Calcium transients evoked by action potentials in frog twitch muscle fibres

Modulation of Skeletal Muscle Contraction by Myosin Phosphorylation

The striated muscle sarcomere is a highly organized and complex enzymatic and structural organelle. Evolutionary pressures have played a vital role in determining the structure-function relationship of each protein within the sarcomere. A key part of this multimeric assembly is the light chain-binding domain (LCBD) of the myosin II motor molecule. This elongated "beam" functions as a biological lever, amplifying small interdomain movements within the myosin head into piconewton forces and nanometer displacements against the thin filament during the cross-bridge cycle. The LCBD contains two subunits known as the essential and regulatory myosin light chains (ELC and RLC, respectively). Isoformic differences in these respective species provide molecular diversity and, in addition, sites for phosphorylation of serine residues, a highly conserved feature of striated muscle systems. Work on permeabilized skeletal fibers and thick filament systems shows that the skeletal myosin light chain kinase catalyzed phosphorylation of the RLC alters the "interacting head motif" of myosin motor heads on the thick filament surface, with myriad consequences for muscle biology. At rest, structure-function changes may upregulate actomyosin ATPase activity of phosphorylated cross-bridges. During activation, these same changes may increase the Ca2+ sensitivity of force development to enhance force, work, and power output, outcomes known as "potentiation." Thus, although other mechanisms may contribute, RLC phosphorylation may represent a form of thick filament activation that provides a "molecular memory" of contraction. The clinical significance of these RLC phosphorylation mediated alterations to contractile performance of various striated muscle systems are just beginning to be understood. © 2017 American Physiological Society. Compr Physiol 7:171-212, 2017.

Quantifying Ca2+ release and inactivation of Ca2+ release in fast- and slow-twitch muscles

The aims of this study were to quantify the Ca(2+) release underlying twitch contractions of mammalian fast- and slow-twitch muscle and to comprehensively describe the transient inactivation of Ca(2+) release following a stimulus. Experiments were performed using bundles of fibres from mouse extensor digitorum longus (EDL) and soleus muscles. Ca(2+) release was quantified from the amount of ATP used to remove Ca(2+) from the myoplasm following stimulation. ATP turnover by crossbridges was blocked pharmacologically (N-benzyl-p-toluenesulphonamide for EDL, blebbistatin for soleus) and muscle heat production was used as an index of Ca(2+) pump ATP turnover. At 20°C, Ca(2+) release in response to a single stimulus was 34 and 84 μmol (kg muscle)(-1) for soleus and EDL, respectively, and increased with temperature (30°C: soleus, 61 μmol kg(-1); EDL, 168 μmol kg(-1)). Delivery of another stimulus within 100 ms of the first produced a smaller Ca(2+) release. The maximum magnitude of the decrease in Ca(2+) release was greater in EDL than soleus. Ca(2+) release recovered with an exponential time course which was faster in EDL (mean time constant at 20°C, 32.1 ms) than soleus (65.6 ms) and faster at 30°C than at 20°C. The amounts of Ca(2+) released and crossbridge cycles performed are consistent with a scheme in which Ca(2+) binding to troponin-C allowed an average of ∼1.7 crossbridge cycles in the two muscles.

STIM1L is a new actin-binding splice variant involved in fast repetitive Ca2+ release

A newly identified splice variant of STIM1 called STIM1L forms constitutive clusters that interact with actin and Orai1 and allows fast repetitive Ca²⁺ release.

Cytosolic Ca²⁺ signals encoded by repetitive Ca²⁺ releases rely on two processes to refill Ca²⁺ stores: Ca²⁺ reuptake from the cytosol and activation of a Ca²⁺ influx via store-operated Ca²⁺ entry (SOCE). However, SOCE activation is a slow process. It is delayed by >30 s after store depletion because stromal interaction molecule 1 (STIM1), the Ca²⁺ sensor of the intracellular stores, must form clusters and migrate to the membrane before being able to open Orai1, the plasma membrane Ca²⁺ channel. In this paper, we identify a new protein, STIM1L, that colocalizes with Orai1 Ca²⁺ channels and interacts with actin to form permanent clusters. This property allowed the immediate activation of SOCE, a characteristic required for generating repetitive Ca²⁺ signals with frequencies within seconds such as those frequently observed in excitable cells. STIM1L was expressed in several mammalian tissues, suggesting that many cell types rely on this Ca²⁺ sensor for their Ca²⁺ homeostasis and intracellular signaling.

Temporal and spatial characteristics of vibrissa responses to motor commands

A mechanistic description of the generation of whisker movements is essential for understanding the control of whisking and vibrissal active touch. We explore how facial-motoneuron spikes are translated, via an intrinsic muscle, to whisker movements. This is achieved by constructing, simulating, and analyzing a computational, biomechanical model of the motor plant, and by measuring spiking to movement transformations at small and large angles using high-precision whisker tracking in vivo. Our measurements revealed a supralinear summation of whisker protraction angles in response to consecutive motoneuron spikes with moderate interspike intervals (5 ms < Deltat < 30 ms). This behavior is explained by a nonlinear transformation from intracellular changes in Ca(2+) concentration to muscle force. Our model predicts the following spatial constraints: (1) Contraction of a single intrinsic muscle results in movement of its two attached whiskers with different amplitudes; the relative amplitudes depend on the resting angles and on the attachment location of the intrinsic muscle on the anterior whisker. Counterintuitively, for a certain range of resting angles, activation of a single intrinsic muscle can lead to a retraction of one of its two attached whiskers. (2) When a whisker is pulled by its two adjacent muscles with similar forces, the protraction amplitude depends only weakly on the resting angle. (3) Contractions of two adjacent muscles sums up linearly for small amplitudes and supralinearly for larger amplitudes. The model provides a direct translation from motoneuron spikes to whisker movements and can serve as a building block in closed-loop motor-sensory models of active touch.

Mechanism of phosphorylation of the regulatory light chain of myosin from tarantula striated muscle

Contraction is modulated in many striated muscles by Ca2+-calmodulin dependent phosphorylation of the myosin regulatory light chain (RLC) by myosin light chain kinase. We have investigated the biochemical mechanism of RLC phosphorylation in tarantula muscle to better understand the basis of myosin-linked regulation. In an earlier study it was concluded that the RLC occurred as two species, both of which could be phosphorylated, potentiating contraction. Here we present evidence that only a single species exists, and that this can be phosphorylated at one or two sites. In relaxed muscle we find evidence for a substantial level of basal phosphorylation at the first site. This is augmented on activation, followed by partial phosphorylation of the second site. We find in addition that Ca2+ has a dual effect on light chain phosphorylation, depending on its concentration. At low concentration (relaxing conditions) only basal phosphorylation is observed, while at higher concentrations (activating conditions) RLC phosphorylation is stimulated. At still higher Ca2+ concentrations we find partial inhibition of RLC phosphorylation, suggesting an additional mechanism by which the muscle cell can fine tune contractile activity by controlling the level of free Ca2+.

Electrical and chemical synaptic transmission as an interacting system

It is proposed that presynaptic potassium efflux triggered by the nerve impulse may generate either excitatory or inhibitory responses depending on the neurotransmitter which more or less steadily impregnates the postsynaptic membrane. The jelly intersynaptic matrix may potentiate the efficiency of inoic intersynaptic signals. The synaptic vesicles are proposed to shuttle mitochondrial ATP towards the presynaptic membrane, thereby supplying the energy necessary to restore the membrane polarity after synaptic transmission. Plain structural data and currently accepted functional antecedents appear to justify the proposal.

