Vol. 288, Issue 2, 455-462, February 1999

Ethanol Counteraction of I₁-Imidazoline but Not Alpha-2 Adrenergic Receptor-Mediated Reduction in Vascular Resistance in Conscious Spontaneously Hypertensive Rats¹

Mahmoud M. El-Mas and Abdel A. Abdel-Rahman

Department of Pharmacology, School of Medicine, East Carolina University, Greenville, North Carolina

Our recent findings have shown that ethanol selectively counteracts decreases in blood pressure (BP) evoked via activation of central I₁-imidazoline receptors but not alpha-2 adrenoceptors in conscious spontaneously hypertensive rats (SHRs). This study investigated the role of sympathetic activity, cardiac output and total peripheral resistance (TPR) in the differential effect of ethanol on centrally mediated hypotension. Changes in plasma norepinephrine (NE), as index of sympathetic activity, BP, heart rate, cardiac index, stroke volume, and TPR elicited by rilmenidine or -methylnorepinephrine (selective I₁ and alpha-2 receptor agonists, respectively) and subsequent ethanol (0.5 or 1 g/kg) or saline, were evaluated in conscious SHRs. Intracisternal rilmenidine (25 µg) or -methylnorepinephrine (-MNE; 4 µg) elicited similar decreases in BP, TPR, and plasma NE, but cardiac index was not changed. Ethanol (0.5 g/kg i.v.) had no effect on hemodynamic responses to rilmenidine or -MNE. The higher dose (1 g/kg i.v.) of ethanol counteracted the hypotensive response to rilmenidine and significantly (P < .05) elevated TPR and plasma NE. In contrast, ethanol (1 g/kg) had no effect on the hypotensive responses to -MNE but significantly (P < .05) elevated plasma NE. However, this increase in NE was approximately one third of the increase evoked by ethanol when given after rilmenidine. These findings suggest that the selective counteraction by ethanol of the hypotension evoked via activation of central I₁ but not alpha-2 receptors may relate, at least in part, to its greater ability to reverse the sympathoinhibition and the associated decrease in vascular resistance mediated by I₁ receptors.