We recently demonstrated that net fluid uptake occurs in the capillary system of the inner medulla. To define the site of fluid uptake, the concentration of protein was determined in plasma from descending vasa recta at the base and tip of the exposed papilla in Munich-Wister rats. The vasa recta plasma-to-arterial plasma protein concentration ratio (VR/P) was 1.43 +/- 0.09 at the base and 1.66 +/- 0.09 at the tip. These results, which indicate fluid loss from the descending vasa recta, are difficult to explain on the basic of hydraulic and oncotic forces alone. The osmolality of the contents of descending vasa recta increased between base and tip (delta = 72 +/- 30 mosmol/kg H2O). If the increase in osmolality of plasma in descending vasa recta lags behind that of the adjacent medullary interstitium, a transcapillary osmotic driving force exists favoring water loss from descending vessels. It is concluded that fluid uptake by the inner medullary circulation occurs beyond descending vasa recta in interconnecting capillaries or ascending vasa recta. In our view the most likely interpretation of these results is that fluid movement across vasa recta in the inner medulla is influenced by three forces: those owing to transcapillary differences in osmotic, oncotic, and hydraulic pressures.

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The vasa recta are thought to play an important role in the transfer of water andsolutes within the renal medulla. Hydraulic pressures were measured in vasa recta onthe surface of the exposed papilla in young Munich Wistar rats, and blood was collected from these microvessels for determination of total protein concentration and calculation of colloid oncotic pressure. In descending vasa recta at the base of the exposed papilla, mean hydraulic pressure was 9.2 plus or minus 0.4 (SE) mmHg and plasma protein concentration averaged 7.1 plus or minus 0.4 g/100 ml. Corresponding valuesin ascending vasa recta at the same level were 7.8 plus or minus 0.4 mmHg and 5.6 plusor minus 0.3 g/100 ml. respectively. The protein concentrations correspond to calculated oncotic pressures of 26 and 18 mmHg in descending and ascending vasa recta, respectively. We interpret these findings as evidence for net water uptake by the vasa recta in the renal inner medulla for which the driving forces are the transcapillary hydraulic and oncotic pressure differences.

The effect of hemorrhage and reinfusion on renal release of prostaglandin E (PGE), arterial [PGE], mixed-venous [PGE], and renal function was observed in anesthetized dogs. Following hemorrhage to 60 mmHg arterial pressure, arterial [PGE] rose significantly from 405 to 740 pg/ml. Renal release of PGE remained near control (8 ng/min), as renal blood flow (RBF) decreased from 4.7 to 2.2 ng/min per gram kidney weight (KW). Mixed-venous [PGE] remained near the control value (960 pg/ml). Reinfusion of shed blood restored RBF to 4.0 ml/min per KW. Renal release of PGE rose significantly to 190 ng/min. Arterial [PGE] remained elevated, but mixed-venous [PGE] was not significantly different from control. Indomethacin, a prostaglandin synthesis inhibitor, caused a significant decrease in renal release of PGE. Arterial [PGE] remained elevated following treatment. The inhibition of PGE release from the kidney by indomethacin indicates that increased renal release of PGE following reinfusion is the result of accelerated PGE synthesis. The data suggest that the elevated arterial [PGE] may be the result of alteration of the handling of PGE by the lung.

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