PHARMACOLOGIC BLOCKADE OF REFLEX VASODILATATION: EFFECTS ON POSTULATED NEUROHUMORAL MECHANISMS

¹ Department of Pharmacology, College of Medicine, University of Iowa, Iowa City, Iowa

The effects of phenoxybenzamine, cocaine and tripelennamine on the release of histamine-C¹⁴ during baroreceptor reflex vasodilatation and the release of norepinephrine-H³ during sympathetic nerve stimulation were studied in isolated, perfused graciis muscles of dogs. Phenoxybenzamine blocked the vasoconstrictor response and increased the amount of norepinephrine-H³ during sympathetic stimulation. It also blocked reflex vasodilatation with a tendency to decrease the amount of histamine-C¹⁴ released into the venous blood. Neither cocaine nor tripelennamine altered the amount of norepinephrine-H³ released during sympathetic stimulation, but tripelennamine blocked the vasoconstrictor response. Both cocaine and tripelennamine reduced the magnitude of reflex vasodilatation, and cocaine reduced the amount of histamine-C¹⁴ released during the reflex. The results with phenoxybenzamine are compatible with blockade of the postulated neurohumoral mechanisms for both the passive and active components of reflex vasodilatation. Cocaine and tripelennamine, however, appear to interfere with the neurohumoral mechanism for only the active component of the reflex. These findings lend further support to the concept of an active component of baroreceptor reflex vasodilatation which is mediated by the release of histamine from sympathetic nerves.