Mild potassium depletion provokes renal sodium retention

Although severe potassium depletion has been shown to cause sodium retention, the effect of small potassium losses on sodium excretion is unknown. Seven healthy male volunteers were given a diet containing 35 mEq sodium and either low (10 mEq) or normal (90 mEq) potassium intake for 9 days. With the low potassium diet plasma potassium concentration decreased by 0.60 +/- 0.15 mEq/L. The cumulative potassium balance with this diet was negative (-124 +/- 22 mEq). Cumulative sodium balance was -82 +/- 43 mEq with the normal potassium diet but was positive (20 +/- 22 mEq) when the same subjects ingested a low potassium diet (P less than 0.05). A significant difference in sodium excretion could be detected as early as day 1 of potassium restriction. To study the effect of potassium depletion on the renal ability to handle a short-term sodium load, on day 10 each subject received a 2 L isotonic saline infusion over a period of 4 hours. Subjects while ingesting a normal potassium diet excreted 20% +/- 4% of the administered load. The same subjects while ingesting a low potassium diet excreted only 9% +/- 3% of the administered load (P less than 0.001). The difference in sodium excretion was not dependent on changes in glomerular filtration rate, renal plasma flow, plasma renin activity, plasma aldosterone, or plasma and urinary catecholamines. We conclude that variations in dietary potassium strikingly modify sodium excretion. Even mild potassium depletion impairs the renal ability to handle a short-term sodium load.