Mediators of nicotine-induced relaxations of the rat gastric fundus

Nitric oxide mediates the inhibitory effect of ethanol on the motility of isolated longitudinal muscle of proximal colon in rats

The aim of the present study was to investigate the effect of ethanol on colon motility in rats and to test the possibility that nitric oxide (NO) mediates this effect. Proximal colon longitudinal muscle strips (LM) (8 x 3 mm) cut parallel to the longitudinal muscle fibres of the colon were isolated and mounted in an organ bath. Ethanol (0.57, 0.87 and 1.30 mmol L(-1)) dose-dependently inhibited the motility of LM. Longitudinal muscle strips from female rats were more sensitive to the inhibitory effect of ethanol than that from male rats. L-NAME (N-nitro-L-arginine methyl ester) (100 micromol L(-1)), AG (aminoguanidine) (10 micromol L(-1)), ODQ (1H-[1,2,4]Oxadiazolo[4,3-a]quinoxalin-1-one) (10 micromol L(-1)) and PTIO (2-Phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide) (200 micromol L(-1)) partly blocked the inhibitory effect of ethanol on LM. Pretreatment with L-NAME, AG, ODQ and PTIO abolished the sex difference of the inhibitory effect of ethanol on LM. Tetrodotoxin (TTX) (10 micromol L(-1)) partly blocked the inhibitory effect but did not influence the sex difference. The relaxation of LM induced by SNP (sodium nitroprusside) (0.1-10 micromol L(-1)) in female rats was greater than that in male rats. In conclusion, ethanol inhibited the colon motility in vitro. This inhibitory effect on LM was mediated by NO through the iNOS - NO - cGMP pathway.

Gastrointestinal Function Regulation by Nitrergic Efferent Nerves

Gastrointestinal (GI) smooth muscle responses to stimulation of the nonadrenergic noncholinergic inhibitory nerves have been suggested to be mediated by polypeptides, ATP, or another unidentified neurotransmitter. The discovery of nitric-oxide (NO) synthase inhibitors greatly contributed to our understanding of mechanisms involved in these responses, leading to the novel hypothesis that NO, an inorganic, gaseous molecule, acts as an inhibitory neurotransmitter. The nerves whose transmitter function depends on the NO release are called "nitrergic", and such nerves are recognized to play major roles in the control of smooth muscle tone and motility and of fluid secretion in the GI tract. Endothelium-derived relaxing factor, discovered by Furchgott and Zawadzki, has been identified to be NO that is biosynthesized from l-arginine by the constitutive NO synthase in endothelial cells and neurons. NO as a mediator or transmitter activates soluble guanylyl cyclase and produces cyclic GMP in smooth muscle cells, resulting in relaxation of the vasculature. On the other hand, NO-induced GI smooth muscle relaxation is mediated, not only by cyclic GMP directly or indirectly via hyperpolarization, but also by cyclic GMP-independent mechanisms. Numerous cotransmitters and cross talk of autonomic efferent nerves make the neural control of GI functions complicated. However, the findingsrelated to the nitrergic innervation may provide us a new way of understanding GI tract physiology and pathophysiology and might result in the development of new therapies of GI diseases. This review article covers the discovery of nitrergic nerves, their functional roles, and pathological implications in the GI tract.

Investigation of the mechanism of nicotine-induced relaxation on the sheep sphincter of Oddi

