Time course of calcium release and removal in skeletal muscle fibers

Intracellular Ca transients in rat cardiac myocytes: role of Na-Ca exchange in excitation-contraction coupling

Membrane current and intracellular Ca concentration ([Ca]i) transients were recorded from isolated rat ventricular myocytes under voltage-clamp control. The cells were dialyzed by the patch pipette solution, which contained the fluorescent Ca indicator indo-1 and 0.5 mM Na. Under these experimental conditions, Ca entry via Na-Ca exchange did not appear to be appreciable even in the absence of extracellular Na. Increasing the duration of voltage-clamp pulses from 5 to 80 ms produced [Ca]i transients of increasing amplitude, while the peak Ca current was not changed. This duration dependence of the [Ca]i transient was most demonstrable at more negative test potentials (e.g., -20 to -30 mV) and was not qualitatively modified by Na-free solutions. This latter result indicates that Ca extrusion by Na-Ca exchange is not responsible for the smaller [Ca]i transients observed when the membrane is repolarized after very brief depolarizations. Although the peak Ca current was not changed by increasing pulse duration, the integrated Ca current was increased. These observations are consistent with a Ca-release mechanism in cardiac excitation-contraction coupling in which 1) the Ca-release process can be modulated by membrane potential or 2) the Ca entering the cell via Ca channels has a preferential access [compared with Ca from the sarcoplasmic reticulum (SR)] to the site(s) that control SR Ca release. The role of Na-Ca exchange in the decline of [Ca]i during relaxation was also explored. Removal of extracellular Na (Nao) resulted in 20% slowing of the decline in [Ca]i during relaxation. From this, we conclude that the Na-Ca exchange competes with SR to remove Ca from the cytoplasm and that under our control conditions the exchanger may account for 20% of this decline. The Nao dependence of relaxation was reduced at more positive membrane potentials and increased by SR Ca loading.

Caffeine slows turn-off of calcium release in voltage clamped skeletal muscle fibers

Myoplasmic free calcium transients delta [Ca2+] were monitored with the calcium indicators antipyrylazo III and fura-2 in voltage clamped cut frog skeletal muscle fibers, in the presence and absence of 0.5 mM caffeine. Without caffeine delta [Ca2+] began to decline within a few milliseconds of fiber repolarization for pulses of all durations. In caffeine delta [Ca2+] continued to rise for 10-60 ms after 10 or 20 ms depolarizing pulses, indicating that the release of calcium from the sarcoplasmic reticulum (SR) continued well after repolarization of transverse tubular (TT) membranes in the presence of caffeine. Caffeine also increased the peak amplitude of delta [Ca2+] for all pulses and slowed the decline of delta [Ca2+] after pulses of all durations. The rate of calcium release from the SR calculated from delta [Ca2+] showed that for 10 ms pulses in caffeine release did not turn off abruptly on repolarization but instead declined to zero with a time constant essentially the same as the time constant for inactivation of SR calcium release during depolarizing pulses in the presence or absence of caffeine. The observed loss of TT membrane potential control of SR calcium release in the presence of caffeine suggests the appearance of a significant component of cytosolic Ca2+-induced calcium release in caffeine.

Single channel characteristics of a high conductance anion channel in "sarcoballs"

Previously undescribed high conductance single anion channels from frog skeletal muscle sarcoplasmic reticulum (SR) were studied in native membrane using the "sarcoball" technique (Stein and Palade, 1988). Excised inside-out patches recorded in symmetrical 200 mM TrisCl show the conductance of the channel's predominant state was 505 +/- 25 pS (n = 35). From reversal potentials, the Pcl/PK ratio was 45. The slope conductance vs. Cl- ion concentration curve saturates at 617 pS, with K0.5 estimated at 77 mM. The steady-state open probability (Po) vs. holding potential relationship produces a bell-shaped curve, with Po values reaching a maximum near 1.0 at 0 mV, and falling off to 0.05 at +/- 25 mV. Kinetic analysis of the voltage dependence reveals that while open time constants are decreased somewhat by increases in potential, the largest effect is an increase in long closed times. Despite the channel's high conductance, it maintains a moderate selectivity for smaller anions, but will not pass larger anions such as gluconate, as determined by reversal-potential shifts. At least two substates different from the main open level are distinguishable. These properties are unlike those described for mitochondrial voltage-dependent anion channels or skeletal muscle surface membrane Cl channels and since SR Ca channels are present in equally high density in sarcoball patches, we propose these sarcoball anion channels originate from the SR. Preliminary experiments recording currents from frog SR anion channels fused into liposomes indicate that either biochemical isolation and/or alterations in lipid environment greatly decrease the channel's voltage sensitivity. These results help underline the potential significance of using sarcoballs to study SR channels. The steep voltage sensitivity of the sarcoball anion channel suggests that it could be more actively involved in the regulation of Ca2+ transport by the SR.

