Abstract
Using in situ constant pressure perfusion of the SA node artery at 100 mmHg, the effect of thymoxamine, which is a salt of (6-acetoxythymoxy) ethyldimethylamine, was investigated in 7 vagotomized dogs weighing 9 to 15 kg. Thymoxamine injected into the SA node artery induced a biphasic chronotropic response, i.e. a deceleration of sinus rate followed by an acceleration. The threshold dose for induction of sinus deceleration by thymoxamine was 3 to 10 μg. This deceleration response was not influenced by atropine treatment. The threshold dose for induction of sinus acceleration by thymoxamine was 10 to 30 μg. This accelerated response was inhibited by propranolol or tetrodotoxin. These results indicate that thymoxamine induces direct depressive action on the SA node pacemaker and the acceleration due to catecholamine release accompanying excitation of sympathetic nerve fibers in the SA node region.
