NORMAL ACID-BASE-EQUILIBRIUM IN ACUTE HYPONATREMIA AND MIXED ALKALOSIS IN CHRONIC HYPONATREMIA INDUCED BY ARGININE-VASOPRESSIN OR 1-DEAMINO-8-D-ARGININE VASOPRESSIN IN RATS

The effects of acute and chronic water intoxication induced by the adm inistration of oral water and arginine vasopressin (AVP) or 1-deamino- 8-D-arginine-vasopressin (DDAVP) on blood acid-base equilibrium and al dosterone, corticosterone, and thyroxine secretion were studied in rat s. Acute hyponatremia (3 hours) was associated with normal bicarbonate and blood acid-base equilibrium and a decrease in aldosterone and thy roxine concentrations, while corticosterone was increased. When simila r levels of hyponatremia (serum sodium 110 mEq/L) were maintained for 24 or 72 hours, a normal serum bicarbonate concentration was observed, but blood acid-base equilibrium showed a mixed respiratory and metabo lic alkalosis. Blood pH was negatively correlated with serum sodium co ncentration (R = -0.65; p < 0.001), as was the metabolic alkalosis (ba se excess; R = -0.64; p < 0.001) and the aldosterone concentration (R = -0.52; p < 0.01), while the PCO2 was positively correlated (R = +0.4 9; p < 0.01). Hyperaldosteronism was similar whether hyponatremia was induced with AVP or DDAVP and was observed even for mild hyponatremia. When hyponatremia was induced by a high water and salt intake (2.5% D -glucose, 0.45% NaCl; 15% body weight), aldosterone concentration was as high (about three times control values) as in rats with similar lev els of hyponatremia but with a salt-free diet. The high salt intake wa s associated with a more severe metabolic alkalosis (base excess +5,5 mEq/L). In chronic hyponatremia, corticosterone and thyroxine values w ere normal. In hyponatremia related to syndrome of inappropriate secre tion of antidiuretic hormone, the normal serum bicarbonate level is an expected observation; as in acute water intoxication, it stays normal . In chronic hyponatremia bicarbonate is also normal, but blood acid-b ase equilibrium shows a mixed respiratory and metabolic alkalosis.