Na/H exchange activities in NHE1-transfected OK-cells : cell polarity and regulation

The human fibroblast, "amiloride-sensitive" Na/H exchanger (NHE1) was transfected into opossum kidney cells (OK cells) (OK/NHE1 cells). Northern blot analysis confirmed that the NHE1 message is expressed in OK/NHE1 cells. In immunoblot analysis, an anti-human NHE1 antibody labelled a membrane protein only present in OK/NHE1 cells. In contrast to the parental cell line containing only an apically located, "amiloride-resistant" Na/H exchange activity, OK/NHE1 cells contain apically and basolaterally located Na/H exchange activities, the apical activity being "amiloride resistant" and the basolateral being "amiloride sensitive". Parathyroid hormone (PTH) inhibited apical transport activity (OK and OK/NHE1 cells) but had no effect on basolateral transport activity (OK/NHE1 cells). Pharmacological activation of protein kinase A (forskolin) decreased both apical and basolateral Na/H exchange activity. Incubation with phorbol ester (exogenous activation of protein kinase C) reduced apical Na/H exchange activity (OK and OK/NHE1 cells) but had only a moderate, inhibitory effect on basolateral Na/H exchange activity (OK/NHE1 cells). These results indicate that transfection of OK cells with human fibroblast NHE1 cDNA encoding an "amiloride-sensitive" form of the Na/H exchanger results in expression of basolaterally located "NHE1-related" transport activity. Regulatory control of intracellular Na/H exchange activities (apically versus basolaterally located) and intercellular Na/H exchange activities (NHE1-related) differs. This may relate to cell-specific properties as well as to exchanger-specific properties.