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

Intracellular Ca2+ changes and Ca2+-activated K+ channel activation induced by acetylcholine at the endplate of mouse skeletal muscle fibres

1. Enzymatically isolated skeletal muscle fibres were used to investigate the effects of applying acetylcholine (ACh) onto the endplate area on intracellular free calcium concentration ([Ca2+]i) measured using the indicator indo-1 and single channel activity using the patch clamp technique. 2. Using a Tyrode solution containing 5 microM tetrodotoxin (TTX) as extracellular solution, ACh applications (at 0.1 or 1 mM) onto the endplate induced intracellular free calcium transients the mean maximal amplitude of which was 360 +/- 30 nM from a mean resting value of 72 +/- 7 nM (n = 13). In cells bathed with a K(+)-rich solution (145 mM K+), applications of ACh (0.1 mM) induced transient rises in [Ca2+]i from a mean resting value of 53 +/- 7 nM to a maximum of 222 +/- 24 nM (n = 33). 3. In cell-attached membrane patches at the endplate membrane of muscle fibres bathed in a K(+)-rich external solution, using a pipette filled with Tyrode solution, external application of 0.1 mM ACh could induce a transient burst opening of channels carrying an outward current of an average amplitude of 4.6 +/- 0.2 pA at 0 mV (n = 8). 4. These channels were characterized as Ca2(+)-activated K+ channels. At 0 mV, in inside-out patches excised from the endplate membrane area, they displayed a conductance of 60 and 224 pS in the presence of Tyrode and K(+)-rich solution in the pipette, respectively. Half-maximum activation was found for a [Ca2+]i close to 4 microM. The channels showed a typical voltage dependence. In outside-out patches these channels were shown to be blocked by 100 nM charybdotoxin (CTX). 5. In fibres bathed in a Tyrode solution containing TTX (5 microM), CTX had no clear effect on the change in membrane voltage, recorded near the endplate with a single intracellular microelectrode, in response to the application of ACh. 6. Although the physiological relevance of this ACh-induced K+ channel activation remains unclear, results suggest that, in the presence of a physiological extracellular [Ca2+], Ca2+ entry through the endplate nicotinic receptors can produce a local increase in [Ca2+]i, sufficient to trigger the opening of Ca2+-activated K+ channels in the adjacent surface membrane.

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.

Evidence for the non-existence of a negative phase in the hump charge movement component (I gamma) in Rana temporaria

1. Charge movement was studied in cut twitch fibres of Rana temporaria with a double Vaseline-gap voltage-clamp technique. 2. When charge movement was measured from stretched fibres (3.5 microns sarcomere length) bathed in a TEA-Cl Ringer solution, the ON transients in the appropriate potential range showed an early I beta component followed by an I gamma hump. The late I gamma hump generally decayed monotonically towards the maintained current level. 3. On some rare occasions, the ON transient showed an undershoot immediately following the I gamma hump before reaching the steady-state level. This dip in current, when it occurred, could only be observed in a very narrow potential range and might not persist until the end of the experiment. 4. A replacement of the Cl- in the external solution by CH3SO3- reduced the magnitude of the dip, suggesting that the dip is ionic in origin. 5. When charge movement was measured in slack fibres (2.2 microns sarcomere length, the probability of observing the dip in current was increased. 6. In some experiments in which the dip in current was very stable, the signal was studied by a sequence of TEST pulses to the same potential but with different durations. It was found that, if the dip in current was included as a negative phase of the I gamma hump, then the amount of ON charge was smaller than that of OFF charge. Also, as the pulse duration was increased progressively so that a longer portion of the dip was recorded, the OFF charge remained constant instead of being decreased.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of conditioning depolarization and repetitive stimulation on Q beta and Q gamma charge components in frog cut twitch fibers

