EFFECT OF INTRACELLULAR FREE CALCIUM MOBILIZATION ON AGGREGATION OF UMBILICAL-CORD BLOOD-PLATELETS

Aggregation activity of platelets in umbilical blood is lower than tha t in adult blood, but the reason for this is not well understood. It h as recently been clarified that calcium plays a role as a second messe nger of platelet aggregation, and that glycoproteins of platelet surfa ce membrane such as glycoprotein Ib and IIb/IIIa are receptors for ago nists inducing aggregation. We examined the concentrations of intracel lular calcium and the membrane glycoproteins of platelets in umbilical and adult blood. The increase of intracellular calcium in umbilical p latelets was lower than that in adult platelets when the aggregation w as induced by ADP, collagen, thrombin and epinephrine. Only calcium io nophore A23187 induced aggregation of both umbilical and adult platele ts. On the other hand, there were no qualitative differences between g lycoproteins I b and IIb/IIIa of these two groups. Therefore, the low aggregation activity of umbilical platelets seems to be due to low res ponsiveness of the intracellular calcium system, not to the disorder o f functional surface membrane glycoprotein.