Evidence for the involvement of a muscarinic receptor in ascorbic acid secretion in the rat stomach

Effect of vitamin C supplement on lead bioaccessibility in contaminated soils using multiple in vitro gastrointestinal assays: Mechanisms and health risks

Effects of vitamin C supplementation on the oral bioaccessibility of lead (Pb) present in contaminated soils were examined using a number of in vitro assays (PBET, SBRC, UBM and IVG). In the presence of vitamin C, an increase in Pb bioaccessibility was observed in the gastric phase by 1.3-fold (30.5%-85.5%) and in the intestinal phase by 3.1-fold (0.9%-58.9%). Lead mobilization was regulated by reductive dissolution of Fe(III) and sequestration of Pb on secondary Fe minerals. Sequential extraction by the Bureau Community of Reference (BCR) provided more evidence that reducible fraction and residual fraction were major contributor of gastric Pb bioaccessibility, as well as reduced fractions in intestinal Pb bioaccessibility. In addition, higher non-carcinogenic risks may occur based on target hazard quotient (THQ ≥ 1). For people exposed to Pb present in soil, the management of vitamin C supplements is of serious concern.

Lost-in-Translation of Metabolic Effects of Inorganic Nitrate in Type 2 Diabetes: Is Ascorbic Acid the Answer?

Beneficial metabolic effects of inorganic nitrate (NO3-) and nitrite (NO2-) in type 2 diabetes mellitus (T2DM) have been documented in animal experiments; however, this is not the case for humans. Although it has remained an open question, the redox environment affecting the conversion of NO3- to NO2- and then to NO is suggested as a potential reason for this lost-in-translation. Ascorbic acid (AA) has a critical role in the gastric conversion of NO2- to NO following ingestion of NO3-. In contrast to AA-synthesizing species like rats, the lack of ability to synthesize AA and a lower AA body pool and plasma concentrations may partly explain why humans with T2DM do not benefit from NO3-/NO2- supplementation. Rats also have higher AA concentrations in their stomach tissue and gastric juice that can significantly potentiate gastric NO2--to-NO conversion. Here, we hypothesized that the lack of beneficial metabolic effects of inorganic NO3- in patients with T2DM may be at least in part attributed to species differences in AA metabolism and also abnormal metabolism of AA in patients with T2DM. If this hypothesis is proved to be correct, then patients with T2DM may need supplementation of AA to attain the beneficial metabolic effects of inorganic NO3- therapy.

Modification of an existing in vitro method to predict relative bioavailable arsenic in soils

The soil matrix can sequester arsenic (As) and reduces its exposure by soil ingestion. In vivo dosing studies and in vitro gastrointestinal (IVG) methods have been used to predict relative bioavailable (RBA) As. Originally, the Ohio State University (OSU-IVG) method predicted RBA As for soils exclusively from mining and smelting sites with a median of 5,636 mg As kg^-1. The objectives of the current study were to (i) evaluate the ability of the OSU-IVG method to predict RBA As for As contaminated soils with a wider range of As content and As contaminant sources, and (ii) evaluate a modified extraction procedure's ability to improve prediction of RBA As. In vitro bioaccessible (IVBA) by OSU-IVG and California Bioaccessibility Method (CAB) methods, RBA As, speciation, and properties of 33 As contaminated soils were determined. Total As ranged from 162 to 12,483 mg kg^-1 with a median of 73 mg kg^-1. RBA As ranged from 1.30 to 60.0% and OSU-IVG IVBA As ranged from 0.80 to 52.3%. Arsenic speciation was predominantly As(V) adsorbed to hydrous ferric oxide (HFO) or iron (Fe), manganese (Mn), and aluminum (Al) oxides. The OSU-IVG often extracted significantly less As in vitro than in vivo RBA As, in particularly for soils from historical gold mining. The CAB method, which is a modified OSU-IVG method extracted more As than OSU-IVG for most soils, resulting in a more accurate predictor than OSU-IVG, especially for low to moderately contaminated soils (<1,500 mg As kg^-1) with RBA As = 0.81 IVBA As + 3.2, r² = 0.91.

