Acid diffusion through rat duodenal mucosal cap

Acid induced duodenal injury occurs immediately following exposure to strong acid but does not worsen despite continuous perfusion with the acid. A thick gelatinous cap is found overlying the duodenum when examined histologically. The purpose of this study was to quantitate the resistance to acid diffusion by native duodenal mucus and the cap material. Rat duodenum was perfused with acid. The mucus from control and acid-perfused animals was harvested and the diffusion coefficient measured. Mucus from the duodenums of anesthetized rats was mixed with bromphenol blue and placed into capillary tubes. The tubes were submersed in 0.15 N HCI at 37 degrees C. The color change from blue to yellow was measured with time as the HCl migrated through the tube. From the distance and time data, the diffusion coefficient could be calculated. Controls of bromphenol blue in water were studied. The diffusion coefficient for HCl in water was 30.1+/-0.9 x 10(-6) cm2/sec, nearly identical to the standard value reported in the literature. For the duodenal mucus adherent to the epithelial surface, it was 1.1+/-0.1 x 10(-6) cm2/sec. Following perfusion with 0.15 N HCl, a 279.6+/-44-microm-thick mucus cap formed over the duodenum that had a diffusion coefficient of 1.8+/-.3 x 10(-8) cm2/sec. In response to injury, the duodenum protects against continued damage by producing a mucoid cap that it highly resistant to acid diffusion.