Pharmacological characterization of the autonomous innervation of the guinea pig tracheobronchial smooth muscle

The effects of nociceptin peptide (N/OFQ)-receptor (NOP) system activation in the airways

The heptadecapeptide nociceptin/orphanin FQ (N/OFQ) is the endogenous ligand for the N/OFQ peptide (NOP) receptor. It is cleaved from a larger precursor identified as prepronociceptin (ppN/OFQ). NOP is a member of the seven transmembrane-spanning G-protein coupled receptor (GPCR) family. ppN/OFQ and NOP receptors are widely distributed in different human tissues. Asthma is a complex heterogeneous disease characterized by variable airflow obstruction, bronchial hyper-responsiveness and chronic airway inflammation. Limited therapeutic effectiveness of currently available asthma therapies warrants identification of new drug compounds. Evidence from animal studies suggests that N/OFQ modulates airway contraction and inflammation. Interestingly up regulation of the N/OFQ-NOP system reduces airway hyper-responsiveness. In contrast, inflammatory cells central to the inflammatory response in asthma may be both sources of N/OFQ and respond to NOP activation. Hence paradoxical dysregulation of the N/OFQ-NOP system may potentially play an important role in regulating airway inflammation and airway tone. To date there is no data on N/OFQ-NOP expression in the human airways. Therefore, the potential role of N/OFQ-NOP system in asthma is unknown. This review focuses on its physiological effects within airways and potential value as a novel asthma therapy.

Neurokinin3 receptor regulation of the airways

Neurokinin(3) (NK(3)) receptors may regulate the airways primarily through actions on the nerves. In the periphery, airway parasympathetic ganglia neurons are depolarized following NK(3) receptor activation resulting subsequently in the facilitation of synaptic transmission. Such an effect may account for the excessive parasympathetic reflex effects (e.g. airway smooth muscle contraction, vascular engorgement, mucus secretion) associated with asthma and chronic obstructive pulmonary disease (COPD). In the central nervous system (CNS), NK(3) receptor activation may regulate airway vagal afferent relay neurons, rendering them hyperresponsive to parallel inputs from glutamate containing afferent nerves. This process is analogous to the process of central sensitization regulating hyperalgesia and pain in somatic tissues. In both the CNS and in the airways, NK(3) receptors are likely activated by either substance P and/or neurokinin A (NKA), both of which are full agonists at NK(3) receptors, as there is little evidence that airway nerves express neurokinin B (NKB). Evidence for other potential sites of regulation by NK(3) receptors in the airways (e.g. vasculature, airway smooth muscle, epithelium, mucus glands) is either inconclusive or conflicting.

Influence of maturation on constrictive response to stimulation of C-fiber afferents in isolated guinea pig airways

We investigated whether the airway constrictive response to stimulation of bronchopulmonary C-fiber afferents is altered during the maturation process. Isometric tension was measured in airway rings isolated from three tracheobronchial locations (intrathoracic trachea and main and hilar bronchi) and compared in mature [M, 407 ± 10 (SE) g body wt, n = 36] and immature (IM, 161 ± 5 g body wt, n = 35) guinea pigs. Our results showed no difference in the ACh (10−5 M)- or KCl (40 mM)-induced contraction between M and IM groups, regardless of the airway location. In sharp contrast, the concentration-response curves of 10−8–10−6 M capsaicin were distinctly lower in IM hilar bronchi; for example, response to the same concentration of capsaicin (10−6 M) was 89.2 ± 15.3% of the response to 10−5 M ACh in IM and 284.7 ± 43.2% in M animals. Similar, but smaller, differences in the bronchoconstrictive response to capsaicin between IM and M groups were also observed in the trachea and main bronchus. Electrical field stimulation induced airway constriction in all three locations in M and IM groups. However, after administration of 10−6 M atropine and 10−6 M propranolol, electrical field stimulation-induced contraction was significantly smaller in the hilar bronchus of IM than M animals, and this difference was not prevented by pretreatment with 5 × 10−5 M indomethacin. Although radioimmunoassay showed no difference in the tissue content of substance P between M and IM airways, the constrictive responses to exogenous substance P and neurokinin A were markedly greater in M airways at all three locations. In conclusion, the constriction of isolated airways evoked by C-fiber stimulation was significantly weaker in the IM guinea pigs, probably because of a less potent effect of tachykinins on the airway smooth muscle.

