Characteristics of aggregation of bovine platelets and clarification of molecular mechanism responsible for a genetic hemorrhagic disease in cattle

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I investigated the characteristics of aggregation of bovine platelets and etiology for a decrease in the collagen-induced aggregation of platelets from Japanese Black cattle affected with Chediak-Higashi syndrome (CHS).1. Characteristics of bovine platelets. Contribution of thromboxane A2 and ADP, which are release following stimulation with collagen, to collagen-induced increase in [Ca^<2+>]_i and aggregation in bovine platelets was small compared to human or rat platelets. Protein kinase C was involved in collagen-induced Ca^<2+> signaling in bovine platelets, whereas this kinase plays a role in aggregation rather than in Ca^<2+> signaling in human platelets.2. Collagen produced inositol 1, 4, 5 -trisphosphate, a Ca^<2+> mobilizing second messenger, in bovine platelets through activation of phospholipase C. Activation of phospholipase C was impaired in CHS platelets.3. Collagen acts on two major receptors on platelets ; GPIa/IIa and GPVI. The response to the GPVI agonist convulxin was normal in CHS platelets indicating that GPVI is normal in these platelets. On the other hand, the response to rhodocytin, a putative GPIa/IIa activator, was greatly depressed in CHS platelets. It was suggested that actin polymerization is involved in the collagen- and rhodocytin-produced Ca^<2+> signaling. Adhesion of platelets to soluble collagen, which is mediated by GPIa/IIa, was normal in CHS platelets. Although it remains to be determined whether rhodocytin acts on GPIa/IIa or the other receptor, the present data suggest that a signal from the rhodocytin-sensitive mechanism is indispensable for full activation of GPVI when collagen aggregates platelets. CHS platelets are devoid of the rhodocytin-sensitive mechanism so that these are useful for clarification of collagen receptors and the crosstalk between receptors.