Separate Mechanisms for the Inhibition of Platelet Aggregation by Adenosine and 2-Cloroadenosine

Abstract

The mechanism by which adenosine (Ado) and 2-cloroadenosine (Cl-Ado) inhibit platelet aggregation is not clear. In order to get some insight into the mode of action of these compounds, we studied the effect of Cl-Ado on the uptake of Ado by intact platelets, the effect of these compounds on the endogenous phosphorylation of specific plasma membrane proteins, and its effect on the carboxymethylation pattern of plasma membrane proteins in intact platelets. Cl-Ado does not modify the uptake of Ado by intact platelets, nor is itself incorporated into the platelet’s pool of nucleotides. Phosphorylation of plasma membrane proteins is not affected by Cl-Ado; however, Ado produces a selective increase in the phosphorylation of one plasma membrane component of glycoproteic nature. As has been reported, phosphorylation of this glycoprotein is also modulated by cAMP (BBA, 455:371, 1976). Although the electrophoretic pattern of carboxymethylated plasma membranes is unaffected by Ado or Cl-Ado, it was found that the former markedly increases the label of all the susceptible proteins, while Cl-Ado selectively protects a single membrane component. Electrophoretically, this component seems to be related to the above mentioned glycoprotein. The results reported suggest that Ado and Cl-Ado interact with different components of the plasma membrane, impairing platelet aggregation through different mechanisms. In the case of Ado, two ways seem operative: a) A cAMP-like stimulation of a specific membrane glycoprotein and b) A more general perturbation of the membrane structure, perhaps through an Ado-carrier complex (Acta Med. Scand. 525:169, 1971). Cl-Ado seems to interact solely on the external surface of the plasma membrane, suggesting that the transmembrane phospho-glycoprotein previously described is in some way closely related to the ADP-receptor of the platelet plasma membrane.