Effects of peptidases on non-adrenergic, non-cholinergic inhibitory responses of tracheal smooth muscle: a comparison with effects on VIP- and PHI-induced relaxation

Role of carbon monoxide in electrically induced non-adrenergic, non-cholinergic relaxations in the guinea-pig isolated whole trachea

BACKGROUND AND PURPOSE

Nitric oxide (NO) and vasoactive intestinal peptide (VIP) are considered transmitters of non-adrenergic, non-cholinergic (NANC) relaxations in guinea-pig trachea, whereas the role of carbon monoxide (CO) is unknown. This study was designed to assess the participation of CO, and to investigate the localization of haem oxygenase-2 (HO-2), the CO-producing enzyme, in tracheal neurons.

EXPERIMENTAL APPROACH

NANC responses to electrical field stimulation (EFS) at 3 and 10 Hz were evaluated in epithelium-free whole tracheal segments as intraluminal pressure changes. Drugs used were: L-nitroarginine methyl ester (L-NAME, 100 microM) to inhibit NO synthase (NOS), alpha-chymotrypsin (2 U ml(-1)) to inactivate VIP, zinc protoporphyrin-IX (ZnPP-IX, 10 microM) to inhibit HO-2, and 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 10 microM), a soluble guanylyl cyclase inhibitor. For immunohistochemistry, tissues were exposed to antibodies to PGP 9.5, a general neuronal marker, HO-2 and NOS, and processed with an indirect immunofluorescence method.

KEY RESULTS

alpha-Chymotrypsin did not affect NANC relaxations. ODQ inhibited NANC responses by about 60%, a value similar to that obtained by combining L-NAME and ZnPP-IX. The combination of ODQ, L-NAME and ZnPP-IX reduced the responses by 90%. Subpopulations of HO-2 positive neurons containing NOS were detected in tracheal sections.

CONCLUSIONS AND IMPLICATIONS

In the guinea-pig trachea, NANC inhibitory responses at 3 and 10 Hz use NO and CO as main transmitters. Their participation is revealed following inhibition of NOS, HO-2 and soluble guanylyl cyclase. The involvement of CO as a relaxing transmitter paves the way for novel therapeutic approaches in the treatment of airway obstruction.

Arginase strongly impairs neuronal nitric oxide-mediated airway smooth muscle relaxation in allergic asthma

BACKGROUND

Using guinea pig tracheal preparations, we have recently shown that endogenous arginase activity attenuates inhibitory nonadrenergic noncholinergic (iNANC) nerve-mediated airway smooth muscle relaxation by reducing nitric oxide (NO) production--due to competition with neuronal NO-synthase (nNOS) for the common substrate, L-arginine. Furthermore, in a guinea pig model of allergic asthma, airway arginase activity is markedly increased after the early asthmatic reaction (EAR), leading to deficiency of agonist-induced, epithelium-derived NO and subsequent airway hyperreactivity. In this study, we investigated whether increased arginase activity after the EAR affects iNANC nerve-derived NO production and airway smooth muscle relaxation.

METHODS

Electrical field stimulation (EFS; 150 mA, 4 ms, 4 s, 0.5-16 Hz)-induced relaxation was measured in tracheal open-ring preparations precontracted to 30% with histamine in the presence of 1 microM atropine and 3 microM indomethacin. The contribution of NO to EFS-induced relaxation was assessed by the nonselective NOS inhibitor Nomega-nitro-L-arginine (L-NNA, 100 microM), while the involvement of arginase activity in the regulation of EFS-induced NO production and relaxation was investigated by the effect of the specific arginase inhibitor Nomega-hydroxy-nor-L-arginine (nor-NOHA, 10 microM). Furthermore, the role of substrate availability to nNOS was measured in the presence of exogenous L-arginine (5.0 mM).

RESULTS

At 6 h after ovalbumin-challenge (after the EAR), EFS-induced relaxation (ranging from 3.2 +/- 1.1% at 0.5 Hz to 58.5 +/- 2.2% at 16 Hz) was significantly decreased compared to unchallenged controls (7.1 +/- 0.8% to 75.8 +/- 0.7%; P < 0.05 all). In contrast to unchallenged controls, the NOS inhibitor L-NNA did not affect EFS-induced relaxation after allergen challenge, indicating that NO deficiency underlies the impaired relaxation. Remarkably, the specific arginase inhibitor nor-NOHA normalized the impaired relaxation to unchallenged control (P < 0.05 all), which effect was inhibited by L-NNA (P < 0.01 all). Moreover, the effect of nor-NOHA was mimicked by exogenous L-arginine.

CONCLUSION

The results clearly demonstrate that increased arginase activity after the allergen-induced EAR contributes to a deficiency of iNANC nerve-derived NO and decreased airway smooth muscle relaxation, presumably via increased substrate competition with nNOS.

Effect of cold storage on cholinergic responses induced by electrical field stimulation in human bronchi

The aim of the present study was to examine the effects of cold storage on the responses induced by electrical field stimulation (EFS) in human bronchial preparations. Responses induced by EFS and acetylcholine were studied in human bronchial rings mounted in organ baths, either on the day of surgery or after storage at 4 degrees C in Krebs-Henseleit solution for 24 and 48 h, respectively. The responses induced by EFS were studied at different voltages (20, 40 and 60 V) and at a range of frequencies (2, 4, 8, 10, 30 and 60 Hz). EFS induced a triphasic response, consisting of a cholinergic contraction, followed by a relaxation and subsequently a slow sustained contraction. The amplitude of the EFS-induced response was enhanced with increasing voltages and increasing frequencies. None of the three EFS-induced phases were significantly altered by cold storage at 24h, whereas storage for 48 h significantly decreased the reactivity of the preparations. Likewise, the contractions induced by acetylcholine were unaltered after 24h, but significantly depressed after 48 h. These results suggest that the reactivity of human bronchial preparations to EFS is not altered when tissues are conserved for 24h, whereas prolonged storage should be avoided.

Arginase attenuates inhibitory nonadrenergic noncholinergic nerve-induced nitric oxide generation and airway smooth muscle relaxation

Background

Recent evidence suggests that endogenous arginase activity potentiates airway responsiveness to methacholine by attenuation of agonist-induced nitric oxide (NO) production, presumably by competition with epithelial constitutive NO synthase for the common substrate, L-arginine. Using guinea pig tracheal open-ring preparations, we now investigated the involvement of arginase in the modulation of neuronal nitric oxide synthase (nNOS)-mediated relaxation induced by inhibitory nonadrenergic noncholinergic (iNANC) nerve stimulation.

Methods

Electrical field stimulation (EFS; 150 mA, 4 ms, 4 s, 0.5 – 16 Hz)-induced relaxation was measured in tracheal preparations precontracted to 30% with histamine, in the presence of 1 μM atropine and 3 μM indomethacin. The contribution of NO to the EFS-induced relaxation was assessed by the nonselective NOS inhibitor L-NNA (0.1 mM), while the involvement of arginase activity in the regulation of EFS-induced NO production and relaxation was investigated by the effect of the specific arginase inhibitor nor-NOHA (10 μM). Furthermore, the role of substrate availability to nNOS in EFS-induced relaxation was measured in the presence of various concentrations of exogenous L-arginine.

Results

EFS induced a frequency-dependent relaxation, ranging from 6.6 ± 0.8% at 0.5 Hz to 74.6 ± 1.2% at 16 Hz, which was inhibited with the NOS inhibitor L-NNA by 78.0 ± 10.5% at 0.5 Hz to 26.7 ± 7.7% at 8 Hz (P < 0.01 all). In contrast, the arginase inhibitor nor-NOHA increased EFS-induced relaxation by 3.3 ± 1.2-fold at 0.5 Hz to 1.2 ± 0.1-fold at 4 Hz (P < 0.05 all), which was reversed by L-NNA to the level of control airways in the presence of L-NNA (P < 0.01 all). Similar to nor-NOHA, exogenous L-arginine increased EFS-induced airway relaxation (P < 0.05 all).

Conclusion

The results indicate that endogenous arginase activity attenuates iNANC nerve-mediated airway relaxation by inhibition of NO generation, presumably by limiting L-arginine availability to nNOS.

Nitric oxide in respiratory diseases

The formation and modulation of nitric oxide (NO) in the lungs is reviewed. Its beneficial and deleterious roles in airways diseases, including asthma, chronic obstructive pulmonary disease, and cystic fibrosis, and in animal models is discussed. The pharmacological effects of agents that modulate NO production or act as NO donors are described. The clinical pharmacology of these agents is described and the therapeutic potential for their use in airways disease is considered.

Vasoactive intestinal polypeptide as mediator of asthma

Vasoactive intestinal polypeptide (VIP) is one of the most abundant, biologically active peptides found in the human lung. VIP is a likely neurotransmitter or neuromodulator of the inhibitory non-adrenergic non-cholinergic airway nervous system and influences many aspects of pulmonary biology. In human airways VIP-immunoreactive nerve fibres are present in the tracheobronchial airway smooth muscle layer, the walls of pulmonary and bronchial vessels and around submucosal glands. Next to its prominent bronchodilatory effects, VIP potently relaxes pulmonary vessels. The precise role of VIP in the pathogenesis of asthma is still uncertain. Although a therapy using the strong bronchodilatory effects of VIP would offer potential benefits, the rapid inactivation of the peptide by airway peptidases has prevented effective VIP-based drugs so far and non-peptide VIP-agonists did not reach clinical use.

