Modulation of Ca2+ signalling in rat atrial myocytes: possible role of the alpha1C carboxyl terminal.
Ca2+ influx through L-type Cav1.2 (alpha1C) 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 (ICa) and termination of release. Although Ca2+ cross-signalling is mediated by high Ca2+ fluxes in the microdomains of alpha1C-RyR complexes, ICa-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 alpha1C in modulating Ca2+ signalling, we tested the effects of introducing two alpha1C C-terminal peptides, LA (1571-1599) and K (1617-1636) on the central alpha1C-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 ICa, LA-peptide did strongly enhance the central Ca2+ transients triggered by ICa at -30 mV (small ICa) but not at +20 mV (large ICa). In contrast, K-peptide had no effect on either ICa 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 alpha1C 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 alpha1C-RyR complexes are intact.
Woo
Sun-Hee
SH
Department of Pharmacology, Georgetown University Medical Center, 3900 Reservoir Road NW, Washington, DC 20057, USA.
Soldatov
Nikolai M
NM
Morad
Martin
M
D000465:Algorithms; D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000502:physiology; D020746:Calcium Channels, L-Type / Q000502:physiology*; D020013:Calcium Signaling / Q000502:physiology*; D002148:Calmodulin-Binding Proteins / Q000502:physiology; D004594:Electrophysiology; D006325:Heart Atria / Q000166:cytology; D007091:Image Processing, Computer-Assisted; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D008564:Membrane Potentials / Q000502:physiology; D018613:Microscopy, Confocal; D032383:Myocytes, Cardiac / Q000502:physiology*; D018408:Patch-Clamp Techniques; D051381:Rats; D044169:Receptors, Calcium-Sensing / Q000502:physiology
D020746:Calcium Channels, L-Type; D002148:Calmodulin-Binding Proteins; C419385:L-type calcium channel alpha(1C); D044169:Receptors, Calcium-Sensing; D002110:Caffeine; D002118:Calcium