Direct and indirect actions of HgCl2 and methyl mercury chloride on permeability and chloride secretion across the rat colonic mucosa

Oral exposure to inorganic mercury or methylmercury elicits distinct pro-inflammatory and pro-oxidant intestinal responses in a mouse model system

Humans are mainly exposed to mercury (Hg) through contaminated foodstuffs. However, the effects of Hg on the intestinal tract have received little attention. We performed a subchronic exposure to inorganic mercury or methylmercury in mice through drinking water (1, 5 or 10 mg/L for four months) to evaluate their intestinal impact. Histological, biochemical and gene expression analyses showed that both Hg species induced oxidative stress in small intestine and colon, while inflammation was mainly detected in the colon. Increased fecal albumin content indicated a compromised epithelial barrier. Mucus production was possibly also affected, as an increase in Muc2 expression was detected. However, differential effects were detected between both Hg species. Activation of p38 MAPK and increased crypt depth were detected in colon only with MeHg. Minor differences in microbiota composition were detected between unexposed and exposed mice. Although significant differences were detected between both Hg species at 10 mg/L, only the relative abundances of low abundance taxa were affected. Concentrations of microbial-derived short-chain fatty acids were decreased, suggesting an effect on microbial metabolism or increased demand by the intestinal epithelium. Results obtained confirm previous in vitro studies and highlights the intestinal mucosa as an initial target of Hg.

Toxic trace elements at gastrointestinal level

Many trace elements are considered essential [iron (Fe), zinc (Zn), copper (Cu)], whereas others may be harmful [lead (Pb), cadmium (Cd), mercury (Hg), arsenic (As)], depending on their concentration and chemical form. In most cases, the diet is the main pathway by which they enter our organism. The presence of toxic trace elements in food has been known for a long time, and many of the food matrices that carry them have been identified. This has led to the appearance of legislation and recommendations concerning consumption. Given that the main route of exposure is oral, passage through the gastrointestinal tract plays a fundamental role in their entry into the organism, where they exert their toxic effect. Although the digestive system can be considered to be of crucial importance in their toxicity, in most cases we do not know the events that occur during the passage of these elements through the gastrointestinal tract and of ascertaining whether they may have some kind of toxic effect on it. The aim of this review is to summarize available information on this subject, concentrating on the toxic trace elements that are of greatest interest for organizations concerned with food safety and health: Pb, Cd, Hg and As.

Thimerosal exposure and the role of sulfation chemistry and thiol availability in autism

Autism spectrum disorder (ASD) is a neurological disorder in which a significant number of the children experience a developmental regression characterized by a loss of previously acquired skills and abilities. Typically reported are losses of verbal, nonverbal, and social abilities. Several recent studies suggest that children diagnosed with an ASD have abnormal sulfation chemistry, limited thiol availability, and decreased glutathione (GSH) reserve capacity, resulting in a compromised oxidation/reduction (redox) and detoxification capacity. Research indicates that the availability of thiols, particularly GSH, can influence the effects of thimerosal (TM) and other mercury (Hg) compounds. TM is an organomercurial compound (49.55% Hg by weight) that has been, and continues to be, used as a preservative in many childhood vaccines, particularly in developing countries. Thiol-modulating mechanisms affecting the cytotoxicity of TM have been identified. Importantly, the emergence of ASD symptoms post-6 months of age temporally follows the administration of many childhood vaccines. The purpose of the present critical review is provide mechanistic insight regarding how limited thiol availability, abnormal sulfation chemistry, and decreased GSH reserve capacity in children with an ASD could make them more susceptible to the toxic effects of TM routinely administered as part of mandated childhood immunization schedules.

Tight junction proteins and oxidative stress in heavy metals-induced nephrotoxicity

Kidney is a target organ for heavy metals. They accumulate in several segments of the nephron and cause profound alterations in morphology and function. Acute intoxication frequently causes acute renal failure. The effects of chronic exposure have not been fully disclosed. In recent years increasing awareness of the consequences of their presence in the kidney has evolved. In this review we focus on the alterations induced by heavy metals on the intercellular junctions of the kidney. We describe that in addition to the proximal tubule, which has been recognized as the main site of accumulation and injury, other segments of the nephron, such as glomeruli, vessels, and distal nephron, show also deleterious effects. We also emphasize the participation of oxidative stress as a relevant component of the renal damage induced by heavy metals and the beneficial effect that some antioxidant drugs, such as vitamin A (all-trans-retinoic acid) and vitamin E (α-tocopherol), depict on the morphological and functional alterations induced by heavy metals.

