Increased pressor function of central vasopressinergic system in hypertensive renin transgenic rats

Renin transgenic hypertensive rats [TGR(mRen2)27] have increased contents of angiotensin II and arginine vasopressin (AVP) in the cardiovascular brain regions. The aim of the present study was to evaluate the effects of centrally released AVP on the regulation of baseline blood pressure in TGR(mRen2)27 rats and to determine the interaction between AVP and angiotensin II in the central control of blood pressure in this model of hypertension.

Three basic series of experiments were performed on 20 TGR(mRen2)27 and 20 Hannover Sprague-Dawley conscious rats, chronically instrumented with lateral cerebral ventricle (LCV) cannulae and femoral artery catheters. In series 1, blood pressure and heart rate were recorded during an LCV infusion of artificial cerebrospinal fluid before and after LCV administration of angiotensin II. In series 2, the effects of an LCV administration of angiotensin 11 (100 ng) on mean arterial pressure and the heart rate were determined during LCV infusion of a selective AVP receptor (V1) antagonist [1-(1-mercapto-4-methylcyclohexaneacetic acid)-8-arginine vasopressin (MeCAAVP) and d(CH2)5[Tyr(Me)2,Ala-NH2(9)]AVP] or a selective angiotensin II type 1 (AT1) receptor antagonist (losartan) or both. In series 3, mean arterial pressure and the heart rate were determined after an LCV injection of either AVP (10 ng) or AVP together with angiotensin II.

The LCV infusions of antagonists to V1 and AT1 receptors caused significant comparable decreases in baseline MAP in TGR(mRen2)27 but not in Sprague-Dawley rats. Angiotensin II elicited significant pressor responses, both in TGR(mRen2)27 and in Sprague-Dawley rats. Blockade of V1 receptors significantly reduced the duration and the maximum amplitude of the central pressor response to angiotensin II in TGR(mRen2)27 rats, whereas in Sprague-Dawley rats the maximum pressor effect was not significantly altered. In both strains, the pressor response to angiotensin II was abolished by blockade of AT1 receptors.

The results indicate that the elevated blood pressure in TGR(mRen2)27 rats is partly caused by increased function of the brain angiotensinergic AT1 and vasopressinergic V1 systems. Centrally released AVP is involved in mediation of the pressor effect exerted by centrally applied angiotensin II in TGR(mRen2)27 rats.