Time course of activation of calcium release from sarcoplasmic reticulum in skeletal muscle

Myoplasmic free calcium transients were measured with antipyrylazo III in voltage clamped segments of frog skeletal muscle fibers and were used to calculate the rate of release (Rrel) of calcium from the sarcoplasmic reticulum. Intramembrane charge movement was measured for the same pulses in the same fibers. During a depolarizing pulse Rrel rose to an early peak and then decayed relatively rapidly but incompletely due to calcium-dependent inactivation (Schneider M.F., and B.J. Simon. 1988. J. Physiol. (Lond.). 405:727-745). Two approaches were used to determine release activation independent of the effects of inactivation: (a) a mathematical correction based on the assumption that inactivation was a process occurring in parallel with and independently of activation; (b) an experimental procedure in which release was maximally inactivated by a large short prepulse and then the remaining noninactivatable component of release was monitored during a subsequent test pulse. Both procedures gave the same time course of activation of release. Release activation paralleled the time course of intramembrane charge movement but was delayed by a few milliseconds.

Inactivation of calcium release from the sarcoplasmic reticulum in frog skeletal muscle

1. The rate of calcium release (Rrel) from the sarcoplasmic reticulum (SR) in voltage clamped segments of frog skeletal muscle fibres was calculated from myoplasmic free calcium transients (delta[Ca2+]) measured with the calcium indicator dye Antipyrylazo III. 2. During a 100-200 ms depolarizing pulse Rrel reached an early peak and then declined markedly. The time course and extent of decline of Rrel were independent of membrane potential over a range of potentials where release activation varied severalfold. 3. For test pulses applied shortly after relatively large or long conditioning pulses Rrel completely lacked the early peak. The peak gradually recovered as the interval between the conditioning and test pulses was increased to 1 s. 4. A latency was often observed before the start of recovery of the peak in Rrel. The latency appeared to be correlated with the time for delta[Ca2+] to fall below a certain level, indicating that recovery of the peak might represent reversal of a calcium-dependent process. It was therefore proposed that the rapid decline in Rrel during a pulse was due to calcium-dependent inactivation of the SR calcium release channels. 5. Inactivation continued to develop during the interval between a relatively large 20 ms conditioning pulse and a test pulse applied 20 ms later. This was as expected for calcium-dependent inactivation of SR calcium release because of the elevated [Ca2+] between the conditioning and test pulses. It was not as expected for external membrane potential-dependent inactivation. 6. Small steady elevations in [Ca2+] due to relatively small 200 ms conditioning pulses produced marked inactivation of Rrel, indicating an apparent dissociation constant for calcium-dependent inactivation only slightly above resting [Ca2+]. 7. All observations could be well simulated by a two-step model for inactivation in which myoplasmic free calcium equilibrates rapidly with a high-affinity calcium receptor on the release channel and then the calcium-receptor complex undergoes a slower conformational change to the inactivated state of the channel. 8. An alternative model in which calcium binds to a soluble receptor (e.g. free calmodulin) and then the calcium-receptor complex binds to and directly inactivates the channel was shown to be formally identical to the preceding model. Either model could closely simulate all observations.