Effect of vasoactive intestinal peptide (VIP)-related peptides on cholinergic neurogenic and direct mucus secretion in ferret trachea in vitro

1 We investigated whether vasoactive intestinal peptide (VIP) and its related peptides, pituitary adenylate cyclase activating peptide (PACAP) and secretin, regulate cholinergic neural mucus secretion in ferret trachea in vitro, using 35SO4 as a mucus marker. We also studied the interaction between VIP and secretin on cholinergic mucus output. 2 VIP (1 and 10 microM) increased secretion, whereas neither PACAP1 - 27, PACAP1 - 38 nor secretin (up to 10 microM) increased mucus output. In contrast, VIP, PACAP1 - 27 and PACAP1 - 38 concentration-dependently inhibited cholinergic neural secretion, with an order of potency of VIP>PACAP 1 - 38>PACAP1 - 27. Neither PACAP1 - 27 nor PACAP1 - 38 altered the secretion induced by acetylcholine (ACh). 3 Secretin increased cholinergic neural secretion with a maximal increase of 190% at 1 microM. This potentiation was blocked by VIP or atropine. Similarly, secretin (1 microM) potentiated VIP (1 microM)-induced mucus output by 160%. Secretin did not alter exogenous ACh-induced secretion. VIP vs secretin competition curves suggested these two peptides were competing reversibly for the same receptor. 4 We conclude that, in ferret trachea in vitro, VIP and PACAPs inhibit cholinergic neural secretion via pre-junctional modulation of cholinergic neurotransmission. VIP and secretin compete for the same receptor, possibly a VIP1 receptor, at which secretin may be a receptor antagonist.

YC-1 potentiates nitric oxide-induced relaxation in guinea-pig trachea

1. The effects of YC-1 (3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole) on tension, levels of cyclic GMP and cyclic AMP were investigated in guinea-pig trachea. We especially studied the combined effect of YC-1 with exogenous or endogenous nitric oxide on these parameters. 2. YC-1 at the concentration 3 or 10 microM, which caused only minor effect by itself, elicited concentration-dependent potentiation of sodium nitroprusside (SNP)-induced tracheal relaxation. This relaxation of YC-1 with SNP was reversed by ODQ. 3. Relaxant responses to electric field stimulation (EFS) in the presence of indomethacin, atropine, guanethidine, alpha-chymotrypsin and histamine were also markedly increased by YC-1 (10 microM). In the presence of L-NAME or ODQ, the relaxant effects to EFS were attenuated and the following addition of YC-1 did not further enhance relaxation. 4. YC-1 (10 microM) or SNP (0.3 microM) alone did not induce significant elevation of cyclic GMP levels in the presence of IBMX, whereas simultaneous application of both compounds markedly elevated the cyclic GMP accumulation. In contrast, the cyclic AMP levels were not altered even at the combination of YC-1 and SNP. Additionally, YC-1 also affected cyclic GMP metabolism, since it inhibited the activity of phosphodiesterase type V in human platelets. 5. YC-1 (30 microM) did not scavenge superoxide anion and had no effect on the removal of superoxide anion by superoxide dismutase in a xanthine/xanthine oxidase system. 6. In conclusion, these results indicate that although YC-1 elicits negligible relaxation of guinea-pig trachea by itself, it can potentiate the relaxant responses of exogenous or endogenous NO. This synergistic response of YC-1 is via the elevation of cyclic GMP contents.

Characterization of non-adrenergic, non-cholinergic inhibitory responses of the isolated guinea-pig trachea: differences between pre- and post-ganglionic nerve stimulation

1 Differences in the mechanism of non-adrenergic, non-cholinergic (NANC) inhibitory responses to preganglionic- and post-ganglionic nerve stimulation were investigated in the guinea-pig isolated trachea. 2 Stimulation of the vagus nerve at frequencies above 4 Hz elicited NANC relaxation of the trachealis muscle. Responses to low frequencies of stimulation (4-8 Hz) were abolished by the nitric oxide (NO) synthase inhibitor L-NOARG (10 microM), while a L-NOARG resistant component was observed at higher stimulus frequencies. The L-NOARG-resistant component of NANC inhibitory responses to higher frequencies of vagus nerve stimulation were significantly attenuated by the proteinase alpha-chymotrypsin (2 U/ml), suggesting that a neuropeptide such as VIP may contribute to NANC responses. 3 When postganglionic nerves were stimulated by electrical field stimulation (EFS), responses were readily elicited at frequencies below 4 Hz. Like responses to vagus nerve stimulation, responses to low frequency (<4 Hz) EFS were abolished by L-NOARG while a L-NOARG-resistant component was apparent at higher stimulus frequencies. 4 The L-NOARG-resistant component of NANC inhibitory responses to EFS was sensitive to alpha-chymotrypsin only if stimuli were delivered in either long trains at a low frequency (4 Hz for 10-30 s) or short trains of high frequency (16 Hz for 2.5-7.5 s). 5 Responses to preganglionic nerve stimulation were approximately 35% of the amplitude of responses to EFS in the same preparations. 6 In conclusion, responses to preganglionic and postganglionic NANC inhibitory nerve stimulation in the guinea-pig trachea differ in maximum amplitude, frequency-response characteristics and the contributions of cotransmitters. We suggest that these differences may be explained by filtering of preganglionic input to postganglionic NANC neurons. These results have implications in all studies where EFS is considered to be representative of physiological stimulation of post-ganglionic nerve stimulation.

Neuroregulation by vasoactive intestinal peptide (VIP) of mucus secretion in ferret trachea: activation of BK(Ca) channels and inhibition of neurotransmitter release

1. The aims of this study were to determine: (1) whether vasoactive intestinal peptide (VIP) regulates cholinergic and 'sensory-efferent' (tachykininergic) 35SO4 labelled mucus output in ferret trachea in vitro, using a VIP antibody, (2) the class of potassium (K+) channel involved in VIP-regulation of cholinergic neural secretion using glibenclamide (an ATP-sensitive K+ (K(ATP)) channel inhibitor), iberiotoxin (a large conductance calcium activated K+ (BK(ca)) channel blocker), and apamin (a small conductance K(ca) (SK(ca)) channel blocker), and (3) the effect of VIP on cholinergic neurotransmission using [3H]-choline overflow as a marker for acetylcholine (ACh) release. 2. Exogenous VIP (1 and 10 microM) alone increased 35SO4 output by up to 53% above baseline, but suppressed (by up to 80% at 1 microM) cholinergic and tachykininergic neural secretion without altering secretion induced by ACh or substance P (1 microM each). Endogenous VIP accounted for the minor increase in non-adrenergic, non-cholinergic (NANC), non-tachykininergic neural secretion, which was compatible with the secretory response of exogenous VIP. 3. Iberiotoxin (3 microM), but not apamin (1 microM) or glibenclamide (0.1 microM), reversed the inhibition by VIP (10 nM) of cholinergic neural secretion. 4. Both endogenous VIP (by use of the VIP antibody; 1:500 dilution) and exogenous VIP (0.1 microM), the latter by 34%, inhibited ACh release from cholinergic nerve terminals and this suppression was completely reversed by iberiotoxin (0.1 microM). 5. We conclude that, in ferret trachea in vitro, endogenous VIP has dual activity whereby its small direct stimulatory action on mucus secretion is secondary to its marked regulation of cholinergic and tachykininergic neurogenic mucus secretion. Regulation is via inhibition of neurotransmitter release, consequent upon opening of BK(Ca) channels. In the context of neurogenic mucus secretion, we propose that VIP joins NO as a neurotransmitter of i-NANC nerves in ferret trachea.

Nitric oxide-mediated relaxation induced by bradykinin in the isolated mouse trachea

We examined the nature of the relaxant effect of bradykinin on mouse isolated tracheal rings. Bradykinin produced a concentration-dependent relaxation in mouse tracheal rings contracted by carbachol. Potentiation of the contractile effect of carbachol and inhibition of the relaxant effect of bradykinin by pretreatment with NG-nitro-L-arginine methyl ester (L-NAME), L-glutamine (L-Gln) and methylene blue (MeB) suggested that the peptide activated the L-arginine nitric oxide (NO) pathway. Part of the relaxant effect of bradykinin was also mediated through the release of cyclooxygenase metabolites of arachidonic acid, as evidenced by the inhibition of this response by lysine acetylsalicylic acid (ASA) pretreatment. Bradykinin also caused a relaxant response in precontracted tracheal rings in the presence of lower but not higher concentrations of K+ (> 60 mM). NG-nitro-L-arginine methyl ester and L-Gln did not alter the contractile effect of K+. K+ channel blockers partially inhibited the relaxant effect of bradykinin in carbachol-induced precontracted tracheal rings. Tetraethylammonium, a non-selective blocker of K+ channels, completely abolished the relaxant response to the peptide. Among the other channel blockers, the inhibitory effect of glibenclamide was slightly greater than that of apamine and iberiotoxin, indicating the involvement of K(ATP) channels in the relaxant response to the peptide. These results suggest that the mechanisms of the relaxation induced by bradykinin in carbachol-induced precontracted mouse tracheal muscle primarily involve activation of L-arginine NO and arachidonic acid cyclooxygenase pathways and secondly K+ channels.

