Thyroxine, renal beta-adrenergic receptors, and dipsogenesis in food-deprived rats

The effect of thyroxine (T4) replacement on the increased renal beta-adrenergic receptor number and the increased beta-adrenergic dipsogenic responsiveness of fasted rats was studied in male Sprague-Dawley rats. Food deprivation significantly decreased serum thyroxine (T4) and triiodothyronine (T3) levels, increased the dipsogenic response to isoproterenol, and elevated renal beta-adrenergic receptor concentration. Daily administration of T4 (40 micrograms/kg) to food-deprived rats restored serum thyroid levels to normal. Thyroxine replacement also reduced the increased beta-adrenergic dipsogenic responsiveness in the food-deprived rats to control levels. In addition, daily administration of thyroxine reduced the beta-adrenergic receptor concentration in renal cortices to that observed in controls. Thyroid treatment tended to decrease the isoproterenol-induced renin release in food-deprived rats and increase the response in the control rats. These results suggest that the relative hypothyroid state observed in the food-deprived rat may be responsible for the increased concentration of renal beta-receptors and the associated activation of the renin-angiotensin system, which may be partially responsible for the observed increased dipsogenic response induced by isoproterenol. Collectively, the data reaffirm the interaction of thyroid hormone and beta-adrenergic responsiveness, although it is of interest that, in regard to renal beta-receptors, the concentrations are decreased to normal by thyroid treatment, whereas previous studies in hypothyroid rats demonstrate an increase to normal of cardiac beta-receptors. This would suggest thyroid hormone may normalize a response in an opposite direction depending on the direction of the disturbance.