Hydraulic and diffusional permeabilities of isolated outer medullary descending vasa recta from the rat

Water permeates many microvessel walls via a pathway shared with small hydrophilic solutes and also via an exclusive water pathway. In outer medullary descending vasa recta (OMDVR), the relation ship between diffusional permeabilities to water and sodium indicates the existence of an exclusive water pathway and suggests that of a shared pathway. We investigated the latter possibility by estimating hydraulic permeability (L-p) and diffusional permeability to [H-3]raffinose (P-raf) in isolated, perfused OMDVR. The product of hydraulic permeability and osmotic reflexion coefficient of albumin (L-p sigma(a)) was 1.56 +/- 0.19 x 10(-6) cm . s(-1) mmHg(-1) (n = 28), calculated from transmural volume fluxes induced by perfusate-to-bath differences in albumin oncotic pressure (Delta Pi(a)). P-raf in the same vessels was 40.1 +/- 1.5 x 10(-5) cm/s when Delta Pi(a) was zero. In separate experiments, sigma(a) was at least 0.89 +/- 0.10 (n = 17). L-p sigma(a) correlates with P-raf, indicating that OMDVR contain a shared pathway for convection driven by Delta Pi(a) and for diffusion of small hydrophilic solutes.