Cigarette smoke-inhibition of neurogenic bronchoconstriction in guinea-pigs in vivo: involvement of exogenous and endogenous nitric oxide

1. We investigated the effect of acute inhalation of cigarette smoke on subsequent non-adrenergic, non-cholinergic (NANC) neural bronchoconstriction in anaesthetized guinea-pigs in vivo by use of pulmonary insufflation pressure (PIP) as an index of airway tone. The contribution of endogenous nitric oxide (NO) was investigated with the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME). The contribution of plasma exudation to the response was investigated with Evans blue dye as a plasma marker. 2. Inhalation of 50 tidal volumes of cigarette smoke or air had no significant effect on baseline PIP. In the presence of propranolol and atropine (1 mg kg(-1) each), electrical stimulation of the vagus nerves in animals given air 30 min previously induced a frequency-dependent increase in PIP above sham stimulated controls (16 fold increase at 2.5 Hz, 24 fold increase at 10 Hz). In contrast, in smoke-exposed animals, the increase in subsequent vagally-induced PIP was markedly less than in the air controls (90% less at 2.5 Hz, 76% less at 10 Hz). 3. L-NAME (10 mg kg[-1]), given 10 min before air or smoke, potentiated subsequent vagally-induced (2.5 Hz) NANC bronchoconstriction by 338% in smoke-exposed animals, but had no significant effect in air-exposed animals. The inactive enantiomer D-NAME (10 mg kg[-1]) had no effect, and the potentiation by L-NAME was partially reversed by the NO-precursor L-arginine (100 mg kg[-1]). Vagal stimulation did not affect the magnitude of vagally-induced bronchoconstriction 30 min later. 4. Cigarette smoke exposure reduced the magnitude of subsequent bronchoconstriction induced by neurokinin A (NKA) by 37% compared with the effect of NKA in air-exposed animals. L-NAME had no significant effect on the smoke-induced inhibition of NKA-induced bronchoconstriction. 5. Vagally-induced plasma exudation in the main bronchi was greater in smoke-exposed animals compared with air-exposed animals (120% greater at 2.5 Hz, 82% greater at 10 Hz). 6. We conclude that cigarette smoke-induced inhibition of subsequent NANC neurogenic bronchoconstriction is not associated with inhibition of airway plasma exudation and is mediated in part via exogenous smoke-derived NO, or another bronchoprotective molecule, and by endogenous NO.

Impairment of neural nitric oxide-mediated relaxation after antigen exposure in guinea pig airways in vitro

Nitric oxide (NO), a neurotransmitter of inhibitory nonadrenergic noncholinergic (iNANC) nerves in airways, is a radical with a short half-life, and its function may be modified by airway inflammation. To test this hypothesis, we examined whether airway allergic inflammation affects iNANC responses mediated by NO in guinea pigs in vitro. Animals sensitized with ovalbumin (OA) were challenged with 0.03% OA (OA group) or saline (saline group) by inhalation on 3 consecutive days. On the day after the final challenge, iNANC responses elicited by electrical field stimulation (2 to 16 Hz) or relaxation responses to 3-morpholinosydnonimine (SIN-1), 10(-8) to 10(-4) M, were obtained in the tracheal strips precontracted by histamine (3 x 10(-6) M) in the presence of atropine and propranolol (both 10(-6) M). The INANC responses of the OA group were significantly attenuated compared with those of the saline group (p < 0.05), and the inhibitory effect of a NO synthase (NOS) inhibitor, Nm-nitro-L-arginine methyl ester, on the INANC responses was abolished in the OA group. SIN-1-induced tracheal smooth muscle relaxation was also significantly affected by antigen exposure (p < 0.05), the effect of which disappeared in the presence of a NO scavenger, carboxy PTIO (3 x 10(-6) M). The impairment of the INANC responses after antigen exposure was significantly restored by superoxide dismutase (1,000 U/ml), especially at lower frequencies. Histochemical demonstration of NADPH-diaphorase-positive nerves representing neural NOS density was not different between the two groups. These results suggest that allergic airway inflammation impairs neural NO-induced relaxation, presumably by inhibiting the access of neural NO to the airway smooth muscle.

Role of nitric oxide in nonadrenergic, noncholinergic relaxation of whole tracheal tube preparations isolated from guinea pigs

1. Frequency-dependent nonadrenergic, noncholinergic (NANC) relaxant responses were induced by transmural stimulation of whole tracheal tube preparations. 2. Responses at lower frequencies (< or = 10 Hz) were abolished by L-nitroarginine methyl ester (L-NAME). 3. Responses at higher frequencies (> or = 20 Hz) consisted of a rapid, short-lasting relaxation, followed by a slow, long-lasting relaxation. The former and the latter were reduced by L-NAME and alpha-chymotrypsin, respectively. 4. alpha-Chymotrypsin had little effect on the magnitude of NANC responses, but reduced the duration of responses at higher frequencies (> or = 20 Hz). 5. The results suggest that NANC relaxation of guinea pig trachea may be mediated primarily by nitric oxide, with and without concomitant release of vasoactive intestinal peptide or related peptides, and nitric oxide may act as predominant mediator providing the magnitude of relaxant response.

ATP and nitric oxide: inhibitory NANC neurotransmitters in the longitudinal muscle-myenteric plexus preparation of the rat ileum

1. The nature of neurotransmitter(s) involved in non-adrenergic non-cholinergic (NANC) relaxations induced by electrical stimulation (10 s trains, 1-8 Hz) was investigated in the precontracted longitudinal muscle-myenteric plexus preparation of the rat ileum. 2. Electrical stimulation of the tissue induced complex responses, consisting of a primary contraction, a primary relaxation, an off-relaxation and a rebound contraction, which were all tetrodotoxin(TTX)-sensitive. 3. Vasoactive intestinal polypeptide (VIP) and carbon monoxide (CO) did not induce relaxations. alpha-Chymotrypsin did not reduce the relaxations induced by electrical stimulation, while zinc protoporphyrin IX had non-specific effects. 4. Nitric oxide (NO) induced concentration-dependent relaxations. NG-nitro-L-arginine methylester (L-NAME) abolished the primary contractions and off-relaxations, while it partially reduced the primary relaxations. 5. ATP induced relaxations and ATP-desensitization of the tissues partially reduced the primary relaxations. Suramin and reactive blue 2 did not consistently influence the primary relaxations. 6. The ATP-induced relaxations were not influenced by L-NAME or TTX. The inhibitory effect of ATP-desensitization and L-NAME did not summate. 7. The cyclic AMP content of the tissue did not increase upon electrical stimulation or after addition of NO or ATP. The cyclic GMP content of the tissue increased upon electrical stimulation and addition of NO, but not after addition of ATP. 8. It is concluded that the relaxation induced by electrical stimulation consists of two types of responses. The off-relaxation is completely nitrergic, while the primary relaxation is mediated by NO, ATP and an as yet unknown transmitter which is not VIP or CO.

Characteristics of vagal reflex-mediated tracheal response induced by bronchoconstriction in guinea pigs

The reflex tracheal response induced by bronchoconstriction was investigated using a newly devised tracheo-bronchi preparation in anesthetized guinea pigs. Tracheal constriction and subsequent dilatation were observed in response to bronchoconstriction induced by the inhalation of 0.001-0.01% histamine and 0.003-0.03% acetylcholine. These tracheal responses were abolished by cervical vagotomy or treatment of the tracheal site with 1% tetrodotoxin. Tracheal constriction and dilatation were significantly inhibited by 0.1% atropine and 1% propranolol, respectively. When high tracheal tone was induced by 0.01% serotonin, the residual tracheal dilatation observed in the presence of propranolol was enhanced, while dilatation was completely inhibited by 1% hexamethonium. Dilatation was also suppressed by 1% N omega-nitro-L-arginine methyl ester (L-NAME) and 1% methylene blue. The tracheal constriction produced by bronchoconstriction was significantly enhanced by propranolol 2 mg/kg, i.v. and L-NAME 10 mg/kg, i.v. These results demonstrate that a vagally mediated reflex tracheal response (constriction followed by dilatation) is induced by bronchoconstriction in anesthetized guinea pigs. Cholinergic nerves may mediate the constriction, and adrenergic and nonadrenergic noncholinergic (NANC) inhibitory nerves may mediate the dilatation. Furthermore, NO may be involved in the NANC reflex tracheal dilatation.

