Caffeine enhances intramembranous charge movement in frog skeletal muscle by increasing cytoplasmic Ca2+ concentration

Voltage modulates halothane-triggered Ca2+ release in malignant hyperthermia-susceptible muscle

Malignant hyperthermia can result from mutations in the ryanodine receptor that favor anesthetic-induced Ca²⁺ release. Zullo et al. find that membrane potential modulates the effect of the volatile anesthetic halothane on skeletal muscle ryanodine receptors possessing the Y524S mutation.

Malignant hyperthermia (MH) is a fatal hypermetabolic state that may occur during general anesthesia in susceptible individuals. It is often caused by mutations in the ryanodine receptor RyR1 that favor drug-induced release of Ca²⁺ from the sarcoplasmic reticulum. Here, knowing that membrane depolarization triggers Ca²⁺ release in normal muscle function, we study the cross-influence of membrane potential and anesthetic drugs on Ca²⁺ release. We used short single muscle fibers of knock-in mice heterozygous for the RyR1 mutation Y524S combined with microfluorimetry to measure intracellular Ca²⁺ signals. Halothane, a volatile anesthetic used in contracture testing for MH susceptibility, was equilibrated with the solution superfusing the cells by means of a vaporizer system. In the range 0.2 to 3%, the drug causes significantly larger elevations of free myoplasmic [Ca²⁺] in mutant (YS) compared with wild-type (WT) fibers. Action potential–induced Ca²⁺ signals exhibit a slowing of their time course of relaxation that can be attributed to a component of delayed Ca²⁺ release turnoff. In further experiments, we applied halothane to single fibers that were voltage-clamped using two intracellular microelectrodes and studied the effect of small (10-mV) deviations from the holding potential (−80 mV). Untreated WT fibers show essentially no changes in [Ca²⁺], whereas the Ca²⁺ level of YS fibers increases and decreases on depolarization and hyperpolarization, respectively. The drug causes a significant enhancement of this response. Depolarizing pulses reveal a substantial negative shift in the voltage dependence of activation of Ca²⁺ release. This behavior likely results from the allosteric coupling between RyR1 and its transverse tubular voltage sensor. We conclude that the binding of halothane to RyR1 alters the voltage dependence of Ca²⁺ release in MH-susceptible muscle fibers such that the resting membrane potential becomes a decisive factor for the efficiency of the drug to trigger Ca²⁺ release.

Reciprocal dihydropyridine and ryanodine receptor interactions in skeletal muscle activation

Dihydropyridine (DHPR) and ryanodine receptors (RyRs) are central to transduction of transverse (T) tubular membrane depolarisation initiated by surface action potentials into release of sarcoplasmic reticular (SR) Ca2+ in skeletal muscle excitation-contraction coupling. Electronmicroscopic methods demonstrate an orderly positioning of such tubular DHPRs relative to RyRs in the SR at triad junctions where their membranes come into close proximity. Biochemical and genetic studies associated expression of specific, DHPR and RyR, isoforms with the particular excitation-contraction coupling processes and related elementary Ca2+ release events found respectively in skeletal and cardiac muscle. Physiological studies of intramembrane charge movements potentially related to voltage triggering of Ca2+ release demonstrated a particular qγ charging species identifiable with DHPRs through its T-tubular localization, pharmacological properties, and steep voltage-dependence paralleling Ca2+ release. Its nonlinear kinetics implicated highly co-operative conformational events in its transitions in response to voltage change. The effects of DHPR and RyR agonists and antagonists upon this intramembrane charge in turn implicated reciprocal rather than merely unidirectional DHPR-RyR interactions in these complex reactions. Thus, following membrane potential depolarization, an orthograde qγ-DHPR-RyR signaling likely initiates conformational alterations in the RyR with which it makes contact. The latter changes could then retrogradely promote further qγ-DHPR transitions through reciprocal co-operative allosteric interactions between receptors. These would relieve the resting constraints on both further, delayed, nonlinear qγ-DHPR charge transfers and on RyR-mediated Ca2+ release. They would also explain the more rapid charging and recovery qγ transients following larger depolarizations and membrane potential repolarization to the resting level.

