Hg2+ signaling in trout hepatoma (RTH-149) cells:: involvement of Ca2+-induced Ca2+ release

Mercury is anon-essential heavy metal affecting intracellular Ca2+ dynamics. We studied the effects of Hg2+ on [Ca2+](i) in trout hepatoma cells (RTH-149). Confocal imaging of fluo-3-loaded cells showed that Hg2+ induced dose-dependent, sustained [Ca2+](i) transient, triggered intracellular Ca2+ waves, stimulated Ca2+-ATPase activity, and promoted InsP(3) production. The effect of Hg2+ was reduced by the Ca2+ channel blocker verapamil and totally abolished by extracellular GSH, but was almost unaffected by cell loading with the heavy metal chelator TPEN or esterified GSH. In a Ca2+-free medium, Hg2+ induced a smaller [Ca2+](i) transient, that was unaffected by TPEN, but was abolished by U73122, a PLC inhibitor, and by cell loading with GDP-betaS, a G protein inhibitor, or heparin, a blocker of intracellular Ca2+ release. Data indicate that Hg2+ induces Ca2+ entry through verapamil-sensitive channels, and intracellular Ca2+ release via a G protein-PLC-InsP(3) mechanism. However, in cells loaded with heparin and exposed to Hg2+ in the presence of external Ca2+, the [Ca2+](i) rise was maximally reduced, indicating that the global effect of Hg2+ is not a mere sum of Ca2+ entry plus Ca2+ release, but involves an amplification of Ca2+ release operated by Ca2+ entry through a CICR mechanism. (C) 2003 Elsevier Ltd. All rights reserved.