Cl-/base exchange and cellular pH regulation in enterocytes isolated from chick small intestine

Intracellular pH (pHi) and Cl-/base exchange activity have been examined in isolated chicken enterocytes, both in the presence and absence of 25 mM HCO3-/5% CO2. Intracellular pH was measured with BCECF, a pH-sensitive carboxyfluorescein derivative. Under resting conditions pHi was 7.17 in Hepes and 7.12 in HCO3(-)-buffered solutions. Cells became more alkaline upon withdrawal of Cl-. Cells depleted of Cl- acidified upon reinstatement of Cl-. These changes were faster in the presence of HCO3- than in its absence. After an alkaline load (removal of HCO3- from the medium) pHi decreases towards base line in the presence of Cl-, but not in its absence. The Cl(-)-dependent pHi changes were prevented by H2DIDS and were unaffected by Na+. The Cl(-)-induced recovery from an alkaline load exhibited simple saturation kinetics, with an apparent Km of 12.5 mM Cl- and maximum velocity of approximately 0.20 pH units min-1. The Cl-/base exchange is functional under resting conditions, as shown by cell alkalinization on exposure to 0.5 mM H2DIDS, both in the presence and in the absence of HCO3-. It is concluded that Cl-/base exchange participates in setting the resting intracellular pH in isolated chicken enterocytes and helps recover from alkaline loads. The exchange operates both in the presence and in the absence of bicarbonate.