Absorption and metabolism of fructose by rat jejunum.

The absorption and metabolism of fructose was investigated in the vascularly perfused jejunum of fructose-fed rats. With 10 mM-glutamate and 10 mM-fructose in the lumen, the viability of the tissue is maintained and fructose is absorbed and utilized at high rates. With 28 mM-fructose in the lumen, glucose appears in the vascular bed. With 10 mM- or 28 mM-fructose in the presence of 10 mM- or mM-glucose in the lumen, the fructose absorption is decreased. From 10 mM- or 28 mM-sucrose in the lumen, fructose uptake is also less than from the equivalent concentration of free fructose. The rate of appearance of fructose in the vascular bed is independent of the source of fructose from which it is derived. In the presence of glucose, either free or as sucrose, there is a marked decrease in the utilization of fructose, defined as the difference between that absorbed by the jejunum and that transported unchanged into the vascular bed. In all cases about half of the carbohydrate absorbed from the lumen is converted into lactate, most of which is secreted into the blood. The absorption of glucose and the rate of vascular appearance of glucose from glucose in the lumen are about 1.5 times greater than those of fructose from fructose in the lumen. It is concluded: firstly, that fructose uptake from the lumen of rat jejunum is determined by its concentration and by the demand for it as a fuel for the intestine, a demand that is severely decreased in the presence of glucose; secondly, that in the vascularly perfused jejunum there is no evident kinetic advantage for uptake of fructose or glucose from sucrose rather than from free monosaccharide in the lumen; thirdly, that some fructose can be converted into glucose.     

Absorption of 3-oxy-methyl-D-glucose by chicken cecum and jejunum in vivo

Jejunal and cecal 3-oxy-methyl-D-glucose (3-OMG) absorption was studied in 4- to 8-week-old chickens by an in vivo perfusion technique (perfusion rate 1.5 ml/min). Total and phloridzin-insensitive 3-OMG absorption was tested for lumenal substrate concentrations ranging from 1.25 to 20 mmol/l. The estimated apparent Michaelis constants in jejunum and cecum were 5.1 and 4.0 mmol/l (Lineweaver-Burk method) and 3.2 and 3.1 mmol/l (visual inspection method), respectively. Vmax were similar in both segments with either method, about 0.3 mumol/cm2 X 5 min. Passive permeability coefficients were the same in both regions (about 45 l/cm2 X 5 min X 10(3)). The transport properties of the cecal epithelium in vivo suggest a role of these intestinal segments in the absorption of nutrients originated from digestive processes.     

Absorption of 2-acetylaminofluorene in the guinea-pig colon

Absorption of the known chemical carcinogen, 2-acetylaminofluorene has been measured in the colon of guinea pigs. Unidirectional influx across the luminal cell membrane was determined in vitro, and transmural absorption across colonic mucosa was evaluated in vivo. The kinetics of unidirectional influx into colon in vitro do not indicate that absorption proceeds by simple diffusion. The observed saturable uptake is indicative of binding of 2-acetylaminofluorene to a cellular component. With 2-acetylaminofluorene present in the lumen in vivo at an initial concentration of 3.5 microM, the rate of absorption decreases over a 20 min period, which also indicates some form of specific interaction between 2-acetylaminofluorene and the intestinal mucosa. We have evaluated the hypothesis that surfactants and a bile salt act as cocarcinogens by increasing the rate of intestinal absorption of 2-acetylaminofluorene. The results lend no support to this possibility.     

水,电解质平衡的激素调节

本文综述心钠素、抗利尿激素和肾素-血管紧张素-醛固酮系统对水、电解质的调节作用及其相互作用。机体内存在排钠、排水和保钠、保水两大类激素,两类激素的平衡,对水、电解质平衡的调节具有重要意义。     

Electrophysiological characterization of chloride secretion across the jejunum and colon of pigs as affected by age and weaning

Hypersecretion of chloride can cause diarrhea, a disease frequently occurring in young pigs, particularly around weaning. We investigated the contribution of different channels to intestinal Cl⁻ secretion as influenced by age and weaning. Jejunal and colonic epithelia from 4-month-old pigs and 4-week-old piglets were incubated in Ussing chambers and stimulated by carbachol and forskolin. Changes in short-circuit currents were taken as measure of electrogenic net Cl⁻ secretion. DIDS or NPPB served to inhibit Ca-activated Cl⁻-channels and outwardly rectifying Cl⁻-channels (ORCC) or cystic fibrosis transmembrane regulator (CFTR), respectively. Depolarizing the basolateral membrane allowed to examine the influence of K⁺-channels on Cl⁻ secretion. Forskolin-stimulated Cl⁻ secretion was mediated by CFTR. ORCC were not involved. Carbachol-induced Cl⁻ secretion could be ascribed to an enhanced driving force due to the opening of K⁺-channels, whereas Ca-dependent Cl⁻ channels seemed not to be involved. In jejunum, piglets showed higher Cl⁻ secretion than pigs. Two days after weaning forskolin induced an I _sc overshoot and a faster increase in G _t. In colon, Cl⁻ secretion was neither influenced by age nor by weaning. The data suggest a disposition of porcine jejunum for a higher Cl⁻ secretion in young and freshly weaned piglets, which might be a natural defense mechanism as well as a predisposing factor for diarrhea.