The liver cell plasma membrane Ca2+ inflow systems exhibit a broad specificity for divalent metal ions.
1. The inflow of Mn2+ across the plasma membranes of isolated hepatocytes was monitored by measuring the quenching of the fluorescence of intracellular quin2, by atomic absorption spectroscopy and by the uptake of 54Mn2+. The inflow of other divalent metal ions was measured using quin2. 2. Under ionic conditions which resembled those present in the cytoplasmic space, Mn2+, Zn2+, Co2+, Ni2+ and Cd2+ each quenched the fluorescence of a solution of Ca2(+)-quin2. 3. The addition of Mn2+, Zn2+, Co2+, Ni2+ or Cd2+ to cells loaded with quin2 caused a time-dependent decrease in the fluorescence of intracellular quin2. Plots of the rate of decrease in fluorescence as a function of the concentration of Mn2+ reached a plateau at 100 microM-Mn2+. 4. The rate of decrease in fluorescence induced by Mn2+ was stimulated by 20% in the presence of vasopressin. The effect of vasopressin was completely inhibited by 200 microM-verapamil. Adrenaline, angiotensin II and glucagon also stimulated the rate of decrease in the fluorescence of intracellular quin2 induced by Mn2+. 5. The rate of decrease in fluorescence induced by Zn2+, Co2+, Ni2+ or Cd2+ was stimulated by between 20 and 190% in the presence of vasopressin or angiotensin II. 6. The rates of uptake of Mn2+ measured by atomic absorption spectroscopy or by using 54Mn2+ were inhibited by about 20% by 1.3 mM-Ca2+o and stimulated by 30% by vasopressin. 7. Plots of Mn2+ uptake, measured by atomic absorption spectroscopy or with 54Mn2+, as a function of the extracellular concentration of Mn2+ were biphasic over the range 0.05-1.0 mM added Mn2+ and did not reach a plateau at 1.0 mM-Mn2+. 8. It is concluded that (i) hepatocytes possess both a basal and a receptor-activated divalent cation inflow system, each of which has a broad specificity for metal ions, and (ii) the receptor-activated divalent cation inflow system is the receptor-operated Ca2+ channel.
Crofts
J N
JN
Department of Medical Biochemistry, Flinders University School of Medicine, Flinders Medical Centre, Bedford Park, Australia.
Barritt
G J
GJ
D000634:Aminoquinolines / Q000378:metabolism; D000818:Animals; D002104:Cadmium / Q000493:pharmacokinetics / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism*; D002413:Cations, Divalent / Q000493:pharmacokinetics*; D002462:Cell Membrane / Q000378:metabolism / Q000648:ultrastructure; D003035:Cobalt / Q000493:pharmacokinetics / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D005453:Fluorescence; D008099:Liver / Q000166:cytology / Q000378:metabolism* / Q000648:ultrastructure; D008297:Male; D008345:Manganese / Q000493:pharmacokinetics / Q000494:pharmacology; D008670:Metals / Q000493:pharmacokinetics*; D051381:Rats; D011919:Rats, Inbred Strains; D013054:Spectrophotometry, Atomic; D013997:Time Factors; D015032:Zinc / Q000493:pharmacokinetics / Q000494:pharmacology
D000634:Aminoquinolines; D015220:Calcium Channels; D002413:Cations, Divalent; D008670:Metals; D002104:Cadmium; D003035:Cobalt; D008345:Manganese; D015032:Zinc; C035739:Quin2; D002118:Calcium