Mechanism of release of calcium from sarcoplasmic reticulum of guinea-pig cardiac cells

1. The mechanisms that control release of Ca2+ from the sarcoplasmic reticulum (SR) of guinea-pig ventricular cells were studied by observing intracellular calcium concentration ([Ca2+]i transients) and membrane currents in voltage-clamped guinea-pig ventricular myocytes perfused internally with Fura-2. 2. Sarcolemmal Ca2+ current was identified through the use of tetrodotoxin (TTX) and Ca2+ channel antagonists (verapamil) and agonists (Bay K 8644). 3. Changes in [Ca2+]i attributable to release of Ca2+ from the SR were identified through the use of ryanodine, which abolishes the ability of the SR to release Ca2+. Ryanodine-sensitive increases in [Ca2+]i could be elicited either by depolarization or by repolarization (from depolarizing pulses to relatively positive membrane potentials). 4. At appropriate voltages, it is the initial fast change in [Ca2+]i elicited by either depolarization or repolarization that is abolished by ryanodine, and is defined here as ryanodine sensitive. 5. The amplitude of the ryanodine-sensitive [Ca2+]i transient elicited by depolarization had a bell-shaped dependence on membrane potential with a maximum of about 500 nM at 10 mV, and with the upper minimum between 60 and 70 mV. Verapamil-sensitive current activated over approximately the same potential range as the [Ca2+]i transient, with a peak amplitude at 10 mV, and a reversal potential of 65 mV. 6. When a holding potential of -68 mV and TTX (30 microM) were used, the most negative pulse potential at which activation of an inward current occurred was -49 mV while changes in [Ca2+]i occurred at -43 mV. 7. Ryanodine-sensitive increases in [Ca2+]i elicited by repolarization (tail transients) were maximal for repolarization to 0 mV. Smaller changes in [Ca2+]i than maximal were elicited by repolarization to both more positive and more negative potentials than 0 mV. The peak amplitude of the verapamil-sensitive tail currents elicited by repolarization increased continuously as the membrane was repolarized to potentials more negative than 60 mV. 8. Increasing depolarizing pulse duration beyond 10-20 ms did not increase the amplitude of the [Ca2+]i transient, but prolonged it. 9. The experimental results are compared to the predictions of two theories on the mechanism of excitation-contraction coupling: Ca2+-induced release of Ca2+ (CICR), as it has been formulated from data in skinned cardiac cells, and a charge-coupled release mechanism (CCRM), as it has been formulated to explain excitation-contraction coupling in skeletal muscle. 10. Some of the results are clearly not consistent with certain features of a charge-coupled release mechanism.(ABSTRACT TRUNCATED AT 400 WORDS)

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)

Simultaneous recording of calcium transients in skeletal muscle using high- and low-affinity calcium indicators

To monitor cytosolic [Ca2+] over a wide range of concentrations in functioning skeletal muscle cells, we have used simultaneously the rapid but relatively low affinity calcium indicator antipyrylazo III (AP III) and the slower but higher affinity indicator fura-2 in single frog twitch fibers cut at both ends and voltage clamped with a double vaseline gap system. When both dyes were added to the end pool solution the cytosolic fura-2 concentration reached a steady level equal to the end pool concentration within approximately 2.5 h, a time when the AP III concentration was still increasing. For depolarizing pulses of increasing amplitude, the fura-2 fluorescence signal approached saturation when the simultaneously recorded AP III absorbance change was far from saturation. Comparison of simultaneously recorded fura-2 and AP III signals indicated that the mean values of the on and off rate constants for calcium binding to fura-2 in 18 muscle fibers were 1.49 x 10(8) M-1 s-1 and 11.9 s-1, respectively (mean KD = 89 nM), if all AP III in the fiber is assumed to behave as in calibrating solution and to be in instantaneous equilibrium with [Ca2+]. [Ca2+] transients calculated from the fura-2 signals using these rate constants were consistent with the [Ca2+] transients calculated from the AP III signals. Resting [Ca2+] or small changes in [Ca2+] which could not be reliably monitored with AP III could be monitored with fura-2 with little or no interference from changes in [Mg2+] or from intrinsic signals. The fura-2 signal was also less sensitive to movement artifacts than the AP III signal. After a [Ca2+] transient the fura-2 signal demonstrated a relatively small elevation of [Ca2+] that was maintained for many seconds.

