Hydroxocobalamin and haemoglobin differentiate between exogenous and neuronal nitric oxide in the rat gastric fundus

Biphasic modulation of NOS expression, protein and nitrite products by hydroxocobalamin underlies its protective effect in endotoxemic shock: downstream regulation of COX-2, IL-1β, TNF-α, IL-6, and HMGB1 expression

Background. NOS/^•NO inhibitors are potential therapeutics for sepsis, yet they increase clinical mortality. However, there has been no in vivo investigation of the (in vitro) ^•NO scavenger, cobalamin's (Cbl) endogenous effects on NOS/^•NO/inflammatory mediators during the immune response to sepsis. Methods. We used quantitative polymerase chain reaction (qPCR), ELISA, Western blot, and NOS Griess assays, in a C57BL/6 mouse, acute endotoxaemia model. Results. During the immune response, pro-inflammatory phase, parenteral hydroxocobalamin (HOCbl) treatment partially inhibits hepatic, but not lung, iNOS mRNA and promotes lung eNOS mRNA, but attenuates the LPS hepatic rise in eNOS mRNA, whilst paradoxically promoting high iNOS/eNOS protein translation, but relatively moderate ^•NO production. HOCbl/NOS/^•NO regulation is reciprocally associated with lower 4 h expression of TNF-α, IL-1β, COX-2, and lower circulating TNF-α, but not IL-6. In resolution, 24 h after LPS, HOCbl completely abrogates a major late mediator of sepsis mortality, high mobility group box 1 (HMGB1) mRNA, inhibits iNOS mRNA, and attenuates LPS-induced hepatic inhibition of eNOS mRNA, whilst showing increased, but still moderate, NOS activity, relative to LPS only. experiments (LPS+D-Galactosamine) HOCbl afforded significant, dose-dependent protection in mice Conclusions. HOCbl produces a complex, time- and organ-dependent, selective regulation of NOS/^•NO during endotoxaemia, corollary regulation of downstream inflammatory mediators, and increased survival. This merits clinical evaluation.

Inhibition of nitric oxide synthase by cobalamins and cobinamides

Cobalamins are important cofactors for methionine synthase and methylmalonyl-CoA mutase. Certain corrins also bind nitric oxide (NO), quenching its bioactivity. To determine if corrins would inhibit NO synthase (NOS), we measured their effects on -L-[(14)C]arginine-to-L-[(14)C]citrulline conversion by NOS1, NOS2, and NOS3. Hydroxocobalamin (OH-Cbl), cobinamide, and dicyanocobinamide (CN(2)-Cbi) potently inhibited all isoforms, whereas cyanocobalamin, methylcobalamin, and adenosylcobalamin had much less effect. OH-Cbl and CN(2)-Cbi prevented binding of the oxygen analog carbon monoxide (CO) to the reduced NOS1 and NOS2 heme active site. CN(2)-Cbi did not react directly with NO or CO. Spectral perturbation analysis showed that CN(2)-Cbi interacted directly with the purified NOS1 oxygenase domain. NOS inhibition by corrins was rapid and not reversed by dialysis with L-arginine or tetrahydrobiopterin. Molecular modeling indicated that corrins could access the unusually large heme- and substrate-binding pocket of NOS. Best fits were obtained in the "base-off" conformation of the lower axial dimethylbenzimidazole ligand. CN(2)-Cbi inhibited interferon-gamma-activated Raw264.7 mouse macrophage NO production. We show for the first time that certain corrins directly inhibit NOS, suggesting that these agents (or their derivatives) may have pharmacological utility. Endogenous cobalamins and cobinamides might play important roles in regulating NOS activity under normal and pathological conditions.

Role of superoxide dismutase enzymes and ascorbate in protection of nitrergic relaxation against superoxide anions in mouse duodenum

AIM

The aim of this study was to investigate whether superoxide dismutase (SOD) enzymes and ascorbate play a role in the protection of the nitrergic relaxation against superoxide anion inhibition in the mouse duodenum.

METHODS

The effects of exogenous SOD, N,No-bis(salicylidene) ethylenediamine chloride (EUK-8; a synthetic cell-permeable mimetic of the manganese SOD [Mn-SOD] and ascorbate on relaxant responses induced by nitrergic nerve stimulation), exogenous nitric oxide (NO), and nitroglycerin were investigated in isolated mouse duodenum tissues.

RESULTS

Diethyldithiocarbamate (DETCA) inhibited the relaxation to exogenous NO and nitroglycerin, but not relaxation to electrical field stimulation (EFS). SOD and ascorbate partially prevented the inhibitory effect of DETCA on relaxation to NO, abut not to nitroglycerin. The DETCA-induced inhibition on nitroglycerin was prevented by EUK-8. Hemoglobin, 2- (4-carboxyphenyl)-4,4,5,5-tetramethylimidazolinel-oxyl-3-oxide, and hydroxocobalamin inhibited the relaxation to NO, but not to EFS and nitroglycerin in the presence of DETCA. Pyrogallol and hydroquinone inhibited the relaxation to NO, but not to EFS and nitroglycerin. This inhibition was prevented by exogenous SOD and ascorbate, but was not prevented by EUK-8. Pyrogallol and hydroquinone did not inhibit the EFS-induced relaxation in the presence of DETCA. Duroquinone and 6-anilino-5.8-quinolinedione inhibited the relaxation to EFS, NO, and nitroglycerin, and this inhibition was prevented by EUK-8.

