L-type Cav1.3 channels regulate ryanodine receptor-dependent Ca2+ release during sino-atrial node pacemaker activity

Sino-atrial node (SAN) automaticity is an essential mechanism of heart rate generation that is still not completely understood. Recent studies highlighted the importance of intracellular Ca2+ ([Ca2+](i)) dynamics during SAN pacemaker activity. Nevertheless, the functional role of voltage-dependent L-type Ca2+ channels in controlling SAN [Ca2+](i) release is largely unexplored. Since Ca(v)1.3 is the predominant L-type Ca2+ channel isoform in SAN cells, we studied [Ca2+](i) dynamics in isolated cells and ex vivo SAN preparations explanted from wild-type (WT) and Ca(v)1.3 knockout (KO) mice (Ca(v)1.3(-/-)). We found that Ca(v)1.3 deficiency strongly impaired [Ca2+](i) dynamics, reducing the frequency of local [Ca2+](i) release events and preventing their synchronization. This impairment inhibited the generation of Ca2+ transients and delayed spontaneous activity. We also used action potentials recorded in WT SAN cells as voltage-clamp commands for Ca(v)1.3(-/-) cells. Although these experiments showed abolished Ca2+ entry through L-type Ca2+ channels in the diastolic depolarization range of KO SAN cells, their sarcoplasmic reticulum Ca2+ load remained normal. beta-Adrenergic stimulation enhanced pacemaking of both genotypes, though, Ca(v)1.3(-/-) SAN cells remained slower than WT. Conversely, we rescued pacemaker activity in Ca(v)1.3(-/-) SAN cells and intact tissues through caffeine-mediated stimulation of Ca2+-induced Ca2+ release. Ca(v)1.3 channels play a critical role in the regulation of [Ca2+](i) dynamics, providing an unanticipated mechanism for triggering local [Ca2+](i) releases and thereby controlling pacemaker activity. Our study also provides an additional pathophysiological mechanism for congenital SAN dysfunction and heart block linked to Ca(v)1.3 loss of function in humans.