The mammalian Na+/H+ antiporters NHE-1, NHE-2, and NHE-3 are electroneutral and voltage independent, but can couple to an H+ conductance.
Na+/H+ exchange in vertebrates is thought to be electroneutral and insensitive to the membrane voltage. This basic concept has been challenged by recent reports of antiport-associated currents in the turtle colon epithelium (Post and Dawson, 1992, 1994). To determine the electrogenicity of mammalian antiporters, we used the whole-cell patch clamp technique combined with microfluorimetric measurements of intracellular pH (pHi). In murine macrophages, which were found by RT-PCR to express the NHE-1 isoform of the antiporter, reverse (intracellular Na(+)-driven) Na+/H+ exchange caused a cytosolic acidification and activated an outward current, whereas forward (extracellular Na(+)-driven) exchange produced a cytosolic alkalinization and reduced a basal outward current. The currents mirrored the changes in pHi, were strictly dependent on the presence of a Na+ gradient and were reversibly blocked by amiloride. However, the currents were seemingly not carried by the Na+/H+ exchanger itself, but were instead due to a shift in the voltage dependence of a preexisting H+ conductance. This was supported by measurements of the reversal potential (Erev) of tail currents, which identified H+ (equivalents) as the charge carrier. During Na+/H+ exchange, Erev changed along with the measured changes in pHi (by 60-69 mV/pH). Moreover, the current and Na+/H+ exchange could be dissociated. Zn2+, which inhibits the H+ conductance, reversibly blocked the currents without altering Na+/H+ exchange. In Chinese hamster ovary (CHO) cells, which lack the H+ conductance, Na+/H+ exchange produced pHi changes that were not accompanied by transmembrane currents. Similar results were obtained in CHO cells transfected with either the NHE-1, NHE-2, or NHE-3 isoforms of the antiporter, indicating that exchange through these isoforms is electroneutral. In all the isoforms tested, the amplitude and time-course of the antiport-induced pHi changes were independent of the holding voltage. We conclude that mammalian NHE-1, NHE-2, and NHE-3 are electroneutral and voltage independent. In cells endowed with a pH-sensitive H+ conductance, such as macrophages, activation of Na(+)-H+ exchange can modulate a transmembrane H+ current. The currents reported in turtle colon might be due to a similar "cross-talk" between the antiporter and a H+ conductance.[1]References
- The mammalian Na+/H+ antiporters NHE-1, NHE-2, and NHE-3 are electroneutral and voltage independent, but can couple to an H+ conductance. Demaurex, N., Orlowski, J., Brisseau, G., Woodside, M., Grinstein, S. J. Gen. Physiol. (1995) [Pubmed]
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