Incubation of human platelets with dBcAMP or PGE1 before thrombin addition blocks the formation of oxygenated products of arachidonic acid by inhibiting its hydrolysis from phospholipids.
Does incubation with dBcAMP or PGE1 prevent thrombin-induced prostaglandin synthesis in human platelet suspensions?
Agents that elevate platelet cAMP levels inhibit the hydrolysis of arachidonic acid from platelet phospholipids, demonstrating that prostaglandin synthesis can be partially dissociated from platelet aggregation and release.
When thrombin is added to washed human platelets, one of its actions results in activation of a phospholipase that hydrolyzes arachidonic acid from phospholipids. The arachidonate is converted to the cyclic endoperoxides (prostaglandin G2 and prostaglandin H2) by fatty acid cyclo-oxygenase. These compounds are then converted to thromboxane A2, also called rabbit aorta-contracting substance, by thromboxane synthetase. These labile, pharmacologically active compounds then break down to inactive products including thromboxane B2 and malonaldehyde. Incubation of platelets with either dibutyryl cyclic adenosine 3',5'-monophosphate (dBcAMP) or prostaglandin E1 (PGE1) before thrombin addition blocks the subsequent formation of oxygenated products of arachidonic acid including thromboxane A2, thromboxane B2, and malonaldehyde. In contrast, when arachidonic acid is added directly to platelets, prior incubation with dBcAMP or PGE1 does not inhibit production of the prostaglandins or their metabolites. Thrombin treatment of platelets also blocks the acetylation of cyclo-oxygenase by aspirin since the hydrolyzed arachidonic acid competes with aspirin for the active site on cyclo-oxygenase. Prior treatment of platelets with dBcAMP or PGE1 reverses the thrombin inhibition of the acetylation of cyclo-oxygenase. We conclude that agents which elevate platelet cAMP levels inhibit the hydrolysis of arachidonic acid from platelet phospholipids. We also find that prostaglandin synthesis can be dissociated, in part, from platelet aggregation and release, and that cAMP has separate actions on these processes. Higher thrombin concentrations are required to stimulate prostaglandin synthesis (0.05-2 U/ml) than are required to induce 14Cserotonin release (0.02-0.1 U/ml). Furthermore, dBcAMP and PGE1 both inhibit platelet aggregation induced by either arachidonic acid or prostaglandin H2 without affecting the production of prostaglandin metabolites from these compounds.
Minkes et al. (Tue,) conducted a other in Healthy donors (in vitro platelet study). dBcAMP or PGE1 vs. Control (no dBcAMP or PGE1) was evaluated on Formation of oxygenated products of arachidonic acid (thromboxane A2, thromboxane B2, and malonaldehyde) after thrombin addition. Incubation of human platelets with dBcAMP or PGE1 before thrombin addition blocks the formation of oxygenated products of arachidonic acid by inhibiting its hydrolysis from phospholipids.