Charge movement was measured in frog cut twitch fibers with the double Vaseline-gap technique. Steady-state inactivation of charge movement was studied by changing the holding potential from -90 mV to a level ranging from -70 to -30 mV. Q beta and Q gamma at each holding potential were separated by fitting the Q-V plot with a sum of two Boltzmann distribution functions. At -70 mV Q beta and Q gamma were inactivated to 54.0% (SEM 2.2) and 82.7% (SEM 3.0) of the amounts at -90 mV. At holding potentials greater than or equal to -60 mV, more Q gamma was inactivated than Q beta, and at -30 mV Q gamma was completely inactivated but Q beta was not. There was no holding potential at which Q beta was unaffected and Q gamma was completely inactivated. The differences between the residual fractions of Q beta and Q gamma are significant at all holding potentials (P less than 0.001-0.05). The plot of the residual fraction of Q beta or Q gamma versus holding potential can be fitted well by an inverted sigmoidal curve that is a mirror image of the activation curve of the respective charge component. The pair of curves for Q gamma correlates well with those for tension generation or Ca release obtained by other investigators. The time courses of the inactivation of Q beta and Q gamma were studied by obtaining several Q-V plots with conditioning depolarizations lasting 1-20 s and separating each Q-V plot into Q beta and Q gamma components by fitting with a sum of two Boltzmann distribution functions. The inactivation time constant of Q beta was found to be 5-10 times as large as that of Q gamma. During repetitive stimulation, prominent I gamma humps could be observed in TEST-minus-CONTROL current traces and normal Q gamma components could be separated from the Q-V plots, whether 20 or 50 mM EGTA was present in the internal solution, whether 2 or 10 stimulations were used, and whether the stimuli were separated by 400 ms or 6 s. Repetitive stimulation slowed the kinetics of the I gamma hump and could shift the Q-V curve slightly in the depolarizing direction in some cases, resulting in an apparent suppression of charge at the potentials that fall on the steep part of the Q-V curve.

A reconstruction of charge movement during the action potential in frog skeletal muscle

The transfer of intramembrane charge during an action potential at 4 degrees C was reconstructed for a model representing the electrical properties of frog skeletal muscle by a cylindrical surface membrane and 16 concentric annuli ("shells") of transverse tubular membrane of equal radial thickness. The lumina of the transverse tubules were separated from extracellular fluid by a fixed series resistance. The quantity, geometrical distribution and steady-state and kinetic properties of charge movement components were described by equations incorporating earlier experimental results. Introducing such nonlinear charge into the distributed model for muscle membrane diminished the maximum amplitude of the action potential within the transverse tubules by 2 mV but increased the maximum size of the after-depolarization by 3-5 mV and also its duration. However, these changes were small in comparison to the 135-mV deflection represented by the action potential. They therefore did not justify altering the values of the electrical parameters adopted by Adrian R.H., and L.D. Peachey (1973. J. Physiol. [Lond.]. 235:103-131.) and used in the present calculations. Cable properties significantly affected the time course and extent of charge movement in each shell during action potential propagation into the tubular system. Q beta charge moved relatively rapidly in all annuli, and did so without significant latency (approximately 0.3 ms) after the surface action potential upstroke. Its peak displacement varied between 53 and 58% (the range representing the difference fiber edge/fiber axis) of the total Q beta charge. This was attained at 5.4-7.3 ms after the stimulus, depending on depth within the tubules. In contrast, q gamma moved after a 1.7-2.9 ms latency and achieved a peak displacement of up to 22-34% of available charge. Both charge movement species could be driven by repetitive (47.7 Hz) action potentials without buildup of charge transfer. Such stimulus frequencies would normally cause tetanus. Latencies in q gamma charge movement in response to an action potential were resolved into (a) propagation of tubular depolarization required to gain the "threshold" of q gamma charge (0.8-1.5 ms) and (b) dielectric loss processes. The latter took consistently around 1.5 ms throughout the tubular system. Taken with (c) the earlier reports of a minimal latency in delta [Ca2+] signals attributed to tubulo-cisternal coupling following voltage sensing (approximately 2 ms: Zhu, P.H., I. Parker, and R. Miledi., 1986. Proc. R. Soc. Lond. B. Biol. Sci. 229:39-46.). these times can be reconciled to the latency (~ 4-5 ms) reported between the onset of the surface action potential and that of delta [Ca2+] signals (Vergara, J., and M. Delay. 1986. Proc. R. Soc. Lond. B. Biol. Sci. 229:97-110.). This is consistent with a relationship between the q gamma system and excitation-contraction coupling whether as an independent event (e.g.,Adrian, R.H., and C.L.-H. Huang. 1984. J. Physiol. (Lond.). 353:419-434.) or as an end reaction following earlier (q beta) transfers of charge (e.g., Horowicz, P., and M.F. Schneider. 1981. J. Physiol. (Lond.). 314:565-593.; Melzer, W., M.F. Schneider, B.J. Simon,and G. Szucs. 1986. J. Physiol. (Lond.). 373:481-512.)