A microspectrophotometric study on the physicochemical properties of antipyrylazo III injected into rat myoballs for measuring free magnesium ion

The accurate measurement of the intracellular concentration of free magnesium ions ([Mg2+]i) is essential for evaluating the role of Mg2+ in cellular functions. Among the specific (compared to fluorescent indicators) metallochromic dyes, antipyrylazo III (APIII) appears to be most suitable for measuring such concentrations in vertebrate cells according to in vitro studies. In this work, the intracellular physicochemical properties of APIII as a Mg2+ indicator were investigated in the cultured rat myoball by means of a microspectrophotometric technique, in order to obtain an accurate measurement of [Mg2+]i. A set of intracellular APIII-Mg2+ calibration spectra was established after permeabilization of the cell membrane with saponin. In comparison with recordings obtained in K+ solutions, the APIII absorption spectra recorded on a myoball exhibited a red shift and an overall change in absorbance, similar to that observed in a protein (bovine serum albumin: BSA) solution. The apparent dissociation constant of APIII for Mg2+ in the myoplasm was found to be 3.16 mM, significantly higher than the 1.86 mM measured in K+ solutions at the same pH (7.35). A stoichiometric ratio of 1:1 was found, however, both in solution and in the myoplasm. In addition, the injected APIII significantly affected the [Mg2+]i of myoballs. The [Mg2+]i was found to be 0.9+/-0.18 mM in 85 myoballs, on the basis of the intracellular calibration spectra obtained at the same myoplasmic APIII concentration ( approximately 2.5 mM), and after correction for the perturbing effect of the dye. It is concluded that an intracellular calibration and recording of whole spectrum are essential for proper interpretations of intracellular dye signals.

Effect of activation of protein kinase C on excitation-contraction coupling in frog twitch muscle fibres

Intracellular Ca2+ transients were recorded from frog twitch muscle fibres in response to voltage-clamp depolarizing pulses, using arsenazo III as an intracellular Ca2+ indicator. The effect of the activation of protein kinase C (PKC) on the Ca2+ transients was studied. With 1 microM phorbol 12,13-dibutyrate (PDBu), a PKC activator, the peak of the Ca2+ transients increased to about 120% of control during the first 0.5 h, and then decreased gradually to a plateau of 44% of control within the following 2 h. This effect of PDBu could be alleviated significantly by PKC inhibitors, 10 microM polymyxin B (PMB) or 30 microM 1-(5-isoquinolinylsulphonyl)-2-methyl-piperazine (H-7). Moreover, PDBu caused an upward shift of the strength/duration curve. In Li(+)-loaded muscle fibres the Ca2+ transients could not fully recover after 80 mM K+ exposure for 15 min, while the post-K+ Ca2- transients could be completely restored in the fibres not loaded with Li+. In the presence of 10 microM PMB or 30 microM H-7, a full restoration of the post-K+ Ca2+ transients was seen in Li(+)-loaded fibres. PMB supplemented after high-K+ exposure also could result in a complete recovery of the post-K+ Ca2+ transients in Li(+)-loaded fibres. The role of PKC in modulating excitation-contraction coupling in frog twitch muscle fibres is clearly indicated, but the mechanism(s) and physiological significance remain to be established.

Non-homogeneous Ca release in isolated frog skeletal muscle fibres

We have examined the spatial distribution of [Ca2+]i during tetanic stimulation in frog skeletal muscle cells using a fluorescence imaging method. We have found a completely unexpected pattern of Ca release: Ca is released forming gradients composed of spots of very significant and slow fluctuations of calcium release. Our findings could be explained if the calcium release process in skeletal muscle is influenced significantly by [Ca2+]i, such as in cardiac muscle, and suggests that the SR/Ca release control can include the established voltage-dependent plus a cardiac-like process of calcium-induced Ca release and a Ca release inhibition by Ca.

Voltage-dependent potentiation of L-type Ca2+ channels due to phosphorylation by cAMP-dependent protein kinase

The force of contraction of motor units in skeletal muscle is graded by changing the discharge rate of motor neurons, and cytosolic calcium transients are similarly increased. During single twitches, contraction is not dependent on extracellular calcium, and L-type Ca2+ channels may only function as voltage sensors for initiating Ca2+ release from the sarcoplasmic reticulum. In contrast, forceful tetanic contractions triggered by action potentials at high frequency (20 to 200 Hz) are dependent on extracellular Ca2+ concentration and sensitive to L-type Ca2+ channel antagonists, but the mechanism of regulation of contractile force is unknown. Here we report a large, voltage- and frequency-dependent potentiation of skeletal muscle L-type Ca2+ currents by trains of high-frequency depolarizing prepulses, which is caused by a shift in the voltage-dependence of channel activation to more negative membrane potentials and requires phosphorylation by cyclic AMP-dependent protein kinase in a voltage-dependent manner. This potentiation would substantially increase Ca2+ influx and contractile force in skeletal muscle fibres in response to tetanic stimuli.

Barnacle muscle: Ca2+, activation and mechanics

In this review, aspects of the ways in which Ca2+ is transported and regulated within muscle cells have been considered, with particular reference to crustacean muscle fibres. The large size of these fibres permits easy access to the internal environment of the cell, allowing it to be altered by microinjection or microperfusion. At rest, Ca2+ is not in equilibrium across the cell membrane, it enters the cell down a steep electrochemical gradient. The free [Ca2+] at rest is maintained at a value close to 200 nM by a combination of internal buffering systems, mainly the SR, mitochondria, and the fixed and diffusible Ca(2+)-binding proteins, as well as by an energy-dependent extrusion system operating across the external cell membrane. This system relies upon the inward movement of Na+ down its own electrochemical gradient to provide the energy for the extrusion of Ca2+ ions. As a result of electrical excitation, voltage-sensitive channels for Ca2+ are activated and permit Ca2+ to enter the cell more rapidly than at rest. It has been possible to determine both the amount of Ca2+ entering by this step, and what part this externally derived Ca2+ plays in the development of force as well as in the free Ca2+ change. The latter can be determined directly by Ca(2+)-sensitive indicators introduced into the cell sarcoplasm. A combination of techniques, allowing both the total and free Ca2+ changes to be assessed during electrical excitation, has provided valuable information as to how muscle cells buffer their Ca2+ in order to regulate the extent of the change in the free Ca2+ concentration. The data indicate that the entering Ca2+ can only make a small direct contribution to the force developed by the cell. The implication here is that the major source of Ca2+ for contraction must be derived from the internal Ca2+ storage sites within the SR system, a view reinforced by caged Ca2+ methods. The ability to measure the free Ca2+ concentration changes within a single cell during activation has also provided the opportunity to analyse, in detail, the likely relations between free Ca2+ and the process of force development in muscle. The fact that the free Ca2+ change precedes the development of force implies that there are delays in the mechanism, either at the site of Ca2+ attachment on the myofibril, or at some later stage in the process of force development that were not previously anticipated.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of spontaneous and action potential-induced calcium transients in developing myotubes in vitro

We have investigated the onset and maturation of action potential- and calcium-induced calcium release from the sarcoplasmic reticulum during the differentiation of excitation-contraction coupling in skeletal muscle. Microfluorometry and video imaging of cultured myotubes loaded with the fluorescent calcium indicator fluo-3 revealed the dynamics, time course, and physiological properties of calcium transients as well as their changes during development. Spontaneous and stimulated contractions in well-differentiated myotubes are accompanied by brief (200-500 ms) increases in the concentration of free cytoplasmic calcium. These transients are modulated by sub-threshold concentrations of caffeine, resulting in a plateau of elevated calcium. Two novel types of calcium transients were observed in non-contracting myotubes. 1) Fast localized transients (FLTs) are radially restricted focal release events that occur spontaneously within the myoplasm at various densities and frequencies. 2) Upon addition of caffeine, propagating calcium waves are generated (35-70 microns/s velocity), which are accompanied by contractures. Aside from caffeine sensitivity, calcium waves and contraction-related sustained release events are similar in amplitude and duration, as well as in their inactivation and refractory properties. Thus, these transients may represent calcium-induced calcium release in quiescent and active myotubes, respectively. Following one calcium-induced calcium release event, myotubes become refractory to new calcium-induced transients; however, action potential-induced transients and FLTs are not blocked. This suggests that these transients occur by distinct release mechanisms and that dual modes of calcium release exist prior to the coupling of calcium release to excitation.

