Coupled gating modifies the regulation of cardiac ryanodine receptors by luminal Ca2 +

Abstract

Cardiac ryanodine receptors (RYR2s) infrequently exhibit coupled gating that is manifested by synchronous opening and closing. To better characterize this phenomenon, we investigated the regulation of coupled RYR2 channels by luminal Ca2+ focusing on effects that are likely mediated by the true luminal activation mechanism. By reconstituting an ion channel into a planar lipid bilayer and using substantially lower concentration of luminal Ba2+ (8mM, the virtual absence of Ca2+) and luminal Ca2+ (8mM), we show that response of coupled RYR2 channels to caffeine at a diastolic cytosolic Ca2+ (90nM) was affected by luminal Ca2+ in a similar manner as for the single RYR2 channel except the gating behavior. Whereas, the single RYR2 channel responded to luminal Ca2+ by prolongation in open and closed times, coupled RYR2 channels seemed to be resistant in this respect. In summary, we conclude that the class of Ca2+ sites located on the luminal face of coupled RYR2 channels that is responsible for the channel potentiation by luminal Ca2+ is functional and not structurally hindered by the channel coupling. Thus, the idea about non-functional luminal Ca2+ sites as a source of the apparent gating resistance of coupled RYR2 channels to luminal Ca2+ appears to be ruled out.