Kinetic isoforms of intramembrane charge in intact amphibian striated muscle

High Time Resolution Analysis of Voltage-Dependent and Voltage-Independent Calcium Sparks in Frog Skeletal Muscle Fibers

In amphibian skeletal muscle calcium (Ca2+) sparks occur both as voltage-dependent and voltage-independent ligand-activated release events. However, whether their properties and their origin show similarities are still in debate. Elevated K+, constant Cl- content solutions were used to initiate small depolarizations of the resting membrane potential to activate dihydropyridine receptors (DHPR) and caffeine to open ryanodine receptors (RyR) on intact fibers. The properties of Ca2+ sparks observed under control conditions were compared to those measured on depolarized cells and those after caffeine treatment. Calcium sparks were recorded on intact frog skeletal muscle fibers using high time resolution confocal microscopy (x-y scan: 30 Hz). Sparks were elicited by 1 mmol/l caffeine or subthreshold depolarization to different membrane potentials. Both treatments increased the frequency of sparks and altered their morphology. Images were analyzed by custom-made computer programs. Both the amplitude (in ΔF/F0; 0.259 ± 0.001 vs. 0.164 ± 0.001; n = 24942 and 43326, respectively; mean ± SE, p < 0.001) and the full width at half maximum (FWHM, in μm; parallel with fiber axis: 2.34 ± 0.01 vs. 1.92 ± 0.01, p < 0.001; perpendicular to fiber axis: 2.08 ± 0.01 vs. 1.68 ± 0.01, p < 0.001) of sparks was significantly greater after caffeine treatment than on depolarized cells. 9.8% of the sparks detected on depolarized fibers and about one third of the caffeine activated sparks (29.7%) overlapped with another one on the previous frame on x-y scans. Centre of overlapping sparks travelled significantly longer distances between consecutive frames after caffeine treatment then after depolarization (in μm; 1.66 ± 0.01 vs. 0.95 ± 0.01, p < 0.001). Our results suggest that the two types of ryanodine receptors, the junctional RyRs controlled by DHPRs and the parajunctional RyRs are activated independently, using alternate ways, with the possibility of cooperation between neighboring release channels.

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.

Ryanodine modification of RyR1 retrogradely affects L-type Ca(2+) channel gating in skeletal muscle

In skeletal muscle, there is bidirectional signalling between the L-type Ca(2+) channel (1,4-dihydropyridine receptor; DHPR) and the type 1 ryanodine-sensitive Ca(2+) release channel (RyR1) of the sarcoplasmic reticulum (SR). In the case of "orthograde signalling" (i.e., excitation-contraction coupling), the conformation of RyR1 is controlled by depolarization-induced conformational changes of the DHPR resulting in Ca(2+) release from the SR. "Retrograde coupling" is manifested as enhanced L-type current. The nature of this retrograde signal, and its dependence on RyR1 conformation, are poorly understood. Here, we have examined L-type currents in normal myotubes after an exposure to ryanodine (200 microM, 1 h at 37 degrees C) sufficient to lock RyR1 in a non-conducting, inactivated, conformational state. This treatment caused an increase in L-type current at less depolarized test potentials in comparison to myotubes similarly exposed to vehicle as a result of a approximately 5 mV hyperpolarizing shift in the voltage-dependence of activation. Charge movements of ryanodine-treated myotubes were also shifted to more hyperpolarizing potentials (approximately 13 mV) relative to vehicle-treated myotubes. Enhancement of the L-type current by ryanodine was absent in dyspedic (RyR1 null) myotubes, indicating that ryanodine does not act directly on the DHPR. Our findings indicate that in retrograde signaling, the functional state of RyR1 influences conformational changes of the DHPR involved in activation of L-type current. This raises the possibility that physiological regulators of the conformational state of RyR1 (e.g., Ca(2+), CaM, CaMK, redox potential) may also affect DHPR gating.

