Cholesterol exchange in platelets, erythrocytes and megakaryocytes

Cholesterol exchange between plasma and human platelets and erythrocytes and guinea pig platelets, erythrocytes and megakaryocytes was studied. The characteristics of exchange of cholesterol between [3H]cholesterol-labeled plasma and human platelets and erythrocytes were similar: exchange per cell was independent of cell concentration in whole plasma, decreased only 2-fold over a wide range of cell concentrations in low concentrations of plasma and approached a plateau at 1/3 normal plasma cholesterol concentration, and there was no net change in the cholesterol content of either cell. The activation energy for exchange for both cells was 47 kJ/mol. In all experiments, erythrocyte cholesterol was labeled to approximately twice the specific activity of platelet cholesterol. Guinea pig megakaryocyte cholesterol exchanged at 25-33% of the rate of guinea pig platelet cholesterol in vitro. Similarly, when guinea pigs were fed [3H]cholesterol, erythrocyte cholesterol specific activity after 24 h was 90%, platelet 50-65%, and megakaryocyte 20-26% that of plasma. Guinea pig platelets incubated with plasma radiolabeled in free and esterified cholesterol incorporated radioactivity from free but not esterified cholesterol. The similarity of free cholesterol exchange in platelets and erythrocytes in vitro and in vivo and the apparent inability of platelets to take up cholesterol esters from lipoproteins suggest that the interaction between normal platelets and normocholesterolemic plasma is limited to cholesterol exchange.