Diffusive transport of solute in the rat medullary microcirculation.

Outer medullary descending vasa recta (OMDVR) permeability to sodium (PNa) is much lower than to urea (Purea). Based on these findings, we hypothesized that sodium and urea diffuse across the OMDFR wall by separate routes. To further test this, we simultaneously perfused OMDVR with 22Na and 36Cl, [14C]urea, [3H]raffinose, or tritiated water. The permeability of OMDVR to 22Na and [3H]raffinose was found to increase markedly and reversibly with perfusion rate. PNa was highly correlated with the permeability to Cl (PCl) and to [3H]raffinose (Praf) (R = 0.90 and 0.95, respectively) but not with Purea (R = 0.23). Praf was also correlated with inulin permeability (PIn) (R = 0.93). The intercepts for the regressions of PNa with PCl and Praf and for Praf with PIn were zero. In contrast, OMDVR with low PNa retained very high diffusional water permeability (PD) and Purea, a finding consistent with separate routes for permeation of those tracers. We previously established that thiourea is a competitive inhibitor of OMDVR urea transport. In the presence of 100 mM thiourea, OMDVR PNa and Purea were correlated (R = 0.71) but retained an intercept much > 0. We conclude that Na, Cl, raffinose, and inulin are likely to traverse the OMDVR wall through a common pathway, whereas specific mechanisms exist to regulate the permeation by urea and water.