Studies on alpha 2-adrenoceptors in rabbits with perinephritis hypertension.

Increased responses to intravenous injections of the full alpha 2-adrenoceptor agonists, alpha-methyl noradrenaline and noradrenaline, were observed in conscious rabbits with perinephritis hypertension. However, responses to the partial agonist, BHT 920, and the antagonist, idazoxan, were unchanged. These increased responses to noradrenaline and alpha-methyl noradrenaline did not appear to be related to the alpha 1- or beta-adrenoceptor activity or to altered clearance of the agonists. The pro-aggregatory response of platelets from hypertensive rabbits and normotensive rabbits to adenosine diphosphate (ADP) was similar, but the sensitivity of platelets from hypertensive rabbits to the adrenaline potentiation of the pro-aggregatory response was increased. This was not related to changes in the number of alpha 2-adrenoceptor binding sites or to changes at platelet beta-adrenoceptors. In contrast the aggregatory responses to the partial agonist, guanabenz, were similar in normotensive and hypertensive animals. In rabbits with perinephritis hypertension responses to phenylethanolamines (full agonists at the alpha 2-adrenoceptor) were increased in vivo in the conscious animal and in vitro in platelets, whereas responses to imidazolines which are partial agonists were similar in normotensive and hypertensive animals. These differences may be related to alterations in the coupling of alpha2-adrenoceptor activation to response in the hypertensive rabbits.