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

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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.