Cytoskeleton-dependent inhibition of the ADP-ribosyl cyclase activity of CD38 in thrombin-stimulated platelets

Stimulation of human platelets with thrombin caused a 42% inhibition of the ADP-ribosyl cyclase activity of membrane CD38. This effect was mediated by the activation of the platelet thrombin receptor rather than by proteolysis of CD38, and was not due to a different distribution of the synthesised nucleotide or to a reduced accessibility of CD38 to the substrate. The inhibitory effect of thrombin required actin polymerisation and was not observed when interaction of CD38 with the cytoskeleton was prevented by cytochalasin D. Finally, we analysed whether cADPR could play a role as a Ca²⁺-mobilising agent in human platelets. Using saponin-permeabilised cells, we found that unlike IP₃, cADPR did not induce any release of Ca²⁺ from intracellular stores. These results indicate that the enzymatic activity of membrane CD38 can be modulated by platelet activation, and that the function of this glycoprotein is probably not related to Ca²⁺ mobilisation.