Monochloramine-induced toxicity and dysregulation of intracellular Zn2+ in parietal cells of rabbit gastric glands

Monochloramine (NH(2)Cl) is a potent, thiol-directed oxidant capable of oxidizing thiol (S-H) residues in a wide variety of proteins. Generated in the stomach by the interaction of bacterial and host products, monochloramine has been shown to dysregulate Ca(2+) homeostasis and disrupt mucosal integrity. In this report, we show that monochloramine also leads to disturbances in intracellular free zinc concentration ([Zn(2+)](i)) in the gastric gland of the rabbit and that the increased Zn(2+) within the cell causes an independent decrease in cell viability. Changes in [Zn(2+)](i) were measured by using the fluorescent reporter FluoZin-3, whereas cell viability was assessed by measuring the conversion of calcein-AM to fluorescent calcein, an assay that is not affected by intracellular oxidation state. Cell death was confirmed using propidium iodide and YO-PRO-1 dye uptake measurements. Our experiments demonstrate that [Zn(2+)](i) is increased in gastric glands exposed to NH(2)Cl and that elevated [Zn(2+)](i) decreases cell viability. Chelation of Zn(2+) with tetrakis-(2-pyridylmethyl) ethylenediamine decreases the toxicity of NH(2)Cl, but only when administered concurrently. These findings suggest that the toxic effect of thiol oxidants present during chronic gastritis is partially due to dysregulation of [Zn(2+)](i) early in the process and that zinc chelation can protect, but not rescue, gastric glands exposed to toxic doses of NH(2)Cl.

Secretory state regulates Zn2+ transport in gastric parietal cell of the rabbit

Secretory compartments of neurons, endocrine cells, and exocrine glands are acidic and contain high levels of labile Zn2+. Previously, we reported evidence that acidity is regulated, in part, by the content of Zn2+ in the secretory [i.e., tubulovesicle (TV)] compartment of the acid-secreting gastric parietal cell. Here we report studies focusing on the mechanisms of Zn2+ transport by the TV compartment in the mammalian (rabbit) gastric parietal cell. Uptake of Zn2+ by isolated TV structures was monitored with a novel application of the fluorescent Zn2+ reporter N-(6-methoxy-8-quinolyl)-para-toluenesulfonamide (TSQ). Uptake was suppressed by removal of external ATP or blockade of H+-K+-ATPase that mediates luminal acid secretion. Uptake was diminished with dissipation of the proton gradient across the TV membrane, suggesting Zn2+/H+ antiport as the connection between Zn2+ uptake and acidity in the TV lumen. In isolated gastric glands loaded with the reporter fluozin-3, inhibition of H+-K+-ATPase arrested the flow of Zn(2+) from the cytoplasm to the TV compartment and secretory stimulation with forskolin enhanced vectorial movement of cytoplasmic Zn2+ into the tubulovesicle/lumen (TV/L) compartment. Our findings suggest that Zn2+ accumulation in the TV/L compartment is physiologically coupled to secretion of acid. These findings offer novel insight into mechanisms regulating Zn2+ homeostasis in the gastric parietal cell and potentially other cells in which acidic subcellular compartments serve signature functional roles.

Monochloramine impairs caspase-3 through thiol oxidation and Zn2+ release

BACKGROUND

Caspase-3, a pro-apoptotic enzyme, represents a class of proteins in which the active site contains reduced thiol (S-H) groups and is modulated by heavy metal cations, such as Zn(2+). We explored the effects of the thiol oxidant monochloramine (NH(2)Cl) on caspase-3 activity within cells of isolated rabbit gastric glands. In addition, we tested the hypothesis that NH(2)Cl-induced alterations of caspase-3 activity are modulated by oxidant-induced accumulation of Zn(2+) within the cytoplasm.

MATERIALS AND METHODS

Isolated gastric glands were prepared from rabbit mucosa by collagenase digestion. Caspase-3 activity was measured colorimetrically in suspensions of healthy rabbit gastric glands, following exposure to various concentrations of NH(2)Cl with or without the zinc chelator TPEN [tetrakis-(2-pyridylmethyl)ethylene diamine] for 1 h, and re-equilibration in Ringer's solution for 5 h. Conversion of procaspase-3 to active caspase-3 was monitored by Western blot.

RESULTS

Monochloramine inhibited caspase-3 activity in a dose-dependent fashion. At concentrations of NH(2)Cl up to 100 microM, these effects were prevented if TPEN was given concurrently and were partly reversed if TPEN was given 1 h later. Caspase-3 activity was preserved by concurrent treatment with a thiol-reducing agent, dithiothreitol.

