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Mechanism for high PCO2 in gastric juice: roles of bicarbonate secretion and CO2 diffusion.

The partial pressure of CO2 (PCO2) in gastric juice often exceeds the PCO2 of blood. CO2 in gastric juice may originate from blood and enter luminal fluid by diffusion, or CO2 may be produced in the lumen of the stomach from the reaction of HCO3- and H+. Because CO2 production from HCO3- is dependent on acid (low pH), we suppressed acid secretion with intravenous cimetidine to estimate to what extent appearance of CO2 in luminal fluid is due to production from HCO3-. When denervated fundic pouches of dogs were distended with saline, the PCO2 of the solution increased to the PCO2 of blood in approximately 20 min, with the initial rate of appearance of CO2 in the pouch solution only minimally affected by cimetidine. Thereafter, PCO2 of luminal fluid continued to increase to 50-60 mmHg in the absence of cimetidine, whereas PCO2 of luminal fluid remained approximately equal to that of blood when cimetidine was infused (P less than 0.001, cimetidine vs. control). The mean pH in the pouch solution remained between 6.3 and 6.9 during cimetidine infusion but decreased to 4.75 without cimetidine (P less than 0.001). In additional experiments, an acidic solution with high PCO2 (242 +/- 3 mmHg) was infused into the fundic pouches. PCO2 in luminal fluid decreased rather slowly toward plasma PCO2, requiring 240 min for luminal fluid PCO2 to decrease to 56 +/- 2 mmHg. Thus the permeability of the gastric mucosa to luminal CO2 was relatively low.(ABSTRACT TRUNCATED AT 250 WORDS)