HCO3--dependent ion transport systems and intracellular pH regulation in colonocytes from the chick

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 (pH(i)) homeostasis. pH(i) was measured with 2',7'-bis(carboxyethyl)-5,6-carboxy-fluorescein (BCECF) at 25 degrees C. Basal pH(i) was 7.16 in HEPES-buffered solutions and 7.06 in those buffered with HCO3-. Removal of external Cl- increased pH(i) and Cl- reinstatement brought the pH(i) towards resting values. These Cl--induced pH(i) changes were Na+-independent, inhibited by H-2-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 H-2-DIDS. This rate of Cl--dependent cell acidification decreased as the pH(i) decreased, with a Hill coefficient value close to 4. Removal of external Na+ decreased pH(i) and readdition of Na+ brought pH(i) towards the control values. The rate of the Na+-induced changes was nor. modified by the presence of HCO3- and was prevented by EIPA and unaffected by H-2-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 pH(i) recovery was inhibited by H-2-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 pH(i) 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 pH(i) recovery from an acid load. (C) 1998 Elsevier Science B.V. All rights reserved.