Extracellular heavy-metal ions stimulate Ca2+ mobilization in hepatocytes

Populations of hepatocytes in primary culture were loaded with fura 2 and t he effects of extracellular heavy-metal ions were examined under conditions that allowed changes in fura 2 fluorescence (R-340/360, the ratio of fluor escence recorded at 340 and 360 nm) to be directly attributed to changes in cytosolic free [Ca2+] ([Ca2+](i)). In Ca2+-free media, Ni2+ [EC50 (concent ration causing 50% stimulation) approximate to 24 +/- 9 mu M] caused revers ible increases in [Ca2+](i) that resulted from mobilization of the same int racellular Ca2+ stores as were released by [Arg(8)]vasopressin. The effects of Ni2+ were not mimicked by increasing the extracellular [Mg2+], by addit ion 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 L a3+ was sustained even in the absence of extracellular Ca2+, probably becau se La3+ also inhibited Ca2+ extrusion. Although Ni2+ entered hepatocytes, f rom 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 Ni 2+-evoked Ca2+ signals and there was no correlation between [Ni2+](i) and t he amplitude of the evoked increases in [Ca2+](i). We conclude that extrace llular Ni2+, Zn2+, Cu2+ and La3+, but not all heavy-metal ions, evoke an in crease in [Ca2+](i) in hepatocytes by stimulating release of the hormone-se nsitive intracellular Ca2+ stores and that they may do so by interacting wi th a specific cell-surface ion receptor. This putative ion receptor may be important in allowing hepatocytes to contribute to regulation of plasma hea vy-metal ions and may mediate responses to Zn2+ released into the portal ci rculation with insulin.