Regional difference in the distribution of L-NAME-sensitive and -insensitive NANC relaxations in cat airway

1. To investigate the distribution profile of functional inhibitory non-adrenergic non-cholinergic (i-NANC) nerves and the contribution of NO to the NANC relaxation in the cat, we studied the effects of N omega-nitro-L-arginine methyl ester (L-NAME) on NANC relaxation elicited by electrical field stimulation (EFS) in the trachea, bronchus and bronchiole. 2. EFS applied to the tracheal smooth muscle during contraction induced by 5-HT (10(-5) M) in the presence of atropine (10(-6) M) and guanethidine (10(-6) M) elicited a monophasic NANC relaxation. By contrast, NANC relaxation elicited in the peripheral airway was biphasic, comprising an initial fast followed by a second slow component and L-NAME (10(-5) M) selectively abolished the first component without affecting the second one. In the trachea, L-NAME (10(-5) M) completely suppressed the monophasic NANC relaxation when single or short repetitive stimuli (< 5) with 1 ms pulse duration were applied. However, at higher repetitive stimuli (> 10) with 1 or 4 ms pulse duration, suppression of NANC relaxation was incomplete. 3. In the small bronchi obtained from L-NAME-pretreated cats, EFS applied during contraction induced by 5-HT (10(-5) M) elicited only the slow component of NANC relaxation which is sensitive to tetrodotoxin. 4. In the peripheral airway, a newly synthesized VIP antagonist (10(-6) M) or alpha-chymotrypsin (1 U ml-1) considerably attenuated the amplitude of L-NAME-insensitive relaxation. 5. Single or repetitive EFS consistently evoked excitatory junction potentials (EJPs) in the central and peripheral airways. When tissues were exposed to atropine (10(-6) M) and guanethidine (10(-6) M), single or repetitive EFS did not alter the resting membrane potential. 6. These results indicate that at least two neurotransmitters, possibly NO or NO-containing compounds and VIP, are involved in i-NANC neurotransmission and the distribution profile of the two components differs in the central and peripheral airway of the cat.

Nitric oxide in the peripheral nervous system

Nitric oxide (NO) is a neurotransmitter and neuromodulator in the central nervous system, but this small labile substance also seems to serve as a peripheral neurotransmitter. Abundant evidence is now available that NO, synthesized from L-arginine by NO synthase (NOS), is a nonadrenergic noncholinergic relaxant transmitter of gastrointestinal smooth muscle. Electrically induced nonadrenergic noncholinergic relaxations are antagonized by NOS inhibitors in vitro and in vivo. In a bioassay superfusion system, the release of a substance with the pharmacological characteristics of NO from a gastrointestinal smooth muscle preparation was detected; also, indirect measurements (e.g. of the NO metabolite nitrite or of the co-product of its synthesis L-citrulline) suggest NO release. Immunohistochemistry with antibodies raised against the neuronal NOS showed immunoreactivity in cell bodies of neurones in the myenteric plexus and in nerve fibres in the muscular layer. These data suggest that nerve endings, innervating smooth muscle, are able to release NO that will penetrate the cells to induce relaxation (i.e. nitrergic neurotransmission). It is unlikely that NO as such is stored and it is generally accepted that it is synthesized on demand when the nerve endings are excited, although the possibility of the release of a NO-containing molecule protecting it from degradation in the junction has been proposed. Other sources than neurones (interstitial cells, smooth muscle cells) for the NO involved in nonadrenergic noncholinergic inhibitory transmission have also been proposed. Using NADPH diaphorase as a marker for neuronal NOS, deficiency of the nitrergic innervation has been shown in isolated tissue from patients with infantile hypertrophic pyloric stenosis, achalasia and Hirschsprung's disease, suggesting that a lack of NO release might be involved in these disorders. Evidence in favour of nitrergic neurotransmission to smooth muscle has also been obtained in the respiratory and lower urinary tract, the corpora cavernosa and some blood vessels.

Evidence for the involvement of cGMP in neural bronchodilator responses in humal trachea

1. We have investigated the correlation between relaxation and changes in cyclic nucleotide content of human tracheal smooth muscle (HTSM) in vitro following inhibitory non-adrenergic non-cholinergic (i-NANC) neural bronchodilator responses evoked by electrical field stimulation (EFS), and compared these with changes seen with sodium nitroprusside (SNP), 3-morpholinosydnonimine (SIN-1) and vasoactive intestinal peptide (VIP). The effects of N omega-nitro-L-arginine methyl ester (L-NAME), Methylene Blue and alpha-chymotrypsin (alpha-CT) were studied. 2. EFS (10 Hz, 1 ms, 40 V for 30 s) evoked a time-dependent relaxation accompanied by a concurrent rise in cGMP, both of which were maximal at 30 s and unaffected by epithelium removal. Levels of cAMP were more variable than those of cGMP and were not significantly changed at any time point. 3. SIN-1 (1 mM) and SNP (100 microM) also produced time-dependent relaxations which were maximal between 2 and 8 min, accompanied by concomitant rises in cGMP; however, these changes were larger than those associated with i-NANC relaxations. cAMP levels were unchanged at all time points. 4. EFS-evoked i-NANC relaxations and cGMP increases (time, t = 30 s) were inhibited by L-NAME. The effects were partially reversed by L-arginine (1 mM), but not by D-arginine. D-NAME and alpha-CT (2 u ml-1) had no effect on either relaxation or cGMP accumulation. Tetrodotoxin (TTX, 3 microM) inhibited both relaxation and cGMP accumulation. 5. VIP (1 microM) also produced a time-dependent relaxation associated with a concurrent rise in cAMP levels with no change in cGMP levels. 6. Methylene Blue (10 microM) partially inhibited EFS (10 Hz)-evoked i-NANC relaxation and cGMP accumulation, and almost completely inhibited both relaxation and cGMP accumulation evoked by SIN-1 (1 mM). Methylene Blue had no significant effect on relaxation or cGMP accumulation evoked by SNP (100 microM). 7. Neural i-NANC relaxations in HTSM are associated with a concurrent selective accumulation of cGMP which is unaffected by epithelium removal. This is inhibited in a stereoselective manner by L-NAME and mimicked by SNP and SIN-1; however, cGMP accumulation was greatly increased with SNP and SIN-1 suggesting compartmentalized changes in cGMP content. VIP also caused relaxation associated with an increase of cAMP; however, no evidence was found for VIP being involved in i-NANC relaxation. Hence nitric oxide (NO), or a NO-containing complex, appears to mediate i-NANC responses in human trachea in vitro.

Role of nitric oxide in non-adrenergic, non-cholinergic relaxation and modulation of excitatory neuroeffector transmission in the cat airway

1. The effects of nitrosocysteine (cys-NO), L-N omega-nitroarginine (L-NNA) and L-N omega-nitro-L-arginine methylester (L-NAME), oxyhaemoglobin and Methylene Blue were observed on the resting membrane potential, muscle tone and excitatory junction potentials (EJPs) of cat tracheal smooth muscle tissue. 2. Cys-NO (10(-9) to 10(-6) M) showed no effect on the resting membrane potential of smooth muscle cells of the cat trachea but it dose-dependently relaxed the tracheal tissue in the presence of 5-HT, atropine and guanethidine. 3. Electrical field stimulation (EFS) applied during contraction evoked by 5-HT in the presence of atropine and guanethidine evoked non-adrenergic, non-cholinergic (NANC) muscle relaxation. L-NNA (10(-4) M) and L-NAME (10(-4) M) completely suppressed the relaxation when single or short repetitive stimuli were applied, but suppression was incomplete with repetitive stimuli of 4 ms pulse duration applied at 20 Hz. A substantial part of the L-NNA- or L-NAME-insensitive relaxation was abolished by tetrodotoxin. 4. Cys-NO dose-dependently suppressed the EJPs without changing the resting membrane potential, and L-NNA, L-NAME, Methylene Blue and oxyhaemoglobin enhanced the amplitude of the EJP to 1.2-1.5 times the control value. 5. EJPs showed some summation when repetitive field stimulation was applied at 20 Hz. L-NNA or L-NAME enhanced the summation, and the mean slopes were increased from 0.61 +/- 0.22 to 2.0 +/- 0.3, or 1.9 +/- 0.2 mV per stimulus. Vasoactive intestinal polypeptide (VIP) antiserum and VIP antagonists further enhanced the summation in the presence of L-NNA. 6. These results indicate that NANC relaxation can be classified into two different components according to the threshold for activation, and nitric oxide is involved in one. The present results also suggest that endogenous or exogenous nitric oxide has a prejunctional action in inhibiting excitatory neuroeffector transmission in addition to a direct action on the smooth muscle cells, presumably by suppressing transmitter release from the vagus nerve.

