Pre-and postjunctional alpha-adrenoceptors in rabbit articular blood vessels

ABSTRACT

Previous in vitro work on rabbit knee joint vessels showed that vasoconstrictor effects of nerve stimulation and administration of a-adrenoceptor agonists were mediated predominantly by alpha1-adrenoceptors. The present experiments were performed to assess the nature of alpha -adrenoceptor subtypes within these blood vessels in vivo. Dose/response relationships for adrenaline and noradrenaline produced a similar pattern of increasing constriction of articular vessels with increasing doses of drug. The alpha 1 agonist phenylephrine also produced dose-dependent constrictor responses which were diminished by prazosin. Using the alpha 2 agonists clonidine and UK -14304, responses in vivo differed from those previously observed in vitro. There was virtually no response to clonidine in vitro while responses were obvious in vivo. Although UK-14304 was found to have small effects in vitro, but only at high doses, this agent exerted more potent effects in vivo, significantly greater than those obtained with phenylephrine. Responses to the alpha 2 agonists were not altered significantly by prazosin but were reduced by rauwolscine. Following injection of UK-14304, the constrictor response to nerve stimulation was reduced. The results suggest that both alpha 1 and alpha 2 adrenoceptors are present postjunctionally within articular blood vessels, and also that prejunctional alpha 2 receptors are present which presumably regulate neurotransmitter release from sympathetic nerve endings in the joint capsule