Calcium release and intramembranous charge movement in frog skeletal muscle fibres with reduced (< 250 microM) calcium content

Whole-cell voltage clamp on skeletal muscle fibers with the silicone-clamp technique

Control of membrane voltage and membrane current measurements are of critical importance for the study of numerous aspects of skeletal muscle physiology and pathophysiology. The silicone-clamp technique makes use of a conventional patch-clamp apparatus to achieve whole-cell voltage clamp of a restricted portion of a fully differentiated adult skeletal muscle fiber. The major part of an isolated muscle fiber is insulated from the extracellular medium with silicone grease and the tip of a single microelectrode connected to the amplifier is then inserted within the fiber through the silicone layer. The method is extremely easy to implement. It represents an alternative to the traditional vaseline-gap isolation and two or three microelectrodes voltage-clamp techniques. The present chapter reviews the benefits of the silicone-clamp technique and provides updated detailed insights into its practical implementation.

Calcium buffering properties of sarcoplasmic reticulum and calcium-induced Ca(2+) release during the quasi-steady level of release in twitch fibers from frog skeletal muscle

Experiments were performed to characterize the properties of the intrinsic Ca²⁺ buffers in the sarcoplasmic reticulum (SR) of cut fibers from frog twitch muscle. The concentrations of total and free calcium ions within the SR ([Ca_T]_SR and [Ca²⁺]_SR) were measured, respectively, with the EGTA/phenol red method and tetramethylmurexide (a low affinity Ca²⁺ indicator). Results indicate SR Ca²⁺ buffering was consistent with a single cooperative-binding component or a combination of a cooperative-binding component and a linear binding component accounting for 20% or less of the bound Ca²⁺. Under the assumption of a single cooperative-binding component, the most likely resting values of [Ca²⁺]_SR and [Ca_T]_SR are 0.67 and 17.1 mM, respectively, and the dissociation constant, Hill coefficient, and concentration of the Ca-binding sites are 0.78 mM, 3.0, and 44 mM, respectively. This information can be used to calculate a variable proportional to the Ca²⁺ permeability of the SR, namely d[Ca_T]_SR/dt ÷ [Ca²⁺]_SR (denoted release permeability), in experiments in which only [Ca_T]_SR or [Ca²⁺]_SR is measured. In response to a voltage-clamp step to −20 mV at 15°C, the release permeability reaches an early peak followed by a rapid decline to a quasi-steady level that lasts ∼50 ms, followed by a slower decline during which the release permeability decreases by at least threefold. During the quasi-steady level of release, the release amplitude is 3.3-fold greater than expected from voltage activation alone, a result consistent with the recruitment by Ca-induced Ca²⁺ release of 2.3 SR Ca²⁺ release channels neighboring each channel activated by its associated voltage sensor. Release permeability at −60 mV increases as [Ca_T]_SR decreases from its resting physiological level to ∼0.1 of this level. This result argues against a release termination mechanism proposed in mammalian muscle fibers in which a luminal sensor of [Ca²⁺]_SR inhibits release when [Ca_T]_SR declines to a low level.

Paradoxical buffering of calcium by calsequestrin demonstrated for the calcium store of skeletal muscle

Contractile activation in striated muscles requires a Ca(2+) reservoir of large capacity inside the sarcoplasmic reticulum (SR), presumably the protein calsequestrin. The buffering power of calsequestrin in vitro has a paradoxical dependence on [Ca(2+)] that should be valuable for function. Here, we demonstrate that this dependence is present in living cells. Ca(2+) signals elicited by membrane depolarization under voltage clamp were compared in single skeletal fibers of wild-type (WT) and double (d) Casq-null mice, which lack both calsequestrin isoforms. In nulls, Ca(2+) release started normally, but the store depleted much more rapidly than in the WT. This deficit was reflected in the evolution of SR evacuability, E, which is directly proportional to SR Ca(2+) permeability and inversely to its Ca(2+) buffering power, B. In WT mice E starts low and increases progressively as the SR is depleted. In dCasq-nulls, E started high and decreased upon Ca(2+) depletion. An elevated E in nulls is consistent with the decrease in B expected upon deletion of calsequestrin. The different value and time course of E in cells without calsequestrin indicate that the normal evolution of E reflects loss of B upon SR Ca(2+) depletion. Decrement of B upon SR depletion was supported further. When SR calcium was reduced by exposure to low extracellular [Ca(2+)], release kinetics in the WT became similar to that in the dCasq-null. E became much higher, similar to that of null cells. These results indicate that calsequestrin not only stores Ca(2+), but also varies its affinity in ways that progressively increase the ability of the store to deliver Ca(2+) as it becomes depleted, a novel feedback mechanism of potentially valuable functional implications. The study revealed a surprisingly modest loss of Ca(2+) storage capacity in null cells, which may reflect concurrent changes, rather than detract from the physiological importance of calsequestrin.

