EFFECT OF CALCIUM AND HYDROGEN-ION ON THE FIBRINOLYTIC-ACTIVITY OF ISOLATED RENAL GLOMERULI FROM RAT

The fibrinolytic activity of glomeruli isolated from rat kidney has been measured with 125I-fibrin-coated tubes. This technique allows the study of the two successive steps of the reaction: the release of the activator of plasminogen from the cells in the presence of isolated glomeruli, and the enzymatic process itself in the presence of the supernatant obtained after a 30-min incubation period with isolated glomeruli and centrifugation. Calcium in the range of 0.01 to 1.0 mM increased the rate of fibrinolysis and shortened the lag-time preceding this effect. The fibrinolytic activity was also measured in the presence of isolated glomeruli as a function of the plasminogen concentration. Calcium at 1 mM clearly increased the maximum velocity but did not change the concentration of plasminogen corresponding to 50% of the maximum. Supernatants were obtained after preincubation of isolated glomeruli with increasing doses of calcium. Addition of these supernatants to the 125I-fibrin-coated tubes produced a progressive increase in the rate of fibrinolysis, depending on the dose of calcium. When calcium was added to the supernatants only, after having discarded the glomeruli, there was no change or, above 0.5 mM calcium, a slight inhibition of the enzymatic reaction itself. The fibrinolytic activity measured in the presence of isolated glomeruli was maximum at a pH of 8. There was a clear-cut interaction in this condition between pH and calcium concentration. This interaction was not observed when the isolated glomeruli had been discarded. These results clearly demonstrate the major role of calcium in the release from glomeruli of an activator of plasminogen.