5-HT-induced neurogenic relaxations of the guinea-pig proximal colon: investigation into the role of ATP and VIP in addition to nitric oxide

In the guinea-pig proximal colon, 5-hydroxytryptamine (5-HT) relaxes the longitudinal muscle by stimulating neuronal 5-HT receptors, which induces the release of nitric oxide (NO). It was investigated whether the inhibitory neurotransmitters adenosine 5'-triphosphate (ATP) and/or vasoactive intestinal polypeptide (VIP) could be involved as well. Antagonists to block the contractile response to 5-HT via 5-HT2, 5-HT3 or 5-HT4 receptors were present throughout the experiments and methacholine was administered to precontract the strips. ATP, VIP and 5-HT induced concentration-dependent relaxations, in the case of 5-HT yielding a non-monophasic concentration-response curve. Tetrodotoxin (TTX; 300 nM), NG-nitro-L-arginine (L-NNA, 100 microM) and their combination did not inhibit the relaxations induced by VIP (up to 0.3 microM) or 0.3-3 microM ATP but reduced those by 10 microM ATP. Suramin (300 microM) strongly inhibited the relaxations to ATP and VIP. L-NNA and suramin also inhibited the relaxations to 5-HT. In the presence of L-NNA (100 microM), suramin did not significantly inhibit the relaxations to 5-HT. Suramin did not affect the relaxations to isoprenaline, nitroglycerin or exogenous NO (1 microM), demonstrating its specificity. Apamin (30 nM) inhibited both the relaxations to ATP (by 70-100%) and to 5-HT; relaxations to isoprenaline were partially inhibited, indicating a non-specific component in the inhibitory action of apamin. However, relaxations to exogenous VIP (up to 0.3 microM), NO (1 microM) and to nitroglycerin were not inhibited. In the presence of L-NNA (100 microM), apamin inhibited the relaxations to 5-HT only at 30 microM. alpha, beta-methylene-ATP (alpha, beta-Me-ATP; 100 microM) did not desensitize the responses to ATP. Reactive blue 2 affected the relaxations to isoprenaline at concentrations necessary to significantly inhibit the relaxations to ATP (i.e. from 10 microM onwards). Thus, it was not possible to test either alpha, beta-Me-ATP or reactive blue 2 against the relaxations to 5-HT. alpha-Chymotrypsin (0.015 mg.ml-1) and trypsin (0.005 mg.ml-1) almost abolished the relaxations to VIP, but did not affect those to isoprenaline and 5-HT. The VIP receptor antagonists [p-Cl-D-Phe6, Leu17]VIP (1 microM) and VIP10-28 (1 and 3 microM) did not affect the concentration-response curve to VIP and were hence not tested against 5-HT. Phosphoramidon (1 microM) had no effect on the relaxations to VIP or 5-HT.(ABSTRACT TRUNCATED AT 400 WORDS)

Peptide histidine isoleucine-like immunoreactivity release from the rat gastric fundus

1. Longitudinal muscle strips from the rat gastric fundus were subjected to in vitro electrical field stimulation (EFS) under non-adrenergic non-cholinergic (NANC) conditions to study the release of peptide histidine isoleucine-like immunoreactivity (PHI-LI) and the correlation between PHI-LI release and NANC relaxation. 2. Different radioimmunoassay (RIA) systems employing C-terminal- and N-terminal-specific anti-PHI sera were used to determine the relative contributions of PHI and its C-terminally extended forms, peptide histidine glycine (PHI-Gly) and peptide histidine valine [PHV(1-42)], to the PHI-LI released by the rat gastric fundus. 3. In the presence of atropine (1 microM) and guanethidine (5 microM), EFS (120 mA, 1 ms, 0.25-32.0 Hz, trains of 2 min) induced frequency-dependent relaxations of 5-hydroxytryptamine (3 microM) pre-contracted strips. 4. EFS at frequencies of 8-32 Hz evoked significant increases in PHI-LI outflow. The increases in PHI-LI outflow evoked by 16-Hz EFS were abolished by tetrodotoxin (3 microM) and by a calcium-free medium, indicating an active release process from intramural nerves. 5. The EFS-induced release of PHI-LI measured with the N-terminal-specific antiserum was significantly greater than that detected with the C-terminal-specific antisera. 6. Sephadex G-25 gel permeation chromatographic analysis was performed on the PHI-LI release in response to 32-Hz EFS. A C-terminal-specific antiserum revealed one peak co-eluting with the rat PHI standard. When PHI-LI was measured with the N-terminal-specific antiserum, two peaks were found that co-eluted with the rat PHV(1-42) and rat PHI-Gly/PHI standards, respectively. 7. The present data suggest that the extended forms of PHI are the primary components of the PHI-LI released by NANC inhibitory neurones in the rat gastric fundus and support a NANC inhibitory neurotransmitter role for PHI and its extended forms in this tissue.

Modulation of neurotransmission in guinea-pig airways by galanin and the effect of a new antagonist galantide

Galanin is localised to sensory nerve fibres and cholinergic nerves in airways. Galantide has been shown to be a novel high affinity antagonist to galanin, since it inhibits galanin-mediated inhibition of glucose-induced insulin secretion and the neuronal action of galanin in the brain. We investigated the effects of galanin on cholinergic and non-adrenergic, non-cholinergic (NANC) responses to electrical field stimulation in guinea-pig airways, and examined whether galantide antagonised the effect of galanin on neurotransmission. Galanin (10(-6)M) had no effect on cholinergic bronchoconstrictor responses and inhibitory NANC relaxation responses in trachea, but significantly inhibited excitatory NANC bronchoconstrictor responses in bronchi which is due to the release of tachykinins. Galantide (10(-8)-10(-6)M) had no effect on the galanin-induced inhibition of the excitatory NANC responses. Galanin may be important in the modulation of excitatory NANC responses but not cholinergic and inhibitory NANC responses in guinea-pig airways. This modulatory effect may be via a different type of galanin receptor than is present in other organs.

Effects of sensitization on vasoactive intestinal polypeptide-induced relaxation and its concentration and binding in guinea-pig airways

We investigated the relaxant effect of vasoactive intestinal polypeptide (VIP) in trachea and lung parenchyma from normal and sensitized guinea-pigs. A technique by which drug access was restricted to either the mucosal or the adventitial surface of tracheal rings was used. In intact trachea, concentration-response curves for VIP entering from the mucosal surface (pD2 = 6.61 +/- 0.06) were displaced to the right compared with those for adventitial entry (pD2 = 6.78 +/- 0.04). Epithelium removal produced a leftward shift (approximately 2.8-fold) in the mucosal VIP concentration-response curve. Sensitization did not alter the responsiveness (maximal effect) or sensitivity (pD2 values) of tracheal rings to VIP irrespective of the surface of drug entry and of the absence or presence of epithelium. VIP-induced relaxation of normal and sensitized lung strips was also similar. Sensitization resulted in a significant decrease in tracheal VIP content (from 2.16 +/- 0.07 in normal to 0.60 +/- 0.08 nmol/mg protein in sensitized trachea; P < 0.05; n = 7) whereas the affinity of both high- and low-affinity binding sites for VIP increased as compared to that of normal trachea. Differences were not found in the binding capacities of normal and sensitized trachea. VIP content and binding did not differ in normal and sensitized lung. In conclusion, immunological sensitization produced changes in VIP tracheal content and binding but neither VIP-induced relaxation of isolated airways nor the influence of epithelium in this response was altered.

Modulation of cholinergic neural bronchoconstriction by endogenous nitric oxide and vasoactive intestinal peptide in human airways in vitro

Human airway smooth muscle possesses an inhibitory nonadrenergic noncholinergic neural bronchodilator response mediated by nitric oxide (NO). In guinea pig trachea both endogenous NO and vasoactive intestinal peptide (VIP) modulate cholinergic neural contractile responses. To identify whether endogenous NO or VIP can modulate cholinergic contractile responses in human airways in vitro, we studied the effects of specific NO synthase inhibitors and the peptidase alpha-chymotrypsin on contractile responses evoked by electrical field stimulation (EFS) at three airway levels. Endogenous NO, but not VIP, was shown to inhibit cholinergic contractile responses at all airway levels but this inhibition was predominantly in trachea and main bronchus and less marked in segmental and subsegmental bronchi. To elucidate the mechanism of this modulation we then studied the effects of endogenous NO on acetylcholine (ACh) release evoked by EFS from tracheal smooth muscle strips. We confirmed that release was neural in origin, frequency dependent, and that endogenous NO did not affect ACh release. These findings show that endogenous NO, but not VIP, evoked by EFS can inhibit cholinergic neural responses via functional antagonism of ACh at the airway smooth muscle and that the contribution of this modulation is less marked in lower airways.

Evidence against vasoactive intestinal polypeptide as the relaxant neurotransmitter in human cavernosal smooth muscle

1. The putative role of vasoactive intestinal polypeptide (VIP) as the relaxant neurotransmitter in human cavernosal smooth muscle has been studied in isolated tissue preparations. 2. Consistent neurogenic relaxations were evoked by electrical field stimulation (EFS; 2-64 pulses/train, 0.8 ms pulse duration, 10 Hz). VIP (0.1-3 microM) relaxed cavernosal smooth muscle in a dose-dependent fashion. Relaxant responses to both EFS and VIP were reduced in tissue from impotent men. 3. Neurogenic relaxant responses were not diminished in the presence of the VIP-inactivating peptidase, alpha-chymotrypsin (alpha-CT, 2 units ml-1). In contrast VIP-induced relaxations were completely abolished. 4. Inhibition of nitric oxide synthase by NG-nitro-L-arginine (30 microM), and of guanylate cyclase by methylene blue (50 microM) caused highly significant reductions of neurogenic relaxant responses whereas VIP-evoked relaxations were unaffected. 5. It is concluded that VIP-evoked relaxations are not mediated by the NO-guanosine 3':5'-cyclic monophosphate (cyclic GMP) pathway and that VIP release is not essential for neurogenic relaxation of human cavernosal smooth muscle. VIP does not therefore act as the major relaxant neurotransmitter in this tissue.

Neuronal control of airways smooth muscle

Sensory afferent nerves relay impulses from the airways to the central nervous system so that appropriate changes in bronchomotor tone and breathing patterns may occur. The dominant efferent control of airways smooth muscle is exerted via bronchoconstrictor parasympathetic cholinergic nerves. In some species this is opposed by bronchodilator sympathetic noradrenergic nerves. In addition, there exist both excitatory bronchoconstrictor and inhibitory bronchodilator non-adrenergic, non-cholinergic pathways. This review examines the role of the different branches of the autonomic nervous system in the control of airways smooth muscle tone with particular reference to modulation of these branches and the interactions which may exist between them.

