PH threshold for human duodenal bicarbonate secretion and diffusion of CO2

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Gastric acid enters the proximal duodenum both as free and buffered H+. The procedures in this study were threefold, (a) to determine the pH threshold for duodenal mucosal bicarbonate secretion, iso-osmolar citric acid (pH 2.5–4.0; H+, 1.1 mmol) was infused; (b) to examine the effect of varying acid loads (H+, 0.4–5.1 mmol); citric acid (pH 3.0) was perfused; and (c) to quantitate duodenal diffusion of CO2, citric acid (pH 5.0) gassed with CO2 (Pco2, 0–210 mm Hg) was tested. Basal bicarbonate secretion was similar on each test day, 230 μmol/cm · h. Citric acid at pH 2.5 and 3.0 increased bicarbonate output equally to about 560 μmol/cm · h (similar to 2 mmol of 100 mmol/L HCl), citric acid at pH 3.5 and 4.0 had no effect. Varying the acid load increased bicarbonate output similarly. Duodenal loss of CO2 was minimal (4%) with infusion of 50 mm Hg Pco2 and increased to approximately 25% (15 mm Hg/min) at higher Pco2 values. It is concluded that (a) the pH threshold for human duodenal mucosal bicarbonate secretion is 3.0; (b) a pH-sensitive, rather than an acid load-sensitive, regulatory process exists, and (c) CO2 loss plateaus at 15 mm Hg/min at a Pco2 of 200 mm Hg.