CONCLUSION

These results suggest that the nitrergic neurotransmission in the mouse duodenum is protected by endogenous tissue antioxidants against superoxide anions, and Mn SOD, in addition to copper/zinc SOD, can protect NO from attack from superoxide anion generators intracellularly. Also, the possibility that the endogenous neurotransmitter may not be the free NO but a NO-containing or NO-generating molecule in the mouse duodenum remains open.

Reactivity of aquacobalamin and reduced cobalamin toward S-nitrosoglutathione and S-nitroso-N-acetylpenicillamine

The reactions of aquacobalamin (Cbl(III)H2O, vitamin B12a) and reduced cobalamin (Cbl(II), vitamin B12r) with the nitrosothiols S-nitrosoglutathione (GSNO) and S-nitroso-N-acetylpenicillamine (SNAP) were studied in aqueous solution at pH 7.4. UV-vis and NMR spectroscopic studies and semiquantitative kinetic investigations indicated complex reactivity patterns for the studied reactions. The detailed reaction routes depend on the oxidation state of the cobalt center in cobalamin, as well as on the structure of the nitrosothiol. Reactions of aquacobalamin with GSNO and SNAP involve initial formation of Cbl(III)-RSNO adducts followed by nitrosothiol decomposition via heterolytic S-NO bond cleavage. Formation of Cbl(III)(NO-) as the main cobalamin product indicates that the latter step leads to efficient transfer of the NO- group to the Co(III) center with concomitant oxidation of the nitrosothiol. Considerably faster reactions with Cbl(II) proceed through initial Cbl(II)-RSNO intermediates, which undergo subsequent electron-transfer processes leading to oxidation of the cobalt center and reduction of the nitrosothiol. In the case of GSNO, the overall reaction is fast (k approximately 1.2 x 10(6) M(-1) s(-1)) and leads to formation of glutathionylcobalamin (Cbl(III)SG) and nitrosylcobalamin (Cbl(III)(NO-)) as the final cobalamin products. A mechanism involving the reversible equilibrium Cbl(II) + RSNO <==> Cbl(III)SR + NO is suggested for the reaction on the basis of the obtained kinetic and mechanistic information. The corresponding reaction with SNAP is considerably slower and occurs in two distinct reaction steps, which result in the formation of Cbl(III)(NO-) as the ultimate cobalamin product. The significantly different kinetic and mechanistic features observed for the reaction of GSNO and SNAP illustrate the important influence of the nitrosothiol structure on its reactivity toward metal centers of biomolecules. The potential biological implications of the results are briefly discussed.

Characterization of the mechanisms of action and nitric oxide species involved in the relaxation induced by the ruthenium complex

Nitric oxide (NO) plays an important role in the control of vascular tone. NO donors have therapeutic use and the most used NO donors, nitroglycerin and sodium nitroprusside have problems in their use. Thus, new NO donors have been synthesized to minimize these undesirable effects. Nytrosil ruthenium complexes have been studied as a new class of NO donors. trans-[RuCl([15]aneN(4))NO](2+), induces vasorelaxation only in presence of reducing agent. In this study, we characterized the mechanisms of vasorelaxation of trans-[RuCl([15]aneN(4))NO](2+) in denuded rat aorta and identified which NO forms are involved in this relaxation. We also evaluated the effect of this NO donor in decreasing the cytosolic Ca(2+) concentration ([Ca(2+)]c) of the vascular smooth muscle cells. Vasorelaxation to trans-[RuCl([15]aneN(4))NO](2+) (E(max): 101.8 +/- 2.3%, pEC(50): 5.03 +/- 0.15) was almost abolished in the presence of the NO* scavenger hydroxocobalamin (E(max): 4.0 +/- 0.4%; P < 0.001) and it was partially inhibited by the NO(-) scavenger L-cysteine (E(max): 79.9 +/- 6.9%, pEC(50): 4.41 +/- 0.06; P < 0.05). The guanylyl cyclase inhibitor ODQ reduced the E(max) (57.7 +/- 4.0%, P < 0.001) and pEC(50) (4.21 +/- 0.42, P < 0.01) and the combination of ODQ and TEA abolished the response to trans-[RuCl([15]aneN(4))NO](2+). The blockade of voltage-dependent (K(v)), ATP-sensitive (K(ATP)), and Ca(2+)-activated (K(Ca) K(+) channels reduced the vasorelaxation induced by trans-[RuCl([15]aneN(4))NO](2+). This compound significantly reduced [Ca(2+)]c (from 100% to 85.9 +/- 3.5%, n = 4). In conclusion, our data demonstrate that this NO donor induces vascular relaxation involving NO* and NO(-) species, that is associated to a decrease in [Ca(2+)]c. The mechanisms of vasorelaxation involve guanylyl cyclase activation, cGMP production and K(+) channels activation.

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.

