EFFECT OF MINERALOCORTICOID ACTIVITY ON TRANSTUBULAR POTASSIUM GRADIENT, URINARY [K] [NA] RATIO, AND FRACTIONAL EXCRETION OF POTASSIUM/

Clinical assessment of potassium derangements may require evaluation o f mineralocorticoid status. Several indirect indices of mineralocortic oid activity based on renal electrolyte excretion have been proposed a nd include the transtubular potassium gradient, urinary [K]/[Na] ratio , and renal fractional excretion of potassium. We studied the impact o f high mineralocorticoid activity versus blocked mineralocorticoid act ivity on these indices in otherwise normal subjects who ingested a def ined diet. Eight normal subjects received either fludrocortisone or sp ironalactone for 4 days. After a washout period of greater than or equ al to 2 weeks, each subject then received the opposite regimen. Subjec ts ingested an identical high-potassium diet during both experimental periods. The renal fractional excretion of potassium and transtubular potassium gradient were calculated using standard formulas, Fludrocort isone caused an increase in body weight and no significant reduction i n serum potassium concentration, while spironolactone decreased body w eight and increased plasma potassium concentration. After 1 or 2 days of treatment with fludrocortisone, the average values for all urinary indices of mineralocorticoid activity were significantly higher than a fter 1 or 2 days of treatment with spironolactone. However, the differ ences between these indices in the fludrocortisone and spironolactone test periods diminished by day 3 and were nonexistent by day 4, In con clusion, the transtubular potassium gradient, [K]/[Na] ratio, and rena l fractional excretion of potassium reflect acute changes in mineraloc orticoid activity, However, these indices do not discriminate between states of high and low mineralocorticoid activity lasting longer than 2 to 3 days. (C) 1998 by the National Kidney Foundation, Inc.