Gastrointestinal peptides activate Na(+)-H+ exchanger in AR42J cells by increasing its affinity for intracellular H+

Regulation of intracellular pH (pHi) was studied by dual wavelength fluorometry in monolayers of pancreatic AR42J cells loaded with the fluorescent probe 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein. In cells superfused with N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid-buffered solution at pH 7.40, basal pHi was determined to be 7.15 +/- 0.13. Na(+)-H+ exchange could be demonstrated in both resting cells and cells subjected to acid loading by use of transient exposure to NH4Cl. Na(+)-H+ exchange was completely blocked by 300 microM amiloride and was dependent on extracellular Na+ (apparent Km = 25 mM). When the concentration of the NH4Cl pulse was varied (0.5-25 mM), the rate of pHi recovery increased as pHi became acidic, reaching a maximum of 0.007 pH units/s at pHi of 6.4. Gastrointestinal hormones, including pentagastrin, cholecystokinin, and bombesin, increased the rate of Na(+)-H+ exchange without affecting cellular buffer capacity (21.5 +/- 1.8 mM/pH unit), thereby leading to an intracellular alkalinization. This was accompanied by a shift in the curve of Na(+)-H+ exchange as a function of pHi to more alkaline values, although the maximum rate of pH recovery was unchanged. Neither protein kinase C nor Ca2+ could be conclusively linked to activation of Na(+)-H+ exchange, raising the possibility of a more direct, receptor-controlled mechanism.