Kinetic separation of charge movement components in intact frog skeletal muscle

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.

Effects of membrane depolarization and changes in extracellular [K(+)] on the Ca (2+) transients of fast skeletal muscle fibers. Implications for muscle fatigue

Repetitive activation of skeletal muscle fibers leads to a reduced transmembrane K⁺ gradient. The resulting membrane depolarization has been proposed to play a major role in the onset of muscle fatigue. Nevertheless, raising the extracellular K⁺ (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{K}}_{\text{o}}^{ + } $$\end{document}) concentration (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ [ {\text{K}}^{ + } ]_{\text{o}} $$\end{document}) to 10 mM potentiates twitch force of rested amphibian and mammalian fibers. We used a double Vaseline gap method to simultaneously record action potentials (AP) and Ca²⁺ transients from rested frog fibers activated by single and tetanic stimulation (10 pulses, 100 Hz) at various \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ [ {\text{K}}^{ + } ]_{\text{o}} $$\end{document} and membrane potentials. Depolarization resulting from current injection or raised \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ [ {\text{K}}^{ + } ]_{\text{o}} $$\end{document} produced an increase in the resting [Ca²⁺]. Ca²⁺ transients elicited by single stimulation were potentiated by depolarization from −80 to −60 mV but markedly depressed by further depolarization. Potentiation was inversely correlated with a reduction in the amplitude, overshoot and duration of APs. Similar effects were found for the Ca²⁺ transients elicited by the first pulse of 100 Hz trains. Depression or block of Ca²⁺ transient in response to the 2nd to 10th pulses of 100 Hz trains was observed at smaller depolarizations as compared to that seen when using single stimulation. Changes in Ca²⁺ transients along the trains were associated with impaired or abortive APs. Raising \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ [ {\text{K}}^{ + } ]_{\text{o}} $$\end{document} to 10 mM potentiated Ca²⁺ transients elicited by single and tetanic stimulation, while raising \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ [ {\text{K}}^{ + } ]_{\text{o}} $$\end{document} to 15 mM markedly depressed both responses. The effects of 10 mM \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{K}}_{\text{o}}^{ + } $$\end{document} on Ca²⁺ transients, but not those of 15 mM \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{K}}_{\text{o}}^{ + } $$\end{document}, could be fully reversed by hyperpolarization. The results suggests that the force potentiating effects of 10 mM \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{K}}_{\text{o}}^{ + } $$\end{document} might be mediated by depolarization dependent changes in resting [Ca²⁺] and Ca²⁺ release, and that additional mechanisms might be involved in the effects of 15 mM \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{K}}_{\text{o}}^{ + } $$\end{document} on force generation.

FPL-64176 alters both charge movement and Ca2+ release properties in amphibian muscle fibres

A number of recent reports have suggested that ryanodine receptor (RyR)-Ca2+ release channels are gated by tubular depolarization in skeletal muscle through their direct coupling to intramembrane dihydropyridine receptor (DHPR)-voltage sensors. The qgama charge movement, which is inhibited by DHPR antagonists, is often regarded as the electrical signature for the voltage sensing process, yet pharmacological modifications of the RyR produce reciprocal upstream kinetic effects on an otherwise conserved qgamma charge. This study investigates the effect of DHPR-specific agonists upon intramembrane charge and the release of intracellularly stored Ca2+. We empirically demonstrate kinetic effects of FPL-64176 upon charge movements that closely resemble the consequences of previous interventions directed instead at the RyR. Increases in extracellular FPL-64176 concentration from 10 to 40 microM converted delayed qgamma transients to monotonic decays indistinguishable from the exponential qbeta current component. Yet total steady-state intramembrane charge and the steepness of its dependence upon test potential closely resembled previous reports from untreated fibres. These changes accompanied an appearance of transient cytosolic [Ca2+] elevations in confocal line-scans in fluo-3-loaded fibres studied in 10mM K+ and 40, but not 10 microM, FPL-64176 that resembled elementary Ca2+ release events ('sparks'). Pharmacological manipulations of the DHPR whose effects on intramembrane charge resembled those from manoeuvres directed at the RyR can thus produce downstream effects upon Ca2+ release.

