THE INTRACELLULAR CALCIUM STORE FUNCTION OF PLATELET GRANULES

The atrial specific granule has been described as an intracellular Ca store (ICaS). It is reasonably proposed that other polypeptide-secreto ry granules could also play the function of ICaS. In the present work, high Ca concentration in the dense granule (dG) and alpha granule (al pha G) of platelets was confirmed by using x-ray microanalysis in both ultrathin cryosections and ethanolic phosphotungstic acid staining se ctions, and Ca2+-ATPase activities were found on the membrane of both the granules. Furthermore, when platelets were activated by A 23187 1 mM, thrombin 2 U/ml or ADP 200 mu M, both the Ca content and number of the granules decreased, which means that the Ca2+ was released and se cretion of the granules increased. It is suggested that the release Ca 2+ raise the cytosolic Ca2+ level and, thus, the secretion of the gran ules is promoted. in addition, it was described that the ultrastructur e of the platelet granules in ultrathin cryosections was somewhat diff erent from that in routine sections.