Characterization of the kinetics of the passive and active transport mechanisms for bile acid absorption in the small intestine and colon of the rat

Bile acid uptake occurs via passive diffusion in all regions of the intestine and via active absorption in the ileum. Determination of the passive permeability coefficient for ionized monomers ((*)P(-)) demonstrated that permeability decreased by a factor of 3.4, 6.8, and 8.1 for the addition of a hydroxyl, glycine, or taurine group, respectively, to the steroid nucleus. Removal of the negative charge increased permeation by a factor of 4.4; however, permeability coefficients for the protonated monomers showed the same relative decrease with addition of a hydroxyl group. The calculated incremental free energies of solution (deltaDeltaF(W-->1)) associated with these additions equaled + 757 (hydroxyl), + 1178 (glycine), and + 1291 (taurine) cal/mole. Passive permeability coefficients for the transverse colon showed the same relative relationships among the various bile acids. After making appropriate corrections for passive permeability across the ileum, apparent values for the maximal transport velocity ((*)V(max)) and Michaelis constant ((*)K(m)) of the active transport system were measured. (*)V(max) depended upon the number of hydroxyl groups on the steroid nucleus; values for the trihydroxy bile acids were high (1543-1906 pmoles/min per cm) while those for the dihydroxy (114-512 pmoles/min per cm) and monohydroxy (45-57 pmoles/min per cm) acids were lower. In contrast, (*)K(m) values were related to whether the bile acid was conjugated; unconjugated bile acids had values ranging from 0.37 to 0.49 mM, while values for the conjugated bile acids were approximately half as high (0.12-0.23 mM).     

In vitro determination of active bile acid absorption in small biopsy specimens obtained endoscopically or surgically from the human intestine

BACKGROUND: In the construction of a Kock reservoir for continent urinary diversion, 70 cm of the distal ileum are used. Impaired absorption of bile acids in these patients might cause diarrhoea. Data on the absorption of bile acids in different parts of the human intestine are limited. METHODS: Biopsies were taken during endoscopy from the duodenum, the terminal ileum or the right colon, and during surgery 10, 50, 100 and 150 cm proximally to the ileo-caecal valve using standard endoscopy biopsy forceps. The biopsy specimens were incubated in vitro with radio-labelled taurocholic acid at 37 degrees C for 22 or 45 min The radioactivity was determined using the liquid scintillation technique. RESULTS: A linear increase in the uptake was observed, with increased concentrations of taurocholic acid between 100 and 500 microm in all specimens tested, that represented passive uptake or unspecific binding. The active uptake could be calculated from the intercept of the line representing passive uptake with the ordinate. The active uptake in the terminal ileum was 3-4 times greater than 100 cm proximal to the valve. CONCLUSIONS: The active absorption of bile acids in humans can be determined in small biopsy specimens taken using standard biopsy forceps during endoscopy or surgery. This method is suitable for clinical studies of bile acid absorption. Active uptake of bile acids not only takes place in the very distal part of the ileum but also to a considerable degree 100 cm proximally to the ileo-colonic valve. This should be taken into account when selecting the ileal segment for continent urinary diversion.     

Nitrogen absorption from isonitrogenous solutions of L-leucyl-L-leucine and L-leucine: a study in the isolated perfused rat small intestine.

1. We assessed the efficacy of nitrogen absorption from luminal L-leucine (1.2 and 12 mmol/l) and from isonitrogenous L-leucyl-L-leucine (0.6 and 6.0 mmol/l) in a preparation of vascularly and luminally perfused rat small intestine by measuring luminal leucyl-leucine disappearance and venous leucine appearance. 2. No intact dipeptide was found in the vascular perfusate. Leucine-nitrogen absorption, as judged by venous leucine appearance, was as efficient from free leucine (29 +/- 3 and 245 +/- 19 ng-atom min-1 g-1) as from leucyl-leucine (27 +/- 4 and 211 +/- 58 ng-atom min-1 g-1). It was not reduced in the presence of glycyl-L-proline or of the brush-border dipeptidase inhibitors alanine-beta-naphthylamide and cilastatin. 3. After one passage of the whole small intestine, only trace amounts of dipeptide, but large amounts of free leucine, were detected in the luminal effluent. Peptidase activity in the luminal effluent was demonstrated in 100,000 g supernatant and was inhibited by p-hydroxy-mercuribenzoate, but not by brush-border dipeptidase inhibitors. 4. We propose that nitrogen absorption from luminal leucyl-leucine may proceed predominantly via intraluminal peptide hydrolysis and subsequent transport of free leucine. Nevertheless, our findings and conclusions are at variance with previous observations and current opinion on intestinal handling of dipeptides, which may be due, in part, to the different methodological approach.     

