 ms interval, is 0.29 for toad and 0.58 for mouse fibres. In toad fibres, recovery from inactivation, to about 80 % of the initial value, occurs with a tau of 32 ms at 22 degrees C; while in mouse fibres recovery from inactivation is almost complete and occurs with a tau of 36 ms under the same conditions. The results indicate that Ca2+ release in enzymatically dissociated mammalian muscle fibres inactivates to a smaller extent than in intact amphibian muscle fibres.

Altered inactivation of Ca2+ current and Ca2+ release in mouse muscle fibers deficient in the DHP receptor gamma1 subunit

Functional impacts of the skeletal muscle-specific Ca2+ channel subunit gamma1 have previously been studied using coexpression with the cardiac alpha1C polypeptide in nonmuscle cells and primary-cultured myotubes of gamma1-deficient mice. Data from single adult muscle fibers of gamma-/- mice are not yet available. In the present study, we performed voltage clamp experiments on enzymatically isolated mature muscle fibers of the m. interosseus obtained from gamma+/+ and gamma-/- mice. We measured L-type Ca2+ inward currents and intracellular Ca2+ transients during 100-ms step depolarizations from a holding potential of -80 mV. Ratiometric Ca2+ transients were analyzed with a removal model fit approach to calculate the flux of Ca2+ from the sarcoplasmic reticulum. Ca2+ current density, Ca2+ release flux, and the voltage dependence of activation of both Ca2+ current and Ca2+ release were not significantly different. By varying the holding potential and recording Ca2+ current and Ca2+ release flux induced by 100-ms test depolarizations to +20 mV, we studied quasi-steady-state properties of slow voltage-dependent inactivation. For the Ca2+ current, these experiments showed a right-shifted voltage dependence of inactivation. Importantly, we could demonstrate that a very similar shift occurred also in the inactivation curve of Ca2+ release. Voltages of half maximal inactivation were altered by 16 (current) and 14 mV (release), respectively. Muscle fiber bundles, activated by elevated potassium concentration (120 mM), developed about threefold larger contracture force in gamma-/- compared with gamma+/+. This difference was independent of the presence of extracellular Ca2+ and likely results from the lower sensitivity to voltage-dependent inactivation of Ca2+ release. These results demonstrate a specific alteration of voltage-dependent inactivation of both Ca2+ entry and Ca2+ release by the gamma1 subunit of the dihydropyridine receptor in mature muscle fibers of the mouse.

How source content determines intracellular Ca2+ release kinetics. Simultaneous measurement of [Ca2+] transients and [H+] displacement in skeletal muscle

In skeletal muscle, the waveform of Ca²⁺ release under clamp depolarization exhibits an early peak. Its decay reflects an inactivation, which locally corresponds to the termination of Ca²⁺ sparks, and is crucial for rapid control. In cardiac muscle, both the frequency of spontaneous sparks (i.e., their activation) and their termination appear to be strongly dependent on the Ca²⁺ content in the sarcoplasmic reticulum (SR). In skeletal muscle, no such role is established. Seeking a robust measurement of Ca²⁺ release and a way to reliably modify the SR content, we combined in the same cells the “EGTA/phenol red” method (Pape et al., 1995) to evaluate Ca²⁺ release, with the “removal” method (Melzer et al., 1987) to evaluate release flux. The cytosol of voltage-clamped frog fibers was equilibrated with EGTA (36 mM), antipyrylazo III, and phenol red, and absorbance changes were monitored simultaneously at three wavelengths, affording largely independent evaluations of Δ[H⁺] and Δ[Ca²⁺] from which the amount of released Ca²⁺ and the release flux were independently derived. Both methods yielded mutually consistent evaluations of flux. While the removal method gave a better kinetic picture of the release waveform, EGTA/phenol red provided continuous reproducible measures of calcium in the SR (Ca_SR). Steady release permeability (P), reached at the end of a 120-ms pulse, increased as Ca_SR was progressively reduced by a prior conditioning pulse, reaching 2.34-fold at 25% of resting Ca_SR (four cells). Peak P, reached early during a pulse, increased proportionally much less with SR depletion, decreasing at very low Ca_SR. The increase in steady P upon depletion was associated with a slowing of the rate of decay of P after the peak (i.e., a slower inactivation of Ca²⁺ release). These results are consistent with a major inhibitory effect of cytosolic (rather than intra-SR) Ca²⁺ on the activity of Ca²⁺ release channels.

