Ϫ -rich fluid in response to stimulation by the adenylyl cyclase-coupled hormone secretin. The Cl Ϫ -rich secretion of pancreatic acinar cells is modified as it flows along the pancreatic ductal system, ultimately producing pancreatic juice with final concentrations of ϳ140 mM HCO 3 Ϫ and ϳ20 mM Cl Ϫ (31). In the guinea pig pancreatic duct, the uptake of bicarbonate across the basolateral membrane is mediated by Na ϩ -HCO 3 Ϫ cotransport (NBC1) and by Na ϩ /H ϩ exchange (19). HCO 3 Ϫ efflux across the apical membrane has been proposed to be mediated by CFTR, in concert with apical Cl Ϫ /HCO 3 Ϫ exchanger activity (9). Proximal pancreatic duct HCO 3 Ϫ secretion by apical Cl Ϫ / HCO 3 Ϫ exchange is favored by the high luminal Cl Ϫ concentrations resulting from pancreatic acinar Cl Ϫ secretion. However, since this proximal fluid flows distally toward the ampullary terminus of the duct, its Cl Ϫ concentration progressively decreases in parallel with a gradual increase in its HCO 3 Ϫ concentration. These inverse changes in Cl Ϫ and HCO 3 Ϫ concentrations are predicted to alter relative contributions of apical membrane Cl Ϫ /HCO 3 Ϫ exchange and CFTR-mediated HCO 3 Ϫ conductance and have been suggested to reflect axial variation in expression and regulation of these transport activities (18). The Cl Ϫ /HCO 3 Ϫ exchangers Slc26a3 and Slc26a6 have been detected in mouse pancreatic duct and in the human pancreatic cell line CFPAC-1 (10, 22), leading to the proposal that Cl Ϫ -dependent HCO 3 Ϫ secretion by pancreatic proximal ducts represents combined activities of Slc26a6 and Slc26a3 anion exchangers, positively regulated by activated CFTR, with reciprocal positive regulation of CFTR by the anion exchangers (4, 5, 22, 23). However, a study in native mouse pancreatic duct suggests negative regulation of CFTR by Slc26a6 (43).Secretin-stimulated HCO 3 Ϫ secretion in guinea pig pancreatic duct is not dependent on elevated luminal [Cl Ϫ ] (21) (brackets denote concentration) and occurs even in the presence of luminal fluid containing 125 mM HCO 3 Ϫ and 23 mM Cl Ϫ (17). As luminal [Cl Ϫ ] falls to or below this level (with corresponding elevation of [HCO 3 Ϫ ]), CFTR Cl Ϫ permeability may fall while that for HCO 3 Ϫ may rise (34). Measurements of membrane potential (V m ) and intracellular pH (pH i ) in luminally perfused interlobular ducts from guinea pig pancreas suggest that CFTR HCO 3 Ϫ conductance can account for observed levels of stimulated bicarbonate secretion (15,18,20). A computational model of pancreatic duct HCO 3 Ϫ secretion predicted for similar conditions that ϳ94% of HCO 3 Ϫ efflux across the apical membrane was mediated by HCO 3 Ϫ conductance (36), although the model did not consider contributions from electrogenic Cl Ϫ /HCO 3 Ϫ exchange. Thus the HCO 3 Ϫ conductance of CFTR could provide the main route for apical HCO 3 Ϫ secretion in distal pancreatic ducts from species in which, like human and guinea pig, pancreatic juice contains high concentrations of HCO 3 Ϫ (ϳ140 mM). Studies of the role of SLC26-media...

