Abstract
Using aequorin-loaded rat platelets stimulated with collagen, we found two phases of Ca2+ mobilization, one coinciding with a shape change and the other with aggregation, which have not yet been detected in quin2-loaded platelets. U46619, a stable analogue of prostaglandin H2, induced only a shape change and a concomitant rapid rise in the cytoplasmic ionized calcium concentration ([Ca12+]). However, upon addition of U46619 to platelets previously stimulated with collagen in the presence of indomethacin, a rapid increase in [Ca12+] and a shape change occurred, and, after about 1min, second increase in [Ca12+] and aggregation occurred. The actions of U46619 were inhibited by an antagonist for the thromboxane A2 (TXA2) receptor. These results suggest that the collagen-induced shape change is initiated by TXA2-induced Ca2+ mobilization, and aggregation is induced by the secondary Ca2+ mobi-lization induced by TXA2 and the occupation of the receptor by collagen.