Voltage-controlled Ca2+ release and entry flux in isolated adult muscle fibres of the mouse

The voltage-activated fluxes of Ca(2+) from the sarcoplasmic reticulum (SR) and from the extracellular space were studied in skeletal muscle fibres of adult mice. Single fibres of the interosseus muscle were enzymatically isolated and voltage clamped using a two-electrode technique. The fibres were perfused from the current-passing micropipette with a solution containing 15 mm EGTA and 0.2 mm of either fura-2 or the faster, lower affinity indicator fura-FF. Electrical recordings in parallel with the fluorescence measurements allowed the estimation of intramembrane gating charge movements and transmembrane Ca(2+) inward current exhibiting half-maximal activation at -7.60 +/- 1.29 and 3.0 +/- 1.44 mV, respectively. The rate of Ca(2+) release from the SR was calculated after fitting the relaxation phases of fluorescence ratio signals with a kinetic model to quantify overall Ca(2+) removal. Results obtained with the two indicators were similar. Ca(2+) release was 2-3 orders of magnitude larger than the flux carried by the L-type Ca(2+) current. At maximal depolarization (+50 mV), release flux peaked at about 3 ms after the onset of the voltage pulse and then decayed in two distinct phases. The slower phase, most likely resulting from SR depletion, indicated a decrease in lumenal Ca(2+) content by about 80% within 100 ms. Unlike in frog fibres, the kinetics of the rapid phase of decay showed no dependence on the filling state of the SR and the results provide little evidence for a substantial increase of SR permeability on depletion. The approach described here promises insight into excitation-contraction coupling in future studies of genetically altered mice.

Determination of depolarisation- and agonist-evoked calcium fluxes on skeletal muscle cells in primary culture

Changes in intracellular calcium concentration ([Ca2+]i) evoked by prolonged depolarisation (120 mM KCl) or by the application of 15 mM caffeine were measured on skeletal muscle cells in primary culture. The extrusion rate (PVmax) of calcium from the myoplasm was determined, which in turn enabled the calculation of the calcium flux (Fl) underlying the measured calcium transients. PVmax was found to increase during differentiation, from 107 +/- 10 microM/s at the early myotube stage to 596 +/- 36 microM/s in secondary myotubes. This was paralleled by a decrease in resting [Ca2+]i from 99 +/- 4 to 51 +/- 2 nM. The depolarisation-evoked Fl rose to peak and then ceased despite the continuous presence of KCl. In contrast, the caffeine-induced Fl showed a peak and a clear steady-level with a peak-to-steady ratio of 5.6 +/- 1.2. Removal of external calcium suppressed the depolarisation--induced flux by 88 +/- 5% indicating that both an influx and a release from the SR underlie the K(+)-evoked calcium transients. Subsequent applications of caffeine resulted in essentially identical fluxes indicating an efficient refilling of the internal stores. Moreover, if a depolarisation-induced calcium transient preceded the second caffeine-evoked release, the latter was significantly larger than the first suggesting that much of the calcium that entered was stored in the SR rather than extruded.

Voltage-dependent Ca2+ fluxes in skeletal myotubes determined using a removal model analysis

The purpose of this study was to quantify the Ca2+ fluxes underlying Ca2+ transients and their voltage dependence in myotubes by using the "removal model fit" approach. Myotubes obtained from the mouse C2C12 muscle cell line were voltage-clamped and loaded with a solution containing the fluorescent indicator dye fura-2 (200 microM) and a high concentration of EGTA (15 mM). Ca2+ inward currents and intracellular ratiometric fluorescence transients were recorded in parallel. The decaying phases of Ca2+-dependent fluorescence signals after repolarization were fitted by theoretical curves obtained from a model that included the indicator dye, a slow Ca2+ buffer (to represent EGTA), and a sequestration mechanism as Ca2+ removal components. For each cell, the rate constants of slow buffer and transport and the off rate constant of fura-2 were determined in the fit. The resulting characterization of the removal properties was used to extract the Ca2+ input fluxes from the measured Ca2+ transients during depolarizing pulses. In most experiments, intracellular Ca2+ release dominated the Ca2+ input flux. In these experiments, the Ca2+ flux was characterized by an initial peak followed by a lower tonic phase. The voltage dependence of peak and tonic phase could be described by sigmoidal curves that reached half-maximal activation at -16 and -20 mV, respectively, compared with -2 mV for the activation of Ca2+ conductance. The ratio of the peak to tonic phase (flux ratio) showed a gradual increase with voltage as in rat muscle fibers indicating the similarity to EC coupling in mature mammalian muscle. In a subgroup of myotubes exhibiting small fluorescence signals and in cells treated with 30 microM of the SERCA pump inhibitor cyclopiazonic acid (CPA) and 10 mM caffeine, the calculated Ca2+ input flux closely resembled the L-type Ca2+ current, consistent with the absence of SR Ca2+ release under these conditions and in support of a valid determination of the time course of myoplasmic Ca2+ input flux based on the optical indicator measurements.