Charge movement and SR calcium release in frog skeletal muscle can be related by a Hodgkin-Huxley model with four gating particles

Charge movement currents (IQ) and calcium transients (delta[Ca2+]) were measured simultaneously in frog skeletal muscle fibers, voltage clamped in a double vaseline gap chamber, using Antipyrylazo III as the calcium indicator. The rate of release of calcium from the SR (Rrel) was calculated from the calcium transients using the removal model of Melzer, W., E. Rios, and M. F. Schneider (1987. Biophys. J. 51:849-863.). IQ and delta [Ca2+] were calculated for 100 ms depolarizing test pulses to membrane potentials from -30 to +20 mV. To eliminate an inactivating component of Rrel, each test pulse was preceded by a large, fixed prepulse to +20 mV. The resulting Rrel records, which represent the noninactivating component of Rrel, were compared with integral of IQdt.(Q), the total charge that moves. The voltage dependence of the steady state Rrel was steeper then that of Q and shifted to the right. During depolarization, the Rrel waveform was similar to that of Q but was delayed by several ms, while, during repolarization, Rrel preceded Q. All of these results could be explained with a Hodgkin-Huxley type model for E-C coupling in which four voltage sensors in the t-tubule membrane which give rise to IQ must all be in their activating positions for the calcium release channel in the SR membrane to open. his model is consistent with the structural architecture of the triadic junction in which four dihydropyridine receptors (the voltage sensors for E-C coupling) in the t-tubule membrane are closely associated with each ryanodine receptor(the calcium release channel) in the SR membrane [Block, B. A., T. Imagawa, K. P. Campbell, and C. Franzini-Armstrong. 1988. J.Cell. Biol. 107:2587-2600.]). Some aspects of this work have appeared in abstract form (Simon, B. J., and D. Hill. 1991. Biophys. J.59:64a. ([Abstr.]).

Calcium-induced inactivation of calcium release from the sarcoplasmic reticulum of skeletal muscle

The ability of myofilament space Ca2+ to modulate Ca2+ release from the sarcoplasmic reticulum (SR) of skeletal muscle was investigated. Single fibers of the frog Rana pipiens belindieri were manually skinned (sarcolemma removed). Following a standard load and pre-incubation in varying myoplasmic Ca2+ concentrations, SR Ca2+ release was initiated by caffeine. Ca2+ release rates were calculated from the changes in absorbance of a Ca2+ sensitive dye, antipyrylazo III. An apparent dissociation constant (Kd) for dye-Ca2+ binding of 8000 microM 2 was determined by comparing the buffering action of the dye with that of ethylenebis(oxonitrilo)tetraacetate (EGTA) using the contractile proteins of the skinned fiber as a measure of free Ca2+. This value for Kd was used in the calculation of Ca2+ release rates. As the myoplasmic space Ca2+ was increased from pCa 7.4, Ca2+ release rates declined sharply such that at pCa 6.9 the calculated release rate was 72 +/- 3% (mean +/- SEM) of control (pCa 8.4). Further increases in myoplasmic Ca2+ from pCa 6.9 to pCa 6.1 did not result in a further decline in release rate. The effect of a decreased driving force on Ca2+ ions was investigated to determine whether it could account for the change in release rates observed. At pCa 6.9, where the greatest degree of inactivation occurred, the measured effects of a change in driving force could account for at most 40% of the observed inactivation. Varying concentrations of Ba2+ and Sr2+ in the myofilament space had no inactivating effect on the SR Ca2+ release rates. The ability of myofilament Ca2+ to inhibit SR Ca2+ release at concentrations normally encountered during muscle activation suggests a role for released Ca2+ as a modulator of the SR Ca2+ channel.

The relationship between Q gamma and Ca release from the sarcoplasmic reticulum in skeletal muscle

