The effects of cadmium on zinc absorption in isolated rat intestinal preparations

The effect of cadmium on zinc absorption was studied using an isolated vascularly and luminally perfused rat intestinal preparation.⁶⁵Zn as well as Zn and Cd (both as the chloride salt) were added to the luminal perfusion medium (LPM) at varying concentrations. Over a 90-min period, the amount of Zn appearing in the vascular perfusion medium (VPM) and that retained by the tissue post-perfusion was estimated. Cd at all levels studied (0.03, 0.10, 1.0, and 10.0 μg/mL) reduced the amount of Zn appearing in the VPM in comparison with control perfusions (no detectable Cd in the LPM) when the initial Zn concentration was 5 μg/mL. Similarly, with an initial Zn concentration of 10 or 20 μg/mL, the amount of Zn appearing in the VPM was reduced when the Cd concentration was 0.1 or 1.0 μg/mL. With these same Zn concentrations, the amount of Zn retained by the tissue was higher when the Cd concentration was 10 μg/mL. These results demonstrate that Cd at low concentrations is capable of reducing Zn appearance in the VPM.     

Inhibition of intestinal copper absorption by divalent cations and low-molecular-weight ligands in the rat

l-histidine (His) has been shown to enhance the inhibitory effect of zinc on intestinal copper absorption. This study was aimed at examining whether this effect of His was also extended to the interactions of other divalent cations: ferrous iron, tin, and cobalt, using an in vivo perfusion system in rats. Copper absorption and intestinal content of this element significantly decreased in the presence of 2 mM His and ferrous iron. Iron accumulation was greater when His was present than when omitted. A fivefold excess of tin inhibited copper absorption only when His was present. Citrate, at the same concentration as His, had no effect on copper absorption, but hepatic copper levels were increased, as compared to the absence of either His or citrate. Addition of 0.5 or 1.0 mM cobaltous salt plus His resulted in a sharp decrease in copper intestinal absorption, with an increase in intestinal tissue retention. These results confirm earlier findings with zinc and His, and suggest that a general phenomenon, either accelerating the removal of copper from the intestinal lumen or increasing, the retention of this element by the intestinal tissue, is a common feature of the interaction between cations of similar electronic configuration to copper and a high-affinity ligand, such as His.     

The role of cernitins in cadmium effect on the absorption processes in rat small intestine

The effect of long-term intoxication of cadmium (administered subcutaneously in a dose of 1 mg Cd/kg of body weight once a week for one month) on the absorption of water, sodium, potassium, glucose, glycine and thiamine in the small intestine of rats was investigated. In addition, the influence of cernitins (special pollen extract) on the action of cadmium intoxication was tested. The cernitins were given by stomach pump in the form of an aqueous solution of Pollitabs Sport tablets in a dose of 1.5 mg of cernitin T-60 and 0.075 mg of cernitin GBX per individual twice a week for two weeks or four weeks according to the group of animals tested. The results indicated that long-term administration of cadmium increased the intestinal absorption of all tested substances. Meanwhile, the application of cernitins reduced the effects of cadmium intoxication upon intestinal absorption and the processes of absorption was close to normal.     

The effects of cadmium and copper on the renal uptake and metallothionein binding of gold in the rat and hamster

Rats and hamsters, (pre)-treated with copper and cadmium, were used to investigate whether species-differences in renal metallothionein synthesis in response to gold were determined by changes in the kidney concentrations of other metals. The effects of both dietary copper limitation and excess on the renal metabolism of gold also were studied in the rat. In this species, all of the pre-treatments affected the renal concentrations of total and metallothionein-bound copper, but none of them altered either the kidney uptake or thionein-binding of gold. Incorporation of zinc into the metallothionein, which accompanied the binding of gold in this fraction of the kidney, however, was influenced slightly by the pretreatments. In hamsters, pretreatment with cadmium, which increased the concentrations of total and thionein-bound zinc in the kidneys, also did not affect the renal uptake of gold, although it increased significantly the binding of gold to the metallothionein fraction of the renal cytosol. This increased binding of gold also was accompanied by further increases in the zinc and copper contents of the metallothionein; the contents of total and thionein-bound cadmium, however, remained essentially unchanged. Concentrations of copper and zinc in the hamster kidney were not affected significantly by subcutaneous administration of copper alone (five daily doses, each of 3.2 mg Cu/kg body wt.), but were increased when gold was given during the copper-treatment. The concentrations of gold, copper and zinc in the renal metallothionein fraction also were increased under these conditions. From these results it seems that kidney metallothionein synthesis in response to gold may be related to the changes in either the concentration or distribution of zinc, rather than copper.     

