Δψ‐dependent gating of Na+/H+ exchange in Halobacterium halobium: a Δg̃mH+‐driven Na+ pump

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Abstract
Na+/H+ antiporter‐mediated 22Na+ transport was studied in envelope vesicles from Halobacterium halobium by manipulating the size of each Δg̃m+ component, ΔpH and Δψ, in the dark. Neither inside alkaline ΔpH nor outwardly directed ΔpNa+, nor a combination could facilitate 22Na+ extrusion from the vesicles. Likewise, Δψ up to 144 mV (inside negative) was not capable of initiating 22Na+ extrusion unless ΔpH existed. This extrusion was facilitated only when approx. 100 mV Δψ (gating potential) was superimposed on ΔpH (either 1 or 2). On the other hand, no uptake of 22Na+ took place even when both inside acidic ΔpH and inwardly directed Na+ gradient were imposed with or without Δψ. Under these conditions, monensin mediated the rapid uptake of 22Na+. The present results indicate that halobacterial Na+/H+ exchange is regulated not only by a Δψ‐dependent gate but also by a certain mechanism to restrict the back flux of Na+, this making the antiporter capable of functioning as an efficient Δg̃mH+‐driven pump for Na+ in a high saline environment.