The correlation between Ca2+ influx and inositol 1, 4, 5-trisphosphate (IP3) formation in platelets stimulated by various agonists.

The relationship between Ca2+ influx (delta [Ca2+]i) and the formation of inositol 1,4,5-trisphosphate (IP3) was investigated in human platelets stimulated by various agonists. Both delta [Ca2+]i and IP3 were increased in proportion to the amount of the agonists (thrombin, ADP, PAF, STA2), the receptors of which were demonstrated in platelets, and were correlated with each other. However, the ratio of delta [Ca2+]i to IP3 was significantly varied among agonists. Furthermore, in thrombin stimulated platelets, IP3 was small at low temperature (20 degrees C) compared with that at high temperature (37 degrees C) in spite of the similar delta [Ca2+]i. Thus, Ca2+ influx in human platelets seems to be regulated directly through the receptor operated mechanism and IP3 may not be involved in it.