The effects of bovine serum albumin and a form of cationised ferritin upon the molecular selectivity of the walls of single frog capillaries.

Single frog mesenteric capillaries have been perfused with Ringer solutions containing the neutral macromolecule Ficoll 70 at a concentration of 40 mg ml-1, while the tissues have been washed with Ringer solution containing no macromolecules but otherwise of similar composition. The effective osmotic pressures (sigma delta II) set up across the capillary wall by the Ficoll 70 were used to assess the wall's molecular selectivity. The hydraulic conductivities, Lp, of the capillary walls were also measured. In seven capillaries the addition of bovine serum albumin (BSA) to the perfusate at concentrations of 1 and 10 mg ml-1 increased sigma delta II from a mean value of 7.14 +/- 1.81 cm H2O to one of 18.71 +/- 2.33 cm H2O and at the same time halved Lp. In another eight capillaries, the addition of a form of cationised ferritin (CF) to the perfusate at a concentration of 1 mg ml-1 increased sigma delta II from a mean value of 5.69 +/- 0.87 cm H2O to one of 16.69 +/- 0.262 cm H2O and reduced Lp to between a third and a half of its original value. In a further seven capillaries, the addition of native ferritin at a concentration of 1 mg ml-1 to the perfusate had no effect on either sigma delta II or Lp. It is suggested that both CF and BSA increase the reflection coefficients of capillary walls to Ficoll 70 by binding to the surface coat of the endothelium. The results are discussed in terms of a development of the fiber matrix theory of Curry and Michel (1980).