EARLY (< 5 SEC) PHOSPHORYLATIONS OF PLATELET PROTEINS FOLLOWING ACTIVATION BY ADP AND ADRENALIN, SEPARATELY AND IN COMBINATION
Understanding the earliest events (< 1 sec) in signal transduction of platelets is important, since there is evicenee that “shape change,” aggregation and secretion can all begin within this period. We have employed a guenched-flow approach to study these early events and found that thrombin can induce rapid phosphorylation of myosin light-chain kinase (20K) and a 47K protein (Blood, 67, 1738, 1986). To investigate the role of rapid phosphorylations in platelet activation, we have studied the influence of adrenalin and ADP during early (0.3 to 5 sec) stimulation. Aggregation in washed human platelets was assessed by following the loss of single particles and phosphorylation by analysing 32P-labeled proteins after gel electrophoresis. 15 µM adrenalin (without ADP) did not initiate significant aggregation or phosphorylation of myosin light chain (MLC). Phosphorylation of the 47K protein was increased by 20% at 5 sec. 0.5 µM ADP did not induce significant aggregation, but increased phosphorylation of MLC by 130% and the 47 protein by 20%. The combination of 0.5 µM ADP and 15 uM adrenalin induced significant aggregation by 0,3 sec (7.6%), which increased to 25.6% by 5 sec. Interestingly, MLC or 47K protein phosphorylation was not increased above control levels. However, the phosphorylation of four other proteins (77K, 102K, 140K and 185K), which previously had been very rapid (<1 sec) and reversible with 0.5 µM ADP alone, was now maintained, peaking at 3 sec. 10 µM ADP caused small sustained increases in phosphorylation of the same proteins. Adrenalin also caused rapid increases in the phosphorylation of 27K, 213 and 250K proteins. High levels of ADP (10 µM) only increased the 213 and 250K proteins; therefore the 27K protein appears adrenalin specific. Analysis of these early platelet phosphorylations will help understand how they are linked to initiation and maintenance of aggregation. Supported by NIH HL-27014.