Mechanistic studies on the interaction of reduced cobalamin (vitamin B12r) with nitroprusside

The electron-transfer reaction between reduced cobalamin (Cbl(II)) and sodium pentacyanonitrosylferrate(II) (sodium nitroprusside, NP), as well as the subsequent processes following the electron-transfer step, were investigated by spectroscopic (UV-vis, (1)H NMR, EPR), electrochemical (CV, DPV) and kinetic (stopped-flow) techniques. In an effort to clarify the complex reaction pattern observed at physiological pH, systematic spectroscopic and kinetic studies were undertaken as a function of pH (1.8-9) and NP concentration (0.0001 - 0.09 M). The kinetics of the electron-transfer reaction was studied under pseudo-first-order conditions with respect to NP. The reaction occurs in two parallel paths of different order, viz. pseudo-first and pseudo-second order with respect to the NP concentration, respectively. The contribution of each path depends on pH and the [NP]/[Cbl(II)] ratio. At low pH and total NP concentration (pH < 3, [NP]/[Cbl(II)] approximately 1), the cyano-bridged successor complex [Cbl(III)-(mu-NC)-Fe(I)(CN)(3)(NO(+))](-) (1(s)()) is the final reaction product formed in an inner-sphere electron transfer reaction that is coupled to the release of cyanide from coordinated nitroprusside. At higher pH, subsequent reactions were observed which involve the attack of cyanide released in the electron transfer step on the initially formed cyano-bridged species, and lead to the formation of Cbl(III)CN and [Fe(I)(CN)(4)(NO(+))](2)(-). The strong dependence of the rate and mechanism of the subsequent reactions on pH is attributed to the large variation in the effective nucleophilicity of the cyanide ligand in the studied pH range. An alternative electron-transfer pathway observed in the presence of excess NP involves the reaction of the precursor complex [Cbl(II)-(mu-NC)-Fe(II)(CN)(4)(NO(+))](2)(-) (1(p)()) with NP to give [Cbl(III)-(mu-NC)-Fe(II)(CN)(4)(NO(+))](-) (2) and reduced nitroprusside, [Fe(CN)(5)NO](3)(-), as the initial reaction products. Analysis of the kinetic data allowed elucidation of the rate constants for the inner- and outer-sphere electron-transfer pathways. The main factors which influence the kinetics and thermodynamics of the observed electron-transfer steps are discussed on the basis of the spectroscopic, kinetic and electrochemical results. A general picture of the reaction pathways that occur on a short (s) and long (min to h) time scale as a function of pH and relative reactant concentrations is derived from the experimental data. In addition, the release of NO resulting from the one-electron reduction of NP by Cbl(II) was monitored with the use of a sensitive NO electrode. The results obtained in the present study are discussed in reference to the possible influence of cobalamin on the pharmacological action of nitroprusside.

Influence of the epithelium on acetylcholine release from parasympathetic nerves of the rat trachea

1. The present study was undertaken to investigate the influence of the airway epithelium on the release of acetylcholine (ACh) from parasympathetic nerves of the rat trachea. Epithelium-intact and epithelium-denuded preparations of rat trachea were incubated with [3H]-choline to incorporate [3H]-ACh into the cholinergic transmitter stores. Release of radiolabelled transmitter ACh was evoked by electrical field stimulation (60 s trains of 1 ms pulses, 5 Hz, 15 V). 2. Field stimulation both of epithelium-intact and epithelium-denuded radiolabelled tracheal preparations evoked an increase in the efflux of radioactivity; however, the mean stimulation-induced (S-I) efflux from epithelium-denuded preparations (2932 +/- 190 d.p.m., n = 9) was approximately 60% of that from epithelium-intact preparations (4802 +/- 820 d.p.m., n = 11). We have shown previously that, in epithelium-intact (but not epithelium-denuded) tracheal preparations, a substantial proportion of the S-I efflux is resistant to tetrodotoxin (1 microM) and to the removal of extracellular Ca2+, indicating that much of the S-I efflux is not caused by exocytotic release of neuronal [3H]-ACh. In epithelium-denuded tracheal preparations, superfused individually, phosphorylcholine (1 and 100 microM) did not alter S-I efflux. In epithelium-intact tracheal preparations, both in the absence and in the presence of atropine (1 microM), neither N(G)-nitro-L-arginine (100 microM), superoxide dismutase (100 units ml(-1)), indomethacin (10 microM), capsaicin (30 microM) nor alpha-chymotrypsin (1 unit ml(-1)) altered S-I efflux. 3. Experiments were also performed using two tracheal preparations superfused in series. When unlabelled epithelium-intact preparations were present in the upper chamber (superfused first), the S-I efflux from radiolabelled epithelium-denuded preparations in the lower chamber (superfused second) did not differ significantly from radiolabelled epithelium-denuded preparations superfused individually. Moreover, there was no significant difference in the S-I efflux from radiolabelled epithelium-denuded preparations in the lower chamber between experiments in which the upper chamber contained epithelium-intact or epithelium-denuded preparations. 4. Field stimulation of epithelium-intact tracheal preparations in the upper chamber with 90, 120 and 300-s periods (trains of 1 ms pulses, 5 Hz, 15 V) did not significantly alter the S-I efflux from radiolabelled epithelium-denuded tracheal preparations in the lower chamber. 5. When introduced into the upper (unlabelled epithelium-intact) and subsequently allowed to superfuse the lower (radiolabelled epithelium-denuded) tracheal preparations, the stable cholinomimetic carbachol (3 microM) markedly reduced the S-I efflux whereas ACh (0.1 and 1 microM) had no significant effect. However, in the presence of the anti-cholinesterase neostigmine (1 microM), ACh (1 microM) significantly reduced S-I efflux, indicating that ACh is subject to rapid hydrolysis by cholinesterase enzymes. When atropine (10 microM) was only exposed to radiolabelled epithelium-denuded preparations in the lower chamber, the inhibitory effects of ACh (1 microM) and carbachol (3 microM) on S-I efflux were prevented. 6. In conclusion, the findings of the present study do not support the notion that the airway epithelium exerts an inhibitory influence on ACh release from parasympathetic nerves of the rat trachea. Alternatively, if epithelium-dependent modulation of cholinergic transmission does occur in the rat trachea, then the mechanism does not appear to involve phosphorylcholine, nitric oxide, superoxide radicals, cyclo-oxygenase products of arachadonic acid, capsaicin-sensitive neuropeptides or vasoactive intestinal peptide. Moreover, the inhibitory effect of carbachol and ACh on transmitter ACh release in the rat trachea appears to be due solely to activation of prejunctional inhibitory muscarinic cholinoceptors on parasympathetic nerves and does not involve the liberation of a putative epithelium-derived inhibitory factor.

