Lymphatics and fibroblasts support intestinal stem cells in homeostasis and injury

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Geometric peculiarities of intestinal surface and efficiency of coupling between membrane hydrolysis and transport of nutrients

Kinetics of the membrane hydrolysis of maltose and of the absorption of the released glucose in the isolated loop of rat small intestine has been examined in a wide range of maltose concentrations (25–200 mM) under the conditions of chronic experiments. The processes studied were simulated by means of mathematical models using two approximations of the villous surface of intestinal epithelium: (i) smooth flat surface with adjoining pre-epithelial (“unstirred”) layer and (ii) folded surface with “unstirred” layer between the folds and partly above them. The results of modeling matched well the experimental data in the whole range of maltose concentrations only in the case of the folded surface. A model with this approximation predicts a closer coupling between maltose hydrolysis and absorption of released glucose as well as a lower glucose concentration in the intestinal lumen than in the case of a flat surface. We conclude that in order to evaluate correctly a relative role of various mechanisms of glucose transport across intestinal epithelium under normal conditions, one should take into account the pre-epithelial layer of the small intestine and geometric peculiarities of its epithelial surface.     

The gastrointestinal tract "tastes" nutrients: evidence from the intestinal taste aversion paradigm

To develop and use a behavioral paradigm for assessments of what nutrient properties are detected by intestinal chemoreceptors, we combined features of the "electronic esophagus" preparation (Elizalde G and Sclafani A. Physiol Behav 47: 63-77, 1990) and the conditioned taste aversion protocol (Garcia J and Koelling RA. Psychon Sci 4: 123-124, 1966). In four experiments, separate groups of food-deprived rats with gastric (experiments 1-4) or duodenal (experiment 4) catheters were infused with either carbohydrates (maltodextrin) or fats (corn oil) into their stomachs or small intestines, either while they consumed nonnutritive flavored solutions (experiments 1 and 2) or in the absence of any intake (experiments 3 and 4). For some animals, one of the macronutrient infusions was paired with lithium chloride injections shown to support conventional conditioned aversions. After training, in various oral preference test trials, animals were given opportunities to taste and consume the nonnutritive solutions that had served as oropharyngeal conditioned stimuli as well as the nutrients that had been infused intragastrically, with or without poisoning, but never sampled by mouth. As previously established, preferences for the nonnutritive flavors were enhanced by association with intragastric infusions of macronutrients, with carbohydrates producing the greater preference. On first exposure to the two macronutrients for oral consumption, animals reduced their intake of the nutrient that had been previously poisoned when it was infused into the gastrointestinal tract. These results, along with additional controls, suggest that nutrient tastes detected in the intestines can be recognized centrally based on oropharyngeal gustatory stimulation.     

A preferred conformation in the vasoactive intestinal peptide (VIP). Molecular architecture of gastrointestinal hormones

The ORD spectrum of the vasoactive intestinal peptide (VIP) in water indicates a preferred conformation with low helix content. Addition of organic solvents, especially of trifluoroethanol, results, even at low solvent concentration, in spectra with pronounced helical character. The readiness of shorter chains, with C-terminal sequences of VIP, to take up helical conformation under the effect of organic solvents parallels their biological activity. This suggests that an “active architecture” may be required for the interaction between hormone and receptor.     

Transport of nutrients and hormones through the blood-brain barrier

The transport of circulating nutrients (glucose, amino acids, ketone bodies, choline, and purines) through the brain endothelial wall, i.e., the blood-brain barrier (BBB), is an important regulatory step in several substrate-limited pathways of brain metabolism. The in vivo kinetics of nutrient transport has been well characterized in the rat, and the kinetic constants of saturable (Km, Vmax) and nonsaturable (KD) transport through the BBB are now known for more than 30 circulating nutrients. The kinetic constants can be used to gain insight into the important rate-limiting role played by BBB nutrient transport in the regulation of brain metabolism and function. Unlike most nutrients, steroid and thyroid hormones circulate tightly bound to plasma proteins. However, owing to favorable kinetic relationships among brain capillary transit times and rates of hormone dissociation from plasma proteins and hormone diffusion through the brain endothelia, the BBB is able to strip hormones off circulating plasma proteins. With regard to peptide hormone, no specific BBB transport systems for peptides have been identified thus far. However, peptides are able to rapidly distribute into brain interstitial space at the circumventricular organs. In addition, specific receptors for insulin are located on the BBB. The presence of BBB peptide receptors provides a mechanism by which circulating peptides may rapidly influence brain function without the peptide crossing the BBB.     

Recent advances in gastrointestinal hormones]

Recent advances in the field of peptide chemistry and gene technology have resulted in an explosive accumulation of information on the chemical structures of gastrointestinal hormones. Based on the information, chemical syntheses of these peptides or their shorter fragments and analogs have been performed. Synthetic peptides related to the hormones have now become important tools in searching for functions of peptides and in producing region-specific antisera to the respective peptides. Using these antisera, hormone-producing cells were clearly identified and the post-translational biosynthetic processings in the cells were demonstrated. Recent immunohistochemical studies have also revealed that cells contain and can release a variety of peptides or amines that are capable of influencing target cells and acting as hormone, neurotransmitter or neuromodulator. In addition, recent studies on galanin and glucagon-like peptide-1 (GLP-1) are described.     

Foliar absorption and transport of inorganic nutrients

All plant cells — those of roots, leaves, or other parts — are capable of absorbing water and solutes as well as gaseous substances. This trait, derived by terrestrial plants during evolution from their ancestral aquatic habitat, is exploited in many agronomic practices. The pathway of entry of nutrients supplied to the leaf involves penetration of cuticular membranes enveloping it, absorption by the cells within, and transport away from the leaf. These processes are affected by humidity, temperature, and physiology of the leaf and influenced by surfactants and growth substances. Foliar injury due to sprays is associated with concentration and nature of the solutes. Considerable knowledge has been acquired in the last few years on the mechanisms of foliar absorption and mobility of several elements. Greater importance is now attached to foliar feeding, when soil‐ground water pollution is attendant with soil fertilization.     

A framework integrating plant growth with hormones and nutrients

It is well known that nutrient availability controls plant development. Moreover, plant development is finely tuned by a myriad of hormonal signals. Thus, it is not surprising to see increasing evidence of coordination between nutritional and hormonal signaling. In this opinion article, we discuss how nitrogen signals control the hormonal status of plants and how hormonal signals interplay with nitrogen nutrition. We further expand the discussion to include other nutrient-hormone pairs. We propose that nutrition and growth are linked by a multi-level, feed-forward cycle that regulates plant growth, development and metabolism via dedicated signaling pathways that mediate nutrient and hormonal regulation. We believe this model will provide a useful concept for past and future research in this field.     

The structure of gastrointestinal hormones.

The primary and secondary structure of the three kinds of gastrointestinal hormones, i.e. gastrin, cholecystokinin-pancreozymin and secretin are discussed.