Differential effects of voltage-dependent inactivation and local anesthetics on kinetic phases of Ca2+ release in frog skeletal muscle

In voltage-clamped frog skeletal muscle fibers, Ca(2+) release rises rapidly to a peak, then decays to a nearly steady state. The voltage dependence of the ratio of amplitudes of these two phases (p/s) shows a maximum at low voltages and declines with further depolarization. The peak phase has been attributed to a component of Ca(2+) release induced by Ca(2+), which is proportionally greater at low voltages. We compared the effects of two interventions that inhibit Ca(2+) release: inactivation of voltage sensors, and local anesthetics reputed to block Ca(2+) release induced by Ca(2+). Holding the cells partially depolarized strongly reduced the peak and steady levels of Ca(2+) release elicited by a test pulse and suppressed the maximum of the p/s ratio at low voltages. The p/s ratio increased monotonically with test voltage, eventually reaching a value similar to the maximum found in noninactivated fibers. This implies that the marked peak of Ca(2+) release is a property of a cooperating collection of voltage sensors rather than individual ones. Local anesthetics reduced the peak of release flux at every test voltage, and the steady phase to a lesser degree. At variance with sustained depolarization, they made p/s low at all voltages. These observations were well-reproduced by the "couplon" model of dual control, which assumes that depolarization and anesthetics respectively, and selectively, disable its Ca(2+)-dependent or its voltage-operated channels. This duality of effects and their simulation under such hypotheses are consistent with the operation of a dual, two-stage control of Ca(2+) release in muscle, whereby Ca(2+) released through multiple directly voltage-activated channels builds up at junctions to secondarily open Ca(2+)-operated channels.

Intracellular Ca(2+) release as irreversible Markov process

In striated muscles, intracellular Ca(2+) release is tightly controlled by the membrane voltage sensor. Ca(2+) ions are necessary mediators of this control in cardiac but not in skeletal muscle, where their role is ill-understood. An intrinsic gating oscillation of Ca(2+) release-not involving the voltage sensor-is demonstrated in frog skeletal muscle fibers under voltage clamp. A Markov model of the Ca(2+) release units is shown to reproduce the oscillations, and it is demonstrated that for Markov processes to have oscillatory transients, its transition rates must violate thermodynamic reversibility. Such irreversibility results in permanent cycling of the units through a ring of states, which requires a source of free energy. Inhibition of the oscillation by 20 to 40 mM EGTA or partial depletion of Ca(2+) in the sarcoplasmic reticulum (SR) identifies the SR [Ca(2+)] gradient as the energy source, and indicates a location of the critical Ca(2+)-sensing site at distances greater than 35 nm from the open channel. These results, which are consistent with a recent demonstration of irreversibility in gating of cardiac Ca(2+) sparks, (Wang, S.-Q., L.-S. Song, L. Xu, G. Meissner, E. G. Lakatta, E. Ríos, M. D. Stern, and H. Cheng. 2002. Biophys. J. 83:242-251) exemplify a cell-wide oscillation caused by coupling between ion permeation and channel gating.

Deterministic inactivation of calcium release channels in mammalian skeletal muscle

Enzymatically dissociated fibres from the extensor digitorum communis muscle of rats were mounted into a double Vaseline gap chamber. The rate of calcium release (R(rel)) from the sarcoplasmic reticulum (SR) and changes in SR permeability to Ca2+ (PSR) were calculated from measured changes in intracellular calcium concentration. Calcium release during a prepulse attenuated the inactivating component of PSR of the subsequent test pulse. The suppression was graded, larger release causing greater suppression, as expected from a calcium-dependent inactivation process. However, if the dissociation constant of the putative inhibitory calcium binding site (Kd) was estimated using different test pulses different affinities were obtained: a smaller test pulse yielded a smaller Kd. Comparing the suppression of the inactivatable component of PSR during the test pulse (suppression) with the inactivatable component during the prepulse (pre-inactivation) revealed a linear relationship with a regression coefficient close to unity. Lowering intracellular magnesium by decreasing its concentration to 25 microM in the internal solution altered the time course of PSR. The maximal peak-to-steady-level ratio was increased to 6.3 +/- 0.4 (n = 10, mean +/- s.e.m.) from a control value of 3.0 +/- 0.2 (n = 19). Despite the apparent change in steady-state inactivation, suppression remained equal to that pre-inactivation. Our results support the view that a depolarizing pulse always recruits the same set of calcium release channels and a portion of these channels undergoes a deterministic inactivation process.

Role of Ca(2+) fluctuations in L6 myotubes in the regulation of the hexokinase II gene

Expression of the hexokinase (HK) II gene in skeletal muscle is upregulated by electrically stimulated muscle contraction and moderate-intensity exercise. However, the molecular mechanism by which this occurs is unknown. Alterations in intracellular Ca(2+) homeostasis accompany contraction and regulate gene expression in contracting skeletal muscle. Therefore, as a first step in understanding the exercise-induced increase in HK II, the ability of Ca(2+) to increase HK II mRNA was investigated in cultured skeletal muscle cells, namely L6 myotubes. Exposure of cells to the ionophore A-23187 resulted in an approximately threefold increase in HK II mRNA. Treatment of cells with the extracellular Ca(2+) chelator EGTA did not alter HK II mRNA, nor was it able to prevent the A-23187-induced increase. Treatment of cells with the intracellular Ca(2+) chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl) ester (BAPTA-AM) also resulted in an approximately threefold increase in HK II mRNA in the absence of ionophore, which was similar to the increase in HK II mRNA induced by the combination of BAPTA-AM and A-23187. In summary, a rise in intracellular Ca(2+) is not necessary for the A-23187-induced increase in HK II mRNA, and increases in HK II mRNA occur in response to treatments that decrease intracellular Ca(2+) stores. Depletion of intracellular Ca(2+) stores may be one mechanism by which muscle contraction increases HK II mRNA.