STIM1-regulated Ca2+ influx across the apical and the basolateral membrane in colonic epithelium

In nonexcitable cells, store-operated Ca(2+) entry is the most important pathway for influx of extracellular Ca(2+) serving as a second messenger in the cytoplasm. The present study investigated the expression, localization and polar distribution of two key components of store-operated Ca(2+) entry identified, e.g., in lymphocytes or epithelial cell lines-STIM1 (stromal interacting molecule 1), working as a Ca(2+) sensor in the endoplasmic reticulum, and Orai1, working as the (or part of the) store-operated Ca(2+) channel in the plasma membrane-in a native intestinal epithelium, i.e., rat colon. Immunohistochemical investigations revealed expression of STIM1 and Orai1 in the rat colonic epithelium. Ca(2+) store depletion led to a translocation of STIM1 both to the basolateral as well as to the apical cell pole as observed by confocal microscopy. A Ca(2+) depletion/repletion protocol was used in Ussing chamber experiments to investigate the contribution of basolateral and apical store-operated Ca(2+) entry to the induction of anion secretion. These experiments revealed that Ca(2+)-dependent anion secretion was induced not only by basolateral Ca(2+) repletion but also, to a lesser extent, by apical Ca(2+) repletion. Both responses were suppressed by La(3+). The effect of basolateral Ca(2+) repletion was significantly inhibited by brefeldin A, a blocker of vesicular transport from the endoplasmic reticulum to the Golgi apparatus. In a final series of experiments, fura-2-loaded HT29/B6 cells were used. A carbachol-induced increase in the cytosolic Ca(2+) concentration was significantly reduced when cells were pretreated with siRNA against STIM1. In conclusion, these results demonstrate that STIM1 as a key component of intracellular Ca(2+) signaling is expressed by rat colonic epithelium and is involved in the regulation not only of basolateral but also of apical Ca(2+) influx.

Independence of net water flux from paracellular permeability in the intestine of Fundulus heteroclitus, a euryhaline teleost

Paracellular permeability and absorptive water flux across the intestine of the euryhaline killifish were investigated using in vitro gut sac preparations from seawater- and freshwater-acclimated animals. The permeability of polyethylene glycol (PEG), a well-established paracellular probe, was measured using trace amounts of radiolabelled oligomers of three different molecular sizes (PEG-400, PEG-900 and PEG-4000) at various times after satiation feeding. All three PEG molecules were absorbed, with permeability declining as a linear function of increasing hydrodynamic radius. Response patterns were similar in seawater and freshwater preparations, though water absorption and PEG-900 permeability were greater in the latter. Despite up to 4-fold variations in absorptive water flux associated with feeding and fasting (highest at 1-3 h, lowest at 12-24 h and intermediate at 1-2 weeks post-feeding), there were no changes in PEG permeability for any size oligomer. When PEG permeability was measured in the opposite direction (i.e. serosal to mucosal) from net water flux, it was again unchanged. HgCl(2) (10(-3) mol l(-1)), a putative blocker of aquaporins, eliminated absorptive water flux yet increased PEG-4000 permeability by 6- to 8-fold in both freshwater and seawater preparations. Experimentally raising the serosal osmolality by addition of 300 mmol l(-1) mannitol increased the absorptive water flux rate 10-fold, but did not alter PEG permeability. Under these conditions, HgCl(2) reduced absorptive water flux by 60% and again increased PEG permeability by 6- to 8-fold in both freshwater and seawater preparations. Clearly, there was no influence of solvent drag on PEG movement. The putative paracellular blocker 2,4,6-triaminopyrimidine (TAP, 20 mmol l(-1)) had no effect on net water flux or PEG permeability. We conclude that PEG and water move by separate pathways; absorptive water transport probably occurs via a transcellular route in the intestine of Fundulus heteroclitus.

The effect of L-ascorbic acid and/or tocopherol supplementation on electrophysiological parameters of the colon of rats chronically exposed to lead

Background

The aim of this study was to assess the effect of diet supplementation with L-ascorbic acid (500 mg/L), tocopherol (3 mg/kg b.w.), and/or a water soluble analog of tocopherol (Trolox) (48 mg/L) on ion transport in the colon of rats subjected to a chronic exposure (9 months) to 0.1% lead acetate in drinking water.

Material/Methods

The electrophysiological parameters of the colon wall were measured with Ussing methods. Lead content in the whole blood was analyzed by graphite furnace atomic absorption spectrometry (GFAAS) using Zeeman correction. L-ascorbic acid and tocopherol in plasma was measured by high performance liquid chromatography. Immunohistochemical reaction was carried out for visualization of occludin, the intracellular tight junction protein.

