Wright T L, Fitz J G, Weisiger R A
Department of Medicine, University of California, San Francisco 94143.
J Biol Chem. 1988 Feb 5;263(4):1842-7.
Non-transferrin-bound iron is efficiently cleared from serum by the liver and may be primarily responsible for the hepatic damage seen in iron-overload states. We tested the hypothesis that transport of ionic iron is driven by the negative electrical potential difference across the liver cell membrane. Extraction of 55Fe-labeled ferrous iron (1 microM) from Krebs bicarbonate buffer by the perfused rat liver was continuously monitored as the transmembrane potential difference (measured using conventional microelectrodes) was altered over the physiologic range by isosmotic ion substitution. Resting membrane potential in Krebs bicarbonate buffer was -28 +/- 1 mV. Perfusion with 1 microM ferrous iron caused a reversible 3 +/- 1 mV depolarization, and higher concentrations of iron caused even greater depolarization. Conversely, depolarization of the liver cells consistently reduced iron extraction. Replacement of sodium with potassium (70 mM) or choline (131 mM) depolarized the hepatocytes to -15 and -20 mV and decreased iron extraction by 28 and 31%, respectively. Perfusion with bicarbonate-free solutions containing tricine buffer (10 mM) reduced the membrane potential to -23 mV and reduced iron extraction by 18%. In contrast, the high basal extraction of iron (91.1 +/- 1.4%) was not further increased by substitution of nitrate for chloride (-46 mV) or infusion of glucagon (-34 mV). All effects were reversible, suggesting that perfusion with 1 microM iron produced little toxicity. These findings are consistent with an electrogenic transport mechanism for uptake of non-transferrin-bound iron that is driven by the transmembrane potential difference.
非转铁蛋白结合铁可被肝脏有效地从血清中清除，并且可能是铁过载状态下所见肝损伤的主要原因。我们验证了以下假说：离子铁的转运是由肝细胞跨膜电位差驱动的。当通过等渗离子置换在生理范围内改变跨膜电位差（使用传统微电极测量）时，持续监测灌注大鼠肝脏从 Krebs 碳酸氢盐缓冲液中提取 55Fe 标记的亚铁离子（1 microM）的情况。Krebs 碳酸氢盐缓冲液中的静息膜电位为 -28 ± 1 mV。用 1 microM 亚铁离子灌注导致可逆的 3 ± 1 mV 去极化，更高浓度的铁导致更大程度的去极化。相反，肝细胞的去极化始终会降低铁的提取。用钾（70 mM）或胆碱（131 mM）替代钠会使肝细胞去极化至 -15 mV 和 -20 mV，并分别使铁提取减少 28% 和 31%。用含有三羟甲基氨基甲烷缓冲液（10 mM）的无碳酸氢盐溶液灌注可将膜电位降低至 -23 mV，并使铁提取减少 18%。相比之下，用硝酸盐替代氯化物（-46 mV）或注入胰高血糖素（-34 mV）并不会进一步提高铁的高基础提取率（91.1 ± 1.4%）。所有效应都是可逆的，这表明用 1 microM 铁灌注产生的毒性很小。这些发现与由跨膜电位差驱动的非转铁蛋白结合铁摄取的电转运机制一致。