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

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

Eugenol-induced contractions of saponin-skinned fibers are inhibited by heparin or by a ryanodine receptor blocker

The effects of eugenol on the sarcoplasmic reticulum (SR) and contractile apparatus of chemically skinned skeletal muscle fibers of the frog Rana catesbeiana were investigated. In saponin-skinned fibers, eugenol (5 mmol/L) induced muscle contractions, probably by releasing Ca(2+) from the SR. The Ca(2+)-induced Ca(2+) release blocker ruthenium red (10 micromol/L) inhibited both caffeine- and eugenol-induced muscle contractions. Ryanodine (200 micromol/L), a specific ryanodine receptor/Ca(2+) release channel blocker, promoted complete inhibition of the contractions induced by caffeine, but only partially blocked the contractions induced by eugenol. Heparin (2.5 mg/mL), an inositol 1,4,5-trisphosphate (InsP3) receptor blocker, strongly inhibited the contractions induced by eugenol but had only a small effect on the caffeine-induced contractions. Eugenol neither altered the Ca(2+) sensitivity nor the maximal force in Triton X-100 skinned muscle fibers. These data suggest that muscle contraction induced by eugenol involves at least 2 mechanisms of Ca(2+) release from the SR: one related to the activation of the ryanodine receptors and another through a heparin-sensitive pathway.

Altered elementary calcium release events and enhanced calcium release by thymol in rat skeletal muscle

The effects of thymol on steps of excitation-contraction coupling were studied on fast-twitch muscles of rodents. Thymol was found to increase the depolarization-induced release of calcium from the sarcoplasmic reticulum, which could not be attributed to a decreased calcium-dependent inactivation of calcium release channels/ryanodine receptors or altered intramembrane charge movement, but rather to a more efficient coupling of depolarization to channel opening. Thymol increased ryanodine binding to heavy sarcoplasmic reticulum vesicles, with a half-activating concentration of 144 micro M and a Hill coefficient of 1.89, and the open probability of the isolated and reconstituted ryanodine receptors, from 0.09 +/- 0.03 to 0.22 +/- 0.04 at 30 micro M. At higher concentrations the drug induced long-lasting open events on a full conducting state. Elementary calcium release events imaged using laser scanning confocal microscopy in the line-scan mode were reduced in size, 0.92 +/- 0.01 vs. 0.70 +/- 0.01, but increased in duration, 56 +/- 1 vs. 79 +/- 1 ms, by 30 micro M thymol, with an increase in the relative proportion of lone embers. Higher concentrations favored long events, resembling embers in control, with duration often exceeding 500 ms. These findings provide direct experimental evidence that the opening of a single release channel will generate an ember, rather than a spark, in mammalian skeletal muscle.

Effects of oxidation and cytosolic redox conditions on excitation-contraction coupling in rat skeletal muscle

