NO(3) (−)-induced pH Changes in Mammalian Cells : Evidence for an NO(3)(−)-H(+) Cotransporter

The effect of NO(3) (−) on intracellular pH (pH(i)) was assessed microfluorimetrically in mammalian cells in culture. In cells of human, hamster, and murine origin addition of extracellular NO(3) (−) induced an intracellular acidification. This acidification was eliminated when the cytosolic pH was clamped using ionophores or by perfusing the cytosol with highly buffered solutions using patch-pipettes, ruling out spectroscopic artifacts. The NO(3) (−)- induced pH change was not due to modulation of Na(+)/H(+) exchange, since it was also observed in Na(+)/H(+) antiport-deficient mutants. Though NO(3) (−) is known to inhibit vacuolar-type (V) H(+)-ATPases, this effect was not responsible for the acidification since it persisted in the presence of the potent V-ATPase inhibitor bafilomycin A(1). NO(3) (−)/HCO(3) (−) exchange as the underlying mechanism was ruled out because acidification occurred despite nominal removal of HCO(3) (−), despite inhibition of the anion exchanger with disulfonic stilbenes and in HEK 293 cells, which seemingly lack anion exchangers (Lee, B.S., R.B. Gunn, and R.R. Kopito. 1991. J. Biol. Chem. 266:11448– 11454). Accumulation of intracellular NO(3) (−), measured by the Greiss method after reduction to NO(2) (−), indicated that the anion is translocated into the cells along with the movement of acid equivalents. The simplest model to explain these observations is the cotransport of NO(3) (−) with H(+) (or the equivalent counter-transport of NO(3) (−) for OH(−)). The transporter appears to be bi-directional, operating in the forward as well as reverse directions. A rough estimate of the fluxes of NO(3) (−) and acid equivalents suggests a one-to-one stoichiometry. Accordingly, the rate of transport was unaffected by sizable changes in transmembrane potential. The cytosolic acidification was a saturable function of the extracellular concentration of NO(3) (−) and was accentuated by acidification of the extracellular space. The putative NO(3) (−)-H(+) cotransport was inhibited markedly by ethacrynic acid and by α-cyano-4-hydroxycinnamate, but only marginally by 4,4′-diisothiocyanostilbene-2,2′ disulfonate or by p-chloromercuribenzene sulfonate. The transporter responsible for NO(3) (−)-induced pH changes in mammalian cells may be related, though not identical, to the NO(3) (−)-H(+) cotransporter described in Arabidopsis and Aspergillus. The mammalian cotransporter may be important in eliminating the products of NO metabolism, particularly in cells that generate vast amounts of this messenger. By cotransporting NO(3) (−) with H(+) the cells would additionally eliminate acid equivalents from activated cells that are metabolizing actively, without added energetic investment and with minimal disruption of the transmembrane potential, inasmuch as the cotransporter is likely electroneutral.