Calcium-induced calcium release in crayfish skeletal muscle

Voltage-induced calcium release in Caenorhabditis elegans body muscles

Type 1 voltage-activated calcium channels (CaV1) in the plasma membrane trigger calcium release from the sarcoplasmic reticulum (SR) by two mechanisms. In voltage-induced calcium release (VICR), CaV1 voltage sensing domains are directly coupled to ryanodine receptors (RYRs), an SR calcium channel. In calcium-induced calcium release (CICR), calcium ions flowing through activated CaV1 channels bind and activate RYR channels. VICR is thought to occur exclusively in vertebrate skeletal muscle while CICR occurs in all other muscles (including all invertebrate muscles). Here, we use calcium-activated SLO-2 potassium channels to analyze CaV1-SR coupling in Caenorhabditis elegans body muscles. SLO-2 channels were activated by both VICR and external calcium. VICR-mediated SLO-2 activation requires two SR calcium channels (RYRs and IP3 Receptors), JPH-1/Junctophilin, a PDZ (PSD95, Dlg1, ZO-1 domain) binding domain (PBD) at EGL-19/CaV1's carboxy-terminus, and SHN-1/Shank (a scaffolding protein that binds EGL-19's PBD). Thus, VICR occurs in invertebrate muscles.

The Impacts of the Multispecies Approach to Caffeine on Marine Invertebrates

Caffeine is one of the most consumed substances by humans through foodstuffs (coffee, tea, drugs, etc.). Its human consumption releases a high quantity of caffeine into the hydrological network. Thus, caffeine is now considered an emergent pollutant sometimes found at high concentrations in oceans and seas. Surprisingly, little research has been conducted on the molecular responses induced by caffeine in marine organisms. We studied, in laboratory conditions, six phylogenetically distant species that perform distinct ecological functions (Actinia equina and Aulactinia verrucosa (cnidarians, predator), Littorina littorea (gastropod, grazer), Magallana gigas (bivalve, filter-feeder), and Carcinus maenas and Pachygrapsus marmoratus (crabs, predator and scavenger)) subjected to caffeine exposure. The antioxidant responses (catalase, CAT; glutathione peroxidase, GPx; superoxide dismutase, SOD), lipid peroxidation (MDA), and the acetylcholinesterase (AChE) activity were estimated when the organisms were exposed to environmental caffeine concentrations (5 μg/L (low), 10 μg/L (high)) over 14 days. Differential levels of responses and caffeine effects were noted in the marine invertebrates, probably in relation to their capacity to metabolization the pollutant. Surprisingly, the filter feeder (M. gigas, oyster) did not show enzymatic responses or lipid peroxidation for the two caffeine concentrations tested. The marine gastropod (grazer) appeared to be more impacted by caffeine, with an increase in activities for all antioxidative enzymes (CAT, GPx, SOD). In parallel, the two cnidarians and two crabs were less affected by the caffeine contaminations. However, caffeine was revealed as a neurotoxic agent to all species studied, inducing high inhibition of AChE activity. This study provides new insights into the sublethal impacts of caffeine at environmentally relevant concentrations in marine invertebrates.

Elementary calcium release events in the skeletal muscle cells of the honey bee Apis mellifera

Calcium sparks are involved in major physiological and pathological processes in vertebrate muscles but have never been characterized in invertebrates. Here, dynamic confocal imaging on intact skeletal muscle cells isolated enzymatically from the adult honey bee legs allowed the first spatio-temporal characterization of subcellular calcium release events (CREs) in an insect species. The frequency of CREs, measured in x–y time lapse series, was higher than frequencies usually described in vertebrates. Honey bee CREs had a larger spatial spread at half maximum than their vertebrate counterparts and a slightly ellipsoidal shape, two characteristics that may be related to ultrastructural features specific to invertebrate cells. In line-scan experiments, the histogram of CREs’ duration followed a bimodal distribution, supporting the existence of both sparks and embers. Unlike in vertebrates, embers and sparks had similar amplitudes, a difference that could be related to genomic differences and/or excitation–contraction coupling specificities in honey bee skeletal muscle fibres. The first characterization of CREs from an arthropod which shows strong genomic, ultrastructural and physiological differences with vertebrates may help in improving the research field of sparkology and more generally the knowledge in invertebrates cell Ca²⁺ homeostasis, eventually leading to a better understanding of their roles and regulations in muscles but also the myotoxicity of new insecticides targeting ryanodine receptors.

Stac protein regulates release of neuropeptides

Neuropeptides are important for regulating numerous neural functions and behaviors. Release of neuropeptides requires long-lasting, high levels of cytosolic Ca²⁺ However, the molecular regulation of neuropeptide release remains to be clarified. Recently, Stac3 was identified as a key regulator of L-type Ca²⁺ channels (CaChs) and excitation-contraction coupling in vertebrate skeletal muscles. There is a small family of stac genes in vertebrates with other members expressed by subsets of neurons in the central nervous system. The function of neural Stac proteins, however, is poorly understood. Drosophila melanogaster contain a single stac gene, Dstac, which is expressed by muscles and a subset of neurons, including neuropeptide-expressing motor neurons. Here, genetic manipulations, coupled with immunolabeling, Ca²⁺ imaging, electrophysiology, and behavioral analysis, revealed that Dstac regulates L-type CaChs (Dmca1D) in Drosophila motor neurons and this, in turn, controls the release of neuropeptides.

Dstac Regulates Excitation-Contraction Coupling in Drosophila Body Wall Muscles

Stac3 regulates excitation-contraction coupling (EC coupling) in vertebrate skeletal muscles by regulating the L-type voltage-gated calcium channel (Ca_v channel). Recently a stac-like gene, Dstac, was identified in Drosophila and found to be expressed by both a subset of neurons and muscles. Here, we show that Dstac and Dmca1D, the Drosophila L-type Ca_v channel, are necessary for normal locomotion by larvae. Immunolabeling with specific antibodies against Dstac and Dmca1D found that Dstac and Dmca1D are expressed by larval body-wall muscles. Furthermore, Ca²⁺ imaging of muscles of Dstac and Dmca1D deficient larvae found that Dstac and Dmca1D are required for excitation-contraction coupling. Finally, Dstac appears to be required for normal expression levels of Dmca1D in body-wall muscles. These results suggest that Dstac regulates Dmca1D during EC coupling and thus muscle contraction.