In this study the effects of oxidation and reduction on various steps in the excitation-contraction (E-C) coupling sequence was examined in mammalian skeletal muscle. In mechanically skinned fast-twitch fibres, electric field stimulation was used to generate action potentials in the sealed transverse-tubular (T-) system, thereby eliciting twitch responses, which are a sensitive measure of Ca2+ release. Treatment of fibres with the oxidant H2O2 (200 microM and 10 mM) for 2-5 min markedly potentiated caffeine-induced Ca2+ release and the force response to partial depolarisation of the T-system (by solution substitution). Importantly, such H2O2 treatment had no effect at all on any aspect of the twitch response (peak amplitude, rate of rise, decay rate constant and half-width), except in cases where it interfered with the T-system potential or voltage-sensor activation, resulting in a reduction or abolition of the twitch response. Exposure to strong thiol reductants, dithiothreitol (DTT, 10 mM) and reduced glutathione (GSH, 5 mM), did not affect the twitch response over 5 min, nor did varying the glutathione ratio (reduced to oxidised glutathione) from the level present endogenously in the cytosol of a rested fibre (30:1) to the comparatively oxidised level of 3:1. In fibres that had been oxidised by H2O2 (10 mM) (or by 2,2'-dithiodipyridine, 100 microM), exposure to GSH (5 mM) caused potentiation of twitch force (by approximately 20 % for H2O2); this effect was due to the increase in the Ca2+ sensitivity of the contractile apparatus that occurs under such circumstances and was fully reversed by subsequent exposure to 10 mM DTT. We conclude that: (a) the redox potential across the sarcomplamsic reticulum has no noticeable direct effect on normal E-C coupling in mammalian skeletal muscle, (b) oxidising the Ca2+-release channels and greatly increasing their sensitivity to Ca2+-induced Ca2+ release does not alter the amount of Ca2+ released by an action potential and (c) oxidation potentiates twitches by a GSH-mediated increase in the Ca2+ sensitivity of the contractile apparatus.

Intramembrane charge movement and L-type calcium current in skeletal muscle fibers isolated from control and mdx mice

Dystrophin-deficient muscle fibers from mdx mice are believed to suffer from increased calcium entry and elevated submembranous calcium level, the actual source and functional consequences of which remain obscure. Here we compare the properties of the dihydropyridine receptor as voltage sensor and calcium channel in control and mdx muscle fibers, using the silicone-voltage clamp technique. In control fibers charge movement followed a two-state Boltzmann distribution with values for maximal charge, midpoint voltage, and steepness of 23 +/- 2 nC/ micro F, -37 +/- 3 mV, and 13 +/- 1 mV (n = 7). Essentially identical values were obtained in mdx fibers and the time course of charge recovery from inactivation was also similar in the two populations (tau approximately 6 s). In control fibers the voltage dependence of the slow calcium current elicited by 100-ms-long pulses gave values for maximal conductance, apparent reversal potential, half-activation potential, and steepness factor of 156 +/- 15 S/F, 65.5 +/- 2.9 mV, -0.76 +/- 1.2 mV, and 6.2 +/- 0.5 mV (n = 17). In mdx fibers, the half-activation potential of the calcium current was slightly more negative (-6.2 +/- 1.2 mV, n = 16). Also, when using longer pulses, the time constant of calcium current decay was found to be significantly larger (by a factor of 1.5-2) in mdx than in control fibers. These changes in calcium current properties are unlikely to be primarily responsible for a dramatic alteration of intracellular calcium homeostasis. They may be speculated to result, at least in part, from remodeling of the submembranous cytoskeleton network due to the absence of dystrophin.

Caffeine-induced immobilization of gating charges in isolated guinea-pig ventricular heart cells

The effects of 10 mM caffeine (CAF) on intramembrane charge movements (ICM) were studied in isolated guinea-pig ventricular heart cells with the whole-cell patch-clamp technique. In the presence of CAF, the properties (voltage dependence, maximum Q(ON) [Q(max)], availability with voltage) of Q(ON) charge activated from -110 mV were barely affected. Following a 100 ms prepulse to -50 mV to decrease the participation of charges originating from Na channels, the voltage dependence of Q(ON) was shifted by 5 mV (negative component) and by 10 mV (positive component) towards negative potentials, and Q(max) was depressed by 16.5%. CAF drastically reduced in a time- and voltage-dependent manner Q(OFF) on repolarization to -50 mV, the effects being greater at positive potentials. CAF-induced Q(OFF) immobilization could be almost entirely removed by repolarization to voltages as negative as -170 mV. In these conditions, the voltage-dependence of Q(OFF) (repolarization to +30 to -170 mV) was shifted by 17 mV (negative component) and 30 mV (positive component) towards negative potentials, suggesting an interconversion into charge 2. Most of CAF effects were suppressed when the sarcoplasmic reticulum (SR) was not functional or when the cells were loaded with BAPTA-AM. We conclude that CAF effects on ICM are likely due to Ca(2+) ions released from the SR, and which accumulate in the subsarcolemmal fuzzy spaces in the vicinity of the Ca channels. Because CAF effects were more pronounced on Q(OFF) than on Q(ON) the channels have likely to open before Ca(2+) ions could affect their gating properties. It is speculated that such an effect on gating charges might contribute to the Ca-induced inactivation of the Ca current.

