The effects of proteins upon the filtration coefficient of individually perfused frog mesenteric capillaries

The filtration coefficients (L_p) of the walls of individually perfused frog mesenteric capillaries have been measured using the methods described by Michel et al. (1974). When a vessel was perfused with frog Ringer solution containing no protein, L_p was four to five times greater than its value for the same capillary perfused with frog plasma. The increase of L_p seen on perfusion with protein-free solutions was readily reversible and could be prevented by adding to the Ringer perfusate bovine serum albumin, bovine γ-globulin and human haemoglobin. The addition of whale myoglobin (0.5 g 100 ml^-1) to the Ringer perfusate did not prevent an increase in L_p. Ringer solutions containing 0.1 g of bovine serum albumin 100 ml^-1 were as effective as those containing higher concentrations of albumin in preventing the increase in L_p, but the L_p of capillaries perfused with solutions containing 0.01 g of albumin 100 ml^-1 did not differ from the values for the same capillaries perfused with protein-free solutions. It was shown that in any given capillary, L_p was independent of the direction of fluid flow across the vessel wall and was the same when the capillary was perfused with 9.0 g of bovine albumin 100 ml^-1 and 3 g of bovine γ-globulin 100 ml^-1. Whereas the effects of L_p are seen within 1 min of removing the protein from the perfusate, no changes in L_p were observed during the first 20 min of perfusion and superfusion of capillaries with solutions free of Ca²⁺ and Mg²⁺. L_p also appeared to be unaffected after 20 min perfusion with neuraminidase. It is suggested that plasma proteins are adsorbed onto capillary walls and greatly modify capillary permeability.