The buzz on caffeine in invertebrates: effects on behavior and molecular mechanisms

A number of recent studies from as diverse fields as plant-pollinator interactions, analyses of caffeine as an environmental pollutant, and the ability of caffeine to provide protection against neurodegenerative diseases have generated interest in understanding the actions of caffeine in invertebrates. This review summarizes what is currently known about the effects of caffeine on behavior and its molecular mechanisms in invertebrates. Caffeine appears to have similar effects on locomotion and sleep in both invertebrates and mammals. Furthermore, as in mammals, caffeine appears to have complex effects on learning and memory. However, the underlying mechanisms for these effects may differ between invertebrates and vertebrates. While caffeine's ability to cause release of intracellular calcium stores via ryanodine receptors and its actions as a phosphodiesterase inhibitor have been clearly established in invertebrates, its ability to interact with invertebrate adenosine receptors remains an important open question. Initial studies in insects and mollusks suggest an interaction between caffeine and the dopamine signaling pathway; more work needs to be done to understand the mechanisms by which caffeine influences signaling via biogenic amines. As of yet, little is known about whether other actions of caffeine in vertebrates, such as its effects on GABAA and glycine receptors, are conserved. Furthermore, the pharmacokinetics of caffeine remains to be elucidated. Overall behavioral responses to caffeine appear to be conserved amongst organisms; however, we are just beginning to understand the mechanisms underlying its effects across animal phyla.

An evaluation of muscle maintenance costs during fiber hypertrophy in the lobster Homarus americanus: are larger muscle fibers cheaper to maintain?

Large muscle fiber size imposes constraints on muscle function while imparting no obvious advantages, making it difficult to explain why muscle fibers are among the largest cell type. Johnston and colleagues proposed the 'optimal fiber size' hypothesis, which states that some fish have large fibers that balance the need for short diffusion distances against metabolic cost savings associated with large fibers. We tested this hypothesis in hypertrophically growing fibers in the lobster Homarus americanus. Mean fiber diameter was 316±11 μm in juveniles and 670±26 μm in adults, leading to a surface area to volume ratio (SA:V) that was 2-fold higher in juveniles. Na(+)/K(+)-ATPase activity was also 2-fold higher in smaller fibers. (31)P-NMR was used with metabolic inhibitors to determine the cost of metabolic processes in muscle preparations. The cost of Na(+)/K(+)-ATPase function was also 2-fold higher in smaller than in larger diameter fibers. Extrapolation of the SA:V dependence of the Na(+)/K(+)-ATPase over a broad fiber size range showed that if fibers were much smaller than those observed, maintenance of the membrane potential would constitute a large fraction of whole-animal metabolic rate, suggesting that the fibers grow large to reduce maintenance costs. However, a reaction-diffusion model of aerobic metabolism indicated that fibers in adults could attain still larger sizes without diffusion limitation, although further growth would have a negligible effect on cost. Therefore, it appears that decreased fiber SA:V makes larger fibers in H. americanus less expensive to maintain, which is consistent with the optimal fiber size hypothesis.

The neuropeptide proctolin potentiates contractions and reduces cGMP concentration via a PKC-dependent pathway

As in many other arthropods, the neuropeptide proctolin enhances contractures of muscles in the crustacean isopod Idotea emarginata. The enhancement of high K+-induced contractures by proctolin (1 micromol l-1) was mimicked upon application of the protein kinase C (PKC) activator phorbol-12-myristate 1-acetate (PMA) and was inhibited by the PKC inhibitor bisindolylmaleimide (BIM-1). The potentiation was not inhibited by H89, a protein kinase A (PKA) inhibitor. Proctolin did not change the intracellular concentration of 3',5'-cyclic adenosine monophosphate (cAMP) whereas it significantly reduced the intracellular concentration of 3',5'-cyclic guanosine monophosphate (cGMP). The reduction of cGMP was not observed in the presence of the PKC inhibitor BIM-1. 8-Bromo-cGMP, a membrane-permeable cGMP analogue, reduced the potentiating effect of proctolin on muscle contracture. We thus conclude that proctolin in the studied crustacean muscle fibres induces an activation of PKC, which leads to a reduction of the cGMP concentration and, consequently, to the potentiation of muscle contracture.

Characterization of single L-type Ca2+ channels in myocytes isolated from the cricket lateral oviduct

The single Ca2+ channel activity was obtained from cell-attached patch recordings with the use of pipettes filled with 100 mM Ba2+ as the charge carrier in myocytes isolated from the lateral oviduct of cricket Gryllus bimaculatus. The following results were obtained. (1) The channel had a unitary conductance of 18 pS. (2) The open time histogram of the channel could be fitted with a single exponential while the closed time histogram could be fitted with the sum of two exponentials, suggesting that there are at least one open state and two closed states for this channel. (3) The open probability of the channel increased with increasing membrane depolarization. (4) The mean current reconstructed by averaging individual current trace responses inactivated slowly and the current-voltage relationship for the peak mean current showed a bell-shaped relation. (5) The dihydropyridine (DHP) Ca2+ antagonist, nifedipine, reduced the mean current by increasing the proportion of "blank" sweeps. On the other hand, the DHP Ca2+ agonist, Bay K 8644, increased the mean current by increasing the mean open-times of the channel. These results confirm a presence of DHP-sensitive L-type Ca2+ channel in myocytes isolated from the lateral oviduct of cricket G. bimaculatus.

Tubular localization of silent calcium channels in crustacean skeletal muscle fibers

Ca2+-induced Ca2+ release (CICR) in the superficial abdominal flexor muscle of the crustacean Atya lanipes appears to be mediated by a local control mechanism similar to that of vertebrate cardiac muscle, but with an unusually high gain. Thus, Ca2+ influx increases sufficiently the local concentration of Ca2+ in the immediate vicinity of the sarcoplasmic reticulum Ca2+ release channels to trigger the highly amplified release of Ca2+ required for contraction, but is too low to generate a macroscopic inward current (i.e., the Ca2+ channels are silent). To determine the localization of the silent Ca2+ Channels, the mechanical, electrophysiological and ultrastructural properties of the muscle were examined before and after formamide treatment, a procedure that produces the disruption of transverse tubules of striated muscle. We found that tubular disruption decreased tension generation by about 90%; reduced inward current (measured as Vmax, the maximum rate of rise of Sr2+ action potentials) by about 80%; and decreased membrane capacitance by about 77%. The results suggest that ca. 80% of the silent Ca2+ channels are located in the tubular system. Thus, these studies provide further evidence to support the local control mechanism of CICR in crustacean skeletal muscle.

