Gonzalez-Begne Mireya, Nakamoto Tetsuji, Nguyen Ha-Van, Stewart Andrew K, Alper Seth L, Melvin James E
Center for Oral Biology, University of Rochester Medical Center, Rochester, New York 14642, USA.
J Biol Chem. 2007 Nov 30;282(48):35125-32. doi: 10.1074/jbc.M707266200. Epub 2007 Sep 21.
Cl(-) influx across the basolateral membrane is a limiting step in fluid production in exocrine cells and often involves functionally linked Cl(-)/HCO(3)(-) (Ae) and Na(+)/H(+) (Nhe) exchange mechanisms. The dependence of this major Cl(-) uptake pathway on Na(+)/H(+) exchanger expression was examined in the parotid acinar cells of Nhe1(-/-) and Nhe2(-/-) mice, both of which exhibited impaired fluid secretion. No change in Cl(-)/HCO(3)(-) exchanger activity was detected in Nhe2-deficient mice. Conversely, Cl(-)/HCO(3)(-) exchanger activity increased nearly 4-fold in Nhe1-deficient mice, despite only minimal or any change in mRNA and protein levels of the anion exchanger Ae2. Acetazolamide completely blocked the increase in Cl(-)/HCO(3)(-) exchanger activity in Nhe1-null mice suggesting that increased anion exchange required carbonic anhydrase activity. Indeed, the parotid glands of Nhe1(-/-) mice expressed higher levels of carbonic anhydrase 2 (Car2) polypeptide. Moreover, the enhanced Cl(-)/HCO(3)(-) exchange activity was accompanied by an increased abundance of Car2.Ae2 complexes in the parotid plasma membranes of Nhe1(-/-) mice. Anion exchanger activity was also significantly reduced in Car2-deficient mice, consistent with an important role of a putative Car2.Ae2 HCO(3)(-) transport metabolon in parotid exocrine cell function. Increased abundance of this HCO(3)(-) transport metabolon is likely one of the multiple compensatory changes in the exocrine parotid gland of Nhe1(-/-) mice that together attenuate the severity of in vivo electrolyte and acid-base balance perturbations.
氯离子（Cl⁻）跨基底外侧膜的内流是外分泌细胞中液体分泌的一个限制步骤，通常涉及功能上相关联的Cl⁻/HCO₃⁻（Ae）和Na⁺/H⁺（Nhe）交换机制。在Nhe1⁻/⁻和Nhe2⁻/⁻小鼠的腮腺腺泡细胞中研究了这条主要的Cl⁻摄取途径对Na⁺/H⁺交换体表达的依赖性，这两种小鼠均表现出液体分泌受损。在Nhe2缺陷小鼠中未检测到Cl⁻/HCO₃⁻交换体活性的变化。相反，在Nhe1缺陷小鼠中，尽管阴离子交换体Ae2的mRNA和蛋白质水平仅有微小变化或无变化，但Cl⁻/HCO₃⁻交换体活性增加了近4倍。乙酰唑胺完全阻断了Nhe1基因敲除小鼠中Cl⁻/HCO₃⁻交换体活性的增加，这表明增加的阴离子交换需要碳酸酐酶活性。事实上，Nhe1⁻/⁻小鼠的腮腺中碳酸酐酶2（Car2）多肽的表达水平更高。此外，增强的Cl⁻/HCO₃⁻交换活性伴随着Nhe1⁻/⁻小鼠腮腺质膜中Car2.Ae2复合物丰度的增加。在Car2缺陷小鼠中阴离子交换体活性也显著降低，这与假定的Car2.Ae2 HCO₃⁻转运代谢体在腮腺外分泌细胞功能中的重要作用一致。这种HCO₃⁻转运代谢体丰度的增加可能是Nhe1⁻/⁻小鼠腮腺外分泌腺中多种代偿性变化之一，这些变化共同减轻了体内电解质和酸碱平衡紊乱的严重程度。