Possible mechanisms for nicotine-induced relaxation were investigated in the isolated sheep's sphincter of Oddi. Sheep's sphincter of Oddi rings were mounted in tissue bath with modified Krebs-Henseleit solution and aerated with 95% oxygen and 5% carbon dioxide. Tension was measured with isometric force transducers, and muscle relaxation was expressed as percent decrease of precontraction induced by carbachol. Nicotine (1 x 10(-5) to 3 x 10(-3) mol/L) produced concentration-dependent relaxation on sphincter of Oddi precontracted by carbachol (10(-6) mol/L). Nicotine-induced relaxation was 72.8 +/- 4.2% of precontraction with carbachol (10(-6) mol/L) (mean pD2 value, 3.76 +/- 0.05 mol/L). Nicotine-induced relaxation was not affected by N(w)-nitro L-arginine methyl ester (L-NAME) (3 x 10(-5) mol/L), methylene blue (10(-5) mol/L), indomethacin (10(-5) mol/L), hexamethonium (10(-5) mol/L), glibenclamide (10(-5) mol/L), 4-aminopyridine (10(-3) mol/L), tetraethylammonium (3 x 10(-4) mol/L), clotrimazole (10(-6) mol/L), 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB) (10(-6) mol/L), and anthracene-9-carboxylate (9-AC) (10(-6) mol/L), but potentiated by bupivacain (10(-5) mol/L). A calcium-antagonizing effect of nicotine was not observed. The results suggest that nicotine-induced relaxation of the sheep's sphincter of Oddi is not mediated by the release of prostaglandins, nitric oxide (NO), or a related substance; by the activation of potassium channels or chloride channels; or by the stimulation of nicotinic cholinoceptors. Potentiation of the nicotine-induced relaxation by bupivacain indicates that blockade of sodium channels may play a role in this relaxation.

Nitric oxide mediates a therapeutic effect of nicotine in ulcerative colitis

BACKGROUND

Ulcerative colitis is a condition of nonsmokers in which nicotine is of therapeutic benefit.

AIMS

To examine the in vitro effect of nicotine on colonic smooth muscle activity and the role of nitric oxide (NO) as a mediator.

METHODS

Nicotine, 1-10 microM, was administered to strips of circular muscle from the distal sigmoid colon of 9 patients with active ulcerative colitis and 18 with colorectal cancer. The effect of electrical field stimulation (EFS) was examined before nicotine was added. Finally L-NAME, a NO synthetase inhibitor, was added before nicotine was administered again.

RESULTS

Muscle strips developed similar spontaneous resting tone. In response to EFS, ulcerative colitis tissue developed lower tensions than the controls. Nicotine significantly reduced the resting tone and peak tension after EFS, with a greater effect in controls. With L-NAME, peak tensions were increased more in ulcerative colitis than controls, and nicotine produced a much smaller reduction.

CONCLUSIONS

Nicotine reduces circular muscle activity, predominantly through the release of nitric oxide-this appears to be 'up-regulated' in active ulcerative colitis. These findings may explain some of the therapeutic benefit from nicotine (and smoking) in ulcerative colitis and may account for the colonic motor dysfunction in active disease.

Subtilisin increases macromolecular efflux from the oral mucosa

The purpose of this study was to determine whether subtilisin, a potent serine proteinase derived from Bacillus species contaminating smokeless tobacco, increases macromolecular efflux from the oral mucosa and, if so, whether local elaboration of bradykinin mediates this response. Using intravital microscopy, I found that suffusion of subtilisin elicits significant, concentration-dependent leaky site formation and an increase in the clearance of fluorescein isothiocyanate-labeled dextran (molecular mass, 70 kDa) from the in situ hamster cheek pouch (P<0.05). Heat-inactivated subtilisin had no significant effects on macromolecular efflux. Subtilisin-induced responses were significantly attenuated by Hoe 140 and NPC 17647, two structurally distinct selective bradykinin B(2) receptor antagonists, but not by des-Arg(9)-[Leu(8)]bradykinin, a selective bradykinin B(1) receptor antagonist, or CP-96,345, a selective neurokinin-1 receptor antagonist. Aprotinin, but not leupeptin, significantly attenuated subtilisin-induced increase in macromolecular efflux. Indomethacin had no significant effects on subtilisin-induced responses. Collectively, these data indicate that subtilisin increases the macromolecular efflux from the in situ hamster cheek pouch in a catalytic-site-dependent fashion through local elaboration of bradykinin. This response does not involve the stimulation of local afferent nerves or the production of prostaglandins.

