Osmotic reflextion coefficients of capillary walls to low molecular weight hydrophilic solutes measured in single perfused capillaries of the frog mesentery.

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Abstract
Individual capillaries of the transilluminated frog mesentery were perfused with suspensions of human red cells in frog Ringer solution containing 1.0 g albumin/100 ml. The outer surface of the mesentery has been washed with normal frog Ringer solution and with frog Ringer solutions made hypertonic by addition of one of the following solutes: NaCl (100 mmol 1-1); urea (100 mmol 1-1); sucrose (20-50 mmol 1-1); cyanocobalamin (8.5 mmol 1-1). The temperature of the mesentery was between 14-16.degree. C in all experiments. With the mesentery superfused with normal Ringer, the filtration coefficient was determined from measurements of the rate of fluid filtration across the capillary wall, at a series of known capillary pressures. Filtration coefficient varied from 0.69 .times. 10-3-4.45 .times. 10-3 .mu.m s-1 H2O-1 with an average value of 1.87 .times. 10-3 .mu.m s-1 cm H2O-1. When the superfusate was made hypertonic by the addition of a test solute, the osmotic reflection coefficient (.sigma.) of the capillary wall to test solute was calculated from the additional rate of filtration, the concentration of test solute in the superfusate and the filtration coefficient. Average values for .sigma. were: NaCl, 0.068 .+-. 0.03 (3 capillaries); urea, 0.071 .+-. 0.015 (4 capillaries); sucrose, 0.115 .+-. 0.023 (7 capillaries); cyanocobalamin, 0.100 .+-. 0.03 (3 capillaries). In further experiments, the osmotic reflection coefficients to NaCl, urea and sucrose were determined in the same capillary. Five technically acceptable experiments were carried out. Although there were differences in the value of .sigma. between different capillaries, in any one capillary values of .sigma. were of the same magnitude and there appeared to be no significant trend with the molecular size of the test solute. Perhaps there is a single pathway for water and small hydrophilic molecules across the capillary wall. The results may be interpreted in terms of an exclusive channel for water in parallel with a channel shared by both water and small hydrophilic molecules. The exclusive water channel may be the membranes and cytoplasm of the endothelial cells and the shared channel may be located in the intercellular junctions. The data suggest the exclusive water channel represents about 10% of the total filtration coefficient in frog mesenteric capillaries. The shared channel shows relatively little restriction to the molecules investigated. Estimates of the volume flow through the 2 channels are made for conditions where hydrostatic pressure differences and osmotic pressure differences are the driving forces.