[Change in cardiac pacemaker activity during interaction of parasympathetic and sympathetic regulatory effects].

Chronotropic effect on the isolated sinus node of the frog heart and the unit activity of the pacemaker cells were studied during separate and joint stimulation of sympathetic and parasympathetic pathways. The joint stimulation entailed parasympathetic bradycardia or augmented the initial negative chronotropic response occurring in case of the parasympathetic stimulation alone. In most cases the joint stimulation diminished the speed of the slow diastolic depolarization and shortened the pacemaker AP. Thus the sympathetic and parasympathetic effects were not antagonistic in relation to the rhythmicity of the sino-atrial pacemaker. The possibility that catecholamines may increase the efficiency of the cholinergic action is discussed.