Alpha adrenoceptors and autonomic mechanisms in perinephritis hypertension in the rabbit.

Increased pressor responses to norepinephrine and other pressor agents have been reported to occur in human essential hypertension and in several animal models of experimental hypertension. These increased responses might be related to the development of hypertension or could be a secondary consequence of the elevation in blood pressure. We have examined pressor responses to alpha-adrenoceptor agonists and to angiotensin II in male New Zealand White rabbits with perinephritic hypertension. Increased pressor responses were observed for the alpha 1 adrenoceptor agonist phenylephrine and the mixed alpha 1/alpha 2-adrenoceptor agonist norepinephrine but not for the alpha 2 adrenoceptor selective agonist guanabenz or angiotensin II. The increase occurred within 7 days of surgery and in some animals was observed when mean arterial pressure was not significantly elevated. It could not readily be attributed to intimal thickening or hypertrophy of the arterial wall, altered basal levels of norepinephrine or epinephrine, changes in norepinephrine clearance, beta-adrenoceptor interactions, or decreased baroreceptor sensitivity. However, the possibility that vascular hypertrophy and decreased baroreflex sensitivity may contribute to the increase at later times cannot be excluded. In all tissues examined, specific prazosin binding was decreased in the older animals and specific clonidine binding was decreased in forebrain. However, these changes were observed in both hypertensive and sham-operated animals and were probably age-related. We believe the increased response to alpha 1-adrenoceptor agonists may be related to changes at a postreceptor site in the coupling of receptor activation to smooth muscle contraction.