Autonomous initiation of pacemaking by L-type Cav1.3 calcium channels in mouse sino-atrial myocytes

Abstract

Cardiac pacemaking relies on the spontaneous electrical activity in the right atrium of sino-atrial myocytes (SANCs). Automaticity in SANCs results from a robust interplay of membrane ion channels activity and intracellular calcium dynamics. However, only a fraction of isolated SANCs exhibit rhythmic firing, whereas most SANCs show irregular (dysrhythmic) firing or remain dormant. To study the capability of L-type Cav1.3 calcium channels to initiate automaticity in dormant SANCs under β-adrenergic stimulation we used a knock-in mouse strain in which the sensitivity of Cav1.2 α1 subunits to dihydropyridines (DHP) was inactivated (Cav1.2DHP-/-). We performed current-clamp recordings on isolated SANCs under isoprenaline (ISO, 100 nM) and in the absence or presence of the DHP blocker Nifedipine (Nife, 3 μM). In rhythmic SANCs, Nife significantly reduced the spontaneous firing under ISO perfusion (ISO: 447 ± 12, ISO + Nife: 233 ± 25 bpm). In dysrhythmic SANCs, Nife significantly reduced the spontaneous rate under ISO in 60% of the cells (ISO: 386 ± 12, ISO + Nife: 188 ± 47 bpm) and completely stopped it in 40% (295 ± 29 bpm to 0). On 25 dormant SANCs, 50% started firing after ISO perfusion (0 to 320 ± 46 bpm). Strikingly, in 75% of them, Nife totally blocked this ISO-induced firing. Moreover, dormant SANCs showed a statistically significant increase in action potential (AP) threshold under ISO compared to dysrhythmic and rhythmic SANCs (dormant: −30.1 ± 2.5, dysrhythmic: −43.3 ± 2.3, rhythmic: −41.2 ± 2.1 mV). No significant difference was observed in the other AP parameters between dormant, dysrhythmic and rhythmic SANCs under ISO. Our results indicate that Cav1.3 channels can generate pacemaker activity autonomously, at least in a particular subpopulation of SANCs, while it is responsible of about half of the automaticity in the others.