Contribution of L-type Ca2+ current to electrical activity in sinoatrial nodal myocytes of rabbits

Abstract

The role of L-type calcium current (I(Ca,L)) in impulse generation was studied in single sinoatrial nodal myocytes of the rabbit, with the use of the amphotericin-perforated patch-clamp technique. Nifedipine, at a concentration of 5 μM, was used to block I(Ca,L). At this concentration, nifedipine selectively blocked I(Ca,L) for 81% without affecting the T-type calcium current (I(Ca,T)), the fast sodium current, the delayed rectifier current (I(K)), and the hyperpolarization-activated inward current. Furthermore, we did not observe the sustained inward current. The selective action of nifedipine on I(Ca,L) enabled us to determine the activation threshold of I(Ca,L), which was around -60 mV. As nifedipine (5 μM) abolished spontaneous activity, we used a combined voltage- and current- clamp protocol to study the effects of ((Ca,L) blockade on repolarization and diastolic depolarization. This protocol mimics the action potential such that the repolarization and subsequent diastolic depolarization are studied in current-clamp conditions. Nifedipine significantly decreased action potential duration at 50% repolarization and reduced diastolic depolarization rate over the entire diastole. Evidence was found that recovery from inactivation of I(Ca,L) occurs during repolarization, which makes I(Ca,L) available already early in diastole. We conclude that I(Ca,L) contributes significantly to the net inward current during diastole and can modulate the entire diastolic depolarization.