Intracellular pH homeostasis in cultured human placental syncytiotrophoblast cells: recovery from acidification

Abstract

Resting or basal intracellular pH (pHi) measured in cultured human syncytiotrophoblast cells was 7.26 ± 0.04 (without HCO3−) or 7.24 ± 0.03 (with HCO3−). Ion substitution and inhibitor experiments were performed to determine whether common H+-transporting species were operating to maintain basal pHi. Removal of extracellular Na+or Cl−or addition of amiloride or dihydro-4,4′-diisothiocyanatostilbene-2,2′-disulfonate (H2DIDS) had no effect. Acidification with the K+/H+exchanger nigericin reduced pHito 6.25 ± 0.15 (without HCO3−) or 6.53 ± 0.10 (with HCO3−). In the presence of extracellular Na+, recovery to basal pHiwas prompt and occurred at similar rates in the absence and presence of HCO3−. Ion substitution and inhibition experiments were also used to identify the species mediating the return to basal pHiafter acidification. Recovery was inhibited by removal of Na+or addition of amiloride, whereas removal of Cl−and addition of H2DIDS were ineffective. Addition of the Na+/H+exchanger monensin to cells that had returned to basal pHielicited a further increase in pHito 7.48 ± 0.07. Analysis of recovery data showed that there was a progressive decrease in ΔpH per minute as pHiapproached the basal level, despite the continued presence of a driving force for H+extrusion. These data show that in cultured syncytial cells, in the absence of perturbation, basal pHiis preserved despite the absence of active, mediated pH maintenance. They also demonstrate that an Na+/H+antiporter acts to defend the cells against acidification and that it is the sole transporter necessary for recovery from an intracellular acid load.