L-TYPE CALCIUM CURRENT IN ROD-SHAPED AND SPINDLE-SHAPED MYOCYTES ISOLATED FROM RABBIT ATRIOVENTRICULAR NODE

The atrioventricular node (AVN) is vital to normal cardiac function. The present report describes the properties of L-type calcium current (I-Ca) in rodand spindle-shaped myocytes isolated from the rabbit AVN. With depolarizing voltage clamps from a holding potential of -40 mV, a rapidly activating I-Ca was observed, which peaked at +10 mV in most cells and exhibited a ''bell-shaped'' current-voltage relation. I-Ca was abolished by nifedipine (2-20 mu M) and cadmium (100-200 mu M) and was greatly reduced by manganese (1 mM). At +10 mV, time to peak I-Ca was 3.3 +/- 0.15 (SE) ms (n = 12) and I-Ca current density was 9.3 +/- 1.2 pA/pF (n = 9). Steady-state activation and inactivation curves for I-Ca showed half-maximal activation at -3.6 mV [slope factor (k) = 6.6 mV] and half-maximal inactivation at -25.8 mV (k = 6.5 mV). The time course of decay of I-Ca during a depolarizing pulse was voltage dependent and biexponential. The time course of recovery of I-Ca from inactivation was also biexponential (with two time constants tau(1) = 194.7 and tau(2) = 907.4 ms). Under current clamp, spontaneous action potentials from AVN cells were blocked by nifedipine as well as by cadmium, suggesting that L-type I-Ca was largely responsible for the action potential upstroke.