Particle Size, Surface Area, and Amorphous Content as Predictors of Solubility and Bioavailability for Five Commercial Sources of Ferric Orthophosphate in Ready-To-Eat Cereal

Ferric orthophosphate (FePO₄) has had limited use as an iron fortificant in ready-to-eat (RTE) cereal because of its variable bioavailability, the mechanism of which is poorly understood. Even though FePO₄ has desirable sensory properties as compared to other affordable iron fortificants, few published studies have well-characterized its physicochemical properties. Semi-crystalline materials such as FePO₄ have varying degrees of molecular disorder, referred to as amorphous content, which is hypothesized to be an important factor in bioavailability. The objective of this study was to systematically measure the physicochemical factors of particle size, surface area, amorphous content, and solubility underlying the variation in FePO₄ bioavailability. Five commercial FePO₄ sources and ferrous sulfate were added to individual batches of RTE cereal. The relative bioavailability value (RBV) of each iron source, determined using the AOAC Rat Hemoglobin Repletion Bioassay, ranged from 51% to 99% (p < 0.05), which is higher than typically reported. Solubility in dilute HCl accurately predicted RBV (R² = 0.93, p = 0.008). Amorphous content measured by Dynamic Vapor Sorption ranged from 1.7% to 23.8% and was a better determinant of solubility (R² = 0.91; p = 0.0002) than surface area (R² = 0.83; p = 0.002) and median particle size (R² = 0.59; p = 0.12). The results indicate that while solubility of FePO₄ is highly predictive of RBV, solubility, in turn, is strongly linked to amorphous content and surface area. This information may prove useful for the production of FePO₄ with the desired RBV.

TEMPOL enhances the antihypertensive effects of sodium nitrite by mechanisms facilitating nitrite-derived gastric nitric oxide formation

Orally administered nitrite exerts antihypertensive effects associated with increased gastric nitric oxide (NO) formation. While reducing agents facilitate NO formation from nitrite, no previous study has examined whether antioxidants with reducing properties improve the antihypertensive responses to orally administered nitrite. We hypothesized that TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl) could enhance the hypotensive effects of nitrite in hypertensive rats by exerting antioxidant effects (and enhancing NO bioavailability) and by promoting gastric nitrite-derived NO generation. The hypotensive effects of intravenous and oral sodium nitrite were assessed in unanesthetized freely moving rats with L-NAME (N(ω)-nitro-L-arginine methyl ester; 100mg/kg; po)-induced hypertension treated with TEMPOL (18mg/kg; po) or vehicle. While TEMPOL exerted antioxidant effects in hypertensive rats, as revealed by lower plasma 8-isoprostane and vascular reactive oxygen species levels, this antioxidant did not affect the hypotensive responses to intravenous nitrite. Conversely, TEMPOL enhanced the dose-dependent hypotensive responses to orally administered nitrite, and this effect was associated with higher increases in plasma nitrite and lower increases in plasma nitrate concentrations. In vitro experiments using electrochemical and chemiluminescence NO detection under variable pH conditions showed that TEMPOL enhanced nitrite-derived NO formation, especially at low pH (2.0 to 4.0). TEMPOL signal evaluated by electron paramagnetic resonance decreased when nitrite was reduced to NO under acidic conditions. Consistent with these findings, increasing gastric pH with omeprazole (30mg/kg; po) attenuated the hypotensive responses to nitrite and blunted the enhancement in plasma nitrite concentrations and hypotensive effects induced by TEMPOL. Nitrite-derived NO formation in vivo was confirmed by using the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (C-PTIO), which blunted the responses to oral nitrite. Our results showed that TEMPOL promotes nitrite reduction to NO in the stomach and enhanced plasma nitrite concentrations and the hypotensive effects of oral sodium nitrite through mechanisms critically dependent on gastric pH. Interestingly, the effects of TEMPOL on nitrite-mediated hypotension cannot be explained by increased NO formation in the stomach alone, but rather appear more directly related to increased plasma nitrite levels and reduced nitrate levels during TEMPOL treatment. This may relate to enhanced nitrite uptake or reduced nitrate formation from NO or nitrite.

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.