An indomethacin-sensitive contraction induced by β-antagonists in guinea pig airways

β-adrenergic receptor (β-AR) antagonists have been associated with increased airway reactivity in asthmatics and potentiation of contractile stimuli in animal models. In the present study, using an in vitro model of tracheal preparations from guinea pigs, we show that the β-AR antagonists propranolol and pindolol induce a smooth muscle contraction. A prerequisite for this contraction is that the airway preparations have been pre-treated with an β-AR agonist. Our data show that the contractile effect of β-AR antagonists is not a simple consequence of reversing the agonist-induced relaxation. Furthermore, the effect seems to be mediated through interaction with β2-ARs since the response is stereo-selective, and the selective β1-AR receptor antagonist atenolol did not induce any contractile response. SQ 29,546, a thromboxane A2 antagonist; MK 886, a lipoxygenase inhibitor; and indomethacin, a cyclooxygenase inhibitor significantly inhibited the contractions of the tracheal preparations induced with propranolol or pindolol. We put forward the hypothesis that the contractile effect of the β-AR antagonist is a consequence of their inverse agonist activity, which is only evident when the receptor population have a higher basal activity. Our results indicate a novel additional explanation for the known side effect, bronchoconstriction, of β-AR antagonist.Key words: beta antagonist, guinea pig trachea, propranolol, formoterol, pindolol, indomethacin.

Pharmacological, pharmacokinetic and clinical properties of olopatadine hydrochloride, a new antiallergic drug

Olopatadine hydrochloride (olopatadine, 11-[(Z)-3-(dimethylamino)propylidene]-6,11-dihydrodibenz[b,e]oxepin-2-acetic acid monohydrochloride) is a novel antiallergic/histamine H1-receptor antagonistic drug that was synthesized and evaluated in our laboratories. Oral administration of olopatadine at doses of 0.03 mg/kg or higher inhibited the symptoms of experimental allergic skin responses, rhinoconjunctivitis and bronchial asthma in sensitized guinea pigs and rats. Olopatadine is a selective histamine H1-receptor antagonist possessing inhibitory effects on the release of inflammatory lipid mediators such as leukotriene and thromboxane from human polymorphonuclear leukocytes and eosinophils. Olopatadine also inhibited the tachykininergic contraction in the guinea pig bronchi by prejunctional inhibition of peripheral sensory nerves. Olopatadine exerted no significant effects on action potential duration in isolated guinea pig ventricular myocytes, myocardium and human ether-a-go-go-related gene channel. Olopatadine was highly and rapidly absorbed in healthy human volunteers. The urinary excretion of olopatadine accounted for not less than 58% and the contribution of metabolism was considerably low in the clearance of olopatadine in humans. Olopatadine is one of the few renal clearance drugs in antiallergic drugs. Olopatadine was shown to be useful for the treatment of allergic rhinitis and chronic urticaria in double-blind clinical trials. Olopatadine was approved in Japan for the treatment of allergic rhinitis, chronic urticaria, eczema dermatitis, prurigo, pruritus cutaneous, psoriasis vulgaris and erythema exsudativum multiforme in December, 2000. Ophthalmic solution of olopatadine was also approved in the United States for the treatment of seasonal allergic conjunctivitis in December, 1996 (Appendix: also in the European Union, it was approved in February 2002).

