Platelets from bleeding Simmental cattle mobilize calcium, phosphorylate myosin light chain and bind normal numbers of fibrinogen molecules but have abnormal cytoskeletal assembly and aggregation in response to ADP.

We have evaluated platelet function in normal Simmental cattle and in those with a congenital, inherited bleeding disorder previously attributed to impaired platelet aggregation. Affected platelets failed to aggregate and secrete in response to ADP and the ionophore A23187, and showed impaired aggregation responses to collagen and ionomycin. Aggregation and secretion of normal and affected platelets was similar in response to thrombin and PMA. Resting cytosolic calcium levels and calcium mobilization in response to ADP and ionomycin were similar in control and four affected animals. Normal and affected bovine platelets phosphorylated myosin light chain and pleckstrin in response to ADP and A23187. Transmission electron microscopy of affected platelets following stimulation with ADP, showed shape change and some degree of centralization of the actomyosin gel. Affected platelets had comparable numbers of GPIIb/IIIa complexes and expressed comparable numbers of fibrinogen receptors as normal platelets in response to ADP. Cytoskeletal assembly in affected platelets was normal in response to PMA but incomplete in response to ADP and A23187. Failure of platelet aggregation in bleeding Simmental cattle is predicted to arise from abnormal cytoskeletal assembly following calcium mobilization and phosphorylation of myosin light chain in response to ADP.