Endogenous vasoactive intestinal peptide and nitric oxide modulate cholinergic neurotransmission in guinea-pig trachea

Guinea-pig tracheal smooth muscle possesses an inhibitory non-adrenergic, non-cholinergic (i-NANC) innervation and the neurotransmitters involved in this response may be vasoactive intestinal peptide (VIP) and nitric oxide (NO). Since i-NANC mechanisms may co-exist with cholinergic nerves we have investigated whether endogenous VIP and NO modulate cholinergic neurotransmission. alpha-Chymotrypsin enhanced the cholinergic contractile responses to electrical field stimulation (EFS at 4 Hz by 38.6 +/- 4.8% (P < 0.05, n = 6) but did not produce a shift in the concentration-response curve to acetylcholine (ACh). L-NG-Nitro-arginine methyl ester (L-NAME) and L-NG-monomethyl arginine (L-NMMA) produced a concentration-dependent enhancement of cholinergic responses to EFS (4 Hz) (at 100 microM, 40.9 +/- 6.6 and 30.2 +/- 5.8%, P < 0.01) with no effect on response curves to ACh. This enhancement was reversed by L-arginine but not D-arginine (1 mM). D-NAME and D-NMMA and L-arginine had no effect on cholinergic neurotransmission. alpha-Chymotrypsin and L-NAME had no effect on excitatory NANC (e-NANC) neural responses in guinea-pig bronchi. These results suggest that endogenous NO and VIP may modulate cholinergic neurotransmission by either functional antagonism at the level of the airway smooth muscle or via a pre-junctional inhibition of ACh release from cholinergic nerve terminals or by both mechanisms.

Regulation of NANC neural bronchoconstriction in vivo in the guinea-pig: involvement of nitric oxide, vasoactive intestinal peptide and soluble guanylyl cyclase

1. We investigated the effect of the nitric oxide (NO) synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME) and the peptidase alpha-chymotrypsin on non-adrenergic, non-cholinergic (NANC neural) bronchoconstriction induced by electrical stimulation of the vagus nerves and by capsaicin in anaesthetized guinea-pigs in vivo using pulmonary insufflation pressure (PIP) as an index of bronchial tone. We also investigated the contribution of soluble guanylyl cyclase (SGC) to NANC neural relaxant mechanisms. 2. In the presence of atropine and propranolol, electrical stimulation of the vagus nerves induced a frequency-dependent increase in PIP above baseline of 67% at 2.5 Hz, of 128% at 5 Hz and of 230% at 10 Hz. L-NAME (1-50 mg kg-1, i.v.), at doses inducing increases in systemic blood pressure, dose-relatedly potentiated NANC bronchoconstriction. At 10 mg kg-1 i.v., L-NAME significantly (P < 0.05) potentiated NANC bronchoconstriction by a further 106% at 2.5 Hz and a further 147% at 5 Hz but did not potentiate the increase in PIP at 10 Hz. L-NAME did not induce bronchoconstriction in sham-stimulated control animals. D-NAME did not potentiate NANC bronchoconstriction. Raising systemic blood pressure with phenylephrine did not potentiate vagally-induced bronchoconstriction (2.5 Hz). 3. The NO precursor L-arginine, but not D-arginine, (100 mg kg-1, i.v.) significantly reversed the potentiation by L-NAME of NANC bronchoconstriction. L-Arginine alone significantly inhibited neurogenic bronchoconstriction at 10 Hz (by 74%); the inhibition of 25% at 2.5 Hz was not significant. 4. L-NAME did not significantly affect the increases in PIP induced by intravenous substance P. neurokinin A (NKA) or capsaicin. 5. The inhibitor of SGC, methylene blue (10 mg kg', i.v.) potentiated (by 110-140%) NANC neural bronchoconstriction induced by lower frequencies of nerve stimulation and reversed the reduction in PIP induced by the SGC activator, sodium nitroprusside (SNP, 1.05 mg kg- 1, i.v.). SNP significantly (P <0.05) reduced by 65% the bronchoconstriction induced by nerve stimulation at 10 Hz. Methylene blue did not effect baseline PIP in sham-stimulated controls. The airway effects of methylene blue and SNP were not associated with their cardiovascular effects. 6. a-Chymotrypsin (2 units kg-', i.v.) significantly potentiated vagally-induced bronchoconstriction by a further 63% at 2.5 Hz, by a further 95.6% at 5 Hz but did not potentiate the increase in PIP at 10 Hz. alpha-Chymotrypsin also potentiated (by 116%) capsaicin-induced bronchoconstriction. Vasoactive intestinal peptide (VIP, 10 ig kg-' i.v. infused over min) significantly reduced by 70% the increase in PIP induced by NKA (0.1 .Lmol kg-' i.v., infused over 30 s). 7. The combination of a-chymotrypsin (2 units kg-', i.v.) and L-NAME (5 mg kg-', i.v.) significantly potentiated NANC bronchoconstriction by a further 304% at 2.5 Hz, an increase in PIP which was greater than that induced by either a-chymotrypsin or L-NAME alone (P <0.05). 8. We conclude that endogenous NO and a bronchodilator peptide, possibly VIP, released in association with nerve stimulation, as well as activation of soluble guanylyl cyclase, regulate the magnitude of NANC neurogenic bronchoconstriction in guinea-pigs in vivo.

On the peptidergic hypothesis for non-adrenergic non-cholinergic innervation in the rat duodenum

1. The nature of the non-adrenergic, non-cholinergic (NANC) transmitter was studied in vitro in the rat duodenum, by use of an isometric-isovolumic preparation. 2. Electrical field stimulation (EFS) induced a tetrodotoxin (TTX)-sensitive fall both in luminal pressure and in isometric tension. 3. Neurotensin (NT) induced TTX-insensitive inhibitory responses similar to those induced by EFS. Vasoactive intestinal peptide (VIP) caused a delayed, slow, concentration-dependent, TTX-insensitive inhibitory effect, detected only by a change in luminal pressure. 4. alpha-chymotrypsin prevented the NT- and VIP-induced inhibitory effects and antagonized the response to EFS. 5. Apamin antagonized the EFS- and NT-induced effects, but failed to affect the relaxation in response to exogenous VIP. 6. Desensitization of NT receptors by exposure to NT (10 nM) for 30 min did not affect the EFS-induced relaxation. 7. These findings provide support for the involvement of a peptide in the NANC relaxation in rat duodenum. However, there is no evidence that NT or VIP are neurotransmitters released from the NANC system in this preparation.

Vasoactive intestinal peptide and neuropeptide Y coexist in non-noradrenergic sympathetic neurons to guinea pig trachea

Vasoactive intestinal peptide (VIP) has been suggested to be a mediator of vagal inhibition of airway tone and it has been assumed that VIP-containing nerve fibres in the airway arise from intrinsic ganglia. We have used a combination of double- and triple-labelling immunohistochemistry, retrograde axonal tracing, organotypic culture and nerve lesion studies, to identify the origin and distribution of neurons containing immunoreactivity (IR) to VIP in guinea pig airway smooth muscle. We also investigated whether immunoreactivity to other neuropeptides coexisted with VIP-IR within these neurons. We found that all VIP-IR nerve fibres in guinea pig tracheal smooth muscle also contained IR to neuropeptide Y (NPY) but not to tyrosine hydroxylase (TH), a marker for noradrenergic neurons. Both VIP-IR and NPY-IR were absent from nerve cell bodies in the tracheal plexus. After maintenance of isolated trachea in organotypic culture for 4 days, to allow degeneration of extrinsic nerve fibres, nerve fibres containing VIP-IR or NPY-IR were almost completely absent from tracheal smooth muscle. Of ganglia known to supply the trachea, coexistence of VIP-IR and NPY-IR was found only in cell bodies of the stellate ganglion. Retrograde tracing studies using the fluorescent tracer, DiI, confirmed that the stellate ganglion was the site of origin of neurons containing VIP-IR and NPY-IR supplying the airways. These neurons projected to the airways from the stellate ganglion both directly through the mediastinum, and via the cervical sympathetic trunk and vagus nerves. These results suggest that nerve fibres containing both VIP-IR and NPY-IR in the tracheal smooth muscle of the guinea pig are derived from non-noradrenergic cell bodies in the stellate ganglion. The absence of VIP-IR from vagal post-ganglionic neurons suggests that VIP cannot be a mediator of vagal inhibitory transmission in tracheal smooth muscle of this species.

The influence of L-NG-nitro-arginine on field stimulation induced contractions and acetylcholine release in guinea pig isolated tracheal smooth muscle

The interaction between parasympathetic and inhibitory non-adrenergic, non-cholinergic nerves in tracheal smooth muscle was investigated by determining the effects of the NO-synthase inhibitor L-NG-nitro-arginine (L-NOARG) on contractions and the associated acetylcholine release elicited by field stimulation of the muscle. At frequencies above 2Hz contractile responses to field stimulation were potentiated by L-NOARG (50 microM). alpha-chymotrypsin pre-treatment potentiated contractile responses at all frequencies, but the effects of L-NOARG were unaltered. The effect of L-NOARG on responses to 5Hz electrical stimulation was not mimicked by D-NOARG, was reversed by L-, but not D-arginine and was unaffected by epithelium removal. L-NOARG did not affect responses to exogenous acetylcholine nor the overflow of 3H from tissues previously loaded with [3H]-choline. It is therefore concluded that field stimulation of tracheal smooth muscle induces the release of an endogenous nitrate, which, by an inhibitory action on smooth muscle, functionally antagonises the concomitantly released parasympathetic neurotransmitter.

