THE EFFECT OF CALCIUM-STORE DEPLETION AND REFILLING WITH VARIOUS BIVALENT-CATIONS ON TYROSINE PHOSPHORYLATION AND MN2-2-LOADED HUMAN PLATELETS( ENTRY IN FURA)

To investigate the possible involvement of tyrosine phosphorylation in the process of store-regulated Ca2+ entry, ionomycin (in the presence of EGTA) was used to deplete the intracellular Ca2+ stores of fura-2- loaded human platelets, and the effect of refilling with Ca2+, Ba2+ or Sr2+ evaluated. Depletion of the intracellular Ca2+ stores resulted i n an increase in protein tyrosine phosphorylation. This increase is re versed when the stores were refilled in Ca2+ or Sr2+, but not Ba2+. Re filling of the stores with Ca2+ or Sr2+, but not Ba2+, suppressed Mn2 entry. These findings support the hypothesis that tyrosine phosphoryl ation plays a role in mediating store-regulated Ca2+ entry in human pl atelets and provides evidence for tyrosine phosphatase activity regula ted by the Ca2+ content of the intracellular stores.