Sodium/proton exchange in the maintenance of intracellular pH in FRTL-5 thyroid cells

In the present study we describe a Na+/H+ exchange system in FRTL-5 rat thyroid cells and its role in the regulation of intracellular pH (pHi). The pHi of these cells was measured in acidic medium (pH 6.5) using the equilibrium distribution of 14C-labeled 5,5'-dimethyl-oxazolidine-2,4-dione. The estimated pHi in 10 separate experiments was 7.30 +/- 0.01 (mean +/- SE). The pHi declined to 6.87 +/- 0.02 when equimolar concentrations of choline+ replaced Na+ in the incubation medium, but was restored when cells were placed in a medium containing 50 mM Na+. The necessity for external Na+ to maintain pHi, when cells are exposed to acidic solutions, is consistent with a Na+/H+ antiport system in these cells. This is further supported by the observation that pHi decreased to 6.70 +/- 0.01 when amiloride, an inhibitor of Na+/H+ exchange, was added, and that this agent prevented the recovery of pHi in Na+-depleted cells after addition of Na+ to the medium. This report also provides additional information concerning the relationship between H+ and I- uptake and suggests that there are two I- transport systems: one that is Na+-dependent, 4,4'-diisothiocyano 2,2'-disulfonic acid stilbene-sensitive, and not associated with intracellular acidification, and the other operative at an external pH of 6.5 and associated with intracellular acidification.