Results

We showed a strong inhibitory effect of lead on the electrophysiological parameters, changes in intestinal permeability, disappearance of junctional occludin, decreased amount of mucus covering the colon surface, and the accumulation of PAS-positive substance in the apical region of the cytoplasm in the absorptive cells.

Conclusions

Supplementation with tocopherol or Trolox did not exert a beneficial influence on the studied parameters. L-ascorbic acid positively influenced the examined electrophysiological parameters, as it cancelled the inhibitory influence of lead on ion transport in the rat colon. L-ascorbic acid also protected against tight junction disruption of epithelial cells in the colon of the lead-treated rats. A similar effect was observed in the group of rats receiving lead and supplemented with L-ascorbic acid plus Trolox.

The protein kinase A pathway contributes to Hg2+-induced alterations in phosphorylation and subcellular distribution of occludin associated with increased tight junction permeability of salivary epithelial cell monolayers

Hg(2+) is commonly used as an inhibitor of many aquaporins during measurements of transcellular water transport. To investigate whether it could also act on the paracellular water transport pathway, we asked whether addition of Hg(2+) affected transport of radiolabeled probes through tight junctions of a salivary epithelial cell monolayer. Inclusion of 1 mM Hg(2+) decreased transepithelial electrical resistance by 8-fold and augmented mannitol and raffinose flux by 13-fold, which translated into an estimated 44% increase in pore radius at the tight junction. These Hg(2+)-induced effects could be partially blocked by the protein kinase A (PKA) inhibitor N-[2-((p-bromocinnamyl) amino) ethyl]-5-isoquinolinesulfonamide, 2HCl (H89), suggesting that both-PKA dependent and PKA-independent mechanisms contribute to tight junction regulation. Western blot analyses showed a 2-fold decrease in tight junction-associated occludin after Hg(2+) treatment and the presence of a novel hyperphosphorylated form of occludin in the cytoplasmic fraction. These findings were corroborated by confocal imaging. The results from this study reveal a novel contribution of the PKA pathway in Hg(2+)-induced regulation of tight junction permeability in the salivary epithelial barrier. Therapeutically, this could be explored for pharmacological intervention in the treatment of dry mouth, Sjögren's syndrome, and possibly other disorders of fluid transport.

Multiple effects of mercury on cell volume regulation, plasma membrane permeability, and thiol content in the human intestinal cell line Caco-2

In a previous study, we characterized Cd-Hg interactions for uptake in human intestinal Caco-2 cells. We pursued our investigations on metal uptake from metal mixtures, focusing on the effects of Hg on cellular homeostasis. A 4-fold higher equilibrium accumulation value of 0.3 micromol/L (203)Hg was measured in the presence of 100 micromol/L unlabeled Hg in the serum-free exposure medium without modification in the initial uptake rate. This phenomenon was eliminated at 4 degrees C. Mercury induced an increase in tritiated water and [(3)H]mannitol uptakes for exposure times greater than 20 min. Incubations for 20 min and 30 min with 100 micromol/L Hg and 2 mmol/L N-ethylmaleimide (NEM) resulted in a 34% and 50% reductions in cellular thiol staining, respectively, with additive effects. Lactate dehydrogenase leakage and live/dead assays confirmed the maintenance of cell membrane integrity in Hg- or NEM-treated cells. We conclude that Hg may alter membrane permeability and increase cell volume without any loss in cell viability. This phenomenon is sensitive to temperature and could involve Hg interaction with membrane thiols, possibly related to solute transport. During metal uptake from metal mixtures, Hg may thus promote the uptake of other toxic metals by increasing cell volume and consequently cell capacity.

Intestinal pathophysiology in autism

Autism is a life-long developmental disorder affecting as many as 1 in 500 children. The causes for this profound disorder are largely unknown. Recent research has uncovered pathology in the gastrointestinal tract of autistic children. The pathology, reported to extend from the esophagus to the colon, is described here along with other studies pointing to a connection between diet and the severity of symptoms expressed in autism. The evidence that there is impaired intestinal permeability in autism is reviewed, and various theories are discussed by which a leaky gut could develop. Lastly, some possible ways in which impaired gastrointestinal function might influence brain function are discussed.

