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

Resting or basal intracellular pH (pH_i) measured in cultured human syncytiotrophoblast cells was 7.26 ± 0.04 (without HCO ₃^-) 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 nigerian reduced pH_i to 6.25 ± 0.15 (without HCO_i) or 6.53 ± 0.10 (with HCO₃^-). In the presence of extracellular Na⁺, recovery to basal pH_i was 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 pH_i after acidification. Recovery was inhibited by removal of Na⁺ or addition of amiloride, whereas removal of Cl^- and addition of H ₂DIDS were ineffective. Addition of the Na⁺/H⁺ exchanger monensin to cells that had returned to basal pH_i elicited a further increase in pH_i to 7.48 ± 0.07. Analysis of recovery data showed that there was a progressive decrease in ΔpH per minute as pH_i approached 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 pH_i is 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.