[Possible participation of slow Na+--Ca++ channels in realizing cholinergic effects on the cardiac pacemaker]

The effect of the Ca++-permeability inhibitors (Mn ions, Verapamil, D-600) on the bioelectrical activity and cholinergic responses of isolated sinus node involved reduction of the pacemaker rhythm within 30-40 min and a complete inhibition of the bioelectrical discharge as well as a diminished amplitude of the AP and a reduced slope of the slow diastolic and the fast depolarization. The findings seem to be due to an inhibiting of the slow Na+-Ca++ influx by Ca++-antagonists. The prolonged APs and the reduced rate of terminal repolarization might be due to effect of the inhibition on potassium permeability. After electrical stimulation of the efferent nerve or acetylcholine application, the cholinergic reactions were markedly reduced with Mn++ D-600 and Verapamil. The participation of slow Na+-Ca++ channels in cholinergic reactions of pacemaker cells is discussed.