Intestinal nitric oxide synthase activity changes during experimental colon obstruction

Nitric oxide bioavailability affects cardiovascular regulation dependent on cardiac nerve status

The sympathetic nervous system and nitric oxide (NO) contribute to regulation of vascular tone, blood flow regulation and cardiac function. Intrinsic cardiac neurons are tonically influenced by locally released NO and exogenous NO donors; however, the role of intact central neural connections remains controversial. We investigated the effects of S-nitroso-N-acetylpenicillamine (SNAP) administered into an intracoronary artery near the ventral interventricular ganglionated plexus (VIVGP) to evaluate distribution of myocardial blood flow (MBF) and ventricular function in normal and acute cardiac decentralized dogs. MBF was measured with microspheres during infusion of SNAP (100μM, IC) after systemic administration of 7-nitroindazole (nNOS blocker) followed by N(ω)-nitro-L-arginine methyl ester (LN; non-selective NOS blocker). Cardiac dynamics were not significantly affected by cardiac decentralization; several of these parameters (aortic systolic and diastolic pressures) were significantly increased after systemic administration of LN. Overall SNAP administered to the VIVGP increased blood flow in the anterior LV wall (vs. posterior LV wall) without affecting other cardiodynamic factors. In cardiac decentralized dogs subepicardial blood flow to the anterior LV wall during LN+SNAP was diminished resulting in a significantly higher inner:outer blood flow ratio (index of blood flow uniformity across the LV wall). LV function was not affected by acute cardiac decentralization; however, LV ejection fraction decreased markedly after LN (reduced NO bioavailability). These results validate earlier claims that reduced NO bioavailability imposes an upper limit on myocardial blood flow regulation and its transmural distribution. These effects are exacerbated after disconnection of intrinsic cardiac neurons from intact central neuron connections.

Epithelial inducible nitric oxide synthase causes bacterial translocation by impairment of enterocytic tight junctions via intracellular signals of Rho-associated kinase and protein kinase C zeta

OBJECTIVE

Gut barrier dysfunction and bacterial translocation occur in various disorders, including intestinal obstruction. Overexpression of inducible nitric oxide synthase is implicated in the pathogenesis of bacterial translocation, of which the molecular mechanism remains unclear. Epithelial permeability is regulated by tight junction reorganization and myosin light chain phosphorylation. Our aim was to investigate the roles of Rho-associated kinase and protein kinase C ζ in epithelial nitric oxide synthase-mediated barrier damage.

DESIGN

Animal study and cell cultures.

SETTING

Research laboratory.

SUBJECTS

BALB/c mice.

INTERVENTIONS

: Mouse distal small intestine was obstructed in vivo by a 10-cm loop ligation in which vehicle, L-Nil (a nitric oxide synthase inhibitor), or Y27632 (a Rho-associated kinase inhibitor) was luminally administered. After obstruction for 24 hrs, intestinal tissues were mounted on Ussing chambers for macromolecular flux. Liver and spleen tissues were assessed for bacterial counts. Caco-2 cells were exposed to 1 mM S-nitroso-N-acetylpenicillamine (a nitric oxide donor) for 24 hrs, and transepithelial resistance and permeability were evaluated.

MEASUREMENTS AND MAIN RESULTS

Mice with intestinal obstruction displayed epithelial barrier dysfunctions, such as permeability rise and bacterial translocation, associated with tight junction disruption and myosin light chain phosphorylation. Increased inducible nitric oxide synthase and phosphorylated protein kinase C ζ were observed in villus epithelium. Enteric instillation of L-Nil and Y27632 attenuated the functional and structural barrier damage caused by intestinal obstruction. L-Nil decreased intestinal obstruction-induced myosin light chain, myosin phosphatase target subunit 1, and protein kinase C ζ phosphorylation, suggesting that inducible nitric oxide synthase is upstream of Rho-associated kinase and protein kinase C ζ signaling. The intestinal phosphorylated myosin light chain level did not increase in inducible nitric oxide synthase(-/-) mice following intestinal obstruction. In vitro studies showed that S-nitroso-N-acetylpenicillamine-induced transepithelial resistance drop and permeability rise was independent of cell apoptosis. Y27632 inhibited S-nitroso-N-acetylpenicillamine-induced myosin light chain phosphorylation and permeability rise. S-nitroso-N-acetylpenicillamine also triggered phosphorylation and membrane translocation of protein kinase C ζ. Inhibitory protein kinase C ζ pseudosubstrate blocked S-nitroso-N-acetylpenicillamine-induced tight junction reorganization, but not myosin light chain phosphorylation.

CONCLUSIONS

Epithelial inducible nitric oxide synthase activates two distinct signals, protein kinase C ζ and Rho-associated kinase, to disrupt tight junctions leading to bacterial influx.

