Evidence for Postjunctional Vascular α2-Adrenoceptors in Peripheral Vascular Regulation in Man

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Abstract
Preliminary studies in healthy normotensive subjects with a selective .alpha.2-adrenoceptor antagonist 2-[2-(1,4-benzodioxanyl)]-2-imidazoline (RX781094) in a dose of 0.2 .mu.g/kg i.v. are described. There was a transient increase in blood pressure and plasma noradrenaline [norepinephrine], 10-20 min after dosing and an associated reduction in heart rate. These observations are consistent with blockade of peripheral presynaptic .alpha.2-adrenoceptors in man. The pressor responses to .alpha.-methylnoradrenaline, which has relatively selective agonist effects on .alpha.2-adrenoceptors, were markedly attenuated between 0.5 and 3.0 h after dosing (dose ratio > 20). The pressor responsiveness to i.v. phenylephrine, a selective .alpha.1-adrenoceptor agonist, was only slightly reduced (dose ratio 1.7 .+-. 0.8), as was the blood pressure rise after i.v. noradrenaline (dose ratio 1.7 .+-. 0.7). The differential effects on the agonist pressor responses are consistent with antagonism by RX781094 of post-junctional .alpha.2-adrenoceptors on vascular smooth muscle.