Cyclic AMP metabolism in cholesterol-rich platelets.

The incorporation of cholesterol into human platelets by means of incubation with cholesterol-rich lecithin dispersions is associated with a decreased fluidity in the phospholipid hydrocarbon core of platelet membranes and an increased sensitivity of platelets to aggregating agents. We examined whether the platelet membrane enzyme, adenylate cyclase is influenced by changes in phospholipid fluidity. After incubation for 5 h at 37” with cholesterol-rich lecithin dispersions, platelets demonstrated a 55% increase in membrane cholesterol content associated with a 5to lo-fold decrease in the sensitivity of these platelets to the aggregation inhibitor and stimulator of adenylate cyclase, prostaglandin E,. However, these platelets were normally sensitive to direct inhibition of epinephrine-induced aggregation by dibutyryl adenosine 3’:5’-monophosphate. Platelet cholesterol content increased in platelets during incubation with cholesterol-rich lecithin dispersions, associated with a parallel increase in both platelet adenosine 3’:5’-monophosphate content and the basal activity of adenylate cyclase. At 5 h, platelet cyclic AMP levels had increased 56% and basal adenylate cyclase activity had increased 2.5-fold. In contrast, platelets incubated with pure lecithin dispersions lost 21% cholesterol and their basal cyclase activity decreased 40%. The adenylate cyclase of control platelets was stimulated 3.5-fold by prostaglandin E, (1.0 PM) and 4.5-fold by sodium fluoride (1.0 mM). In contrast, the adenylate cyclase of cholesterol-rich platelets was stimulated neither by prostaglandin E, nor by sodium fluoride. Adenylate cyclase in cholesterol-depleted platelets was stimulated normally by prostaglandin E,. The kinetic constants of adenosine 3’:5’-monophosphate phosphodiesterases (EC 3.1.4.17) of cholesterol-rich platelets were normal. These studies demonstrate that incorporation of cholesterol into platelet membranes is associated with a diminished inhibitory effect of prostaglandin E, on platelet aggregation. This appears to be related to the inability of prostaglandin E, to stimulate adenylate cyclase and therefore adenosine 3’:5’-monophosphate production in these platelets. The loss of hormone and fluoride responsiveness as well as the increased basal activity of adenylate cyclase in cholesterol-rich platelets may be due to the physical effects of cholesterol on platelet membrane phospholipid.