Kynurenines and intestinal neurotransmission: the role of N-methyl-D-aspartate receptors

Gastrointestinal neuroprotection involves the net effect of many mechanisms which protect the enteral nervous system and its cells from death, dysfunction or degeneration. Neuroprotection is also a therapeutic strategy, aimed at slowing or halting the progression of primary neuronal loss following acute or chronic diseases. The neuroprotective properties of a compound clearly have implications for an understanding of the mechanism of dysfunctions and for therapeutic approaches in a number of gastrointestinal diseases.This paper focused on the roles of glutamate and N-methyl-D-aspartate (NMDA) receptors in the intrinsic neuronal control of gastrointestinal motility; the consequences of inflammation on gastrointestinal motility changes; and the involvement of tryptophan metabolites (especially kynurenic acid) in the regulatory function of the enteral nervous system and the modulation of the inflammatory response. Common features in the mechanisms of action, illustrative evidence from animal models, and experimental neuroprotective therapies making use of the currently available possibilities are also discussed.Overall, the evidence suggests that gastrointestinal neuroprotection against inflammation and glutamate-induced neurotoxicity may be mediated synergistically through the blockade of NMDA receptors and the inhibition of neuronal nitric oxide synthase activity and xanthine oxidoreductase-dependent superoxide production. These components are likewise significant factors in the pathomechanism of gastrointestinal inflammatory diseases and inflammation-linked motility alterations. Inhibition of the enteric NMDA receptors by kynurenic acid or its analogues may provide a novel option via which to influence intestinal hypermotility and inflammatory processes simultaneously.

N-Methyl-D-aspartate receptor antagonism decreases motility and inflammatory activation in the early phase of acute experimental colitis in the rat

BACKGROUND

Inflammatory bowel diseases are accompanied by severe motility disorders. The aim of our study was to investigate whether the blockade of peripheral N-methyl-D-aspartate (NMDA)-sensitive glutamate receptors (NMDA-Rs) alters motility changes in chemically induced acute colitis and how this modulation is accomplished.

METHODS

The inflammatory and motility changes in 2,4,6-trinitrobenzenesulfonic acid (TNBS)-induced colitis were studied in anaesthetized Wistar rats following treatment with the natural NMDA-R antagonist kynurenic acid (KynA) or SZR-72, a blood-brain barrier-permeable synthetic KynA analogue. The macrohaemodynamics, serosal microcirculation (visualized by intravital videomicroscopy), plasma levels of tumour necrosis factor alpha (TNF-alpha), inflammatory enzyme activities (xanthine oxidoreductase (XOR), myeloperoxidase (MPO) and nitric oxide synthase (NOS)), and colonic motility (with a strain-gauge technique) were evaluated 17 h after colitis induction and compared with the control conditions.

KEY RESULTS

The TNBS enema induced a systemic hyperdynamic circulatory reaction, increased the serosal capillary blood flow, significantly elevated the mucosal XOR, MPO and NOS activities and augmented the colonic motility relative to the controls. The NMDA-R antagonist treatment with KynA or SZR-72 significantly reduced the XOR, NOS and MPO activities, decreased the motility and increased the tone of the colon.

CONCLUSIONS & INFERENCES

These data demonstrate a potential modulatory mechanism of NMDA-R in altered colonic motility in TNBS colitis. Inhibition of the enteric NMDA-Rs may provide a therapeutic option via which to influence intestinal hypermotility, microcirculatory changes and inflammatory activation simultaneously.

Kynurenic acid inhibits intestinal hypermotility and xanthine oxidase activity during experimental colon obstruction in dogs

Kynurenic acid (KynA), an endogenous antagonist of N-methyl-d-aspartate (NMDA) glutamate receptors, protects the central nervous system in excitotoxic neurological diseases. We hypothesized that the inhibition of enteric glutamate receptors by KynA may influence dysmotility in the gastrointestinal tract. Group 1 of healthy dogs served as the sham-operated control, in group 2, the animals were treated with KynA, while in groups 3 and 4 mechanical colon obstruction was maintained for 7 h. Group 4 was treated with KynA at the onset of ileus. Hemodynamics and motility changes were monitored, and the activities of xanthine oxidoreductase (XOR) and myeloperoxidase (MPO) were determined from tissue samples. Colon obstruction induced a hyperdynamic circulatory reaction, significantly elevated the motility index and increased the mucosal leucocyte accumulation and the XOR activity. The KynA treatment augmented the tone of the colon, permanently decreased the motility index of the giant colonic contractions and reduced the increases in XOR and MPO activities. These effects were concomitant with the in vitro inhibition of XOR activity. In conclusion, KynA antagonizes the obstruction-induced motility responses and XOR activation in the colon. Inhibition of enteric NMDA receptors may provide an option to influence intestinal hypermotility and inflammatory changes.