CALCIUM-ACTIVATED SODIUM AND CHLORIDE FLUXES MODULATE PLATELET VOLUME - ROLE OF CA2+ STORES

An increase in cytosolic ionized Ca2+ concentration ([Ca2+](i)) initiates volume changes in various types of cells. In response to increases in [Ca2+](i) most cell types contract by efflux of K+ and Cl-, whereas platelets expand in response to rises in [Ca2+](i). This study examined the importance of the source of Ca2+, the flux of ions responsible for the volume change, and the role of Ca2+-dependent protein kinases in regulating these ionic fluxes. The baseline platelet volume was independent of extracellular Ca2+ but when stimulated by the Ca2+ ionophore A-23187 (50 nM) the volume increased in both the presence and absence of extracellular Ca2+ (1.18 +/- 0.08 vs. 0.83 +/- 0.06 fl, respectively). The increased volume was caused by the gain of Na+ and Cl-. Na+ entered through both conductive and nonconductive (Na+/H+ exchange) pathways, whereas the influx of Cl- was conductive and inhibited by the Cl- channel blocker 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid. The Ca2+-induced volume change was blocked by both calmodulin and protein kinase inhibitors. Thus the activation of Ca2+-dependent protein kinases promotes platelet swelling by stimulating Na+ and Cl- influx.