Calcium transients in skeletal muscle fibres under isometric conditions and during and after a quick stretch

The transient change in the sarcoplasmic concentration of Ca2+ was measured in intact fibres isolated from the anterior tibial muscle of the frog Litoria moorei. The fibres had been injected with the calcium-sensitive dye arsenazo III and the change of the calcium concentration was calculated from the changes in light absorbance at 570, 600 and 720 nm wavelengths. Absorbance and force were measured under three different conditions: (1) during a normal isometric twitch, (2) when a quick ramp-and-hold stretch had been applied to the fibre during onset of the contraction, and (3) when the fibre was allowed to contract isometrically at a length corresponding to the final length of the stretch. A method was devised to neutralize most of the movement artefacts encountered in such measurements. While the quick stretch caused substantial increase in the level and the duration of the contractile force such as originally described in whole muscle by A. V. Hill, the calcium transients appeared basically unaffected. It thus seems that the mechanism behind the phenomenon of the force enhancement lies at a step in the excitation-contraction coupling subsequent to the calcium release. From the present results, however, it is not clear whether the phenomenon is caused by an increase in the level of activation of the calcium-dependent regulatory system, or whether it is to be found in the acto-myosin interaction itself. The latter alternative would be consistent with the stiffness measurements published earlier.

Imaging of calcium transients in skeletal muscle fibers

Epifluorescence images of Ca2+ transients elicited by electrical stimulation of single skeletal muscle fibers were studied with fast imaging techniques that take advantage of the large fluorescence signals emitted at relatively long wavelengths by the dyes fluo-3 and rhod-2 in response to binding of Ca2+ ions, and of the suitable features of a commercially available CCD video camera. The localized release of Ca2+ in response to microinjection of InsP3 was also monitored to demonstrate the adequate space and time resolutions of the imaging system. The time resolution of the imager system, although limited to the standard video frequency response, still proved to be adequate to investigate the fast Ca2+ release process in skeletal muscle fibers at low temperatures.

Dependence of intracellular free calcium and tension on membrane potential and intracellular pH in single crayfish muscle fibres

The dependence of intracellular free calcium ([Ca2+]i) and tension on membrane potential and intracellular pH (pHi) was studied in single isolated fibres of the crayfish claw-opener muscle using ion-selective microelectrodes. Tension (T) was quantified as a percentage of the maximum force, or as force per cross-sectional area (N/cm2). In resting fibres, pHi had a mean value of 7.06. Contractions evoked by an increase extracellular potassium [( K+]0) produced a fall in pHi of 0.01-0.05 units. The lowest measured levels of resting [Ca2+]i corresponded to a pCai (= -log [Ca2+]i) of 6.8. Intracellular Ca2+ transients recorded during K(+)-induced contractions did not reveal any distinct threshold for force development. Both the resting [Ca2+]i and resting tension were decreased by an intracellular alkalosis and increased by an acidosis. The sensitivity of resting tension to a change in pHi (quantified as -dT/dpHi) showed a progressive increase during a fall in pHi within the range examined (pHi 6.2-7.5). The pHi/[Ca2+]i and pHi/tension relationships were monotonic throughout the multiphasic pHi change caused by NH4Cl. A fall of 0.5-0.6 units in pHi did not produce a detectable shift in the pCai/tension relationship at low levels of force development. The results indicate that resting [Ca2+]i is slightly higher than the level required for contractile activation. They also show that the dependence of tension on pHi in crayfish muscle fibres is attributable to a direct H+ and Ca2+ interaction at the level of Ca2+ sequestration and/or transport. Finally, the results suggest that in situ, the effect of pH on the Ca2+ sensitivity of the myofibrillar system is not as large as could be expected on the basis of previous work on skinned crustacean muscle fibres.

Intracellular free magnesium in frog skeletal muscle studied with a new type of magnesium-selective microelectrode: interactions between magnesium and sodium in the regulation of [Mg]i

The application of a new type of intracellular magnesium-selective microelectrode based on the neutral carrier ETH5214 to measure intracellular free magnesium ([Mg]i) in frog skeletal muscle fibers is reported. At room temperature (18-20 degrees C) the average values for [Mg]i was 0.93 mmol/l (pMgi = 3.03 +/- 0.42, SD; n = 38 experiments). The regulation of [Mg]i was studied by measuring [Mg]i and [Na]i with ion-selective microelectrodes during alterations of the membrane potential and the transmembrane sodium and magnesium gradients. Depolarization by increasing external [K] from 2.5 mmol/l to 12.5 mmol/l did not significantly influence [Mg]i. Increasing extracellular [Mg] from 1 mmol/l to 10 and 20 mmol/l caused a concentration-dependent rise in [Mg]i and a decrease in [Na]i, whereas removal of external magnesium did not affect [Mg]i. Removal of external [Na] caused an increase in [Mg]i and a decrease of [Na]i. The results show that [Mg]i in frog skeletal muscle is not in thermodynamic equilibrium and suggest that a Na/Mg exchange mechanism may be involved in maintaining low levels of [Mg]i.

Effect of prolonged in vitro lithium treatment on calcium transients in frog twitch muscle fibres and its reversal by exogenous myo-inositol

Using arsenazo III as an intracellular indicator to monitor the calcium transients elicited by voltage-clamp depolarizing pulse, the effect of prolonged in vitro lithium treatment on excitation--contraction coupling in frog twitch muscle fibres was investigated. Incubation in 10 mM Li+ Ringer's solution for 2 days caused a 46% increase in the amplitude of the calcium transients, while treatment with 30 mM Li+ for 2 days produced a depression of 44%. Shortening the bathing time to 1 day, the decrease of calcium transients caused by 30 mM Li+ was reversed to a small increase. For the 2-day incubation, both the increase in the amplitude with 10 mM and the decrease with 30 mM Li+ were abolished by the presence of 1 mM myo-inositol in the bathing medium. These results imply that the turnover of inositol phospholipids is involved in regulating excitation-contraction coupling in skeletal muscle fibres.

Rapid relaxation of single frog skeletal muscle fibres following laser flash photolysis of the caged calcium chelator, diazo-2

Diazo-2 is a calcium chelator based on BAPTA [(1989) J. Biol. Chem., in press], whose electron withdrawing diazoacetyl group may be rapidly (2000 s-1) converted photochemically to an electron donating carboxymethyl group by exposure to near ultraviolet light, producing an increase in its calcium affinity (Kd changes from 2.2 microM to 0.073 microM) without steric modification of the metal binding site. Photolysis of a 2 mM solution of this compound with a brief flash of light from a frequency-doubled ruby laser (347 nm) caused single skinned muscle fibres from the semitendinosus muscle of the frog Rana temporaria to relax with a mean half-time of 60.4 +/- 5 ms (range 30-100 ms, n = 15) at 12 degrees C, which is faster than the relaxation observed in intact muscles (half-time 133 ms at 14 degrees C [(1986) J. Mol. Biol. 188, 325-342]) and similar to the rate of the fast phase of tension decay in intact single fibres (20 s-1 at 10 degrees C [(1982) J. Physiol. 329, 1-20]).