Calcium release in frog cut twitch fibers exposed to different ionic environments under voltage clamp

Calcium release was measured in highly stretched frog cut twitch fibers mounted in a double Vaseline-gap voltage clamp chamber, with the internal solution containing 20 mM EGTA plus 0.4 or 1.8 mM added calcium. Rise in myoplasmic [Ca(2+)] was monitored with antipyrylazo III as the indicator at a temperature of 13 to 14 degrees C. The waveform of calcium release rate (Rel) computed from the absorbance change showed an early peak (Rel(p)) followed by a maintained phase (Rel(m)). Each Rel(p)-versus-V plot was fitted with a Boltzmann distribution function. The maximum value of Rel(p) (Rel(p,max)) was compared in various calcium-containing external solutions. The average value in a Cl(-) solution was about one-third larger than those in a CH(3)SO(3)(-) or gluconate solution, whereas the values in the CH(3)SO(3)(-) and gluconate solutions had no statistically significant difference. In external solutions containing CH(3)SO(3)(-) or gluconate, a replacement of the Ca(2+) with Mg(2+) reduced Rel(p,max) by 30 to 50%, on average. The values of Rel(p, max) also had no statistically significant difference among calcium-free external solutions containing different impermeant anions. An increase of the nominal free [Ca(2+)] in the end-pool solution from a reduced to the normal physiological level increased the value of Rel(p,max), and also slowed the decay of the maintained phase of the Rel waveform. The Rel waveforms in the Cl(-) and CH(3)SO(3)(-) solutions were compared in the same fiber at a fixed potential. CH(3)SO(3)(-) increased the time to peak, reduced Rel(p), and increased Rel(m), and the effects were partially reversible. Under the hypothesis that the decay of the peak was due to calcium inactivation of calcium release, the inactivation was larger in Cl(-) than in CH(3)SO(3)(-), in qualitative agreement with the ratio of Rel(p) in the two solutions. Under the alternative hypothesis that the peak and the maintained phase were separately gated by calcium and depolarization, respectively, then CH(3)SO(3)(-) appeared to decrease the calcium-gated component and increase the voltage-gated component.

Differential effects of caffeine and perchlorate on excitation-contraction coupling in mammalian skeletal muscle

1. Enzymatically dissociated single muscle fibres of the rat were studied under voltage clamp conditions in a double Vaseline gap experimental chamber. Intramembrane charge movement and changes in intracellular calcium concentration ([Ca2+]i) were measured and the rate of calcium release (Rrel) from the sarcoplasmic reticulum (SR) was calculated. This enabled the determination of SR permeability and thus the estimation of the transfer function between intramembrane charge movement and SR permeability. 2. Perchlorate (3 mM) shifted the membrane potential dependence of intramembrane charge movement to more negative voltages without any effect on the steepness or on the maximal available charge. The drug increased SR permeability at every membrane potential but did not alter the peak-to-steady level ratio. It also increased the slope of the transfer function, indicating a more efficient coupling between the voltage sensors and the ryanodine receptors. 3. Caffeine (1 mM), on the other hand, increased SR permeability without altering the voltage dependence of intramembrane charge movement. It neither prolonged the depolarization-induced increase in [Ca2+]i at short pulse durations nor altered the time to peak of Rrel. The augmentation of SR permeability by the drug was more pronounced during the peak caffeine response than during its steady level. This was manifested in a leftward shift of the transfer function rather than an increase in its slope. 4. These observations indicate that perchlorate and caffeine alter the coupling between the voltage sensors and SR calcium release channels in mammalian skeletal muscle. They do not, however, share a common mechanism for enhancing the depolarization-induced release of calcium from the SR.

Coordination between Ca2+ release and subsequent re-uptake in the sarcoplasmic reticulum

