Response of Heart Rate to Acute Administration of Isoproterenol in Rats Treated Chronically With Norethynodrel, Ethinyl Estradiol, and Both Combined1

Abstract

Chronic (17-20 wk) administration of ethinyl estradiol alone (36 .mu.g/kg per day), and in combination (27 or 50 .mu.g/kg per day) with several doses of norethynodrel (135, 165 or 233 .mu.g/kg per day), attenuated the increase in heart rate accompanying acute s.c. administration of the .beta.-adrenergic agonist, l-isoproterenol (50 or 100 .mu.g/kg), to female rats. Dietary administration of an oral contraceptive containing mestranol and norethynodrel (7.5 mg/kg food) was also accompanied by an attenuated response to isoproterenol. A significant inverse linear relationship was observed between the logarithm of the dose of estrogen received by each group and either heart rate or change in heart rate measured at 10, 20 and 30 min after administration of isoproterenol. The antagonistic relationship between the dose of estrogen administered chronically and responsiveness of heart rate to a test dose of isoproterenol suggests a reduced .beta.-adrenergic responsiveness in estrogen-treated rats.