Prostacyclin-lipoprotein interactions. Studies on human platelet aggregation and adenylate cyclase

The in vitro effects of different lipoprotein fractions (VLDL, LDL and HDL) on human washed platelet aggregation, induced by collagen and thrombin, were evaluated in the presence and absence of PGI2. Although VLDL and LDL increased the platelet aggregation while HDL showed an opposite effect, none of the tested lipoprotein fractions affected the potency of PGI2 as inhibitor of platelet aggregation (IC50). In addition, studies were performed to evaluate the effects of lipoproteins on adenylate cyclase activity in human platelet membranes. The three lipoprotein classes inhibited both basal and PGI2-stimulated adenylate cyclase without affecting the EC50 for PGI2. This inhibitory activity was not specifically elicited by any protein or lipid since neither bovine serum albumin nor a lipid emulsion (Intralipid) displayed any inhibition. The effect on adenylate cyclase elicited by VLDL, LDL and HDL does not seem to be correlated with the activity on platelet aggregation. It is concluded that mediators other than cAMP might be involved in the control of platelet function by lipoproteins.