Micropuncture study of renal tubular transfer of sodium chloride in the rat

¹ Department of Physiology, Cornell University Medical College, New York City

Micropuncture studies were made in the rat on the relationship between net water and net sodium movement across proximal tubular epithelium during strong osmotic diuresis induced by intravenous infusion of a 20% mannitol solution. Results obtained in the mannitol series were compared with data obtained in rats during infusion of 0.9% saline. Collected tubular fluid and appropriate plasma samples were analyzed for C¹⁴ inulin or creatinine, chloride and radiosodium. The latter was injected intravenously several hours prior to tubular puncture. Net water reabsorption across the proximal tubule was less than under nondiuretic conditions. The concentration of radiosodium and of chloride in proximal tubular fluid was found to be below that of plasma in osmotic diuresis. Much steeper gradients for these ions were found across distal tubular epithelium indicating that parts other than the proximal convolution participate in the establishment of the over-all concentration gradients. Since net sodium movement occurred against an electrochemical potential gradient during osmotic diuresis, this study provides evidence for active transport of sodium ions across the proximal tubular wall.