L-type Cav1.3 calcium channels are key actors in beta-adrenergic triggered automaticity in dormant mouse sino-atrial myocytes

Abstract

Cardiac pacemaking relies on the spontaneous electrical activity of sino-atrial myocytes (SANCs) in the right atrium arising from a robust interplay of membrane ion channels activity (membrane clock) and intracellular calcium dynamics (calcium clock). Strikingly, most of isolated SANCs exhibit a dormant state (no apparent spontaneous activity) whereas only a small fraction shows regular firing. It has recently been shown that these non-firing SANCs are also present in intact SAN and can switch into a firing mode under beta-adrenergic stimulation. However, their functional relevance in vivo is unknown. To study the role of L-type Ca v 1.3 calcium channels in dormant SANCs using a knock-in mouse strain in which the sensitivity of Ca v 1.2 α1 subunits to dihydropyridines (DHP) was inactivated (Ca v 1.2 DHP−/− ). We performed voltage and current-clamp recordings and confocal live imaging on isolated SANCs under isoprenaline (ISO, 100 nM) in the absence or presence of the DHP blocker Nifedipine (Nife, 3 μM). Preliminary results seem to show less L-type calcium current I Ca,L in dormant SANCs (D) compared to rhythmic SANCs (R) (D: −6.3 ± 1.2, R: −15.6 pA/pF). Dormant SANCs also have less diastolic calcium (D: 96.9 ± 12, R: 218 ± 51.2 a.u, P = 0.14) but significant higher sarcoplasmic reticulum calcium load (D: 6.3 ± 0.6, R: 3.1 ± 0.3 a.u, P = 0.004). Interestingly, ISO perfusion could only switch half of dormant SANCs into a firing mode in both action potential (AP) and calcium transients (CT) recordings whereas the others remained dormant. Further Nife perfusion completely stopped this ISO-induced firing switching the cells back into a dormant state for the vast majority of them (75% in AP-recorded SANCs and 100% in CT-recorded SANCs). Taken together, these results indicate that Cav1.3 channels play a key role in sustaining the coupling of membrane and calcium clock in dormant SANCs under beta-adrenergic stimulation.