Demonstration of an intrinsic renal adaptation for K+ conservation in short-term K+ depletion

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Abstract
Renal potassium conservation occurs within 3 days of potassium deprivation by a mechanism that appears to be independent of mineralocorticoids and sodium and anion excretion. To examine whether the mechanism involves an intrinsic renal adaptation, urinary potassium excretion was measured in isolated perfused kidneys from rats maintained on a normal or K+-free diet for 3 days. Perfusions were carried out with a K+ concentration that averaged 3.5 mM and with glucose (5 mM) as the only substrate. Both absolute (UKV) and fractional K+ excretion (FEK) were substantially less in kidneys from animals on K+-free diets compared with controls. These differences in K+ excretion were not explained by changes in GFR, urine flow rate, urine pH, or sodium, chloride, or ammonium excretion. The K+ content of renal tissue was not different in the perfused and nonperfused kidneys from rats receiving K+-free diets compared with controls. Suppression of K+ excretion by amiloride (10(-4) M) suggested that, as in vivo, tubular secretion of K+ in the perfused normal kidney accounts for 90% of the urinary K+ excretion and that tubular K+ secretion is reduced in isolated kidneys from animals on a K+-free diet. Further studies of isolated kidneys from adrenalectomized (ADX) rats receiving aldosterone and dexamethasone replacement and fed a normal or K-free diet also revealed significantly lower UKV and FEK in kidneys from animals on K+-free diets. K+ content of renal tissue was not different in the ADX animals on a K+-free diet compared with ADX rats on normal K+ intake. These studies indicate that within 72 h of dietary K+ deprivation an intrinsic renal adaptive process to conserve potassium is activated that is independent of renal potassium content, aldosterone, and urinary factors that can alter K+ excretion such as flow, pH, ammonium, sodium, and anions. This regulatory mechanism, which has a substantial influence on potassium excretion, remains to be elucidated.