Effect of albumin on the osmotic pressure exerted by myoglobin across capillary walls in frog mesentery.

1. Individual capillaries in mesenteries of pithed frogs were perfused sequentially with two frog Ringer solutions containing myoglobin at a concentration of 70 mg/ml. The first perfusate solution contained bovine serum albumin at a concentration of 10 mg/ml. The second perfusate contained no albumin. The modified Landis micro-occlusion technique (Michel, Mason, Curry & Tooke, 1974) was used to measure the capillary hydraulic conductivity and the effective osmotic pressure of the myoglobin solution across the capillary wall under conditions where the transcapillary concentration difference was established by steady-state ultrafiltration. 2. In six capillaries the effective osmotic pressure of the myoglobin solution at 20-22 degrees C was 15.9 +/- 2.2 (S.E. of mean) cmH2O when albumin was in the perfusate and 10.1 +/- 1.1 cmH2O after albumin was removed from the perfusate. 3. The hydraulic conductivity of the capillary wall increased from a mean value of 6.5 +/- 1.4 X 10(-7) cm/(s cmH2O) when albumin was in the perfusate to 15.6 +/- 3.8 X 10(-7) cm/(s cmH2O) during perfusion with Ringer solution containing myoglobin alone. 4. Control experiments to measure steady-state filtration rates at pressures 10-30 cmH2O were carried out to check the assumption in the method that the reduction in the measured osmotic pressure of myoglobin was the result of changes in the properties of the molecular sieve within the capillary wall and was not simply the result of solute accumulation on the tissue side of the capillary wall. 5. The measured reduction in the effective osmotic pressure of myoglobin when albumin was removed from the perfusate does not conform to the hypothesis (Crone, 1984) that the tight segment of the intercellular junction is the principal molecular filter for myoglobin in the wall of frog mesenteric capillaries. 6. Our results do conform to the hypothesis that a network of fibrous molecules, reinforced by adsorbed albumin, forms the principal molecular filter at the capillary wall in frog mesenteric capillaries.