Effects of nicotine, dimethylphenylpiperazinium and tetramethylammonium on smooth muscles from feline and human gastric corpus

Both excitatory and inhibitory intrinsic neurons could be found within the gastric wall, both of them receiving innervation from vagal fibres and being sensitive to nicotine. The effects of three nicotine receptor agonists, nicotine, tetramethylammonium (TMA) and 1,1-dimethyl-4-phenylpiperazinium (DMPP), on contractile activity of preparations isolated from feline and human gastric corpus wall were investigated. While DMPP (3.5x10(-8) to 5.9x10(-4)m) did not affect either spontaneous contractions or basal tension of isolated preparations from both species, TMA produced concentration-dependent tonic contractions of both circular and longitudinal isolated preparations from human (3.66x10(-5) to 5.10x10(-3)m) and feline (6. 1x10(-7) to 2.1x10(-3)m) stomach. On the other hand, nicotine (4. 1x10(-8) to 7.0x10(-4)m) produced concentration-dependent relaxation of only circular isolated preparations from feline gastric corpus. The effect of nicotine was sensitive to mecamylamine, and not to pancuronium, while the effect of TMA was sensitive to both mecamylamine and pancuronium. Although in our experiments DMPP had no effect, its excitatory action on gastric intrinsic neurons through the hexamethonium-insensitive pathway had already been described. The results of our study suggest that two different types of ganglion nicotine receptor exist together within the wall of feline stomach: (1) type N(N1)which is involved in relaxation and is sensitive only to nicotine and mecamylamine, and not to DMPP, TMA and pancuronium; (2) and type N(N2)which is involved in contraction of gastric muscle and sensitive to DMPP, TMA, mecamylamine and pancuronium, and not to nicotine.

Nitric oxide modulates release of noradrenaline in guinea-pig gastric fundus

The interaction between nitric oxide (NO) and the release of [(3)H]noradrenaline ([(3)H]NA) in conditions of non-activated and activated nicotinic receptors in guinea-pig gastric fundus preincubated with [(3)H]NA was studied. Nicotinic receptor agonist, 1,1-dimethyl-4-phenyl-piperazinium iodide (DMPP) (100 microM) significantly increased the resting release of [(3)H]NA. NO-synthase inhibitor, N(omega)-nitro-L-arginine (L-NNA) (100 microM) significantly decreased DMPP-induced release of [(3)H]NA. Field electrical stimulation (FES) (2Hz; 1 ms; 360 st) significantly increased the release of [(3)H]NA above the basal levels. L-NNA significantly decreased the stimulation-evoked release of [(3)H]NA. DMPP increased the stimulation-evoked release of [(3)H]NA, effect which was significantly decreased by L-NNA. The data suggests that endogenous NO increases the release of [(3)H]NA, evoked either by activation of the nicotinic receptors or by electrical stimulation in guinea-pig gastric fundus.

Smokeless tobacco potentiates VIP-induced DNA synthesis and inactivates NEP 24.11 in oral keratinocytes

The purpose of this study was to determine whether exposure of cultured chemically transformed hamster oral keratinocytes (HCPC-1) to an aqueous extract of smokeless tobacco (STE) potentiates DNA synthesis elicited by vasoactive intestinal peptide (VIP), an autocrine neuropeptide, and, if so, whether this response is associated with inactivation of neutral endopeptidase 24.11 (NEP 24. 11), an ectoenzyme that cleaves and inactivates VIP very effectively, in these cells. I found that STE and VIP each elicited a modest, albeit significant, increase in DNA synthesis in cultured HCPC-1 cells (P < 0.05). However, incubation of HCPC-1 cells with STE together with VIP evoked a significant, concentration- dependent increase in DNA synthesis that was mediated by VIP receptors. The effects of STE and VIP were synergistic. Maximal response was observed after a 48-h incubation. STE significantly attenuated NEP 24.11 activity in HCPC-1 cells at a time when VIP-induced DNA synthesis was maximal. Collectively, these data indicate that STE potentiates VIP-induced DNA synthesis in cultured oral keratinocytes, and that this response is temporally related to STE-induced inactivation of NEP 24.11 in these cells. I suggest that NEP 24.11 modulates the mitogenic effects of smokeless tobacco in the oral epithelium, in part, by inactivating VIP.