Evidence for nonvesicular nitric oxide release evoked by nerve activation

The gaseous nature of nitric oxide (NO) has led to the general assumption that its release from neurons during nerve stimulation is independent of vesicular storage. However, recent findings have shown that NO can exist intracellularly as part of more stable bioactive molecules, suggesting that the role of vesicular exocytosis for NO release cannot be excluded simply based on the chemical nature of NO itself. We have used botulinum toxin B (BTX B) to directly address the role of vesicular exocytosis for NO release. BTX B cleaves the synaptic vesicle protein synaptobrevin/VAMP, and by this inhibits Ca++-mediated exocytic release of neurotransmitters. As a target organ we used the guinea-pig enteric nervous system, which innervates the gastrointestinal tract, and in which both classical neurotransmitters as well as NO are released and influence smooth muscle activity. As expected, BTX B (0.1 microM) blocked the nerve stimulation-induced cholinergic and tachykininergic smooth muscle contractions, and markedly inhibited the nerve stimulation-evoked release of [3H]-choline. In contrast, BTX B (0.1 microM) had no effect on nerve stimulation-evoked relaxations, which were equally inhibited by an NO-synthase inhibitor as well as by a selective inhibitor of soluble guanylyl cyclase. In addition, nerve stimulation-evoked NO synthase-dependent outflow of NO/NO2- was unaffected by BTX B (0.1 microM). These findings suggest that the neuronal release of endogenous NO is independent of intact synaptobrevin/VAMP, and therefore provide further evidence that nerve-mediated release of further NO is nonvesicular.

Influence of bilirubin and other antioxidants on nitrergic relaxation in the pig gastric fundus

1. The influence of several antioxidants (bilirubin, urate, ascorbate, alpha-tocopherol, glutathione (GSH), Cu/Zn superoxide dismutase (SOD) and the manganese SOD mimic EUK-8) on nitrergic relaxations induced by either exogenous nitric oxide (NO; 10(-5) M) or electrical field stimulation (4 Hz; 10 s and 3 min) was studied in the pig gastric fundus. 2. Ascorbate (5x10(-4) M), alpha-tocopherol (4x10(-4) M), SOD (300 - 1000 u ml(-1)) and EUK-8 (3x10(-4) M) did not influence the relaxations to exogenous NO. In the presence of GSH (5x10(-4) M), the short-lasting relaxation to NO became biphasic, potentiated and prolonged. Urate (4x10(-4) M) and bilirubin (2x10(-4) M) also potentiated the relaxant effect of NO. None of the antioxidants influenced the electrically evoked relaxations. 3. 6-Anilino-5,8-quinolinedione (LY83583; 10(-5) M) had no influence on nitrergic nerve stimulation but nearly abolished the relaxant response to exogenous NO. Urate and GSH completely prevented this inhibitory effect, while it was partially reversed by SOD and bilirubin. Ascorbate, alpha-tocopherol and EUK-8 were without effect. 4. Hydroquinone (10(-4) M) did not affect the electrically induced nitrergic relaxations, but markedly reduced NO-induced relaxations. The inhibition of exogenous NO by hydroquinone was completely prevented by urate and GSH. SOD and ascorbate afforded partial protection, while bilirubin, EUK-8 and alpha-tocopherol were ineffective. 5. Hydroxocobalamin (10(-4) M) inhibited relaxations to NO by 50%, but not the electrically induced responses. Full protection versus this inhibitory effect was obtained with urate, GSH and alpha-tocopherol. 6. These results strengthen the hypothesis that several endogenous antioxidant defense mechanisms, enzymatic as well as non-enzymatic, might play a role in the nitrergic neurotransmission process.

