Agents that elevate the concentration of cAMP in platelets inhibit the formation of a NaDodSO4-resistant complex between thrombin and a 40-kDa protein.

Abstract

We studied the influence of prostaglandin El and theophylline on the ability of rabbit or human platelets to form NaDodSO4-resistant complexes between 1251-labeled thrombin and a platelet protein of -40 kDa. Control platelets formed two types of these complexes, one that sedimented with the platelets and one that was found in the suspension medium. There were 30-40 sedimentable complexes per platelet and about three times this number of soluble complexes. Pretreatment of rabbit or human platelets with prostaglandin El and theophylline decreased the formation of both types of complex by as much as 60-80%. The pretreatment was particularly effective when low doses of thrombin were used. When added to control platelets, such doses of thrombin caused the formation of sedimentable complexes and the concomitant formation of inositol trisphosphate. However, when added to platelets that had been pretreated with prostaglandin El and theophylline, low doses of thrombin had little or no effect on either reaction.Thrombin, a serine protease, is a potent activator of several platelet responses, including the very early hydrolysis of phosphatidylinositol 4,5-bisphosphate to inositol 1,4,5-trisphosphate and diacylglycerol (1-3). It presumably stimulates these responses by interacting with a receptor on the platelet surface, which then mediates stimulus transduction. However, the precise identity of the receptor remains to be established, and how thrombin interacts with the receptor to promote stimulus transduction is unknown. What is known is that thrombin can bind to several different platelet proteins, including a glycoprotein of 170 kDa, referred to as GP1b (4), and a protein of about 40 kDa, which appears to be a protease nexin (5). It is also known that thrombin can hydrolyze at least one platelet protein, a glycoprotein referred to as GP V (6), and that thrombin that has been inactivated by covalent modification binds to platelets without stimulating them (7). This suggests, but does not prove, that thrombin first binds to platelets and then activates them by a mechanism that involves a proteolytic cleavage reaction.We recently demonstrated that treatment of rabbit or human platelets with prostaglandin E1 (PGE1) and theophylline or other agents that elevate the concentration of cAMP decreases the ability of these platelets to bind thrombin (8). This raised the possibility that these agents might be used to identify cAMP-sensitive components of a functional thrombin receptor complex. To explore this possibility we decided to test the ability of PGE1 and theophylline to inhibit the binding of thrombin to known platelet proteins and began by focusing attention on the protein of 40 kDa. This protein seemed of special interest because it is the only platelet protein that has been identified so far that binds thrombin only when the latter is proteolytically active (9). It forms a NaDodSO4-resistant complex with thrombin that may be an intermediate in a proteolytic cleavage reaction because the complex resembles one tha...

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Regulators of Platelet cAMP Levels: Clinical and Therapeutic Implications

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Prostacyclin and Sodium Nitroprusside Inhibit the Activity of the Platelet Inositol 1,4,5-Trisphosphate Receptor and Promote Its Phosphorylation

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