Intestinal absorption of zinc: sodium-metal-ligand interactions

Association of lead-exposure risk and family income with childhood brain outcomes

Socioeconomic factors influence brain development and structure, but most studies have overlooked neurotoxic insults that impair development, such as lead exposure. Childhood lead exposure affects cognitive development at the lowest measurable concentrations, but little is known about its impact on brain development during childhood. We examined cross-sectional associations among brain structure, cognition, geocoded measures of the risk of lead exposure and sociodemographic characteristics in 9,712 9- and 10-year-old children. Here we show stronger negative associations of living in high-lead-risk census tracts in children from lower- versus higher-income families. With increasing risk of exposure, children from lower-income families exhibited lower cognitive test scores, smaller cortical volume and smaller cortical surface area. Reducing environmental insults associated with lead-exposure risk might confer greater benefit to children experiencing more environmental adversity, and further understanding of the factors associated with high lead-exposure risk will be critical for improving such outcomes in children.

Proabsorptive effect of gum arabic in isotonic solutions orally administered to rats: effect on zinc and other solutes

We have previously shown that the addition of gum arabic (GA) to oral rehydration solution (ORS) enhances water and electrolyte absorption during jejunal perfusion in rats under anesthesia. This study investigates whether GA by oral administration could be equally effective in rats. Isotonic solutions containing 25 g/L GA (AG), or without GA (A0) were administered via oral tube to lightly anesthetized adult female rats. Similar experiments were conducted with hypertonic solutions containing no GA (B0), or either 10 (B10) or 50 g/L GA (B50). Blood concentrations of sodium, glucose, glutamate, zinc, and tritiated water were determined at 0, 15, 30, 60, 90, 120, and 180 minutes, and results between treatments were compared. Administration of the isotonic, GA-containing solution (AG) resulted in a higher blood zinc level than with the isotonic GA-free solution (A0) from 15 minutes throughout 180 minutes. Blood zinc at 15 minutes (means +/- SEM) was as follows: for A0: 69.3 +/- 2.0, for AG: 83.4 +/- 3.5 nmol/L, P=0.002. At 180 minutes, A0: 52.6 +/- 1.8; AG: 68.1 +/- 4.6 nmol/L, P=0.004. The corresponding areas under the curve (AUC) were as follows: for A0: 10,737 +/- 214; for AG: 13,919 +/- 765 nmol x min/L, P<0.001). Glucose, glutamate, sodium, and tritiated water body distribution presented no differences in blood concentrations. For sodium and tritiated water body distribution, there was a significant time effect (P<0.0001). In hypertonic solutions, blood zinc levels declined over time, possibly due to their osmotic, counter-absorptive action, thus obscuring possible opposite effects of GA. GA appears to be an effective enhancer of zinc absorption when orally administered in isotonic solutions to laboratory animals. This proabsorptive capacity could be attributed to some of the physicochemical and biochemical properties of GA and suggest possible applications of GA in liquid formulas and solid food preparations.

Effect of heavy metals on the uptake of [3H]-L-histidine by the polychaete Nereis succinea

Integumentary uptake of [3H]-L-histidine by Nereis succinea was measured in the presence and absence of selected heavy metals and the amino acid L-leucine in 60% artificial seawater (ASW). The time course of 10 microM [3H]-L-histidine uptake into worms over a 60 min incubation was approximately doubled in the presence of 0.5 microM zinc and when calcium in the incubation medium was reduced from 6 mM to 5 microM the stimulatory effect of zinc on amino acid accumulation was reduced and uptake under the latter conditions was approximately half that of the control. Zinc stimulation of [3H]-L-histidine influx was a hyperbolic function of zinc concentration over the range 0 to 50 microM metal and displayed an apparent activation or affinity constant of 385+/-127 nM Zn(2+). The hyperbolic stimulatory effect of 1 microM Zn(2+) on the time course of 10 microM [3H]-L-histidine uptake was abolished in the presence of 25 microM L-leucine, suggesting that this amino acid shared the same transport system as [3H]-L-histidine and acted as a potential competitive inhibitor. Influx of [3H]-L-histidine was a hyperbolic function of external amino acid concentration and displayed an apparent affinity constant (Km) of 23.71+/-5.02 microM and an apparent aximal velocity (J(max)) of 4701+/-449 pmol/g dry wt.x15 min. Addition of 0.5 microM zinc resulted in a four-fold increase in J(max) and a doubling of K(m), suggesting the effect of the metal was mostly on the rate of amino acid transport. [3H]-L-histidine influx was mildly stimulated by Fe(2+) (0.5 microM), but was unaffected by either Ag(+) or Al(3+) (both at 0.5 microM). These results suggest that [3H]-L-histidine uptake into worm integument may take place by the classical Na(+)-independent L-transport system shared by L-leucine and regulated by exogenous calcium and other divalent metal concentrations.

