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

Mercury is a non-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 InsP₃ 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-βS, 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-InsP₃ 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.