L-type Ca_v1.3 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_v1.3 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_v1.3 knockout (KO) mice (Ca_v1.3^−/−).

We found that Ca_v1.3 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_v1.3^−/− 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. β-Adrenergic stimulation enhanced pacemaking of both genotypes, though, Ca_v1.3^−/− SAN cells remained slower than WT. Conversely, we rescued pacemaker activity in Ca_v1.3^−/− SAN cells and intact tissues through caffeine-mediated stimulation of Ca²⁺-induced Ca²⁺ release.

Ca_v1.3 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_v1.3 loss of function in humans.