THE STUDY AND ASSAY OF SUBSTANCES AFFECTING INTESTINAL ABSORPTION IN THE MOUSE

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.

Choice of anesthetic for intestinal absorption and secretion experiments using rats

The net rates of glucose and water absorption from, and secretion of water into, the lumen of the rat small intestine were compared using alpha-chloralose, halothane, pentobarbitone, and urethane as anesthetics. Water and glucose transport were determined by recirculation of perfusion fluid through the lumen of the jejunum. Net fluid secretion was stimulated by intraarterial (i.a.) infusion of prostaglandin E1 (PGE1) and vasoactive intestinal peptide (VIP). The results show that the anesthetics used exert only minimal influence on the physiological rates of glucose and water absorption and there is no correlation between the small differences in glucose and water absorption among the anesthetics. However, when the fluid secretory mechanism of the mucosa is stimulated with either PGE1 or VIP, pentobarbitone allows the greatest rates of secretion, but halothane prevents secretion. It is suggested that of the anesthetics used, pentobarbitone is the most appropriate for intestinal secretion experiments.

Stimulation of water and sodium secretion and inhibition of glucose absorption from the rat jejunum during intraarterial infusions of prostglandins

The effect of prostaglandins, isoprenaline and dibutyryl cyclic adenosine monophosphate (dibut cAMP) on the net transfer of water and Na+ and glucose absorption have been studied in the anaesthetized rat. The lumen of the jejunum was recirculated with a solution of normal saline containing D (+) glucose and phenosulphonphthalein. The superior mesenteric artery was perfused extracorporeally and drugs were infused into the arterial blood. Prostaglandin E1 (PGE1) at infusion rates of 2-5 X 10(-9) and 10(-8) mol/min induced a net secretion of water and Na+ which was significantly different from the infused control mean. Both water and Na+ secretion were dose-related. PGE1 induced slight but significant inhibition of glucose absorption at 10(-8) mol/min. This could either be a direct effect or secondary to solvent drag. PGE2 (2-5 X 10(-9) mol/min) and PGF2alpha (8 X 10(-9) mol/min) induced water and Na+ secretion, and inhibited glucose absorption but not significantly. Isoprenaline (10(-9) mol/min) and dibut cAMP (8 X 10(-9) mol/min) did not alter water, Na+ or glucose absorption.

Inhibition of intestinal absorption by different samples of cetrimide and the homologous alkyl series C10-C20

The effects of different samples of cetrimide on the intestinal absorption of D-glucose, sodium butyrate and DL-methionine were compared with those of homologous compounds C(10)-C(20) of the same alkyl series, by in vivo intraluminal perfusion in the mouse, rat and guinea-pig. Two samples of “cetrimide” had different inhibitory activities in the mouse and rat but were equiactive in the guinea-pig in which both samples caused glucose reversal. None of the homologues, C(10)-C(20), paralleled the activity of either of the cetrimide samples exactly. It is concluded that cetrimide should be assayed in the rat before its use in studies of absorption and protein binding.