Regulation of Na/H excharger activity

Na/H交换剂（NHE）中存在几种异构体。一般认为，在上皮细胞中，对阿米洛利抑制具有抗性的NHE-3位于顶膜，而对阿米洛利抑制敏感的NHE-1位于基底外侧膜。我们在培养的肾上皮细胞（OK细胞和LLC-PK 1细胞）中表征了NHE活性及其调节。我们在OK细胞中除了已知的顶膜中的NHE活性之外，还鉴定了基底外侧膜中的NHE活性。由于两种膜中的NHE对阿米洛利具有抗性，并且其最大活性被甲状旁腺激素抑制，因此两种膜中的NHE似乎是NHE-3。因此，在OK细胞中，NHE-3不仅可以在顶膜中表达，而且可以在基底侧膜中表达。基底膜和顶膜中NHE对胞浆[H^+]的亲和力不同。因此，NHE-3的特性可以根据NHE-3所在的一侧而变化。由于转染人NHE-1 cDNA的OK细胞仅在基底侧膜表达NHE-1活性，因此NHE-1可能具有内在信号以靶向上皮细胞的基底侧膜。LLC-PK 1细胞的顶膜NHE活性对阿米洛利比典型的NHE-3更敏感。为了阐明NHE-1活性是否可能在顶膜以及基底外侧膜中表达，我们检测了NHE-1反义寡核苷酸的作用。由于用反义寡核苷酸转染仅抑制LLC-PK 1细胞中的基底外侧NHE活性，因此仅基底外侧膜中的NHE由NHE-1 mRNA编码。

Several isoforms exist in Na/H exchangers (NHE) . It is generally believed that NHE-3, which is resistant to amiloride inhibition, is located in the apical membrane and NHE-1, which is sensitive to amiloride inhibition, is located in the basolateral membrane in the epithelial cells. We characterized NHE activity and its modulation in the cultured renal epithelial cells (OK cells and LLC-PK1 cells) .We identified NHE activity in the basolateral membrane in addition to well known NHE activity in the apical membrane in OK cells. Since NHE in the both membranes is resistant to amiloride and its maximum activity is inhibited by parathyroid hormone, NHE in the both membranes seems to be NHE-3. Thus, NHE-3 can be expressed not only in the apical membrane but also in the basolateral membrane in OK cells. The affinity for cytosolic [H^+] of NHE in the basolateral membrane is different from that in the apical membrane. Thus, the character of NHE-3 may vary depending on the side where NHE-3 is located. Since OK cells transfected with human NHE-1 cDNA expressed NHE-1 activity only in the basolateral membrane, NHE-1 may have intrinsic signal to target itself to the basolateral membrane in the epithelial cells.NHE activity in the apical membrane of LLC-PK1 cells is more sensitive to amiloride than typical NHE-3. To clarify whether NHE-1 activity might be expressed in the apical membrane as well as in the basolateral membrane, we examined the effects of NHE-1 antisense oligonucleotide. Since transfection with antisense oligonucleotide inhibited only basolateral NHE activity in LLC-PK1 cells, NHE in the only basolateral membrane is encoded by NHE-1 mRNA.