Actions of caffeine on fast- and slow-twitch muscles of the rat

1. The effects of caffeine (0.2-20 mmol l-1) have been examined on calcium transients (measured with aequorin) and isometric force in intact bundles of fibres from soleus (slow-twitch) and extensor digitorum longus (EDL; fast-twitch) muscles of the rat. 2. At 25 degrees C, threshold caffeine concentration for an observable increase in resting [Ca2+]i was 0.2 and 1.0 mmol l-1 for soleus and EDL muscles respectively. Increases in resting force were first detectable at about 0.5 mmol l-1 caffeine for soleus muscles and 5.0 mmol l-1 caffeine for EDL muscles and occurred in the range 0.2-0.4 mumol l-1 [Ca2+]i for soleus and 0.7-0.9 mumol l-1 for EDL. 3. Caffeine potentiated the twitch responses of soleus and EDL in a dose-related manner. The soleus was more sensitive in this respect, with 50% potentiation occurring at 1 mmol l-1 caffeine compared with 3.5 mmol l-1 for the EDL. Concentrations of caffeine below 2 mmol l-1 potentiated Ca2+ transients associated with twitches in both soleus and EDL muscles with no apparent change in the decay rate constant. 4. High concentrations of caffeine (greater than 2 mmol l-1) further potentiated peak Ca2+ in the EDL but depressed it in the soleus. The rate of decay of the Ca2+ transient in high caffeine was significantly prolonged in the soleus but remained unaffected in the EDL. 5. The phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX) had little effect on force or [Ca2+]i at concentrations known to significantly increase intracellular cyclic AMP levels. 6. The Ca2+ transient during fused tetani was characterized by an initial peak, a decline to a plateau level and sometimes a gradual rise towards the end of the stimulus train. Peak [Ca2+]i during normal tetani ranged between 1.1 and 2.4 mumol l-1 in the soleus and 1.9 and 4.0 mumol l-1 in the EDL. 7. Caffeine potentiated both force and [Ca2+]i during tetanus. Since the increase of the Ca2+ transient was significantly greater than potentiation of force, it is likely that saturation of myofilaments occurs. The primary effect of caffeine on the Ca2+ transient was an elevation of the plateau phase. 8. Caffeine concentrations below 5 mmol l-1 potentiate twitch and tetanic force in both fast- and slow-twitch mammalian skeletal muscles primarily by increasing both the basal and stimulus-evoked release of Ca2+ from the sarcoplasmic reticulum.

Regulation of calcium concentration in voltage-clamped smooth muscle cells

The regulation of intracellular calcium concentration in single smooth muscle cells was investigated by simultaneously monitoring electrical events at the surface membrane and calcium concentration in the cytosol. Cytosolic calcium concentration rose rapidly during an action potential or during a voltage-clamp pulse that elicited calcium current; a train of voltage-clamp pulses caused further increases in the calcium concentration up to a limit of approximately 1 microM. The decline of the calcium concentration back to resting levels occurred at rates that varied with the calcium concentration in an apparently saturable manner. Moreover, the rate of decline at any given calcium concentration was enhanced after a higher, more prolonged increase of calcium. The process responsible for this enhancement persisted for many seconds after the calcium concentration returned to resting levels. Thus, the magnitude and duration of a calcium transient appear to regulate the subsequent calcium removal.

Calcium transients evoked by electrical stimulation of smooth muscle from guinea-pig ileum recorded by the use of Fura-2

1. Intracellular free calcium levels were recorded in strips of longitudinal smooth muscle from guinea-pig ileum, by the use of the fluorescent calcium indicator Fura-2. 2. The resting intracellular free calcium concentration was estimated to be 210 nM. Many muscle strips showed spontaneous bursts of contractions, accompanied by bursts of calcium transients. Following these the calcium level often fell transiently below the resting level. The spontaneous transients were unaffected by tetrodotoxin (TTX) and atropine. 3. Field electrical stimulation of muscle strips evoked a series of calcium transients comprising: (i) an initial rise in free calcium, reaching a peak within 20-30 ms of stimulation, (ii) a second rise in calcium, beginning after a few hundred milliseconds, and finally (iii) a decline in calcium to below the resting level, persisting for a few seconds. The mean peak increase in free calcium above the resting level during components (i) and (ii) was, respectively, 130 and 200 nM. The mean decrease in free calcium during the third component was to 20 nM below the resting level. 4. The short-latency calcium transient required relatively long stimuli for activation, and was not blocked by TTX and atropine. The long-latency transient was selectively activated by brief stimuli, and was abolished by TTX and atropine. Thus, the short-latency component probably arose because of direct electrical stimulation of muscle fibres, while the long-latency component was due to stimulation of muscarinic nerves. 5. The first detectable increase in tension began about 100 ms after the peak of the initial calcium transient. Contractions associated with the long-latency calcium transient were much larger than those associated with the short-latency transient, even in muscle strips where the calcium levels were similar for both transients. 6. Removal of calcium in the bathing solution caused the resting intracellular calcium level to fall, following an initial rise accompanied by increased spontaneous transients. Electrically evoked contractions and calcium transients were abolished in calcium-free solution, and by the addition of verapamil or diltiazem to normal Krebs solution.

Fura-2 calcium transients in frog skeletal muscle fibres

1. Intact single twitch fibres from frog muscle were mounted at long sarcomere spacing (3.5-4.2 microns) on an optical bench apparatus for the measurement of absorbance and fluorescence signals following the myoplasmic injection of either or both of the Ca2+ indicator dyes Fura-2 and Antipyrylazo III. Dye-related signals were measured at 16-17 degrees C in fibres at rest and stimulated electrically to give a single action potential or brief train of action potentials. 2. The apparent diffusion constant of Fura-2 in myoplasm, Dapp, was estimated from Fura-2 fluorescence measured as a function of time and distance from the site of dye injection. On average (N = 7), Dapp was 0.36 x 10(-6) cm2 s-1, a value nearly 3-fold smaller than expected if all the Fura-2 was freely dissolved in the myoplasmic solution. The small value of Dapp is explained if approximately 60-65% of the Fura-2 molecules were bound to relatively immobile sites in myoplasm. 3. In resting fibres the fraction of Fura-2 in the Ca2+-bound form was estimated to be small, on average (N = 11) 0.06 of total dye. However, because of the large fraction of Fura-2 not freely dissolved in myoplasm, and the indirect method employed for estimating Ca2+-bound dye, calibration of the resting level of myoplasmic free Ca2+ ([Ca2+]) from the fraction of Ca2+-bound dye was not considered reliable. 4. In response to a single action potential, large changes in Fura-2 fluorescence (delta F) and absorbance (delta A) were detected, which had identical time courses. As expected, the directions of these transients corresponded to an increase in Ca2+-dye complex. For wavelengths, lambda, between 380 and 460 nm, peak delta A(lambda) was closely similar to the Ca2+-dye difference spectrum for Fura-2 determined in in vitro calibrations. Beer's law was used to calibrate the concentration of Ca2+-dye complex formed during activity (delta[CaFura-2]) from the delta A(lambda) signal. Peak delta[CaFura-2] was found to vary between 0.01 and 0.4 mM, depending on the total concentration of injected Fura-2 ([Fura-2T]), which ranged as high as 0.9 mM. 5. In fibres in which peak delta[CaFura-2] was less than 0.06 mM, delta[CaFura-2] had a limiting minimal half-width of 50-60 ms. However, as peak delta[CaFura-2] increased (up to 0.3-0.4 mM), delta[CaFura-2] half-width became markedly prolonged (up to 150-200 ms), indicative of a strong buffering action of large concentrations of Fura-2 on the underlying [Ca2+] transient (delta[Ca2+]).(ABSTRACT TRUNCATED AT 400 WORDS)

Inositol trisphosphate, calcium and muscle contraction

The identity of organelles storing intracellular calcium and the role of Ins(1,4,5)P3 in muscle have been explored with, respectively, electron probe X-ray microanalysis (EPMA) and laser photolysis of 'caged' compounds. The participation of G-protein(s) in the release of intracellular Ca2+ was determined in saponin-permeabilized smooth muscle. The sarcoplasmic reticulum (SR) is identified as the major source of activator Ca2+ in both smooth and striated muscle; similar (EPMA) studies suggest that the endoplasmic reticulum is the major Ca2+ storage site in non-muscle cells. In none of the cell types did mitochondria play a significant, physiological role in the regulation of cytoplasmic Ca2+. The latency of guinea pig portal vein smooth muscle contraction following photolytic release of phenylephrine, an alpha 1-agonist, is 1.5 +/- 0.26 s at 20 degrees C and 0.6 +/- 0.18 s at 30 degrees C; the latency of contraction after photolytic release of Ins(1,4,5)P3 from caged Ins(1,4,5)P3 is 0.5 +/- 0.12 s at 20 degrees C. The long latency of alpha 1-adrenergic Ca2+ release and its temperature dependence are consistent with a process mediated by G-protein-coupled activation of phosphatidylinositol 4,5 bisphosphate (PtdIns(4,5)P2) hydrolysis. GTP gamma S, a non-hydrolysable analogue of GTP, causes Ca2+ release and contraction in permeabilized smooth muscle. Ins(1,4,5)P3 has an additive effect during the late, but not the early, phase of GTP gamma S action, and GTP gamma S can cause Ca2+ release and contraction of permeabilized smooth muscles refractory to Ins(1,4,5)P3. These results suggest that activation of G protein(s) can release Ca2+ by, at least, two G-protein-regulated mechanisms: one mediated by Ins(1,4,5)P3 and the other Ins(1,4,5)P3-independent. The low Ins(1,4,5)P3 5-phosphatase activity and the slow time-course (seconds) of the contractile response to Ins(1,4,5)P3 released with laser flash photolysis from caged Ins(1,4,5)P3 in frog skeletal muscle suggest that Ins(1,4,5)P3 is unlikely to be the physiological messenger of excitation-contraction coupling of striated muscle. In contrast, in smooth muscle the high Ins(1,4,5)P3-5-phosphatase activity and the rate of force development after photolytic release of Ins(1,4,5)P3 are compatible with a physiological role of Ins(1,4,5)P3 as a messenger of pharmacomechanical coupling.

