Impaired renal and extrarenal potassium adaptation in old rats

Abstract

Young (3 to 4 months) and old (21 to 22 months) rats were fed either a regular or high potassium (K) diet. After acute potassium chloride infusion, the fraction of infused K excreted (K efficiency) was similar in rats on a normal diet (57 ± 3%, young, vs. 61 ± 2%, old). With high K feeding there was a significant increase in the young, 69 ± 4%, but not in the old rats, 62 ± 2%. Na-K ATPase activity was markedly reduced in the renal medulla of old rats on a regular or high K diet. In addition, the response to acute K loading was compared in acutely nephrectomized rats. In the young rats on a regular diet plasma K increased from 3.72 ± 0.09 to 5.28 ± 0.16 mEq/liter while with K ingestion the increase was significantly less, 3.62 ± 0.07 to 4.75 ± 0.12 mEq/liter. In the old rats plasma K increased similarly on a regular or high K diet, 3.68 ± 0.10 to 5.68 ± 0.33 mEq/liter and 3.76 ± 0.06 to 5.97 ± 0.30 mEq/liter, respectively. Thus, old rats have impaired renal and extrarenal adaptation, but they have a normal response to an acute K challenge. A reduction in Na-K ATPase may account for the defect in renal adaptation in the aged rats.