Hypoxia/Reoxygenation Effects on Ion Transport across Rat Colonic Epithelium

Gasotransmitters do not prevent changes in transepithelial ion transport induced by hypoxia followed by reoxygenation

OBJECTIVES

How gaseous signalling molecules affect ion transport processes contributing to the physiological functions of the gastrointestinal tract under hypoxic conditions still needs to be clarified. The objective of the present study was to characterize the impact of gaseous signalling molecules on parameters of colonic ion transport during a hypoxia/reoxygenation cycle and the remaining secretory capacity of the epithelium after such a cycle.

METHODS

Short-circuit current (Isc) and tissue conductance (Gt) recordings in Ussing chamber experiments were performed on rat colon samples using CORM-2 (putative CO donor; 35 and 350 µM), sodium nitroprusside (NO donor; 100 µM), NaHS (fast H2S donor; 10 - 1,000 µM), GYY 4137 (slow H2S donor; 50 µM) and Angeli's salt (HNO donor; 100 µM) as donors for gasotransmitters. Inhibition of endogenous synthesis of H2S was operated by inhibitors of cystathionin-γ-lyase, i.e. dl-propargylglycine (1 mM) or β-cyano-l-alanine (5 mM), and the inhibitor of cystathionine-β-synthase, amino-oxyacetate (5 mM).

RESULTS

The fast gasotransmitter donors NaHS, sodium nitroprusside and Angeli's salt, administered 5 min before the onset of hypoxia, induced an increase in Isc. The response to the subsequently applied hypoxia was characterized by a decrease in Isc, which tended to be reduced only in the presence of the lowest concentration of NaHS (10 µM) tested. Reoxygenation resulted in a slow increase in Isc, which was unaffected by all donors or inhibitors tested. The stable acetylcholine derivative carbachol (50 µM) was administered at the end of each hypoxia/reoxygenation cycle to test the secretory capacity of the epithelium. Pretreatment of the tissue with the putative CO donor CORM-2 suppressed the secretory response induced by carbachol. The same was observed when cystathionin-γ-lyase and cystathionin-γ-synthase were inhibited simultaneously. Under both conditions, Gt drastically increased suggesting an impaired tissue integrity.

CONCLUSIONS

The present results demonstrate that none of the exogenous gasotransmitter releasing drugs significantly ameliorated the changes in epithelial ion transport during the hypoxia/reoxygenation cycle ex vivo. In contrast, the putative CO donor CORM-2 exerted a toxic effect on the epithelium. The endogenous production of H2S, however, seems to have a protective effect on the mucosal integrity and the epithelial transport functions, which - when inhibited - leads to a loss of the secretory ability of the mucosa. This observation together with the trend for improvement observed with a low concentration of the H2S donor NaHS suggests a moderate protective role of low concentrations of H2S under hypoxic conditions.

Effects of the gaseous signalling molecule nitroxyl (HNO) on myenteric neurons governing intestinal motility

OBJECTIVES

The main function of myenteric neurons is the control of gut motility. As we recently showed that nitroxyl (HNO) induces intestinal smooth muscle relaxation, it was of interest to evaluate the effects of this signalling molecule on myenteric neurons in order to distinguish its properties in regard to myocytes.

METHODS

Myenteric neurons isolated from the ileum of 4-10 days old rats were used. HNO-induced changes in intracellular concentration of Ca2+ or membrane potential and ion currents were measured using the Ca2+-sensitive fluorescent dye fura-2 AM or by electrophysiological whole-cell recordings, respectively. Changes in intracellular thiol groups pool were evaluated using thiol tracker violet. Angeli's salt was used as HNO donor.

RESULTS

The HNO donor Angeli's salt induced a significant increase in the cytosolic Ca2+ concentration at the concentration 50 µM and a membrane hyperpolarization from a resting membrane potential of -56.1 ± 8.0 mV to -63.1 ± 8.7 mV (n=7). Although potassium channels primarily drive membrane potential changes in these cells, outwardly rectifying potassium currents were not significantly affected by 50 µM Angeli's salt. Fast inward sodium currents were slightly but not significantly reduced by HNO. In more sensitive cells, HNO tended to reduce the pool of thiol groups.

CONCLUSIONS

As in the case of smooth muscle cells, HNO causes hyperpolarization of myenteric neurons, an effect also associated with an increase in intracellular Ca2+ concentration. Pathways other than activation of potassium currents appear to drive the hyperpolarization evoked by HNO.