Differential effects of sarcoplasmic reticular Ca(2+)-ATPase inhibition on charge movements and calcium transients in intact amphibian skeletal muscle fibres

A hypothesis in which intramembrane charge reflects a voltage sensing process allosterically coupled to transitions in ryanodine receptor (RyR)-Ca(2+) release channels as opposed to one driven by release of intracellularly stored Ca(2+) would predict that such charging phenomena should persist in skeletal muscle fibres unable to release stored Ca(2+). Charge movement components were accordingly investigated in intact voltage-clamped amphibian fibres treated with known sarcoplasmic reticular (SR) Ca(2+)-ATPase inhibitors. Cyclopiazonic acid (CPA) pretreatment abolished Ca(2+) transients in fluo-3-loaded fibres following even prolonged applications of caffeine (10 mM) or K(+) (122 mM). Both CPA and thapsigargin (TG) transformed charge movements that included delayed (q(gamma)) "hump" components into simpler decays. However, steady-state charge-voltage characteristics were conserved to values (maximum charge, Q(max) approximately equal to 20-25 nC microF(-1); transition voltage, V* approximately equal to -40 to-50 mV; steepness factor, k approximately equal to 6-9 mV; holding voltage -90 mV) indicating persistent q(gamma) charge. The features of charge inactivation similarly suggested persistent q(beta) and q(gamma) charge contributions in CPA-treated fibres. Perchlorate (8.0 mM) restored the delayed kinetics shown by "on" q(gamma) charge movements, prolonged their "off" decays, conserved both Q(max) and k, yet failed to restore the capacity of such CPA-treated fibres for Ca(2+) release. Introduction of perchlorate (8.0 mM) or caffeine (0.2 mM) to tetracaine (2.0 mM)-treated fibres, also known to restore q(gamma) charge, similarly failed to restore Ca(2+) transients. Steady-state intramembrane q(gamma) charge thus persists with modified kinetics that can be restored to its normally complex waveform by perchlorate, even in intact muscle fibres unable to release Ca(2+). It is thus unlikely that q(gamma) charge movement is a consequence of SR Ca(2+) release rather than changes in tubular membrane potential.

Calcium waves induced by hypertonic solutions in intact frog skeletal muscle fibres

1. Regenerative Ca2+ waves and oscillations indicative of calcium-induced calcium release (CICR) activity were induced in fully polarized, fluo-3-loaded, intact frog skeletal muscle fibres by exposure to hypertonic Ringer solutions. 2. The calcium waves persisted in fibres exposed to EGTA-containing solutions, during sustained depolarization of the membrane potential or following treatment with the dihydropyridine receptor (DHPR)-blocker nifedipine. 3. The waves were blocked by the ryanodine receptor (RyR)-specific agents ryanodine and tetracaine, and potentiated by caffeine. 4. In addition to these pharmacological properties, the amplitudes, frequency and velocity of such hypertonicity-induced waves closely resembled those of Ca2+ waves previously described in dyspedic skeletal myocytes expressing the cardiac RyR-2. 5. Quantitative transmission and freeze-fracture electronmicroscopy demonstrated a reversible cell shrinkage, transverse (T)-tubular luminal swelling and decreased T-sarcoplasmic reticular (SR) junctional gaps in fibres maintained in and then fixed using hypertonic solutions. 6. The findings are consistent with a hypothesis in which RyR-Ca2+ release channels can be partially liberated from their normal control by T-tubular DHPR-voltage sensors in hypertonic solutions, thereby permitting CICR to operate even in such fully polarized skeletal muscle fibres.

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.

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.

A slow calcium-dependent component of charge movement in Rana temporaria cut twitch fibres

1. Charge movement was studied in highly stretched frog cut twitch fibres in a double Vaseline-gap voltage-clamp chamber, with the internal solution containing either 0.1 mM EGTA or 20 mM EGTA plus 1. 8 mM total Ca2+. 2. Fibres were stimulated with TEST pulses lasting 100-400 ms. Replacement of the external Cl- with an 'impermeant' anion, such as SO42-, CH3SO3-, gluconate or glutamate, greatly reduced the calcium-dependent Cl- current in the ON segment and generated a slowly decaying inward OFF current in charge movement traces. 3. Application of 20 mM EGTA to the internal solution abolished the slow inward OFF current, implying that the activation of the current depended on the presence of Ca2+ in the myoplasm. The possibility that the slow inward OFF current was carried by cations flowing inwards or anions flowing outwards was studied and determined to be unlikely. 4. During a long (2000 ms) TEST pulse, a slowly decaying ON current was also observed. When the slow ON and OFF currents were included as parts of the total charge movement, ON-OFF charge equality was preserved. This slow capacitive current is named Idelta. 5. When Cl- was the major anion in the external solution, the OFF Idelta was mostly cancelled by a slow outward current carried by the inflow of Cl-. 6. The OFF Idelta component showed a rising phase. The average values of the rising time constants in CH3SO3- and SO42- were similar and about half of that in gluconate. 7. The OFF Idelta component in CH3SO3- had a larger magnitude and longer time course than that in SO42-. The maximum amount of Qdelta in CH3SO3- was about three times as much as that in SO42-, whereas the voltage dependence of Qdelta was similar in the two solutions. 8. Since the existence of Qdelta depends on the presence of Ca2+ in the myoplasm, it is speculated that Qdelta could be a function of intracellular calcium release.