In vivo characterisation of intestinal zinc uptake in freshwater rainbow trout

Knowledge of the uptake mechanisms and metabolism of metals is essential for understanding the factors governing metal toxicity, discerning means by which acclimation and homeostasis may be achieved and characterising interactions between the metal of interest and other environmental moieties. Zinc is both an important aquatic contaminant and a vital micronutrient. The physiological characterisation of dietary zinc absorption in fish has, therefore, important implications for environmental protection and aquaculture. The present study aimed to elucidate the mechanism of intestinal zinc uptake in freshwater rainbow trout (Oncorhynchus mykiss), using an in vivo cannulation technique. Only a saturable component of zinc uptake, with a concentration giving half-maximal rate of accumulation (K0.5) of 309 μmol l–1, and a maximal rate of accumulation (Jmax) of 933 nmol kg–1 h–1, was described. This characterised the intestine as a low-affinity, high-capacity zinc absorption pathway. Physiological mechanisms appear to regulate zinc uptake. Intestinal mucus was one important regulatory locus, promoting zinc uptake at low concentrations yet buffering the animal against high luminal zinc loads. Regulatory mechanisms also seemed to limit subepithelial zinc accumulation. Experiments using ethylene glycol tetraacetic acid (EGTA) to wash the intestinal lumen following zinc perfusion exhibited a higher proportion of loosely associated zinc at higher perfused concentrations. This was attributed to saturation of the uptake process or efflux from the subepithelium. Two distinct pathways for passage of zinc across the epithelium were discerned, with post-intestinal transfer possibly mediated by sulphydryl groups, as illustrated by N-ethylmaleimide perfusion experiments. Putative roles of zinc transporters and/or intracellular-binding proteins are discussed.

Effect of aging on zinc and histidine transport across rat intestinal brush-border membranes

The effects of aging on intestinal absorption of zinc and L-histidine were investigated in adult (10-month-old) and senescent (30-month-old) Wistar rats' brush-border membrane vesicles isolated from jejunum and ileum. Kinetic parameters of the zinc transport by the jejunal brush-border membrane were Jmax = 126 +/- 24 nmol.min-1.mg-1 protein and Km = 490 +/- 126 microM (10-month-old rats, n = 7). The transport of zinc was the same in the jejunum and the ileum of adult animals. In senescent rats, the zinc uptake was significantly lower in the distal part of the intestine than in the proximal one. A comparison of zinc uptake in 10- and 30-month-old rats showed that the transport capacity of the jejunum did not change with age but the ileal transport capacity decreased by 50%. This reduced uptake was associated with an increased cholesterol content of the brush-border membrane. The major site of L-histidine absorption was the jejunum, in both the 10- and 30-month-old animals. L-Histidine was co-transported with Na+. The kinetic parameters of the L-histidine carrier in the presence of Na+ were Jmax = 6.5 +/- 1.0 nmol.min-1.mg-1 protein and Km = 190 +/- 29 microM in the jejunum of 10-month-old rats (n = 12). Increasing the extra-vesicular concentration of zinc (0 --> 1 mM) reduced the uptake of L-histidine, and conversely increasing the concentration of L-histidine (0 --> 1 mM) reduced that of zinc: there was no evidence of transport of a complexed form [zinc-L-histidine] in brush-border membranes of the small intestine. During aging, the transport capacity of L-histidine by the jejunum decreased, whereas the ileal transport capacity was conserved. The modifications of absorptive capacity for zinc and L-histidine at the membrane level (loss of ileal function for zinc, and loss of jejunal function for amino acid) indicate that the normal aging of intestinal epithelial cells cannot be regarded as a decline in the overall transport of nutriments but as a combination of highly specific modifications of the various transport systems.

