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

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.