[Effect of acetylcholine on the firing rate and action potential shape of pacemaker cells].

A study was made of the action of exogenous acetylcholine on the isolated pacemaker of the Rana temporaria heart. Bioelectrical activity of individual cells and sum total activity of the preparation were recorded. Exogenous acetylcholine was found not only to inhibit, but also to accelerate the rhythm of pacemaker cells' discharge. As a rule, positive chronotropic effects were observed when relatively low acetycholine concentrations were used. Parasympathetic acceleration was accompanied by an increase in the rate of slow diastolic depolarization; this is in favour of the active mechanism of this process. It is suggested that low and high concentrations of acetylcholine could change the transmembrane ionic currents in a different way.