Effects of phentolamine, phenoxybenzamine and desipramine on clonidine-induced blockade of cardiac acceleration in the dog.

Clonidine reduced cardiac acceleration induced by low frequency electrical stimulation of cardiac sympathetic nerve fibers in anesthetized and vagotomized dogs. This effect of clonidine was abolished by short periods of high frequency electrical stimulation. The inhibitory effect of clonidine was observed with 10- as well as with 90-sec periods of electrical stimulation of sympathetic nerve fibers; it was antagonized by phentolamine, phenoxybenzamine and desipramine. This suggests the presence of alpha-adrenoceptors at the peripheral sympathetic nerve ending in the dog.