Intracellular pH regulation in cultured embryonic chick heart cells: Na+-dependent Cl- /HCO3- exchange

Abstract

The contribution of Cl-/HCO3- exchange to intracellular pH (pHi) regulation in cultured chick heart cells was evaluated using ion-selective microelectrodes to monitor pHi, Na+ (aNai), and Cl- (aCli) activity. In (HCO3- + CO2)-buffered solution steady-state pHi was 7.12. Removing (HCO3- + CO2) buffer caused a SITS (0.1 mM)-sensitive alkalinization and countergradient increase in aCli along with a transient DIDS-sensitive countergradient decrease in aNai. SITS had no effect on the rate of PHi recovery from alkalinization. When (HCO3- + CO2) was reintroduced the cells rapidly acidified, aNai increased, aCli decreased, and pHi recovered. The decrease in aCli and the pHi recovery were SITS sensitive. Cells exposed to 10 mM NH4Cl became transiently alkaline concomitant with an increase in aCli and a decrease in aNai. The intracellular acidification induced by NH4Cl removal was accompanied by a decrease in aCli and an increase in aNai that led to the recovery of pHi. In the presence of (HCO3- + CO2) addition of either amiloride (1 mM) or DIDS (1 mM) partially reduced pHi recovery, whereas application of amiloride plus DIDS completely inhibited the PHi recovery and the decrease in aCli. Therefore, after an acid load pHi recovery is HCO3o- and Nao- dependent and DIDS sensitive (but not Cao2+ dependent). Furthermore, SITS inhibition of Na+-dependent Cl- /HCO3- exchange caused an increase in aCli and a decrease in the 36Cl efflux rate constant and pHi. In (HCO3- + CO2)-free solution, amiloride completely blocked the pHi recovery from acidification that was induced by removal of NH4Cl. Thus, both Na+/H+ and Na+-dependent Cl-/HCO3- exchange are involved in pHi regulation from acidification. When the cells became alkaline upon removal of (HCO3- + CO2), a SITS-sensitive increase in pHi and aCli was accompanied by a decrease of aNai, suggesting that the HCO3- efflux, which can attenuate initial alkalinization, is via a Na+-dependent Cl- /HCO3- exchange. However, the mechanism involved in pHi regulation from alkalinization is yet to be established. In conclusion, in cultured chick heart cells the Na+-dependent Cl-/HCO3- exchange regulates pHi response to acidification and is involved in the steady-state maintenance of pHi.