Inactivation of excitation-contraction coupling in rat extensor digitorum longus and soleus muscles

K contractures and two-microelectrode voltage-clamp techniques were used to measure inactivation of excitation-contraction coupling in small bundles of fibers from rat extensor digitorum longus (e.d.l.) and soleus muscles at 21 degrees C. The rate of spontaneous relaxation was faster in e.d.l. fibers: the time for 120 mM K contractures to decay to 50% of maximum tension was 9.8 +/- 0.5 s (mean +/- SEM) in e.d.l. and 16.8 +/- 1.7 s in soleus. The rate of decay depended on membrane potential: in e.d.l., the 50% decay time was 14.3 +/- 0.7 s for contractures in 80 mM K (Vm = 25 mV) and 4.9 +/- 0.4 s in 160 mM K (Vm = -3 mV). In contrast to activation, which occurred with less depolarization in soleus fibers, steady state inactivation required more depolarization: after 3 min at -40 mV in 40 mM K, the 200 mM K contracture amplitude in e.d.l. fell to 28 +/- 10% (n = 5) of control, but remained at 85 +/- 2% (n = 6) of control in soleus. These different inactivation properties in e.d.l. and soleus fibers were not influenced by the fact that the 200 mM K solution used to test for steady state inactivation produced contractures that were maximal in soleus fibers but submaximal in e.d.l.: a relatively similar depression was recorded in maximal (200 mM K) and submaximal (60 and 80 mM K) contracture tension. A steady state "pedestal" of tension was observed with maintained depolarization after K contracture relaxation and was larger in soleus than in e.d.l. fibers. The pedestal tension was attributed to the overlap between the activation and inactivation curves for tension vs. membrane potential, which was greater in soleus than in e.d.l. fibers. The K contracture results were confirmed with the two-microelectrode voltage clamp: the contraction threshold increased to more positive potentials at holding potentials of -50 mV in e.d.l. or -40 mV in soleus. At holding potentials of -30 mV in e.d.l. or 0 mV in soleus, contraction could not be evoked by 15-ms pulses to +20 mV. Both K contracture and voltage-clamp experiments revealed that activation in soleus fibers occurred with a smaller transient depolarization and was maintained with greater steady state depolarization than in e.d.l. fibers. The K contracture and voltage-clamp results are described by a model in which contraction depends on the formation of a threshold concentration of activator from a voltage-sensitive molecule that can exist in the precursor, activator, or inactive states.

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)

Effects of extracellular calcium on calcium movements of excitation-contraction coupling in frog skeletal muscle fibres

1. The effect of low extracellular free calcium ion concentration ([Ca2+]o) on the transient changes in cytoplasmic [Ca2+] associated with membrane depolarization (Ca2+ transients) was studied on single cut skeletal muscle fibres of the frog, voltage clamped in a double-Vaseline-gap chamber. The Ca2+ transients were monitored with the dye Antipyrylazo III diffused intracellularly. 2. The Ca2+ transients were substantially reduced in external salines with low [Ca2+] (10(-5) M or less and Mg2+ substituted for Ca2+). This decrease was more noticeable at late times during 100 ms or longer depolarizing pulses. 3. The rates of the processes that remove Ca2+ from the myoplasmic solution were not altered by the low [Ca2+]o. This implies that the input flux of Ca2+ into the myoplasm was reduced. 4. The Ca2+ input flux, equal to release flux from the sarcoplasmic reticulum (SR) plus Ca2+ influx via the T-tubule membrane Ca2+ channel, was derived from the Ca2+ transient. In low [Ca2+]o the peak input flux was reduced by 45% (n = 16 fibres) and decayed more rapidly during a depolarizing pulse. 5. The reduction in Ca2+ influx via the T-tubule membrane Ca2+ channel due to the reduced [Ca2+]o could not account for more than 5% of the reduction in Ca2- input flux, which was thus interpreted as an actual reduction of release from the SR. 6. The inward (T-tubular) Ca2+ current was not associated with this effect of extracellular Ca2+ as the effect was voltage independent at high intracellular voltages at which the Ca2+ inward current was strongly voltage dependent. 7. Low [Ca2+]o made Ca2+ release more readily inactivatable; the effect of low [Ca2-]o is best described as a left shift by 29 mV of the 'inactivation curve' of Ca2+ release, relating peak release flux to membrane holding potential. 8. The reduction of Ca2+ release by low [Ca2+]o was not accompanied by changes in the voltage dependence of Ca2+ release or in the threshold voltage for just-detectable release. 9. The results are consistent with a primary effect of Ca2+ on the T-tubular-membrane voltage sensor of excitation-contraction coupling.