Spark- and ember-like elementary Ca2+ release events in skinned fibres of adult mammalian skeletal muscle

1. Using laser scanning confocal microscopy, we show for the first time elementary Ca2+ release events (ECRE) from the sarcoplasmic reticulum in chemically and mechanically skinned fibres from adult mammalian muscle and compare them with ECRE from amphibian skinned fibres. 2. Hundreds of spontaneously occurring events could be measured from individual single skinned mammalian fibres. In addition to spark-like events, we found ember-like events, i.e. long-lasting events of steady amplitude. These two different fundamental release types in mammalian muscle could occur in combination at the same location. 3. The two peaks of the frequency of occurrence for ECRE of mammalian skeletal muscle coincided with the expected locations of the transverse tubular system within the sarcomere, suggesting that ECRE mainly originate at triadic junctions. 4. ECRE in adult mammalian muscle could also be identified at the onset of the global Ca2+ release evoked by membrane depolarisation in mechanically skinned fibres. In addition, the frequency of ECRE was significantly increased by application of 0.5 mM caffeine and reduced by application of 2 mM tetracaine. 5. We conclude that the excitation-contraction coupling process in adult mammalian muscle involves the activation of both spark- and ember-like elementary Ca2+ release events.

Cyclopiazonic acid and thapsigargin reduce Ca2+ influx in frog skeletal muscle fibres as a result of Ca2+ store depletion

We have investigated the influence of the sarcoplasmic reticulum (SR) Ca2+ content on the retrograde control of skeletal muscle L-type Ca2+ channels activity by ryanodine receptors (RyR). The effects of cyclopiazonic acid (CPA) and thapsigargin (TG), two structurally unrelated inhibitors of SR Ca(2+)-adenosine triphosphatase (ATPase), were examined on the SR Ca2+ content, the calcium current and contraction in single frog semitendinosus fibres using the double mannitol-gap technique. At moderate concentrations that only partially inhibited Ca2+ sequestration by the SR, CPA (2-4 microM) induces a concentration dependent depression of contraction and Ca2+ current amplitudes. When Ba2+ is the charge carrier, the inward current is not changed by CPA suggesting that this Ca(2+)-pump inhibitor does not directly affect dihydropyridine Ca2+ channels. Similar effects were obtained with TG (1-5 microM). Changes in Ca2+ currents and contraction were accompanied by a reduced Ca2+ loading of the SR. We attribute the modulation of the Ca2+ current to the selective inhibition of the SR Ca2+ ATPase, resulting in a decreased Ca2+ release and thereby a reduced activation of calcium inward currents. This is therefore taken to represent a calcium release-dependent modulation of skeletal muscle L-type Ca2+ channels.

Charge movements in intact amphibian skeletal muscle fibres in the presence of cardiac glycosides

