Enhanced fluid uptake in frog mesenteric capillaries associated with plasmin perfusion.

1. We have measured the permeability of single capillaries of the mesenteries of decerebrated frogs, before and during perfusion with solutions containing the fibrinolytic enzyme, plasmin. 2. The hydraulic permeability (Lp) and the effective oncotic pressure exerted across the vessel walls (sigma delta pi) were measured using the method of Michel (1980). The vessels were sequentially perfused with a control frog Ringer solution containing either Ficoll 70 or bovine serum albumin (BSA) at concentrations of 40 mg ml-1 or a mixture of Ficoll 70 (40 mg ml-1) and BSA (10 mg ml-1), and then with a second Ringer perfusate containing plasmin (1 mg ml-1) but in all other respects identical to the control solution. 3. In sixteen out of seventeen experiments, perfusion with plasmin increased sigma delta pi. In eleven of these experiments the increase was very large such that sigma delta pi exceeded the in vitro value for perfusate oncotic pressure. 4. In the same seventeen vessels plasmin perfusion was associated with a fall in Lp from a mean value of 10.3 x 10(-7) cm s-1 cmH2O-1 to one of 7.7 x 10(-7) cm s-1 cmH2O-1. The fall in Lp was not significant. 5. In five of the seventeen vessels, a second control perfusion was made after exposure to plasmin. There was no evidence that Lp had increased above or sigma delta pi had fallen below the initial control value. 6. In a further six experiments, the effects of plasmin were investigated in the absence of other perfusate macromolecules. No significant changes in Lp or sigma delta pi were observed. 7. In a further eight vessels, the effects of plasmin on fluid filtration were investigated with the tissues cold and then at room temperature. In all eight vessels plasmin reduced filtration or increased fluid reabsorption to a greater extent when the tissue temperature was 17 degrees C than when it was 4 degrees C. 8. The large increases in sigma delta pi which we have observed during perfusion of single vessels with plasmin-containing solutions are consistent with the development of substantial local osmotic gradients at the capillary wall following the enzyme's action upon substrates at the endothelial cell surface, one of which could be fibrin. Alternatively, plasmin might stimulate endothelial cells to liberate molecules which locally amplify the oncotic pressure exerted by the perfusate macromolecules. These effects are more marked at room temperature than at 4 degrees C.