Effects of superoxide anion generators and thiol modulators on nitrergic transmission and relaxation to exogenous nitric oxide in the sheep urethra

The effects of superoxide anion generators, the nitric oxide (NO) scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoine-1-oxyl 3-oxide (carboxy-PTIO), the specific guanylate cyclase inhibitor 1H-[1,2,4]-oxadiazole-[4,3-a]-quinoxalin-1-one (ODQ), and thiol modulating agents were investigated on relaxations induced by nitrergic stimulation and exogenous NO addition in the sheep urethra. Methylene blue (MB, 10 microM), pyrogallol (0.1 mM) and xanthine (X, 0.1 mM)/xanthine oxidase (XO, 0.1 u ml(-1)) inhibited NO-mediated relaxations, without affecting those induced by nitrergic stimulation. This resistance was not diminished following inhibition of endogenous Cu/Zn superoxide dismutase (Cu/Zn SOD) with diethyldithiocarbamic acid (DETCA, 3 mM), which almost abolished tissue SOD activity. Carboxy-PTIO (0.1 - 0.5 mM) inhibited NO-mediated relaxations but had no effect on responses to nitrergic stimulation, which were not changed by treatment with ascorbate oxidase (2 u ml(-1)). Relaxations to NO were reduced, but not abolished, by ODQ (10 microM), while nitrergic responses were completely blocked. The thiol modulators, ethacrynic acid (0.1 mM), diamide (1.5 mM), or 5,5'-dithio-bis (2-nitrobenzoic acid) (DTNB, 0. 5 mM), and subsequent treatment with dithiothreitol (DTT, 2 mM) had no effect on responses to nitrergic stimulation or NO. In contrast, N-ethylmaleimide (NEM, 0.2 mM) markedly inhibited both relaxations. L-cysteine (L-cys, 0.1 mM) had no effect on responses to NO, while it inhibited those to nitrergic stimulation, in a Cu/Zn SOD-independent manner. Our results do not support the view that the urethral nitrergic transmitter is free NO, and the possibility that another compound is acting as mediator still remains open. British Journal of Pharmacology (2000) 129, 53 - 62

Effects of hydroxocobalamin and carboxy-PTIO on nitrergic transmission in porcine anococcygeus and retractor penis muscles

The effects of carboxy-PTIO and hydroxocobalamin were studied on nitrergic transmission in anococcygeus and retractor penis muscles taken during post mortem examination from young male pigs. In both muscles under resting conditions, electrical field stimulation (EFS) caused contractions that were sensitive to tetrodotoxin (1 microM) and were greatly inhibited by prazosin (1 microM) and guanethidine (10-30 microM), but were not significantly affected by atropine (1 microM). In the anococcygeus muscle, but not in the retractor penis muscle, guanethidine produced a prolonged contraction. After tone was raised by guanethidine in the anococcygeus or by phenylephrine (1 microM) in the presence of guanethidine in the retractor penis, EFS caused tetrodotoxin-sensitive relaxations. The EFS-induced relaxations were abolished by the NO synthase inhibitor N(G)-L-nitro-arginine methyl ester (L-NAME; 100 microM) and its effect was partly overcome by L-arginine (1 mM), indicating it was mediated by nitrergic nerves. Carboxy-PTIO (0.1-1 mM) had no significant effect in reducing stimulation-induced nitrergic relaxations in either muscle. However, hydroxocobalamin (0.1-1 mM) caused concentration-dependent reductions of nitrergic relaxations in both muscles. Relaxations to exogenous nitric oxide (1 microM) in both muscles were abolished by carboxy-PTIO (0.3 mM) and hydroxocobalamin (0.1 mM). There were no differences in reactivity to carboxy-PTIO or hydroxocobalamin between anococcygeus and retractor penis muscles from the same species (pig). The finding also confirms earlier observations that the nitrergic transmitter is generally resistant to the NO-scavenger carboxy-PTIO.

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.

Physiological features of visceral smooth muscle cells, with special reference to receptors and ion channels

Visceral smooth muscle cells (VSMC) play an essential role, through changes in their contraction-relaxation cycle, in the maintenance of homeostasis in biological systems. The features of these cells differ markedly by tissue and by species; moreover, there are often regional differences within a given tissue. The biophysical features used to investigate ion channels in VSMC have progressed from the original extracellular recording methods (large electrode, single or double sucrose gap methods), to the intracellular (microelectrode) recording method, and then to methods for recording from membrane fractions (patch-clamp, including cell-attached patch-clamp, methods). Remarkable advances are now being made thanks to the application of these more modern biophysical procedures and to the development of techniques in molecular biology. Even so, we still have much to learn about the physiological features of these channels and about their contribution to the activity of both cell and tissue. In this review, we take a detailed look at ion channels in VSMC and at receptor-operated ion channels in particular; we look at their interaction with the contraction-relaxation cycle in individual VSMC and especially at the way in which their activity is related to Ca2+ movements and Ca2+ homeostasis in the cell. In sections II and III, we discuss research findings mainly derived from the use of the microelectrode, although we also introduce work done using the patch-clamp procedure. These sections cover work on the electrical activity of VSMC membranes (sect. II) and on neuromuscular transmission (sect. III). In sections IV and V, we discuss work done, using the patch-clamp procedure, on individual ion channels (Na+, Ca2+, K+, and Cl-; sect. IV) and on various types of receptor-operated ion channels (with or without coupled GTP-binding proteins and voltage dependent and independent; sect. V). In sect. VI, we look at work done on the role of Ca2+ in VSMC using the patch-clamp procedure, biochemical procedures, measurements of Ca2+ transients, and Ca2+ sensitivity of contractile proteins of VSMC. We discuss the way in which Ca2+ mobilization occurs after membrane activation (Ca2+ influx and efflux through the surface membrane, Ca2+ release from and uptake into the sarcoplasmic reticulum, and dynamic changes in Ca2+ within the cytosol). In this article, we make only limited reference to vascular smooth muscle research, since we reviewed the features of ion channels in vascular tissues only recently.

