The role of nitric oxide in the regulation of macromolecular transport in rat jejunum

Expression of Bis in the mouse gastrointestinal system

The Bcl-2 interacting death suppressor (Bis) protein is known to be involved in a variety of pathophysiological conditions. We recently generated bis-deficient mice, which exhibited early lethality with typical nutritional deprivation status. To further investigate the molecular basis for the malnutrition phenotype of bis deficient mice, we explored Bis expression in the digestive system of normal mice. Western blot analysis and quantitative real time reverse transcription polymerase chain reaction analysis indicated that Bis expression is highest in the esophagus, followed by the stomach, colon, jejunum and ileum. Immunohistochemical data indicated that Bis expression is restricted to the stratified squamous epitheliums in the esophagus and forestomach, and was not notable in the columnar epitheliums in the stomach, small intestine and colon. In addition, strong Bis immunoreactivity was detected in the striated muscles surrounding the esophagus and smooth muscles at a lesser intensity throughout the gastrointestinal (GI) tract. Ganglionated plexuses, located in submucous layers, as well as intermuscular layers, were specifically immunoreactive for Bis. Immunofluorescence studies revealed that Bis is co-localized in glial fibrillary acidic protein-expressing enteric glial cells. Immunostaining with neuron specific esterase antibodies indicate that Bis is also present in the cell bodies of ganglions in the enteric nervous system (ENS). Our findings indicate that Bis plays a role in regulating GI functions, such as motility and absorption, through modulating signal transmission between the ENS and smooth muscles or the intestinal epitheliums.

Neural regulation of intestinal nutrient absorption

The nervous system and the gastrointestinal (GI) tract share several common features including reciprocal interconnections and several neurotransmitters and peptides known as gut peptides, neuropeptides or hormones. The processes of digestion, secretion of digestive enzymes and then absorption are regulated by the neuro-endocrine system. Luminal glucose enhances its own absorption through a neuronal reflex that involves capsaicin sensitive primary afferent (CSPA) fibres. Absorbed glucose stimulates insulin release that activates hepatoenteric neural pathways leading to an increase in the expression of glucose transporters. Adrenergic innervation increases glucose absorption through α1 and β receptors and decreases absorption through activation of α2 receptors. The vagus nerve plays an important role in the regulation of diurnal variation in transporter expression and in anticipation to food intake. Vagal CSPAs exert tonic inhibitory effects on amino acid absorption. It also plays an important role in the mediation of the inhibitory effect of intestinal amino acids on their own absorption at the level of proximal or distal segment. However, chronic extrinsic denervation leads to a decrease in intestinal amino acid absorption. Conversely, adrenergic agonists as well as activation of CSPA fibres enhance peptides uptake through the peptide transporter PEPT1. Finally, intestinal innervation plays a minimal role in the absorption of fat digestion products. Intestinal absorption of nutrients is a basic vital mechanism that depends essentially on the function of intestinal mucosa. However, intrinsic and extrinsic neural mechanisms that rely on several redundant loops are involved in immediate and long-term control of the outcome of intestinal function.

Lack of evidence in vivo for nitrergic inhibition by Escherichia coli (STa) enterotoxin of fluid absorption from rat proximal jejunum

Fluid absorption from the proximal jejunum of the anaesthetised rat was measured in vivo by fluid recovery. As expected, heat stable (STa) enterotoxin from E. coli reduced fluid absorption. Neither intraperitoneal L-NAME, thought to inhibit a putative neurally mediated action of STa, nor similar doses of D-NAME, ameliorated the inhibitory effect on jejunal fluid absorption of STa. Luminally perfused 10 mM sodium nitroprusside (SNP) had no effect on fluid absorption when expressed per gram dry weight per hour but reduced fluid absorption when expressed per cm length per hour. Similarly, 80 but not 40 mg/Kg of L-NAME reduced fluid absorption when expressed per cm length per hour, while the same dose of D-NAME did not. L-NAME and SNP significantly increased the wet weight to dry weight and the length to dry weight ratio of perfused loops. We conjecture that smooth muscle relaxation caused by these compounds increases interstitial fluid volumes that can be misconstrued as changes in absorption when this is expressed per cm length or per tissue wet weight. When fluid absorption is expressed per gram dry weight of tissue, there is no evidence for a role of nitric oxide in normal or STa inhibited fluid absorption.

