Calcium-cadmium interaction on sugar absorption across the rabbit jejunum

The element Cd is considered to have no biological function and is highly toxic to humans and animals. Toxic effects of this metal upon cell membrane structure and function have been shown. On the other hand, Ca is an essential element in a wide variety of cellular activities. The present study was initiated to research whether the interaction between Ca and Cd could affect D-galactose absorption across the rabbit jejunum in vitro. In media with Ca²⁺, when CdCl₂ was present at 0.5 or 1 mM, Cd was found to significantly reduce the sugar absorption. In Ca²⁺-free media, where CaCl₂, was omitted and replaced isotonically with choline chloride, the sugar transport was not modified by Cd, but when CaCl₂ was replaced isotonically with MgCl₂, the inhibition is observed. Verapamil at 10⁻⁶ M (blocking mainly Ca²⁺ transport) did not modify the inhibitory effect of cadmium on D-galactose transport. When 10⁻⁶ M of A 23187 (Ca²⁺ specific ionophore) was added in media with/without Ca²⁺; CdCl₂ produced no change in D-galactose transport. These results suggest that Ca and Cd could have affinity for the same chemical groups of enterocyte membrane, which would be related with the intestinal absorption of D-galactose.     

Study of interaction between calcium and zinc ond-galactose intestinal transport

Zinc is an essential trace element necessary to life. This metal may exert some of its physiological effects by acting directly on cellular membranes, either by altering permeability or by modulating the activity of membrane-bound enzymes. On the other hand, calcium is an essential element in a wide variety of cellular activities. The aim of the present work was to study a possible interaction between zinc and calcium on intestinal transport ofd-galactose in jejunum of rabbit in vitro. In media with Ca²⁺, when ZnCl₂ was present at 0.5 or 1 mM, zinc was found to reduce thed-galactose absorption significantly. In Ca²⁺-free media, where CaCl₂ was omitted and replaced isotonically with choline chloride, the sugar transport was not modified by zinc. Verapamil at 10⁻⁶ M (blocking mainly Ca²⁺ transport) did not modify the inhibitory effect of zinc ond-galactose transport. When 10⁻⁶ M of A 23187 (Ca²⁺-specific ionophore) was added with/without Ca²⁺ to the media, ZnCl₂ produced no change in sugar transport. These results could suggest a possible interaction of calcium and zinc for the same chemical groups of membrane, which could affect the intestinal absorption of sugars.     

TLR2, TLR3, and TLR4 activation specifically alters the oxidative status of intestinal epithelial cells

Intestinal inflammatory diseases are the result of multiple processes, including mucosal oxidative stress and perturbed homeostasis between commensal bacteria and mucosal immunity. Toll-like receptors (TLRs) recognize molecular-associated microorganisms' patterns and trigger innate immunity responses contributing to intestinal homeostasis and inflammatory responses. However, TLRs effects on redox balance in intestinal mucosa remain unknown. Therefore, the present study analyzes the effect of TLR2, TLR3, and TLR4 on both oxidative damage of lipids and proteins, and the activity of antioxidant enzymes in enterocyte-like Caco-2 cells. The results show that the activation of these TLRs increased lipid and protein oxidation levels; however, the effect on the antioxidant enzymes activity is different depending on the TLR activated. These results suggest that the activation of TLR2, TLR3, and TLR4 might affect intestinal inflammation by not only their inherent innate immunity responses, but also their pro-oxidative effects on intestinal epithelial cells.     