The L-type voltage-dependent Ca2+ channel EGL-19 controls body wall muscle function in Caenorhabditis elegans

Caenorhabditis elegans is a powerful model system widely used to investigate the relationships between genes and complex behaviors like locomotion. However, physiological studies at the cellular level have been restricted by the difficulty to dissect this microscopic animal. Thus, little is known about the properties of body wall muscle cells used for locomotion. Using in situ patch clamp technique, we show that body wall muscle cells generate spontaneous spike potentials and develop graded action potentials in response to injection of positive current of increasing amplitude. In the presence of K+ channel blockers, membrane depolarization elicited Ca2+ currents inhibited by nifedipine and exhibiting Ca2+-dependent inactivation. Our results give evidence that the Ca2+ channel involved belongs to the L-type class and corresponds to EGL-19, a putative Ca2+ channel originally thought to be a member of this class on the basis of genomic data. Using Ca2+ fluorescence imaging on patch-clamped muscle cells, we demonstrate that the Ca2+ transients elicited by membrane depolarization are under the control of Ca2+ entry through L-type Ca2+ channels. In reduction of function egl-19 mutant muscle cells, Ca2+ currents displayed slower activation kinetics and provided a significantly smaller Ca2+ entry, whereas the threshold for Ca2+ transients was shifted toward positive membrane potentials.

Voltage-clamp analysis of membrane currents and excitation-contraction coupling in a crustacean muscle

A single fibre preparation from the extensor muscle of a marine isopod crustacean is described which allows the analysis of membrane currents and simultaneously recorded contractions under two-electrode voltage-clamp conditions. We show that there are three main depolarisation-gated currents, two are outward and carried by K+, the third is an inward Ca2+ current, I(Ca). Normally, the K+ currents which can be isolated by using K+ channel blockers, mask I(Ca). I(Ca) activates at potentials more positive than -40 mV, is maximal around 0 mV, and shows strong inactivation at higher depolarisation. Inactivation depends on current rather than voltage. Ba2+, Sr2+ and Mg2+ can substitute for Ca2+. Ba2+ currents are about 80% larger than Ca2+ currents and inactivate little. The properties of I(Ca) characterise it as a high threshold L-type current. The outward current consists primarily of a fast, transient A current, I(K(A)) and a maintained, delayed rectifier current, I(K(V)). In some fibres, a small Ca2+-dependent K+ current is also present. I(K(A)) activates fast at depolarisation above -45 mV, shows pronounced inactivation and is almost completely inactivated at holding potentials more positive than -40 mV. I(K(A)) is half-maximally blocked by 70 microM 4-aminopyridine (4-AP), and 70 mM tetraethylammonium (TEA). I(K(V)) activates more slowly, at about -30 mV, and shows no inactivation. It is half-maximally blocked by 2 mM TEA but rather insensitive to 4-AP. Physiologically, the two K+ currents prevent all-or-nothing action potentials and determine the graded amplitude of active electrical responses and associated contractions. Tension development depends on and is correlated with depolarisation-induced Ca2+ influx mediated by I(Ca). The voltage dependence of peak tension corresponds directly to the voltage dependence of the integrated I(Ca). The threshold potential for contraction is at about -38 mV. Peak tension increases with increasing voltage steps, reaches maximum at around 0 mV, and declines with further depolarisation.

Peptide-induced Ca(2+) movements in a tonic insect muscle: effects of proctolin and periviscerokinin-2

Although most of the characterized insect neuropeptides have been detected by their actions on muscle contractions, not much is known about the mechanisms underlying excitation-contraction coupling. Thus we initiated a pharmacological study on the myotropic action of the peptides periviscerokinin-2 (PVK-2) and proctolin on the hyperneural muscle of the cockroach Periplaneta americana. Both peptides required extracellular Ca(2+) to induce muscle contraction, and a blockage of sarcolemmal Ca(2+) channels by Mn(2+) or La(3+) inhibited myotropic effects. The peptides were able to induce contractions in dependence on the extracellular Ca(2+) concentration in muscles depolarized with high K(+) saline. A reduction of extracellular Na(+), K(+), or Cl(-) did not effect peptide action. Nifedipine, an L-type Ca(2+)-channel blocker, partially blocked the response to both peptides but to a much lesser extent than contractions evoked by elevated K(+). Using calcium imaging with fluo-3, we show that proctolin induces an increase of the intracellular Ca(2+) concentration. In calcium-free saline, no increase of the intracellular Ca(2+) concentration could be detected. The inhibiting effect of ryanodine, thapsigargin, and TMB-8 on peptide-induced contractions suggests that Ca(2+) release from the sarcoplasmic reticulum plays a major role during peptide-induced contractions. Preliminary experiments suggest that the peptides do not employ cyclic nucleotides as second messengers, but may activate protein kinase C. Our results indicate that the peptides induce Ca(2+) influx by an activation or modulation of dihydropyridine-sensitive and voltage-independent sarcolemmal Ca(2+) channels. Ca(2+)-induced Ca(2+) release from intracellular stores, but not inositol trisphosphate-induced Ca(2+) release, seems to account for most of the observed increase in intracellular Ca(2+). Additionally, both peptides were able to potentiate glutamate-induced contractions at threshold concentrations.

Effect of nitric oxide on calcium-induced calcium release in coronary arterial smooth muscle

The present study was designed to determine whether nitric oxide (NO)-induced reduction of [Ca(2+)](i) is associated with Ca(2+)-induced Ca(2+) release (CICR) in coronary arterial smooth muscle cells (CASMCs). Caffeine was used as a CICR activator to induce Ca(2+) release in these cells. The effects of NO donor, sodium nitroprusside (SNP), on caffeine-induced Ca(2+) release were examined in freshly dissociated bovine CASMCs using single cell fluorescence microscopic spectrometry. The effects of NO donor on caffeine-induced coronary vasoconstriction were examined by isometric tension recordings. Caffeine, a CICR or ryanodine receptor (RYR) activator, produced a rapid Ca(2+) release with a 330 nM increase in [Ca(2+)](i). Pretreatment of the CASMCs with SNP, CICR inhibitor tetracaine or RYR blocker ryanodine markedly decreased caffeine-induced Ca(2+) release. Addition of caffeine to the Ca(2+)-free bath solution produced a transient coronary vasoconstriction. SNP, tetracaine and ryanodine, but not guanylyl cyclase inhibitor, ODQ, significantly attenuated caffeine-induced vasoconstriction. These results suggest that CICR is functioning in CASMCs and participates in the vasoconstriction in response to caffeine-induced Ca(2+) release and that inhibition of CICR is of importance in mediating the vasodilator response of coronary arteries to NO.

