Effects of beta-agonist and antagonist on the pulmonary circulation and the pulmonary pressor response to 5-HT.

We investigated the effects of beta-agonist and antagonist (isoproterenol, propranolol) on the pulmonary circulation and on the pulmonary pressor response to 5-HT in an isolated canine lung lobe. The pulmonary vessels were dilated by isoproterenol at doses up to 200 micrograms, but were constricted by alpha-adrenoceptor stimulation at relatively high doses. The mechanism by which isoproterenol inhibited the 5-HT response is probably related to the stimulation of beta-adrenoceptors at the lower doses and to the stimulation of alpha-adrenoceptors at the higher doses. Propranolol alone had no effect on pulmonary vascular tone, but inhibited the 5-HT response markedly at doses high, possibly by directly blocking the 5-HT receptors.