CALCIUM THRESHOLD IN HUMAN PLASMA CLOTTING KINETICS

In vitro clotting kinetics of citrate human blood plasma under its tit ration with calcium ions are studied. Contact activation (Factor XIa plasma kallikrein) is shown to be independent of calcium while non-li near growth of thrombin concentration is demonstrated at calcium conce ntrations higher than 0.25 mM under conditions of contact activation c aused by quartz walls of the measuring cell. Thrombin generation kinet ics are well fitted with an exponential function. Power index of the e xponential function steeply rises as calcium concentration increases f rom 0.25 to 0.5 mM and reaches plateau at higher concentrations. At fr ee calcium concentrations under 0.25 mM thrombin level does not grow r emaining lower than 30 pM. So, blood coagulation system behaves in a t hreshold manner under calcium concentration changes. The threshold con centration of free calcium is equal to 0.25 +/- 0.05 mM.