Fast BK-type channel mediates the Ca(2+)-activated K(+) current in crayfish muscle

The role of the Ca(2+)-activated K(+) current (I(K(Ca))) in crayfish opener muscle fibers is functionally important because it regulates the graded electrical activity that is characteristic of these fibers. Using the cell-attached and inside-out configurations of the patch-clamp technique, we found three different classes of channels with properties that matched those expected of the three different ionic channels mediating the depolarization-activated macroscopic currents previously described (Ca(2+), K(+), and Ca(2+)-dependent K(+) currents). We investigated the properties of the ionic channels mediating the extremely fast activating and persistent I(K(Ca)). These voltage- and Ca(2+)-activated channels had a mean single-channel conductance of approximately 70 pS and showed a very fast activation. Both the single-channel open probability and the speed of activation increased with depolarization. Both parameters also increased in inside-out patches, i.e., in high Ca(2+) concentration. Intracellular loading with the Ca(2+) chelator bis(2-aminophenoxy) ethane-N, N,N',N'-tetraacetic acid gradually reduced and eventually prevented channel openings. The channels opened at very brief delays after the pulse depolarization onset (<5 ms), and the time-dependent open probability was constant during sustained depolarization (< or =560 ms), matching both the extremely fast activation kinetics and the persistent nature of the macroscopic I(K(Ca)). However, the intrinsic properties of these single channels do not account for the partial apparent inactivation of the macroscopic I(K(Ca)), which probably reflects temporal Ca(2+) variations in the whole muscle fiber. We conclude that the channels mediating I(K(Ca)) in crayfish muscle are voltage- and Ca(2+)-gated BK channels with relatively small conductance. The intrinsic properties of these channels allow them to act as precise Ca(2+) sensors that supply the exact feedback current needed to control the graded electrical activity and therefore the contraction of opener muscle fibers.

Cross-coupling between voltage-dependent Ca2+ channels and ryanodine receptors in developing ascidian muscle blastomeres

1. Ascidian blastomeres of muscle lineage express voltage-dependent calcium channels (VDCCs) despite isolation and cleavage arrest. Taking advantage of these large developing cells, developmental changes in functional relations between VDCC currents and intracellular Ca2+ stores were studied. 2. Inactivation of ascidian VDCCs is Ca2+ dependent, as demonstrated by two pieces of evidence: (1) a bell-shaped relationship between prepulse voltage and amplitude during the test pulse in Ca2+, but not in Ba2+, and (2) the decay kinetics of Ca2+ currents (ICa) obtained as the size of tail currents. 3. During replacement in the external solution of Ca2+ with Ba2+, the inward current appeared biphasic: it showed rapid decay followed by recovery and slow decay. This current profile was most evident in the mixed bath solution (2 % Ca2+ and 98 % Ba2+, abbreviated to '2Ca/98Ba'). 4. The biphasic profile of I2Ca/98Ba was significantly attenuated in caffeine and in ryanodine, indicating that Ca2+ release is involved in shaping the current kinetics of VDCCs. After washing out the caffeine, the biphasic pattern was reproducibly restored by depolarizing the membrane in calcium-rich solution, which is expected to refill the internal Ca2+ stores. 5. The inhibitors of endoplasmic reticulum (ER) Ca2+-ATPase (SERCAs) cyclopiazonic acid (CPA) and thapsigargin facilitated elimination of the biphasic profile with repetitive depolarization. 6. At a stage earlier than 36 h after fertilization, the biphasic profile of I2Ca/98Ba was not observed. However, caffeine induced a remarkable decrease in the amplitude of I2Ca/98Ba and this suppression was blocked by microinjection of the Ca2+ chelator BAPTA, showing the presence of caffeine-sensitive Ca2+ stores at this stage. 7. Electron microscopic observation shows that sarcoplasmic membranes (SR) arrange closer to the sarcolemma with maturation, suggesting that the formation of the ultrastructural machinery underlies development of the cross-coupling between VDCCs and Ca2+ stores.

Dynamic regulation of intracellular calcium signals through calcium release channels

After the seminal work of Ebashi and coworkers which established the essential role of the intracellular Ca2+ concentration ([Ca2+]i) in the regulation of skeletal muscle contraction, we have witnessed an explosive elongation of the list of cell functions that are controlled by the [Ca2+]i. In numerous instances, release of intracellular Ca2+ stores plays important roles in Ca2+ signalling which displays significant variation in spatio-temporal pattern. There are two families of Ca2+ release channels, ryanodine receptors and inositol 1,4,5-trisphosphate (IP3) receptors. These Ca2+ release channels are structurally and functionally similar. In particular, the activity of both types of channels is regulated by the [Ca2+]i. The [Ca2+]i dependence of the Ca2+ release channel activity provides both types of channels with properties of a Ca2+ signal amplifier. This function of the ryanodine receptor is important in striated muscle excitation-contraction coupling, whereas that of the IP3 receptor seems to be the basis of the generation of Ca2+ waves. Thus the wide variety of Ca2+ signalling patterns seem to be critically dependent on the [Ca2+]i dependence of the Ca2+ release channels.

Muscle-specific functions of ryanodine receptor channels in Caenorhabditis elegans

Ryanodine receptor channels regulate contraction of striated muscle by gating the release of calcium ions from the sarcoplasmic reticulum. Ryanodine receptors are expressed in excitable and non-excitable cells of numerous species, including the nematode C. elegans. Unlike vertebrates, which have at least three ryanodine receptor genes, C. elegans has a single gene encoded by the unc-68 locus. We show that unc-68 is expressed in most muscle cells, and that the phenotypic defects exhibited by unc-68 null mutants result from the loss of unc-68 function in pharyngeal and body-wall muscle cells. The loss of unc-68 function in the isthmus and terminal bulb muscles of the pharynx causes a reduction in growth rate and brood size. unc-68 null mutants exhibit defective pharyngeal pumping (feeding) and have abnormal vacuoles in the terminal bulb of the pharynx. unc-68 is required in body-wall muscle cells for normal motility. We show that UNC-68 is localized in body-wall muscle cells to flattened vesicular sacs positioned between the apical plasma membrane and the myofilament lattice. In unc-68 mutants, the vesicles are enlarged and densely stained. The flattened vesicles in body-wall muscle cells thus represent the C. elegans sarcoplasmic reticulum. Morphological and behavioral phenotypes of unc-68 mutants suggest that intracellular calcium release is not essential for excitation-contraction coupling in C. elegans.

