Renal potassium handling in aging rats.

Aging is often related to electrolyte disorders. This study was designed to evaluate the renal capacity for K+ handling of aging rats submitted to both extreme situations of K+ depletion and K+ loading. Aging rats were submitted to metabolic assessment, measurement of plasma aldosterone and determination of Na+,K+-ATPase and H+,K+-ATPase activity in the inner medullary collecting duct (IMCD). During K+ depletion, aging rats showed low values of urine K+ excretion. After 2 weeks of dietary K+ restriction, the plasma K+ levels of these animals reached 2.9+/-0.1 mEq/l without acid-base disorders. During K+ loading, aging rats showed a significant increase in urinary K+ excretion, with plasma K+ remaining at normal levels (4.7+/-0.1 mEq/l). Plasma aldosterone levels were low in control aging rats but K+ intake modulated the levels of this hormone, with K+ depletion reducing it and K+ loading significantly increasing it. Na+,K+-ATPase activity increased only in the initial portion of the IMCD of aging rats after 2 weeks of K+ depletion. In the distal portion of the IMCD, Na+,K+-ATPase activity did not change. H+,K+-ATPase remained unchanged in both the proximal and distal portions of the IMCD of aging rats. During K+ loading, there was no change in Na+,K+-ATPase or H+,K+-ATPase activity in the proximal or distal portions of the IMCD of aging rats. This study suggests that the senescent kidney responded to K+ conservation and excretion under the two extremes of low and high K+ intake.