Effects of manganese ions and diltiazem on the spontaneous action potential of the canine atrio-ventricular node cell.

Effects of Mn2+ (10(-4) to 10(-3) M) and diltiazem (10(-7) to 10(-6) M) on the spontaneous action potential were studied in small preparations of the canine atrio-ventricular (A-V) node. Mn2+ and diltiazem decreased the maximum rate of rise (Vmax), amplitude of the action potential and rate of depolarization of the slow diastolic depolarization in a concentration-dependent manner. The spontaneous action potentials were depressed by the drugs, and were changed into abortive action potentials. Decrease in the rate of depolarization of the slow diastolic potential was much more prominent with Mn2+ than with diltiazem. These results indicate that the slow inward current may not play an important role in the initiation of the slow diastolic depolarization of the A-V node cell.