Many in vitro studies have examined the transport characteristics of the pars recta. Yet, because this nephron segment is inaccessible to direct micropuncture, its in vivo reabsorptive capacity remains unknown. In superficial nephrons, water movement between the late proximal and early distal tubule is limited to the pars recta and thin descending limb of Henle’s loop. Net water reabsorption in this latter segment is dependent on a favorable osmotic gradient generated by the hypertonic medullary interstitium. Accordingly, if this hypertonicity is abolished, net water movement between the late proximal and early distal tubule is limited to the pars recta. Since sodium reabsorption is isotonic along the pars recta, a comparison of fractional water delivery to the late proximal and early distal tubule provides an index of sodium reabsorption by this nephron segment. Therefore, furosemide was administered to 7 Munich-Wistar rats and micropuncture samples were obtained from the late proximal and early distal tubules. Ascending vasa recta (AVR) samples were also obtained and the ratio of AVR/plasma osmolality were determined and averaged 1.04, indicating that furosemide abolished medullary hypertonicity. The fractional delivery of filtrate to the late proximal tubule averaged 40.2% and that to the early distal tubule 30.0%, indicating that 10.2% of the filtered sodium was re-absorpbed along the pars recta. The administration of benzolamide, a carbonic anhydrase inhibitor, decreased this value to 5% of the filtered load.

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