CONCLUSIONS

At pathologically relevant concentrations, NH(2)Cl impairs caspase-3 activity through oxidation of its thiol groups. Independently from its thiol oxidant effects on the enzyme, NH(2)Cl-induced accumulation of Zn(2+) in the cytoplasm is sufficient to restrain endogenous caspase-3 activity. Our studies suggest that some bacterially generated oxidants, such as NH(2)Cl, impair host pathways of apoptosis through release of Zn(2+) from endogenous pools.

Report of the 2007 Education Committee Panel on simulation in alimentary tract surgery

At the 2007 Digestive Diseases Week, the Education Committee of the Society for Surgery of the Alimentary Tract sponsored a symposium entitled "Simulation in Gastrointestinal Surgery." Four panelists presented perspectives on the role of simulation in education and training of medical students and residents in operations of Gastrointestinal Surgery in the gastrointestinal surgeon's practice, in the certification of centers for training and maintenance of skills, and in the credentialing of surgeons in new procedures. The consensus of the panel is that over the next several years, simulation will play an expanding role in all of these spheres of the GI surgeon's activities.

Influence of imaging on the negative appendectomy rate in pregnancy

Appendectomy is the most common non-gynecologic surgery performed during pregnancy. Little data exist on the accuracy of imaging studies in the diagnosis of appendicitis in pregnancy. The objective of this study was to evaluate the probability of ultrasound and computed tomography (CT) scan in diagnosing appendicitis in pregnancy, as reflected in the negative appendectomy rate. We retrospectively reviewed the charts of 86 pregnant women who underwent an appendectomy between January 1, 1997 and January 1, 2006. Patients were divided into three groups: clinical evaluation, ultrasound, and ultrasound followed by a CT scan. The clinical evaluation group had 13 patients, with a negative appendectomy rate of 54% (7/13). Fifty-five patients underwent an ultrasound alone, with a negative appendectomy rate 36% (20/55). In the ultrasound/CT group (n=13), the negative appendectomy rate was 8% (1/13). There was a significant reduction in the negative appendectomy rate in the ultrasound/CT scan group compared to clinical evaluation group (54 vs 8%, p<0.05). This reduction was not achieved in the ultrasound group when compared to the clinical evaluation group or the ultrasound/CT group (p=0.05). A significant reduction was achieved when the ultrasound/CT group was compared to the patients in the ultrasound only group who had a normal or inconclusive ultrasound (p<0.05). Our data documents a very high negative appendectomy rate in the pregnant patient. We recommend an ultrasound followed by a CT scan in patients with a normal or inconclusive ultrasound.

Lymphatic drainage of the peritoneal space: a pattern dependent on bowel lymphatics

BACKGROUND

Understanding lymph drainage patterns of the peritoneum could assist in staging and treatment of gastrointestinal and ovarian malignancies. Sentinel lymph nodes (SLNs) have been identified for solid organs and the pleural space. Our purpose was to determine whether the peritoneal space has a predictable lymph node drainage pattern.

METHODS

Rats received intraperitoneal injections of near-infrared (NIR) fluorescent tracers: namely, quantum dots (designed for retention in SLNs) or human serum albumin conjugated with IRDye800 (HSA800; designed for lymphatic flow beyond the SLN). A custom imaging system detected NIR fluorescence at 10 and 20 minutes and 1, 4, and 24 hours after injection. To determine the contribution of viscera to peritoneal lymphatic flow, additional cohorts received bowel resection before NIR tracer injection. Associations with appropriate controls were assessed with the chi(2) test.

RESULTS

Quantum dots drained to the celiac, superior mesenteric, and periportal lymph node groups. HSA800 drained to these same groups at early time points but continued flowing to the mediastinal lymph nodes via the thoracic duct. After bowel resection, both tracers were found in the thoracic, not abdominal, lymph node groups. Additionally, HSA800 was no longer found in the thoracic duct but in the anterior chest wall and diaphragmatic lymphatics.

CONCLUSIONS

The peritoneal space drains to the celiac, superior mesenteric, and periportal lymph node groups first. Lymph continues via the thoracic duct to the mediastinal lymph nodes. Bowel lymphatics are a key determinant of peritoneal lymph flow, because bowel resection shifts lymph flow directly to the intrathoracic lymph nodes via chest wall lymphatics.