Effect of hydroxocobalamin on vasodilatations to nitrergic transmitter, nitric oxide and endothelium-derived relaxing factor in guinea-pig basilar artery

In endothelium-denuded guinea-pig isolated basilar artery preparations, hydroxocobalamin (30, 100 and 300 microM) concentration-dependently inhibited the vasodilator responses to exogenous nitric oxide (NO), whereas the vasodilator responses to nitrergic nerve stimulation were slightly reduced by high (100 and 300 microM) but not by the low (30 microM) concentration of hydroxocobalamin. Vasodilatation in response to sodium nitroprusside (10-100 nM) was totally abolished by 300 microM hydroxocobalamin. In endothelium-intact preparations, vasodilator responses to acetylcholine (0.3-3 microM) were significantly reduced or abolished by hydroxocobalamin (30-300 microM). The mean reduction by hydroxocobalamin of relaxations to acetylcholine was significantly greater than that of the equivalent response evoked by nitrergic nerve stimulation. The findings suggest that the nitrergic transmitter in the guinea-pig basilar artery may be quantitatively less susceptible than the endothelium-derived relaxing factor to the NO scavenger hydroxocobalamin.

Nitric oxide and the non-adrenergic non-cholinergic neurotransmission

In the early 1960s, the first evidence was reported demonstrating neurally mediated responses in the presence of adrenergic and cholinergic antagonists, leading to the introduction of the concept of non-adrenergic non-cholinergic neurotransmission. The inhibitory component of this part of the autonomic nervous system has been illustrated in numerous organ systems mediating a wide range of physiological events. Since the discovery of these nerves, several substances have been proposed as putative neurotransmitter, with ATP and vasoactive intestinal polypeptide as main candidates. Finally, the ongoing research on the nature of the substance released by these nerves has generated the nitrergic theory proposing nitric oxide as putative neurotransmitter. By now, increasing evidence is reported to support the idea that inhibitory neurons release more neurotransmitters, interacting with each other at pre- and/or postsynaptic levels.

The inactivation of methionine synthase in isolated rat hepatocytes by sodium nitroprusside

Methionine synthase, the enzyme that catalyses the transfer of a methyl group from 5-methyl tetrahydrofolate to homocysteine via the cofactor methylcobalamin, is one of the two established mammalian enzymes that utilise a biologically active vitamin B-12 derivative. Through its substrates, products and downstream metabolites, methionine synthase is directly involved in the sulphur amino acid pathways, polyamine biosynthesis, biological methylations and one-carbon-unit transfers. Rat liver methionine synthase was shown to be inactivated by the nitric oxide donor sodium nitroprusside. The inactivation occurred during the treatment of isolated rat hepatocytes in a time-dependent and dose-dependent manner with an apparent IC50 value of 170 microM. Highly purified rat liver methionine synthase was inactivated in a partially irreversible manner with an apparent IC50 value of 10 microM. The inactivation has been attributed to nitric oxide released by sodium nitroprusside. Since biomolecules possessing transition state metals are targets for nitric oxide, the possibility of a nitric oxide-cobalamin interaction could explain the observed inactivation. Nitric oxide is directly involved in different aspects of liver metabolic functions both under physiological and pathological conditions like sepsis and inflammation. The nitric-oxide-induced inactivation of methionine synthase could offer a rational explanation for the cellular and cytotoxic effects of this highly reactive molecule.

Effect of thiol modulators and Cu/Zn superoxide dismutase inhibition on nitrergic relaxations in the rat gastric fundus

1. The effects of superoxide anion generators before and after treatment with inhibitors of Cu/Zn superoxide dismutase (Cu/Zn SOD) and the effects of thiol-modulating agents were investigated on nitrergic relaxations to electrical stimulation of non-adrenergic non-cholinergic (NANC) nerves of the rat gastric fundus and on relaxations to authentic nitric oxide (NO) and nitroglycerin. 2. The superoxide anion generators, pyrogallol (30 microM) and duroquinone (30-60 microM), significantly inhibited the relaxations to NO (0.03-3 microM) but not nitrergic relaxations to NANC nerve stimulation (0.5-8 Hz) or those to ATP (10 microM). Treatment of the rat gastric fundus with the inhibitors of Cu/Zn SOD, diethyldithiocarbamate (DETC, 1 mM for 2 h) or triethylenetetramine (TETA, 100 microM for 2 h) had no effect on the relaxations to NANC nerve stimulation (1-8 Hz), NO (0.03-3 microM) or on those to ATP (10 microM). 3. After treatment of the rat gastric fundus with DETC (1 mM) but not after treatment with TETA (100 microM), pyrogallol (30 microM) and duroquinone (30-60 microM) significantly inhibited the nitrergic relaxations to electrical stimulation (0.5-8 Hz) and those to NO (0.03-3 microM). This inhibitory effect of pyrogallol and duroquinone was prevented by addition of exogenous SOD (250 units ml-1). Pyrogallol but not duroquinone also inhibited the NO-independent relaxations to ATP (10 microM). 4. The thiol modulators, buthionine sulphoximine (1 mM for 2 h) and ethacrynic acid (30 microM for 2 h), significantly inhibited the relaxations to nitroglycerin (0.03-3 microM) but had no effect on the nitrergic relaxations to electrical stimulation (0.5-8 Hz) or on those to NO (0.03-10 microM) and ATP (10 microM). The thiol modulators, sulphobromophthalein (100 microM for 2 h) and diamide (30-100 microM for 2 h) did not affect the relaxations to nitroglycerin, or those to NANC nerve stimulation and NO. 5. In summary, thiol modulators significantly inhibited the thiol-dependent relaxations to nitroglycerin but not those to NANC nerve stimulation or NO. Relaxations to nitrergic stimulation were decreased by superoxide anion generators only after inhibition of Cu/Zn SOD. These results suggest that the nitrergic NANC neurotransmitter in the rat gastric fundus is not a nitrosothiol but more likely free NO, which is protected from breakdown by tissue SOD.

