Extracellular heavy-metal ions stimulate Ca2+ mobilization in hepatocytes

Populations of hepatocytes in primary culture were loaded with fura 2 and the effects of extracellular heavy-metal ions were examined under conditions that allowed changes in fura 2 fluorescence (R-340/360, the ratio of fluorescence recorded at 340 and 360 nm) to be directly attributed to changes in cytosolic free [Ca2+] ([Ca2+](i)). In Ca2+-free media, Ni2+ [EC50 (concentration causing 50% stimulation) approximate to 24 +/- 9 mu M] caused reversible increases in [Ca2+](i) that resulted from mobilization of the same intracellular Ca2+ stores as were released by [Arg(8)]vasopressin. The effects of Ni2+ were not mimicked by increasing the extracellular [Mg2+], by addition of MnCl2, CoCl2 or CdCl2, or by decreasing the extracellular pH from 7.3 to 6.0; nor were they observed in cultures of smooth muscle, endothelial cells or pituitary cells. CuCl2 (80 mu M), ZnCl2 (80 mu M) and LaCl3 (5 mM) mimicked the ability of Ni2+ to evoke Ca2+ mobilization. The response to La3+ was sustained even in the absence of extracellular Ca2+, probably because La3+ also inhibited Ca2+ extrusion. Although Ni2+ entered hepatocytes, from the extent to which it quenched fura 2 fluorescence the free cytosolic [Ni2+] ([Ni2+](i)) was estimated to be < 5 nM at the peak of the maximal Ni2+-evoked Ca2+ signals and there was no correlation between [Ni2+](i) and the amplitude of the evoked increases in [Ca2+](i). We conclude that extracellular Ni2+, Zn2+, Cu2+ and La3+, but not all heavy-metal ions, evoke an increase in [Ca2+](i) in hepatocytes by stimulating release of the hormone-sensitive intracellular Ca2+ stores and that they may do so by interacting with a specific cell-surface ion receptor. This putative ion receptor may be important in allowing hepatocytes to contribute to regulation of plasma heavy-metal ions and may mediate responses to Zn2+ released into the portal circulation with insulin.