Red wine-dependent reduction of nitrite to nitric oxide in the stomach

Nitrite may be a source for nitric oxide (*NO), particularly in highly acidic environments, such as the stomach. Diet products contribute also with reductants that dramatically increase the production of *NO from nitrite. Red wine has been attributed health promoting properties largely on basis of the reductive antioxidant properties of its polyphenolic fraction. We show in vitro that wine, wine anthocyanin fraction and wine catechol (caffeic acid) dose- and pH-dependently promote the formation of *NO when mixed with nitrite, as measured electrochemically. The production of *NO promoted by wine from nitrite was substantiated in vivo in healthy volunteers by measuring *NO in the air expelled from the stomach, following consumption of wine, as measured by chemiluminescence. Mechanistically, the reaction involves the univalent reduction of nitrite, as suggested by the formation of *NO and by the appearance of EPR spectra assigned to wine phenolic radicals. Ascorbic and caffeic acids cooperate in the reduction of nitrite to *NO. Moreover, reduction of nitrite is critically dependent on the phenolic structure and nitro-derivatives of phenols are also formed, as suggested by caffeic acid UV spectral modifications. The reduction of nitrite may reveal previously unrecognized physiologic effects of red wine in connection with *NO bioactivity.

Upregulation of sodium-dependent vitamin C transporter 2 expression in adrenals increases norepinephrine production and aggravates hyperlipidemia in mice with streptozotocin-induced diabetes

The hyperglycemia and hyperoxidation that characterize diabetes lead to reduced vitamin C (L-ascorbic acid, AA) levels in diabetic humans and animals. We examined the possibility that diabetes-induced low plasma AA levels impair AA distribution to various tissues and that these changes are closely related to the development of diabetic complications. AA levels were markedly decreased in the plasma and increased in the adrenals of mice with streptozotocin (STZ)-induced diabetes. Consistently with these results, in [1-(14)C]AA accumulation assays, the efficiency of [1-(14)C]AA accumulation was significantly higher in the adrenals (which had the greatest ability to accumulate [1-(14)C]AA) of diabetic mice than in those of controls. Expression of sodium-dependent vitamin C transporter (SVCT)-2, a transporter of AA, was upregulated in diabetic adrenals. Furthermore, increased AA incorporation into the diabetic adrenals by SVCT-2 led to increased plasma norepinephrine, triglyceride and free fatty acid levels in mice with STZ-induced diabetes. Therefore, oversupplementation with AA could be deleterious in diabetic patients, because overexpression of adrenal SVCT-2 in diabetes could lead to excessive AA uptake, thus enhancing norepinephrine production and exacerbating some diabetic complications. Interestingly, however, treatment with AA dose-dependently abolished the increased expression of adrenal SVCT-2 and normalized the abovementioned plasma parameters in diabetic mice. These results suggest SVCT-2-mediated increases in AA uptake by the adrenals followed by excessive production of plasma norepinephrine may play a pivotal role in the development of diabetic complications.

Increased Lipid Peroxidation and Ascorbic Acid Utilization in Testis and Epididymis of Rats Chronically Exposed to Lead

The hypothesis has been recently presented that lead may exert its negative effect at least partially through the increase of reactive oxygen species (ROS) level in tissues. However, little is known about the influence of lead intoxication on equilibrium between generation and elimination of ROS in the male reproductive system. Sexually mature male Wistar rats were given ad libitum 1% of aqueous solution of lead acetate (PbAc) for 9 months. Significantly higher lead concentrations were found in blood [median 7.03 (Q25-Q75: 2.99-7.65) versus 0.18 (0.12-0.99) microg dl-1, P < 0.01], caput epididymis [median 5.51 (Q25-Q75: 4.31-7.83) versus 0.51 (0.11-0.80) microg g-1 d.m., P < 0.001], cauda epididymis [median 5.88 (Q25-Q75: 4.06-8.37) versus 0.61 (0.2 - 1.08) microg g-1 d.m., P < 0.001] and testis [median 1.81 (Q25-Q75: 0.94-2.31) versus 0.17 (0.03-0.3) microg g-1 d.m., P < 0.01] of lead-intoxicated rats when compared to the control. The concentration of ascorbyl radical, generated in vitro from L: -ascorbic acid (present in tissues in vivo) was measured by means of Electron Paramagnetic Resonance (EPR) spectroscopy. The EPR signal of ascorbyl radical in caput epididymis, cauda epididymis, testis and liver of lead acetate-treated animals revealed a significant decrease by 53%, 45%, 40% and 69% versus control tissues, respectively. Plasma L: -ascorbic acid content measured by high performance liquid chromatography (HPLC) method and total antioxidant status (TAS) measured by means of spectrophotometry were also significantly lower in the intoxicated versus control animals (28% and 21%, respectively). In the group exposed to lead the concentration of lipid peroxide in homogenates of the reproductive system organs was significantly elevated versus control group. It can be assumed that the lower EPR signal was caused by decreased tissue concentrations of L: -ascorbic acid. The latter may have resulted from consumption of ascorbic acid for scavenging of ROS excess in tissues of animals chronically exposed to lead.

