Ovariectomy abolishes ethanol-induced impairment of baroreflex control of heart rate in conscious rats

Abstract

Our previous studies have shown that ethanol attenuates baroreflex control of heart rate in male rats. The present study investigated whether this effect of ethanol is gender-related, and whether it involves hormonal factors. The effect of intragastric administration of ethanol or equal volume of water on baroreflex-mediated decreases in heart rate in response to increments in blood pressure evoked by phenylephrine were evaluated in conscious age-matched male and female Sprague–Dawley rats as well as in ovariectomized rats. Baroreflex curves relating changes in blood pressure and associated heart rate responses were constructed, and the slopes of the regression lines were taken as a measure of baroreflex sensitivity. Phenylephrine (1–16 μg kg −1, i.v.) elicited dose-dependent pressor responses that were similar in all groups of rats. However, the associated reflex bradycardic responses depended on the rat preparation and the dose of ethanol employed. In water-treated (control) animals, significantly ( P<0.05) lesser reflex bradycardic responses were observed in female compared with male rats (baroreflex sensitivity, −1.21±0.12 vs. −1.67±0.12 beats min −1 mmHg −1). Ovariectomy resulted in a further reduction in baroreflex sensitivity (−0.82±0.06 beats min −1 mmHg −1), suggesting a favorable role for ovarian hormones in baroreflex modulation. In male rats, ethanol (0.25, 0.5, or 1 g kg −1, intragastric) elicited dose-related decreases in reflex bradycardic responses. The reduction in the regression coefficient obtained by the two higher doses (0.5 and 1 g kg −1) of ethanol was statistically significant compared with control values. The ability of ethanol to reduce baroreflex sensitivity appears to be gender-independent as it was similarly demonstrated in intact female rats. In contrast, ethanol had no effect on reflex bradycardic responses in ovariectomized rats at any of the doses tested. The data suggest that ethanol reduces baroreflex control of heart rate irrespective of the rat gender. Further, the lack of an effect of ethanol on baroreflex sensitivity in ovariectomized rats may suggest a role for ovarian hormones in ethanol-evoked baroreflex attenuation.