Department of Physiology, Emory University School of Medicine, Atlanta, GA 30322, USA.
Exp Physiol. 2011 Aug;96(8):745-55. doi: 10.1113/expphysiol.2011.057844. Epub 2011 May 13.
The electroneutral Na(+)-HCO(3)(-) cotransporter NBCn1 (SLC4A7) contributes to intracellular pH maintenance and transepithelial HCO(3)(-) movement. In this study, we expressed NBCn1 in Xenopus oocytes and examined the effect of NBCn1 on oocyte NH(4)(+) transport by analysing changes in membrane potential, current and intracellular pH mediated by NH(4)Cl. In the presence of HCO(3)(-)/CO(2), applying NH(4)Cl (20 mm) produced intracellular acidification of oocytes. The acidification was faster in oocytes expressing NBCn1 than in control oocytes injected with water; however, NH(4)Cl-mediated membrane depolarization was smaller in oocytes expressing NBCn1. In HCO(3)(-)/CO(2)-free solution, NH(4)Cl produced a smaller inward current in NBCn1-expressing oocytes (56% inhibition by 20 mm NH(4)Cl, measured at --60 mV), while minimally affecting intracellular acidification. The inhibition of the current by NBCn1 was unaffected when BaCl(2) replaced KCl. Current-voltage relationships showed a positive and nearly linear relationship between NH(4)Cl-mediated current and voltage, which was markedly reduced by NBCn1. Large basal currents (before NH(4)Cl exposure) were produced in NBCn1-expressing oocytes owing to the previously characterized channel-like activity of NBCn1. Inhibiting this channel-like activity by Na(+) removal abolished the inhibitory effect of NBCn1 on NH(4)Cl-mediated currents. The currents were progressively reduced over 72-120 h after NBCn1 cRNA injection, during which the channel-like activity was high. These results indicate that NBCn1 stimulates NH(4)(+) transport by its Na(+)-HCO(3)(-) cotransport activity, while reducing NH(4)(+) conductance by its channel-like activity.
电中性的 Na(+)-HCO(3)(-)共转运蛋白 NBCn1（SLC4A7）有助于维持细胞内 pH 值和跨上皮 HCO(3)(-)转运。在这项研究中，我们在非洲爪蟾卵母细胞中表达了 NBCn1，并通过分析 NH(4)Cl 介导的膜电位、电流和细胞内 pH 变化来研究 NBCn1 对卵母细胞 NH(4)(+)转运的影响。在 HCO(3)(-)/CO(2)存在的情况下，施加 NH(4)Cl（20mm）会导致卵母细胞内酸化。与对照卵母细胞（水注射）相比，表达 NBCn1 的卵母细胞中的酸化更快；然而，表达 NBCn1 的卵母细胞中 NH(4)Cl 介导的膜去极化更小。在 HCO(3)(-)/CO(2) 无溶液中，NH(4)Cl 在表达 NBCn1 的卵母细胞中产生较小的内向电流（用 20mm NH(4)Cl 测量时抑制 56%，在 --60 mV 时），而对内酸化的影响最小。用 BaCl(2) 代替 KCl 时，NBCn1 对电流的抑制作用不受影响。电流-电压关系显示，NH(4)Cl 介导的电流与电压之间呈正相关且近乎线性，NBCn1 显著降低了这种相关性。由于 NBCn1 先前表现出的通道样活性，表达 NBCn1 的卵母细胞中产生了较大的基础电流（在暴露于 NH(4)Cl 之前）。通过去除 Na(+) 抑制这种通道样活性消除了 NBCn1 对 NH(4)Cl 介导电流的抑制作用。在 NBCn1 cRNA 注射后 72-120 小时期间，电流逐渐减少，在此期间通道样活性很高。这些结果表明，NBCn1 通过其 Na(+)-HCO(3)(-)共转运活性刺激 NH(4)(+) 转运，同时通过其通道样活性降低 NH(4)(+) 电导。