The influence of perchlorate ions on complex charging transients in amphibian striated muscle

1. The effects of perchlorate ions on intramembrane charge movements were examined under different conditions of ryanodine receptor (RyR) modification in intact voltage-clamped amphibian skeletal muscle fibres studied in the gluconate-containing solutions previously reported to emphasize the features of q gamma at the expense of those of the q beta charge. 2. The introduction of graded increases in perchlorate concentration to the experimental solutions selectively shifted the threshold of appearance of the q gamma 'hump' currents to more negative test potentials at which they actually appeared in the absence of prior q beta transients at perchlorate concentrations of 4.0-8.0 mM. Such findings suggested that the delayed (q gamma) transitions can take place independently of any previous exponential (q beta) decay. 3. These kinetic effects were accompanied by hyperpolarizing shifts in the transition potentials (V*) of the steady-state voltage dependences of either the overall or the isolated q gamma charge. These shifts were graded with concentration and reached their maximum effects at 4.0-8.0 mM perchlorate. However, both the total charge (Qmax) and the steepness factor (k) remained conserved at values consistent with a system that included significant contributions from the steeply voltage-sensitive q gamma component (overall charge: Qmax approximately 19-21 nC microF-1, k approximately 7-9 mV; q gamma component alone: Qmax approximately 10-12 nC microF-1, k approximately 4-6 mV). This contrasts with earlier reports on the effects of perchlorate in fibres that were studied in sulphate- or methanesulphonate-containing extracellular solutions. 4. Perchlorate (8.0 mM) restored the 'hump' waveform associated with q gamma charge movements that had previously been obliterated by the prior application of fully effective (0.1 mM) concentrations of either ryanodine or daunorubicin. 5. Perchlorate similarly reversed the positive shift in the transition potential of the q gamma component that was brought about by such RyR modification (from V* approximately -40 mV back to V* approximately -60 mV). In contrast, the values of either Qmax (overall charge, 19-21 nC microF-1; q gamma component, 10-13 nC microF-1) or k (overall charge, 7-9 mV; q gamma component, 4-6 mV) remained conserved through all these experimental manoeuvres. 6. The inclusion of perchlorate also reversed the action of 2 mM tetracaine and restored delayed q gamma transients to an extent that was graded with concentration (0.5-8.0 mM perchlorate). There was an accompanying recovery of the steeply voltage-dependent steady-state (q gamma) component consistent with a competitive interaction between these agents upon the q gamma intramembrane charge. 7. The present findings suggest that perchlorate exerts a specific action upon the q gamma charge in independent transitions that are driven by the tubular membrane field. Its interactions with the known RyR inhibitors that nevertheless conserve both the charge and its voltage sensitivity suggest a primary action upon the RyR that in turn exerts reciprocal actions upon the voltage sensor.