Voltage-gated and Ca2+-activated conductances mediating and controlling graded electrical activity in crayfish muscle

Crayfish opener muscle fibers provide a unique preparation to quantitatively evaluate the relationships between the voltage-gated Ca2+ (ICa) and Ca2+-activated K+ (IK(Ca)) currents underlying the graded action potentials (GAPs) that typify these fibers. ICa, IK(Ca), and the voltage-gated K+ current (IK) were studied using two-electrode voltage-clamp applying voltage commands that simulated the GAPs evoked in current-clamp conditions by 60-ms current pulses. This methodology, unlike traditional voltage-clamp step commands, provides a description of the dynamic aspects of the interaction between different conductances participating in the generation of the natural GAP. The initial depolarizing phase of the GAP was due to activation of the ICa on depolarization above approximately -40 mV. The resulting Ca2+ inflow induced the activation of the fast IK(Ca) (<3 ms), which rapidly repolarized the fiber (<6 ms). Because of its relatively slow activation, the contribution of IK to the GAP repolarization was delayed. During the final steady GAP depolarization ICa and IK(Ca) were simultaneously activated with similar magnitudes, whereas IK aided in the control of the delayed sustained response. The larger GAPs evoked by higher intensity stimulations were due to the increase in ICa. The resulting larger Ca2+ inflow increased IK(Ca), which acted as a negative feedback that precisely controlled the fiber's depolarization. Hence IK(Ca) regulated the Ca2+-inflow needed for the contraction and controlled the depolarization that this Ca2+ inflow would otherwise elicit.

Identification of a two EF-hand Ca2+ binding domain in lobster skeletal muscle ryanodine receptor/Ca2+ release channel

The lobster skeletal muscle Ca2+ release channel, known also as the ryanodine receptor, is composed of four polypeptides of approximately 5000 amino acids each, like its mammalian counterparts. Clones encoding the carboxy-terminal region of the lobster ryanodine receptor were isolated from a lobster skeletal muscle cDNA library. Analysis of the deduced 1513 carboxy-terminal amino acid sequence suggests a cytoplasmic Ca2+ binding domain consisting of two EF-hand Ca2+ binding motifs (amino acid residues 594-656). The Ca2+ binding properties of this domain were assessed by preparing bacterial fusion proteins with sequences from the lobster Ca2+ binding domain and the corresponding sequences of the rabbit cardiac and skeletal muscle ryanodine receptors. The lobster skeletal muscle fusion protein bound 45Ca2+ in Ca2+ overlays, and bound two Ca2+ under equilibrium binding conditions with a Hill dissociation constant (KH) of 0.9 mM and coefficient (nH) of 1.4. Rabbit skeletal and cardiac fusion proteins bound two Ca2+ with KHs of 3.7 and 3.8 mM and nHs of 1.1 and 1.3, respectively. Similar to results previously reported for the mammalian RyRs, the lobster RyR was activated by micromolar Ca2+ and inhibited by millimolar Ca2+, as determined in single-channel and [3H]ryanodine binding measurements. These results suggest that the two EF-hand Ca2+ binding domain of the lobster Ca2+ release channel as well as the corresponding regions of the mammalian channels may play a role in Ca2+ inactivation of sarcoplasmic reticulum Ca2+ release.

The effect of tetracaine on stimulated contractions, sarcoplasmic reticulum Ca2+ content and membrane current in isolated rat ventricular myocytes

1. The effects of tetracaine were examined on rat ventricular myocytes. In both field-stimulated and voltage-clamped cells tetracaine (100-200 microM) produced an initial decrease of contraction before a recovery towards the control level. Removal of tetracaine produced a transient overshoot of contraction to levels greater than the control. 2. The transient decrease of contraction produced by tetracaine was accompanied by a small transient increase in the integral of the L-type Ca2+ current and a larger transient decrease of the Na+-Ca2+ exchange current on repolarization. These are attributed to decreased systolic release of Ca2+. On removal of tetracaine there was an increase of the Na+-Ca2+ exchange current. Before the addition of tetracaine, calculated Ca2+ influx and efflux across the sarcolemma were approximately equal. On adding tetracaine, efflux was transiently less than influx and, on removal of tetracaine, efflux was greater than influx. 3. These changes in Ca2+ fluxes result in an increase of cell Ca2+ during exposure to tetracaine. The calculated magnitude of this increase was equal to that measured directly by applying caffeine (20 mM) to release sarcoplasmic reticulum (SR) Ca2+ and integrating the resulting Na+-Ca2+ exchange current. 4. It is concluded that the effects of tetracaine can be accounted for by depression of calcium-induced Ca2+ release (CICR). The response is transient because the inhibition is compensated for by an increase of SR Ca2+ content such that there is no steady-state effect on the magnitude of the systolic Ca2+ transient. The consequences of this result for the effects of other modulators of CICR are discussed.

Contraction and relaxation in the absence of a sarcoplasmic reticulum: muscle fibres in the small pelagic tunicate Doliolum

Previous ultrastructural observations suggested that Doliolum muscle fibres apparently lacked both sarcoplasmic reticulum and transverse tubular-system. External Ca2+ is required for contraction, caffeine does not evoke contraction, nor does it increase intracellular Ca2+ level. Ryanodine at 50 microM has no effect on electrically-evoked contractions. Further, electrical stimulation in external solutions lacking Na+ leads to sustained contracture. We conclude that intracellular Ca2+ stores are absent in these rapid obliquely-striated fibres, and that reduction in internal Ca2+ levels following contraction depends upon Na(+)-Ca2+ exchange across the sarcolemma.

A dihydropyridine-sensitive voltage-dependent calcium channel in the sarcolemmal membrane of crustacean muscle

Single-channel currents through calcium channels in muscle of a marine crustacean, the isopod Idotea baltica, were investigated in cell-attached patches. Inward barium currents were strongly voltage-dependent, and the channels were closed at the cell's resting membrane potential. The open probability (Po) increased e-fold for an 8.2 mV (+/- 2.4, n = 13) depolarization. Channel opening were mainly brief (< 0.3 ms) and evenly distributed throughout 100-ms pulses. Averaged, quasimacroscopic currents showed fast activation and deactivation and did not inactivate during 100-ms test pulses. Similarly, channel activity persisted at steadily depolarized holding potentials. With 200 mM Ba2+ as charge carrier, the average slope conductance from the unitary currents between +30 and +80 mV, was 20 pS (+/- 2.6, n = 12). The proportion of long openings, which were very infrequent under control conditions, was greatly increased by preincubation of the muscle fibers with the calcium channel agonist, the dihydropyridine Bay K8644 (10-100 microM). Properties of these currents resemble those through the L-type calcium channels of mammalian nerve, smooth muscle, and cardiac muscle cells.

Calcium in close quarters: microdomain feedback in excitation-contraction coupling and other cell biological phenomena

Researchers have made good progress in unraveling the molecular mechanisms of excitation-contraction (EC) coupling in striated muscle. Despite this progress, paradoxes abound. In skeletal muscle, the existence of a mechanical coupling between membrane charge movement and activation of sarcoplasmic reticulum (SR) release channels is essentially established, but the contribution of Ca(2+)-induced Ca2+ release (CICR) to the transient and steady-state components of Ca2+ release remains controversial. In cardiac muscle, the role of CICR as the primary mechanism of EC coupling is well established, but the stability and tight coupling between membrane Ca2+ current and release are paradoxical. Answers may lie in microdomain issues, and the examination of discrete elementary release events, although quantitative treatments are needed. This review explores the theoretical and experimental methods used and the observations made in the study of microdomain Ca2+.

