Abstract
We studied inactivation of Ca(2+)-induced Ca(2+) release (CICR) via ryanodine receptors (RyRs) in bullfrog sympathetic neurons. The rate of rise in [Ca(2+)](i) due to CICR evoked by a depolarizing pulse decreased markedly within 10-20 ms to a much slower rate despite persistent Ca(2+) entry and little depletion of Ca(2+) stores. The Ca(2+) entry elicited by the subsequent pulse within 50 ms, during which the [Ca(2+)](i) level remained unchanged, did not generate a distinct [Ca(2+)](i) rise. This mode of [Ca(2+)](i) rise was unaffected by a mitochondrial uncoupler, carbonyl cyanide p-trifluromethoxy-phenylhydrazone (FCCP, 1 microm). Paired pulses of varying interval and duration revealed that recovery from inactivation became distinct >or= 50 ms after depolarization and depended on [Ca(2+)](i). The inactivation was prevented by BAPTA (>or= 100 microm) but not by EGTA (<or= 10 mM), whereas the activation was less affected by BAPTA. When CICR was partially activated, some of the non-activated RyRs were also inactivated directly. Thus, the inactivation in these neurons is induced by Ca(2+) binding to the high-affinity regulatory sites residing very close to Ca(2+) channels and/or RyRs, although the sites for activation are located much closer to those Ca(2+) sources. The rate of [Ca(2+)](i) decay after the pulse decreased with increasing pulse duration longer than 10 ms, and this was abolished by BAPTA. Thus, some mechanism counteracting Ca(2+) clearance is induced after full inactivation and potentiated during the pulse. Possible models for RyR inactivation were proposed and the roles of inactivation in Ca(2+) signalling were discussed.