Asymmetric membrane currents and fluxes of Ca2+ release were determined in skeletal muscle fibers voltage clamped in a Vaseline-gap chamber. The conditioning pulse protocol 1 for suppressing Ca2+ release and the "hump" component of charge movement current (I gamma), described in the first paper of this series, was applied at different test pulse voltages. The amplitude of the current suppressed during the ON transient reached a maximum at slightly suprathreshold test voltages (-50 to -40 mV) and decayed at higher voltages. The component of charge movement current suppressed by 20 microM tetracaine also went through a maximum at low pulse voltages. This anomalous voltage dependence is thus a property of I gamma, defined by either the conditioning protocol or the tetracaine effect. A negative (inward-going) phase was often observed in the asymmetric current during the ON of depolarizing pulses. This inward phase was shown to be an intramembranous charge movement based on (a) its presence in the records of total membrane current, (b) its voltage dependence, with a maximum at slightly suprathreshold voltages, (c) its association with a "hump" in the asymmetric current, (d) its inhibition by interventions that reduce the "hump", (e) equality of ON and OFF areas in the records of asymmetric current presenting this inward phase, and (f) its kinetic relationship with the time derivative of Ca release flux. The nonmonotonic voltage dependence of the amplitude of the hump and the possibility of an inward phase of intramembranous charge movement are used as the main criteria in the quantitative testing of a specific model. According to this model, released Ca2+ binds to negatively charged sites on the myoplasmic face of the voltage sensor and increases the local transmembrane potential, thus driving additional charge movement (the hump). This model successfully predicts the anomalous voltage dependence and all the kinetic properties of I gamma described in the previous papers. It also accounts for the inward phase in total asymmetric current and in the current suppressed by protocol 1. According to this model, I gamma accompanies activating transitions at the same set of voltage sensors as I beta. Therefore it should open additional release channels, which in turn should cause more I gamma, providing a positive feedback mechanism in the regulation of calcium release.

The effect of phenylglyoxal on contraction and intramembrane charge movement in frog skeletal muscle

1. The effects of the arginine-specific protein-modifying reagent, phenylglyoxal, on contraction and intramembrane charge movement were studied in cut single fibres from frog skeletal muscle, using the double-Vaseline-gap voltage clamp technique. 2. The strength-duration curve for pulses which produced microscopically just-detectable contractions was shifted to more positive potentials and longer durations following treatment of fibres with phenylglyoxal. Caffeine-induced contractures were not blocked. 3. The amount of charge moved by large depolarizing pulses from -100 mV holding potential (charge 1) declined during the phenylglyoxal treatment with a single-exponential time course (tau = 7 min). Linear capacitance did not change significantly over the entire experiment. Inhibition of charge movement was predominantly irreversible. 4. Slow bumps (Q gamma) observed in charge movement current transients recorded before phenylglyoxal treatment, using either large test pulses or small steps superimposed on test pulses, were absent from currents recorded after treatment. The current removed by phenylglyoxal contained the bump (Q gamma) and a small fast transient (Q beta). 5. The amount of charge moved by large depolarizing pulses from -100 mV was reduced 20-50% following phenylglyoxal treatment. Charge moved by pulses to potentials more negative than -40 mV was relatively unaffected. The magnitude and voltage range of this inhibitory effect were the same whether the reagent was applied at -100 mV or at 0 mV holding potential. 6. A phenylglyoxal-sensitive component of charge was isolated which had a much steeper voltage dependence than the total charge movement or the charge remaining after treatment. 7. Charge recorded during hyperpolarizing pulses from 0 mV holding potential (charge 2) was reduced very little (less than 5%) at any potential by phenylglyoxal treatments at either 0 or -100 mV. 8. The phenylglyoxal reaction with charge 2 was kinetically different from the reaction with charge 1. 9. The effects of phenylglyoxal on charge 1 and charge 2 both measured in the same fibre were compared. Whether phenylglyoxal was applied at -100 mV, or at 0 mV the results were the same: charge 1 was inhibited much more (25-60%) than charge 2(2-12%). 10. The results presented here indicate that arginyl residues have a functional role in the voltage-sensing mechanism of excitation-contraction coupling and support the hypothesis that the dihydropyridine receptor is the voltage sensor molecule.

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

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

Anatomical distribution of voltage-dependent membrane capacitance in frog skeletal muscle fibers

