Localization and Characterization of Sodium Transport Along the Renal Tubule

¹ From the Department of Physiology, School of Medicine, University of Michigan, Ann Arbor, Michigan

The proximal tubular Na transport system was investigated using our stop flow analysis. During varying degrees of osmotic diuresis in dogs the ureter of one kidney was occluded for 2 1/2–8 minutes. Following occlusion serial urine samples were collected and analyzed for Na, creatinine, PAH or glucose. Using urinary creatinine concentrations as an index of water movement the mass of Na and water reabsorbed from the proximal segments during occlusion was calculated. In all experiments the calculations showed that the Na concentration in the proximal reabsorbate was of the same concentration which existed in plasma. Such behavior is inconsistent with a proximal Na pump which would otherwise reabsorb a supraplasma-level Na solution away from mannitol and Na in the tubule. The data suggest that Na and water transport out of the proximal tubules may be a passive process initiated by the colloidal osmotic pressure differences which exist between the intratubular urine and the peritubular capillary blood. A passive mechanism for Na and water reabsorption from the proximal segment resolves many difficulties faced by a Na pump.