Transport of Sodium and Urea in Outer Medullary Descending Vasa Recta

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We dissected and perfused outer medullary vasa recta (OMVR) from vascular bundles in the rat.Permeabilities of sodium (PNa) and urea (P (u)) were simultaneously determined from the lumen-to-bath efflux of Sodium-22 and (Carbon-14)urea. PNa and Pu were also measured by in vivo microperfusion of descending (DVR) and ascending vasa recta (AVR) at the papillary tip of Munich-Wistar rats. In some OMVR PNa was indistinguishable from zero. The mean+/-SE of PNa (x10-5, cm/s) in OMVR was 76+/-9. Pu in OMVR was always very high (x10-5, cm/s), 360+/-14. There was no correlation between OMVR PNa and Pu. Inner medullary AVR and DVR had PNa of 115+/-10 and 75+/-10, respectively, and Pu of 121+/-10 and 76+/-11, respectively. PNa and Pu in papillary vasa recta were always nearly identical and highly correlated. Transport of (Carbon-14)urea in OMVR was reversibly inhibited by addition of unlabeled urea or phloretin to the bath and lumen, providing evidence for carrier-mediated transport. These data suggest that sodium and urea might traverse the wall of inner medullary vasa recta by a paracellular pathway while urea also crosses by a transcellular route in OMVR. Electron microscopic examination of seven in vitro perfused OMVR revealed no fenestrations and exposure of these vessels to 10 micromolars calcium ionophore A23187 or 1 nM angiotensin II resulted in reversible contraction, suggesting that in vitro perfused OMVR are DVR only. (J. Clin. Invest. 1994. 93:212-222.) Key words: angiotensin. microcirculation. microperfusion. rat. urinary concentration

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