Different types of asymmetric distribution of hydrosoluble and liposoluble substances at the two sides of a mucosal intestinal preparation

Effect of changing intestinal flow rate on a measurement of intestinal permeability

BACKGROUND/AIMS

The flow rate of fluid through the proximal small intestine varies widely under normal physiological conditions. The aim of this study was to assess the effect of changes in flow rate on the passive permeability of the aqueous paracellular pathway of the human jejunum.

METHODS

Normal subjects were studied in vivo during constant perfusion of 30-cm loops of jejunum at flow rates of 5, 10, or 20 mL/min. The permeability ratio of L-xylose/urea was used to assess apparent permeability of the mucosa and to calculate the average pore radius of the aqueous pathway for passive diffusion.

RESULTS

Increasing jejunal flow rate from 5 to 20 mL/min significantly decreased the L-xylose/urea permeability ratio from 0.35 to 0.23 and decreased average calculated pore radius of the diffusion pathway from 13 A to 8 A.

CONCLUSIONS

Increases in flow rate in the normal physiological range decrease the estimated pore size of normal healthy jejunal mucosa. Because increasing flow rate is known to increase exposure of luminal fluid to the intervillus space, the results of this study are best explained by postulating that cells lining the sides of villi are less permeable than cells lining the villus tips.

Occurrence, absorption and metabolism of short chain fatty acids in the digestive tract of mammals

Short chain fatty acids (SCFA) also named volatile fatty acids, mainly acetate, propionate and butyrate, are the major end-products of the microbial digestion of carbohydrates in the alimentary canal. The highest concentrations are observed in the forestomach of the ruminants and in the large intestine (caecum and colon) of all the mammals. Butyrate and caproate released by action of gastric lipase on bovine milk triacylglycerols ingested by preruminants or infants are of nutritional importance too. Both squamous stratified mucosa of rumen and columnar simple epithelium of intestine absorb readily SCFA. The mechanisms of SCFA absorption are incompletely known. Passive diffusion of the unionized form across the cell membrane is currently admitted. In the lumen, the necessary protonation of SCFA anions could come first from the hydration of CO2. The ubiquitous cell membrane process of Na+-H+ exchange can also supply luminal protons. Evidence for an acid microclimate (pH = 5.8-6.8) suitable for SCFA-protonation on the surface of the intestinal lining has been provided recently. This microclimate would be generated by an epithelial secretion of H+ ions and would be protected by the mucus coating from the variable pH of luminal contents. Part of the absorbed SCFA does not reach plasma because it is metabolized in the gastrointestinal wall. Acetate incorporation in mucosal higher lipids is well-known. However, the preponderant metabolic pathway for all the SCFA is catabolism to CO2 except in the rumen wall where about 80% of butyrate is converted to ketone bodies which afterwards flow into bloodstream. Thus, SCFA are an important energy source for the gut mucosa itself.

Intestinal transport of weak electrolytes. Evidence in favor of a three-compartment system

A study has been made of the transmural fluxes of benzoic, phenylacetic, and pentanoic acids, benzylamine, hexylamine, and D-amphetamine across rat jejunum incubated in vitro. The M to S fluxes of the weak acids were greater than their corresponding S to M fluxes, and the S to M fluxes of the weak bases were larger than their M to S fluxes. These patterns of asymmetric movements were observed when the transmural electrical potential difference was clamped at 0 mV, and when the pH values of the mucosal and serosal fluids were identical. The effects of a weak acid on the fluxes of other weak electrolytes were qualitatively similar when the effector weak acid was added to the mucosal fluid, and when it was added to the serosal fluid. But the effects of a weak base on the fluxes of other weak electrolytes were dependent upon its location, and the interactions observed when the effector weak base was added to the mucosal fluid were qualitatively different than those seen when it was added to the serosal fluid. The interactions between weak electrolytes could readily be explained in terms of the function of a system of three compartments in series, in which the pH of the intermediate compartment is greater than that of the bulk phases. But these observations could not be explained in terms of an analogous system involving an intermediate compartment of low pH, or in terms of a carrier mediated system. The transport function of the three-compartment system can be described in the form of an equation, and it is found that a pH difference of less than 0.5 unit may explain our observations on weak electrolyte transport.

Handling of glycerides of acetic acid by rat small intestine in vitro

1. When mono-, di- and triacetins are incubated with sacs of rat everted intestine, they enter the epithelial cells and are hydrolysed to free glycerol and acetic acid.2. The rate-limiting step in the process is the entry of glyceride into the epithelial cell.3. The three acetins enter the epithelial cell at the same rate, and the mechanism of this remains unknown.4. The acetate released appears in higher concentrations on the serosal side, and the relation of this to the mechanism for transfer of volatile fatty acids is discussed.5. It is not necessary to postulate a special mechanism for entry of volatile fatty acids into the cell.