Presynaptic calcium currents at voltage-clamped excitor and inhibitor nerve terminals of crayfish

1. A two-electrode voltage clamp was used to record calcium currents from the excitatory and inhibitory nerve terminals that innervate the crayfish (Procambarus spp.) opener muscle. Other voltage-dependent currents were blocked with tetrodotoxin, 3,4-diaminopyridine, 4-aminopyridine and tetraethylammonium. 2. The presynaptic calcium current at both excitatory and inhibitory synapses was blocked by cadmium and omega-agatoxin IVA but was not affected by omega-conotoxin GVIA, omega-conotoxin MVIIC or nifedipine, suggesting that the calcium currents flow through P-type calcium channels. 3. Current-voltage (I-V) relations at both excitatory and inhibitory synapses are similar, with current activation near -40 mV, peak current near -10 mV and current reversal at membrane potentials greater than +25 mV. I-V relations were scaled along the current axis by partial calcium current blockade with cobalt, suggesting that series resistance and space-clamp errors were small. 4. A subset of terminals on one muscle fibre was locally superfused with a physiological saline containing barium; the rest of the preparation was superfused with a physiological saline containing calcium channel antagonists. Under such conditions the characteristics of the I-V relation were very similar to the I-V relations recorded when the entire preparation was bathed in physiological levels of calcium, suggesting that the space clamp was adequate. 5. Calcium channel activation, as determined from tail current analyses, was similar when the entire preparation was bathed in physiological levels of calcium or if terminals on one muscle fibre were locally superfused with barium. 6. During a 30 ms depolarization, calcium currents inactivated to a greater extent in inhibitory than in excitatory terminals. The inactivation was of small magnitude (< 20%) and was eliminated by intracellular injection of the calcium chelator BAPTA, suggesting that the inactivation was calcium dependent. 7. These data show that biophysical and pharmacological properties of calcium currents at crayfish neuromuscular junctions resemble those found at stellate synapses in squid.

unc-68 encodes a ryanodine receptor involved in regulating C. elegans body-wall muscle contraction

Striated muscle contraction is elicited by the release of stored calcium ions through ryanodine receptor channels in the sarcoplasmic reticulum. ryr-1 is a C. elegans ryanodine receptor homologue that is expressed in body-wall muscle cells used for locomotion. Using genetic methods, we show that ryr-1 is the previously identified locus unc-68. First, transposon-induced deletions within ryr-1 are alleles of unc-68. Second, transformation of unc-68 mutants with ryr-1 genomic DNA results in rescue of the Unc phenotype. unc-68 mutants move poorly, exhibiting an incomplete flaccid paralysis, yet have normal muscle ultrastructure. The mutants are insensitive to the paralytic effects of ryanodine, and lack detectable ryanodine-binding activity. The Unc-68 phenotype suggests that ryanodine receptors are not essential for excitation-contraction coupling in nematodes, but act to amplify a (calcium) signal that is sufficient for contraction.

Mechanism of the ATP-induced rise in cytosolic Ca2+ in freshly isolated smooth muscle cells from human saphenous vein

Effects of exogenous adenosine 5'-triphosphate (ATP) were studied by measurements of intracellular Ca2+ concentration ([Ca2+]i) and membrane currents in myocytes freshly isolated from the human saphenous vein. At a holding potential of -60 mV, ATP (10 microM) elicited a transient inward current and increased [Ca2+]i. These effects of ATP were inhibited by alpha,beta-methylene adenosine 5'-triphosphate (AMP-CPP, 10 microM). The ATP-gated current corresponded to a non-selective cation conductance allowing Ca2+ entry. The ATP-induced [Ca2+]i rise was abolished in Ca(2+)-free solution and was reduced to 30.1 +/- 5.5% (n = 14) of the control response when ATP was applied immediately after caffeine, and to 23.7 +/- 3.8% (n = 11) in the presence of thapsigargin. The Ca(2+)-induced Ca2+ release blocker tetracaine inhibited the rise in [Ca2+]i induced by both caffeine and ATP, with apparent inhibitory constants of 70 microM and 100 microM, respectively. Of the ATP-induced increase in [Ca2+]i 29.3 +/- 3.9% (n = 8) was tetracaine resistant. It is concluded that the effects of ATP in human saphenous vein myocytes are only mediated by activation of P2x receptor channels. The ATP-induced [Ca2+]i rise is due to both Ca2+ entry and Ca2+ release activated by Ca2+ ions that enter the cell through P2x receptor channels.

Suppression of calcium release by calcium or procaine in voltage clamped rat skeletal muscle fibres

1. Calcium transients were measured in 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-seal Vaseline gap technique. Resting calcium was monitored with fura-2 and the calcium transients were measured with antipyrylazo III. The rate of release of calcium from the sarcoplasmic reticulum was calculated from the calcium transient records. The temperature was 14-17 degrees C. 2. The steady-state calcium dependence of inactivation of release was studied with a two-pulse protocol in which 200 ms prepulses of different amplitudes elevated the internal calcium concentration to various levels. The inactivation of release was then measured in the test pulse that followed the prepulses. The calcium concentration at which the inactivation of release are half-maximal was approximately 0.22 microM, the average number of bound calcium ions needed to cause inactivation was about three per release channel and the amount of release that could be inactivated was, on average, 2.48 times the steady level of release during the test pulses. 3. Procaine (0.3mM) reversibly decreased the amplitude and the rate of rise of the calcium transient. Both the peak and the steady level of release were decreased by about 50%. The shape of the release waveform was not modified.