Components of nonlinear capacitance, or charge movement, were localized in the membranes of frog skeletal muscle fibers by studying the effect of 'detubulation' resulting from sudden withdrawal of glycerol from a glycerol-hypertonic solution in which the muscles had been immersed. Linear capacitance was evaluated from the integral of the transient current elicited by imposed voltage clamp steps near the holding potential using bathing solutions that minimized tubular voltage attenuation. The dependence of linear membrane capacitance on fiber diameter in intact fibers was consistent with surface and tubular capacitances and a term attributable to the capacitance of the fiber end. A reduction in this dependence in detubulated fibers suggested that sudden glycerol withdrawal isolated between 75 and 100% of the transverse tubules from the fiber surface. Glycerol withdrawal in two stages did not cause appreciable detubulation. Such glycerol-treated but not detubulated fibers were used as controls. Detubulation reduced delayed (q gamma) charging currents to an extent not explicable simply in terms of tubular conduction delays. Nonlinear membrane capacitance measured at different voltages was expressed normalized to accessible linear fiber membrane capacitance. In control fibers it was strongly voltage dependent. Both the magnitude and steepness of the function were markedly reduced by adding tetracaine, which removed a component in agreement with earlier reports for q gamma charge. In contrast, detubulated fibers had nonlinear capacitances resembling those of q beta charge, and were not affected by adding tetracaine. These findings are discussed in terms of a preferential localization of tetracaine-sensitive (q gamma) charge in transverse tubule membrane, in contrast to a more even distribution of the tetracaine-resistant (q beta) charge in both transverse tubule and surface membranes. These results suggest that q beta and q gamma are due to different molecules and that the movement of q gamma in the transverse tubule membrane is the voltage-sensing step in excitation-contraction coupling.

Physiological role and selectivity of the in situ potassium channel of the sarcoplasmic reticulum in skinned frog skeletal muscle fibers

The role of K+ as a counterion during Ca2+ release from the sarcoplasmic reticulum (SR) has been investigated. An optical technique using the Ca2+-sensitive dye antipyrylazo III monitored Ca2+ release from skinned (sarcolemma removed) muscle fibers of the frog. Skinned fibers were used since the removal of the sarcolemma allows direct access to the SR membrane. Releases were stimulated by caffeine, which activates Ca2+ release directly by binding to a receptor on the SR. Two different methods were used to decrease the SR K+ conductance so that its effect on Ca2+ release could be assessed: (a) the SR K+ channel blocker, 1,10-bis-quanidino-n-decane (bisG10) was used to eliminate current pathways and (b) substitution of the impermeant ion choline for K+ was used to decrease charge carriers. Both bisG10 and choline substitution caused a concentration-dependent decrease in the Ca2+ release rate. Therefore we conclude that K+ is an important counterion for Ca2+ during its release from the SR. The selectivity of the in situ SR K+ channel to several monovalent cations was determined by substituting them for K+ and comparing their effect on Ca2+ release. The substituted ions were expected to affect Ca2+ release in proportion to their ability to support a counterion flux, which is, in turn, a function of their relative conductance through the SR K+ channel. The selectivity sequence determined by these experiments was K+ = Rb+ = Na+ greater than Cs+ greater than Li+ greater than choline.

Fura-2 calcium transients in frog skeletal muscle fibres

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

Voltage sensors of the frog skeletal muscle membrane require calcium to function in excitation-contraction coupling

1. Intramembrane charge movements and changes in intracellular Ca2+ concentration (Ca2+ transients) elicited by pulse depolarization were measured in frog fast twitch cut muscle fibres under voltage clamp. 2. Extracellular solutions with very low [Ca2+] and 2 mM-Mg2+ , shown in the previous paper to reduce Ca2+ release from the sarcoplasmic reticulum (SR), were found to cause two changes in charge movement: (a) a decrease (-12 nC/microF) in the charge that moves during depolarizing pulses from -90 to 0 mV, termed here 'charge 1'; (b) an increase (+7 nC/microF) in the charge moved by hyperpolarizing pulses from -90 to -180 mV, termed 'charge 2'. 3. The increase in charge moved by hyperpolarizing pulses was correlated (r = 0.64) with the decrease in charge moved by depolarizing pulses and both were correlated with the inhibition of Ca2+ release recorded in the same fibres. 4. The low Ca2+ solutions caused a shift to more negative voltages of the dependence relating charge movement and holding potential (VH). This shift is of similar magnitude (about 22 mV) and direction as the shift in the curve relating Ca2+ release flux to VH (previous paper). 5. In solutions with normal [Ca2+] a conditioning depolarization to 0 mV, of 2 s duration, placed 100 ms before a test pulse from -70 to 0 mV, reduced by 30% the amount of charge displaced by the test pulse. Conditioning pulses of 1 s or less caused potentiation of charge movement by up to 30%. 6. In low Ca2+ solutions, reduction of charge was observed at all durations of the conditioning pulse. The duration for half-inhibition was near 200 ms. 7. An extracellular solution with no metal cations caused a more radical inhibition than the low Ca2+ solutions that contained Mg2+. The inhibition of Ca2+ release was essentially complete (90-100%). The charge moved by a pulse to 0 mV was reduced by 20 nC/microF and the charge moved by a pulse to -170 mV increased 8 nC/microF. This shows that Mg2+ supports excitation-contraction (E-C) coupling to some extent. 8. A state model of the voltage sensor of E-C coupling explains qualitatively the observations in both papers.(ABSTRACT TRUNCATED AT 400 WORDS)