Ricinoleic acid effect on the electrical activity of the small intestine in rabbits.

Using myoelectric recording techniques, we examined the myoelectric effects of castor oil; ricinoleic acid (cis isomer), the active ingredient of castor oil; and ricinelaidic acid (trans isomer) in the small intestine of New Zealand white rabbits. Ricinoleic acid, 2 microgram/kg per min (6mM), was perfused into a distal 12-cm ileal loop. An abnormal myoelectric pattern developed that was similar to the alteration in the electrical activity that has previously been reported for cholera enterotoxin. Castor oil, 0.85 ml/kg, had a similar effect. Ricinelaidic acid, 2 microgram/kg per min, induced no activity. A second preparation consisted of an intraluminal perfusion of ricinoleic acid, 2 microgram/kg per min, into the first section of the duodenum. The abnormal myoelectric pattern was observed in the jejunum and the ileum but not the duodenum. The mean onset time for the development of this altered myoelectric state for all experiments was 3.5 h. These studies suggest that an active motility component in addition to the secretory state exists throughout the small intestine that is exposed to castor oil or ricinoleic acid.     

Visualization of the mucosal villi on double-contrast barium studies of the small intestine by using a high molecular fraction of guaran

Guaran is a water-soluble polysaccharide from the seed endosperm ofCyanopsis tetragonolobus. A high molecular fraction of guaran, when added to barium, enhances barium coating of the intestinal mucosa. This study reports its use to demonstrate the mucosal villi of the small intestine during double-contrast barium examination in the rat and in the human. In the animal model, individual villi were identified surrounded by pools of barium. In the human, a granular pattern caused by the villi was seen. Individual villi were visible with high-definition radiography. This was confirmed by microscopic studies of biopsy specimens in which the villi were shown to have the same diameters as the structures seen on high-definition radiography.     

The mechanisms of sodium absorption in the human small intestine

The present studies were designed to characterize sodium transport in the jejunum and ileum of humans with respect to the effects of water flow, sodium concentration, addition of glucose and galactose, and variations in aniomic composition of luminal fluid. In the ileum, sodium absorption occurred against very steep electrochemical gradients (110 mEq/liter, 5-15 mv), was unaffected by the rate or direction of water flow, and was not stimulated by addition of glucose, galactose, or bicarbonate. These findings led to the conclusion that there is an efficiently active sodium transport across a membrane that is relatively impermeable to sodium. In contrast, jejunal sodium (chloride) absorption can take place against only the modest concentration gradient of 13 mEq/liter, was dramatically influenced by water movement, and was stimulated by addition of glucose, galactose, and bicarbonate. The stimulatory effect of glucose and galactose was evident even when net water movement was inhibited to zero by mannitol. These observations led to the conclusion that a small fraction of jejunal sodium absorption was mediated by active transport coupled either to active absorption of bicarbonate or active secretion of hydrogen ions. The major part of sodium absorption, i.e. sodium chloride absorption, appeared to be mediated by a process of bulk flow of solution along osmotic pressure gradients. The stimulatory effect of glucose and galactose, even at zero water flow, was explained by a model in which the active transport of monosaccharide generates a local osmotic force for the absorption of solution (NaCl and water) from the jejunal lumen, which, in the presence of mannitol, is counterbalanced by a reverse flow of pure solvent (H(2)O) through a parallel set of channels which are impermeable to sodium. Support for the model was obtained by the demonstration that glucose and bicarbonate stimulated the absorption of the nonactively transported solute urea even when net water flow was maintained at zero by addition of mannitol to luminal contents.     