Dual actions of tetracaine on intramembrane charge in amphibian striated muscle

1. The effects of graded concentrations of tetracaine on the steady-state and kinetic properties of intramembrane charge were examined in intact voltage-clamped amphibian muscle fibres. 2. The micromolar tetracaine concentrations that were hitherto reported to abolish Ca2+ transients in skeletal muscle failed to affect significantly the steady-state charge. Maximal reductions of such intramembrane charge required relatively high, 1-2 mM, concentrations of tetracaine. 3. The plots of maximum charge against tetracaine concentration suggested a saturable 1:1 drug binding that spared a fixed amount of tetracaine-resistant (q beta) charge but inhibited a discrete fraction of susceptible (q gamma) charge with a KD between 0.1 and 0.2 mM. 4. The q beta charge thus isolated by 2 mM tetracaine was conserved through a wide range of applied test voltages and pulse durations and regardless of whether the imposed transition from the holding potential (-90 mV) to the test potential took place in one or more steps. 5. Similarly, 'on' and 'off' q beta currents that were elicited by voltage steps from fixed conditioning to varying test levels mapped onto non-linear phase-plane trajectories that nevertheless depended uniquely upon voltage. In contrast, the currents that followed voltage steps made from varying prepulse levels to fixed -90 or -20 mV test potentials mapped onto identical q beta phase-plane trajectories that were independent of the prepulse history. 6. The charge movements that followed strong depolarizing voltage clamp steps to test potentials in the range -50 to 0 mV approximated simple monotonic decays that could empirically be described by a single time constant. Nevertheless, a complete inhibition of a tetracaine-sensitive (q gamma) charge movement by 2 mM tetracaine that left only q beta charge, sharply altered both the magnitude and the voltage dependence of these time constants. This establishes a distinct contribution of the q gamma species to overall charge kinetics even at such test voltages. 7. Under such a criterion for the voltage dependence of charging kinetics, even the micromolar (0.05-0.2 mM) tetracaine concentrations that failed to markedly alter the steady-state charge consistently increased the charging time constants yet did not influence their voltage sensitivity. 8. These findings demonstrate the existence of separate kinetic and steady-state effects of tetracaine on intramembrane charge movements, at micromolar and millimolar anaesthetic concentrations, respectively. These parallel earlier effects of tetracaine that have been reported upon the transient and sustained components of sarcoplasmic reticular Ca2+ release. They also establish that maximally effective concentrations of tetracaine isolate a single distinct species of conserved (q beta) intramembrane charge.

A surface potential change in the membranes of frog skeletal muscle is associated with excitation-contraction coupling

1. Voltage changes and intramembrane charge movements in the transverse tubule membranes (T-system) of frog fast twitch muscle fibres were compared using the potentiometric dye WW-375 and a Vaseline-gap voltage clamp. As shown previously, the potentiometric dye reports a dynamic surface potential change that occurs on the myoplasmic face of the T-system membranes when the macroscopic potential applied across the surface membrane exceeds the mechanical threshold (about -60 mV). 2. The voltage dependence of the extra surface potential change and charge movement were found to be similar. Both activated with a sigmoid voltage dependence centred around -35 to -40 mV, and saturated at voltages above 0 mV. Both processes inactivated upon sustained depolarization, with a mid-point for inactivation of -40 mV. 3. Pharmacological agents which alter charge movement and excitation-contraction (E-C) coupling altered the non-linear surface potential change in a parallel manner. Perchlorate, which potentiates charge movement and E-C coupling, slowed the activation and deactivation of both charge movement and the non-linear surface potential change at voltages above -40 mV, and shifted the voltage dependence of both processes by 13 14 mV to more negative voltages. Dantrolene, which depresses charge movement and E-C coupling, shifted the voltage dependence of both processes to more positive voltages. Nifedipine, which suppresses charge movement and E-C coupling, reduced the magnitude of both charge movement and the non-linear surface potential change. 4. The non-linear surface potential change remained after the sarcoplasmic reticulum (SR) was depleted of Ca2+, suggesting that it is not a consequence of Ca2+ release. 5. These results suggest that the non-linear surface potential change is closely associated with movements of the voltage sensor (dihydropyridine (DHP) receptor) that control E-C coupling and/or signal transduction across the triadic junction. We propose that the movement of charged intramembrane domains of the DHP receptor which generate charge movement drive a subsequent movement of charged intracellular molecular domains that move within about 1 nm of the T-system membrane to generate a measurable change in surface charge. For example, the postulated mobile surface charges could be on an intracellular domain of the voltage sensor or closely associated protein, or could be a charged molecular domain of a protein that associates/dissociates with T-system membrane or DHP receptor during E-C coupling.

