Induction of platelet activation by cobra venom factor from Naja naja atra in rat

AIM: To study the effects of cobra venom factor (CVF) on platelets of rat platelet rich plasma (PRP) and to elucidate its cellular mechanism. METHODS: PRP was used to measure platelet aggregation and ATP-release simultaneously; prothrombinase, intracellular free calcium, and cAMP were assayed using chromogenic substrate, fluorophor Fura-2, and RIA respectively. RESULTS: From 19.5 nmol L-1 to 617 nmol . L-1 CVF induced platelet aggregation and ATP release concentration-dependently. The ATP release induced by CVF 195 nmol L-1 was independent of aggregation and was much weaker than that induced by thrombin 1 U/ml. CVF 195 nmol L-1 increased the prothrombinase activity on platelet surface in a time-dependent manner. SZ-l, SZ 21, and SZ-22, which are monoclonal antibodies against glycoproteins I b/IX, III a, and II b located on platelet membranes, inhibited CVF-induced platelet aggregation. CVF 195 nmol . L-1 also elevated moderately intracellular free calcium ion from (141 +/- 46) to (240 +/- 64) nmol . L-1 in Fura-2 AM loaded platelets, and decreased intracellular cAMP. CONCLUSION: Complement activator CVF induced platelet aggregation and ATP release, and increased prothrombinase activity on platelet surface. These actions were dependent on fibrinogen receptors on the platelet surface, elevation in intracellular free calcium ion, and reduction in cAMP.