We here report the results of our recent effort to produce, in the isolated sarcoplasmic reticulum (SR), a biphasic Ca2+ release and Ca2+ re-uptake transient and to resolve the kinetic relationship between Ca2+ release and re-uptake of the released Ca2+. Ca2+ release from the SR was induced by polylysine (the ryanodine receptor-specific Ca2+ release trigger) at various levels of calcium loading, or at various doses of the trigger. The changes in the Ca2+ concentration in the reaction solution and in the lumenal Ca2+ concentration were determined by stopped-flow spectroscopy using fluo-3 and mag-fura-2AM, respectively. At higher levels of calcium loading (>150 nmol/mg), polylysine induced monophasic Ca2+ release curves (without an appreciable re-uptake phase) as reported in most studies in the literature. However, lowering the calcium loading level to an intermediate range (100-150 nmol/mg) produced the desired biphasic transient curves consisting of Ca2+ release and Ca2+ re-uptake phases. Under these conditions, the increase in the polylysine concentration resulted in the increase of both the rate of Ca2+ release and that of re-uptake of the released Ca2+. The maximal rate of Ca2+ release and that of re-uptake showed a parallel relationship in the polylysine concentration range of 0-10 microM. This indicates that Ca2+ release from the SR and re-uptake of the released Ca2+ via the SR Ca2+ pump are well-coordinated processes. The changes in the lumenal Ca2+ concentration during the release and re-uptake reaction were monitored at an optimum level of calcium loading while clamping the extravesicular Ca2+ concentration at a constant value. There was again a tight correlation between Ca2+ release (decrease of the lumenal Ca2+ concentration) and re-uptake (increase of the lumenal Ca2+ concentration), indicating that acceleration of the re-uptake is controlled by the rate of decrease of the lumenal Ca2+ concentration. We propose that one of the mechanisms, by which the mode of coordination between the two components of the biphasic Ca2+ transient (viz. Ca2+ release via the ryanodine receptor and Ca2+ re-uptake via the SR Ca2+ pump) is controlled, is the change in the Ca2+ concentration gradient across the SR membrane.

Effects of 2,3-butanedione monoxime on excitation-contraction coupling in frog twitch fibres

10 and 30 mM 2,3-butanedione monoxime (BDM) applied extracellularly to voltage-clamped frog skeletal muscle twitch fibres suppressed both Ca2+ release flux and intramembranous charge movement. Both effects could be clearly separated. The early peak of the Ca2+ release flux was suppressed at every test voltage. The steady level attained at the end of a 100 ms clamp depolarization was relatively spared for lower depolarizing pulses, but was as suppressed as the peak at voltages above -20 mV. The intramembranous charge movement was affected mainly in the I gamma component. The drug had a distinct effect on the kinetics of the intramembranous charge movement current around the threshold for Ca2+ release. The three kinetic components of I gamma were simultaneously affected. For more positive depolarizations where the kinetic effect was not evident, the oxime had no significant effect on the charge moved. Under conditions in which I gamma was absent (i.e. stretched fibres, intracellular solutions containing 6 to 10 mM BAPTA), treatment with 10 mM BDM had a small, not significant suppressive effect on the maximum charge moved (Qmax), while it affected Ca2+ release significantly. When 10 mM BDM was applied in the presence of 0.2 mM tetracaine, the local anaesthetic-resistant Ca2+ release flux was not further suppressed by the oxime.

Numerical methods to determine calcium release flux from calcium transients in muscle cells

Several methods are currently in use to estimate the rate of depolarization-induced calcium release in muscle cells from measured calcium transients. One approach first characterizes calcium removal of the cell. This is done by determining parameters of a reaction scheme from a fit to the decay of elevated calcium after the depolarizing stimulus. In a second step, the release rate during depolarization is estimated based on the fitted model. Using simulated calcium transients with known underlying release rates, we tested the fidelity of this analysis in determining the time course of calcium release under different conditions. The analysis reproduced in a satisfactory way the characteristics of the input release rate, even when the assumption that release had ended before the start of the fitting interval was severely violated. Equally good reconstructions of the release rate time course could be obtained when the model used for the analysis differed in structure from the one used for simulating the data. We tested the application of a new strategy (multiple shooting) for fitting parameters in nonlinear differential equation systems. This procedure rendered the analysis less sensitive to ill-chosen initial guesses of the parameters and to noise. A locally adaptive kernel estimator for calculating numerical derivatives allowed good reconstructions of the original release rate time course from noisy calcium transients when other methods failed.

Local control model of excitation-contraction coupling in skeletal muscle

This is a quantitative model of control of Ca release from the sarcoplasmic reticulum in skeletal muscle, based on dual control of release channels (ryanodine receptors), primarily by voltage, secondarily by Ca (Ríos, E., and G. Pizarro. 1988. 3:223-227). Channels are positioned in a double row array of between 10 and 60 channels, where exactly half face voltage sensors (dihydropyridine receptors) in the transverse (t) tubule membrane (Block, B.A., T. Imagawa, K.P. Campbell, and C. Franzini-Armstrong. 1988. 107:2587-2600). We calculate the flux of Ca release upon different patterns of pulsed t-tubule depolarization by explicit stochastic simulation of the states of all channels in the array. Channels are initially opened by voltage sensors, according to an allosteric prescription (Ríos, E., M. Karhanek, J. Ma, A. González. 1993. 102:449-482). Ca permeating the open channels, diffusing in the junctional gap space, and interacting with fixed and mobile buffers produces defined and changing distributions of Ca concentration. These concentrations interact with activating and inactivating channel sites to determine the propagation of activation and inactivation within the array. The model satisfactorily simulates several whole-cell observations, including kinetics and voltage dependence of release flux, the "paradox of control," whereby Ca-activated release remains under voltage control, and, most surprisingly, the "quantal" aspects of activation and inactivation (Pizarro, G., N. Shirokova, A. Tsugorka, and E. Ríos. 1997. 501:289-303). Additionally, the model produces discrete events of activation that resemble Ca sparks (Cheng, H., M.B. Cannell, and W.J. Lederer. 1993. 262:740-744). All these properties result from the intersection of stochastic channel properties, control by local Ca, and, most importantly, the one dimensional geometry of the array and its mesoscopic scale. Our calculations support the concept that the release channels associated with one face of one junctional t-tubule segment, with its voltage sensor, constitute a functional unit, termed the "couplon." This unit is fundamental: the whole cell behavior can be synthesized as that of a set of couplons, rather than a set of independent channels.

