REGULATION OF THE RECEPTOR FOR PLATELET-ACTIVATING-FACTOR ON HUMAN PLATELETS

Human platelets possess about 300 receptors for platelet-activating factor (PAF) per cell with a K(d) of about 0.2 nM. In the present study we investigated whether these receptors are subject to intracellular control mechanisms. Preincubation with the protein kinase C inhibitor staurosporine had no effect, and also agents that increase cyclic AMP failed to change the binding of [H-3]PAF. The Ca2+-calmodulin inhibitors W-7 and sphingosine decreased PAF binding by 50-80 %. Inhibition of energy metabolism induced a fall in adenylate energy charge {AEC = ([ATP] + 1/2[ADP])([ATP + ADP + AMP])} and an almost parallel decrease in specific [H-3]PAF binding without changing the K(d). Restoration of the AEC restored the [H-3]PAF binding. Abrupt arrest of energy metabolism during binding of [H-3]PAF left the binding unchanged until the metabolic ATP level had decreased by about 90 %. These data indicate that PAF receptors on human platelets are under close intracellular control, possibly via a Ca2+-calmodulin-dependent phosphorylation/dephosphorylation process.