CALCIUM MOBILIZATION CONTROLS TYROSINE PROTEIN-PHOSPHORYLATION INDEPENDENTLY OF THE ACTIVATION OF PROTEIN-KINASE-C IN HUMAN PLATELETS

We have investigated the regulation of tyrosine proteins phosphorylati on by intracellular Ca2+ level ([Ca2+](i)) and protein kinase C (PKC) during platelet stimulation. We found that chelation of extracellular calcium completely prevented phosphorylation of tyrosine proteins indu ced by thapsigargin and phorbol 12-myristate 13-acetate (PMA), whereas , when induced by thrombin, it prevented a subset of tyrosine proteins . The selective inhibition of PKC by GF 109203X did not abolish tyrosi ne protein phosphorylation when induced by thrombin and thapsigargin. The results suggest that in human platelets tyrosine protein phosphory lation is dependent on [Ca2+](i), although direct PKC activation can a lso induce phosphorylation of tyrosine proteins.