Effects of sphingosine 1-phosphate on excitation-contraction coupling in mammalian skeletal muscle

Sphingosine 1-phosphate (S1P) activates a subset of plasma membrane receptors of the endothelial differentiation gene family (EdgRs) in many cell types. In C2C12 myoblasts, exogenous S1P elicits Ca2+ transients by activating voltage-independent plasma membrane Ca2+ channels and intracellular Ca2+-release channels. In this study, we investigated the effects of exogenous S1P on voltage-dependent L-type Ca2+ channels in skeletal muscle fibers from adult mice. To this end, intramembrane charge movements (ICM) and L-type Ca2+ current (I(Ca)) were measured in single cut fibers using the double Vaseline-gap technique. Our data showed that submicromolar concentrations of S1P (100 nM) caused a approximately 10-mV negative shift of the voltage threshold and transition voltages of q(gamma) and q(h) components of ICM, and of I(Ca) activation and inactivation. Biochemical studies showed that EdgRs are expressed in skeletal muscles. The involvement of EdgRs in the above S1P effects was tested with suramin, a specific inhibitor of Edg-3Rs. Suramin (200 microM) significantly reduced, by approximately 90%, the effects of S1P on ICM and I(Ca), suggesting that most of S1P action occurred via Edg-3Rs. Moreover, SIP at concentration above 10 microM elicited intracellular Ca2+ transients in muscle fibers loaded with the fluorescent Ca2+ dye Fluo-3, as detected by confocal laser scanning microscopy.

Association of the Igamma and Idelta charge movement with calcium release in frog skeletal muscle

Charge movement and calcium transient were measured simultaneously in stretched frog cut twitch fibers under voltage clamp, with the internal solution containing 20 mM EGTA plus added calcium and antipyrylazo III. When the nominal free [Ca2+]i was 10 nM, the shape of the broad I(gamma) hump in the ON segments of charge movement traces remained invariant when the calcium release rate was greatly diminished. When the nominal free [Ca2+]i was 50 nM, which was close to the physiological level, the I(gamma) humps were accelerated and a slow calcium-dependent I(delta) component (or state) was generated. The peak of ON I(delta) synchronized perfectly with the peak of the calcium release rate whereas the slow decay of ON I(delta) followed the same time course as the decay of calcium release rate. Suppression of calcium release by TMB-8 reduced the amount of Q(delta) concomitantly but not completely, and the effects were partially reversible. The same simultaneous suppression effects were achieved by depleting the sarcoplasmic reticulum calcium store with repetitive stimulation. The results suggest that the mobility of Q(delta) needs to be primed by a physiological level of resting myoplasmic Ca2+. Once the priming is completed, more I(delta) is mobilized by the released Ca2+ during depolarization.

The effect of extracellular tonicity on the anatomy of triad complexes in amphibian skeletal muscle

Ultrastructural features of tubular-sarcoplasmic (T-SR) triad junctions and measures of cell volume following graded increases of extracellular tonicity were compared under physiological conditions recently shown to produce spontaneous release of intracellularly stored Ca2+ in fully polarized amphibian skeletal muscle fibres. The fibres were fixed using solutions of equivalent tonicities prior to processing for electron microscopy. The resulting anatomical sections demonstrated a partially reversible cell shrinkage corresponding to substantial increases in intracellular solute or ionic strength graded with extracellular tonicity. Serial thin sections through triad structures confirmed the presence of geometrically close but anatomically isolated transverse (T-) tubular and sarcoplasmic reticular (SR) membranes contrary to earlier suggestions for the development of luminal continuities between these structures in hypertonic solutions. They also quantitatively demonstrated accompanying decreases in T-SR distances, increased numbers of sections that showed closely apposed T and SR membranes, tubular luminal swelling and reductions in luminal volume of the junctional SR, all correlated with the imposed increases in extracellular osmolarity. Fully polarized fibres correspondingly showed elementary Ca(2+)-release events ('sparks', in 100 mM-sucrose-Ringer solution), sustained Ca2+ elevations and propagated Ca2+ waves (> or = 350-500 mM sucrose) following exposure to physiological Ringer solutions of successively greater tonicities. These were absent in hypotonic, isotonic or less strongly hypertonic (approximately 50 mM sucrose-Ringer) solutions. Yet exposure to hypotonic solutions also disrupted T-SR junctional anatomy. It increased the tubular diameters and T-SR distances and reduced their area of potential contact. The spontaneous release of intracellularly stored Ca2+ thus appears more closely to correlate with the expected changes in intracellular solute strength or a reduction in absolute T-SR distance rather than disruption of an optimal anatomical relationship between T and SR membranes taking place with either increases or decreases in extracellular tonicity.

