GREATER EFFECTIVENESS OF PHENOXYBENZAMINE IN BLOCKING VASOCONSTRICTOR RESPONSES TO CENTRAL SYMPATHETIC STIMULATION THAN TO NOREPINEPHRINE ADMINISTRATION IN THE CAT

¹ Departamento de Ciencias Fisiológicas, Facultad de Medicine, Universidad del Valle, Cali, Colombia

Vasoconstrictor responses were elicited in autoperfused hindquarters of cats both neurogenically, by stimulation of the sympathetic pressor center, and by exogenous administration of norepinephrine. Stimulus-effect and dose-effect relationships were determined, and the effectiveness of different doses of phenoxybenzamine in modifying these relationships was studied. Phenoxybenzamine produced a significantly greater blockade of responses evoked by central sympathetic stimulation than of similar responses produced by norepinephrine administration. Experiments in which a hindlimb of the cat with intact innervation was perfused independently with oxygenated blood from a reservoir indicate that the selective blockade by phenoxybensamine of centrally evoked responses is not due to a central inhibitory effect of the drug. The greater reduction of neurogenic response cannot be attributed to an activation of beta adrenergic receptors by bulbar stimulation because pretreatment of cats with propranonol did not modify the degree of differential blockade by phenoxybenzamine. Other possible mechanisms for the differential effect of phenoxybenzamine are discussed.