The role of bile and digestive juices in cadmium effect on the absorption processes in rat small intestine

The effects of bile and digestive juices was studied on the intestinal absorption of water, sodium and glucose in the small intestine of rats after their intoxication with one dose of cadmium 1.33 mg/kg of body weight injected intravenously. The investigations were carried out on 60 rats by the method of intestinal perfusion. The obtained results showed that cadmium inhibited the intestinal absorption of these substances. Bile and digestive juices abolished partially this effect during their physiological secretion. After administration of cholagogues no protective role of bile and digestive juices was observed alleviating the toxic effects of cadmium, and the intestinal absorption was even more reduced.     

Inhibition of L-threonine intestinal absorption in rabbits by cadmium

Cadmium compounds are widely spread in the environment. Animal exposure to cadmium compounds occurs mainly through foods or drinks contaminated by this metal. Cadmium has been shown to produce several negative effects on the gastrointestinal tract such as inhibition on sugars and amino acids absorption. The aim of the present work was to study the inhibitory characteristics of cadmium on L-threonine intestinal absorption in rabbits in order to understand about this malabsorption of nutrients. Our results show that L-thereonine tissue accumulation as well as mucosal to serosal transepithelial fluxes are decreased in a dose-dependent manner in rabbit jejunum. Amino acid diffusion across the intestinal epithelium was not affected by cadmium. A noncompetitive mechanism and a partial reversion by dithioerythritol (thiol groups protector) is described for this inhibition.     

The effect of cysteine and 2,4-dinitrophenol on heme and nonheme absorption in a rat intestinal model

Previous studies have showed that purified heme iron forms insoluble polymers that are poorly absorbed. The presence of peptides and of amino acids maintaining heme iron in a soluble form could improve its bioavailability. The digestive uptake and transfer of a concentrated hydrolysate of heme peptides (HPH) and of iron gluconate (Gluc) at 100 μM were compared in vitro in a Ussing chamber. The effects of an enhancing amino acid (L-cysteine) on the uptake and transfer of both forms were assessed. An inhibitor of the oxidative phosphorylation (2,4-dinitrophenol; DNP) was used to differentiate the active and passive mechanisms of the absorption. The mucosal uptake (%Tot) and enterocyte transfer (%S) of the two sources of iron did not differ. DNP significantly reduced %Tot and %S of both forms. Cysteine significantly enhanced %Tot and %S of HPH and Gluc, partly corrected the inhibition exerted by DNP on %Tot of HPH and %S of both forms, and fully restored %Tot of Gluc. In presence of peptides produced by globin hydrolysis, the absorption of hemoglobin iron was efficient; it was mostly energy dependent and, therefore, should have occurred by a regulated transcellular pathway. Cysteine enhanced the passive uptake of iron and the passive processes involved in the enterocyte transfer of the common pool made of both sources (heme and nonheme) of iron. These results showed that heme iron can be purified and concentrated without impairing its digestive absorption, provided it remains in presence of peptides or amino acids.     