Biogenic Selenium Nanoparticles Synthesized by Lactobacillus casei ATCC 393 Alleviate Acute Hypobaric Hypoxia-Induced Intestinal Barrier Dysfunction in C57BL/6 Mice

Exposure to hypobaric hypoxia at high altitude will cause different tissue and organ damage over a long period of time. Studies have shown that hypobaric hypoxia can cause severe primary intestinal barrier dysfunction, and then cause multiple organ dysfunction. Our previous research showed that selenium nanoparticles (SeNPs) synthesized by Lactobacillus casei ATCC 393 (L. casei ATCC 393) can effectively alleviate intestinal barrier dysfunction caused by oxidative stress and inflammation in mice. This study was conducted to investigate the protective effect of biological SeNPs synthesized by L. casei ATCC 393 on intestinal barrier function in acute hypobaric hypoxic stress mice. The results showed that compared with the hypobaric hypoxic, the SeNPs synthesized by L. casei ATCC 393 by oral administration could effectively alleviate the shortening of intestinal villi, which decreased the level of diamine oxidase (DAO) and myeloperoxidase (MPO), and the expression level of tight junction protein in ileum was increased. In addition, SeNPs significantly increased the activities of superoxide dismutase (SOD), cyclooxygenase (COX-1) and glutathione peroxidase (GPx), and decreased the level of malondialdehyde (MDA), and inhibit the increase of hypoxia related factor. SeNPs effectively regulate the intestinal microecology disorder caused by hypobaric hypoxia stress, and maintain the intestinal microecology balance. In addition, oral administration of SeNPs had better protective effect on intestinal barrier function of mice under hypobaric hypoxia stress. These results suggested that SeNPs synthesized by L. casei ATCC 393 can effectively alleviate the damage of intestinal barrier function under acute hypobaric hypoxic stress, which may be closely related to the antioxidant activity of SeNPs.

Butyrate Protects Porcine Colon Epithelium from Hypoxia-Induced Damage on a Functional Level

The large intestinal epithelium is confronted with the necessity to adapt quickly to varying levels of oxygenation. In contrast to other tissues, it meets this requirement successfully and remains unharmed during (limited) hypoxic periods. The large intestine is also the site of bacterial fermentation producing short-chain fatty acids (SCFA). Amongst these SCFA, butyrate has been reported to ameliorate many pathological conditions. Thus, we hypothesized that butyrate protects the colonocytes from hypoxic damage. We used isolated porcine colon epithelium mounted in Ussing chambers, incubated it with or without butyrate and simulated hypoxia by changing the gassing regime to test this hypothesis. We found an increase in transepithelial conductance and a decrease in short-circuit current across the epithelia when simulating hypoxia for more than 30 min. Incubation with 50 mM butyrate significantly ameliorated these changes to the epithelial integrity. In order to characterize the protective mechanism, we compared the effects of butyrate to those of iso-butyrate and propionate. These two SCFAs exerted similar effects to butyrate. Therefore, we propose that the protective effect of butyrate on colon epithelium under hypoxia is not (only) based on its nutritive function, but rather on the intracellular signaling effects of SCFA.

Hypoxia attenuate ionic transport in the isolated gill epithelium of Carcinus maenas

The gills are osmorespiratory organs of aquatic organisms and the prime target of environmentally induced hypoxia. We have studied the impact of severe hypoxia (0.5 mg O₂/L) on the ionic transport across posterior gills of Carcinus maenas acclimated to 12 ppt seawater (DSW). The short-circuit current (Isc) across hemilamellae from gills i.e. active ion transport was studied in micro Ussing chambers. Hypoxia induced by deoxygenation of the basolateral side, and not the apical side, resulted in time-dependent inhibition of Isc and full recovery of Isc after reoxygenation. Exposure of the crabs to severe 7 h hypoxia decreased the specific activity of Na⁺,K⁺-ATPase in the gills by 36%. Full recovery of enzyme activity occurred in fasted crabs after 3 days of reoxygenation. The intensity of Western blotting bands was not different in the gills of oxygenated, hypoxic and reoxygenated crabs. The reversible, nonspecific blocker of K⁺ channels Cs and hypoxia inhibited over 90% of Isc which is after reoxygenation fully recovered. The specific blocker of Cl^- channels NPPB [5-nitro-2-(3-phenylpropylamino)benzoic acid] blocked Isc by 68.5%. Only the rest of not inhibited Isc in aerated saline was blocked by hypoxia and recovered after reoxygenation. The activity of the antioxidant enzyme catalase was not changed during hypoxia and reoxygenation kept the high enzyme activity in the gills at the level of crabs acclimated to DSW. As a response to hypoxia the presence of 2 mM H₂O₂ induce an initial slight transient decrease of Isc followed by a rise and after reoxygenation fully recovered Isc. Incubation of hemilamellae with the antioxidant derivative Trolox did not affect the inhibition of Isc by hypoxia.

