Quantal Properties of Voltage-Dependent Ca²⁺ Release in Frog Skeletal Muscle Persist After Reduction of [Ca²⁺] in the Sarcoplasmic Reticulum

Abstract

In skeletal muscle, the Ca²⁺ release flux elicited by a voltage clamp pulse rises to an early peak that inactivates rapidly to a much lower steady level. Using a double pulse protocol the fast inactivation follows an arithmetic rule: if the conditioning depolarization is less than or equal to the test depolarization, then decay (peak minus steady level) in the conditioning release is approximately equal to suppression (unconditioned minus conditioned peak) of the test release. This is due to quantal activation by voltage, analogous to the quantal activation of IP3 receptor channels. Two mechanisms are possible. One is the existence of subsets of channels with different sensitivities to voltage. The other is that the clusters of Ca²⁺-gated Ryanodine Receptor (RyR) β in the parajunctional terminal cisternae might constitute the quantal units. These Ca²⁺-gated channels are activated by the release of Ca²⁺ through the voltage-gated RyR α channels. If the RyR β were at the basis of quantal release, it should be modified by strong inhibition of the primary voltage-gated release. This was attained in two ways, by sarcoplasmic reticulum (SR) Ca²⁺ depletion and by voltage-dependent inactivation. Both procedures reduced global Ca²⁺ release flux, but SR Ca²⁺ depletion reduced the single RyR current as well. The effect of both interventions on the quantal properties of Ca²⁺ release in frog skeletal muscle fibers were studied under voltage clamp. The quantal properties of release were preserved regardless of the inhibitory maneuver applied. These findings put a limit on the role of the Ca²⁺-activated component of release in generating quantal activation.

Graphical Abstract

Appendix

In order to validate the findings presented in this study a methodological point is necessary to be addressed. In previous studies of this phenomenon (Pizarro et al 1997; Szentesi et al 2000) the release flux was corrected by the depletion of the SR [Ca²⁺] with the method introduced by Schneider et al (1989). In keeping with this approach, we also used this method, albeit modified, in the current study. As explained in the Methods section, the procedure assumes that the release flux is proportional to the total Ca²⁺ content in the SR and that the permeability reaches a steady level at the end of a 100 ms voltage clamp pulse.

The two basic assumptions of the method are not free of problems. First, the assumption that the driving force is proportional to the total Ca²⁺ in the SR requires that Ca²⁺ buffering in the SR is linear in order to make free [Ca²⁺] always proportional to total [Ca²⁺]. This is not what was recently reported (Pape et al 2007; Pizarro and Olivera 2020). A second problem is that the assumption of a steady permeability after the fast inactivation is not fully supported by the available data (Sztretye et al 2011; Olivera and Pizarro 2018).

In some cases, the quantal activation is already apparent in not corrected data. It could be evident even in the Ca²⁺ transients, when a combination of high EGTA (10 mM or higher) and a fast indicator is used. Despite this, most of the time it is necessary to correct the data for depletion in order to observe the deterministic properties of inactivation. This is because the release during the conditioning pulse produces enough depletion to reduce the peak of the release in the test pulse beyond the effect of inactivation. Therefore, it is relevant to rule out if the correction itself might artificially create the phenomenon.

This was explored by means of simulations. We used two synthetic permeability waveforms, one assuming deterministic inactivation and one assuming that e percentage of inactivation is the same in the conditioning and in the test pulse.

The simulations were carried out as those performed by Pizarro and Olivera (2020). Then, the release flux obtained was corrected using the method of Schneider et al (1989). Two conditions were simulated, with quantal activation (i.e.: decay equal to suppression) and the non quantal case where the suppression was twice the decay. The simulations shown are meant to represent the reference condition. It was assumed that these conditions were those reported in the study of Pizarro and Olivera (2020), with 0.4 mM resting free [Ca²⁺] in the SR. The intra SR Ca²⁺ buffering was assumed to be due to cooperative Ca²⁺ binding following a Hill equation with k_d = 852 μM, n_H = 2.38 and 46.7 mM total binding sites.

In the simulations in Fig. 6 the permeability time course showed a moderate slow decay after the fast inactivation. This time course is based on reports by Olivera and Pizarro (2018) and Sztretye et al (2011). An experimentally determined permeability (taken from Fig. 6E, record labeled 1 in Olivera and Pizarro 2018), shown in the inset in gray trace, was fitted with a mathematical function (Eq. 43 in Pizarro and Olivera 2020). The fit is shown in the inset, in black, superimposed to the experimental trace. This fit was used in the simulations to obtain the release during the unconditioned test. To represent the conditioned release the amplitude of the fast inactivating component was changed. The release flux during the conditioning pulse was assumed to reach a steady state and to have a higher peak to steady ratio. The amplitude of the permeability (actually, it is the permeability multiplied by the ratio between the SR surface and the myoplasmic volume, with unit of ms⁻¹) was set to reproduce the intensity of the release flux observed. The outcome of the simulations is shown in figure 6. In panels A, B and C the quantal properties were assumed. D, E and F correspond to the non quantal case. A and D are the permeability time courses that drive the simulations. B and D are the release flux waveforms obtained. The corresponding corrected release time course are shown C and F. The initial [Ca] SR used in the correction was 1100 μM. As shown in the figure, if the decay and suppression were equal to begin with, the correction yields also an equality. Conversely, if deterministic inactivation was not assumed in the permeability to drive the simulation the corrected records did not show a deterministic behavior. The recovery of the assumed properties, even though not all the tenets of the correction procedure are correct, is somehow remarkable. It is necessary to point out that if the decay is beyond 30% of the value reached after the fast inactivation the correction method tend to overcompensate, making the suppression less than the decay despite they were assumed equal in the simulations.

Fig. 6

Simulations performed to explore the effect of the depletion correction procedure on the quantal properties of release. A and C show the synthetic permeability time courses with (red ) and without (black) conditioning pulses. Equality between decay and suppression is assumed for the simulations in panels A, B and C. Suppression double than decay is assumed in the simulations shown in D, E and F. In the inset it is shown how the time course of the permeability was obtained, by fitting the time course of an experimentally obtained permeability record ( in grey, taken from figure 6, panel E, Olivera and Pizarro, 2018). Note that there is a slow decay of the permeability after the fast inactivation. Thus, a steady level of permeability was not assumed. B and E show the release flux through this permeability assuming that flux is proportional to free [Ca2+] SR and a cooperative Ca2+ buffer in the SR with parameters B = 46.7 mM, nH = 2.38 and kd = 852 μM and an assumed resting free [Ca2+] SR =0.4 mM. In B the simulated release records, suppression is bigger than decay as a consequence of the depletion in free [Ca2+] SR due to the conditioning pulse. C shows the corrected release flux traces performing the correction with the method of Schneider et al (1989). The equality between decay and suppression is recovered. In E and F release flux and corrected release flux are shown respectively. In this case, when decay and suppression were assumed drastically different the correction does not produce the quantal properties. Therefore, the quantal properties are not an artifact produced by the correction procedure, even though none of the assumptions of the correction method were fulfilled

Therefore, if the intra SR Ca²⁺ buffering properties we previously reported (Olivera and Pizarro 2018; Pizarro and Olivera 2020) apply in this study, it seems reasonable to accept that the quantal properties that we observed were not an artifact of the correction method, despite the assumptions underneath it were not fulfilled. Similar simulations, not shown, in a partially depleted SR gave the same outcome. The SR depleted condition corresponds to 0.1 mM free Ca²⁺, k_d = 330 μM n_H = 2.2 and 28.07 mM total binding sites (Pizarro and Olivera 2020).