Alpha 1-and alpha 2-adrenoceptor mediated vasoconstriction in the forearm of normotensive and hypertensive subjects.

alpha 1- And alpha 2-adrenoceptor mediated vasoconstriction were studied in 13 patients with essential hypertension and 13 age-matched normotensive controls. This was done by comparing the changes in forearm blood flow induced by intra-arterial infusion of the selective alpha 1- and alpha 2-adrenoceptor agonists methoxamine and B-HT 933, the catecholamines adrenaline and noradrenaline, and the selective alpha 2-adrenoceptor antagonist yohimbine in both study groups. The catecholamines were infused in the presence of propranolol in order to prevent beta-adrenergic effects. Forearm blood flow was measured by plethysmography. All agonists produced a dose-dependent vasoconstriction which was more pronounced in the hypertensive patients, although the difference was significant only for the infusion of the catecholamines and for the combined effects of methoxamine and B-HT 933. No preference was found for either the alpha 1- or alpha 2-adrenoceptor mediated vasoconstriction in the greater response of the hypertensive patients. These results could well be explained by structural vascular changes, secondary to the elevated blood pressure. No evidence was found for increased alpha 2-adrenoceptor mediated basal vascular tone in patients with essential hypertension, since yohimbine tended to induce a greater vasodilatation in the normotensive subjects.