GASTRIC MUCUS OF THE GUINEA-PIG - PROTON CARRIER AND DIFFUSION BARRIER

Proton transport with the gastic mucus was investigated in the guinea pig in vitro by use of three experimental series. In series I, pH prof iles were obtained in the mucus and mucosa of a gastric explant with f ine-tipped double-barreled microelectrodes. With a luminal pH of 1.8, pH increased across this layer to similar to 6 at the epithelial surfa ce. Thickness of the gastric mucous gel layer increased continuously b y 170 +/- 100 mu m/h in the unstimulated and by 450 +/- 120 mu m/h in the histamine-stimulated preparation (means +/- SD). In series II, fre sh guinea pig gastric mucus was obtained from the gastric mucosa and t itrated at 10 degrees C from pH 6.5 to 0.7, followed by an incubation period of 30 min at 37 degrees C. During this incubation period, a spo ntaneous acidic shift was observed, corresponding to a proton release from the mucus of 130 +/- 19 mM. This proton release could be blocked by the pepsin inhibitor pepstatin and was not observed when titrating down to only pH 3. Buffer values calculated as the mean slope of the t itration curves in the pH range of 7 to 3 averaged 40 mM/pH unit. In s eries III, when titration was repeated with purified porcine mucin, no proton release was observed during incubation at pH 1.0, unless pepsi nogen (375 U/ml) had been added before titration. Proton release durin g incubation at pH 1.0 and 37 degrees C in the presence of pepsinogen averaged 50 mM. The data suggest that protons secreted by the gastric mucosa are buffered by the continuously secreted mucus and transported , bound to the proteins of the mucus, toward the gastric lumen. During this transport, pepsinogen is converted within the mucus to pepsin. P epsin modifies the buffering properties of the mucus, whereby protons are released from the protein binding. Thus the mucus forms a vehicle for proton transport toward the gastric lumen while, at the same time, constituting a diffusion barrier to prevent proton backdiffusion towa rd the gastric epithelium.