Glutamate mediated responses in isolated trachea preparations from control and ovalbumin sensitized guinea-pigs

We investigated whether the glutamergic system plays a role in isolated trachea from control and ovalbumin-sensitized guinea-pigs. Electrical field stimulation induced contractile responses in control group, but electrical field stimulation produced relaxation responses in ovalbumin-challenged guinea-pigs. The responses induced by electrical field stimulation in both groups were completely abolished by tetrodotoxin, but unaffected by hexamethonium. DL-2-amino-5-phosphono-valeric acid (D-AP5) caused a concentration-dependent statistically significant inhibition in the contractile responses to electrical field stimulation50 (EFS50) in control guinea-pigs. But in the ovalbumin-challenged groups, D-AP5 did not cause any significant effect on the relaxation response to frequency of field stimulation (EFS50). N(G)-monmethyl-L-argine caused a significant inhibition in the relaxation effect of EFS50. L- and D-glutamate and N-methyl-D-aspartic acid (NMDA) alone had no effect on the resting tension on the trachea in both groups. Carbachol produced concentration-dependent contractile responses in ovalbumin-challenged groups. These results suggested that responses to electrical field stimulation in control groups might be due to NMDA receptor-mediated release of any substance on prejunctional neurones and, alternatively, NMDA might exert a modulatory effect on any substance at prejunctional level. Also, responses to electrical field stimulation in ovalbumin-challenged guinea-pigs might not be mediated by NMDA but rather by increasing the production of nitric oxide by inducible nitric oxide synthase.

Nociceptin effects in the airways

The opioid-like heptadecapeptide nociceptin (NC) has the following effects in the airways (investigated in isolated tracheae and bronchi from guinea pig or rat): the electric field stimulation (EFS)-induces release of acetylcholine (ACh), the tachykinin substance P (SP) and calcitonin gene-related peptide (CGRP) is reduced after pretreatment with NC, and EFS-induced tachykinergic nonadrenergic-noncholinergic (NANC) bronchoconstriction is inhibited by NC. Both the NC-mediated inhibition of neurotransmission and of smooth muscle contraction occurred in a concentration-dependent manner. Because these effects were naloxone-insensitive, were blocked by the NC receptor antagonist [F/G]NC(1-13)NH(2), and could be mimicked by the NC analogs, NCNH(2) and NC(1-13)NH(2), it is thought that they are distinct from the classic opioid receptors. That these pharmacological actions of NC are of relevance for airway physiology is highly probable given the presence of NC-immunoreactivity in the nerve fibers of the airways and of opioid-like receptor (ORL-1) transcripts in the jugular ganglia, from where the tachykinin-containing afferents arise.

Tachykinin NK(2) receptors facilitate acetylcholine release from guinea-pig isolated trachea

The release of newly synthesised [3H]acetylcholine was evoked by electrical field stimulation (5 Hz, 600 pulses) of epithelium-deprived guinea-pig trachea strips after sensory neuropeptides depletion with 3 microM capsaicin. The selective tachykinin NK(2) receptor agonist [betaAla(8)]neurokinin A-(4-10) increased in a concentration-dependent manner the electrically-induced release of [3H]acetylcholine. The facilitatory effect was antagonised by the selective non-peptide tachykinin NK(2) receptor antagonist, SR 48968 (apparent pK(B) 8.9). The tachykinin NK(1) and NK(3) receptor agonists substance P methyl ester and senktide (both 10 and 100 nM), respectively, did not affect the evoked release of [3H]acetylcholine. It is concluded that the cholinergic nerves of guinea-pig trachea are endowed with prejunctional facilitatory tachykinin receptors of the NK(2) subtype.

