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

The role of L-type calcium current (I-Ca,I-L) in impulse generation was stu died in single sinoatrial nodal myocytes of the rabbit, with the use of the amphotericin-perforated patch-clamp technique. Nifedipine, at a concentrat ion of 5 mu M, was used to block I-Ca,I-L. At this concentration, nifedipin e selectively blocked I-Ca,I-L for 81% without affecting the T-type calcium current (I-Ca,I-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 nife dipine on I-Ca,I-L enabled us to determine the activation threshold of I-Ca ,I-L which was around -60 mV. As nifedipine (5 mu M) abolished spontaneous activity, we used a combined voltage- and current-clamp protocol to study t he effects of I-Ca,I-L blockade on repolarization and diastolic depolarizat ion. This protocol mimics the action potential such that the repolarization and subsequent diastolic depolarization are studied in current-clamp condi tions. Nifedipine significantly decreased action potential duration at 50% repolarization and reduced diastolic depolarization rate over the entire di astole. Evidence was found that recovery from inactivation of I-Ca,I-L occu rs during repolarization, which makes I-Ca,I-L available already early in d iastole. We conclude that I-Ca,I-L contributes significantly to the net inw ard current during diastole and can modulate the entire diastolic depolariz ation.