title: pH threshold for human duodenal bicarbonate secretion and diffusion of CO2.
abstract: 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 mumol/cm.h. Citric acid at pH 2.5 and 3.0 increased bicarbonate output equally to about 560 mumol/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.
authors.affiliation: Department of Medicine, UCSD Medical Center, University of California.
authors.forename: S P
authors.initials: SP
authors.lastname: Feitelberg
authors.forename: D L
authors.initials: DL
authors.lastname: Hogan
authors.forename: M A
authors.initials: MA
authors.lastname: Koss
authors.forename: J I
authors.initials: JI
authors.lastname: Isenberg
mesh_terms: D000328:Adult; D001639:Bicarbonates / Q000378:metabolism*; D002245:Carbon Dioxide / Q000378:metabolism*; D004058:Diffusion; D004386:Duodenum / Q000378:metabolism*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007413:Intestinal Mucosa / Q000378:metabolism; D008297:Male; D008875:Middle Aged
chemical_list: D001639:Bicarbonates; D002245:Carbon Dioxide