Human platelet CD38 is a multifunctional ectoenzyme catalysing the synthesis and hydrolysis of cADP-ribose (cADPR), a recently identified calcium-mobilizing agent that acts independently of d-myo-inositol 1,4,5-trisphosphate and is known to be expressed by human platelets. The present work shows that ADP-ribosyl cyclase activity is exclusively a membrane activity, of which the major part is located in plasma membranes and a small part in internal membranes. In broken cells, cyclase activity was insensitive to the presence of calcium and was not modulated by agonists such as thrombin or ADP, whereas in intact cells thrombin increased cADPR formation by 30%, an effect due to fusion of granules with the plasma membrane. In order to assess the role of cADPR as a calcium-mobilizing agent, vesicles were prepared from internal membranes and loaded with 45CaCl2. These vesicles were efficiently discharged by IP3 in a dose-dependent manner, but were not responsive to cADPR or ryanodine in the presence or absence of calmodulin. Thus cADPR is unlikely to play a role in intracellular calcium release in human blood platelets.

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