Internal citrate ions reduce the membrane potential for contraction threshold in mammalian skeletal muscle fibers

An effect of internal citrate ions on excitation-contraction coupling in skeletal muscle is described. The threshold for contraction was measured in rat extensor digitorum longus, (EDL), and soleus muscle fibers using a two microelectrode voltage clamp technique with either KCl-filled or K3 citrate-filled current electrodes. Contraction thresholds were stable for many minutes with KCl current electrodes. In contrast, thresholds fell progressively towards the resting membrane potential, by as much as -15 mV over a period of 10 to 20 min of voltage-clamp with citrate current electrodes. In addition, prepulse inhibition was suppressed, subthreshold activation enhanced and steady-state inactivation shifted to more negative potentials. Fibers recovered slowly from these effects when the citrate electrode was withdrawn and replaced with a KCl electrode. The changes in contraction threshold suggest that citrate ions act on the muscle activation system at an intracellular site, since the citrate permeability of the surface membrane is probably very low. An internal citrate concentration of 5 mM was calculated to result from citrate diffusion out of the microelectrode into the recording area for 20 min. 5 mM citrate added to an artificial cell lowered the free calcium concentration from 240 to 31 microM. It is suggested that citrate modifies excitation-contraction coupling either by acting upon an anion-dependent step in activation or by reducing the free calcium and/or free magnesium concentration in the myoplasm.

Depletion of calcium from the sarcoplasmic reticulum during calcium release in frog skeletal muscle

1. Free intracellular calcium transients (delta[Ca2+] were monitored in cut segments of frog skeletal muscle fibres voltage clamped in a double Vaseline-gap chamber and stretched to sarcomere lengths that eliminated fibre movement. The measured calcium transients were used to calculate the rate of calcium release from the sarcoplasmic reticulum (s.r.) as previously described (Melzer, Rios & Schneider, 1984, 1987). 2. Conditioning pulses were found to suppress the rate of calcium release in test pulses applied after the conditioning pulse. Various combinations of conditioning and test pulses were used to investigate the basis of the suppression of calcium release by the conditioning pulse. 3. Using a constant test pulse applied at varying intervals after a constant conditioning pulse, recovery from suppression of release was found to occur in two phases. During the fast phase of recovery, which was completed within about 1 s, the rate of calcium release was smaller and had a different wave form than the unconditioned control release. The early peak in release that is characteristic of the control release wave form was absent or depressed. During the slow phase of recovery, which required about 1 min for completion, the release wave form was the same as control but was simply scaled down compared to the control. 4. Conditioning pulses also slowed the rate of decay of delta[Ca2+] after a constant test pulse, probably due to an increased occupancy by calcium of slowly equilibrating myoplasmic sites that bind some of the calcium released by the conditioning pulse. Since calcium binding to these sites contributes to the decay of delta[Ca2+], their increased occupancy would slow the decay of delta[Ca2+] following the test pulse. This effect was used to estimate the calcium occupancy of the slowly equilibrating sites. 5. Comparison of the time course of the slow recovery from suppression of release following a constant conditioning pulse with the time course of the loss of calcium from the slowly equilibrating myoplasmic calcium binding sites indicated that the two processes occurred in parallel. 6. Using a set 1 s recovery period and a constant test pulse but varying the amplitude and/or duration of the conditioning pulse, the degree of slowly recovering suppression of release was found to be directly related to the amount of calcium remaining outside of the s.r. at the start of the test pulse. 7. Points 3, 5 and 6 above indicate that the slow recovery from suppression of release may be due to slow recovery from depletion of calcium from the s.r.(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.

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.

