Modulation of Ca2+ signalling in rat atrial myocytes:: possible role of the α1C carboxyl terminal

Ca2+ influx through L-type Ca(v)1.2 (alpha(1C)) Ca2+ channels is a critical step in the activation of cardiac ryanodine receptors (RyRs) and release of Ca2+ via Ca2+-induced Ca2+ release(CICR). The released Ca2+, in turn, is the dominant determinant of inactivation of the Ca2+ current (I-Ca) and termination of release. Although Ca2+ cross-signalling is mediated by high Ca2+ fluxes in the microdomains of alpha(1C)-RyR complexes, I-Ca-gated Ca2+ cross-signalling is surprisingly resistant to intracellular Ca2+ buffering and has steeply voltage-dependent gain, inconsistent with a strict CICR mechanism, suggesting the existence of additional regulatory step(s). To explore the possible regulatory role of the carboxyl (C)-terminal tail of alpha(1C) in modulating Ca2+ signalling, we tested the effects of introducing two alpha(1C) C-terminal peptides, LA (1571-1599) and K (1617-1636) on the central alpha(1C)-unassociated Ca2+-release sites of atrial myocytes, using rapid (240 Hz) two-dimensional confocal Ca2+ imaging. The frequency of spontaneously activating central sparks increased by approximately fourfold on dialysing LA- but not K-peptide into myocytes voltage-clamped at -80 mV. The rate but not the magnitude of caffeine (10 mm)-triggered central Ca2+ release was significantly accelerated by LA- but not K-peptide. Individual Ca2+ spark size and flux were larger in LA- but not in K-peptide-dialysed myocytes. Although LA-peptide did not change the amplitude or inactivation kinetics of I-Ca, LA-peptide did strongly enhance the central Ca2+ transients triggered by I-Ca at -30 mV (small I-Ca) but not at +20 mV (large I-Ca). In contrast, K-peptide had no effect on either I-Ca or the local Ca2+ transients. LA-peptide with a deleted calmodulin-binding region (LM1-peptide) had no significant effects on the central spark frequency but suppressed spontaneous spark frequency in the periphery. Our results indicate that the calmodulin-binding LA motif of the alpha(1C) C-terminal tail may sensitize the RyRs, thereby increasing their open probability and providing for both the voltage-dependence of CICR and the higher frequency of spark occurrence in the periphery of atrial myocytes where the native alpha(1C)-RyR complexes are intact.