Effect of Cu2+ on relaxations to the nitrergic neurotransmitter, NO and S-nitrosothiols in the rat gastric fundus

1. The effects of addition of Cu2+ and chelation of Cu2+ were studied on relaxations in response to S-nitrosothiols and on relaxations to non-adrenergic non-cholinergic (NANC) nerve stimulation, nitric oxide (NO) and glyceryl trinitrate (GTN) in the rat gastric fundus. 2. The S-nitrosothiols S-nitroso-L-cysteine (NOCys, 1-300 nM), S-nitrosoglutathione (GSNO, 0.01-3 microM) and S-nitroso-N-acetyl-D,L-penicillamine (SNAP, 0.01-3 microM) induced concentration-dependent relaxations of the rat gastric fundus muscle strip. The relaxant potencies of the S-nitrosothiols were NOCys > SNAP > GSNO. Relaxations to NOCys were transient and comparable to those to NANC nerve stimulation and NO whereas relaxations to GSNO and SNAP were sustained. The relaxations to NOCys, GSNO and SNAP were significantly and concentration-dependently enhanced by CuSO4 (3-30 microM). The order of relaxant potency in the presence of CuSO4 was reversed to GSNO approximately SNAP > NOCys. 3. In the presence but not in the absence of 0.1 microM GSNO, CuSO4 (1 microM) induced a rapid and transient relaxation which was inhibited by the superoxide radical generator, pyrogallol (30 microM). CuCl2 but not FeSO4 mimicked the effect of CuSO4. 4. Electrical stimulation (0.5-8 Hz) of the rat gastric fundus strips induced frequency-dependent relaxations which were previously shown to be nitrergic in nature and which were not affected by CuSO4 (3-30 microM). Relaxations to NO (3-100 nM) and GTN (0.01-1 microM) were not affected by 3 and 10 microM CuSO4 but were inhibited by 30 microM CuSO4. 5. The Cu2+ chelator, bathocuproine (3-30 microM) significantly and concentration-dependently inhibited the relaxations to NOCys (0.01-3 microM), GSNO (0.01-10 microM) and SNAP (0.01-3 microM). The inhibitory effect of 10 microM bathocuproine was reversed by 3 microM CuSO4. 6. Bathocuproine (3-30 microM) had no effect on the relaxations to NANC nerve stimulation (0.5-8 Hz) or on the concentration-response curve to NO (0.01-0.3 microM), whereas relaxations to GTN (0.01-1 microM) were significantly inhibited by 30 microM bathocuproine. 7. From these results we conclude that relaxations to S-nitrosothiols and to nitrergic stimulation of the rat gastric fundus are differentially affected by addition and chelation of Cu2+, suggesting that the nitrergic NANC neurotransmitter in the rat gastric fundus is not an S-nitrosothiol but is more likely to be free nitric oxide.

Influence of superoxide dismutase inhibition on the discrimination between NO and the nitrergic neurotransmitter in the rat gastric fundus

1. The influence of diethyldithiocarbamate (DETCA), that irreversibly inhibits Cu/Zn-containing superoxide dismutase, on the inability of 6-anilino-5,8-quinolinedione (LY83583), hypoxanthine/xanthine oxidase, hydroquinone and hydroxocobalamin to reduce electrically-induced NANC relaxations in the rat gastric fundus was investigated. 2. Longitudinal muscle strips of the rat gastric fundus were mounted for auxotonic recording in the presence of atropine and guanethidine and tone was raised by administration of prostaglandin F2 alpha DETCA (3 x 10(-3) M) slightly reduced the short-lasting relaxations induced by 10(-5) M exogenous nitric oxide (NO) and transmural electrical stimulation for 10 s at 4 Hz but this effect was not influenced by 1000 u ml-1 superoxide dismutase (SOD). 3. DETCA (3 x 10(-5) -3 x 10(-3) M) concentration-dependently potentiated the inhibitory effect of LY83583 upon the electrically-induced relaxations, although this was less pronounced than the inhibition of the NO-induced relaxations. The inhibition of the electrically-induced non-adrenergic non-cholinergic (NANC) relaxations was not reversed by SOD while that of the NO-induced relaxations was partially reversed. 4. The inhibitory effect of hypoxanthine/xanthine oxidase, hydroquinone and hydroxocobalamin on the electrically-induced NANC relaxations in the presence of DETCA (3 x 10(-3) M) was not different from the inhibitory effect of DETCA alone. 5. It was concluded that the differentiating effect of LY83583 between exogenous NO and the endogenous nitrergic neurotransmitter is partially related to protection of the endogenous nitrergic neurotransmitter by high levels of intracellular superoxide dismutase. This mechanism does not hold for hydroquinone and hydroxocobalamin, as they still discriminate between exogenous NO and the endogenous nitrergic neurotransmitter in the presence of DETCA. The possibility that the endogenous nitrergic neurotransmitter is not free NO in the rat gastric fundus therefore remains open.

