Intracellular pH regulation in intestinal and renal epithelial cells

Abstract

The current study examines the presence of the Na+/HCO3− cotransporter and of the Cl−/HCO3− exchanger in chicken colonocytes and their role in cytosolic pH (pHi) homeostasis. pHi was measured with 2′,7′-bis(carboxyethyl)-5,6-carboxyfluorescein (BCECF) at 25°C. Basal pHi was 7.16 in HEPES-buffered solutions and 7.06 in those buffered with HCO3−. Removal of external Cl− increased pHi and Cl− reinstatement brought the pHi towards resting values. These Cl−-induced pHi changes were Na+-independent, inhibited by H2-DIDS and faster in the presence than in the absence of HCO3−. Cells recovered from alkaline loads by a mechanism that was Cl−-dependent, Na+-independent and inhibited by H2-DIDS. This rate of Cl−-dependent cell acidification decreased as the pHi decreased, with a Hill coefficient value close to 4. Removal of external Na+ decreased pHi and readdition of Na+ brought pHi towards the control values. The rate of the Na+-induced changes was not modified by the presence of HCO3− and was prevented by EIPA and unaffected by H2-DIDS. In the presence of EIPA cells partially recovered from a moderate acid load only when both Na+ and HCO3− were present. The EIPA resistant Na+- and bicarbonate-dependent pHi recovery was inhibited by H2-DIDS and occurred at equal rates in both Cl−-containing and Cl−-free solutions. It is concluded that in chicken colonocytes bathed in HCO3−-buffered solutions, both the Na+/H+ exchanger and the Cl−/HCO3− exchanger participate in setting the resting pHi value. The latter transporter helps the cells to recover from alkaline loads and the first transporter, together with the Na+/HCO3− cotransporter, is involved in pHi recovery from an acid load.