Characterization of voltage gated L-type Ca2+ channels in the sinoatrial node

Pacemaking of the sinoatrial node (SAN) controls cardiac rhythmicity. Dysregulation of L-type Ca²⁺ currents in SAN cells causes cardiac arrhythmia. Both Ca_v1.2 and Ca_v1.3 channels mediate Ca²⁺ currents and are activated during the SAN action potential. Whether these channels exhibit differences in modulation and localization in the SAN is unknown. We hypothesize that the channels contribute to different phases of the SAN action potential and therefore have distinguishable impact on cardiac pacemaking.  A clear separation of each channel’s role in pacemaking could allow for targeted development of pharmaceuticals leading to better treatment for cardiac arrhythmia.

Cells were immunflourescently labeled with antibodies specific to Ca_v1.2, Ca_v1.3 and Ryanodine Receptors (RyR). Confocal imaging of these cells indicates that Cav1.2 is mainly localized in the cell membrane, while Cav1.3 colocalizes with the RyR in transverse tubules. Electrophysiology experiments were performed on isolated SAN cells of genetically altered mice (rendering Cav1.2 channel insensitive to nimodipine) to characterize distinct properties of Ca_v1.2 and Ca_v1.3 currents. Depolarizations (500ms to -10 mV) led to enhanced voltage dependent inactivation of Ca_v1.2 (~55%) versus Ca_v1.3 (~36%, t-test, p=0.01). Using a prepulse (200ms to +80mV) protocol we were able to unmask voltage dependent facilitation (VDF - 105% of original current) of Ca_v1.3, which became evident during recovery from inactivation after 450ms of the prepulse (Cav1.2 remained inactivated ~15%).

The novelty of our work lies in the functional and spatial separation of two closely related calcium channels in SAN cells and how they help shape the AP and successively control pacemaking.