CALCIUM STIMULATION OF PROCOAGULANT ACTIVITY IN HUMAN ERYTHROCYTES - ATP DEPENDENCE AND THE EFFECTS OF MODIFIERS OF STIMULATION AND RECOVERY

The human erythrocyte membrane is generally considered to have no proc oagulant activity. The normal membrane is characterized as having an a symmetric distribution of phospholipid species such that negatively ch arged and aminophospholipids are predominantly located on the inner le aflet of the membrane bilayer. Elevation of cytoplasmic Ca2+ in erythr ocytes produces an assortment of biochemical and structural responses that include diminished phospholipid asymmetry and an elevation in pro coagulant activity. Maintenance of the normal asymmetric distribution of phospholipid species is believed to be largely mediated by a phosph olipid translocase mechanism. We have utilized a recently developed si ngle-step kinetic assay of procoagulant activity to investigate the me chanisms of Ca2+ stimulation of procoagulant activity and recovery fro m the procoagulant state upon removal of Ca2+. This study demonstrated that stimulation of procoagulant activity by elevated cytoplasmic Ca2 + is greatly diminished in ATP-depleted erythrocytes. Phospholipid tra nslocase inhibitors failed to fully inhibit recovery from the procoagu lant state after removal of Ca2+. The data indicate that recovery of e ndogenous lipid from a procoagulant configuration may not be entirely mediated by the phospholipid translocase. Additionally, the data are i nconsistent with the phospholipid translocase mediating the Ca2+-induc ed elevation of procoagulant activity, although the involvement of oth er protein(s) is indicated.