Regulation of Na + /H + and Cl − /HCO 3 − antiports in vero cells

The effect of serum, phorbol‐12‐myristate‐13‐acetate (TPA), and forskolin on the activity Na + /H + antiport and the Na + ‐coupled and Na + ‐independent Cl − / HCO 3 − antiport was studied in Vero cells by measuring 22 Na + and 36 Cl − fluxes and changes in cytosolic pH (pH i ). The Na + ‐independent Cl − /HCO 3 − antiport, which acts as an acidifying mechanism, is strongly pH‐sensitive. In serum‐starved cells it is activated at alkaline cytosolic pH, with a half‐maximal activity at pH i ∼7.20. Incubation with serum increased the activity of the Na + ‐independent Cl − /HCO 3 − antiport at pH i values from 6.8 to 7.2. Thus serum appeared to alter the pH i sensitivity of this antiporter such that the threshold value for activation of the antiport was shifted to a more acidic value. Na + /H + antiport was somewhat stimulated initially by addition of serum, but further incubation with serum (> 45 min) decreased its activity. The activity of the Na + ‐coupled Cl − /HCO 3 − antiport, which is the major alkalinizing antiport in Vero cells, was not altered by short‐term incubation with serum (< 10 min) but decreased after prolonged incubation (> 45 min). Our findings with TPA and forskolin indicate that the effect of serum is partly mediated by the protein kinase C pathway, whereas the cyclic adenosine monophosphate pathway does not appear to play an important role. The net effect of serum on the pH i ‐regulating antiports was a slight decrease in intracellular pH.