Comparison of the effects of hydroxocobalamin and oxyhaemoglobin on responses to NO, EDRF and the nitrergic transmitter

1. The effects of ranges of concentrations of oxyhaemoglobin (0.01-30 microM) and hydroxocobalamin (1-100 microM) were compared for their abilities to reduce relaxant responses to EDRF released by acetylcholine in endothelium-intact rat aortic rings, the nitergic transmitter in rat anococcygeus muscles, and NO in aqueous solution in both tissues (aortic rings were denuded of endothelium). 2. The concentrations of oxyhaemoglobin producing 50% reduction of responses to EDRF and NO in rat aorta correspond closely, the IC50 values being 0.13 +/- 0.02 microM and 0.11 +/- 0.02 microM respectively. 3. Oxyhaemoglobin was equally effective in inhibiting responses to NO in anococcygeus muscles and in aortic rings with an IC50 of 0.14 +/- 0.05 microM. However, responses to the nitrergic transmitter were considerably less sensitive to inhibition by oxyhaemoglobin, the IC50 being 19.7 +/- 5.1 microM. 4. The IC50 values for hydroxocobalamin in inhibiting responses to EDRF and NO in aorta were 3.4 +/- 0.2 microM and 8.4 +/- 0.63 microM, respectively, but it was less effective against responses to NO in anococcygeus muscles the IC50 being 46 +/- 9.6 microM. However, even in the highest concentration used (100 microM), it did not reduce responses to the nitrergic transmitter. 5. The findings are compatible with the views that EDRF is NO, but suggest that the nitergic transmitter in the rat anococcygeus muscle does not behave like free NO.

Nitric oxide synthase activity and non-adrenergic non-cholinergic relaxation in the rat gastric fundus

1. In the presence of atropine (1 microM) and guanethidine (5 microM), electrical field stimulation (EFS, 120 mA, 1 ms, 0.5-16.0 Hz, trains of 2 min) induced frequency-dependent relaxations of 5-hydroxytryptamine (3 microM)-precontracted longitudinal muscle strips from the rat gastric fundus. 2. L-Citrulline concentrations were measured in the incubation medium of precontracted strips before and after EFS to investigate nitric-oxide (NO) synthase activity and its possible relation to non-adrenergic non-cholinergic (NANC) relaxation. 3. Basal NO synthase activity was reflected by the finding of prestimulation levels of L-citrulline of approximately 30 nM. These levels were unaffected by tetrodotoxin (3 microM) and NG-nitro-D-arginine methyl ester (D-NAME, 100 microM), slightly reduced by a calcium-free medium and halved by NG-nitro-L-arginine methyl ester (L-NAME, 100 microM). 4. EFS evoked significant, frequency-dependent increases in bath levels of L-citrulline at all frequencies tested. The increases evoked by 16-Hz EFS were abolished by tetrodotoxin (3 microM), a calcium-free medium and L-NAME (100 microM) but not by D-NAME (100 microM). 5. L-NAME (0.1 microM-1.0 mM) produced significant reduction of 4-Hz EFS-induced L-citrulline production (100% inhibition at 10 microM), but had less marked effects on basal production (approximately 50% reduction at 100 microM) and 4-Hz EFS-induced NANC relaxation (approximately 50% reduction at 1 mM). 6. L-Arginine (1 mM), but not D-arginine (1 mM), increased basal L-citrulline levels and reversed the inhibitory effect of L-NAME (10 microM). 7. These findings represent clear biochemical evidence of both basal and EFS-stimulated NO synthase activity in the rat gastric fundus.

Discrimination by the NO-trapping agent, carboxy-PTIO, between NO and the nitrergic transmitter but not between NO and EDRF

1. The effects of carboxy-PTIO, a scavenger of free radical nitric oxide (NO), were studied on endothelium-dependent relaxations of rat aorta and nitrergic nerve stimulation-induced relaxations of anococcygeus muscle and gastric fundus strips to test the hypothesis that endothelium-derived relaxing factor (EDRF) and the transmitter released by nitrergic nerves is free radical NO. 2. Carboxy-PTIO (10-300 microM) produced concentration-dependent reductions of relaxations elicited by exogenous NO, and relaxations mediated by EDRF released by acetylcholine and ATP in rings of rat aorta. The inhibitory effect of carboxy-PTIO was removed by washing the tissues. 3. In the rat anococcygeus muscle, carboxy-PTIO (10-300 microM) produced concentration-dependent reductions of relaxations to exogenous NO; however, in concentrations up to 2000 microM it did not reduce relaxations elicited by nitrergic nerve stimulation (1-2 Hz), in fact, concentrations of 300 microM or more slightly enhanced them. 4. In rat gastric fundus strips, carboxy-PTIO (100 and 300 microM) reduced relaxations to exogenous NO, but relaxations elicited by stimulation of the nitrergic component of non-adrenergic, non-cholinergic nerves were not affected. 5. These results suggest that EDRF is free radical NO and may be designated EDNO, but the transmitter released from nitrergic nerves does not appear to be identical to EDNO and may not be free radical NO.