Nicotinic acetylcholine receptor blocking effect of guanethidine in the rat gastric fundus

1 Guanethidine is commonly used as a drug to investigate adrenergic neurotransmission and, in combination with atropine, to realize non-adrenergic non-cholinergic (NANC) conditions. Previous studies suggested a nicotinic acetylcholine receptor blocking effect of guanethidine. Therefore, we investigated the effect of increasing concentrations of guanethidine (0.1-100 microM) on nicotine-induced relaxations of longitudinal muscle strips of rat gastric fundus. 2 In the presence of 1 microM atropine and 3 microM guanethidine, nicotine (30 microM) induces a fast and sustained relaxation which is partly inhibited by the nitric oxide synthase inhibitors Nomega-nitro-L-arginine (L-NOARG) and Nomega-nitro-L-arginine methyl ester (L-NAME) (both 30 and 100 microM). One microM tetrodotoxin (TTX) completely blocks this nicotine-induced relaxation. 3 High concentrations of guanethidine (> or =10 microM), but not adrenoceptor blockade by the alpha-adrenoceptor antagonist phentolamine in combination with the beta-adrenoceptor antagonist nadolol (both 3 microM), inhibit the nicotine-induced relaxation. 4 Guanethidine (0.1-100 microM) has no effect on relaxations induced by electrical field stimulation (EFS; 1-8 Hz), nitric oxide (NO; 0.01-1 microM), vasoactive intestinal polypeptide (VIP; 0.1-10 nM) or isoprenaline (1-10 nM). 5 We conclude that high concentrations of guanethidine (> or =10 microM) block nicotine-induced NANC relaxations of longitudinal muscle strips of the rat gastric fundus most likely at the level of the nicotinic acetylcholine receptor.

Nitric oxide (NO) increases acetylcholine release from and inhibits smooth muscle contraction of guinea-pig gastric fundus

Experiments were carried out to investigate the interaction between nitric oxide (NO) and cholinergic neurotransmission in smooth muscle strips of guinea-pig gastric fundus. Electrical field stimulation (2 Hz, 1 ms, 360 shocks) evoked atropine-sensitive contractions. Dimethylphenylpiperazinium (DMPP) (100 microM), a nicotinic receptor agonist, reversed the stimulation-evoked contraction and resulted in relaxation. Nomega-nitro-L-arginine (L-NNA) (100 microM), an NO synthase inhibitor, significantly increased the amplitude of stimulation-evoked contraction and abolished the effect of DMPP. Electrical stimulation increased the release of [3H]acetylcholine ([3H]ACh) from the tissue strips above the basal levels. Neither L-NNA (100 microM) nor DMPP (100 microM) alone influenced the basal release of [3H]ACh. Nomega-nitro-L-arginine (100 microM) decreased the electrical stimulation-evoked release of [3H]ACh. Dimethylphenylpiperazinium increased the stimulation-evoked release of [3H]ACh but had no effect in the presence of L-NNA. It is suggested that in guinea-pig gastric fundus, endogenous NO released in response to field stimulation has an opposite effect at the pre- and postsynaptic sites: it increases the release of ACh from cholinergic nerve terminals but reduces smooth muscle responses to ACh.

Parallel pathways mediate inhibitory effects of vasoactive intestinal polypeptide and nitric oxide in canine fundus

1. The gastric adaptation reflex is activated by the release of non-adrenergic, non-cholinergic (NANC) inhibitory transmitters, including nitric oxide (NO) and vasoactive intestinal polypeptide (VIP). The role of NO in this reflex is not disputed, but some investigators suggest that NO synthesis is stimulated by VIP in post-junctional cells or in nerve terminals. We investigated whether the effects of these transmitters are mediated by independent pathways in the canine gastric fundus. 2. VIP and NO produced concentration-dependent relaxation of the canine fundus. Nomega-nitro-L-arginine (L-NNA) reduced relaxation induced by electrical field stimulation (EFS; 0.5-8 Hz), but had no effect on responses to exogenous VIP and sodium nitroprusside (SNP, 10 microM). 3. Oxyhaemoglobin reduced relaxations produced by EFS and SNP. Oxyhaemoglobin also reduced relaxation responses to low concentrations of VIP (<10 nM), but these effects were non-specific and mimicked by methaemoglobin which had no effect on nitrergic responses. 4. A blocker of guanylyl cyclase, 1H-[1,2,4]oxidiazolo [4,3,-a]quinoxalin-1-one, (ODQ) inhibited responses to EFS, SNP and DETA/NONOate (an NO.donor), but had no effect on responses to VIP. cis-N-(2-phenylcyclopentil)-azacyclotridec-1en-2-amine monohydrochloride (MDL 12,330A), a blocker of adenylyl cyclase, reduced responses to EFS, VIP and forskolin, but did not affect responses to SNP. 5. Levels of cyclic GMP were enhanced by the NO donor S-nitroso-n-acetylpenicillamine (SNAP) but were unaffected by VIP (1 microM). The increase in cyclic GMP in response to SNAP was blocked by ODQ. 6. The results suggest that at least two transmitters, possibly NO and VIP, mediate relaxation responses in the canine fundus. NO and VIP mediate responses via cyclic GMP- and cyclic AMP-dependent mechanisms, respectively. No evidence was found for a serial cascade in which VIP is coupled to NO-dependent responses.