运动对小肠铁吸收及一氧化氮的调节作用

目的:运动性低铁状态在运动员中发生率明显高于一般人群,但补铁效果很不理想。运动时一氧化氮对铁代谢的调节活动以及功能关系已成为近年研究的热点,对了解运动导致铁缺乏的原因以及运动对铁代谢影响的生理或病理生理机制尤为关键,且有利于运动员铁状态的改善。资料来源:应用计算机检索PUBMED1994-01/2005-10期间的相关文章,检索词为“exercise,iron,nitricoxide”,并限定文章语言种类为“English”。同时计算机检索万方数据库2000-01/2005-10期间的相关文章,检索词为“运动,铁,一氧化氮”,并限定文章语言种类为中文。资料选择:对资料进行初审,并查看每篇文献后的引文。纳入标准:文章所述内容应与运动时一氧化氮对铁代谢的调节活动以及功能关系的研究相关。排除标准:重复研究或Meta分析类文章。资料提炼:共收集到60篇相关文献,28篇文献符合纳入标准,排除的32篇文献为内容陈旧或重复。符合纳入标准的28篇文献中,4篇涉及铁代谢与一氧化氮的联系,8篇涉及运动过程中铁代谢与一氧化氮的关系,8篇涉及一氧化氮对小肠生理和病理研究的意义,7篇涉及小肠黏膜细胞对铁的吸收情况。资料综合:铁是生命体中不可缺少的过渡金属,在对运动员及动物运动模型的铁代谢研究中发现,运动可导致运动性铁缺乏,甚至发生铁缺乏性贫血。小肠黏膜是机体铁转运的第一步,弄清楚铁在小肠黏膜处的吸收机制是解决这个问题的关键,几个与铁吸收有关的大分子蛋白的结构的掌握,为研究运动状态下小肠黏膜铁吸收情况奠定了理论基础。虽然运动性低铁状态在运动员中发生率明显高于一般人群,但运动员补铁效果不理想,耐力运动员体内铁储存量是否趋向于低水平仍有争议。运动对铁代谢的影响,一方面认为是很多原因引起的铁负平衡的结果,若是这种缺铁状况,对机体进行补铁应是有效的,应对运动员进行常规补铁。另一方面认为是机体正常的铁调节结果,应当尽量减少补铁,以免增加铁毒性。结论:血清中的铁含量与一氧化氮水平呈负相关,补充一氧化氮合成酶抑制剂L-NAME可明显改善低铁状态,由此假设一氧化氮可能是改善铁代谢的因素之一。小肠黏膜上皮是机体铁转运的第一步,一氧化氮是否影响此处铁的转运有待于进一步分析。     

运动对小肠铁吸收及一氧化氮的调节作用

目的：运动性低铁状态在运动员中发生率明显高于一般人群，但补铁效果很不理想。运动时一氧化氮对铁代谢的调节活动以及功能关系已成为近年研究的热点，对了解运动导致铁缺乏的原因以及运动对铁代谢影响的生理或病理生理机制尤为关键，且有利于运动员铁状态的改善。 资料来源：应用计算机检索PUBMED 1994—01／2005-10期间的相关文章，检索词为“exercise，iron，nitric oxide”，并限定文章语言种类为“English”。同时计算机检索万方数据库2000-01／2005-10期间的相关文章，检索词为“运动，铁，一氧化氮”，并限定文章语言种类为中文。 资料选择：对资料进行初审，并查看每篇文献后的引文。纳入标准：文章所述内容应与运动时一氧化氮对铁代谢的调节活动以及功能关系的研究相关。排除标准：重复研究或Meta分析类文章。 资料提炼：共收集到60篇相关文献，28篇文献符合纳入标准，排除的32篇文献为内容陈旧或重复。符合纳入标准的28篇文献中，4篇涉及铁代谢与一氧化氮的联系，8篇涉及运动过程中铁代谢与一氧化氮的关系，8篇涉及一氧化氮对小肠生理和病理研究的意义，7篇涉及小肠黏膜细胞对铁的吸收情况。 资料综合：铁是生命体中不可缺少的过渡金属，在对运动员及动物运动模型的铁代谢研究中发现，运动可导致运动性铁缺乏，甚至发生铁缺乏性贫血。小肠黏膜是机体铁转运的第一步，弄清楚铁在小肠黏膜处的吸收机制是解决这个问题的关键，几个与铁吸收有关的大分子蛋白的结构的掌握，为研究运动状态下小肠黏膜铁吸收情况奠定了理论基础。虽然运动性低铁状态在运动员中发生率明显高于一般人群，但运动员补铁效果不理想，耐力运动员体内铁储存量是否趋向于低水平仍有争议。运动对铁代谢的影响，一方面认为是很多原因引起的铁负平衡的结果，若是这种缺铁状况，对机体进行补铁应是有效的，应对运动员进行常规补铁。另一方面认为是机体正常的铁调节结果，应当尽量减少补铁，以免增加铁毒性。 结论：血清中的铁含量与一氧化氮水平呈负相关，补充一氧化氮合成酶抑制剂L—NAME可明显改善低铁状态，由此假设一氧化氮可能是改善铁代谢的因素之一。小肠黏膜上皮是机体铁转运的第一步，一氧化氮是否影响此处铁的转运有待于进一步分析。