Effects of cromakalim on neurally-mediated responses of guinea-pig tracheal smooth muscle

1. The ability of cromakalim to modulate several different types of neuroeffector transmission has been assessed in guinea-pig isolated trachea. 2. In trachea treated with propranolol (10(-6) M) and indomethacin (2.8 x 10(-6) M), stimulation of the extrinsic vagal nerves evoked contractions which were blocked by hexamethonium (5 x 10(-4) M) or by tetrodotoxin (TTX; 10(-6) M). Cromakalim (10(-5) M) caused a two fold rightward shift of the frequency-response curve. 3. In carinal trachea treated with propranolol and indomethacin, transmural stimulation evoked an initial, rapid contraction followed by a more sustained secondary contraction. The initial, rapid contractile response was virtually ablated by atropine (10(-6) M) or by TTX but was resistant to hexamethonium. Cromakalim (10(-8)-10(-5) M) caused a concentration-dependent rightward shift of the frequency-response curve for the initial contraction. 4. In carinal trachea treated with atropine, propranolol and indomethacin, transmural stimulation evoked only the secondary (non-adrenergic, non-cholinergic (NANC] contractile responses. These were markedly reduced by TTX but were resistant to hexamethonium. Cromakalim (10(-8)-10(-5) M) suppressed the NANC contractile responses in a concentration-dependent manner. This action could be offset by glibenclamide (10(-6) M). 5. In trachea treated with atropine, histamine (10(-4) M), propranolol and indomethacin, transmural stimulation evoked NANC relaxant responses. Cromakalim (up to 10(-5) M) was without effect on the frequency-response curve for the stimulation of NANC inhibitory nerves. 6. Tested on trachea bathed by drug-free Krebs solution, cromakalim (10(-7)-10(-5) M) caused concentration-dependent suppression of tracheal tone. In trachea treated with propranolol and indomethacin, cromakalim (10- 7-1O- 5 M) caused concentration-dependent antagonism of acetylcholine (ACh). In trachea treated with atropine, propranolol and indomethacin, cromakalim (up to 10- 5M) failed to antagonize effects of either histamine or substance P.7. It is concluded that cromakalim can inhibit cholinergic (excitatory) neuroeffector transmission in the trachea but only at a concentration having demonstrable inhibitory activity against the action of exogenous ACh and the spontaneous tone of the airways smooth muscle. In contrast, cromakalim may depress NANC excitatory (putative peptidergic) neuroeffector transmission at a concentration below that exerting inhibitory activity on airways smooth muscle. Cromakalim does not concurrently depress NANC inhibitory neuroeffector transmission. Depression of NANC excitatory neuroeffector transmission could explain the ability of cromakalim to suppress airway hyperreactivity or bronchial asthma at doses lacking direct relaxant effect on airways smooth muscle.

Effects of hyperosmolarity on human isolated central airways

1. We studied the effect of hyperosmolarity on human isolated airways because a better understanding of the effect of hyperosmolarity on the human airway wall may improve insight into the pathophysiology of hyperosmolarity-induced bronchoconstriction in asthma. 2. In cartilaginous bronchial rings dissected from fresh human lung tissue, hyperosmolar krebs-Henseleit buffer (450 mosM, extra sodium chloride added) evoked a biphasic response: a rapid relaxation phase (peak after 5.0 +/- 0.3 min) followed by a slow contraction phase (peak after 25.4 +/- 0.8 min). 3. With the histamine (H1) receptor antagonist mepyramine, the contraction phase was reduced to 41.2% of the control value (P less than 0.001), with atropine to 50.0% (P less than 0.01), with the local anaesthetic lignocaine to 48.7% (P less than 0.05) and with mepyramine together with atropine to 19.2% (P less than 0.001). 4. With the inhibitor of neutral metalloendopeptidase, phosphoramidon, the contraction phase increased to 128.0% of the control value (P less than 0.05) and after removal of the epithelium to 131.8% (P less than 0.05). 5. Indomethacin, the leukotriene C4/D4 (LTC4/D4) antagonist FPL 55712 or the blocker of nerve conduction, tetrodotoxin, had no effect on the contractile phase. 6. The relaxation phase was not altered by any of these drugs nor by epithelial denudation. The relaxation phase was also unchanged in the presence of alpha-chymotrypsin, which degrades muscle relaxing peptides such as vasoactive intestinal peptide. 7. Hyperosmolar buffer slightly increased the sensitivity and maximal response to methacholine as well as the cholinergic twitch to electric field stimulation. 8. We conclude that hyperosmolarity releases acetylcholine, histamine and neuropeptides in the human airway wall in sufficient quantities to contract airway smooth muscle. This release itself or its effect on airway muscle is modulated by the airway epithelium. The mechanism of the relaxation phase may be an unknown smooth muscle relaxing substance or a direct effect on the airway muscle, related to ion fluxes.

The release of a non-prostanoid inhibitory factor from rabbit bronchus detected by co-axial bioassay

1. Methacholine relaxed phenylephrine-contracted aorta of the rat with the endothelium intact. This effect was inhibited by haemoglobin, methylene blue, gossypol, phenidone and L-NG-nitroarginine methyl ester (L-NAME). Rat aorta denuded of endothelium failed to relax in response to methacholine, histamine and the peptidoleukotrienes C4, D4 and E4. 2. Methacholine and histamine but not leukotrienes C4, D4 and E4 relaxed phenylephrine-contracted rat aorta without endothelium when surrounded by rabbit epithelium-intact bronchus. The muscarinic antagonist atropine antagonized the methacholine-induced relaxation. 3. Removal of the epithelium either mechanically or chemically, abolished methacholine-induced relaxation of rat aorta in the co-axial bioassay. These data indicate that the epithelium is responsible for the observed relaxant effect to methacholine and histamine. 4. The cyclo-oxygenase inhibitor, indomethacin, the phospholipase A2 inhibitor, mepacrine and the lipoxygenase inhibitor, nordihydroguaiaretic acid (NDGA), failed to inhibit methacholine-induced relaxation of rat aorta in the co-axial bioassay. This indicates that the epithelium-derived inhibitory factor (EpDIF) is not a product of the cyclo-oxygenase or lipoxygenase pathway or a product derived from activation of phospholipase A2. 5. Haemoglobin, methylene blue, phenidone, gossypol and L-NAME failed to inhibit the relaxation of rat aorta in the co-axial bioassay. These results demonstrate that EpDIF detected in the co-axial bioassay is not endothelium-derived relaxing factor (EDRF) or nitric oxide. Similarly, catalase was without effect. 6. EpDIF is unlikely to be a peptide since papain and alpha-chymotrypsin failed to alter the methacholine-induced relaxation of rat aorta in the co-axial bioassay. Furthermore, thiorphan, captopril and aprotinin were also without effect, suggesting that EpDIF is not a substrate for airway peptidases. 7. The results presented in this paper demonstrate the release of a vasoactive epithelium-derived inhibitory factor (EpDIF) from rabbit intrapulmonary bronchi by use of a co-axial bioassay preparation.

Modulation of neural bronchoconstrictor responses in the guinea pig respiratory tract by vasoactive intestinal peptide

Vasoactive intestinal peptide (VIP) is localised to cholinergic nerves in airways. We have investigated the effects of VIP on both cholinergic and non-adrenergic, non-cholinergic (NANC) neuronal bronchoconstrictor responses to electrical field stimulation (EFS) in guinea pig airways and on cholinergic neurotransmission following sensory nerve depletion. VIP significantly attenuated the cholinergic bronchoconstrictor responses to EFS in trachea (EC50 values in upper and lower trachea of 3.7 +/- 0.2 x 10(-9) M and 8.6 +/- 0.3 x 10(-9) M, respectively) and bronchi (31.2 +/- 1.6% inhibition in main and 15.1 +/- 3.3% in hilar bronchi at 10(-7) M VIP) and the NANC bronchoconstrictor responses to EFS in bronchi (with maximum inhibitions of 93.1 +/- 1.8% at 3 x 10(-8) M VIP in main and 40.2 +/- 5.3% at 10(-8) M in hilar bronchi). VIP at 10(-7) M, but not at 10(-10) M, significantly attenuated the contractile responses to exogenously applied ACh in trachea (EC50 values of 4.9 +/- 0.2 x 10(-6) M in the absence and 8.4 +/- 0.4 x 10(-5) M in the presence of VIP 10(-7) M VIP) to SP in main bronchi (EC50 values of 5.7 +/- 0.2 x 10(-8) M in the absence vs. 7.3 +/- 0.3 x 10(-7) M in the presence of 10(-7) M VIP). Since the inhibition of these neural responses is greater than the inhibition of the equivalent responses elicited by the exogenous transmitters, this indicates that VIP may modulate release of acetylcholine from cholinergic nerves and of neuropeptides from sensory nerves, in addition to a post-junctional functional antagonist action.

