Effect of metabolizable sugars on the mucosal surface pH of rat intestine

In a layer juxtaposed to the mucosal surface of distal duodenum and of jejunum, and designated as "acid microclimate," the proton concentration is significantly higher than in the bulk phase of the intestinal lumen. The aim of the present study was to quantify the effect of different sugars on the mucosal surface pH value when these substrates were either added to the in vitro preparation of rat distal duodenal and mid-jejunal segments or administered by intragastric intubation and by i.v. infusion. All pH measurements were carried out in vitro by surface microelectrodes in oxygenated phosphate buffer. Glucose, fructose, mannose, galactose and 3-O-methylglucose were applied in vitro, glucose, maltose, fructose, galactose, 3-O-methylglucose and whey protein for the intragastric intubation and glucose, fructose and galactose for the intravenous infusion. Only substrates that could be metabolized in the intestinal epithelia were able to depress surface pH to a considerable extent. Substrate concentration at which half-maximum decrease of surface pH was obtained (K delta pH) was calculated to be 4.5 mM for glucose, 14.5 mM for fructose and 34.5 mM for mannose. The study was the first to demonstrate that metabolizable sugars are not only able to lower surface pH of intestinal epithelia when they are added in vitro but that they are also effective after intragastric intubation or i.v. infusion. It is plausible that generation of the protons, which are secreted into the acid microclimate zone, is closely related to the intracellular metabolization of glucose. The existence of a high proton concentration on the immediate mucosal surface could be of considerable significance for the absorption of weak electrolytes, a group to which different nutrients belong.