Kinetic isoforms of intramembrane charge in intact amphibian striated muscle

The effects of the ryanodine receptor (RyR) antagonists ryanodine and daunorubicin on the kinetic and steady-state properties of intramembrane charge were investigated in intact voltage-clamped frog skeletal muscle fibers under conditions that minimized time-dependent ionic currents. A hypothesis that RyR gating is allosterically coupled to configurational changes in dihydropyridine receptors (DHPRs) would predict that such interactions are reciprocal and that RyR modification should influence intramembrane charge. Both agents indeed modified the time course of charging transients at 100-200-microM concentrations. They independently abolished the delayed charging phases shown by q gamma currents, even in fibers held at fully polarized, -90-mV holding potentials; such waveforms are especially prominent in extracellular solutions containing gluconate. Charge movements consistently became exponential decays to stable baselines in the absence of intervening inward or other time-dependent currents. The steady-state charge transfers nevertheless remained equal through the ON and the OFF parts of test voltage steps. The charge-voltage function, Q(VT), shifted by approximately +10 mV, particularly through those test potentials at which delayed q gamma currents normally took place but retained steepness factors (k approximately 8.0 to 10.6 mV) that indicated persistent, steeply voltage-dependent q gamma contributions. Furthermore, both RyR antagonists preserved the total charge, and its variation with holding potential, Qmax (VH), which also retained similarly high voltage sensitivities (k approximately 7.0 to 9.0 mV). RyR antagonists also preserved the separate identities of q gamma and q beta species, whether defined by their steady-state voltage dependence or inactivation or pharmacological properties. Thus, tetracaine (2 mM) reduced the available steady-state charge movement and gave shallow Q(VT) (k approximately 14 to 16 mV) and Qmax (VH) (k approximately 14 to 17 mV) curves characteristic of q beta charge. These features persisted with exposure to test agent. Finally, q gamma charge movements showed steep voltage dependences with both activation (k approximately 4.0 to 6.5 mV) and inactivation characteristics (k approximately 4.3 to 6.6 mV) distinct from those shown by the remaining q beta charge, whether isolated through differential tetracaine sensitivities, or the full approximation of charge-voltage data to the sum of two Boltzmann distributions. RyR modification thus specifically alters q gamma kinetics while preserving the separate identities of steady-state q beta and q gamma charge. These findings permit a mechanism by which transverse tubular voltage provides the primary driving force for configurational changes in DHPRs, which might produce q gamma charge movement. However, they attribute its kinetic complexities to the reciprocal allosteric coupling by which DHPR voltage sensors and RyR-Ca2+ release channels might interact even though these receptors reside in electrically distinct membranes. RyR modification then would still permit tubular voltage change to drive net q gamma charge transfer but would transform its complex waveforms into simple exponential decays.

A slow component of intramembranous charge movement during sarcoplasmic reticulum calcium release in frog cut muscle fibers

Cut muscle fibers from Rana temporaria were mounted in a double Vaseline-gap chamber and equilibrated with an end-pool solution that contained 20 mM EGTA and 1.76 mM Ca (sarcomere length, 3.3-3.8 microns; temperature, 14-16 degrees C). Sarcoplasmic reticulum (SR) Ca release, delta[CaT], was estimated from changes in myoplasmic pH (Pape, P.C., D.-S. Jong, and W.K. Chandler. 1995. J. Gen. Physiol. 106:259-336). The maximal value of delta[CaT] obtained during a depleting depolarization was assumed to equal the SR Ca content before stimulation, [CaSR]R (expressed as myoplasmic concentration). After a depolarization to -55 to -40 mV in fibers with [CaSR]R = 1,000-3,000 microM, currents from intramembranous charge movement, Icm, showed an early I beta component. This was followed by an I gamma hump, which decayed within 50 ms to a small current that was maintained for as long as 500 ms. This slow current was probably a component of Icm because the amount of OFF charge, measured after depolarizations of different durations, increased according to the amount of ON charge. Icm was also measured after the SR had been depleted of most of its Ca, either by a depleting conditioning depolarization or by Ca removal from the end pools followed by a series of depleting depolarizations. The early I beta component was essentially unchanged by Ca depletion, the I gamma hump was increased (for [CaSR]R > 200 microM), the slow component was eliminated, and the total amount of OFF charge was essentially unchanged. These results suggest that the slow component of ON Icm is not movement of a new species of charge but is probably movement of Q gamma that is slowed by SR Ca release or some associated event such as the accompanying increase in myoplasmic free [Ca] that is expected to occur near the Ca release sites. The peak value of the apparent rate constant associated with this current, 2-4%/ms at pulse potentials between -48 and -40 mV, is decreased by half when [CaSR]R approximately equal to 500-1,000 microM, which gives a peak rate of SR Ca release of approximately 5-10 microM/ms.