1. Intramembrane charge movements were examined in intact voltage-clamped amphibian muscle fibres following treatment with cardiac glycosides in the hypertonic gluconate-containing solutions hitherto reported to emphasise the features of q(gamma) at the expense of q(beta) charge. 2. The application of chlormadinone acetate (CMA) at concentrations known selectively to block Na(+)-K(+)-ATPase conserved the steady-state voltage dependence of intramembrane charge, contributions from delayed (q(gamma)) charging transients, and their inactivation characteristics brought about by shifts in holding potential. 3. The addition of either ouabain (125, 250 or 500 nM) or digoxin (5 nM) at concentrations previously reported additionally to influence excitation-contraction coupling similarly conserved the steady-state charge-voltage relationships, Q(V), in fully polarised fibres to give values of maximum charge, Q(max), transition voltage, V*, and steepness factor, k, that were consistent with a persistent q component as reported on earlier occasions (Q(max) approximately = 25-27 nC F-1, V* approximately = -45 to -50 mV, k approximately = 7-9 mV). 4. In both cases shifts in holding potential from -90 to -50 mV produced a partial inactivation that separated steeply and more gradually voltage-dependent charge components in agreement with previous characterisations. 5. However, charge movements that were observed in the presence of either digoxin or ouabain were monotonic decays in which delayed (q(gamma)) transients could not be distinguished from the early charging records. These features persisted despite the further addition of chlormadinone acetate over a 10-fold concentration range (5-50 microM) known to displace ouabain from the Na(+)-K(+)-ATPase. 6. Ouabain (500 nM) restored the steady-state charge movement that was previously abolished by the addition of 2.0 mM tetracaine in common with previous results of using ryanodine receptor (RyR)-specific agents. 7. Perchlorate (8.0 mM) restored the delayed 'on' relaxations and increased the prominence of the 'off' decays produced by q(gamma) charge following treatment with cardiac glycosides. This was accompanied by a negative (approximately 10-15 mV) shift in the steady-state charge-voltage relationship but an otherwise conserved maximum charge, Q(max), and steepness factor, k, in parallel with previously reported effects of perchlorate following treatments with RyR-specific agents. 8. The features of cardiac glycoside action thus parallel those of other agents that act on RyR-Ca(2+) release channels yet influence the kinetics but spare the steady-state properties of intramembrane charge.

Modification of excitation-contraction coupling by 4-chloro-m-cresol in voltage-clamped cut muscle fibres of the frog (R. pipiens)

1. The effect of 5 microM 4-chloro-m-cresol (4-CmC) on voltage-controlled Ca2+ release was studied in cut muscle fibres of the frog loaded with internal solutions containing 15 mM EGTA. Fibres were voltage clamped using a double Vaseline gap system, and Ca2+ signals were recorded with the fluorescent indicator dye fura-2 2. Resting intracellular free Ca2+ concentration increased from 61 to 100 nM upon application of 4-CmC. 3. Both peak rate of release of intracellularly stored Ca2+ and the steady level attained after 50 ms of depolarization increased, but the potentiation of the latter was more pronounced (by a factor of 1.7 versus 1.3). The voltage of half-maximal activation remained unchanged. 4. Non-linear intramembranous charge movements showed no significant change in voltage dependence while the maximal charge displaced by depolarization increased by 25 %. 5. The dependence of peak release flux on total intramembranous charge was not different in 4-CmC, but for the steady level of release the steepness of the relation increased by a factor of 1.3. 6. The stimulating effect of 5 microM 4-CmC on depolarization-induced Ca2+ release resembled the potentiation by 0.5 mM caffeine. However, 0.5 mM caffeine increased the peak and steady levels of the release rate by a similar factor and caused no increase in the resting free calcium concentration, indicating different modes of action of the two substances. 7. Neither 5 microM 4-CmC nor 0.5 mM caffeine led to a loss of voltage control of Ca2+ release during repolarization after short depolarizations, as has been reported previously for caffeine. Potentiated Ca2+ release could be terminated by repolarization as fast as under control conditions both with 15 mM and 0.1 mM internal EGTA. 8. The effects of 4-CmC may result from a direct opening of the release channel combined with an enhancement of the transduction mechanism that couples channel opening to displacement of voltage sensor charges.

