β-Adrenergic responsiveness of rats treated chronically with isoproterenol

Abstract

The effect of chronic administration of isoproterenol on isoproterenol-induced thirst and isoproterenol-induced changes in heart rate and selected organ weights of male rats was studied. Administration of 25 μg isoproterenol/kg, s.c., in saline daily for 10 days was accompanied by a significant attenuation of the characteristic increase in water intake following a challenging dose of isoproterenol (25 μg/kg, s.c.) on the 11th day. Administration of 25 μg isoproterenol/kg, s.c., every 2nd, 3rd, or 4th day for 10 days was without significant effect on water intake following isoproterenol (25 μg/kg, s.c.) on the 11th day. Administration of 25 μg isoproterenol/kg, s.c., every day for 10 days led to a slight increase in cardiac responsiveness to a challenging dose of isoproterenol (25 μg/kg) on the 11th day. Chronic treatment with this low dose of isoproterenol for 10 days was also accompanied by a significant increase in the ratio of heart weight to body weight but no significant changes in the ratio of kidney, adrenal, thyroid, spleen, or interscapular brown fat to body weight. Thus, daily administration of the β-adrenergic agonist isoproterenol for 10 days can alter β-adrenergic responsiveness in the rat with β1 (heart rate) and β2 (thirst) mediated responses showing opposite effects. In addition, the results suggest that tests of β-adrenergic responsiveness must be assessed in terms of the frequency of administration of the agonist.