Transtubular Water Movement in the Isolated Doubly-Perfused Bullfrog Kidney

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Abstract
Deuterium oxide (D2O) was perfused through either the arterial or the portal system of the isolated doubly-perfused bullfrog kidney. Admixture of the two fluids at the level of the peritubular capillaries makes it possible to regulate independently the composition of the glomerular filtrate and the peritubular fluid. The appearance of large amounts of labeled water in the urine when D2O was perfused through the portal system demonstrates a bidirectional water movement across the renal tubule. Furthermore the urine D2O concentration was, on the average, between 80–90% that of the renal vein perfusate (the latter was assumed to be similar in composition to peritubular capillary fluid). The apparent failure to reach complete equilibrium was shown to be due to an inhomogeneous flow distribution within the kidney. When the flow pattern was determined and the urine D2O content predicted on that basis, 100% equilibration of the isotope was established. These results are in agreement with the prediction, based on present concepts of proximal tubule water reabsorption, that tubule fluid should be 90% equilibrated before 1% of the proximal tubule is traversed.

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