'Quantal' calcium release operated by membrane voltage in frog skeletal muscle

1. Ca2+ transients and Ca2+ release flux were determined optically in cut skeletal muscle fibres under voltage clamp. 'Decay' of release during a depolarizing pulse was defined as the difference between the peak value of release and the much lower steady level reached after about 100 ms of depolarization. Using a double-pulse protocol, the inactivating effect of release was measured by 'suppression', the difference between the peak values of release in the test pulse, in the absence and presence of a conditioning pulse that closely preceded the test pulse. 2. The relationship between decay and suppression was found to follow two simple arithmetic rules. Whenever the conditioning depolarization was less than or equal to the test depolarization, decay in the conditioning release was approximately equal to suppression of the test release. Whenever the conditioning depolarization was greater than that of the test, suppression was complete, i.e. test release was reduced to a function that increased monotonically to a steady level. The steady level was the same with or without conditioning. 3. These arithmetic rules suggest that inactivation of Ca2+ release channels is strictly and fatally linked to their activation. More than a strict linkage, however, is required to explain the arithmetic properties. 4. The arithmetic rules of inactivation result in three other properties that are inexplicable with classical models of channel gating: constant suppression, incremental inactivation and increment detection. These properties were first demonstrated for inositol trisphosphate (IP3)-sensitive channels and used to define IP3-induced release as quantal. In this sense, it can now be stated that skeletal muscle Ca2+ release is activated by membrane voltage in a quantal manner. 5. For both classes of intracellular Ca2+ channels, one explanation of the observations is the existence of subsets of channels with different sensitivities (to voltage or agonist dose). In an alternative explanation, channels are identical, but have a complex repertoire of voltage- or dose-dependent responses.

Activation of Ca2+ release by caffeine and voltage in frog skeletal muscle

1. Using a fast flow, computer-controlled, two-Vaseline-gap chamber, single muscle fibres were subjected to 'pulses' of caffeine at Ca2+ releasing concentrations, combined with voltage-clamp depolarizations, while monitoring intracellular [Ca2+]. 2. Ca2+ release flux elicited by caffeine reached 2.5 mM s-1, or less, after 3 s of exposure, then decayed to zero. The caffeine-releasable pool of sarcoplasmic reticulum (SR) Ca2+ was 2.9 +/- 0.4 mM (mean +/- S.E.M., n = 10). 3. In parallel with release induced by caffeine, release induced by voltage pulses applied during a caffeine exposure increased in the first second of exposure, then decreased, to abolition after 5 s. 4. The amount of Ca2+ releasable by depolarizing pulses was always equal to the amount of Ca2+ in the caffeine-releasable pool. Therefore, there is a single releasable Ca2+ pool. This pool is well stirred-it takes much more time to lose its Ca2+ by release than to diffusionally homogenize its [Ca2+]. Its depletion explains quantitatively the decay of release induced by caffeine or voltage during an exposure to caffeine. 5. A 1.5 s pulse to 10 mV, applied during exposure to caffeine, resulted in large Ca2+ release and, upon repolarization, termination of the caffeine-induced release. This is similar to repolarization-induced stop of caffeine contracture (RISC) in embryonic murine myoballs. The permeability elicited by caffeine (ratio of flux to calcium in the releasable pool) was not affected by depolarizing pulses. Therefore, the mechanism of the RISC-like effect was Ca2+ depletion. 6. Caffeine-induced release did not depend on the holding potential. 7. Whether caffeine was present or not, release activated by voltage remained always under voltage control, ending rapidly upon repolarization. A depolarizing pulse induced a release permeability with an early peak, followed by decay to a steady level. Caffeine (10 mM) shifted the mid-activation voltage of both peak and steady components by -15 mV and increased the steepness of their voltage dependence by 15%. The maximum permeability increased by 30% for the peak and 25% for the steady component (n = 5). These results neither support nor disprove the hypothesis that the peak of Ca2+ release is activated by Ca2+. 8. The similar potentiation by caffeine of both components of release, the continued ability of voltage to control release in the presence of caffeine, and its failure to alter caffeine-induced permeability indicate that caffeine and the voltage sensor enhance independently the channel's tendency to open.