Nitrergic and purinergic mechanisms and their interactions for relaxation of the rat internal anal sphincter

1. The NANC neuronal mechanisms for relaxations of the rat internal anal sphincter in response to electrical field stimulation (EFS) were studied in isolated preparations in the presence of atropine (1 microM), propranolol (3 microM) and phentolamine (3 microM). 2. EFS-induced relaxations were abolished by tetrodotoxin (1 microM) and reduced to 64% of control by the guanylate cyclase inhibitor ODQ (1 microM), but were not significantly reduced by the nitric oxide synthase inhibitor L-NAME (100 microM) or oxyhaemoglobin (10 microM). However, in the presence of tubocurarine (10 microM) or apamin (0.1 microM), L-NAME or oxyhaemoglobin greatly reduced or abolished EFS-induced relaxations. 3. The EFS-induced relaxations were mimicked by NO (10-100 microM) and by ATP (3-10 mM). The relaxations elicited by these agents were not affected by tetrodotoxin, L-NAME, tubocurarine or apamin. However, ATP-induced relaxations were reduced by the combination of L-NAME with tubocurarine or apamin. 4. Nicotine (10-100 microM) produced concentration-dependent relaxations that were abolished by tubocurarine (10 microM) or hexamethonium (200 microM). After desensitisation to nicotine (100 microM) and in its continued presence, the addition of L-NAME (100 microM) resulted in almost complete abolition of EFS-induced relaxations. 5. It is suggested that tubocurarine, hexamethonium and desensitisation to nicotine have an apamin-like action in the rat internal anal sphincter, the main effect being blockade of a purinergic component of the relaxant transmission process. 6. The findings suggest that both nitrergic and purinergic transmissions are involved in EFS-induced NANC relaxations of the rat internal anal sphincter, and there appears to be a complex interaction between these two pathways of transmission.

Non-adrenergic non-cholinergic relaxation of the rat stomach

1. Receptive and adaptive relaxations of the proximal third of the stomach are reflex responses that enable the stomach to accommodate large volumes with minimal increases in intraluminal pressure. The smooth muscle relaxations are termed non-adrenergic non-cholinergic (NANC). 2. Nitric oxide (NO) and vasoactive intestinal polypeptide (VIP) are considered to be the principal neurotransmitters of NANC relaxation of the rat stomach. NO appears to be mainly responsible for the speed of the relaxation and VIP appears to be responsible for the duration. 3. Studies indicate that inhibitory neurons may also release other neurotransmitters, such as adenosine triphosphate (ATP) and peptide histidine isoleucine (PHI). 4. NANC relaxation of the rat stomach is a complex phenomenon that appears to involve many neurotransmitters, each with a specific role.

Influence of a selective guanylate cyclase inhibitor, and of the contraction level, on nitrergic relaxations in the gastric fundus

