Na⁺-H⁺ exchangers (NHE) are localized in both basolateral and apical membranes of various epithelia and 8 isoforms have been identified. NHE3 is localized in the apical membrane and mediates HCO₃^– absorption in kidney proximal tubule and H⁺-coupled dipeptide absorption in the small intestine. NHE activity was detected at the apical membrane of the main pancreatic duct from mice (1). In the present study, to investigate the role of apical NHE in HCO₃^– secretion from pancreatic duct cells, we examined the activity of apical NHE in interlobular pancreatic duct segments isolated from normal and ΔF mice, a cystic fibrosis mouse model in which the ΔF508 mutation was introduced in the mouse CFTR (cystic fibrosis transmembrane conductance regulator) and pancreatic HCO₃^– secretion is impaired. Interlobular duct segments (∼100 μm in diameter) were isolated by collagenase digestion and microdissection as described previously (2). The lumen was microperfused and intracellular pH (pH_i) was measured by microfluorometry at 37oC in ducts loaded with the pH-sensitive fluoroprobe BCECF (3). Both the bath and lumen were perfused with HCO₃^–-free Hepes-buffered solutions. The duct cells were acid-loaded with a 2-min pulse of 20 mM NH₄⁺, which was followed by a Na⁺-free solution in both the bath and lumen. The rate of pH_i recovery after re-addition of Na⁺ to the luminal solution was calculated as a measure of the activity of apical Na⁺-H⁺ exchange. Averaged data are presented as the mean ± SD. Tests for significant differences were made with Student’s t test. The rate of pH_i recovery (dpH/dt) (dependent on luminal Na⁺, independent of HCO₃^–) was 0.12 ± 0.01 pH unit/min (n = 8) in wild type (WT/WT) ducts, which was completely inhibited by 100 μM HOE642, an inhibitor of NHE to 0.005 ± 0.003 pH unit/min (n = 6, p < 0.01). Forskolin (1 μM), an activator of adenylate cyclase, reduced the apical NHE activity to 0.05 ± 0.01 pH unit/min (n = 9, p < 0.01). The apical NHE activity in cystic fibrosis (ΔF/ΔF) ducts was 0.20 ± 0.01 pH unit/min (n = 6), which was significantly (p < 0.01) higher than that in wild type ducts and was accelerated to 0.66 ± 0.11 pH unit/min (n = 6, p < 0.01) by application of forskolin. In interlobular duct cells from mice pancreas, the activity of apical NHE was suppressed by functional CFTR and it was stimulated by cAMP in the absence of functional CFTR. These data suggest that the inhibitory regulation of apical NHE by CFTR does not work in cystic fibrosis pancreatic duct, which may lead to acidification of pancreatic juice.