Regulation of Na/H excharger activity

Na/H交换器(NHE)中存在多种异构体。一般认为，耐阿米洛利抑制的NHE-3位于上皮细胞的顶膜，而对阿米洛利抑制敏感的NHE-1位于基底膜。我们研究了培养的肾上皮细胞(OK细胞和LLC-PK1细胞)中NHE活性及其调节。我们发现，在OK细胞中，除了已知的顶膜NHE活性外，NHE活性还存在于基底膜。由于两种膜上的NHE对阿米洛利具有抗药性，并且其最大活性被甲状旁腺激素抑制，因此两种膜上的NHE似乎都是NHE-3。因此，在OK细胞中，NHE-3不仅在顶膜上表达，在基底膜上也有表达。NHE基侧膜与顶膜对胞浆[H^+]的亲和力不同。因此，NHE-3的特性可能会因NHE-3所在的一侧而异。由于转染人NHE-1基因的OK细胞仅在基底膜表达NHE-1活性，NHE-1在上皮细胞中可能具有靶向基底膜的内在信号，LLC-PK1细胞顶膜中的NHE活性对阿米洛利比典型的NHE-3更敏感。为了弄清NHE-1活性是否可能在顶膜和基侧膜中表达，我们检测了NHE-1反义寡核苷酸的作用。由于反义寡核苷酸只抑制LLC-PK1细胞基底侧膜的NHE活性，因此NHE-1mRNA只编码基底侧膜上的NHE。

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.