Regulation of copper absorption by copper availability in the Caco-2 cell intestinal model

Relatively little is known about the individual steps in intestinal copper absorption and whether or how they may be regulated. Polarized Caco-2 cell monolayers with tight junctions offer an already tested model in which to study intestinal metal transport. This model was used to examine potential effects of cellular copper availability on copper absorption. Uptake and transport were determined on application of (64)Cu(II) to the brush border. In the range of 0.2-2 micro M, uptake was dose dependent and was approximately 20% of dose/90 min. Overall transport of (64)Cu across the basolateral surface was approximately 0.3%. When cellular copper levels were depleted 40% by 18-h pretreatment with the specific copper chelator triethylenetetraamine, uptake and overall transport were markedly increased, going to 80 and 65% of dose, respectively. Cellular retention of (64)Cu fell fourfold, from 6 to 1.5%. Depletion of copper with the chelator was rapid and preceded initial changes in uptake and overall transport by 4 h. A lesser depletion of cellular copper (13%) failed to enhance copper uptake but doubled the rate of overall transport, as measured with (64)Cu and by atomic absorption. As previously reported, preexposure of the cells to excess copper (10 micro M, 18 h) also enhanced copper uptake ( approximately 3-fold). In contrast, ascorbate (10-1,000 micro M) failed to significantly alter uptake and transport of 1 micro M (64)Cu. Our findings are consistent with the concepts that, in the low physiological range, copper availability alters the absorption capacity of the intestine to support whole body homeostasis and that basolateral transport is more sensitively regulated than uptake.     

The effects of cytoskeletal inhibitors on intestinal iron absorption in the rat

An established and validated method using loops of intestine in vivo in rats was used to study the effects of cytoskeletal inhibitors on iron absorption. Radioactive iron instilled into the loop of intestine pretreated with test substance was monitored in the blood and, after death, ferritin loading with radioactive iron was measured on density gradients of mucosal cell homogenates and absorbed iron in the carcass was determined. Colchicine, vincristine and cytochalasin B all caused dose- and time-dependent inhibition of iron absorption, and the effects of cytochalasin B were reversible within 1 h. It is not known which cellular component is the vehicle for the transcellular movement of iron from the intestinal lumen onto plasma transferrin; however, this study showed that the uptake of iron by ferritin in an iron-absorbing loop of intestine paralleled the actual absorption of iron into the carcass. This phenomenon did not occur in non-iron-absorbing intestinal and was inhibited by the action of the cytoskeletal inhibitors in the iron-absorbing region. Previously we had shown that iron uptake into cells and onto cellular transferrin was virtually the same throughout the small intestine, irrespective of the iron-absorbing capacity of the region. The results of this study therefore suggest that iron absorption depends on an intact cytoskeletal system and that ferritin in the iron-absorbing cell is able to load from the pool of iron committed to transcellular movement onto plasma transferrin.     

Accumulation of cadmium in rat liver cadmium binding protein following single and repeated cadmium administration.

The accumulation of cadmium in rat liver cadmium binding protein induced by single and repeated intraperitoneal injections of CdCl2 and the de novo biosynthesis of CdBP were studied by using 109Cd to measure cadmium binding in the CdBP and 35S incorporation as indicator of protein synthesis. The biosynthesis of CdBP is controlled by the cadmium concentrations. For single doses up to 1 mg Cd2+/Kg b.w. about 50% of the cadmium is present in the soluble fraction of liver bound to CdBP and the incorporation of 35S-cysteine is linear with the cadmium concentration. When single doses ranging from 1 to 3 mg Cd2+/Kg b.w. are administered the fractions of both 35S-cysteine and cadmium incorporated into de novo synthesized CdBP gradually decrease. For single doses higher than 3 mg Cd2+/Kg b.w. the biosynthesis capability is maximum and 20 mug Cd/g liver can be incorporated into the de novo biosynthesized CdBP. When rats are treated every day with amounts of cadmium of about 0.8 mg Cd2+/Kg b.w. for up to 8 days a dose-proportional increase in both Cd incorporation and CdBP biosynthesis are observed. This shows a cumulative incorporation of cadmium in the de novo biosynthesized CdBP. Experiments carried out by injecting 65ZnCl2 and 203HgCl2 every day showed that they are not accumulated like cadmium and do not induce the biosynthesis of rat liver CdBP after repeated administration over 7 days.     

