CADP-RIBOSE FORMATION BY BLOOD-PLATELETS IS NOT RESPONSIBLE FOR INTRACELLULAR CALCIUM MOBILIZATION

Human platelet CD38 is a multifunctional ectoenzyme catalysing the syn thesis and hydrolysis of cADP-ribose (cADPR), a recently identified ca lcium-mobilizing agent that acts independently of D-myo-inositol 1,4,5 -trisphosphate and is known to be expressed by human platelets. The pr esent work shows that ADP-ribosyl cyclase activity is exclusively a me mbrane activity, of which the major part is located in plasma membrane s and a small part in internal membranes. In broken cells, cyclase act ivity was in sensitive to the presence of calcium and was not modulate d by agonists such as thrombin or ADP, whereas in intact cells thrombi n 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 c alcium-mobilizing agent, vesicles were prepared from internal membrane s and loaded with (CaCl2)-Ca-45. These vesicles were efficiently disch arged by IP3 in a dose-dependent manner, but were not responsive to cA DPR or ryanodine in the presence or absence of calmodulin. Thus cADPR is unlikely to play a role in intracellular calcium release in human b lood platelets.