Evaluation of the NMR-MOUSE as a new method for continuous functional monitoring of the small intestine during different perfusion states in a porcine model

Objective

The study aim was to evaluate a small low-field NMR (nuclear magnetic resonance) scanner, the NMR-MOUSE^®, for detecting changes in intestinal diffusion under different (patho-) physiological perfusion states.

Methods

Laparotomy was performed on 8 female landrace pigs (body weight 70±6 kg) and the feeding vessels of several intestinal loops were dissected. Successively, the intestinal loops were examined using O2C (oxygen to see, LEA Medizintechnik GmbH, Giessen, Germany) for microcirculatory monitoring and the NMR-MOUSE^® for diffusion measurement (fast and slow components). On each loop the baseline measurement (physiological perfusion) was followed by one of the following main procedures: method 1 –ischemia; method 2 –flow reduction; method 3 –intraluminal glucose followed by ischemia; method 4 –intraluminal glucose followed by flow reduction. Additionally, standard perioperative monitoring (blood pressure, ECG, blood gas analyses) and histological assessment of intestinal biopsies was performed.

Results

There was no statistical overall time and method effect in the NMR-MOUSE measurement (fast component: p_time = 0.6368, p_method = 0.9766, slow component: p_time = 0.8216, p_method = 0.7863). Yet, the fast component of the NMR-MOUSE measurement showed contrary trends during ischemia (increase) versus flow reduction (decrease). The slow-to-fast diffusion ratio shifted slightly towards slow diffusion during flow reduction. The O2C measurement showed a significant decrease of oxygen saturation and microcirculatory blood flow during ischemia and flow reduction (p < .0001). The local microcirculatory blood amount (rHb) showed a significant mucosal increase (p_{Clamping(method 1)} = 0.0007, p_{Clamping(method 3)} = 0.0119), but a serosal decrease (p_{Clamping(method 1)} = 0.0119, p_{Clamping(method 3)} = 0.0078) during ischemia. The histopathological damage was significantly higher with increasing experimental duration and at the end of methods 3 and 4 (p < .0001,Fisher-test).

Conclusion

Monitoring intestinal diffusion changes due to different perfusion states using the NMR-MOUSE is feasible under experimental conditions. Despite the lack of statistical significance, this technique reflects perfusion changes and therefore seems promising for the evaluation of different intestinal perfusion states in the future. Beforehand however, an optimization of this technology, including the optimization of the penetration depth, as well as further validation studies under physiological conditions and including older animals are required.

The effects of hypoxia on active ionic transport processes in the gill epithelium of hyperregulating crab, Carcinus maneas

Effects of hypoxia on the osmorespiratory functions of the posterior gills of the shore crab Carcinus maenas acclimated to 12ppt seawater (DSW) were studied. Short-circuit current (Isc) across the hemilamella (one epithelium layer supported by cuticle) was substantially reduced under exposure to 1.6, 2.0, or 2.5mg O₂/L hypoxic saline (both sides of epithelium) and fully recovered after reoxygenation. Isc was reduced equally in the epithelium exposed to 1.6mg O₂/L on both sides and when the apical side was oxygenated and the basolateral side solely exposed to hypoxia. Under 1.6mg O₂/L, at the level of maximum inhibition of Isc, conductance was decreased from 40.0mScm^-2 to 34.7mScm^-2 and fully recovered after reoxygenation. Isc inhibition under hypoxia and reduced ⁸⁶Rb⁺ (K⁺) fluxes across apically located K⁺ channels were caused preferentially by reversible inhibition of basolaterally located and ouabain sensitive Na⁺,K⁺-ATPase mediated electrogenic transport. Reversible inhibition of Isc is discussed as decline in active transport energy supply down regulating metabolic processes and saving energy during oxygen deprivation. In response to a 4day exposure of Carcinus to 2.0mg O₂/L, hemolymph Na⁺ and Cl^- concentration decreased, i.e. hyperosmoregulation was weakened. Variations of the oxygen concentration level and exposure time to hypoxia lead to an increase of the surface of mitochondria per epithelium area and might in part compensate for the decrease in oxygen availability under hypoxic conditions.