The effect of gastric inhibitory polypeptide on intestinal glucose absorption and intestinal motility in mice

Gastric inhibitory polypeptide (GIP) is released from the small intestine upon meal ingestion and increases insulin secretion from pancreatic β cells. Although the GIP receptor is known to be expressed in small intestine, the effects of GIP in small intestine are not fully understood. This study was designed to clarify the effect of GIP on intestinal glucose absorption and intestinal motility. Intestinal glucose absorption in vivo was measured by single-pass perfusion method. Incorporation of [¹⁴C]-glucose into everted jejunal rings in vitro was used to evaluate the effect of GIP on sodium-glucose co-transporter (SGLT). Motility of small intestine was measured by intestinal transit after oral administration of a non-absorbed marker. Intraperitoneal administration of GIP inhibited glucose absorption in wild-type mice in a concentration-dependent manner, showing maximum decrease at the dosage of 50 nmol/kg body weight. In glucagon-like-peptide-1 (GLP-1) receptor-deficient mice, GIP inhibited glucose absorption as in wild-type mice. In vitro examination of [¹⁴C]-glucose uptake revealed that 100 nM GIP did not change SGLT-dependent glucose uptake in wild-type mice. After intraperitoneal administration of GIP (50 nmol/kg body weight), small intestinal transit was inhibited to 40% in both wild-type and GLP-1 receptor-deficient mice. Furthermore, a somatostatin receptor antagonist, cyclosomatostatin, reduced the inhibitory effect of GIP on both intestinal transit and glucose absorption in wild-type mice. These results demonstrate that exogenous GIP inhibits intestinal glucose absorption by reducing intestinal motility through a somatostatin-mediated pathway rather than through a GLP-1-mediated pathway.     

The role of insulin in the intestinal absorption of glucose in the rat.

1. Acute pre-treatment with either mannoheptulose or streptozotocin--both compounds acting as powerful suppressors of insulin secretion--caused a significant decrease on the in vivo rate of intestinal glucose absorption following an intragastric [U-14C]glucose administration. 2. Mannoheptulose treatment also lowered the rate of whole-body oxidation of the administered tracer. 3. Insulin had no effect on the metabolic fate of [U-14C]glucose by isolated enterocytes. 4. However, the rate of glucose uptake, measured by the oxidation of [1-14C]glucose to 14CO2 in the presence of phenazine methosulphate, was decreased by insulin at concentrations of 50-200 munits/ml. 5. In addition, the rate of transport of [U-14C]glucose by brush-border membrane vesicles was also inhibited by insulin at high concentrations (100-1000 munits/ml). 6. This indicated that insulin acts by inhibiting glucose transport in isolated in vitro preparations. 7. Acute pre-treatment with either mannoheptulose or streptozotocin caused a significant decrease in the rate of gastric emptying, measured as the distribution of [3H]insulin along the gastrointestinal tract, following an intragastric glucose load. 8. It is concluded that insulin secretion modulates intestinal glucose absorption in vivo by enhancing gastric emptying in spite of the inhibitory effects of glucose transport observed with in vitro preparations.     

The effect of gastric inhibitory polypeptide on intestinal glucose absorption and intestinal motility in mice

Holocarboxylase synthetase (HLCS) catalyzes the covalent binding of biotin to both carboxylases in extranuclear structures and histones in cell nuclei, thereby mediating important roles in intermediary metabolism, gene regulation, and genome stability. HLCS has three putative translational start sites (methionine-1, -7, and -58), but lacks a strong nuclear localization sequence that would explain its participation in epigenetic events in the cell nucleus. Recent evidence suggests that small quantities of HLCS with a start site in methionine-58 (HLCS58) might be able to enter the nuclear compartment. We generated the following novel insights into HLCS biology. First, we generated a novel HLCS fusion protein vector to demonstrate that methionine-58 is a functional translation start site in human cells. Second, we used confocal microscopy and western blots to demonstrate that HLCS58 enters the cell nucleus in meaningful quantities, and that full-length HLCS localizes predominantly in the cytoplasm but may also enter the nucleus. Third, we produced recombinant HLCS58 to demonstrate its biological activity toward catalyzing the biotinylation of both carboxylases and histones. Collectively, these observations are consistent with roles of HLCS58 and full-length HLCS in nuclear events. We conclude this report by proposing a novel role for HLCS in epigenetic events, mediated by physical interactions between HLCS and other chromatin proteins as part of a larger multiprotein complex that mediates gene repression.