The administration of lipopolysaccharide, in vivo, induces alteration in L-leucine intestinal absorption

The objective of the present study was to determine the alterations in L-leucine intestinal uptake by intravenous administration of Lipopolysaccharide (LPS), which is a constituent of gram negative bacterial, causative agent of sepsis. The amino acid absorption in LPS treated rabbits was reduced compared to the control animals. The LPS effect on the amino acid uptake was due to an inhibition of the Na+-dependent system of transport, through both reduction of the apparent capacity transport (Vmax) and diminution of the Na+/K-ATPase activity. The results have also shown that the LPS decreases the mucosal to serosal transepithelial flux and the transport across brush border membrane vesicles of L-leucine. The study of possible intracellular mechanisms implicated in the LPS effect, showed that the second messengers calcium, protein kinase C and c-AMP did not play any role in this effect. However, the absence of ion chloride in the incubation medium removes the LPS inhibition and the intracellular tissue water was affected by the LPS treatment. Therefore, the inhibition in the L-leucine intestinal absorption, by intravenous administration of LPS, could be mainly produced by the secretagogue action of this endotoxin on the gut.     

The annexin A3-membrane interaction is modulated by an N-terminal tryptophan

The crystal structure of annexin A3 (human annexin III) solved recently revealed a well-ordered folding of its N-terminus with the side chain of tryptophan 5 interacting with residues at the extremity of the central pore. Since the pore of annexins has been suggested as the ion pathway involved in membrane permeabilization by these proteins, we investigated the effect of the N-terminal tryptophan on the channel activity of annexin A3 by a comparative study of the wild-type and the W5A mutant in structural and functional aspects. Calcium influx and patch-clamp recordings revealed that the mutant exhibited an enhanced membrane permeabilization activity as compared to the wild-type protein. Analysis of the phospholipid binding behavior of wild-type and mutant protein was carried out by cosedimentation with lipids and inhibition of PLA(2) activity. Both methods reveal a much stronger binding of the mutant to phospholipids. The structure is very similar for the wild-type and the mutant protein. The exchange of the tryptophan for an alanine results in a disordered N-terminal segment. Urea-induced denaturation of the wild-type and mutant monitored by intrinsic fluorescence indicates a separate unfolding of the N-terminal region which occurs at lower urea concentrations than unfolding of the protein core. We therefore conclude that the N-terminal domain of annexin A3, and especially tryptophan 5, is involved in the modulation of membrane binding and permeabilization by annexin A3.     

Effect of zinc on L-threonine transport across the jejunum of rabbit

Zinc is an essential trace element for life. Many metalloenzymes involved in the metabolism of carbohydrates, lipids, protein, and nucleic acids require zinc for their functions. The aim of this study was to characterize how zinc acts on the intestinal amino acid absorption in rabbit. Results obtained show that zinc inhibits both L-threonine accumulation in the jejunum tissue, and mucosal-to-serosal transepithelial flux of this amino acid in a dose-dependent way. The inhibition does not increase by a 10-min previous intestinal exposure of the mucosa to the heavy metal, and is not reversed by washing the intestinal tissue with saline solution or 10mM EDTA, but is appreciably reversed with 10mM dithioerythritol. Zinc seems not to modify amino acid diffusion across the intestinal epithelium. The inhibition of intestinal amino acid transport by zinc seems to be of a competitive type, and appears to be a result of impairment of the active transport that is altered by its binding to proteins (prevailing to thiol groups) of the brush-border membrane of enterocytes.     

Effect of serotonin on D-galactose transport across the rabbit jejunum

The patterns of storage and release of serotonin found in the enterochromaffin cells of the intestinal mucosa suggest that this hormone may be an important modulator of intestinal functions. Serotonin has been shown to produce secretion of water and electrolytes in rabbit ileum, but the hormone does not appear to interact significantly with other transport processes. The aim of the present study was to determine the effect of serotonin on D-galactose absorption in rabbit jejunum. The results obtained show that serotonin (10(-8) and 10(-6)M) partially reduced (by 20 and 40% respectively) D-galactose uptake across the mucosal border. This effect was concentration-dependent, and it seemed to be caused by the inhibition of Na+-dependent sugar transport. Methysergide, an antagonist of serotonin which binds with receptor 2 of serotonin, blocked the effect of serotonin. These findings suggest that serotonin may act as a regulator of sugar intestinal absorption, and that this serotonin regulation could be mediated by a direct or indirect action of the complex serotonin-receptor, which may inhibit the Na+-dependent transport system of sugars located in the brush-border membrane.