A general procedure for determining the rate of calcium release from the sarcoplasmic reticulum in skeletal muscle fibers

A general procedure for using myoplasmic calcium transients measured with a metallochromic indicator dye to calculate the time course of calcium release from the sarcoplasmic reticulum in voltage-clamped skeletal muscle fibers is described and analyzed. Explicit properties are first assigned to all relatively rapidly equilibrating calcium binding sites in the myoplasm so that the calcium content (CaF) in this pool of "fast" calcium can be calculated from the calcium transient. The overall properties of the transport systems and relatively slowly equilibrating binding sites that remove calcium from CaF are then characterized experimentally from the decay of CaF following fiber repolarization. The rate of calcium release can then be calculated as dCaF/dt plus the rate of removal of calcium from CaF. Two alternatives are assumed for the component of CaF that is due to fast binding sites intrinsic to the fiber: a linear instantaneous buffer or a set of binding sites having properties similar to thin filament troponin. Both assumptions yielded similar calcium release wave forms. Three alternative methods for characterizing the removal system are presented. The choice among these or other methods for characterizing removal can be based entirely on convenience since any method that reproduces the decay of CaF following fiber repolarization will give the same release wave form. The calculated release wave form will be accurate provided that the properties assumed for CaF are correct, that release turns off within a relatively short time after fiber repolarization, that the properties of the slow removal system are the same during and after fiber depolarization, and that possible spatial nonuniformities of free or bound calcium do not introduce major errors.

Effects of hypertonic solutions on calcium transients in frog twitch muscle fibres

1. The effects of hypertonic solutions on excitation-contraction (e.-c.) coupling in frog skeletal muscle fibres were investigated using Arsenazo III to monitor intracellular calcium transients in voltage-clamped fibres. 2. In solutions made hypertonic with sucrose or sodium chloride, the size of the Arsenazo signal evoked by a 5 ms depolarization to 0 mV was little altered by increases in tonicity up to about twice normal, but declined in higher tonicities, and was almost completely suppressed at 4 times normal tonicity. 3. The latency to onset of the Arsenazo signal was increased in hypertonic solutions (2.3 and 3.1 times normal tonicity), but the decay time constant of the signal was little changed with tonicities up to 2.3 times normal. 4. The rheobase potential for a just-detectable Arsenazo signal was shifted about 4 mV more negative by increases in tonicity up to 2.3 times normal, but further increases reversed the direction of the shift, and in 3.95 times normal tonicity the rheobase was 10 mV more positive than in normal Ringer solution. 5. With short (less than 10 ms) pulse durations the depolarization needed to elicit a threshold Arsenazo signal increased steeply with increasing tonicity. Changes in the strength-duration curve could be accounted for by an increase in the time constant for build-up of a hypothetical coupler in the e.-c. coupling process. 6. Solutions of about twice normal tonicity are commonly used to suppress muscle contraction. Since the size of the Arsenazo signal was only slightly reduced by this tonicity, the main effect is presumably on the contractile proteins.

Intrinsic optical and passive electrical properties of cut frog twitch fibers

This article describes a new apparatus for making simultaneous optical measurements on single muscle fibers at three different wavelengths and two planes of linear polarization. There are two modes of operation: mode 1 measures the individual absorbances of light linearly polarized along and perpendicular to the fiber axis, and mode 2 measures retardation (or birefringence) and the average of the two absorbance components. Although some intact frog twitch fibers were studied, most experiments used cut fibers (Hille, B., and D. T. Campbell. 1976. Journal of General Physiology. 67:265-293) mounted in a double-Vaseline-gap chamber (Kovacs, L., E. Rios, and M. F. Schneider. 1983. Journal of Physiology. 343:161-196). The end-pool segments were usually exposed for 2 min to 0.01% saponin. This procedure, used in subsequent experiments to make the external membranes in the end pools permeable to Ca indicators (Maylie, J., M. Irving, N. L. Sizto, G. Boyarsky, and W. K. Chandler. 1987. Journal of General Physiology. 89:145-176; Maylie, J., M. Irving, N. L. Sizto, and W. K. Chandler. 1987. Journal of General Physiology. 89:41-143), was routinely employed so that all our cut fiber results would be comparable. A simple method, which does not require microelectrodes, allowed continual estimation of a fiber's membrane (rm) and internal longitudinal (ri) resistances as well as the external resistance (re) under the Vaseline seals. The values of rm and ri obtained from cut fibers with this method agree reasonably well with values obtained from intact fibers using microelectrode techniques. Optical measurements were made on resting and action potential-stimulated fibers. The intrinsic fiber absorbance, defined operationally as log10 of the ratio of incident light to transmitted light intensity, was similar in intact and cut preparations, as were the changes that accompanied stimulation. On the other hand, the resting birefringence and the peak of the active change in cut fibers were, respectively, only 0.8 and 0.7 times the corresponding values in intact fibers. Both the amplitude and the half-width of the active retardation signal increased considerably during the time course of cut fiber experiments; a twofold increase in 2 h was not unusual. Such changes are probably due to a progressive alteration in the internal state of the cut fibers.

Comparison of arsenazo III optical signals in intact and cut frog twitch fibers

The Ca indicator arsenazo III was introduced into cut frog twitch fibers by diffusion from end-pool segments rendered permeable by saponin. After 2-3 h, the arsenazo III concentration at the optical recording site in the center of a fiber reached two to three times that in the end-pool solutions. Thus, arsenazo III was bound to or taken up by intracellular constituents. The time course of indicator appearance was fitted by equations for diffusion plus linear reversible binding; on average, 0.73 of the indicator was bound and the free diffusion constant was 0.86 x 10(-6) cm2/s at 18 degrees C. When the indicator was removed from the end pools, it failed to diffuse away from the optical site as rapidly as it had diffused in. The wavelength dependence of resting arsenazo III absorbance was the same in cut fibers and injected intact fibers. After action potential stimulation, the active Ca and dichroic signals were similar in the two preparations, which indicates that arsenazo III undergoes the same changes in absorbance and orientation in both cut and intact fibers. Ca transients in freshly prepared cut fibers appeared to be similar to those in intact fibers. As a cut fiber experiment progressed, however, the Ca signal changed. With action potential stimulation, the half-width of the signal gradually increased, regardless of whether the indicator concentration was increasing or decreasing. This increase was usually not accompanied by any change in the amplitude of the Ca signal at a given indicator concentration or by any obvious deterioration in the electrical condition of the fiber. In voltage-clamp experiments near threshold, the relation between peak [Ca] and voltage usually became less steep with time and shifted to more negative potentials. All these changes were also observed in cut fibers containing antipyrylazo III (Maylie, J., M. Irving, N. L. Sizto, and W. K. Chandler. 1987. Journal of General Physiology. 89:83-143). They are considered to represent a progressive change in the physiological state of a cut fiber during the time course of an experiment.