Mechanisms of zinc transport into pig small intestine brush-border membrane vesicles

1. The purpose of the present work was to examine certain membrane transport mechanisms likely to carry zinc across the brush-border membrane of pig small intestine, isolated in a vesicular form. 2. In initial velocity conditions, saturation kinetics revealed a great effect of pH on zinc transport: optimal conditions were observed with an intravesicular pH of around 6.6 with or without a H+ gradient; however, this did not allow us to conclude the existence of a neutral exchange between Zn2+ and H+ ions. 3. By measuring 36Cl uptakes, the presence of the Cl(-)-HCO3- or Cl(-)-OH-antiporter with typical 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) sensitivity was detected in vesicles; zinc did not alter this anionic exchange activity. A 65Zn time course, performed in conditions identical with those for 36Cl uptake, was DIDS insensitive and was greatly inhibited by an outward OH- gradient. This could argue against a transport of zinc as a complex with Cl- and HCO3- through the anion antiporter. 4. When external Cl- and HCO3- were replaced by SCN-, able to form a Zn(SCN)4(2-) complex, we observed a stimulating effect of outward HCO3- gradients on 65Zn uptake but neither DIDS nor diphenylamine-2-carboxylate (DPC) inhibited the transport in these conditions. This suggested that the intestinal anion antiporter was not a major route for zinc reabsorption. 5. The tripeptide Gly-Gly-His at low concentrations stimulated 65Zn uptake, then inhibited it in a dose-dependent manner either in the presence of an inward H+ gradient or in the presence of a membrane potential 'negative inside' or in both situations. These conditions are necessary for the active transport of the peptide and this strongly suggests that zinc can be transported as a [Gly-Gly-His-Zn] complex, utilizing the peptide carrier system.

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.

Inhibition of D-galactose transport across the small intestine of rabbit by zinc

Zinc is an essential trace element necessary to life. Many metallo-enzymes involved in the metabolism of carbohydrates, lipids, proteins, and nucleic acids require zinc for their functions. The aim of this study was to characterize how zinc acts on the intestinal sugar absorption in rabbit. Results obtained show that zinc decreases both D-galactose accumulation in the jejunum tissue, and mucosal to serosal transepithelial flux of this sugar, in a dose-dependent way. Furthermore, zinc seems not to modify sugar diffusion across the intestinal epithelium. The inhibition of intestinal sugar transport by zinc seems to be of a competitive type and it is reversed in high proportion with dithioerythritol (thiol groups protector). The results suggest that zinc decreases carrier-mediated intestinal sugar absorption.

Distinct mechanisms of zinc uptake at the apical and basolateral membranes of caco-2 cells

Zinc uptake mechanisms at the apical and basolateral membrane borders of caco-2 cells were examined. This human-derived cell line possesses many morphological and functional characteristics of absorptive small intestinal cells. By day 14, confluent and well-differentiated monolayers were formed when the cells were grown on porous polycarbonate filters. Labelled zinc was placed on the apical or basal side of the monolayer and its uptake by the cells, as well as its transport across the monolayer, were measured. Zinc uptake by the cells from the apical side was found to be a saturable process (Kt = 41 microM; Vmax = 0.3 nmols/cm2/10 min) with a diffusional term at higher concentrations (1.0 sec/cm). Apical uptake was not affected by metabolic inhibitors or potential zinc ligands. Zinc uptake from the basolateral side was concentration dependent (Kd = 1.3 sec/cm) and was partially inhibited (30%) by ouabain and vanadate, suggesting that the (Na-K)-ATPase on the basolateral membrane is involved in the serosal uptake of zinc by the cell. Transport of zinc across the monolayers from the apical or basolateral compartment was concentration dependent and was not affected by metabolic inhibitors. Zinc transport from the basolateral side was greater than 2-fold greater than apical transport. Hence, separate mechanisms can be distinguished with respect to zinc uptake at the apical and basolateral membranes of caco-2 cells.

In vitro release and intestinal absorption of physostigmine salicylate from submicron emulsions

The in vitro release of physostigmine salicylate (PS) from a submicron emulsion and an aqueous solution was studied using the dialysis bag method. These formulations were then perfused to various locations along the rat small intestine (proximal, mid, and distal jejunum), and two lengths (10 and 55 cm). The disappearance of PS from the luminal compartment and its appearance in the blood compartment was monitored. In the in vitro drug release from emulsion experiments, a biphasic appearance of PS in the sink solution was observed, suggesting a possible sustained release from the emulsion. However, absorption data from perfusion studies did not correlate with this in vitro observation. No significant difference was found in absorption from emulsion versus solution in the mid jejunum where PS absorption was maximal. The difference between the two liquid formulations was observed only in those intestinal segments where the absorption was relatively low [absorption rate values of 4.6 +/- 0.86 and 9.98 +/- 2.04 (log%/min) x 10(-3) in the proximal and distal parts of the small intestine, respectively, as compared with 14.0 +/- 1.2-14.8 +/- 1.1 (log%/min) x 10(-3) in the mid jejunum]. In the distal part of the rat small intestine, PS was absorbed significantly better from solution than from the submicron emulsion. Cholinesterase activity in blood samples collected after intestinal perfusion with emulsion or solution revealed lower enzyme activity following emulsion administration.