Ascorbic acid secretion in the human stomach and the effect of gastrin

AIM: To investigate the changes of gastric mucosal ascorbic acid secretion in patients with nonulcer dyspepsia and the effect of gastrin on it, and to relate any observed changes to H. pylori infection and mucosal histology.

METHODS: Ascorbic acid secretions in patients were examined by collecting continuously gastric juice for one hour after having aspirated and discarded fasting gastric juice. Using the clearance rate (mL/min) of ascorbic acid from blood to gastric juice represented ascorbic acid secretion in the gastric mucosa. Ascorbic acid concentrations in plasma and juice were measured by ferric reduced method.

RESULTS: Gastric ascorbic acid secretions in H. pylori -positive patients (1.46 mL/min, range 0.27-3.78) did not significantly differ from those in H. pylori -negative patients (1.25 mL/min, 0.47-3.14) (P > 0.05). There were no significant differences in ascorbic acid secretions between patients with mild (1.56 mL/min, 0.50-3.30), moderate (1.34 mL/min, 0.27-2.93) and severe (1.36 mL/min, 0.47-3.78) inflammation (P > 0.05). There were no significant differences in ascorbic acid secretions between patients without activity (1.45 mL/min, 0.27-3.14) and with mild (1.32 mL/min, 0.61-2.93), moderate (1.49 mL/min, 0.50-3.78) and severe (1.43 mL/min, 0.51-3.26) activity of chronic gastritis either (P > 0.05). Ascorbic acid secretions in patients with severe atrophy (0.56 mL/min, 0.27-1.20) were markedly lower than those in patients with out atrophy (1.51 mL/min, 0.59-3.30) and with mild (1.43 mL/min, 0.53-3.78) and moderate (1.31 mL/min, 0.47-3.16) atrophy (P < 0.005). There was a significant negative correlation between ascorbic acid secretion and severity of atrophy (correlation coefficient = -0.43, P < 0.005). After administration of pentagastrin, ascorbic acid secretions were markedly elevated (from 1.39 mL/min, 0.36-2.96 to 3.53 mL/min, 0.84-5.91) (P < 0.001).

CONCLUSION: Ascorbic acid secretion in gastric mucosa is not affected by H. pylori infection. Gastric ascorbic acid secretion is markedly related to the severity of atrophy, whereas not related to the severity of inflammation and activity. Gastrin may stimulate gastric ascorbic acid secretion. A decreased ascorbic acid secretion may be an important factor in the link between atrophic gastritis and gastric carcinogenesis.

S-nitrosothiols and the nitrergic neurotransmitter in the rat gastric fundus: effect of antioxidants and metal chelation

