EVIDENCE FOR A SYMPATHETIC ALPHA RECEPTOR BLOCKING ACTION OF BETA RECEPTOR BLOCKING AGENTS

¹ Pharmacological Research Unit, Council of Scientific and Industrial Research and the Department of Pharmacology, Medical College, Baroda, India

Pronethalol, dichloroisoproterenol and d-, dl- and l-N-isopropyl-p-nitrophenylethanolamine blocked the contractile responses of the rabbit aortic strip to epinephrine and/or norepinephrine. Propranolol blocked the contractile responses of the rabbit aortic strip to methoxamine. Dichloroisoproterenol and pronethalol did not alter the responses to 5-hydroxytryptamine, angiotensin or acetylcholine but blocked the responses to histamine. Propranolol and N-isopropyl-p-nitrophenylethanolamine blocked the responses to 5-hydroxytryptamine and histamine but did not affect those to angiotensin and potentiated those to acetylcholine. Antagonism of the excitatory effects of epinephnine and norepinephnine by pronethalol, dichlonoisoproterenol and N-isopropyl-p-nitrophenylethanolamine and that of methoxamine by propranolol fulfilled the conditions of competitive antagonism. Antagonism by propranolol of the excitatory effects of histamine did not fulfill the conditions of competitive antagonism.