NA-H+ EXCHANGE AND INTRACELLULAR PH REGULATION IN COLONOCYTES FROM THE CHICK()

The involvement of Na+-H+ exchange in chicken colonocyte homeostasis w as investigated. Colonocyte pH (pH(i)) was measured with 2',7'-bis(car boxyethyl)-5,6-carboxyfluore (BCECF). The proton ionophore FCCP reduce d basal pH(i), indicating that cytosolic [H+] is not at electrochemica l equilibrium across the membrane. External Na+ removal decreased pH(i ) and subsequent addition of Na+ returns pH(i) towards its control val ue. The rate of pH(i) recovery from an acid load was Na+-dependent (K- m for Na+, 24 mM) and inhibited by EIPA (IC50, 0.18 mu M). The initial rate of Na+-dependent cell alkalization increased as the pH(i) decrea sed from 7.2 to 6.6 (Hill coefficient, 1.88). Radioisotope flux studie s revealed that an outwardly directed proton gradient transiently stim ulated Na+ uptake into BBMV isolated from the chick colon. EIPA and am iloride inhibited pH gradient-driven Na+ uptake (IC50 of 4 mu M and 32 mu M, respectively). The K-m for Na+ of pH gradient-driven Na+ uptake was 6.8 mM. The Hill coefficient of the relationship between the init ial rate of pH-driven Na+ uptake and the intravesicular pH was 0.70. I t is concluded that a Na+-H+ exchanger is involved in pH(i) homeostasi s in chicken colonocytes and that these cells possess at least two typ es of Na+-H+ antiporters with different sensitivity to EIPA and differ ent kinetic parameters.