1. The effects of the antioxidants ascorbic acid and alpha-tocopherol and of the metal chelator ethylenediaminetetraacetic acid (EDTA) were studied on relaxations in response to S-nitrosothiols, authentic nitric oxide (NO) and nitrergic non-adrenergic non-cholinergic stimulation of the rat gastric fundus. 2. The S-nitrosothiols S-nitrosocysteine (1-100 nM), S-nitrosoglutathione (0.01-3 microM) and S-nitroso-N-acetylpenicillamine (0.01-3 microM) induced concentration-dependent relaxations of the rat gastric fundus muscle strips, which were precontracted with prostaglandin F2alpha. The relaxations to all S-nitrosothiols were concentration-dependently enhanced by the antioxidants ascorbic acid (0.1-3 microM) and alpha-tocopherol (3-30 microM) and inhibited by the metal chelator EDTA (26 microM). 3. Ascorbic acid and alpha-tocopherol alone did not induce a relaxation of the precontracted rat gastric fundus muscle strip. However, when ascorbic acid (1 microM) or alpha-tocopherol (1 microM) were injected in the organ bath 1 minute after S-nitrosoglutathione (0.1 microM) or after S-nitroso-N-acetylpenicillamine (0.1 microM), they induced an immediate, sharp and transient relaxation. This relaxation was inhibited by the superoxide generator pyrogallol (2 microM). Such a relaxation to ascorbic acid or alpha-tocopherol was not observed in the presence of S-nitrosocysteine (10 nM). 4. Electrical field stimulation (0.5-4 Hz) of the precontracted rat gastric fundus strips induced frequency-dependent nitrergic relaxations which were mimicked by authentic NO (3-300 nM) and by acidified sodium nitrite NaNO2 (0.3-10 microM). Ascorbic acid (0.33-3 microM), alpha-tocopherol (3-30 microM) or EDTA (26 microM) did not affect the relaxations to nitrergic stimulation, NO or NaNO2. 5. In summary, relaxations to S-nitrosothiols in the rat gastric fundus are enhanced by the antioxidants ascorbic acid and alpha-tocopherol and inhibited by the metal chelator EDTA. However, relaxations to nitrergic stimulation of the rat gastric fundus or those to authentic NO were not affected by the antioxidants or by the metal chelator. These results indicate that antioxidants and metal chelators have a different effect on the biological activity of S-nitrosothiols and on that of the nitrergic neurotransmitter. Therefore, our results suggest that S-nitrosothiols do not act as intermediate compounds in nitrergic neurotransmission in the rat gastric fundus.

Release of the antioxidants ascorbate and urate from a nitrergically-innervated smooth muscle

1. The main object of the present study was to determine whether ascorbate, an antioxidant which has been shown to protect nitric oxide (NO) from attack by scavenger molecules, might be released from nitrergically-innervated smooth muscle; ascorbate release from the rat anococcygeus was measured by use of h.p.l.c. with electrochemical detection. 2. Incubation of rat anococcygeus muscles in normal physiological salt solution (PSS; 30 min) resulted in release of ascorbate into the bathing medium (7.7 +/- 0.9 nmol g-1 tissue). This release was increased by 96% when muscles were incubated in high K+ (70 mM) PSS. The resting release of ascorbate was unaffected by tetrodotoxin (TTX; 1 microM), omega-conotoxin GVIA (10 nM) or omission of calcium ions from the PSS (with addition of 0.2 mM EGTA), but all three procedures attenuated the increased release observed under depolarizing conditions. Resting release of ascorbate was unaffected by glutamate (100 microM), aspartate (100 microM), gamma-aminobutyric acid (100 microM) or carbachol (50 microM). 3. A second h.p.l.c. peak, which always preceded the ascorbate peak, was identified as urate. Urate release from the anococcygeus, following 30 min incubation in normal PSS, was 64.6 +/- 12.7 nmol g-1 tissue but, unlike ascorbate, urate release was unchanged in high K+ PSS. In functional experiments, urate (100-400 microM) partially protected NO (15 microM)-induced relaxations of the rat anococcygeus from inhibition by 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (carboxy-PTIO; 50 microM), but not from inhibition by hydroquinone or duroquinone (both 100 microM). 4. Muscles chemically sympathectomized with 6-hydroxydopamine (6-OHDA, 500 microM; 2 h) still exhibited release of ascorbate (2.5 +/- 0.4 nmol g-1 tissue) and urate (22.2 +/- 2.9 nmol g-1 tissue); in both cases the release was similar to that observed in time-matched control tissues not exposed to 6-OHDA. High K+ PSS produced a TTX-sensitive increase in release of ascorbate, but not urate, from 6-OHDA-treated muscles. 5. The results demonstrate that significant amounts of ascorbate and urate are released from the rat anococcygeus muscle. Ascorbate, but not urate, release appears to be enhanced by activation of nerves which are resistant to 6-OHDA pretreatment. Since both antioxidants can protect NO from attack by scavenger molecules, their release in nitrergically-innervated tissues may be important for the provision of the correct redox environment to allow NO to fulfill its proposed neurotransmitter role.