Enhancement of ovalbumin-induced antibody production and mucosal mast cell response by mercury

Food contaminants may contribute to the recent increased incidence of food allergies. We have investigated this hypothesis experimentally. It was our objective to determine whether toxicity to the intestinal tissue by orally applied mercury (Hg) could modulate the immune response to food allergens. Effective mechanisms were studied with functional immunological and toxicological parameters. Brown Norway rats were immunized intraperitoneally by ovalbumin (OVA). Before oral challenge with OVA, immunized and non-immunized animals were exposed to HgCl2. Immunological responses were measured by enzyme-linked immunosorbent assays [anti-OVA-IgE and-IgG, rat mast cell protease II (RMCPII), interferon-gamma, interleukin-4, lymphocyte proliferation] and by flow cytometry (lymphocyte subpopulations). Toxicity of Hg to the intestinal barrier was determined by measuring viability, DNA damage and induction of glutathione S-transferase in isolated intestinal epithelial cells and lymph node cells, and by measuring permeability, short-circuit current and tissue conductance of the intact intestinal epithelium. A single high oral dose of HgCl2 enhanced the serum concentrations of anti-OVA-IgE and IgG (P < 0.05) and of RMCPII (P < 0.05) in immunized rats. The treatment resulted in a higher number of CD4/CD25+ T cells in the lymph nodes (P < 0.05). The multiple application of low HgCl2 doses (5 x 0.2 mg/kg body weight) only resulted in an elevated RMCPII serum concentration (P < 0.05). Neither treatment schedules impaired proliferation and cytokine production of lymphocytes. In non-immunized rats only minor immunological changes were observed. Oral HgCl2 induced genotoxic damage in lymph node cells and in jejunal epithelial cells (P < 0.05). Moreover, HgCl2 increased the permeability of intestinal epithelial tissue and of Caco-2 monolayers and was genotoxic and cytotoxic to isolated intestinal epithelial cells in vitro. In conclusion, these studies indicate that the food contaminant Hg can stimulate the immune response to OVA in immunized rats. One possible mechanism could be the toxicity of Hg to the intestinal epithelial and the lymph node cells. Whether humans with allergies respond to high oral doses of Hg in a similar way needs to be investigated in further studies.

Renal oxidant injury and oxidant response induced by mercury

The role of oxidative stress in mercuric chloride (HgCl2)-induced nephrotoxicity is uncertain and controversial. We demonstrate that I.L.C-PK1 cells, exposed to HgCl2, generate massive amounts of hydrogen peroxide, the latter completely quenched by the hydrogen peroxide scavenger, pyruvate. HgCl2 exerts a dose-dependent cytotoxicity which is attenuated by pyruvate and catalase. Cellular generation of hydrogen peroxide arises, at least in part, from mitochondria since mitochondrial rates of generation of hydrogen peroxide increase in response to HgCl2; HgCl2 also provokes a shift in absorbance spectra in rhodamine 123 loaded-mitochondria and stimulates mitochondrial state 4 respiration. HgCl2, applied for one hour, impairs cellular vitality as demonstrated by the MTT assay, an assay dependent in part on mitochondrial function. HgCl2 impairs function in other organelles such as lysosomes that maintain a transmembrane proton gradient; these latter effects are partially attenuated by pyruvate. We complement these in vitro findings with in vivo evidence demonstrating that HgCl2 stimulates renal generation of hydrogen peroxide. The functional significance of such generation of hydrogen peroxide was evaluated in rats deficient in selenium and vitamin E, a nutrient deficiency that impairs the scavenging of hydrogen peroxide and promotes the toxicity of this oxidant. In these rats serum creatinine values were significantly higher on sequential days following the administration of HgCl2. To probe the renal response to oxidative stress induced by HgCl2, we examined hydrogen peroxide-scavenging enzymes and redox-sensitive genes. Catalase activity was unaltered whereas glutathione peroxidase activity was decreased, effects that may contribute to the net renal generation of hydrogen peroxide. The redox sensitive enzyme, heme oxygenase, was markedly up-regulated in the kidney in response to HgCl2. HgCl2 also induced members of the bcl family, bcl2 and bclx, genes that protect against apoptosis and oxidant injury. In another model of oxidant-induced renal injury, the glycerol model, bcl2 mRNA was not induced at 6 and 24 hours after the administration of glycerol. In summary, we demonstrate that HgCl2 potently stimulates renal generation of hydrogen peroxide in vitro and in vivo and such generation of peroxide contributes to renal dysfunction in vitro and in vivo. We also demonstrate that in response to HgCl2, redox sensitive genes are expressed including heme oxygenase and members of the bcl family.

