AJP - Heart and Circulatory Physiology, Vol 249, Issue 4 891-H898, Copyright © 1985 by American Physiological Society
Evidence for existence of postjunctional alpha 1- and alpha 2-adrenoceptors in cat pulmonary vascular bed
A. L. Hyman and P. J. Kadowitz
The subtypes of postjunctional alpha-adrenoceptors in the feline pulmonary
vascular bed were studied 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-adrenoceptor agonists, phenylephrine and methoxamine, and the alpha
2-adrenoceptor agonists, UK 14,304 and BHT 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 BHT 933. Yohimbine, an alpha
2-adrenoceptor blocker, decreased responses to UK 14,304 and BHT 933
without altering responses to phenylephrine or methoxamine. The same
pattern of blockade was observed in animals pretreated with
6-hydroxydopamine, an agent that destroys the integrity of adrenergic nerve
terminals. However, in propranolol-treated animals, prazosin antagonized
responses to phenylephrine and methoxamine without altering responses to UK
14,304 or BHT 933, and the selectivity of the blocking effects of yohimbine
were preserved. Responses to intralobar injections of norepinephrine 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
norepinephrine in the cat pulmonary vascular bed are due mainly to
activation of alpha 1-adrenoceptors.
