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

Abstract

Abstract Funding Acknowledgements Type of funding sources: Other. Main funding source(s): Ecole doctorale Background 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. Purpose 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-/-). Methods We performed current and voltage-clamp recordings on isolated SANCs under isoprenaline (ISO, 100 nM) and in the absence or presence of the DHP blocker Nifedipine (Nife, 3 µM). Results Nife significantly reduced the spontaneous firing under ISO perfusion in all rhythmic SANCs (ISO: 447 ± 12, ISO + Nife: 233 ± 25 bpm) and 60% of dysrhythmic SANCs (ISO: 386 ± 12, ISO + Nife: 188 ± 47 bpm) whereas it completely stopped it in the remaining 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. Interestingly, these cells exhibited a significantly slower rate and a slower slope of the diastolic depolarization under ISO perfusion compared to the remaining 25% dormant SANCs in which Nife only reduced the 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. Conclusion Our results seem to point at a difference of expression in ionic channels (Cav1.3, HCN4) within isolated SANCs. Preliminary results on If density support this hypothesis with a lesser density in dormant SANCs compared to dysrhythmic SANCs. These results also tend to indicate that Cav1.3 channels can generate pacemaker activity autonomously, at least in a particular subpopulation of SANCs.