Arginine does not exacerbate markers of inflammation in cocultures of human enterocytes and leukocytes

Enteral arginine supplementation in the critically ill has become a matter of controversy. In this study, we investigated effects of the addition of 0.4 and 1.2 mmol/L arginine in a coculture model on markers of inflammation, enterocyte layer integrity, and amino acid transport. In this model, a monolayer of intestinal epithelial cells (Caco-2) separated compartments with nonpathogenic Escherichia coli and mononuclear leukocytes. Activation of enterocytes and leukocytes was assessed by the measurement of nitric oxide, TNF-alpha, IL-6, IL-8, IL-10, and IFN-gamma. Further outcomes were the transepithelial flux of 22 amino acids, their catabolism, and the integrity of the enterocyte layer assessed as permeability of fluorescein dextran (M(r) 4400). Bacterial stimulation of intestinal epithelial cells enhanced the basolateral concentration of nitric oxide and all cytokines measured. Supplementation with arginine did not affect epithelial integrity, production of any of the cytokines investigated, or the amount of nitric oxide. The amino acid used primarily by nonstimulated intestinal epithelial cells cocultured with leukocytes was glutamine. Activation of IEC with bacteria significantly enhanced the catabolism of serine, asparagine, and lysine, and reduced glutamine catabolism. Addition of arginine increased ornithine formation and moderately reduced transepithelial transport of methionine and other amino acids. Hence, arginine supplementation does not interfere with inflammation-associated cross-talk between human enterocytes and leukocytes. Because it also does not seem to affect the integrity of enterocyte layers, a detrimental role of arginine during septic-like conditions seems unlikely.

Induction of nitric oxide synthase and subsequent production of nitric oxide not involved in interferon-gamma-induced hyperpermeability of Caco-2 intestinal epithelial monolayers

Caco-2 cell monolayers exposed to 1000 U/ml interferon-gamma (IFN-gamma) for 6 days elicited inducible nitric oxide synthase (iNOS) expression and increased translayer permeability. This iNOS increase was blocked by pyrrolidinedithiocarbamate (an inhibitor of iNOS induction) but it did not suppress the hyperpermeability response. Furthermore, 2,2'-(hydroxynitrosohydrazino) bis-ethanamine (a NO donor) did not increase monolayer permeability. Therefore, IFN-gamma-induced hyperpermeability is not due to its induction of iNOS activity and resulting increases in NO levels.

Role of nitric oxide and cyclic guanosine 3',5'-monophosphate in the estrogen regulation of cervical epithelial permeability

Treatment of cultured human cervical epithelia on filters with 17beta-estradiol increases paracellular permeability in a time- and dose-related manner (EC50, 1.1 nM). The objective of the present study was to understand the molecular mechanisms of estrogen action. In cultured human cervical epithelial cells the nitric oxide (NO) donors sodium nitroprusside (SNP) and N-[ethoxycarbonyl]-3-[4-morpholinyl]sydnoneimine (SIN-I) and the cell-permeable cGMP analog 8-bromo-cGMP (8-Br-cGMP) increased paracellular permeability. In estrogen-treated cells SNP and 8-Br-cGMP increased permeability to a lesser degree than in estrogen-deprived cells, suggesting that NO and cGMP mediate the effect of estrogen on permeability. Tamoxifen blocked the estrogen-induced increase in permeability, but it had no effect on increases in permeability that were induced by SNP or by 8-Br-cGMP. LY-83583 (blocker of guanylate cyclase) attenuated the effect of SNP, whereas KT-5823 (blocker of cGMP-dependent protein kinase) abrogated the effects of both SNP and 8-Br-cGMP. Treatment with 17beta-estradiol increased NO release and cellular cGMP in a dose-related manner (EC50, approximately 1 nM), and the effects were inhibited by tamoxifen. Treatment with SNP increased cGMP maximally, even in estrogen-deficient cells. LY-83583 blocked the estrogen-induced increase in cGMP, but neither LY-83583 nor KT-5823 had a significant effect on the estrogen-induced increases in NO release and cellular cGMP. The NO synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester decreased NO release, and pretreatment of cells with L-arginine reversed the effect. Cultured human cervical epithelial cells express messenger RNA for the NOS isoforms endothelial NOS (ecNOS), brain NOS, and inducible NOS. 17beta-Estradiol up-regulated ecNOS messenger RNA, and tamoxifen blocked the effect. Based on these results we suggest that the effect of estradiol on permeability involves four signaling steps: 1) activation of estrogen receptors, 2) increase in ecNOS transcription and up-regulation of NO activity, 3) NO activation of guanylate cyclase and increase in cGMP, and 4) cGMP activation of cGMP-dependent protein kinase.

