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 (R340/360, 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) approximately 24+/-9 microM] caused reversible increases in [Ca2+]i that resulted from mobilization of the same intracellular Ca2+ stores as were released by [Arg8]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 microM), ZnCl2 (80 microM) 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.
McNulty
T J
TJ
Department of Pharmacology, University of Cambridge, Tennis Court Road, Cambridge CB2 1QJ, UK.
Taylor
C W
CW
D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D001127:Arginine Vasopressin / Q000494:pharmacology; D019256:Cadmium Chloride / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002460:Cell Line; D002478:Cells, Cultured; D002712:Chlorides / Q000494:pharmacology; D003035:Cobalt / Q000494:pharmacology; D003300:Copper / Q000494:pharmacology; D005029:Ethylenediamines / Q000494:pharmacology; D005453:Fluorescence; D016257:Fura-2 / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007811:Lanthanum / Q000494:pharmacology; D008099:Liver / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D008274:Magnesium / Q000378:metabolism; D008297:Male; D017895:Manganese Compounds / Q000494:pharmacology; D019216:Metals, Heavy / Q000378:metabolism / Q000494:pharmacology*; D009532:Nickel / Q000378:metabolism / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D017967:Zinc Compounds / Q000494:pharmacology
D002712:Chlorides; D005029:Ethylenediamines; D017895:Manganese Compounds; D019216:Metals, Heavy; D017967:Zinc Compounds; C028521:lanthanum chloride; D001127:Arginine Vasopressin; D003035:Cobalt; C022838:nickel chloride; D007811:Lanthanum; D003300:Copper; D009532:Nickel; C016837:zinc chloride; D000255:Adenosine Triphosphate; C018021:cobaltous chloride; D008274:Magnesium; D019256:Cadmium Chloride; C025340:manganese chloride; C044387:N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine; D002118:Calcium; D016257:Fura-2