Effect of experimental diabetes on GABA-mediated inhibition of neurally induced contractions in rat isolated trachea

1. In the present study, we investigated the effect of GABA and selective GABA agonists and antagonists on neurally induced tracheal contractions in streptozotocin (STZ) diabetic rats. 2. Contractile responses to electrical field stimulation (EFS) in rat tracheal rings were completely abolished by atropine and tetrodotoxin, but were unaffected by the ganglion blocker hexamethonium, indicating that they were mediated via neuronal release of acetylcholine (ACh). 3. Contractions induced by EFS, but not by exogenous ACh, were inhibited by GABA and the selective GABA(B) receptor agonist baclofen, but not by the selective GABA(A) receptor agonist 3-aminopropane sulphonic acid. The inhibitory effects of GABA or baclofen were not affected by the GABA(A) antagonist bicuculline, but were significantly reversed by the GABA(B) antagonist phaclofen. 4. The inhibitory effects of both GABA and baclofen were found to be significantly greater in trachea from control rats compared with tissues from diabetic rats. 5. Non-adrenergic, non-cholinergic relaxation responses elicited by EFS in precontracted tracheal rings from diabetic and control rats were similar in magnitude and were unaffected by GABA or GABA analogues. 6. These results suggest that GABA decreases the response to EFS by directly inhibiting the evoked release of ACh through GABA(B) receptors in rat trachea and that STZ-induced diabetes causes an impairment in the inhibitory effect of GABA on neurally induced contractions in this tissue.

Central control of the cardiovascular and respiratory systems and their interactions in vertebrates

This review explores the fundamental neuranatomical and functional bases for integration of the respiratory and cardiovascular systems in vertebrates and traces their evolution through the vertebrate groups, from primarily water-breathing fish and larval amphibians to facultative air-breathers such as lungfish and some adult amphibians and finally obligate air-breathers among the reptiles, birds, and mammals. A comparative account of respiratory rhythm generation leads to consideration of the changing roles in cardiorespiratory integration for central and peripheral chemoreceptors and mechanoreceptors and their central projections. We review evidence of a developing role in the control of cardiorespiratory interactions for the partial relocation from the dorsal motor nucleus of the vagus into the nucleus ambiguus of vagal preganglionic neurons, and in particular those innervating the heart, and for the existence of a functional topography of specific groups of sympathetic preganglionic neurons in the spinal cord. Finally, we consider the mechanisms generating temporal modulation of heart rate, vasomotor tone, and control of the airways in mammals; cardiorespiratory synchrony in fish; and integration of the cardiorespiratory system during intermittent breathing in amphibians, reptiles, and diving birds. Concluding comments suggest areas for further productive research.

A comparison of sensory nerve function in human, guinea-pig, rabbit and marmoset airways

We have investigated the role of sensory nerves in regulating airway smooth muscle function in the guinea-pig, marmoset, rabbit and man. Tissue levels of the sensory neuropeptides CGRP and substance P in the airways of the guinea-pig were significantly greater compared with the rabbit and marmoset. The relative order of tissue content was guinea-pig >>> rabbit = marmoset. Marmoset bronchial and tracheal preparations responded weakly to exogenously administered substance P and neurokinin A but contracted to methacholine and demonstrated atropine-sensitive cholinergic responses. In marmoset, rabbit and human airway preparations, capsaicin mediated weak contractile responses to exogenously administered capsaicin. However, high concentrations of capsaicin elicited a relaxation response that was epithelium-independent, cyclo-oxygenase-insensitive, not involving nitric oxide and not dependent on the activation of capsaicin-sensitive afferents. These results suggest that rabbit and marmoset airways respond functionally in a similar way to human airway preparations and maybe more relevant than guinea-pig airways with regard to understanding the role of sensory neuropeptides in airways.

The effects of 8-hydroxy-2-(di-n-propylamino)tetralin on the cholinergic contraction in guinea pig and human airways in vitro