The secretin-stimulated human pancreatic duct secretes HCO3−-rich fluid essential for normal digestion. Optimal stimulation of pancreatic HCO3− secretion likely requires coupled activities of the cystic fibrosis transmembrane regulator (CFTR) anion channel and apical SLC26 Cl−/HCO3− exchangers. However, whereas stimulated human and guinea pig pancreatic ducts secrete ∼140 mM HCO3− or more, mouse and rat ducts secrete ∼40–70 mM HCO3−. Moreover, the axial distribution and physiological roles of SLC26 anion exchangers in pancreatic duct secretory processes remain controversial and may vary among mammalian species. Thus the property of high HCO3− secretion shared by human and guinea pig pancreatic ducts prompted us to clone from guinea pig pancreatic duct cDNAs encoding Slc26a3, Slc26a6, and Slc26a11 polypeptides. We then functionally characterized these anion transporters in Xenopus oocytes and human embryonic kidney (HEK) 293 cells. In Xenopus oocytes, gpSlc26a3 mediated only Cl−/Cl− exchange and electroneutral Cl−/HCO3− exchange. gpSlc26a6 in Xenopus oocytes mediated Cl−/Cl− exchange and bidirectional exchange of Cl− for oxalate and sulfate, but Cl−/HCO3− exchange was detected only in HEK 293 cells. gpSlc26a11 in Xenopus oocytes exhibited pH-dependent Cl−, oxalate, and sulfate transport but no detectable Cl−/HCO3− exchange. The three gpSlc26 anion transporters exhibited distinct pharmacological profiles of 36Cl− influx, including partial sensitivity to CFTR inhibitors Inh-172 and GlyH101, but only Slc26a11 was inhibited by PPQ-102. This first molecular and functional assessment of recombinant SLC26 anion transporters from guinea pig pancreatic duct enhances our understanding of pancreatic HCO3− secretion in species that share a high HCO3− secretory output.

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Cellular mechanisms underlying the impairment of pancreatic fluid and electrolyte secretion in diabetes were examined using interlobular ducts isolated from rat pancreas. Fluid secretion was assessed by monitoring changes in luminal volume. HCO3(-) uptake across the basolateral membrane was estimated from the recovery of intracellular pH following an acid load. Exposure to high glucose concentrations inhibited fluid secretion and reduced the rate of basolateral HCO3(-) uptake in secretin-stimulated ducts isolated from normal rats. In ducts isolated from streptozotocin-treated diabetic rats, fluid secretion and basolateral HCO3(-) uptake were also severely impaired but could be largely reversed by incubation in normal-glucose solutions. Sodium-dependent glucose cotransporter 1 (SGLT1), glucose transporter (GLUT)1, GLUT2, and GLUT8 transcripts were detected by reverse transcriptase polymerase chain reaction in isolated ducts. Raising the luminal glucose concentration in microperfused ducts caused a depolarization of the membrane potential, consistent with the presence of SGLT1 at the apical membrane. Unstimulated ducts filled with high-glucose solutions lost luminal fluid by a phlorizin-sensitive mechanism, indicating that pancreatic ducts are capable of active glucose reabsorption from the lumen via SGLT1. In ducts exposed to high glucose concentrations, continuous glucose diffusion to the lumen and active reabsorption via SGLT1 would lead to elevation of intracellular Na+ concentration and sustained depolarization of the apical membrane. These two factors would tend to inhibit the basolateral uptake and apical efflux of Cl(-) and HCO3(-) and could therefore account for the impaired fluid and electrolyte secretion that is observed in diabetes.

Although cystic fibrosis is rare in Japanese, measurement of sweat Cl(-) has suggested mild dysfunction of cystic fibrosis transmembrane conductance regulator (CFTR) in some patients with chronic pancreatitis. In the present study, we have investigated the association of CFTR variants and chronic pancreatitis in Japanese and the functional characteristics of a Japanese- and pancreatitis-specific CFTR variant, L1156F. Seventy patients with alcoholic chronic pancreatitis, 18 patients with idiopathic chronic pancreatitis, and 180 normal subjects participated. All exons and their boundaries and promoter region of the CFTR gene were sequenced. Human embryonic kidney-293 cells were transfected with three CFTR variants (M470V, L1156F, and M470V+L1156F), and the protein expression was examined. Xenopus laevis oocytes were injected with the CFTR variants, and bicarbonate (HCO3 (-)) transport activity was examined. CFPAC-1 cells were transfected with the CFTR variants and Cl(-)/HCO3 (-) exchange activity was examined. Six variants (E217G, I556V, M470V, L1156F, Q1352H, and R1453W) were identified in the coding region of the CFTR gene. Cystic fibrosis-causing mutations were not found. The allele frequencies of L1156F and Q1352H in alcoholic chronic pancreatitis (5.0 and 7.9%) were significantly (P < 0.01) higher than those in normal subjects (0.6 and 1.9%). L1156F was linked with a worldwide CFTR variant, M470V. Combination of M470V and L1156F significantly reduced CFTR expression to ∼60%, impaired CFTR-mediated HCO3 (-)/Cl(-) transport activity to 50-60%, and impaired CFTR-coupled Cl(-)/HCO3 (-) exchange activity to 20-30%. The data suggest that the Japanese-specific CFTR variant L1156F causes mild dysfunction of CFTR and increases the risk of alcoholic chronic pancreatitis in Japanese.