Chronic ethanol feeding potentiates alpha1-adrenoceptor responsiveness in SHR aortas

Previous studies including ours demonstrated a hypotensive response to ethanol in spontaneously hypertensive rats (SHRs). In this study, we investigated whether this hypotensive effect of ethanol involves alterations in vascular alpha1-adrenergic receptor responsiveness. The contractile responses to the alpha1-receptor agonist phenylephrine were evaluated in aortic rings obtained from pair-fed SHRs receiving liquid diet with or without ethanol (2.5% or 5%, w/v) for 3 months. The responses were measured in aortas with and without endothelium to determine the role of the endothelium in the observed responses. The liquid diet intake was similar in the control and ethanol groups throughout the study whereas the body weight was significantly reduced by ethanol. Cumulative addition of phenylephrine (1 x 10(-9)-1 x 10(-4) M) caused concentration-related contractile responses. These responses were significantly reduced after endothelium denudation suggesting a role for the endothelium in the modulation of alpha1-receptor responsiveness. Ethanol (2.5% and 5%) caused significant and concentration-related increases in the contractile responses elicited by phenylephrine but not KCl. The maximum contraction (Emax) caused by phenylephrine in rings obtained from SHRs treated with 2.5% and 5% ethanol amounted to 413.6 +/- 26.3 and 513.0 +/- 46.7 mg tension/mg tissue, respectively, compared with 383.6 +/- 35.2 mg tension/mg tissue in control rings. The enhancement of alpha1 contractions by ethanol was virtually abolished in rings pretreated with the alpha1-receptor antagonist prazosin, suggesting upregulation of alpha1-receptors in aortas of ethanol-fed rats. Endothelium denudation also abolished ethanol-evoked increases in phenylephrine contractions. These findings suggest that chronic ethanol feeding upregulates aortic alpha1-receptors, which may be a consequence of chronic alpha1-receptor blockade by ethanol. The latter may account, at least in part, for the hypotensive response elicited by ethanol in SHRs.