PLATELETS FROM BLEEDING SIMMENTAL CATTLE MOBILIZE CALCIUM, PHOSPHORYLATE MYOSIN LIGHT-CHAIN AND BIND NORMAL NUMBERS OF FIBRINOGEN MOLECULESBUT 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 attrib uted to impaired platelet aggregation. Affected platelets failed to ag gregate and secrete in response to ADP and the ionophore A23187, and s howed impaired aggregation responses to collagen and ionomycin. Aggreg ation and secretion of normal and affected platelets was similar in re sponse to thrombin and PMA. Resting cytosolic calcium levels and calci um mobilization in response to ADP and ionomycin were similar in contr ol and four affected animals. Normal and affected bovine platelets pho sphorylated myosin light chain and pleckstrin in response to ADP and A 23187. Transmission electron microscopy of affected platelets followin g stimulation with ADP, showed shape change and some degree of central ization of the actomyosin gel. Affected platelets had comparable numbe rs of GPII/IIIa complexes and expressed comparable numbers of fibrinog en receptors as normal platelets in response to ADP. Cytoskeletal asse mbly in affected platelets was normal in response to PMA but incomplet e in response to ADP and A23187. Failure of platelet aggregation in bl eeding Simmental cattle is predicted to arise from abnormal cytoskelet al assembly following calcium mobilization and phosphorylation of myos in light chain in response to ADP.