ENDOTHELIN-3 REDUCES CA2+-UPTAKE AND CA2+ CONTENT OF PLATELET INTERNAL STORES

In stimulated platelets, endothelin-3 (ET-3) has been previously shown to attenuate Ca2+ mobilization. Using the calcium indicator chlortetracycline, the present study demonstrates that 0.5 mu M ET-3 produced a 24% reduction in the Ca2+ pool mobilized by A23187. ET-3 up to 1 mu M dose-dependently decreased the initial velocity and steady state value of Ca-45(2+)-uptake into platelet membrane vesicules (p<0.001). In addition, ET-3 partially reversed the inhibitory effects of half maximally effective concentrations of thapsigargin and 2,5-di-(t-butyl)-1,4-benzohydroquinone, two specific inhibitors of the sarco/endoplasmic reticulum Ca2+-ATPases. These results suggest that ET-3 is functionally coupled to Ca2+-pumps of the dense tubular system. Based on these findings, we propose that ET-3 decreases the activity of Ca2+-pumps in the dense tubular system which accumulates less Ca2+, leading to lowered Ca2+ release in response to agonists. (C) 1995 Academic Press, Inc.