Electrical field stimulation of guinea pig tracheal strips and human bronchial rings, in vitro, evokes a cholinergic contraction mediated by the release of acetylcholine. 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) is a 5-HT1A and 5-HT7 agonist. In this study, we have investigated whether 8-OH-DPAT could modulate the cholinergic contraction in guinea pig and human airways in vitro. 8-OH-DPAT (1 to 30 microM) produced a concentration-dependent inhibition of the cholinergic contraction in guinea pig tracheal strips with a maximal inhibition of 75.8% +/- 4. 7% (30 microM, 0.5 Hz). Pretreatment of the tissues with the 5- HT1/2/7 antagonist methysergide (10 to 30 microM) significantly attenuated the inhibitory effects of 8-OH-DPAT (10 to 30 microM) on the cholinergic contraction. Pretreatment with ketanserin (10 microM), a 5-HT2 antagonist, tropisetron (1 microM), a 5-HT3/4 antagonist, SDZ 216-525 (1 to 10 microM) and pindobind (10 microM), both selective 5-HT1A antagonists, or capsaicin (10 microM), which depletes sensory nerves from neuropeptides, had no effect on the inhibition of the cholinergic contraction by 8-OH-DPAT (10 to 30 microM). 5-carboxamidotryptamine (5-CT) (10 to 100 microM), a 5-HT1/2/7 agonist, partially mimicked the inhibitory effects of 8-OH-DPAT on the cholinergic contraction. 8-OH-DPAT (10 to 30 microM) also inhibited the cholinergic contraction in human bronchial rings in vitro with a maximal inhibition of 46.2% +/- 7.2% (30 microM, 1 Hz). SDZ 216-525 (10 microM) had no effect, whereas methysergide (30 microM) partially prevented the effect of 8-OH-DPAT in human airways. 8-OH-DPAT (30 microM) did not displace the concentration-response curve to acetylcholine (10 nM-30 mM) in guinea pig and human airways in vitro. These results suggest that 8-OH-DPAT inhibits the cholinergic contraction in guinea pig and human airways in vitro through stimulation of prejunctional atypical 5-HT receptors, possibly of the 5-HT7 subtype, located on postganglionic cholinergic nerves.

Neurokinin A-LI release after antigen challenge in guinea-pig bronchial tubes: influence of histamine and bradykinin

1. Our aim was to determine if antigen challenge stimulates sensory nerves and provokes the release of tachykinins. The involvement of histamine and bradykinin was studied by using specific receptor antagonists. Capsaicin-induced responses were also examined. Experiments were performed in vitro on tracheal and bronchial preparations from ovalbumin-sensitized guinea-pigs. 2. Characterization of ovalbumin-induced contraction, with regard to histamine and bradykinin, was carried out on airway ring preparations in the presence of phosphoramidon. The histamine H1 receptor antagonist pyrilamine reduced allergen-induced bronchial contractions by about 30%, whereas the bradykinin B2 receptor antagonist icatibant (Hoe 140) did not significantly affect the response. Combined treatment with pyrilamine (1 microM) and icatibant (0.1 microM) reduced the contractions by about 80%, indicating a synergistic inhibitory action. Tracheal preparations were not significantly affected by treatments, neither were capsaicin-induced contractions. 3. To study the outflow of tachykinins, we used a perfused bronchial-tube preparation, allowing simultaneous measurement of smooth muscle tension and mediator release. Neurokinin A-like immunoreactivity (NKA-LI) and substance P-like immunoreactivity (SP-LI) were determined by radioimmunoassay. 4. The results of the perfusion study showed an increased outflow of NKA-LI into the perfusate in response to ovalbumin (127% of basal) challenge. SP-LI determined in some of the samples showed a much lower amount (40 to 70 times lower) of SP-LI than NKA-LI. Treatment with icatibant and pyrilamine, separately and in combination, significantly reduced the ovalbumin-induced NKA-LI outflow by 38%, 26% and 22%, respectively. 5. Capsaicin-induced outflow (124% of basal) was not significantly affected by treatments (icatibant 121%, pyrilamine 107% and combined treatment 111% of basal). However, when pyrilamine was present the increased outflow was not statistically significant. 6. In conclusion, we found that allergen provocation of guinea-pig bronchi caused an increased outflow of NKA-LI that was reduced by treatment with both pyrilamine and icatibant. These findings demonstrate that the allergen-induced release of histamine and bradykinin stimulate sensory nerves and thereby increase outflow of tachykinins that contribute to the allergic reaction.

