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Recent models of the urinary concentrating mechanism have postulated that urea in the medullary interstitium creates a transtubular concentration gradient for sodium between fluid at the end of the descending limb of Henle's loop and the medullary interstitium, favoring the passive outward movement of sodium from Henle's thin ascending limb. These experiments were designed to determine whether such a gradient normally exists. Young nondiuretic Munich-Wistar rats were prepared for micropuncture of the exposed left renal papilla. Samples of loop of Henle fluid and vasa recta plasma (assumed to reflect the composition of interstitial fluid) were obtained from adjacent sites. Loop fluid values in 21 comparisons from 18 rats (mean +/- SE) were: sodium 344 +/- 12 meq/liter; potassium, 26 +/- 2 meq/liter; osmolality, 938 +/- 37 mosmol/kg H23. Vasa recta plasma values (in corresponding units of measurement) were: sodium, 284 +/- 11; potassium, 34 +/- 2; osmolality, 935 +/- 34. Mean values of paired differences (loop fluid minus vasa recta plasma) were: delta sodium, 60 +/- 11.1 (P less than 0.001); delta potassium, -8.0 +/- 2.1 (P less than 0.001); delta osmolality, 4 +/- 16 (NS). Corrected for plasma water, the loop fluid minus vasa recta differences (in milliequivalents per kilogram H2O) were: delta sodium, 40 +/- 11.4 (P less than 0.005); delta potassium, -9.7 +/- 1.9 (P less than 0.001). We interpret these findings to indicate that in the papilla of nondiuretic rats, a significant difference in sodium concentration exists across the thin loop of Henle favoring outward movement of sodium, which confirms a key requirement of the passive models. A concentration difference for potassium in the reverse direction was also observed.

Evidence for a concentration gradient favoring outward movement of sodium from the thin loop of Henle.