Na+-dependent transporters mediate HCO3- salvage across the luminal membrane of the main pancreatic duct

To study the roles of Na+-dependent H+ transporters, we characterized H+ ef flux mechanisms in the pancreatic duct in wild-type, NHE2(-/-), and NHE3(-/ -) mice. The pancreatic duct expresses NHE1 in the basolateral membrane, an d NHE2 and NHE3 in the luminal membrane, but does not contain NHE4 or NHE5. Basolateral Na+-dependent H+ efflux in the microperfused duct was inhibite d by 1.5 mu M of the amiloride analogue HOE 694, consistent with expression of NHE1, whereas the luminal activity required 50 mu M HOE 694 for effecti ve inhibition, suggesting that the efflux might be mediated by NHE2. Howeve r, disruption of NHE2 had no effect on luminal transport, while disruption of the NHE3 gene reduced luminal Na+-dependent H+ efflux by similar to 45%. Notably, the remaining luminal Na+-dependent H+ efflux in ducts from NHE3- /- mice was inhibited by 50 mu M HOE 694. Hence, similar to 55% of luminal H+ efflux (or HCO3- influx) in the pancreatic duct is mediated by a novel, HOE 694-sensitive, Na+-dependent mechanism. H+ transport by NHES and the no vel transporter is inhibited by cAMP, albeit to different extents. We propo se that multiple Na+-dependent mechanisms in the luminal membrane of the pa ncreatic duct absorb Na+ and HCO3- to produce a pancreatic juice that is po or in HCO3- and rich in Cl- during basal secretion. Inhibition of the trans porters during stimulated secretion aids in producing the HCO3--rich pancre atic juice.