Minimal latency of calcium release in frog twitch muscle fibres

Intracellular release of calcium in frog skeletal muscle fibres was monitored by the use of arsenazo III, in response to voltage clamped depolarizing pulses. A latency of a few milliseconds was evident between the onset of depolarization and the first detectable rise in the arsenazo-calcium signal, and this decreased logarithmically as the depolarization was increased. The minimal latency with strong depolarization (to +20 to +100 mV) was about 2 ms at 5 degrees C. This delay appears to be sufficiently long to be compatible with a chemically mediated coupling mechanism between depolarization and calcium release from the sarcoplasmic reticulum.

Properties of the metallochromic dyes Arsenazo III, Antipyrylazo III and Azo1 in frog skeletal muscle fibres at rest

Intact single twitch fibres from frog muscle were isolated and mounted in a normal Ringer solution (16 degrees C) on an optical bench apparatus for measuring fibre absorbance as a function of the wave-length and polarization of the incident light. Fibre absorbance was measured in resting fibres both in the absence and in the presence of one of three metallochromic dyes: Arsenazo III, Antipyrylazo III and Azo1. In the absence of dye, the fibre intrinsic absorbance, Ai(lambda), measured as a function of wave-length, lambda, was well described by the equation: Ai(lambda) = Ai(lambda long) (lambda long/lambda)X, where lambda long is a reference wave-length selected to lie beyond the absorbance band of the dyes and X is the exponential index. For wave-lengths between 480 and 810 nm, the average value of X was 1.1 for 0 deg polarized light (electric vector parallel to the fibre axis) and 1.3 for 90 deg polarized light (electric vector perpendicular to the fibre axis). The intrinsic absorbance at 0 deg, Ai,0(lambda), was somewhat larger than the intrinsic absorbance at 90 deg, Ai,90(lambda); for example, on average (n = 6), Ai,0 (810 nm) was 0.22, whereas Ai,90 (810 nm) was 0.016. Following dye injection, dye-related absorbance was estimated from the measured total fibre absorbance by subtracting the component attributable to the intrinsic absorbance; additionally, for comparison with in vitro calibrations as a function of wave-length, myoplasmic dye absorbance was corrected for the steady change in dye-concentration with time that was attributable to dye diffusion. In fibres injected with either Arsenazo III or Antipyrylazo III, the dye-related absorbance measured with 0 deg light, A0(lambda), was found to be significantly greater than that measured with 90 deg light, A90(lambda), indicating the presence of a resting 'dichroic' signal, A0(lambda)-A90(lambda), attributable to bound and oriented dye molecules. On average, the lower limit estimated for the percentage of oriented dye was 2.8-3.0% for Antipyrylazo III and 1.5-1.8% for Arsenazo III, the population differences between the two dyes being statistically significant. The actual percentage of bound and oriented dye molecules is likely to be considerably larger for both dyes. For Arsenazo III, the wave-length dependence of the dichroic signal was not distinguishably different from the 'isotropic' signal, defined as (A0(lambda) + 2A90(lambda))/3. which represents the average spectrum of all the dye molecules independent of orientation.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of tetanus duration on the free calcium during the relaxation of frog skeletal muscle fibres

Single fibres were dissected from the tibialis anterior muscle of the frog and injected with the photoprotein aequorin. Tension and the light emission of the injected aequorin (a function of the free intracellular calcium concentration) were recorded both at rest and during tetanus relaxation. The level of light emission from resting single fibres corresponded to a free intracellular calcium concentration ([Ca2+]i) of 100 nM (+/- 40 nM, n = 4). The time course of the decline in light was examined during the three periods of muscle relaxation: period 1 during the slow phase of tension relaxation, period 2 during the exponential phase of relaxation and period 3 after the completion of force relaxation. The time course of the decline in light (after a correction for the kinetics of the aequorin reaction) showed that [Ca2+]i declines exponentially with a rate constant of 25 s-1 (+/- 1.7, n = 3) after a single stimulus at 10 degrees C. With increasing tetanus duration, the rate of decline of [Ca2+]i decreased during period 1. It is suggested that this decrease in the rate of decline of [Ca2+]i results from an intracellular calcium buffer (which takes up calcium in parallel with the sarcoplasmic reticulum) becoming loaded with calcium during the tetanus. Throughout period 2 [Ca2+]i was elevated above resting levels. The level of [Ca2+]i during this period varied from fibre to fibre but could be as high as 1 microM. The mean level of [Ca2+]i during this period also depended on the tetanus duration. A long-lasting elevation in [Ca2+]i was observed during period 3, [Ca2+]i returning towards resting levels with an approximately exponential time course. During this period the level of [Ca2+]i (at a given time after the last stimulus) depended on the tetanus duration. It is suggested that this long-lasting elevation in [Ca2+]i reflects the release of calcium from the intracellular calcium buffer described above. The results suggest that the rate of decline of [Ca2+]i after a few seconds of tetanic stimulation can be explained by the rate of calcium sequestration by the sarcoplasmic reticulum. The increased rate of decline of [Ca2+]i after shorter periods of stimulation may be explained by the presence of a buffer that takes up calcium in parallel with the sarcoplasmic reticulum. The later release of calcium from this buffer gives rise to the long-lasting elevation in [Ca2+]i during period 3. The slow kinetics of calcium binding and release by this buffer appear compatible with published data on the kinetic properties of parvalbumin.

Structural changes during activation of frog muscle studied by time-resolved X-ray diffraction

The pattern given by contracting frog muscle can be followed with high time resolution using synchrotron radiation as a high-intensity X-ray source. We have studied the behaviour of the second actin layer-line (axial spacing of approximately 179 A) at an off-meridional spacing of approximately 0.023 A-1, a region of the diagram that is sensitive to the position of tropomyosin in the thin filaments. In confirmation of earlier work, we find that there is a substantial increase in the intensity of this part of the pattern during contraction. We find that the reflection reaches half its final intensity about 17 milliseconds after the stimulus at 6 degrees C. The changes in the equatorial reflections, which arise from movement of crossbridges towards the thin filaments, occur with a delay of about 12 to 17 milliseconds relative to this change in the actin pattern. In over-stretched muscle, where thick and thin filaments no longer overlap, the changes in the actin second layer-line still take place upon stimulation with a time course and intensity similar to that observed at full overlap. This indicates that tropomyosin movement, in response to calcium binding to troponin, is the first structural step in muscular contraction, and is the prerequisite for myosin binding. A change in intensity similar to that found in contracting muscle is seen in rigor, where tropomyosin is probably locked in the active position. During relaxation the earlier stages in the decrease in intensity of the second actin layer-line take place significantly sooner after the last stimulus than tension decay. In over-stretched muscles the intensity decay is appreciably faster than in the same muscles at rest length, where attached crossbridges may interfere with the return of tropomyosin to its resting position.

The removal of myoplasmic free calcium following calcium release in frog skeletal muscle

Transient changes in intracellular free calcium concentration (delta [Ca2+]) in response to pulse depolarizations were monitored in isolated segments of single frog skeletal muscle fibres cut at both ends and voltage clamped at a holding potential of -90 mV in a double-Vaseline-gap chamber. Calcium transients were monitored optically using the metallochromic indicator dye Antipyrylazo III (APIII), which entered the fibre by diffusion from the solution applied to the cut ends. Optical artifacts due to fibre movement were minimized or eliminated by stretching the fibres to sarcomere lengths at which there was little or no overlap of thick and thin contractile filaments. Remaining movement-independent optical changes intrinsic to the fibre and unrelated to the dye were monitored at 850 nm, where free and dye-bound APIII have no absorbance. These 850 nm signals scaled by lambda -1.2 were used to remove intrinsic components from the signals at 700 or 720 nm, wave-lengths at which the APIII absorbance increases when calcium is bound. The corrected 700 or 720 nm signals were used to calculate delta [Ca2+]. The decay of delta [Ca2+] following fibre repolarization at the termination of a depolarizing pulse was well described by a single exponential plus a constant. The exponential rate constant for the decay of delta [Ca2+] decreased and the final 'steady' level that delta [Ca2+] appeared to be approaching increased with increasing amplitude and/or duration of the depolarizing pulse. Both the decreasing decay rate and the build up of the 'steady' level can be accounted for using a two-component model for the removal of free calcium from the myoplasm. One component consists of a set number of a single type of saturable calcium binding site in the myoplasm. The second component is a non-saturable, first-order uptake mechanism operating in parallel with the saturable binding sites. The removal model parameter values were adjusted to fit simultaneously the decay of delta [Ca2+] after pulses of various amplitudes and durations in a given fibre. The basic procedure was to track delta [Ca2+] during each pulse when an undetermined calcium release was occurring, but to calculate the decay of delta [Ca2+] starting 14 ms after repolarization when release was assumed to be negligible. After appropriate selection of parameter values, the model reproduced most aspects of the decay of delta [Ca2+].(ABSTRACT TRUNCATED AT 400 WORDS)