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.

L-type Ca2+ channel and ryanodine receptor cross-talk in frog skeletal muscle

The dihydropyridine receptors (DHPRs)/L-type Ca2+ channels of skeletal muscle are coupled with ryanodine receptors/Ca2+ release channels (RyRs/CRCs) located in the sarcoplasmic reticulum (SR). The DHPR is the voltage sensor for excitation-contraction (EC) coupling and the charge movement component q gamma has been implicated as the signal linking DHPR voltage sensing to Ca2+ release from the coupled RyR. Recently, a new charge component, qh, has been described and related to L-type Ca2+ channel gating. Evidence has also been provided that the coupled RyR/CRC can modulate DHPR functions via a retrograde signal. Our aim was to investigate whether the newly described qh is also involved in the reciprocal interaction or cross-talk between DHPR/L-type Ca2+ channel and RyR/CRC. To this end we interfered with DHPR/L-type Ca2+ channel function using nifedipine and 1-alkanols (heptanol and octanol), and with RyR/CRC function using ryanodine and ruthenium red (RR). Intramembrane charge movement (ICM) and L-type Ca2+ current (ICa) were measured in single cut fibres of the frog using the double-Vaseline-gap technique. Our records showed that nifedipine reduced the amount of q gamma and qh moved by approximately 90% and approximately 55%, respectively, whereas 1-alkanols completely abolished them. Ryanodine and RR shifted the transition voltages of q gamma and qh and of the maximal conductance of ICa by approximately 4-9 mV towards positive potentials. All these interventions spared q beta. These results support the hypothesis that only q gamma; and qh arise from the movement of charged particles within the DHPR/L-type Ca2+ channel and that these charge components together with ICa are affected by a retrograde signal from RyR/CRC.

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.

Separation of charge movement components in mammalian skeletal muscle fibres

1. Intramembrane charge movements, I(ICM), were measured in rat skeletal muscle fibres in response to voltage steps from a -90 mV holding potential to a wide test voltage range (-85 to 30 mV), using a double Vaseline-gap voltage-clamp technique. Solutions were designed to minimise ionic currents. Ca(2+) current was blocked by adding Cd(2+) (0.8 mM) to the external solution. In a subset of experiments Cd(2+) was omitted to determine which components of the charge movement best correlated with L-type Ca(2+) channel gating. 2. Detailed kinetic analysis of I(ICM) identified two major groups of charges. The first two components, designated Q(a) and Q(b), were the only charges moved by small depolarising steps. The second group of components, Q(c) and Q(d), showed a more positive voltage threshold, -35.6 +/- 2.0 mV, (n = 6) in external solution with Cd(2+), and -41.1 +/- 2.0 mV (n = 12) in external solution without Cd(2+). Notably, in external solution without Cd(2+) the voltage threshold of Ca(2+) current, I(Ca), activation had a similar value, being -38.1 +/- 2.4 mV. 3. The sum of three Boltzmann functions, Q(1), Q(2) and Q(3), showing progressively more positive transition voltages, could be fitted to charge versus voltage, Q(ICM)-V, plots. The three Boltzmann terms identified three charge components: Q(1) described the shallow voltage-dependent Q(a) and Q(b) charges, Q(2) and Q(3) described the steep voltage-dependent Q(c) and Q(d) charges. 4. In external solution without Cd(2+) the charge kinetics changed: a slow decaying phase was replaced by a pronounced delayed hump. Moreover, the transition voltages of the individual steady-state charge components were shifted towards negative potentials (from 6.3 to 8.2 mV). Nevertheless, the overall charge and steepness factors were conserved. 5. In conclusion, these experiments allowed a clear separation of four components of intramembrane charge movements in rat skeletal muscle, showing that there are no fundamental differences with respect to charge movement components between amphibian and mammalian twitch muscle. Moreover, Q(c) and Q(d) charge are correlated with L-type Ca(2+) channel gating.

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.