Possible mechanism for increased beta-adrenergic dipsogenic response in food-deprived rats

The dipsogenic responsiveness to isoproterenol was studied in food-deprived male rats. Unstimulated water intake was similar between control and fasted groups, and parallel dose-response curves for the dipsogenic response induced by isoproterenol (10-50 micrograms/kg) were observed. A twofold elevation in dipsogenic responsiveness was observed in the fasted rats, and this enhanced response was correlated with a dose-dependent increase in plasma renin activity when compared with the control rats after administration of isoproterenol. beta-Adrenergic receptor binding assays were performed on both heart and renal cortical tissues. In the heart the receptor concentration was decreased after food deprivation, whereas the affinity of the receptor for the beta-adrenergic antagonist [125I]CYP remained unchanged. On the other hand, an increased beta-adrenergic receptor concentration without change in affinity was found in renal cortices of fasted rats. Results from these receptor binding studies correlated with the attenuated chronotrophic response and the increased dipsogenic response to beta-adrenergic stimulation in the fasted rat. Therefore stimulation of these increased renal receptors by isoproterenol could result in an enhanced activation of the renin-angiotensin system and thus be a factor responsible for the increased dipsogenic response induced by isoproterenol observed in the fasted rats.