Effect of apomorphine on the canine SA node

A constant pressure perfusion of the sinus node artery was performed in 10 in situ and in 3 isolated SA node preparations. The selective administration of apomorphine caused a negative chronotropic effect which was not blocked by treatment with atropine. The sinus deceleration was usually produced by apomorphine, 10-300 mug, while sinus irregularity and finally atrioventricular nodal rhythm in the in situ preparation or sinus arrest in the isolated one resulted from doses above 1 mg. In the in situ preparation at a higher dose from 300 mug to 1 mg, a slight sinus acceleration was occasionally induced following sinus deceleration; In this time, a fall in blood pressure was usually observed; This acceleration response to apomorphine was inhibited by propranolol. From these results, it is concluded that apomorphine induces direct depressive action on the SA node and, in extremely high doses, a slight sinus acceleration may be induced by reflex mechanism.