Filtration coefficients and osmotic reflexion coefficients of the walls of single frog mesenteric capillaries.

Abstract

Single capillaries in the mesentery of pithed frogs were perfused with frog Ringer solutions containing various concentrations of bovine serum albumin and myoglobin. Filtration coefficients (Lp) of the capillary wall were determined from measurements of fluid filtration rate at a series of different capillary pressures. The osmotic reflexion coefficients (.sigma.) to albumin and myoglobin were determined by comparing the effective osmotic pressure exerted by these solutes across the capillary walls with their osmotic pressures in a membrane osmometer. Lp and .sigma. to albumin were measured in 18 vessels at different sites in the capillary bed with the tissue temperature in the range of 20-24.degree. C. Lp varied from 1.5 .times. 10-3 to 15 .times. 10-3 .mu.m s-1 cm H2O-1 having higher mean value in 9 venous capillaries (11.33 .times. 10-3 .mu.m s-1 cm H2O-1) than in 9 arterial and mid-capillaries (4.83 .times. 10-3 .mu.m s-1 cm H2O-1). For all 18 vessels .sigma. to albumin had a mean value of 0.816. There was no correlation between Lp and .sigma.. The mean value of .sigma. for the venous capillaries was 0.841 and the other 9 vessels 0.802. The osmotic reflexion coefficient to myoglobin was measured in 7 different capillaries and was found to have a mean value of 0.348 at 20-24.degree. C. The Lp of the capillaries varied from 3.0 .times. 10-3 to 10.5 .times. 10-4 .mu.m s-1 cm H2O-1. There was no correlation between .sigma. for myoglobin and Lp. The .sigma. for urea in 8 capillaries at 20-24.degree. C was measured (.sigma. for albumin was also measured in 2 of these vessels). The mean value of .sigma. for urea was 0.061. The exclusive water channel was calculated at 0.209 .times. 10-3 .mu.m s-1 cm H2O-1. The effects of temperature on Lp were investigated in 7 capillaries. When tissue temperature changed slowly (less than 2.degree. C min-1) the changes of Lp were similar to the reciprocal changes in the viscosity of water. The estimates of Lp and .sigma. for myoglobin and serum albumin are discussed in terms of the classical pore theory and in terms of a theory that the molecular sieving properties of the capillary wall reside in a matrix of molecular fibers which covers the endothelial cells and fills the channels through or between them. A quantitative theory of a fiber matrix membrane is given.