1. The influence of the soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ) on non-adrenergic non-cholinergic (NANC) relaxations and the possible role of a nerve-derived hyperpolarizing factor in NANC relaxation were investigated in the rat gastric fundus. 2. ODQ (10(-6) and 10(-5) M) concentration-dependently inhibited the short-lasting relaxations by NO (2 x 10(-6) M-10(-4) M) administered as a bolus without influencing the relaxation by 3 x 10(-8) M isoprenaline. The relaxation by an infusion of NO was reduced to the same extent by 10(-6) and 10(-5) M ODQ. 3. The electrically induced short-lasting and sustained relaxations (40 V, 1 ms, 0.5-16 Hz, 10 s trains at 2 min interval or cumulative increase in the frequency every 2 min) in NANC conditions were inhibited to a similar extent by 10(-6) and 10(-5) M ODQ, and by the NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME; 3 x 10(-4) M). 4. ODQ (10(-6) M) and L-NAME (3 x 10(-4) M), administered after 5, 10 or 20 min of long-term stimulation, reversed the relaxation to a similar extent (approximately 50% at 2 Hz and 20% at 8 Hz). 5. When the tissues were contracted to 40% of maximum by adapting the concentration of prostaglandin F2alpha (PGF2alpha), the inhibitory effect of 3 x 10(-4) M L-NAME on relaxations induced by train and cumulative stimulation was the same as when tissues were contracted with 3 x 10(-7) M PGF2alpha. 6. The findings of this study illustrate that the relaxation by exogenous and endogenous NO in the rat gastric fundus is due to activation of soluble guanylate cyclase. During long-term electrical stimulation, the partial contribution of NO to NANC relaxation is maintained but it is small at higher frequencies of stimulation. Evidence for the contribution of a nerve-derived hyperpolarizing factor to NANC relaxation was not obtained.

Modulation of electrically evoked responses in rat duodenum by activation of nicotinic cholinoceptors

1. The effect of activation of nicotinic cholinoceptors in rat duodenal segments following electrical field stimulation (EFS) was investigated. 2. Electrical field stimulation elicited a two-component response: transient relaxation followed by contraction. The EFS-evoked response was tetrodotoxin (TTX; 1 mumol/L) sensitive. The relaxation component was NG-nitro-L-arginine (L-NNA; 100 mumol/L) sensitive, while the contractile response was atropine (1 mumol/L) sensitive. 3. 1,1-Dimethyl-4-phenyl-piperazinium iodide (DMPP; 20 mumol/L) induced relaxation of spontaneously active preparations that was L-NNA sensitive. L-Arginine (1 mmol/L) reversed the effects of L-NNA on DMPP-induced relaxation. 4. When EFS was applied, DMPP increased the amplitude of the relaxation component of the response and reduced the contractile component. 5. In the presence of L-NNA, the effect of DMPP on the relaxation component of the response to EFS was reduced, but the contractile response was not affected. L-Arginine partly reduced this effect of L-NNA. 6. Neither propranolol (1 mumol/L) nor yohimbine (1 mumol/L) had any effect on the actions of DMPP on EFS-evoked responses, but prazosin (1 mumol/L) strongly reduced the effect of DMPP on the contractile component of the response to EFS and slightly reduced the effect of DMPP on the relaxation response. 7. Histochemical studies demonstrated that, in the myenteric plexus of the rat duodenum, there are many reduced nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d)-positive neurons and that their number decreased after treatment with L-NNA. In the presence of L-arginine and L-NNA, the number of NADPH-d-positive neurons was similar to that found in control samples. 8. The data suggest that activation of nicotinic cholinoceptors modulates EFS-evoked responses in the rat duodenum as a result of the potentiation of nitrergic and adrenergic neurotransmission.

Involvement of vasoactive intestinal polypeptide in nicotine-induced relaxation of the rat gastric fundus

1. Nicotine-induced relaxation and release of vasoactive intestinal polypeptide (VIP)- and peptide histidine isoleucine (PHI)-like immunoreactivity (LI) were measured in longitudinal muscle strips from the rat gastric fundus. 2. Under non-cholinergic conditions (0.3 microM atropine), nicotine (3-300 microM) produced concentration-dependent relaxations of the 5-hydroxytryptamine (3 microM)-precontracted strips. Under non-adrenergic non-cholinergic (NANC) conditions (0.3 microM atropine + 1 microM phentolamine + 1 microM nadolol), relaxations induced by sub-maximal nicotine concentrations (10 and 30 microM) were significantly smaller, while that produced by the highest concentration used (300 microM) was similar to that seen under non-cholinergic conditions. 3. Re-exposure to the same nicotine concentration 1 h later induced smaller relaxations, indicating desensitization. The reductions seen in the second responses were proportional to the concentration used. 4. Under non-cholinergic conditions, the relaxant response to 30 microM nicotine was abolished by hexamethonium (100 microM) and significantly reduced by tetrodotoxin (TTX, 3 microM). The TTX-resistant component was not observed under NANC conditions. 5. NANC relaxation induced by 30 microM nicotine was significantly reduced by a specific anti-VIP serum (approximately 35% less than that seen with normal rabbit serum). 6. Nicotine (30-300 microM) caused significant, concentration-dependent increases in the outflow of VIP- and PHI-LI from the strips; these effects were also diminished with re-exposure. The increases in both types of immunoreactivity evoked by nicotine (300 microM) were abolished by hexamethonium (300 microM), TTX (3 microM) and a calcium-free medium. 7. These findings indicate that VIP and possibly PHI are involved in NANC relaxation of the rat gastric fundus induced by nicotine.