Ca2+ release from the sarcoplasmic reticulum compared in amphibian and mammalian skeletal muscle

Puzzled by recent reports of differences in specific ligand binding to muscle Ca2+ channels, we quantitatively compared the flux of Ca2+ release from the sarcoplasmic reticulum (SR) in skeletal muscle fibers of an amphibian (frog) and a mammal (rat), voltage clamped in a double Vaseline gap chamber. The determinations of release flux were carried out by the "removal" method and by measuring the rate of Ca2+ binding to dyes in large excess over other Ca2+ buffers. To have a more meaningful comparison, the effects of stretching the fibers, of rapid changes in temperature, and of changes in the Ca2+ content of the SR were studied in both species. In both frogs and rats, the release flux had an early peak followed by fast relaxation to a lower sustained release. The peak and steady values of release flux, Rp and Rs, were influenced little by stretching. Rp in frogs was 31 mM/s (SEM = 4, n = 24) and in rats 7 +/- 2 mM/s (n = 12). Rs was 9 +/- 1 and 3 +/- 0.7 mM/s in frogs and rats, respectively. Transverse (T) tubule area, estimated from capacitance measurements and normalized to fiber volume, was greater in rats (0.61 +/- 0.04 microns-1) than in frogs (0.48 +/- 0.04 micron-1), as expected from the greater density of T tubuli. Total Ca in the SR was estimated as 3.4 +/- 0.6 and 1.9 +/- 0.3 mmol/liter myoplasmic water in frogs and rats. With the above figures, the steady release flux per unit area of T tubule was found to be fourfold greater in the frog, and the steady permeability of the junctional SR was about threefold greater. The ratio Rp/Rs was approximately 2 in rats at all voltages, whereas it was greater and steeply voltage dependent in frogs, going through a maximum of 6 at -40 mV, then decaying to approximately 3.5 at high voltage. Both Rp and Rs depended strongly on the temperature, but their ratio, and its voltage dependence, did not. Assuming that the peak of Ca2+ release is contributed by release channels not in contact with voltage sensors, or not under their direct control, the greater ratio in frogs may correspond to the relative excess of Ca2+ release channels over voltage sensors apparent in binding measurements. From the marked differences in voltage dependence of the ratio, as well as consideration of Ca(2+)-induced release models, we derive indications of fundamental differences in control mechanisms between mammalian and amphibian muscle.

Role of calcium feedback in excitation-contraction coupling in isolated triads

There is a considerable controversy in the literature concerning the effects of higher concentrations of calcium chelators (e.g. BAPTA (1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) or fura-2) on the intracellular Ca2+ transients in muscle. We induced calcium release from sarcoplasmic reticulum (SR) in the triad preparation by chemical depolarization of the T-tubule in the presence of various concentrations of BAPTA-calcium buffer ([Ca2+] = 0.1 microM) and investigated the effects of the BAPTA concentration on the time courses of conformational changes in the junctional foot protein (JFP) and calcium release from SR. Upon stimulation, the JFP underwent biphasic conformational changes, as determined by stopped-flow fluorometry of the JFP-bound conformational probe. The first phase of protein conformational change, which preceded calcium release from SR, was virtually unaffected by the BAPTA concentration. However, the magnitude of the second phase increased in an inversely proportional fashion to the BAPTA concentration. An abrupt increase in [Ca2+] from 0.1 microM up to 1.0 microM (delta Ca2+), concurrently with T-tubule depolarization, produced biphasic protein conformational changes: a delta Ca(2+)-independent first phase and a delta Ca(2+)-dependent second phase. Similar Ca2+ jump experiments under non-depolarizing conditions produced a slow monophasic conformational change equivalent to the second phase described above. These results suggest that the first phase of protein conformational change represents the activation of JFP by T-tubule depolarization to induce calcium release, and the second phase the secondary activation by the released Ca2+. Activation of the JFP by the released Ca2+ resulted in an acceleration of both (i) the rate of initial calcium release, and (ii) the subsequent attenuation of calcium release. The acceleration of both was suppressed by higher concentrations of BAPTA. These results provide a reasonable explanation for both of the apparently contradictory views in the literature; high concentrations of calcium buffer (a) suppress the initial activation and (b) prevent the subsequent attenuation of calcium release.

Properties and roles of an intramembranous charge mobilized at high voltages in frog skeletal muscle

