The sinoatrial node of isolated atria from rabbits was explored with the ultramicroelectrode technique of intracellular recording.
Either acetylcholine or l-epinephrine was added at the site of electrode impalement in order to observe the changes in configuration of pacemaker potentials associated with changing heart rate.
The primary effect of acetylcholine during retardation of rate was to reduce the slope of the pacemaker cell prepotential and the magnitude of the action potential. Epinephrine acceleration was associated with an increased slope of prepotential. Action potential magnitude was not significantly changed.
In the presence of either drug, there was evidence of reversible migration of pacemaker activity.