Nature of alpha 1 and postjunctional alpha 2 adrenoceptors in the pulmonary vascular bed

The subtypes of postjunctional alpha adrenoceptors in the feline pulmonary vascular bed were studied by using selective alpha-adrenoceptor agonists and antagonists. Under conditions of controlled pulmonary blood flow and constant left atrial pressure, intralobar injections of the alpha 1 agonists phenylephrine and methoxamine, and the alpha 2 agonists UK 14,304 and B-HT 933, increased lobar arterial pressure in a dose-related manner. Prazosin, an alpha 1-adrenoceptor antagonist, reduced responses to phenylephrine and methoxamine to a greater extent than responses to UK 14,304 and B-HT 933. Yohimbine, an alpha 2 blocker, decreased responses to UK 14,304 and B-HT 933 without altering responses to phenylephrine or methoxamine. The same pattern of blockade was observed in animals pretreated with 6-hydroxydopamine, an adrenergic neuronal blocking agent. However, in propranolol-treated animals, prazosin antagonized responses to phenylephrine and methoxamine without altering responses to UK 14,304 or B-HT 933, and the selectivity of the blocking effects of yohimbine were preserved. Responses to intralobar injections of norepinephrine (NE) were markedly decreased by prazosin, whereas yohimbine had only a small effect. These data suggest the presence of both postjunctional alpha 1 and alpha 2 adrenoceptors mediating vasoconstriction in the pulmonary vascular bed. These results also indicate that the vasoconstrictor responses to injected NE in the cat pulmonary vascular bed result mainly from activation of alpha 1 adrenoceptors.