1. Membrane Ca2+ currents (ICa), intramembranous charge movement currents and changes in intracellular Ca2+ concentrations were recorded in voltage clamped cut skeletal muscle fibres of the frog. Intra- and extracellular solutions, designed to prevent ionic current, and use of the saponin-permeabilization procedure made possible the measurement of transfer of intramembranous charge up to high positive potentials. 2. Substantial charge moved at positive potentials. This charge was shown to be intramembranous in four tests of charge conservation, demonstrating that the total displacement of charge depended only on the initial and final voltages, and not on the history or pathway of intermediate voltages. 3. On average, in twenty-three cells, the charge moved at 50 mV was 31 +/- 1.9 nC microF-1 (mean +/- S.E.M.), and at 0 mV was 25 +/- 1.5 nC microF-1. Approximately one-fifth of the total charge moved above 0 mV. 4. The charge that moved at high voltage could be fitted, in most cases, with a Boltzmann distribution function. In twenty of twenty-three cells, the total charge distribution could be fitted as the sum of two Boltzmann terms; the high voltage term was centred at 11 +/- 3.9 mV, with a steepness factor of 12 +/- 1.6 mV and a magnitude of 8.6 +/- 1.1 nC microF-1. The low voltage term was centered at -43 +/- 2.1 mV, with a steepness factor of 7.7 +/- 0.6 mV and a magnitude of 22 +/- 1.8 nC microF-1. Thus, the high voltage component comprised about one-quarter of the mobile charge. In four cells it was possible to fit the sum of three Boltzmann terms to the distribution of mobile charge; the parameters of the high voltage term then were similar to those found by fitting the sum of two Boltzmann terms to the same data. 5. The voltage dependence of activation of ICa was determined in a buffered 2 mM Ca2+ external solution, from the tails of ionic current at -30 mV, after activating pulses to various voltages, the duration of which was sufficient to reach the peak of inward current. The voltage dependence was described by a Boltzmann function centred at 2.6 +/- 6.9 mV (n = 6), with a steepness factor of 20 +/- 1.4 mV. The voltages at which the high voltage charge moved were roughly the same as those at which ICa was activated. 6. Calcium release from the sarcoplasmic reticulum was determined from the Ca2+ transients. Calcium release continued to increase at potentials above 0 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

Ca2+ modulation of sarcoplasmic reticulum Ca2+ release in rat skeletal muscle fibers

Ca2+ transients and the rate of Ca2+ release (dCaREL/dt) from the sarcoplasmic reticulum (SR) in voltage-clamped, fast-twitch skeletal muscle fibers from the rat were studied with the double Vaseline gap technique and using mag-fura-2 and fura-2 as Ca2+ indicators. Single pulse experiments with different returning potentials showed that Ca2+ removal from the myoplasm is voltage independent. Thus, the myoplasmic Ca2+ removal (dCaREM/dt) was studied by fitting the decaying phase of the Ca2+ transient (Melzer, Ríos & Schneider, 1986) and dCaREL/dt was calculated as the difference between dCa/dt and dCaREM/dt. The fast Ca2+ release decayed as a consequence of Ca2+ inactivation of Ca2+ release. Double pulse experiments showed inactivation of the fast Ca2+ release depending on the prepulse duration. At constant interpulse interval, long prepulses (200 msec) induced greater inactivation of the fast Ca2+ release than shorter depolarizations (20 msec). The correlation (r) between the myoplasmic [Ca2+]i and the inhibited amount of Ca2+ release was 0.98. The [Ca2+]i for 50% inactivation of dCaREL/dt was 0.25 microM, and the minimum number of sites occupied by Ca2+ to inactivate the Ca2+ release channel was 3.0. These data support Ca2+ binding and inactivation of SR Ca2+ release.

Calcium transients and calcium release in rat fast-twitch skeletal muscle fibres

1. Calcium transients were recorded from cut segments of fast-twitch rat skeletal muscle fibres stretched to 3.7-4.0 microns per sarcomere and voltage clamped at a holding potential of -80 mV using the double Vaseline-gap technique. Calcium transients were monitored simultaneously with the two calcium indicators antipyrylazo III (AP III) and fura-2. AP III was used to record the calcium changes in response to 10-200 ms depolarizing pulses to different membrane potentials while fura-2 monitored the slow decay of the transient (during 16-20 s) and the resting calcium concentration. Experiments were performed at 14-17 degrees C. 2. For 50-100 ms depolarizing pulses calcium transients were first detected between -30 and -20 mV in a total of twenty-one fibres. The transients recorded with AP III showed a plateau for small pulses (-20 mV) and a steady increase during stronger pulses (-10 mV and more positive). Upon repolarization the transients decayed towards the baseline. The signal recorded simultaneously with fura-2 showed a continuous increase of the transient during the pulses at all membrane potentials. The amplitude of the calcium transients for the large pulses could not be followed with fura-2 due to saturation of the dye. 3. The signals obtained with both dyes were used to determine the kinetics of the calcium-fura-2 reaction inside the fibres. The mean values of the kinetic parameters were: the on rate constant (kon) = 5.1 x 10(8) M-1s-1, the off rate constant (koff) = 26 s-1, and koff/kon (KD) = 69.7 nM. 4. The fast phase of decay of the calcium transients after the pulses was studied from the records obtained with AP III. For depolarizing pulses of the same duration, the rate of decay of the transients after the pulse was slower the stronger the depolarization. For pulses to the same membrane potential, the rate of decay was slower the longer the pulse duration. Both stimulating patterns indicated saturation of the removal system in the muscle fibres due to occupancy of slowly equilibrating myoplasmic calcium binding sites by released calcium. 5. The fast phase of decay of the signals obtained with AP III was well fitted with a model of the system for removing calcium from the myofilament space. 6. The rate of calcium release (Rrel) from the sarcoplasmic reticulum was calculated once the removal system was characterized in the same fibre.(ABSTRACT TRUNCATED AT 400 WORDS)