Ontogenetic development of proline intestinal transport in the domestic fowl

1. The proline disappearance from the jejunal and ileal lumen of chickens aged from 1 day to 15 weeks were studied using a perfusion method "in vivo". 2. A decrease in proline transport was observed from younger to older animals. The 2 intestinal segments showed different behaviours, the 1-day and 1-week-old animals showed the same value in the jejunum, whereas in the ileum a progressive decrease in proline transport was observed from the first week of life. 3. The differences observed in the 2 segments could be attributed to the different rates of growth of the jejunum and ileum with age; the jejunum showed a peak of growth in the second week of life whereas the ileum showed a peak of growth in the first week. Proline transport in the jejunum decreased until the fifth week and remained constant thereafter.

Induction of renin release from isolated glomeruli by inorganic mercury(II)

Mercury(II) ions are known to accumulate in the kidney and their effect upon the renin-angiotensin system has also been described. The question, however, whether mercury(II) also exerts direct effect on the juxtaglomerular cells (JGC) to induce renin release remained to be answered. Suspension of isolated glomeruli was used to measure the mercury(II)-induced renin release in vitro. The glomeruli were isolated from female BALBc mice. HgCl2 was found to be capable of inducing renin release directly from JGC. The effect is concentration-dependent (P < 0.05, r = 0.914 and P < 0.01, r = 0.982, with and without Neutral Red vital staining) and becomes apparent already at a mercury(II) ion concentration as low as 1 microM. The renin-releasing effect of the mercury ion is to be inhibited by dithiothreitol (DTT) (renin activity 20.37 vs. 2.60 ng/ml.h in supernatant) as well as the elevated osmotic concentration of the incubating bath medium (20.37 vs. 6.84 ng/ml.h). This suggests that certain membrane sulfhydryl groups are implicated in the process on the one hand, and it is also in accordance with the known sensitivity of the renin granules to osmotic pressure on the other hand. Light and electron micrographs also demonstrate the direct, effective role of Hg(II) in the renin release process. Therefore, it is assumed that apart from its influence on tubulo-glomerular feedback a direct way of action of mercury(II) on renin release must also be taken into account.

Nickel absorption from perfused rat jejunal and ileal segments

The intestinal absorption of Ni2+ was studied in isolated perfused jejunal and ileal segments of rats, by a method which allows continuous sampling of the absorbates. The results showed that the Ni(2+)-absorption proceeds at a much higher rate in the jejunum than in the ileum. Several observations indicate that Ni2+ is absorbed actively in the jejunum. There are indications in the literature that Ni2+ at least partly may share the transport mechanism for iron across the intestinal mucosa and our results may reflect the participation of Ni2+ in this absorptive process. The transfer of Ni2+ across the ileal epithelium may occur by passive diffusion. Addition of Zn2+, Co2+, Cd2+ or Hg2+ to the jejunal perfusates affected the Ni(2+)-absorption to varying extents. Thus, Zn2+ had minor effects on the Ni(2+)-absorption. Co2+ decreased the Ni(2+)-concentration in the absorbates, possibly by interfering with Ni2+ in the iron transfer process. Addition of Cd2+ or Hg2+ to the perfusates resulted in decreased jejunal water absorption. Hg2+ also depressed the glucose absorption. These results show that Cd2+ and Hg2+ at low concentrations are toxic to the jejunal mucosal cells. Thus, these metals can inhibit the amount of Ni2+ transferred across the intestinal mucosa by decreasing the volume of the absorbate.

Chloride secretion induced by mercury and cadmium: action sites and mechanisms

The actions of two mercury compounds, HgCl2 and methyl mercury chloride (MeHg), and of CdCl2 on the epithelium of the rat colon were studied with the whole-cell patch-clamp technique and the Ussing chamber. MeHg (50 microM) induced an increase of membrane outward current (I(out)) in enterocytes of isolated crypts patched from the basolateral side. This action was inhibited by a Cl- channel blocker and a K+ channel blocker, indicating an increase of both the Cl- and the K+ conductance. In contrast, HgCl2 (50 microM) did not affect I(out), whereas CdCl2 (50 microM) decreased it slightly. In mucosal preparations all three compounds induced a concentration-dependent increase in short-circuit current (Isc) when administered to the serosal, i.e., contraluminal side. Sensitivity to chloride transport blockers and anion replacement experiments revealed that the increase in Isc represented Cl- secretion. In contrast to the actions of luminally applied mercury compounds, the increase of tissue conductance (Gt) was only small. Tetrodotoxin and indomethacin suppressed the effect of the metal compounds on Isc and Gt, while atropine diminished it only partly. This indicates that the secretory action of these heavy metals has not only a direct effect on epithelial cells but is also mediated by prostaglandins and cholinergic and noncholinergic neurons.