Thiol-oxidant monochloramine mobilizes intracellular Ca2+ in parietal cells of rabbit gastric glands

In Helicobacter pylori-induced gastritis, oxidants are generated through the interactions of bacteria in the lumen, activated granulocytes, and cells of the gastric mucosa. In this study we explored the ability of one such class of oxidants, represented by monochloramine (NH(2)Cl), to serve as agonists of Ca(2+) accumulation within the parietal cell of the gastric gland. Individual gastric glands isolated from rabbit mucosa were loaded with fluorescent reporters for Ca(2+) in the cytoplasm (fura-2 AM) or intracellular stores (mag-fura-2 AM). Conditions were adjusted to screen out contributions from metal cations such as Zn(2+), for which these reporters have affinity. Exposure to NH(2)Cl (up to 200 microM) led to dose-dependent increases in intracellular Ca(2+) concentration ([Ca(2+)](i)), in the range of 200-400 nM above baseline levels. These alterations were prevented by pretreatment with the oxidant scavenger vitamin C or a thiol-reducing agent, dithiothreitol (DTT), which shields intracellular thiol groups from oxidation by chlorinated oxidants. Introduction of vitamin C during ongoing exposure to NH(2)Cl arrested but did not reverse accumulation of Ca(2+) in the cytoplasm. In contrast, introduction of DTT or N-acetylcysteine permitted arrest and partial reversal of the effects of NH(2)Cl. Accumulation of Ca(2+) in the cytoplasm induced by NH(2)Cl is due to release from intracellular stores, entry from the extracellular fluid, and impaired extrusion. Ca(2+)-handling proteins are susceptible to oxidation by chloramines, leading to sustained increases in [Ca(2+)](i). Under certain conditions, NH(2)Cl may act not as an irritant but as an agent that activates intracellular signaling pathways. Anti-NH(2)Cl strategies should take into account different effects of oxidant scavengers and thiol-reducing agents.

Intracellular Ca(2+) and Zn(2+) signals during monochloramine-induced oxidative stress in isolated rat colon crypts

During acute exacerbations of inflammatory bowel diseases, oxidants are generated through the interactions of bacteria in the lumen, activated granulocytes, and cells of the colon mucosa. In this study we explored the ability of one such class of oxidants, represented by monochloramine (NH(2)Cl), to serve as agonists of Ca(2+) and Zn(2+) accumulation within the colonocyte. Individual colon crypts prepared from Sprague-Dawley rats were mounted in perfusion chambers after loading with fluorescent reporters fura 2-AM and fluozin 3-AM. These reporters were characterized, in situ, for responsiveness to Ca(2+) and Zn(2+) in the cytoplasm. Responses to different concentrations of NH(2)Cl (50, 100, and 200 microM) were monitored. Subsequent studies were designed to identify the sources and mechanisms of NH(2)Cl-induced increases in Ca(2+) and Zn(2+) in the cytoplasm. Exposure to NH(2)Cl led to dose-dependent increases in intracellular Ca(2+) concentration ([Ca(2+)](i)) in the range of 200-400 nM above baseline levels. Further studies indicated that NH(2)Cl-induced accumulation of Ca(2+) in the cytoplasm is the result of release from intracellular stores and basolateral entry of extracellular Ca(2+) through store-operated channels. In addition, exposure to NH(2)Cl resulted in dose-dependent and sustained increases in intracellular Zn(2+) concentration ([Zn(2+)](i)) in the nanomolar range. These alterations were neutralized by dithiothreitol, which shields intracellular thiol groups from oxidation. We conclude that Ca(2+)- and Zn(2+)-handling proteins are susceptible to oxidation by chloramines, leading to sustained, but not necessarily toxic, increases in [Ca(2+)](i) and [Zn(2+)](i). Under certain conditions, NH(2)Cl may act not as a toxin but as an agent that activates intracellular signaling pathways.

Anatomy and Physiology of the Stomach

In this article, key concepts in gastric anatomy and physiology are reviewed. Attention is given to historical development of concepts of acid secretion, to the role of stomach in digestion, and to the mechanisms that protect gastric mucosa from acid and hostile luminal conditions. Evolving ideas that may influence understand-ing of the physiologic consequences of emerging therapeutics, and procedures that target anatomy or function of the stomach are also reviewed.

Divalent cations regulate acidity within the lumen and tubulovesicle compartment of gastric parietal cells

BACKGROUND & AIMS

Until recently, it has not been possible to evaluate factors that regulate the acidity of the microenvironment within the tubulovesicles and luminal (TV/L) spaces of the gastric gland. The goal of this study was to develop a method for monitoring the mechanisms that regulate acidity in the TV/L compartment.

METHODS

Isolated rabbit gastric glands (intact or permeabilized with S. aureus alpha-toxin) were loaded with a recently characterized fluorescent dye, LysoSensor Yellow-Blue DND 160 (Molecular Probes, Eugene, OR), which localizes to highly acidic compartments and can be used to monitor acidity ratiometrically.

