Interaction between angiotensin III and α₂-adrenoceptors of the medulla oblongata involved in cardiovascular regulation in the rat

We evaluated, in adult, male Sprague-Dawley rats anesthetized with pentobarbital sodium, possible interaction between angiotensin III (AIII) and the α₂-adrenoceptors in the medulla oblongata that are involved in cardiovascular regulation. The hypotensive and negative chronotropic and inotropic actions of the α₂-adrenoceptor agonist, guanabenz, were used as our experimental index. Intracerebroventricular (i.c.v.) administration of AIII (100 or 200 pmol) significantly attenuated the cardiovascular suppressive effects of the aminoguanidine compound (25 or 50 μg/kg, i.v.). Bilateral microinjection of AIII (20 or 40 pmol) to the nucleus reticularis gigantocellularis (NRGC), a medullary site believed to be intimately related to the antihypertensive action of guanabenz, produced similar results. In addition, i.c.v. administered AIII (200 pmol) altered the effects of guanabenz on the arterial pressure-related neurons in the NRGC, in a manner that paralleled the blunted vasodepressive action of the aminoguanidine compound by the heptapeptide. When applied microiontophoretically, AIII also significantly decreased the responsiveness of arterial pressure-related neurons in the NRGC to guanabenz. These findings suggest that AIII may interact with the α₂-adrenoceptors located in the NRGC that are involved in central cardiovascular regulation.