Inactivation of Ca2+ release channels (ryanodine receptors RyR1 and RyR2) with rapid steps in [Ca2+] and voltage

The transient responses of sheep cardiac and rabbit skeletal ryanodine receptors (RyRs) to step changes in membrane potential and cytosolic [Ca2+] were measured. Both cardiac and skeletal RyRs have two voltage-dependent inactivation processes (tau approximate to 1-3 s at +40 mV) that operate at opposite voltage extremes. Approximately one-half to two-thirds of RyRs inactivated when the bilayer voltage was stepped either way between positive and negative values. Inactivation was not detected (within 30 s) in RyRs with P-o less than 0.2, Inactivation rates increased with intraburst open probability (P-o) and in proportion to the probability of a long-lived, RyR open stale (P-OL). RyR inactivation depended on P-OL and not on the particular activator (Ca2+ (mu M), ATP, caffeine, and ryanodine), inhibitor (mM Ca2+ and Mg2+), or gating mode. The activity of one-half to two-thirds of RyRs declined (i.e., the RyRs inactivated) after [Ca2+] steps from subactivating (0.1 mu M) to activating (1-100 mu M) levels. This was due to the same inactivation mechanism responsible for inactivation after voltage steps. Both forms of inactivation had the same kinetics and similar dependencies on P-o and voltage. Moreover, RyRs that failed to inactivate after voltage steps also did not inactivate after [Ca2+] steps. The inactivating response to [Ca2+] steps (0.1-1 mu M) was not RyRs "adapting" to steady [Ca2+] after the step, because a subsequent step from I to 100 mu M failed to reactivate RyRs.