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The duodenum must absorb ∼450 mmol of H+ per 24 h in order to decrease [H+] of the luminal content by 6 log orders over its 15 cm length (Feldman & Colturi, 1984). HCl in the duodenal lumen is neutralized by HCO3− secreted by the pancreas and duodenal epithelium, generating extremely high luminal CO2 pressures (PCO2 > 30 kPa) which dissipate by the proximal jejunum (Rune & Henriksen, 1969; Winship & Robinson, 1974). Gastric mucosal CO2 and H+ permeability is low, since pyloric obstruction leads to severe metabolic alkalosis due to the inability of the stomach to absorb substantial quantities of H+ or CO2 (Gamble & Ross, 1925; Javaheri & Nardell 1981). Thus, the duodenum is the major site for intestinal H+ and CO2 absorption. Transmucosal bulk absorption of CO2 and H+ is likely to follow the sequence of transport across the apical cell membrane into the cytosol, transport across the basolateral membrane of the epithelium into the subepithelial interstitium, followed by transport into the portal vein. The mechanism for CO2 and H+ transport across cell membranes remains incompletely understood. In terms of transapical CO2 and H+ transport, one accepted model involves conversion of luminal H+ and HCO3− to CO2 and H2O, diffusion of CO2 across the apical plasma membrane, and hydration of CO2 to HCO3− and H+ in the cytoplasm. This process, sometimes termed the Jacobs-Stewart cycle after its original description in red blood cells (Jacobs & Stewart, 1942), requires the presence of intracellular and extracellular carbonic anhydrases (CAs) and a plasma membrane anion exchanger. Since the duodenal epithelium has abundant cytoplasmic and membrane-associated CA activity (Sugai et al. 1994; Lonnerholm et al. 1989; Parkkila et al. 1994; Saarnio et al. 1998; Purkerson & Schwartz, 2005; Leppilampi et al. 2005) and apical anion exchangers (Wang et al. 2002; Spiegel et al. 2003), we hypothesized that most of the excess duodenal luminal H+ was absorbed through neutralization of secreted HCO3−, yielding luminal CO2. CO2 is the molecular species that then traverses the apical membrane, which enters the cytoplasm, and is hydrated to H2CO3, which then dissociates in the cytoplasm to H+ and HCO3−. HCO3− is transported into the lumen whereas H+ is transported into the submucosal space by membrane transport proteins. In essence, luminal H+ is transported through the apical membrane as CO2, but HCO3− is simultaneously secreted in its anionic form. Several observations support this hypothesis. We, and others have found that luminal acidification or elevation of luminal PCO2 provokes several epithelial responses, such as acidification of the cytoplasm and subepithelial interstitial fluid, increased mucosal blood flow, increased HCO3− secretion, and increased mucus secretion (Flemstrom & Kivilaakso, 1983; Flemstrom, 1994; Seno et al. 1998; Paimela et al. 1990, 1992; Akiba & Kaunitz, 1999; Akiba et al. 2000, 2001a,b, 2006). Since these responses appear to be dependent on acidification of the subepithelial space, and since elevated luminal CO2 or H+ provoke similar responses, it appears that luminal CO2 must be converted to subepithelial H+, which then signals these protective mechanisms (Allen & Flemstrom, 2005). The further characterization of transmucosal H+ and CO2 movement thus has larger implications for the understanding of mucosal protective mechanisms and the signalling pathways coordinating mucosal responses to acid perfusion. In order to further test our hypothesis, we devised a system in which the movement of CO2 and H+ between lumen, mucosa and the portal vein was measured. In order to determine the contribution of cytoplasmic and extracellular CAs towards H+ and CO2 movement, we used cell-permeant and -impermeant CA inhibitors. Finally, transmucosal tracer carbon movement was measured using 13C. Our results support our hypothesis that luminal H+, neutralized by secreted HCO3−, is converted to CO2 prior to entry into the cytoplasm of the epithelial cells, and that cellular CO2 is reconverted to H+, which then is transported into the portal vein. |
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