NO increases permeability of cultured human cervical epithelia by cGMP-mediated increase in G-actin

Human cervical epithelial cells express mRNA for the nitric oxide (NO) synthase (NOS) isoforms ecNOS, bNOS, and iNOS and release NO into the extracellular medium. N(G)-nitro-L-arginine methyl ester (L-NAME), an NOS inhibitor, and Hb, an NO scavenger, decreased paracellular permeability; in contrast, the NO donors sodium nitroprusside (SNP) and N-(ethoxycarbonyl)-3-(4-morpholinyl)sydnonimine increased paracellular permeability across cultured human cervical epithelia on filters, suggesting that NO increases cervical paracellular permeability. The objective of the study was to understand the mechanisms of NO action on cervical paracellular permeability. 8-Bromo-cGMP (8-BrcGMP) also increased permeability, and the effect was blocked by KT-5823 (a blocker of cGMP-dependent protein kinase), but not by LY-83583 (a blocker of guanylate cyclase). In contrast, LY-83583 and KT-5823 blocked the SNP-induced increase in permeability. Treatment with SNP increased cellular cGMP, and the effect was blocked by Hb and LY-83583, but not by KT-5823. Neither SNP nor 8-BrcGMP had modulated cervical cation selectivity. In contrast, both agents increased fluorescence from fura 2-loaded cells in the Ca(2+)-insensitive wavelengths, indicating that SNP and 8-BrcGMP stimulate a decrease in cell size and in the resistance of the lateral intercellular space. Neither SNP nor 8-BrcGMP had an effect on total cellular actin, but both agents increased the fraction of G-actin. Hb blocked the SNP-induced increase in G-actin, and KT-5823 blocked the 8-BrcGMP-induced increase in G-actin. On the basis of these results, it is suggested that NO acts on guanylate cyclase and stimulates an increase in cGMP; cGMP, acting via cGMP-dependent protein kinase, shifts actin steady-state toward G-actin; this fragments the cytoskeleton and renders cells more sensitive to decreases in cell size and resistance of the lateral intercellular space and, hence, to increases in permeability. These results may be important for understanding NO regulation of transcervical paracellular permeability and secretion of cervical mucus in the woman.

Macromolecular transport across the rabbit proximal and distal colon

BACKGROUND

Although many studies have investigated macromolecular uptake in the stomach and small intestine, little is known about macromolecular uptake in the colon.

AIMS

To investigate the mechanisms involved in the transport of large antigenically intact macromolecules across the proximal and distal colonic epithelium in the rabbit.

METHODS

The mucosal to serosal movement of bovine serum albumin (BSA) was examined in modified Ussing chambers under short circuited conditions. The mucosal surface was exposed to varying concentrations of BSA, and after a 50 minute equilibration period, the mucosal to serosal flux of immunologically intact BSA was determined by ELISA. Total BSA flux was determined by the transport of radiolabelled 125I-BSA.

RESULTS

Intact BSA transport in proximal and distal colonic tissue showed saturable kinetics. Intact BSA transport in the proximal and distal segment was 7% and 2% of the total 125I-BSA flux respectively. Immunologically intact BSA transport in the distal segment was significantly less than that in the proximal segment. Intact BSA transport in the proximal colon was significantly reduced following treatment with sodium fluoride, colchicine, and tetrodotoxin. Cholinergic blockade had no effect on the uptake of intact BSA.

CONCLUSION

The findings indicate that the transport of intact macromolecules across the proximal and distal large intestine is a saturable process. Further, intact BSA transport in the proximal colon is an energy dependent process that utilises microtubules and is regulated by the enteric nervous system.