Influence of Dietary Sodium on Renal Function in Patients with Chronic Renal Disease

Abstract

This study was undertaken to determine to what degree patients with chronic renal disease are able to conserve salt and maintain renal function in the presence of salt-restriction. Ten patients with chronic glomeruloneph- ritis, chronic pyelonephritis, arteriolar nephrosclerosis, and poly-cystic kidney disease were studied with standard renal clearances after a week each of regular, low and high salt-intakes. Plasma and extracellular fluid volumes and renal handling of Na and K were also measured. With salt-restriction, glomerular filtration rate (inulin clearance) decreased in every patient an average of 32%; renal plasma flow (PAH clearance) was reduced an average of 25%; plasma volume (Evan''s blue dye) diminished an average of 15%; and extracellular fluid volume (S-35) decreased an average of 11%. With salt-replacement, these changes returned towards control levels. Serum sodium levels fell an average of 3.8 mEq per liter on a low-salt diet. Only 5 patients could respond normally to salt-restriction by reabsorb-ing 99% or more of filtered sodium. In 6 of 9 subjects, potassium excretion increased with salt-restriction; the 3 whose potassium excretion did not increase were the patients with the best residual renal function. These results suggest that patients with chronic renal disease have a proximal tubular defect in the ability to reabsorb sodium; the distal tubule responds normally. Because of negative sodium balance, plasma and extracellular fluid volumes diminish, which then result in decreased glomerular filtration rates and renal plasma flows.