Net increase of platelet membrane tyrosine specific-protein kinase activity by phorbol myristate acetate

Tyrosine protein kinase (TPK) activity in rabbit platelets after stimulation by phorbol myristate acetate (PMA) or thrombin was directly estimated by 32P incorporation from (gamma-32P)ATP (adenosine triphosphate) into synthetic peptide angiotensin II. By PMA-treatment a net increase of TPK activity was obtained, while thrombin acted on the TPK quickly but stimulation was limited within the range attained by the control after lengthy incubation. The responsive TPK to these stimulators was localized mainly in membrane but much less in cytosol. The specific activity of the particulate TPK was low in the sonicate of control ice cold platelets but increased about 6-fold when the platelets were incubated at 37 degrees C. On a brief contact of platelets with PMA at 37 degrees C the TPK was fully activated and reached a maximum value about 130% of the control. Determination of phosphotyrosine phosphatase in the stimulated platelet sonicate revealed that its participation in the above described increase of 32P-incorporation was meagre. The quick response suggested a possible role of TPK in the signal transduction through the platelet cell membrane.