Micropuncture study of renal excretion of water, K, Na, and Cl in the rat

¹ Department of Biochemistry, Woman's Medical College of Pennsylvania, Philadelphia, Pennsylvania

Inulin, K, Na, and Cl were determined on identical samples of proximal tubule fluid, urine, and serum in nondiuretic rats. Tubule fluid-serum ratios for inulin showed that at least two-thirds of the filtered water was reabsorbed proximally. Potassium ratios remained at 1, suggesting incomplete reabsorption. Early proximal sodium and chloride ratios were close to 1, but more distal regions had ratios of approximately 1.1 and 1.4, respectively. pH measurements on tubule fluid of nondiuretic and diuretic rats demonstrated proximal acidification with values as low as 6.7. Under conditions of extreme mannitol diuresis further acidification was negligible, whereas in nondiuretic rats urine pH was below 6.