Net transtubular movement of water and urea in saline diuresis

¹ Department of Medicine, University of North Carolina, School of Medicine, Chapel Hill, North Carolina

Anesthetized rats were given urea-C¹⁴ intravenously and infused with 5% NaCl solution containing inulin-methoxy-H³. Fluid was collected by micropuncture from surface tubules for simultaneous determination of H³ and C¹⁴ activities and osmolality. Inulin clearances, plasma and urine sodium, urine volume, and urea excretion were all greatly increased, but fractional water and urea reabsorption in the proximal convolution was similar to that in nondiuretic rats. Inulin fluid/plasma ratios in hypotonic early distal samples were slightly lower than those observed in nondiuretic rats. Urea/inulin ratios in distal samples and in ureteral urine were similar to the late proximal convolution, indicating little or no addition of urea in the loop of Henle or loss from distal convolution or collecting ducts, in contrast to nondiuretic rats. These results suggest that in the proximal convolution sodium (and therefore water) reabsorption is proportional to the rate of delivery of sodium to the tubule. Furthermore, the absence of demonstrable net transtubular movement of urea beyond the proximal convolution under conditions in which concentration gradients are reduced is consistent with the hypothesis that urea movement is largely passive.

Key Words: renal tubule • rat kidney • tubular water reabsorption • tubular urea reabsorption • tubular sodium reabsorption

Submitted on July 24, 1963