CALCIUM MODULATION OF THE SPONTANEOUS ACTIVITY AND THE IONIC CURRENTS IN RABBIT ATRIO-VENTRICULAR NODAL CELLS

Abstract

Modulation by changing the extracellular Ca²⁺ concentration ([Ca]ₒ) of the electrophysiological activity in isolated rabbit atrio-ventricular (AV) nodal cells was investigated using the two-microelectrode voltage-clamp technique. Low [Ca]ₒ (0.9 mM or Ca²⁺-free) decreased the rate of spontaneous beating, but the maximum rate of depolarization was significantly enhanced. Simultaneously, the [Ca]ₒ decline tended to increase the action potential amplitude, prolong the action potential duration (at 50% repolarization), and shorten the cycle length. The maximum diastolic potential was hyperpolarized. But these factors were not significant. In contrast, elevation of [Ca]ₒ to 5.4 or 10 mM (from 1.8 mM) transiently stimulated, but then inhibited the spontaneous activity. The effects on the action potential parameters were rather depressant. In voltage-clamp experiments, the [Ca]ₒ elevation increased the maximum onductances for the slow inward current and the delayed outward K⁺ current, whereas the [Ca]ₒ decline decreased them. An arrhythmia occurred in 4 out of 6 preparations only at 10 mM [Ca]ₒ. These results indicate that [Ca]ₒ would modulate the electrical activity of the AV nodal cells due to conductance changes in the ionic channels, but high [Ca]ₒ induces rather the depressant effects.