REGULATION OF CYTOSOLIC CALCIUM BY COLLAGEN IN SINGLE HUMAN PLATELETS

1 There is controversy in the literature as to whether collagen is able to induce directly a rise in cytosolic calcium concentration ([Ca2+](i)) in human platelets. We have addressed this question by observing the cytosolic calcium response of single fura-2-loaded human platelets settling onto a collagen-coated surface using dynamic fluorescence ratio imaging. 2 Following a short lag phase after adherence to collagen fibres, platelets underwent a rapid rise in cytosolic calcium from basal values of 80 +/- 13 nM (n = 24) to a peak of 475 +/- 42 nM (n = 24) which was sustained for the remaining period of the experiment. 3 The tyrphostin protein tyrosine kinase inhibitor, ST271, reduced substantially the proportion of platelets which exhibited a rise in [Ca2+](i) on adherence to collagen and transformed the response in remaining cells to one of oscillations. 4 In contrast, and as a control for collagen, laminin-coated surfaces induced adherence of human platelets without elevating intracellular [Ca2+]; the cells however remained responsive to ADP. 5 We conclude that collagen directly induces a rise in cytosolic calcium in single human platelets through a tyrosine kinase-mediated pathway.