Role of water balance in the enhanced potassium excretion and hypokalaemia of rats with diabetes insipidus.

Abstract

The role of H2O balance in the hypokalemia of rats with diabetes insipidus (DI rats) was studied. After a 3 day balance study, DI rats had a lower muscle K content, and plasma [K+], and the urinary K excretion in response to oral KCl loading was reduced when compared to normal rats. The hypokalemia was associated with elevated K concentrations in renal medulla and papilla when compared to values in normal Long-Evans rats. During a 9 day balance study, urinary K excretion was higher than that of normal rats on days 1-3, but not different on days 4-9; this transient elevation was observed in DI rats on normal, high and low K diets. On a low K diet, the urinary K excretion of DI rats fell to mininmal levels, making unlikely the existence of a renal defect in K handling. Muscle K content and plasma [K+] were normal after 9 days in metabolism cages. This spontaneous reversal of the hypokalemia of DI rats was associated with increased H2O content of renal medulla and papilla, and decreased K concentration in these zones. The effect of acute mild dehydration on K handling of DI rats was evaluated. Water deprivation for 1-8 h was sufficient to raise the urinary K excretion of DI rats above that of DI rats drinking ad lib. Renal tissue [K+] was significantly increased after 8 h of dehydration. Water deprivation also enhanced the response of DI rats to an oral KCl load. Two days of chronic dehydration in the form of H2O rationing also significantly enhanced the urinary K excretion of DI rats. Chronic mild dehydration may be responsible for the modest K deficiency observed in DI rats via alterations in renal tissue [K+] and consequently in urinary K excretion. Correction of dehydration during prolonged periods in metabolism cages may account for the spontaneous reversal of the hypokalemic condition.