Water flow across the walls of single muscle capillaries in the frog, Rana pipiens

Individual capillaries of the transilluminated frog muscle cutaneous pectoris were perfused with suspensions of human red cells in frog Ringer solution containing 1 g/dl bovine serum albumin. The modified Landis technique (Michel, Mason, Curry & Tooke, 1974) was used to measure hydraulic conductivities of the capillary wall. Sucrose osmotic reflexion coefficients of the capillary wall were measured in four capillaries when the superfusate contained 100 mM-sucrose. All experiments were made at 22-24 degrees C. The hydraulic conductivity of arterial capillaries varied from 0.3 to 1.26 X 10(-7) cm/(s cmH2O) with a mean of 0.79 X 10(-7) cm/(s cmH2O) (six capillaries). The hydraulic conductivities of mid-capillaries varied from 0.43 to 1.86 X 10(-7) cm/(s cmH2O) with a mean value of 0.72 X 10(-7) cm/(s cmH2O) (six capillaries). The mean reflexion coefficient to sucrose was 0.12 +/- 0.05 (S.D.). The measured reflexion coefficients to sucrose conform to the hypothesis that 90% of the transcapillary water flow crosses the capillary wall via the principal hydrophilic pathway. The remaining 10% crosses via an exclusive water pathway. The distribution of water flow is similar to that previously described in frog mesenteric capillaries. The mean value of the hydraulic conductivity of frog muscle capillaries is about one-seventh the mean value of the hydraulic conductivity of frog mesenteric capillaries measured at the same temperature. The result conforms to the hypothesis that only a small fraction (mean 10%) of the area of junctional contact between adjacent endothelial cells is available for water and solute exchange in frog muscle capillaries. The hydraulic and diffusional conductances per unit length of open junction appear to be very similar when muscle capillaries are compared to mesenteric capillaries in the frog. Our results lead us to speculate that structures within the intercellular junctions determine the extent of open junction and may modulate the hydraulic conductivity of both the principal water pathway and the exclusive water pathway.