Evaluation of an electrochemical microprobe for direct NO measurement in the rat gastric fundus

Nitric oxide (NO) is an inhibitory nonadrenergic, noncholinergic (NANC) neurotransmitter in the rat gastric fundus and is released upon electrical or pharmacological stimulation of the inhibitory NANC neurons. In this study, it was attempted to measure the release of NO from the rat gastric fundus upon electrical stimulation or administration of nicotine directly via an electrochemical probe (ISO-NO). The system was evaluated by adding exogenous NO. Addition of exogenous NO induced concentration-dependent relaxation of the rat gastric fundus and an increase in the ISO-NO probe baseline current. The concentration of NO detected by the ISO-NO probe was lower than the concentration of NO administered. When no tissue was present, higher concentrations of NO were detected than in the presence of a tissue. In the absence of 95% O2/5% CO2 the concentration of NO detected was highest. Electrical stimulation induced relaxations of the rat gastric fundus which were reduced by NG-nitro-L-arginine methylester (L-NAME). An increase in the ISO-NO probe baseline current was also observed, but this was duc to nonspecific effects as the response also occurred without a tissue present and was not sensitive to L-NAME. Nicotine induced relaxations, which were reduced by L-NAME, but the ISO-NO probe baseline current remained unaltered, even in the presence of L-arginine plus superoxide dismutase. It can be concluded that it is not possible to detect directly the NO release from the rat gastric fundus upon electrical or pharmacological stimulation of the NANC neurons with the ISO-NO probe.

Non-adrenergic non-cholinergic neuron stimulation in the cat lower esophageal sphincter

Both electrical field stimulation and nicotine produced non-adrenergic non-cholinergic (NANC) relaxation of the circular muscle strips from the cat lower esophageal sphincter in the presence of 5 microM guanethidine and 5 microM scopolamine. Low-frequency stimulation (2 Hz, 0.2 ms duration, supramaximal current intensity, 20-s train) provoked a transient relaxation, while at high-frequency stimulation (20 Hz) a slow restoration to the resting tone was observed. Blockade of nitric oxide (NO) synthesis by 1 mM N omega -nitro-L-arginine decreased by 20% the amplitude of the 20 Hz-induced relaxation and changed the pattern of relaxation, making it similar to the sustained relaxation evoked by exogenously applied vasoactive intestinal peptide (VIP). After chymotrypsin (4 U/ml), the pattern of the high-frequency-induced relaxation resembled that of the low-frequency-induced relaxation. Similarly, chymotrypsin changed the shape of nicotine-provoked relaxation, increasing the speed of restoration to the resting tone. We suggest that the fast relaxation elicited in cat lower esophageal sphincter by electrical field stimulation or nicotine is initiated by NO. The slow restoration to the resting tone in the case of high-frequency- or nicotine-induced relaxation seems to be due to the release of VIP or VIP-like peptides. The possibility of participation of another transmitter(s) involved in NANC relaxation should not be excluded.