Post-synaptic calcium influx at the giant synapse of the squid during activation by glutamate

Changes in free calcium were monitored in the post-synaptic axon of the giant synapse of the squid, using the calcium indicators aequorin and Arsenazo III. The peak size of the calcium-dependent optical signals recorded from aequorin and Arsenazo III both showed a linear relation with the amount of calcium injected ionophoretically into the axon, but the Arsenazo signal had a slower time course than the aequorin. Ionophoretic application of glutamate to the post-synaptic axon depolarized the axon and caused a rise in intracellular free calcium. Aequorin signals were detected in natural sea water, and their size increased when the calcium concentration in the sea water was raised. Arsenazo signals could be detected only in high-calcium (55 mM) sea water. Intracellular calcium signals were detected also during bath application of several glutamate analogues, including kainate, ibotenate, and aspartate. The peak amplitude of the intracellular calcium signal, monitored with both indicators, increased with increasing ionophoretic glutamate dose, and varied linearly with the integral of the glutamate-induced membrane depolarization. No calcium signals were detected when depolarizations, similar to those produced by glutamate, were induced by current injection in the absence of glutamate. We conclude that glutamate increases the calcium permeability of the post-synaptic membrane, independently of the glutamate-induced depolarization. The glutamate-induced depolarization and the rise in intracellular free calcium increased roughly linearly as the membrane potential was made more negative. Extrapolation of these data indicated that the glutamate depolarization would reduce to zero at about -30 mV, while the calcium signals would be suppressed at about +50 mV.

Calcium entry into voltage-clamped presynaptic terminals of squid

Voltage-clamp measurements of Ca current and Arsenazo III measurements of intracellular Ca concentration were used to assess Ca ion entry into voltage-clamped presynaptic terminals of squid 'giant' synapses. Depolarization of voltage-clamped terminals filled with Arsenazo III produced absorbance changes consistent with intracellular accumulation of Ca ions. These intracellular Ca transients had a bell-shaped dependence on presynaptic potential and were maximal at approximately -10 mV. Arsenazo III signals recorded from the proximal portion of voltage-clamped presynaptic terminals had a dependence on command potential which was shifted relative to signals recorded from other presynaptic regions. Micro-electrode measurements of presynaptic membrane potential showed that during voltage-clamp depolarizations the proximal region was less depolarized than the rest of the presynaptic terminal. This indicates that voltage-clamped presynaptic terminals may be poorly controlled at their proximal region due to current flow into the adjacent axon. This poor control can cause heterogeneous Ca entry into the presynaptic terminal and thus heterogeneous release of transmitter along the terminal. Application of Ca ions from an extracellular pipette positioned near the distal end of the presynaptic terminal was used to restrict Ca entry to this well-controlled region. Local Ca application decreased the contribution of release from the poorly controlled proximal region to synaptic transfer curves. Presynaptic Ca currents were derived by correcting membrane currents for leakage and capacitive currents and other currents measured in the absence of Ca application. Ca currents measured in this way activated along a sigmoidal time course and did not inactivate for depolarizations as long as 25 ms. Peak Ca currents occurred at approximately -10 mV and inward Ca currents had an apparent 'reversal potential' near +60 mV. Ca channel activation, assessed with tail current measurements, was half-maximal at -13 mV and maximal at +20 mV. Simultaneous measurements of presynaptic Ca currents and Arsenazo III transients revealed a quantitative correspondence between Ca current integrals and Arsenazo III signal amplitude. This suggests that both methods provide reliable measures of Ca ion entry into presynaptic terminals under these conditions.

Voltage dependence of membrane charge movement and calcium release in frog skeletal muscle fibres

Voltage dependent membrane charge movement (gating current) and the release of Ca2+ from intracellular stores have been measured simultaneously in intact frog skeletal muscle fibres. Charge movement was measured using the three microelectrode voltage clamp technique. Ca2+ release was measured using the metallochromic indicator dye arsenazo III. Fibres were bathed in 2.3 X hypertonic solutions to prevent contraction. Rb+, tetraethylammonium and tetrodotoxin (TTX) were used to eliminate voltage-dependent ionic currents. The maximum rate of Ca2+ release from the sarcoplasmic reticulum in response to voltage-clamp step depolarizations to 0 mV was calculated using the dye-related parameters of model 2 of Baylor et al. (1983) and a method described in the Appendix for calculating a scaling factor (1 + p) that accounts for the additional Ca2+ buffering power of the indicator dye. The estimates of the maximum rate of Ca2+ release at 5-6 degrees C ranged from 3 to 19 microM ms-1 in the 17 fibres examined. The mean value was 8.9 +/- 1.1 microM ms-1 (S.E.M.) The maximum rate of Ca2+ release was linearly related to the magnitude of the nonlinear membrane change moved during suprathreshold depolarizing steps. The voltage dependence of charge movement and the maximum rate of Ca2+ releases were nearly identical at 6 degrees C. The voltage-dependence of the delay between the test step and the onset of Ca2+ release could be adequately described by an equation having the same functional form as the voltage dependence of nonlinear charge movement. The relationship between the test pulse voltage and the delay was shifted to more negative voltages and to shorter delays as the temperature was raised from 6 degrees C to 15 degrees C. The inactivation of Ca2+ release was found to occur at more negative holding voltages and to be more steeply voltage dependent than the immobilization of nonlinear membrane charge movement. The above data are discussed using the 'hypothetical coupler' model of excitation-contraction coupling (Miledi et al., 1983b) applied to the specific case in which each mobile charge group controls the gating of one Ca2+ release site in the sarcoplasmic reticulum.

Kinetic investigations in single muscle fibres using luminescent and fluorescent Ca2+ probes

The Ca2+-sensitive photoprotein aequorin and the Ca2+-dependent fluorescent indicators quin 2 and TnCDANZ have been used to investigate contractile processes in single crustacean muscle fibres. The investigations with quin 2 indicate that the free Ca2+ rises to a maximum value before peak force as with aequorin light (approximately 200 msec delay at 12 degrees C) and subsequently decays more slowly, unlike the majority of the aequorin signal, although an aequorin 'tail' signal remains. The resting quin 2 fluorescence from the cell suggests an upper limit of 348 nM for the resting calcium concentration. Experiments with TnCDANZ indicate that this fluorescence response rises rapidly but then the rate of rise slows to reach a maximum value at a time when peak force is achieved and then the fluorescence signal decays more slowly than force. The latter result implies that Ca2+ is attached to the Ca2+-specific sites of TnC when externally recorded force is small.