Evidence that part of the NANC relaxant response of guinea-pig trachea to electrical field stimulation is mediated by nitric oxide

1. The nitric oxide (NO) synthesis inhibitors NG-monomethyl L-arginine (L-NMMA) and L-nitroarginine methyl ester (L-NAME) reduced relaxations of guinea-pig tracheal smooth muscle elicited by stimulation of intramural non-adrenergic, non-cholinergic (NANC) nerves, but D-NMMA had no effect. L-NAME was 10-30 times more potent than L-NMMA. Relaxations produced by sodium nitroprusside and vasoactive intestinal polypeptide (VIP) were not affected by L-NMMA or L-NAME. 2. The inhibitory effect of L-NMMA on NANC-mediated relaxations was partially reversed by L-arginine but was not affected by D-arginine. 3. VIP antibody and alpha-chymotrypsin abolished or greatly reduced the relaxant action of VIP and reduced relaxations elicited by stimulation of NANC nerves; the residual NANC relaxation was further reduced by L-NAME. 4. The results suggest that NO and VIP are mediators of NANC-induced relaxations of guinea-pig tracheal smooth muscle. We propose the term 'nitrergic' to describe transmission processes which are mediated by NO.

Modulation of cholinergic neurotransmission by the peptide VIP, VIP antiserum and VIP antagonists in dog and cat trachea

1. Comparative studies on the effects of vasoactive intestinal polypeptide (VIP), commercially available VIP antiserum or VIP antagonists [Ac-Tyr1, D-Phe2]-GRF(1-29)-NH2 and [4-Cl-D-Phe6, Leu17]-VIP on excitatory neuroeffector transmission in the dog and cat trachea were performed with microelectrode, double sucrose-gap, and tension recording methods. 2. VIP (10(-11)-10(-9) M) had no effect on the resting membrane potential or on the input resistance of the smooth muscle cells of dog and cat trachea. However, with increased concentrations (greater than 10(-8) M) VIP hyperpolarized the membrane and decreased the input resistance of the membrane in both tissues. 3. VIP (10(-10)-10(-7) M) dose-dependently reduced the amplitude of the contractions evoked through the nervous structure excited by field stimulation in the combined presence of indomethacin (10(-5) M) and guanethidine (10(-6) M) in the dog, and in the presence of guanethidine (10(-6) M) in cat trachea. In parallel with actions on twitch contractions, VIP (10(-11)-10(-7) M) reduced the amplitude of the excitatory junction potentials (EJPs) evoked through the nervous structure excited by single pulse field stimulation in both tissues. 4. VIP (10(-9) M) had no effect on the post-junctional response of smooth muscle cells to exogenous acetylcholine (ACh) (10(-9)-10(-5) M). 5. During repetitive field stimulation at the stimulus frequency of 0.033-0.1 Hz, the amplitude of the EJPs was gradually reduced, and VIP (10(-9) M) enhanced this depression phenomenon in the dog and cat trachea. 6. EJPs also showed summation when repetitive field stimulation was applied at high frequency (20 Hz) in the dog trachea. The slope of the relationship between the relative amplitude of the EJP and number of stimuli at 20 Hz was 2.2 +/- 0.4 mV/stimulation (n = 4) in the dog trachea. However, in the cat trachea, summation of EJPs was not prominent, giving a mean slope of 0.6 +/- 0.2 mV/stimulation (n = 6) measured by the microelectrode method. VIP (10(-9) M) shifted downward the relationship between the relative amplitude of the EJP and the number of stimuli at 20 Hz in both tissues. 7. Overnight incubation with VIP antiserum (10(-6) g/ml) had little effect on the depression of the EJP in the dog and cat trachea, or the summation of the EJP observed in the dog trachea.(ABSTRACT TRUNCATED AT 400 WORDS)

L-NG-nitro arginine inhibits non-adrenergic, non-cholinergic relaxations of guinea-pig isolated tracheal smooth muscle

The effects of L-NG-nitro arginine (L-NOARG) on alpha-chymotrypsin-resistant, non-adrenergic, non-cholinergic (NANC) relaxations of guinea-pig tracheal smooth muscle have been examined. L-NOARG (1-100 microM), but not D-NOARG (100 microM), inhibited the NANC relaxations in a concentration-related manner. The effects of L-NOARG were partially reversed by L-arginine but not D-arginine. L-NOARG was without effect on acetylcholine-induced contractile responses of the trachea or on relaxations produced by vasoactive intestinal peptide, sodium nitroprusside or isoprenaline. These results suggest that an endogenous nitrate may contribute to NANC relaxations of tracheal smooth muscle.

Effects of epithelium removal on relaxation of airway smooth muscle induced by vasoactive intestinal peptide and electrical field stimulation

1. We have studied the effect of epithelium removal on relaxation of guinea-pig isolated tracheal smooth muscle induced by vasoactive intestinal peptide (VIP) or stimulation of non-adrenergic, non-cholinergic (NANC) inhibitory nerves. Also examined were the effects of inhibitors of neutral endopeptidase (NEP) and angiotensin-converting enzyme (ACE). 2. Epithelium removal produced a 3.6 +/- 0.4 fold leftward shift in the VIP concentration-response curve. The supersensitivity to VIP, following epithelium removal was abolished by phosphoramidon or thiorphan (NEP inhibitors), but unaffected by captopril (an ACE inhibitor). In intact trachea, the NEP inhibitors produced leftward shifts in the VIP curves similar to those produced by epithelium removal. 3. In contrast to responses to exogenous VIP, neurogenic NANC inhibitory responses to electrical field stimulation were affected neither by epithelial denudation nor by the peptidase inhibitors. 4. As in previous studies, epithelium removal increased tracheal sensitivity to isoprenaline. This was not altered by pretreatment with a cocktail of peptidase inhibitors. Thus, the effect of the NEP inhibitors on responses to VIP appears to be relatively specific. 5. These data indicate that exogenous VIP is a substrate for airway NEP, since inhibition of the enzyme potentiates the peptide. This is further evidence that the airway epithelium provides a source for the metabolism of mediators. 6. In guinea-pig trachea the NEP responsible for cleaving VIP may be located largely in the epithelial layer, since NEP inhibition was without effect on sensitivity to VIP in epithelium-denuded preparations. If VIP is a NANC inhibitory neurotransmitter in this tissue its degradation endogenously does not appear to involve epithelial NEP.

Effects of peptidases on non-adrenergic, non-cholinergic inhibitory responses of tracheal smooth muscle: a comparison with effects on VIP- and PHI-induced relaxation

1. The effects of peptidase enzymes on non-adrenergic, non-cholinergic (NANC) inhibitory responses of guinea-pig trachea to electrical field stimulation (EFS), and on relaxations induced by vasoactive intestinal peptide (VIP) and peptide histidine isoleucine (PHI) have been examined. 2. alpha-Chymotrypsin reduced both the magnitude and, particularly, the duration of the inhibitory response to EFS, whereas papain reduced only the magnitude. Aprotinin, a peptidase inhibitor prevented the effects of alpha-chymotrypsin but was without effect on papain. 3. alpha-Chymotrypsin and papain both abolished relaxant responses to exogenous VIP and PHI. The action of alpha-chymotrypsin was prevented by aprotinin, whereas that of papain was not affected. 4. The peptidases were without effect on concentration-response curves to methacholine or to isoprenaline. It was also observed that, in the absence of the peptidases, aprotinin had no effect on inhibitory responses either to EFS or to exogenous VIP and PHI. 5. It is suggested that neuropeptides, possibly VIP and PHI, released during EFS of guinea-pig trachea, partly mediate NANC relaxations, and that their action may be inhibited by peptidases. However, the lack of effect of aprotinin alone, on responses to EFS, suggests that, if endogenous peptidases are important in terminating the action of neuropeptides, they are resistant to the effect of this particular peptidase inhibitor. It is further suggested that neurogenic relaxation of guinea-pig trachea is also partly mediated by a substance, possibly non-peptide, other than VIP or PHI.

Modulation of cholinergic neurotransmission by vasoactive intestinal peptide and peptide histidine isoleucine in guinea-pig tracheal smooth muscle

There is increasing evidence that vasoactive intestinal peptide (VIP) and peptide histidine isoleucine (PHI) are non-adrenergic, non-cholinergic (NANC) inhibitory neurotransmitters in airway smooth muscle. The possibility that VIP and PHI may also have neuromodulatory effects on excitatory responses, mediated by cholinergic nerves, to electrical field stimulation (EFS) was studied in guinea-pig isolated trachea. VIP (0.5 nM) pre-junctionally, inhibited the release of acetylcholine (ACh), whereas post-junctionally, responses to methacholine (MCh) were enhanced. At a maximum relaxant concentration (100 nM), VIP inhibited cholinergic neurotransmission both pre- and post-junctionally. Similarly, PHI (30 nM) inhibited neuronal ACh release, but enhanced transmitter action post-junctionally. At 3 microM, PHI inhibited ACh release. VIP- and PHI-induced inhibition of EFS was not affected by methysergide, pyrilamine, naloxone, phentolamine and propranolol. These data suggest that, in airway smooth muscle VIP and PHI may modulate cholinergic transmission via specific receptors.