Detection of nitric oxide release from canine enteric neurons

Previous pharmacological and immunohistochemical studies have suggested that nitric oxide (NO) is a mediator of enteric neuromuscular transmission in the canine proximal colon. The present study demonstrates the release of nitric oxide (NO) and/or its oxidation products (collectively termed NOx) following stimulation of intrinsic neurons with the nicotinic agonist 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP). Strips of muscularis externa, including the ganglionated myenteric and submucosal plexuses were suspended under tension in modified Krebs solution. DMPP (1-100 microM) produced concentration-dependent inhibition of circular muscle contractile activity and this effect was antagonized by L-nitroarginine methyl ester or L-nitroarginine (100 microM). Tetrodotoxin (TTX, 1 microM) significantly reduced mechanical responses to DMPP (10 microM) but had no effect on responses to high concentrations of DMPP (100 microM). Aliquots of the bathing medium were assayed for NOx after regeneration of NO from NO2- and/or NO3-. NO was measured as chemiluminescence produced by reaction with ozone. Detection was linear over the range 6-25,000 pmol of added NO2- or NO3-. In the absence of drugs, a basal release of 1113.5 +/- 100.4 pmol NOx/g tissue (n = 27) was detected after a 30-min incubation period. DMPP (1-100 microM) stimulated a concentration-dependent increase in NOx to 5099.9 +/- 430 pmol/g per 30 min (n = 17) at 100 microM. NOx release was inhibited by an inhibitor of nitric oxide synthase activity (N(G)-monomethyl-L-arginine) or by reduction in extracellular Ca2+ concentration. DMPP-stimulated NOx accumulation was significantly reduced, but not abolished by TTX (1 microM). These results provide further evidence that NO is released following stimulation of intrinsic neurons in canine proximal colon and support previous suggestions that nicotinic agonists may directly stimulate terminals of NO-releasing neurons.

Role of nitric oxide in mediating non-adrenergic non-cholinergic relaxation of the cat ileocecal sphincter

The participation of nitric oxide (NO) in field stimulation- or nicotine-evoked non-adrenergic non-cholinergic (NANC) relaxation of cat ileocecal sphincter was studied in vitro. During a 30 microM noradrenaline-induced contraction, both the application of electrical field stimulation (2-20 Hz, 0.2 ms, supramaximal current intensity, 10 s duration) and (-)-nicotine (10-500 microM) produced a tetrodotoxin-sensitive relaxation. The maximal relaxation was observed at 10 Hz or 100 microM (-)-nicotine. In 12 out of 19 strips the pretreatment with N omega-nitro-L-arginine (100 microM) decreased the amplitude of the field stimulation-evoked relaxation, while in the remaining strips the relaxation was transformed into a contraction. By increasing the concentration of N omega-nitro-L-arginine up to 1 mM all strips responded to field stimulation with a frequency-dependent tetrodotoxin-resistant contraction. N omega-Nitro-L-arginine (100 microM) completely inhibited the nicotine-induced relaxation. L-Arginine (1 mM) restored the amplitude of both field stimulation- and nicotine-evoked relaxations. These data indicate that NO appears to be involved in both field stimulation- and nicotine-evoked NANC relaxations. Evidence has been obtained for the existence of tetrodotoxin-resistant NANC contraction in cat ileocecal sphincter.

Relationship between nitric oxide and vasoactive intestinal polypeptide in enteric inhibitory neurotransmission

Although considerable evidence suggests that NO serves as a neurotransmitter in gastrointestinal muscles, it is unlikely to be the only substance involved in enteric inhibitory neurotransmission. Vasoactive intestinal polypeptide (VIP) is known to be expressed by inhibitory motor neurons in the gut, and it appears to be co-localized with nitric oxide synthase (NOS) in a subpopulation of enteric neurons. These data suggest that NO and VIP may be parallel neurotransmitters. Others have suggested that VIP is the primary inhibitory transmitter, and it stimulates production of NO in smooth muscle cells. In this "serial cascade" model NO is a paracrine substance. We performed experiments on circular muscles and cells from the canine proximal colon to further test the idea that NO and VIP are parallel neurotransmitters and to determine the validity of the serial cascade model in these muscles. We found that NO-independent inhibitory effects were unmasked when excitatory and NO-dependent inhibitory responses were blocked. NO-independent inhibitory effects were reduced by alpha-chymotrypsin and blocked by tetrodotoxin. NOS- and VIP-like immunoreactivities were co-localized in enteric neurons and varicose fibers in the circular muscle layer. Similar to several other reports we found no evidence for a constitutive NOS in smooth muscle cells. Several aspects of the serial cascade model were not supported by our results: (i) the electrical and mechanical effects of VIP did not depend upon NO synthesis; (ii) VIP-induced changes in [Ca2+]i did not depend upon NO synthesis; and (iii) VIP did not cause the release of NO from canine colonic muscles. These results are consistent with the hypothesis that NO and VIP are co-transmitters, released in parallel from enteric inhibitory nerves.