In vitro calibration of the equilibrium reactions of the metallochromic indicator dye antipyrylazo III with calcium

The equilibrium reactions of the metallochromic indicator dye Antipyrylazo III with calcium at physiological ionic strength have been investigated spectrophotometrically. Dye absorbance as a function of wavelength was measured at various total dye and calcium concentrations. Analysis of the absorbance spectra indicated that at pH 6.9 at least three calcium:dye complexes form, with 1:1, 1:2, and possibly 2:2 stoichiometries. The dissociation constant and the changes in dye extinction coefficients on formation of the 1:2 complex, the main complex which forms when Antipyrylazo III is used to study cytoplasmic calcium transients, have been characterized.

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.

Involvement of dihydropyridine receptors in excitation-contraction coupling in skeletal muscle

The transduction of action potential to muscle contraction (E-C coupling) is an example of fast communication between plasma membrane events and the release of calcium from an internal store, which in muscle is the sarcoplasmic reticulum (SR). One theory is that the release channels of the SR are controlled by voltage-sensing molecules or complexes, located in the transverse tubular (T)-membrane, which produce, as membrane voltage varies, 'intramembrane charge movements', but nothing is known about the structure of such sensors. Receptors of the Ca-channel-blocking dihydropyridines present in many tissues, are most abundant in T-tubular muscle fractions from which they can be isolated as proteins. Fewer than 5% of muscle dihydropyridines are functional Ca channels; there is no known role for the remainder in skeletal muscle physiology. We report here that low concentrations of a dihydropyridine inhibit charge movements and SR calcium release in parallel. The effect has a dependence on membrane voltage analogous to that of specific binding of dihydropyridines. We propose specifically that the molecule that generates charge movement is the dihydropyridine receptor.

Properties of the metallochromic dyes Arsenazo III, Antipyrylazo III and Azo1 in frog skeletal muscle fibres at rest

Intact single twitch fibres from frog muscle were isolated and mounted in a normal Ringer solution (16 degrees C) on an optical bench apparatus for measuring fibre absorbance as a function of the wave-length and polarization of the incident light. Fibre absorbance was measured in resting fibres both in the absence and in the presence of one of three metallochromic dyes: Arsenazo III, Antipyrylazo III and Azo1. In the absence of dye, the fibre intrinsic absorbance, Ai(lambda), measured as a function of wave-length, lambda, was well described by the equation: Ai(lambda) = Ai(lambda long) (lambda long/lambda)X, where lambda long is a reference wave-length selected to lie beyond the absorbance band of the dyes and X is the exponential index. For wave-lengths between 480 and 810 nm, the average value of X was 1.1 for 0 deg polarized light (electric vector parallel to the fibre axis) and 1.3 for 90 deg polarized light (electric vector perpendicular to the fibre axis). The intrinsic absorbance at 0 deg, Ai,0(lambda), was somewhat larger than the intrinsic absorbance at 90 deg, Ai,90(lambda); for example, on average (n = 6), Ai,0 (810 nm) was 0.22, whereas Ai,90 (810 nm) was 0.016. Following dye injection, dye-related absorbance was estimated from the measured total fibre absorbance by subtracting the component attributable to the intrinsic absorbance; additionally, for comparison with in vitro calibrations as a function of wave-length, myoplasmic dye absorbance was corrected for the steady change in dye-concentration with time that was attributable to dye diffusion. In fibres injected with either Arsenazo III or Antipyrylazo III, the dye-related absorbance measured with 0 deg light, A0(lambda), was found to be significantly greater than that measured with 90 deg light, A90(lambda), indicating the presence of a resting 'dichroic' signal, A0(lambda)-A90(lambda), attributable to bound and oriented dye molecules. On average, the lower limit estimated for the percentage of oriented dye was 2.8-3.0% for Antipyrylazo III and 1.5-1.8% for Arsenazo III, the population differences between the two dyes being statistically significant. The actual percentage of bound and oriented dye molecules is likely to be considerably larger for both dyes. For Arsenazo III, the wave-length dependence of the dichroic signal was not distinguishably different from the 'isotropic' signal, defined as (A0(lambda) + 2A90(lambda))/3. which represents the average spectrum of all the dye molecules independent of orientation.(ABSTRACT TRUNCATED AT 400 WORDS)

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)

