The esophageal submucosal glands (SMG) secrete HCO(3)(-) and mucus into the esophageal lumen, where they contribute to acid clearance and epithelial protection. This study characterized the ion transport mechanisms linked to HCO(3)(-) secretion in SMG. We localized ion transporters using immunofluorescence, and we examined their expression by RT-PCR and in situ hybridization. We measured HCO(3)(-) secretion by using pH stat and the isolated perfused esophagus. Using double labeling with Na(+)-K(+)-ATPase as a marker, we localized Na(+)-coupled bicarbonate transporter (NBCe1) and Cl(-)-HCO(3)(-) exchanger (SLC4A2/AE2) to the basolateral membrane of duct cells. Expression of cystic fibrosis transmembrane regulator channel (CFTR) was confirmed by immunofluorescence, RT-PCR, and in situ hybridization. We identified anion exchanger SLC26A6 at the ducts' luminal membrane and Na(+)-K(+)-2Cl(-) (NKCC1) at the basolateral membrane of mucous and duct cells. pH stat experiments showed that elevations in cAMP induced by forskolin or IBMX increased HCO(3)(-) secretion. Genistein, an activator of CFTR, which does not increase intracellular cAMP, also stimulated HCO(3)(-) secretion, whereas glibenclamide, a Cl(-) channel blocker, and bumetanide, a Na(+)-K(+)-2Cl(-) blocker, decreased it. CFTR(inh)-172, a specific CFTR channel blocker, inhibited basal HCO(3)(-) secretion as well as stimulation of HCO(3)(-) secretion by IBMX. This is the first report on the presence of CFTR channels in the esophagus. The role of CFTR in manifestations of esophageal disease in cystic fibrosis patients remains to be determined.

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Rhbg is a membrane glycoprotein that is involved in NH 3/NH4 ϩ transport. Several models have been proposed to describe Rhbg, including an electroneutral NH 4 ϩ /H ϩ exchanger, a uniporter, an NH 4 ϩ channel, or even a gas channel. In this study, we characterized the pH sensitivity of Rhbg expressed in Xenopus oocytes. We used twoelectrode voltage clamp and ion-selective microelectrodes to measure NH 4 ϩ -induced [and methyl ammonium (MA ϩ )] currents and changes in intracellular pH (pHi), respectively. In oocytes expressing Rhbg, 5 mM NH4Cl (NH3/NH 4 ϩ ) at extracellular pH (pHo) of 7.5 induced an inward current, decreased pHi, and depolarized the cell. Raising pHo to 8.2 significantly enhanced the NH 4 ϩ -induced current and pHi changes, whereas decreasing bath pH to 6.5 inhibited these changes. Lowering pHi (decreased by butyrate) also inhibited the NH 4 ϩ -induced current and pHi decrease. In oocytes expressing Rhbg, 5 mM methyl amine hydrochloride (MA/MA ϩ ), often used as an NH4Cl substitute, induced an inward current, a pHi increase (not a decrease), and depolarization of the cell. Exposing the oocyte to MA/MA ϩ at alkaline bath pH (8.2) enhanced the MA ϩ -induced current, whereas lowering bath pH to 6.5 inhibited the MA ϩ current completely. Exposing the oocyte to MA/MA ϩ at low pHi abolished the MA ϩ -induced current and depolarization; however, pHi still increased. These data indicate that 1) transport of NH 4 ϩ and MA/MA ϩ by Rhbg is pH sensitive; 2) electrogenic NH 4 ϩ and MA ϩ transport are stimulated by alkaline pHo but inhibited by acidic pHi or pHo; and 3) electroneutral transport of MA by Rhbg is likely but is less sensitive to pH changes. NH 4 ϩ transport; Rh B glycoprotein; methyl ammonium; ammonium/ hydrogen exchange; ammonium channel; gas channel SEVERAL STUDIES HAVE SHOWN that Rhbg and Rhcg are involved in transport of NH 3 /NH 4 ϩ (1, 11, 12, 14 -16, 20, 24, 27, 28 The physiological roles of Rhbg and Rhcg in acid-base transport and in renal NH 3 /NH 4 ϩ handling are becoming more evident. Rhcg is highly expressed in the epididymis, where it may be involved in the mechanisms of acidifying the epididymal fluid needed for maturation of sperm (3). Interestingly, Rhcg knockout mice were found to have lower pH of the epididymal fluid compared with wild type. In the kidney, chronic metabolic acidosis increased Rhcg protein expression in cells of the medullary collecting duct (CD) (25). The same study indicated an increase in apical and basolateral Rhcg expression. Metabolic studies demonstrated that mice lacking Rhcg had impaired urinary NH 4 ϩ excretion in response to acid loads (3). A recent study on mice with renal CD-specific Rhcg deletion (CD-KO) (9) showed that urinary NH 4 ϩ excretion was less in KO mice than in control mice and that after acid loading CD-KO mice developed more severe metabolic acidosis than control mice. Studies on Rhbg showed that genetic deletion of pendrin, an apical Cl (2) generated intercalated cell-specific Rhbg KO mice and demonstrated that when acid loaded urinar...

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