How source content determines intracellular Ca2+ release kinetics. Simultaneous measurement of [Ca2+] transients and [H+] displacement in skeletal muscle

In skeletal muscle, the waveform of Ca²⁺ release under clamp depolarization exhibits an early peak. Its decay reflects an inactivation, which locally corresponds to the termination of Ca²⁺ sparks, and is crucial for rapid control. In cardiac muscle, both the frequency of spontaneous sparks (i.e., their activation) and their termination appear to be strongly dependent on the Ca²⁺ content in the sarcoplasmic reticulum (SR). In skeletal muscle, no such role is established. Seeking a robust measurement of Ca²⁺ release and a way to reliably modify the SR content, we combined in the same cells the “EGTA/phenol red” method (Pape et al., 1995) to evaluate Ca²⁺ release, with the “removal” method (Melzer et al., 1987) to evaluate release flux. The cytosol of voltage-clamped frog fibers was equilibrated with EGTA (36 mM), antipyrylazo III, and phenol red, and absorbance changes were monitored simultaneously at three wavelengths, affording largely independent evaluations of Δ[H⁺] and Δ[Ca²⁺] from which the amount of released Ca²⁺ and the release flux were independently derived. Both methods yielded mutually consistent evaluations of flux. While the removal method gave a better kinetic picture of the release waveform, EGTA/phenol red provided continuous reproducible measures of calcium in the SR (Ca_SR). Steady release permeability (P), reached at the end of a 120-ms pulse, increased as Ca_SR was progressively reduced by a prior conditioning pulse, reaching 2.34-fold at 25% of resting Ca_SR (four cells). Peak P, reached early during a pulse, increased proportionally much less with SR depletion, decreasing at very low Ca_SR. The increase in steady P upon depletion was associated with a slowing of the rate of decay of P after the peak (i.e., a slower inactivation of Ca²⁺ release). These results are consistent with a major inhibitory effect of cytosolic (rather than intra-SR) Ca²⁺ on the activity of Ca²⁺ release channels.

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

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

Recruitment of Ca(2+) release channels by calcium-induced Ca(2+) release does not appear to occur in isolated Ca(2+) release sites in frog skeletal muscle

Ca(2+) release from the sarcoplasmic reticulum (SR) in skeletal muscle in response to small depolarisations (e.g. to -60 mV) should be the sum of release from many isolated Ca(2+) release sites. Each site has one SR Ca(2+) release channel activated by its associated T-tubular voltage sensor. The aim of this study was to evaluate whether it also includes neighbouring Ca(2+) release channels activated by Ca-induced Ca(2+) release (CICR). Ca(2+) release in frog cut muscle fibres was estimated with the EGTA/phenol red method. The fraction of SR Ca content ([Ca(SR)]) released by a 400 ms pulse to -60 mV (denoted f(Ca)) provided a measure of the average Ca(2+) permeability of the SR associated with the pulse. In control experiments, f(Ca) was approximately constant when [Ca(SR)] was 1500-3000 microM (plateau region) and then increased as [Ca(SR)] decreased, reaching a peak when [Ca(SR)] was 300-500 microM that was 4.8 times larger on average than the plateau value. With 8 mM of the fast Ca(2+) buffer BAPTA in the internal solution, f(Ca) was 5.0-5.3 times larger on average than the plateau value obtained before adding BAPTA when [Ca(SR)] was 300-500 microM. In support of earlier results, 8 mM BAPTA did not affect Ca(2+) release in the plateau region. At intermediate values of [Ca(SR)], BAPTA resulted in a small, if any, increase in f(Ca), presumably by decreasing Ca inactivation of Ca(2+) release. Since BAPTA never decreased f(Ca), the results indicate that neighbouring channels are not activated by CICR with small depolarisations when [Ca(SR)] is 300-3000 microM.

Extra activation component of calcium release in frog muscle fibres

In addition to activating more Ca(2+) release sites via voltage sensors in the t-tubular membranes, it has been proposed that more depolarised voltages enhance activation of Ca(2+) release channels via a voltage-dependent increase in Ca-induced Ca(2+) release (CICR). To test this, release permeability signals in response to voltage-clamp pulses to two voltages, -60 and -45 mV, were compared when Delta[Ca(2+)] was decreased in two kinds of experiments. (1) Addition of 8 mM of the fast Ca(2+) buffer BAPTA to the internal solution decreased release permeability at -45 mV by > 2-fold and did not significantly affect Ca(2+) release at -60 mV. Although some of this decrease may have been due to a decrease in voltage activation at -45 mV - as assessed from measurements of intramembranous charge movement - the results do tend to support a Ca-dependent enhancement with greater depolarisations. (2) Decreasing SR (sarcoplasmic reticulum) Ca content ([Ca(SR)]) should decrease the Ca(2+) flux through an open channel and thereby Delta[Ca(2+)]. Decreasing [Ca(SR)] from > 1000 microM (the physiological range) to < 200 microM decreased release permeability at -45 mV relative to that at -60 mV by > 6-fold, an effect shown to be reversible and not attributable to a decrease in voltage activation at -45 mV. These results indicate a Ca-dependent triggering of Ca(2+) release at more depolarised voltages in addition to that expected by voltage control alone. The enhanced release probably involves CICR and appears to involve another positive feedback mechanism in which Ca(2+) release speeds up the activation of voltage sensors.