A reappraisal of the Ca2+ dependence of fast inactivation of Ca2+ release in frog skeletal muscle.

Two procedures to inhibit Ca(2+) release designed to differentiate between local and common pool mechanisms for the Ca(2+) dependent, fast inactivation of Ca(2+) release in skeletal muscle of the frog were compared. Inhibition by voltage dependent inactivation of Ca(2+) release, without modification of the single channel current of the Ryanodine Receptor (RyR) and the [Ca(2+)] close to the open pore, produced a reduction in the rate of inactivation linearly related to the reduction in the peak of Ca(2+) release flux. Linear fits in the individual fibers were performed, giving average values (+/-SEM, N = 8) of the best fit parameters of 5.75 x 10(-3) +/- 7.35 x 10(-4 )microM(-1) for the slope and 0.07 +/- 0.015 ms(-1) for the ordinate intercept. Inhibition of Ca(2+) release by reducing the Ca content of the sarcoplasmic reticulum (SR) involves reduction of the Ca(2+) current through the single RyR. The reduction in rate of inactivation also followed linearly the reduction in Ca(2+) peak release flux. The average values (+/-SEM) of the best fit parameters of linear fits were 14 x 10(-3) +/- 3.76 x 10(-3 )microM(-1) and 0.019 +/- 0.006 ms(-1) (N = 7) for slope and ordinate intercept respectively. The differences between both parameters were statistically significant (by t test, at P = 0.05). The extent of inactivation, measured by the peak/final Ca(2+) release flux ratio, was differentially affected by the two procedures. Inhibition by voltage dependent inactivation, despite slowing down the fast inactivation, increased the peak/final Ca(2+) release flux ratio. In contrast, depletion of the SR reticulum reduced it. If the fast inactivation is driven by the high [Ca(2+)] attained locally, close to the open pore of the RyR, the inhibition of Ca(2+) release due to voltage dependent inactivation should not modify the rate of inactivation while inhibition by SR Ca(2+) depletion should reduce it. A process driven by [Ca(2+)] in a common pool should depend on the overall Ca(2+) release independently of how it was modified. In this case both inhibitory procedures should reduce the inactivation rate similarly. Our findings are generally consistent with a common pool process. The differences between the two protocols could be understood if the organization of RyR in junctional and parajunctional release units is considered.