The influence of caffeine on intramembrane charge movements in intact frog striated muscle

1. The influence of caffeine, applied over a 25-fold range of concentrations, on intramembrane charge movements was examined in intact voltage-clamped amphibian muscle fibres studied in the hypertonic gluconate-containing solutions that were hitherto reported to emphasize the features of qgamma at the expense of those of qbeta charge. 2. The total charge, Qmax, the transition voltage, V*, and the steepness factor, k, of the steady-state charge-voltage relationships, Q(V), were all conserved to values expected with significant contributions from the steeply voltage-dependent qgamma species (Qmax approximately 20 nC microF-1, V* approximately -50 mV, k approximately 8 mV) through all the applications of caffeine concentrations between 0.2 and 5.0 mM. This differs from recent reports from studies in cut as opposed to intact fibres. 3. The delayed transients that have been attributed to transitions within the qgamma charge persisted at low (0.2 mM) and intermediate (1.0 mM) caffeine concentrations. 4. In contrast, the time courses of such qgamma currents became more rapid and their waveforms consequently merged with the earlier qbeta decays at higher (5.0 mM) reagent concentrations. The charging records became single monotonic decays from which individual contributions could not be distinguished. This suggests that caffeine modified the kinetic properties of the qgamma system but preserved its steady-state properties. These findings thus differ from earlier reports that high caffeine concentrations enhanced the prominence of delayed transient components in cut fibres. 5. Caffeine (5.0 mM) and ryanodine (0.1 mM) exerted antagonistic actions upon qgamma charge movements. The addition of caffeine restored the delayed time courses that were lost in ryanodine-containing solutions, reversed the shift these produced in the steady-state charge-voltage relationship but preserved both the maximum charge, Qmax, and the steepness, k, of the steady-state Q(V) relationships. 6. Caffeine also antagonized the actions of tetracaine on the total available qgamma charge, but did so only at the low and not at the high applied concentrations. Thus, 0.2 mM caffeine restored the steady-state qgamma charge, the steepness of the overall Q(V) function and the appearance of delayed qgamma charge movements that had been previously abolished by the addition of 2.0 mM tetracaine. 7. In contrast, the higher applied (1.0 and 5.0 mM) caffeine concentrations paradoxically did not modify these actions of tetracaine. The total charge and voltage dependence of the Q(V) curves, and the amplitude and time course of charge movements remained at the reduced values expected for the tetracaine-resistant qbeta charge. 8. These results permit a scheme in which caffeine acts directly upon ryanodine receptor (RyR)-Ca2+ release channels whose consequent activation then dissociates them from the tubular dihydropyridine receptor (DHPR) voltage sensors that produce qgamma charge movement, with which they normally are coupled in reciprocal allosteric contact.

Imaging caffeine-induced Ca2+ transients in individual fast-twitch and slow-twitch rat skeletal muscle fibers

Fast-twitch and slow-twitch rat skeletal muscles produce dissimilar contractures with caffeine. We used digital imaging microscopy to monitor Ca2+ (with fluo 3-acetoxymethyl ester) and sarcomere motion in intact, unrestrained rat muscle fibers to study this difference. Changes in Ca2+ in individual fibers were markedly different from average responses of a population. All fibers showed discrete, nonpropagated, local Ca2+ transients occurring randomly in spots about one sarcomere apart. Caffeine increased local Ca2+ transients and sarcomere motion initially at 4 mM in soleus and 8 mM in extensor digitorum longus (EDL; approximately 23 degrees C). Ca2+ release subsequently adapted or inactivated; this was surmounted by higher doses. Motion also adapted but was not surmounted. Prolonged exposure to caffeine evidently suppressed myofilament interaction in both types of fiber. In EDL fibers, 16 mM caffeine moderately increased local Ca2+ transients. In soleus fibers, 16 mM caffeine greatly increased Ca2+ release and produced propagated waves of Ca2+ (approximately 1.5-2.5 microns/s). Ca2+ waves in slow-twitch fibers reflect the caffeine-sensitive mechanism of Ca2(+)-induced Ca2+ release. Fast-twitch fibers possibly lack this mechanism, which could account for their lower sensitivity to caffeine.

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+.

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

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