RESULTS

In resting glands, the pH of the TV/L compartment was approximately 3.4. Moderate alkalizations ( approximately 0.5 to 1.0 pH unit alkalization) were observed during exposure to inhibitors of the apical H(+)/K(+) ATPase (omeprazole and SCH28080), thereby unmasking a stable, low-level leak of H(+) ions from the TV/L compartment. Similar changes were observed in alpha-toxin permeabilized glands following depletion of ATP in the cytoplasm. In intact and permeabilized glands, we used the cell-permeant, divalent cation chelator, tetrakis-(2-pyridylmethyl)ethylenediamine (TPEN) to probe the effects of lowering divalent cation content of the TV/L compartment. Exposure to relatively low concentrations (20 micromol/L, 50 micromol/L) of TPEN reversibly promoted H(+) leakage. At these concentrations, simultaneous inhibition using SCH28080 led to marked enhancement of the rate of alkalization.

CONCLUSIONS

The effects of low-dose TPEN suggests that acidity within the TV/L compartment of the gastric gland may be regulated, at least in part, by its content of divalent cations such as Zn(2+), for which TPEN has high affinity.

Partitioning of aquaporin-4 water channel mRNA and protein in gastric glands

Immunolocalization studies in proximal, middle, and distal stomach indicated that aquaporin-4 (AQP4) protein is localized only in parietal cells located in the middle or deep regions of the gastric glands. In studies using in situ hybridization, AQP4 mRNA failed to localize in parietal cells but was identified in neighboring mucosal cells that were triangular in shape and smaller than parietal cells in size, and in columnar cells at the base of the gastric gland. This spatial separation of mRNA and protein was also observed in other species and with other kind of mRNA/protein. In neonatal and adolescent rats, the appearance of morphologically mature parietal cells was preceded by identification of mRNA-bearing triangular cells. Cells harboring both protein and mRNA were observed in postnatal rats and in the pyloric region of the glandular stomach, during induced hypergastrinaemia. The results suggest that such cells represent a transition between those that bear only mRNA and those that are terminally differentiated, expressing proteins that are related to acid secretion.

Luminal regulation of Na(+)/H(+) exchanger gene expression in rat ileal mucosa

It is well recognized that ileostomy patients suffer from chronic depletion of Na(+) through the stoma effluent. In this study we evaluated the effects of ileostomy on messenger RNA levels that encode different Na(+)/H(+) exchanger isoforms (NHE-2 and NHE-3). Loop ileostomies were created in Sprague-Dawley rats. Segments of diverted ileum were harvested for quantitation of mRNA levels encoding these isoforms and the Na(+)/K(+) ATPase in mucosal scrapings and for immunofluorescence microscopy, specifically of the NHE-3 protein. Our studies indicate that as early as 8 days after diversion, NHE-3 gene expression is selectively attenuated in poststomal ileal mucosa. Mucosal morphology remains undisturbed, and the distribution of protein expression along the crypt/villus axis is not altered. Infusion of Na(+) or the enterocyte nutrient, glutamine, into the lumen of the diverted segment restores or even augments mRNA levels for NHE-3, again without altering the histologic appearance or distribution of the protein along the crypt/villus axis. These effects are specific because nonpolar osmolytes (mannitol) and related organic nutrients not specific for the enterocyte (i.e., butyrate) have no effect on mRNA levels of NHE-3. Further work is required to understand how the early changes in mRNA contribute to mucosal function and response to luminal diversion.

Asymmetrical, agonist-induced fluctuations in local extracellular [Ca(2+)] in intact polarized epithelia

We recently proposed that extracellular Ca(2+) ions participate in a novel form of intercellular communication involving the extracellular Ca(2+)-sensing receptor (CaR). Here, using Ca(2+)-selective microelectrodes, we directly measured the profile of agonist-induced [Ca(2+)]ext changes in restricted domains near the basolateral or luminal membranes of polarized gastric acid-secreting cells. The Ca(2+)-mobilizing agonist carbachol elicited a transient, La(3+)-sensitive decrease in basolateral [Ca(2+)] (average approximately 250 microM, but as large as 530 microM). Conversely, carbachol evoked an HgCl2-sensitive increase in [Ca(2+)] (average approximately 400 microM, but as large as 520 microM) in the lumen of single gastric glands. Both responses were significantly reduced by pre-treatment with sarco-endoplasmic reticulum Ca(2+) ATPase (SERCA) pump inhibitors or with the intracellular Ca(2+) chelator BAPTA-AM. Immunofluorescence experiments demonstrated an asymmetric localization of plasma membrane Ca(2+) ATPase (PMCA), which appeared to be partially co-localized with CaR and the gastric H(+)/K(+)-ATPase in the apical membrane of the acid-secreting cells. Our data indicate that agonist stimulation results in local fluctuations in [Ca(2+)]ext that would be sufficient to modulate the activity of the CaR on neighboring cells.