Diminished metabolic responsiveness to beta-adrenergic stimulation in estradiol benzoate-treated rats

Abstract

Earlier studies have shown that chronic treatment with estradiol benzoate reduced the responsiveness of heart rate, drinking, and tail-skin temperature to administration of the beta-adrenergic agonist, isoproterenol. The objective of the present study was to assess the relationship between the isoproterenol-induced elevations in rate of oxygen consumption and tail-skin temperature in adult ovariectomized rats treated chronically with estradiol benzoate (28 micrograms/day) and untreated, ovariectomized controls. Isoproterenol (0, 10, 25, and 50 micrograms/kg, s.c.) elevated both rate of oxygen consumption and tail-skin temperature in ovariectomized and ovariectomized plus estradiol benzoate-treated rats in a dose-related fashion. However, in both cases the response of the estradiol-treated group was less than that of controls. A comparison of the regressions of rate of oxygen consumption versus dose of isoproterenol administered to each group revealed significant differences between the intercepts, but not the slopes. A similar comparison with respect to the regression of tail-skin temperature versus dose of isoproterenol administered to each group revealed significant differences between both the slopes and intercepts of the relationships. A comparison of the regressions of rate of oxygen consumption versus tail-skin temperature for control and estrogen-treated rats revealed significant differences between both slopes and intercepts of the relationships. The latter comparison suggests a reduced change in tail-skin temperature for a given change in rate of oxygen consumption in the estrogen-treated group. It is concluded from these results that the reduction in metabolic response to beta-adrenergic stimulation in estrogen-treated rats explains, in part at least, the attenuated response of tail-skin temperature. The tail of the rat is important in short-term temperature regulation. When threatened acutely with overheating, the rat uses its tail as a heat radiator, increasing its skin blood flow, temperature, and heat loss. Administration of isoproterenol to rats at doses of 50-100 micrograms/kg, subcutaneously can increase tail-skin temperature as much as 6 degrees C. This response is mediated by beta-adrenoceptors since it can be either attenuated or prevented by prior administration of propranolol. It serves as a simple and reliable test of beta-adrenergic responsiveness in rats. Chronic treatment with the oral contraceptive, Enovid (7.5 mg/kg food) attenuated this response. Chronic administration of the estrogenic component (ethinyl estradiol, 48-50 micrograms/kg/day) of Enovid also attenuated the response.(ABSTRACT TRUNCATED AT 400 WORDS)