Arsenazo III calcium transients and latency relaxation in frog skeletal muscle fibres at different sarcomere lengths

Single, intact, frog muscle fibres were injected electrophoretically with a Ca2+-sensitive metallochromic dye, Arsenazo III, to a local concentration of 1.2-1.5 mmol/l. The intracellular concentration of free Mg2+, estimated photometrically in the presence of approximately millimolar Arsenazo III, was 3-4 mmol/l in fibres at rest. Ca2+-related changes in dye absorbance were characterized in vitro using 1 mM-Arsenazo III in solutions approximating the intracellular ionic environment. Isometric twitch contractions and related changes in light transmittance of dye-injected regions of fibre were recorded at 2.4 and 3.0 micron sarcomere lengths, at 15 degrees C. A method was developed for separating Ca2+ transients from larger, movement-related optical changes recorded as compound signals during fibre contraction. Decreases in twitch amplitude by about one-third following dye injection, together with the in vitro characteristics of the dye, suggested that millimolar intracellular Arsenazo III acted as a major Ca2+ buffer and inhibited the activation of contractile filaments. The onset of both the Ca2+ transient and latency relaxation occurred at virtually the same time in the twitch response and neither of those transition times was altered significantly with changes in sarcomere length from 2.4 to 3.0 micron. The amount of activation Ca2+ released in dye-injected regions of fibres following a single stimulus was about 0.3 mmol/l at 2.4 micron sarcomere length. The rate of rise and the amplitude of the Ca2+ transient were reversibly decreased with increase in sarcomere length from 2.4 to 3.0 micron. That finding is reviewed in relation to other evidence indicating length dependence of the intracellular release and distribution of activation Ca2+ up to 3.9 micron sarcomere length.

The maximum velocity of shortening during the early phases of the contraction in frog single muscle fibres

The maximum velocity of shortening (Vmax) was determined at preset times during the development and the plateau of isometric tetani in single fibres isolated from the tibialis anterior muscle of the frog. Experiments were performed at low temperature (3.6-6 degrees C) and at about 2.25 micron sarcomere length. The controlled velocity release method was used. Vmax was measured by determining the lowest velocity of release required to keep the tension at zero. Extreme care was taken in dissection and mounting of the fibres in order to make the passive series compliance very small. The value of Vmax at the end of the latent period for the development of isometric tension (at 4.5 degrees C about 10 ms after the beginning of the stimulus volley) was already the same as later during either the tension rise or at the plateau of isometric tetani. These results show that the value of Vmax of intact fibres is independent of time and activation subsequent to the latent period, and suggest that the cycling rate of the crossbridges may thus attain its steady-state value just at the end of the isometric latent period.

Extracellular ions and excitation-contraction coupling in frog twitch muscle fibres

Intracellular calcium transients were recorded from voltage-clamped frog twitch muscle fibres using Arsenazo III. The possible role of extracellular ions in excitation-contraction (e.-c.) coupling was examined using ion substitutions and blocking drugs in the bathing medium. Parameters measured included the Arsenazo response size to a standard depolarizing pulse (5 ms, 0 mV) and the strength-duration curve for threshold Arsenazo signal. Addition of tetrodotoxin (TTX) decreased the response size to small (-30 mV, 5 ms), but not large (+30 mV, 10 ms) depolarizations, probably because of poor voltage clamp of the tubular membrane in the absence of TTX. Clamping TTX-treated fibres with the wave form of a recorded action potential gave an Arsenazo response similar to that elicited by the normal action potential (at 10 degrees C). Complete substitution of sodium (by choline, lithium or Tris) or chloride (by methyl sulphate or maleate) in the bathing solution gave no appreciable changes in the size of the Arsenazo response. Reduction of extracellular free [Ca2+] to low levels using EGTA caused a slight reduction in the calcium signal elicited by the standard depolarization (to 74% after a few hours, and to 62% after 2 days; temperature 5-10 degrees C). The strength-duration curve was unchanged. Arsenazo responses about 75% of the control size could be elicited in high potassium solution (42 mM-K2SO4) by strong (+80 mV, 20 ms) depolarizations, after re-polarizing the fibres to -90 mV for a few minutes. The voltage dependence of activation was shifted to more positive potentials in this solution. Tetraethylammonium (TEA) bromide at a concentration of 20 mM did not alter the Arsenazo signal, whilst 120 mM-TEA reduced the response by 25%. 3,4-diaminopyridine (DAP) reduced the size of the Arsenazo signal at a concentration of 5 mM, and caused spontaneous release of calcium from the sarcoplasmic reticulum (s.r.) in the absence of membrane potential changes. The Arsenazo signal elicited by an action potential was enhanced by 1 mM-DAP, because of prolongation of the action potential, but was depressed by higher concentrations. We conclude that e.-c. coupling does not involve the influx of any external ions into the muscle fibre. If a current flow between the T-tubules and the s.r. is involved in e.-c. coupling, then this is probably carried by an efflux of potassium ions.

Model of calcium movements during activation in the sarcomere of frog skeletal muscle

A model of calcium movement during activation of frog skeletal muscle is described. The model was based on the half sarcomere of a myofibril and included compartments representing the terminal cisternae, the longitudinal sarcoplasmic reticulum, the extramyofibrillar space, and the myofibrillar space. The calcium-binding proteins troponin, parvalbumin, and calsequestrin were present in appropriate locations and with realistic binding kinetics. During activation a time-dependent permeability in the terminal cisternal wall led to calcium release into the myoplasm and its diffusion through the myoplasm longitudinally and radially was computed. After adjustment of three parameters, the model produced a myoplasmic free-calcium concentration that was very similar to those recorded experimentally with calcium indicators. The model has been used to demonstrate the importance of parvalbumin in the relaxation of skeletal muscle, to describe the time course and magnitude of calcium gradients associated with diffusion across the sarcomere, and to estimate the errors associated with the use of aequorin as an intracellular calcium indicator in muscle.

Time course of calcium release and removal in skeletal muscle fibers

The transient increase in free myoplasmic calcium concentration due to depolarization of a skeletal muscle fiber is the net result of the release of calcium from the sarcoplasmic reticulum (SR) and its simultaneous removal by binding to various sites and by reuptake into the SR. We present a procedure for empirically characterizing the calcium removal processes in voltage-clamped fibers and for using such characterization to determine the time course of SR calcium release during a depolarizing pulse. Our results reveal a decline of the SR calcium release rate during depolarization that was not anticipated from simple inspection of the calcium transients.

Sarcoplasmic reticulum calcium release in frog skeletal muscle fibres estimated from Arsenazo III calcium transients

Single twitch fibres, dissected from frog muscle, were injected with the metallochromic dye Arsenazo III. Changes in dye-related absorbance measured at 650 or 660 nm were used to estimate the time course of myoplasmic free [Ca2+] following either action potential stimulation or voltage-clamp depolarization (temperature, 15-17 degrees C). The amplitude of the Ca2+ transient decreased when fibres were stretched to sarcomere spacings approaching 4 microns. The effect appeared to be less marked in H2O Ringer than in D2O Ringer, where a reduction of about 40% was observed in going from 3.0 microns to 3.7-3.9 microns. In fibres heavily injected with dye (1.5-2.2 mM-dye) at least 0.1 mM-Ca2+ was complexed with Arsenazo III following a single action potential, implying that at least 0.1 mM-Ca2+ was released from the sarcoplasmic reticulum (s.r.) into the myoplasm. Computer simulations were carried out to estimate the flux of Ca2+ between the s.r. and myoplasm (in fibres containing no more that 0.8 mM-dye). The amounts and time courses of Ca2+ bound to the Ca2+-regulatory sites on troponin and to the Ca2+, Mg2+ sites on parvalbumin were estimated from the free [Ca2+] wave form and the law of mass action. In the computations the total myoplasmic [Ca2+] was taken as the total amount of Ca2+ existing either as free ion or as ion complexed with dye, troponin or parvalbumin. The time derivative of total myoplasmic [Ca2+] was used as an estimate of net Ca2+ flux (release minus uptake) from the s.r. into myoplasm. Rate constants for formation of cation: receptor complex were taken from published values. For the Ca2+-regulatory sites on troponin, three sets of rate constants, corresponding to two values of dissociation constant (0.2 and 2 microM) were used. Each set of three simulations was carried out both with and without parvalbumin. The simulations show that following action potential stimulation, 0.2-0.3 mM-Ca2+ enters the myoplasm from the s.r. The wave form of s.r. Ca2+ release is early and brief compared with the wave form of free [Ca2+]. Neither the selection of troponin rate constants nor the inclusion of parvalbumin has much effect on the shape of the release wave form; the main effect of varying these parameters is to change the magnitude. After the initial, rapid phase of Ca2+ release from the s.r. there is a longer, maintained period of Ca2+ uptake.(ABSTRACT TRUNCATED AT 400 WORDS)