Perchlorate and the relationship between charge movement and contractile activation in frog skeletal muscle fibres

1. The effects of perchlorate ions (1-8 mM) on intramembrane charge movement, myoplasmic Antipyrylazo III Ca2+ transients and contractile activation were examined in voltage-clamped cut skeletal muscle fibres of the frog. 2. Perchlorate shifted both the voltage dependence of charge movement and the rheobase of the strength-duration relation for contraction threshold towards more negative membrane potentials. 3. Both charge movements and myoplasmic Ca2+ transients were much slower at the new rheobase in the presence of perchlorate than in the control solution but there was no change in the threshold amount of charge or in the calculated peak binding of Ca2+ to troponin C. 4. The peak release rate had a steeper voltage dependence than the non-linear charge, but a lower concentration (2 mM) of perchlorate shifted both voltage dependences equally without altering the maxima in the amount of charge and in the rate of Ca2+ release. 5. The voltage dependence of the difference between total charge and charge at the threshold of Ca2+ transients agreed well with the voltage dependence of the rate of Ca2+ release in both the presence and absence of perchlorate. 6. It is concluded that the effect of perchlorate on contractile activation can be accounted for by its action on the intramembrane charge movement responsible for contraction, without significant effects on subsequent Ca2+ release from the sarcoplasmic reticulum or on Ca2+ binding to regulatory sites of troponin C.

Potassium contractures and asymmetric charge movement in extensor digitorum longus and soleus muscles from thyrotoxic rats

Potassium contractures and asymmetric charge movement were recorded from extensor digitorum longus (EDL) and soleus muscle from normal rats and rats that had been made thyrotoxic by daily intraperitoneal injections of triiodothyronine (150 micrograms kg-1) for two to three weeks. Potassium contracture tension (relative to tetanic tension) in thyrotoxic rats was greater in EDL muscles and smaller in soleus muscles than in normal rats. As the relationship between membrane potential and potassium concentration was unaltered by thyroid treatment, it was concluded that the changes in potassium contracture tension were due to changes in excitation-contraction coupling. In thyrotoxic rats there was an average negative shift of -5 mV in the voltage sensitivity of tension in EDL fibres and a positive shift of 5 mV in soleus. As a result, the tension-membrane potential curves for EDL and soleus fibres essentially coincided. There was a corresponding average negative shift of -4 mV in the voltage sensitivity of asymmetric charge in EDL fibres, and a positive shift of 4 mV in soleus fibres from thyrotoxic rats. The dependence of asymmetric charge movement on membrane potential became essentially the same in EDL and soleus fibres from thyrotoxic rats. The maximum asymmetric charge in soleus fibres increased from an average value of 6.5 nC microF-1 in normal rats (n = 33) to 8.9 nC microF-1 in thyrotoxic rats (n = 32; p less than 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)

Intramembrane charge movement in frog skeletal muscle fibres. Properties of charge 2

1. Membrane currents were measured in cut skeletal muscle fibres voltage-clamped in a double Vaseline gap in solutions that had impermeant ions substituted for Na+, K+ and Cl-. The fibres were maintained at a holding potential of 0 mV. Pulses to positive voltages elicited outward currents that were proportional to voltage at all times; these were used to estimate linear capacitive currents, which in turn were used in the construction of non-linear current transients. 2. Large negative-going pulses elicited proportionally larger inward currents that decayed during the pulse with voltage-dependent kinetics. A portion of the non-linear current could be eliminated by solutions containing EGTA, as well as by large negative conditioning pulses of 200 ms or more. This portion was probably an inward Ca2+ current. 3. The non-linear current remaining in EGTA-containing solutions had characteristics of intramembrane charge movement ('charge 2'). This charge depended on voltage according to a two-state Boltzmann function of average parameters Qmax = 47.7 nC/microF, V = -115 mV, K = 21.5 mV (seven fibres). 4. The charge movement current transients were single-exponential decays (after a short rising phase) with time constants (tau) that depended on voltage (V). A single-barrier Eyring rate model described well the dependence of time constant on voltage. This fit permitted an independent estimate of a transition voltage, V, and a slope parameter K related to apparent valence of the mobile particle. The values of V and K that best fitted the kinetic data were close to the corresponding values estimated from the charge vs. voltage distribution. 5. Effective capacitance was measured by the transfer of capacitive charge by a small pulse superimposed on a variable pre-pulse. The capacitance was found to depend on pre-pulse voltage. The voltage dependence of the capacitance was as expected from the properties of charge 2 measured independently in the same fibres. 6. The presence of charge 2, defined as charge that moves in a very negative voltage range, was compared on the same fibres in a depolarized and a normally polarized (holding potential = -100 mV) situation. All fibres had less charge 2 at a holding potential of -100 mV (14 nC/microF average reduction). In these fibres charge 1, explored with pulses from -70 mV to 0 mV, was greater at a holding potential of -100 mV (18 nC/microF average increase).(ABSTRACT TRUNCATED AT 400 WORDS)