Effects of action potential duration on excitation-contraction coupling in rat ventricular myocytes. Action potential voltage-clamp measurements

Although each of the fundamental processes involved in excitation-contraction coupling in mammalian heart has been identified, many quantitative details remain unclear. The initial goal of our experiments was to measure both the transmembrane Ca2+ current, which triggers contraction, and the Ca2+ extrusion due to Na(+)-Ca2+ exchange in a single ventricular myocyte. An action potential waveform was used as the command for the voltage-clamp circuit, and the membrane potential, membrane current, [Ca2+]i, and contraction (unloaded cell shortening) were monitored simultaneously. Ca(2+)-dependent membrane current during an action potential consists of two components: (1) Ca2+ influx through L-type Ca2+ channels (ICa-L) during the plateau of the action potential and (2) a slow inward tail current that develops during repolarization negative to approximately -25 mV and continues during diastole. This slow inward tail current can be abolished completely by replacement of extracellular Na+ with Li+, suggesting that it is due to electrogenic Na(+)-Ca2+ exchange. In agreement with this, the net charge movement corresponding to the inward component of the Ca(2+)-dependent current (ICa-L) was approximately twice that during the slow inward tail current, a finding that is predicted by a scheme in which the Ca2+ that enters during ICa is extruded during diastole by a 3 Na(+)-1 Ca2+ electrogenic exchanger. Action potential duration is known to be a significant inotropic variable, but the quantitative relation between changes in Ca2+ current, action potential duration, and developed tension has not been described in a single myocyte. We used the action potential voltage-clamp technique on ventricular myocytes loaded with indo 1 or rhod 2, both Ca2+ indicators, to study the relation between action potential duration, ICa-L, and cell shortening (inotropic effect). A rapid change from a "short" to a "long" action potential command waveform resulted in an immediate decrease in peak ICa-L and a marked slowing of its decline (inactivation). Prolongation of the action potential also resulted in slowly developing increases in the magnitude of Ca2+ transients (145 +/- 2%) and unloaded cell shortening (4.0 +/- 0.4 to 7.6 +/- 0.4 microns). The time-dependent nature of these effects suggests that a change in Ca2+ content (loading) of the sarcoplasmic reticulum is responsible. Measurement of [Ca2+]i by use of rhod 2 showed that changes in the rate of rise of the [Ca2+]i transient (which in rat ventricle is due to the rate of Ca2+ release from the sarcoplasmic reticulum) were closely correlated with changes in the magnitude and the time course of ICa-L.(ABSTRACT TRUNCATED AT 400 WORDS)

Release of Ca2+ by noradrenaline and ATP from the same Ca2+ store sensitive to both InsP3 and Ca2+ in rat portal vein myocytes

1. Changes in cytosolic free Ca2+ concentration ([Ca2+]i) induced by noradrenaline (NA) and ATP were investigated using indo-1 microspectrofluorimetry in single smooth muscle cells of rat portal vein. 2. Activation of P2x-purinoceptors by ATP (10 microM) increased [Ca2+]i from 92 +/- 7 nM (n = 18) to 557 +/- 30 nM (n = 11). In the presence of NA (10 microM), the ATP-induced rise in [Ca2+]i was reduced to 23.6 +/- 1.5% (n = 7) of the control response (in the absence of NA). 3. Tetracaine (10 microM to 2 mM) inhibited in a concentration-dependent manner the Ca(2+)-induced Ca2+ release (CICR) evoked by 5 mM caffeine. In the presence of 1 mM tetracaine, the rise in [Ca2+]i induced by ATP (10(-8)-10(-4) M) was strongly inhibited. A tetracaine-resistant rise in [Ca2+]i, corresponding to 26.4 +/- 2.3% (n = 14) of control values, was recorded in response to 10 microM ATP. 4. The amplitude of the NA-induced [Ca2+]i rise depended on NA concentrations (10(-8)-10(-5) M) and was not modified by tetracaine (1 mM). 5. This study suggests that Ca2+ ions released through the InsP3 receptor-channel upon NA application do not activate CICR and the InsP3- and Ca(2+)-sensitive Ca2+ store appears to represent, at least functionally, a single releasable Ca2+ pool.

The effect of 2,3-butanedione 2-monoxime (BDM) on ventricular trabeculae from the avian heart

2,3-butanedione 2-monoxime (BDM, 3-30 mM) decreased twitch force of intact ventricular trabeculae isolated from 19-day embryonic chick hearts in a dose-dependent manner. The responses to BDM were rapid and reversible. In an attempt to determine the cellular basis for the inhibitory effect of BDM, experiments were carried out on skinned muscle fibres and isolated myocytes. In trabeculae skinned with Triton X-100, BDM depressed maximum calcium activated force (Fmax) with an IC50 of 14 mM. At 3 mM BDM, the proportional decrease in twitch force in intact tissue was similar to that of Fmax in skinned tissue. At higher BDM concentrations (10 and 30 mM), however, the proportional decrease in twitch force was greater than that of Fmax. BDM (up to 10 mM) had no effect on the normalized force-pCa relationship. In saponin-skinned preparations, BDM (3 and 30 mM) released calcium from the fully loaded sarcoplasmic reticulum to a slightly greater extent in the absence of calcium (pCa 8.5) than in the presence of a fixed level of free calcium (pCa 5.5). Whole cell patch clamping of freshly isolated chick myocytes demonstrated that BDM caused a dose-dependent decrease in the T- and L-type calcium current. Therefore, at low BDM concentrations (3 mM), the decrease in twitch force can be ascribed predominantly to depression of the contractile apparatus while, at higher concentrations of BDM, there is an additional inhibitory effect of BDM on excitation-contraction coupling.

Photoreleased inositol 1,4,5-trisphosphate-induced response in single smooth muscle cells of rat portal vein

1. The Ca2+ release in response to inositol 1,4,5-trisphosphate (InsP3) was studied in single patch-clamped smooth muscle cells of rat portal vein. InsP3 was photochemically produced from a caged InsP3 precursor included in the pipette solution. Changes in internal Ca2+ concentration ([Ca2+]i) were monitored by measuring Ca(2+)-activated K+ current. 2. Photoreleased InsP3 evoked a transient K+ current which was abolished when 10 mM EGTA or 5 mg ml-1 heparin was included in the pipette. The amplitude and time course of the K+ current responses depended on the light-flash intensity. The amplitude increased, and the latency and the time to peak decreased, with increasing flash intensity, suggesting that the amount of released Ca2+ varied as a function of the amount of InsP3 photoreleased. 3. The K+ current response to photolysis of caged InsP3 was abolished in the presence of 10 mM caffeine; conversely, caffeine was inefficient at inducing at K+ current when applied immediately after a light flash of maximal intensity. 4. The time course of the recovery of the K+ response evoked by a light flash of supramaximal intensity was similar to that obtained for the 10 mM caffeine-induced K+ current. The response recovered to 50% of control with an interval (t1/2) of about 10 s between pulses. The time course of the recovery of submaximal response to photoreleased InsP3 was considerably slower (t1/2 = 1 min), and did not correspond to that obtained for a response of similar amplitude evoked by 2 mM caffeine. 5. Responses to photoreleased InsP3 obtained after the cells were bathed for 3 min in Ca(2+)-free solution were compared with those obtained in 2 mM Ca2+ solution. Responses to light flashes of submaximal intensity were proportionally more inhibited than those evoked by supramaximal stimulations. 6. In portal vein smooth muscle cells, the InsP3-sensitive Ca2+ store seems also to be sensitive to caffeine. Our results suggest that the InsP3-induced Ca2+ release was modulated by regulatory mechanisms.