The removal of myoplasmic free calcium following calcium release in frog skeletal muscle

Transient changes in intracellular free calcium concentration (delta [Ca2+]) in response to pulse depolarizations were monitored in isolated segments of single frog skeletal muscle fibres cut at both ends and voltage clamped at a holding potential of -90 mV in a double-Vaseline-gap chamber. Calcium transients were monitored optically using the metallochromic indicator dye Antipyrylazo III (APIII), which entered the fibre by diffusion from the solution applied to the cut ends. Optical artifacts due to fibre movement were minimized or eliminated by stretching the fibres to sarcomere lengths at which there was little or no overlap of thick and thin contractile filaments. Remaining movement-independent optical changes intrinsic to the fibre and unrelated to the dye were monitored at 850 nm, where free and dye-bound APIII have no absorbance. These 850 nm signals scaled by lambda -1.2 were used to remove intrinsic components from the signals at 700 or 720 nm, wave-lengths at which the APIII absorbance increases when calcium is bound. The corrected 700 or 720 nm signals were used to calculate delta [Ca2+]. The decay of delta [Ca2+] following fibre repolarization at the termination of a depolarizing pulse was well described by a single exponential plus a constant. The exponential rate constant for the decay of delta [Ca2+] decreased and the final 'steady' level that delta [Ca2+] appeared to be approaching increased with increasing amplitude and/or duration of the depolarizing pulse. Both the decreasing decay rate and the build up of the 'steady' level can be accounted for using a two-component model for the removal of free calcium from the myoplasm. One component consists of a set number of a single type of saturable calcium binding site in the myoplasm. The second component is a non-saturable, first-order uptake mechanism operating in parallel with the saturable binding sites. The removal model parameter values were adjusted to fit simultaneously the decay of delta [Ca2+] after pulses of various amplitudes and durations in a given fibre. The basic procedure was to track delta [Ca2+] during each pulse when an undetermined calcium release was occurring, but to calculate the decay of delta [Ca2+] starting 14 ms after repolarization when release was assumed to be negligible. After appropriate selection of parameter values, the model reproduced most aspects of the decay of delta [Ca2+].(ABSTRACT TRUNCATED AT 400 WORDS)

Kinetics of rapid Ca2+ release by sarcoplasmic reticulum. Effects of Ca2+, Mg2+, and adenine nucleotides

A radioisotope flux-rapid-quench-Millipore filtration method is described for determining the effects of Ca2+, adenine nucleotides, and Mg2+ on the Ca2+ release behaviour of "heavy" sarcoplasmic reticulum (SR) vesicles. Rapid 45Ca2+ efflux from passively loaded vesicles was blocked by the addition of Mg2+ and ruthenium red. At pH 7 and 10(-9) M Ca2+, vesicles released 45Ca2+ with a low rate (k = 0.1 s-1). An increase in external Ca2+ concentration to 4 microM or the addition of 5 mM ATP or the ATP analogue adenosine 5'-(beta,gamma-methylenetriphosphate) (AMP-PCP) resulted in intermediate 45Ca2+ release rates. The maximal release rate was observed in media containing 4 microM Ca2+ and 5 mM AMP-PCP and had a first-order rate constant of 30-100 s-1. Mg2+ partially inhibited Ca2+- and nucleotide-induced 45Ca2+ efflux. In the absence of AMP-PCP, 45Ca2+ release was fully inhibited at 5 mM Mg2+ or 5 mM Ca2+. The composition of the release media was systematically varied, and the flux data were expressed in the form of Hill equations. The apparent n values of activation of Ca2+ release by ATP and AMP-PCP were 1.6-1.9. The Hill coefficient of Ca2+ activation (n = 0.8-2.1) was dependent on nucleotide and Mg2+ concentrations, whereas the one of Mg2+ inhibition (n = 1.1-1.6) varied with external Ca2+ concentration. These results suggest that heavy SR vesicles contain a "Ca2+ release channel" which is capable of conducting Ca2+ at rates comparable with those found in intact muscle. Ca2+, AMP-PCP (ATP), and Mg2+ appear to act at noninteracting or interacting sites of the channel.