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

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

Contractile inactivation in frog skeletal muscle fibers. The effects of low calcium, tetracaine, dantrolene, D-600, and nifedipine

Short muscle fibers (approximately 1.5 mm) of Rana pipiens were voltage-clamped with a two-microelectrode technique at a holding potential of -100 mV. Using conditioning depolarizing ramps, with slopes greater than 0.2 mV/s, partially inactivated responses are obtained at threshold values between -55 and -35 mV. With slopes equal to or slower than 0.1 mV/s, one inactivates contraction without ever activating it. When the membrane potential is brought slowly to values more positive than about -40 mV, test pulses, applied on top of the ramps, bringing the membrane potential to values up to +100 mV, are ineffective in eliciting contractile responses, which indicates complete inactivation. After inactivation, contractile threshold is shifted by perhaps 10 mV, to about -40 mV. The sensitivity of fibers to depolarizing ramps is increased by D-600 (50 microM), dantrolene (50 microM), tetracaine (100 microM), and low calcium (10(-8) M). In the presence of these agents, complete inactivation was obtained using ramp slopes of 1, 0.8, 0.4, and 0.2 mV/s, respectively. Nifedipine was less effective. With D-600, once inactivation had been induced, no repriming occurred after repolarization to -100 mV, and partial recovery occurred after washing out the drug. With low calcium, tetracaine, and nifedipine, the tension-voltage relationship was not affected, whereas the steady state inactivation curve (obtained in repriming experiments) was shifted by 10-25 mV toward more negative potentials. With D-600, the activation curve was not modified, whereas the inactivation curve could not be obtained, because of repriming failure. With dantrolene, the inactivation curve was not affected, whereas the activation curve was shifted toward less negative potentials and peak tension diminished, depending on the pulse duration. The results indicate that it is possible to induce complete inactivation without activation, and to differentiate activation and inactivation parameters pharmacologically, which suggests that the two are separate processes.

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

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

Asymmetric charge movement in polarized and depolarized muscle fibres of the rabbit

1. The Vaseline-gap technique was used to record asymmetric charge movement in single white sternomastoid fibres of the rabbit, both when the fibres were normally polarized (holding potential, -90 mV) and when they were subjected to prolonged depolarization (holding potential, 0 mV). 2. In normally polarized fibres, 10 microM-nifedipine suppressed substantial charge movement (charge 1) without any prior depolarization, showing that the asymmetric charge does not have to be activated for the drug to exert its action. 3. Examination of the charge moved over a potential range from -140 to +10 mV confirmed that nifedipine affects only that charge generated at potentials more positive than -60 mV. 4. It was shown that there was charge movement in fibres subjected to prolonged depolarization (charge 2), with the greatest movement of charge occurring at about -71 mV, and that this charge was unaffected by nifedipine. 5. The total capacitive charge, that is, the sum of 'linear' capacitive and asymmetric charges, moved between -90 and -80 mV, was the same in polarized and depolarized fibres. Furthermore, the amount of asymmetric charge moved between potentials in the range -140 to -70 mV was the same in polarized and depolarized fibres. These observations indicate that the asymmetric charge moved between -140 and -70 mV in polarized fibres (charge 1) and depolarized fibres (charge 2) are in fact the same charge movement. 6. These results are inconsistent with the hypothesis that prolonged depolarization or nifedipine can cause a transition between charge 1 and charge 2. The results also suggest that the charge movement involved in calcium release may be quite different from that usually assumed.