Ca(2+)-dependent negative control mechanism for Ca(2+)-induced Ca2+ release in crayfish muscle

The mechanism of termination of Ca(2+)-induced Ca2+ release (CICR) from the sarcoplasmic reticulum has been investigated in voltage clamped cut crayfish muscle fibres loaded with rhod-2. During depolarizing steps evoking calcium current (ICa), Ca2+ release was first activated. Then the release rapidly (tau approximately 6 ms) declined, as evidenced by the rate of change of the intracellular fluorescence signal representing a Ca2+ transient. The rapid termination of release was not accounted for by inactivation of the trigger ICa or depletion of Ca2+ from the SR, since the rate at which release declined was constant under conditions where the rate of ICa inactivation and the amount of Ca2+ released varied widely. Pre-elevations of [Ca2+]i with prepulses or photolysis of caged Ca2+ caused depression of Ca2+ release during a subsequent test pulse. When the rate of ICa onset was varied by applying voltage ramps with different slopes, currents with fast onset elicited larger Ca2+ release than calcium currents with slower onset, even though the amplitude of the currents was the same. These results suggest that a Ca(2+)-dependent negative control mechanism exists which mediates the termination of CICR independently of the duration of the trigger ICa and before significant depletion of Ca2+ in the SR occurs.

The fastest contracting muscles of nonmammalian vertebrates express only one isoform of the ryanodine receptor

The skeletal muscles of chickens, frogs, and fish have been reported to express two isoforms (alpha and beta) of the sarcoplasmic reticulum calcium release channel (ryanodine receptor or RYR), while mammals express only one. We have studied patterns of RYR isoform expression in skeletal muscles from a variety of fish, reptiles, and birds with immunological techniques. Immunoblot analysis with a monoclonal antibody that recognizes both nonmammalian RYR isoforms and a polyclonal antibody specific to the alpha isoform show two key results: (a) two reptilian orders share with mammals the pattern of expressing only the alpha (skeletal) RYR isoform in skeletal muscle; and (b) certain functionally specialized muscles of fish and birds express only the alpha RYR isoforms. While both isoforms are expressed in the body musculature of fish and birds, the alpha isoform is expressed alone in extraocular muscles and swimbladder muscles. The appearance of the alpha RYR isoform alone in the extraocular muscles and a fast-contracting sonic muscle in fish (toadfish swimbladder muscle) provides evidence that this isoform is selectively expressed when rapid contraction is required. The functional and phylogenetic implications of expression of the alpha isoform alone are discussed in the context of the mechanism and evolution of excitation-contraction coupling.

Potentiation of sarcoplasmic reticulum Ca2+ release by 2,3-butanedione monoxime in crustacean muscle

The effect of the chemical phosphatase 2,3-butanedione monoxime (BDM) on various aspects of excitation/contraction coupling in crustacean muscle was investigated. Despite having a depressant effect on vertebrate skeletal and cardiac muscle, BDM was a potentiator of contraction in crustacean muscle. At concentrations of 1-3 mM BDM caused an increase of potassium contractures in bundles of fibers isolated from crayfish muscle. At higher concentrations BDM caused oscillatory contractions by itself. In single voltage-clamped cut muscle fibers loaded with rhod-2, BDM (0.5-2 mM) potentiated the magnitude and duration of intracellular Ca2+ transients elicited by depolarization. At the same time BDM did not affect the rate of Ca2+ removal from the myoplasm under conditions where Ca2+ release was blocked by tetracaine. Nor did BDM increase Ca2+ entry; in fact it caused a decrease in the amplitude of the inward Ca2+ current (ICa). In microsomes isolated from lobster muscle, BDM also potentiated Ca2+ release induced by caffeine and at higher concentrations (above 3 mM) induced release by itself. At the same time it had little effect on Ca2+ uptake. These results indicate that BDM potentiates Ca2+ release in crustacean muscle possibly by dephosphorylation of the Ca(2+)-release channel.

Effects of repeated tetanic stimulation on excitation-contraction coupling in cut muscle fibres of the frog

1. The effects of prolonged intermittent fatiguing stimulation were studied on various steps of excitation-contraction (E-C) coupling in cut single frog muscle fibres using the triple Vaseline voltage clamp and the fluorescent Ca2+ indicator rhod-2. 2. There were two phases of changes in amplitude of Ca2+ transients during fatiguing stimulation: first a 5-10% increase, then a larger decrease. The decrease in amplitude of Ca2+ transients was accompanied by a slowing down of the rate of decay of the transients and by an increase in resting [Ca2+]. 3. A complete recovery of both amplitude and time course of Ca2+ transients as well as of the resting [Ca2+] occurred within 1-3 min after cessation of fatiguing stimulation. 4. The changes in Ca2+ release signals during fatiguing stimulation were accompanied by decreases in the amplitude and the rate of decay of the action potentials as well as by a decrease in resting potential. However, these alterations are not likely to contribute to fatigue significantly, since fibres stimulated under voltage-clamp conditions, when the T-tubule voltage sensor is activated directly by applied voltage steps, showed similar fatiguability to fibres stimulated by action potentials under current-clamp conditions. 5. Simultaneous measurements of intramembrane charge movement and [Ca2+] revealed that the decrease in sarcoplasmic reticulum (SR) Ca2+ release during fatiguing stimulation is not accompanied by any significant change in charge movement. 6. These results suggest that fatigue caused by repeated tetanic stimulation develops primarily at the level of SR Ca2+ release with only small possible additional effects at the level of membrane excitability and action potential propagation along the surface/T-tubule membrane. The T-tubule voltage sensor with this type of stimulation is virtually fatigue resistant.

Procaine effects on single sarcoplasmic reticulum Ca2+ release channels

The effects of the Ca(2+)-induced Ca2+ release blocker procaine on individual sarcoplasmic reticulum Ca2+ release channels have been examined in planar lipid bilayers. Procaine did not reduce the single channel conductance nor appreciably shorten the mean open times of the channel; rather, it increased the longest closed time. These results indicated that procaine interacted selectively with a closed state of the channel rather than with an open state. Gating of the sarcoplasmic reticulum Ca2+ release channel was described by a modified scheme of Ashley and Williams (1990. J. Gen. Physiol. 95:981-1005), including an additional long-lived closed state. Computer simulations determined that procaine was more likely to interact with this long-lived Ca(2+)-bound closed state of the channel rather than with other states of the channel. Simulations with the same model were also able to reproduce a prominent Ca(2+)-sensitive transition between "random" and "bursting" forms of gating of the channel, variations of which may account for "gearshift" behavior reported in studies with this and other single channels.