Increases in muscle Ca2+ mediate changes in acetylcholinesterase and acetylcholine receptors caused by muscle contraction

The synthesis of acetylcholinesterase (AcChoE; acetylcholine acetylhydrolase, EC 3.1.1.7) and of acetylcholine receptors (AcChoR) by cultured rat muscle fibers is influenced strongly by the level of muscle contractile activity. If fibers are grown in the presence of tetrodotoxin (TTX) to block spontaneous contraction, the total amount of AcChoE decreases markedly, as does the percentage of AcChoE assembled as the collagen-tailed presumed synaptic form of the enzyme. Under these conditions, however, the number of AcChoR increases. We demonstrate here that each effect of TTX can be prevented by treating the muscle cells with the calcium ionophore A23187. Thus, cells treated with A23187 and TTX have 30- to 40-fold higher levels of collagen-tailed AcChoE and lower levels of AcChoR by a factor of 4-5 than do cells grown in TTX alone. These results suggest that an increase in muscle cytoplasmic Ca2+ mediates the known effects of muscle contraction on these cholinergic macromolecules.

Estimation of intracellular [Ca2+] by nonlinear indicators. A quantitative analysis

When spatial gradients of intracellular free [Ca2+] are present, intracellular calcium indicators that have a nonlinear response to [Ca2+] may yield an estimate of [Ca2+] that differs from the spatial average [Ca2+]. We present two rules that provide (a) general criteria to distinguish those classes of indicators that will yield an overestimate of spatial average [Ca2+] from those that will yield an underestimate, and (b) limits on the extent to which spatial average [Ca2+] might be over- or underestimated. These rules are used to interpret quantitatively the aequorin luminescence signals obtained from cardiac ventricular myocardium.

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

Voltage dependent membrane charge movement (gating current) and the release of Ca2+ from intracellular stores have been measured simultaneously in intact frog skeletal muscle fibres. Charge movement was measured using the three microelectrode voltage clamp technique. Ca2+ release was measured using the metallochromic indicator dye arsenazo III. Fibres were bathed in 2.3 X hypertonic solutions to prevent contraction. Rb+, tetraethylammonium and tetrodotoxin (TTX) were used to eliminate voltage-dependent ionic currents. The maximum rate of Ca2+ release from the sarcoplasmic reticulum in response to voltage-clamp step depolarizations to 0 mV was calculated using the dye-related parameters of model 2 of Baylor et al. (1983) and a method described in the Appendix for calculating a scaling factor (1 + p) that accounts for the additional Ca2+ buffering power of the indicator dye. The estimates of the maximum rate of Ca2+ release at 5-6 degrees C ranged from 3 to 19 microM ms-1 in the 17 fibres examined. The mean value was 8.9 +/- 1.1 microM ms-1 (S.E.M.) The maximum rate of Ca2+ release was linearly related to the magnitude of the nonlinear membrane change moved during suprathreshold depolarizing steps. The voltage dependence of charge movement and the maximum rate of Ca2+ releases were nearly identical at 6 degrees C. The voltage-dependence of the delay between the test step and the onset of Ca2+ release could be adequately described by an equation having the same functional form as the voltage dependence of nonlinear charge movement. The relationship between the test pulse voltage and the delay was shifted to more negative voltages and to shorter delays as the temperature was raised from 6 degrees C to 15 degrees C. The inactivation of Ca2+ release was found to occur at more negative holding voltages and to be more steeply voltage dependent than the immobilization of nonlinear membrane charge movement. The above data are discussed using the 'hypothetical coupler' model of excitation-contraction coupling (Miledi et al., 1983b) applied to the specific case in which each mobile charge group controls the gating of one Ca2+ release site in the sarcoplasmic reticulum.

Use of a metallochromic indicator to study intracellular calcium movements in skeletal muscle

The transient increase in free myoplasmic calcium concentration due to depolarization of a skeletal muscle fibre is the net result of the release of calcium from the sarcoplasmic reticulum (SR) and its simultaneous removal by binding to various sites and by reuptake into the SR. We review here procedures recently developed in this laboratory for empirically characterizing the calcium removal processes in voltage-clamped fibres and for using such characterization to determine the time course of SR calcium release during a depolarizing pulse.