PRE- AND POSTJUNCTIONAL α-ADRENOCEPTORS IN RABBIT ARTICULAR BLOOD VESSELS

Previous in vitro work on rabbit knee joint vessels showed that vasoconstrictor effects of nerve stimulation and administration of α-adrenoceptor agonists were mediated predominantly by α1-adrenoceptors5,9 The present experiments were performed to assess the nature of α-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 u} agonist phenylephrine also produced dosedependent constrictor responses which were diminished by prazosin. Using the α2 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 α2 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 α1 and α2 adrenoceptors are present postjunctionally within articular blood vessels, and also that prejunctional α2 receptors are present which presumably regulate neurotransmitter release from sympathetic nerve endings in the joint capsule.