Calcium signals recorded from cut frog twitch fibers containing tetramethylmurexide

The Ca indicator tetramethylmurexide was introduced into cut fibers, mounted in a double-Vaseline-gap chamber, by diffusion from the end-pool solutions. The indicator diffused rapidly to the central region of a fiber where optical recording was done and, if removed, diffused away equally fast. The time course of concentration suggests that, on average, a fraction 0.27 of indicator was reversibly bound to myoplasmic constituents and the free diffusion constant was 1.75 x 10(-6) cm2/s at 18 degrees C. The shape of the resting absorbance spectrum suggests that a fraction 0.11-0.15 of tetramethylmurexide inside a fiber was complexed with Ca. After action potential stimulation, there was a rapid transient change in indicator absorbance followed by a maintained change of opposite sign. The wavelength dependence of both changes matched a cuvette Ca-difference spectrum. The amplitude of the early peak varied linearly with indicator concentration and corresponded to an average rise in free [Ca] of 17 microM. These rather diverse findings can be explained if the sarcoplasmic reticulum membranes are permeable to Ca-free indicator. Both Ca-free and Ca-complexed indicator inside the sarcoplasmic reticulum would appear to be bound by diffusion analysis and the Ca-complexed form would be detected by the resting absorbance spectrum. The transient change in indicator absorbance would be produced by myoplasmic Ca reacting with indicator molecules that freely diffuse in myoplasmic solution. The maintained signal, which reports Ca dissociating from indicator complexed at rest, would come from changes within the sarcoplasmic reticulum. A method, based on these ideas, is described for separating the two components of the tetramethylmurexide signal. The estimated myoplasmic free [Ca] transient has an average peak value of 26 microM at 18 degrees C. Its time course is similar to, but possibly faster than, that recorded with antipyrylazo III (Maylie, J., M. Irving, N. L. Sizto, and W. K. Chandler. 1987. Journal of General Physiology. 89:83-143).

Charge movement and depolarization-contraction coupling in arthropod vs. vertebrate skeletal muscle

Voltage-dependent charge movement has been characterized in arthropod skeletal muscle. Charge movement in scorpion (Centuroides sculpturatus) muscle is distinguishable from that in vertebrate skeletal muscle by criteria of kinetics, voltage dependence, and pharmacology. The function of scorpion charge movement is gating of calcium channels in the sarcolemma, and depolarization-contraction coupling relies on calcium influx through these channels.

Intramembrane charge movement and calcium release in frog skeletal muscle

Intramembrane charge movement and myoplasmic free calcium transients (delta[Ca2+]) were monitored in voltage-clamped segments of isolated frog muscle fibres cut at both ends and mounted in a double Vaseline-gap chamber. The fibres were stretched to sarcomere lengths of 3.5-4.6 micron to minimize mechanical movement and the related optical artifacts. The over-all calcium removal capability of each fibre was characterized by analysing the decay of delta[Ca2+] following pulses of several different amplitudes and durations. The rate of sarcoplasmic reticulum (s.r.) calcium release was then calculated for each delta[Ca2+] using the calcium removal properties determined for that fibre. The calculated calcium release wave form reached a relatively early peak and then declined appreciably during a 100-150 ms depolarizing pulse. The voltage dependence of the peak rate of calcium release was steeper and was centred at more positive membrane potentials than the steady-state voltage dependence of charge movement in the same fibres. A considerable fraction of the total intramembrane charge was moved at potentials at which delta[Ca2+] and calcium release were only a few per cent of maximum. This 'subthreshold' charge may correspond to charge moved in preliminary transitions that precede a final charge transition that activates release. A 'stepped on' pulse protocol was used to experimentally separate the subthreshold charge movement from the charge movement of the final transitions that may control calcium release. The stepped on pulse consisted of a set 50 ms pre-pulse to a potential just at or below the potential for detectable delta[Ca2+] followed immediately by a test pulse of varying amplitude and duration. For a wide range of test pulse amplitudes and durations in the stepped on protocol the peak rate of calcium release was linearly related to the charge movement during the test pulse. This result points to a tight control of activation of s.r. calcium release by intramembrane charge movement. The voltage dependence of both charge movement and of the rate of calcium release could be fitted simultaneously with a three-state, two-transition sequential model in which charge moves in both transitions but only the final transition activates s.r. calcium release. A model with three identical and independent charged gating particles per channel gave an equally good fit to the data. Both models closely fit the charge movement and release data except within about 10 mV of the voltage at which release became detectable, where release varied more steeply with membrane potential than predicted by either model.(ABSTRACT TRUNCATED AT 400 WORDS)