Yamaguchi Soichiro, Ishikawa Toru
Laboratory of Physiology, Department of Biomedical Sciences, Graduate School of Veterinary Medicine, Hokkaido University, Sapporo 060-0818, Japan.
Biochem Biophys Res Commun. 2008 Nov 7;376(1):100-4. doi: 10.1016/j.bbrc.2008.08.104. Epub 2008 Aug 30.
NBCe1-B, a major splice variant of the electrogenic Na+--HCO3- cotransporter (NBCe1) fulfills basic cellular functions including regulation of intracellular pH and epithelial HCO3- secretion. However, its cellular regulatory mechanism still remains elusive. Here, we provide evidence for the first time that NBCe1-B activity can be controlled by intracellular Mg2+ (Mg2+(i)), the physiologically most abundant intracellular divalent cation. Using the whole-cell patch-clamp technique, we found that recombinant NBCe1-B currents expressed in HEK293 and NIH3T3 cells were inhibited voltage-independently by Mg2+(i) in a concentration-dependent manner (K(i) approximately 0.01 mM). The Mg2+(i) inhibition was partially relieved by truncation of the NBCe1-B specific N-terminal region (K(i) approximately 0.3 mM), and was also observed for native electrogenic Na+--HCO3- cotransporter current in bovine parotid acinar cells that endogenously express NBCe1-B (K(i) approximately 1 mM). These results suggest that Mg2+ may be a cytosolic factor that limits intrinsic cotransport activity of NBCe1-B in mammalian cells.
NBCe1-B是电中性Na⁺-HCO₃⁻共转运体（NBCe1）的一种主要剪接变体，它执行包括调节细胞内pH值和上皮细胞HCO₃⁻分泌在内的基本细胞功能。然而，其细胞调节机制仍然不清楚。在这里，我们首次提供证据表明，NBCe1-B的活性可以受细胞内Mg²⁺（Mg²⁺(i)）调控，Mg²⁺(i)是细胞内生理上最丰富的二价阳离子。使用全细胞膜片钳技术，我们发现，在HEK293和NIH3T3细胞中表达的重组NBCe1-B电流被Mg²⁺(i)以浓度依赖性方式非电压依赖性抑制（抑制常数K(i)约为0.01 mM）。通过截短NBCe1-B特定的N端区域，Mg²⁺(i)的抑制作用部分得到缓解（K(i)约为0.3 mM），并且在天然表达NBCe1-B的牛腮腺腺泡细胞中的电中性Na⁺-HCO₃⁻共转运体电流中也观察到这种抑制作用（K(i)约为1 mM）。这些结果表明，Mg²⁺可能是一种限制哺乳动物细胞中NBCe1-B固有共转运活性的胞质因子。
