RESTORATION BY CORTICOSTEROIDS OF THE HYPERALDOSTERONISM IN HYPONATREMIC RATS WITH PANHYPOPITUITARISM

1. In the syndrome of inappropriate secretion of antidiuretic hormone, hyponatraemia is associated with a normal bicarbonate concentration d espite dilution. This normal bicarbonate concentration is related to t he development of a hyperaldosteronism, which is attributed to a direc t stimulation of the zona glomerulosa by the hyponatraemic state. Some workers have suggested that, to develop this hyperaldosteronism requi res the presence of a pituitary factor. To determine whether the pitui tary gland plays a role in this hyponatraemia-induced hyperaldosteroni sm, water intoxication was performed for 24 h in normal and in panhypo pituitaric rats. 2. In normal rats, hyponatraemia (108 mmol/l), induce d by the administration of 1-desamino-8-D-arginine vasopressin and 2.5 % D-glucose-0.45% NaCl by gavage (15% body weight) was associated with a mild increase in bicarbonate concentration, and blood acid-base equ ilibrium showed a mixed metabolic and respiratory alkalosis (pH 7.57, partial pressure of CO2 29 mmHg, base excess +5.5 mmol/l), and aldoste rone concentration was increased 3-fold as compared with the control v alue. When hyponatraemia (110 mmol/l) was induced in a similar manner in panhypopituitaric rats, we observed a very low aldosterone concentr ation (< 50 pg/ml) and a compensated respiratory alkalosis (pH 7.45, p artial pressure of CO2 30 mmHg, base excess -2.6 mmol/l). The restorat ion of a hyperaldosteronaemic state in this group of rats was related essentially to corticosteroid intake. 3. These data suggest that corti costeroids play a critical role in the development of hyponatraemia-re lated hyperaldosteronism, a phenomenon not necessarily dependent on a pituitary factor.