Tachykinin receptors mediating non-cholinergic contraction of the guinea-pig isolated main bronchus in response to field stimulation

1. The primary aim of the present study was to classify the receptors activated by tachykinins released by field stimulation of intramural nerve terminals of the guinea-pig isolated main bronchus by using the novel non-peptide NK1 and NK2 receptor-selective antagonists SR 140333 and SR 48968, respectively. 2. Log concentration-response curves to substance P (SP), the NK1 receptor-selective agonist [Sar9, Met(O2)11]-SP and the NK2 receptor-selective agonist [Nle10]-neurokinin (NK) A(4-10) were constructed in the presence of indomethacin (2 mumol/L) and phosphoramidon (5 mumol/L). Substance P was the least potent of these agonists. 3. In left and right main bronchi, SR 140333 (100 nmol/L) antagonized concentration-related contractions evoked by SP yielding pKB values of 8.02 and 7.68, respectively. SR 140333 (10 nmol/L) antagonized the effects of [Sar9, Met(O2)11]-SP on the left bronchus with a pKB value of 8.04. 4. SR 48968 (100 nmol/L) antagonized the effects of SP yielding pKB estimates of 7.88 (left bronchus) and 7.31 (right bronchus). 5. [Nle10]-NKA(4-10) was more potent in the left than in the right main bronchus. SR 48968 (0.1-10 nmol/L) antagonized the effects of [Nle10]-NKA(4-10) on the left bronchus with pKB estimates of 8.26-10.25. 6. In the presence of indomethacin (2 mumol/L), phosphoramidon (5 mumol/L) and atropine (1 mumol/L), electrical field stimulation (EFS; 30 V, 1 ms, 15 s at 1, 3, 10 and 30 Hz) produced prolonged contractions. SR 48968 (0.1 mumol/L) markedly reduced responses to stimulation, whereas SR 140333 (0.3 mumol/L) caused a small but significant rightward displacement of the log frequency-response curve. In combination, these concentrations of SR 48968 and SR 140333 produced complete inhibition to field stimulation at 10 Hz. 7. These results indicate that: (i) both NK1 and NK2 receptor subtypes are present in the guinea-pig main bronchi with the left bronchus being more sensitive to an NK2 receptor agonist; and (ii) EFS of the main bronchus leads to frequency dependent contractions due to the release of tachykinin(s) that predominantly activate an NK2 receptor.

Enhancement of tachykinin-induced contractions of guinea pig isolated bronchus by corticotropin-releasing factor

The effect of corticotropin-releasing factor (CRF) on contractions of guinea pig isolated airways in response to electrical or chemical stimulation of tachykinergic nerve fibers was studied. CRF (1 microgram/ml) caused a 70% enhancement of the peak magnitude of the response to electrical field stimulation (EFS). CRF had a similar effect on contractions of the isolated bronchus evoked by capsaicin. CRF also potentiated contractions evoked by exogenously applied substance P. This effect was selective, as CRF has no effect on contractions evoked by neurokinin A or the substance P analog ASMSP. The potentiation of the substance P-induced contractions of airway smooth muscle was blocked by the CRF receptor antagonist alpha helical (9-41) CRF. These data support the hypothesis that CRF enhances the airway smooth muscle response to stimulation of tachykinin-containing nerve fibers and that this effect is due to a post-junctional mechanism of action.

Effects of the nitric oxide-donor, GEA 3175, on guinea-pig airways

This investigation characterized the smooth muscle relaxing effect of a novel nitric oxide (NO)-releasing substance, GEA 3175 (1,2,3,4-oxatriazolium, 3-(3-chloro-2-methylphenyl)-5-[[(4-methylphenyl)sulfonyl]amino], hydroxide inner salt) on guinea-pig trachea. GEA 3175 caused a concentration-dependent relaxation of tracheal smooth muscle precontracted with acetylcholine. This effect was reversed by both okadaic acid, an inhibitor of serine/threonine-specific phosphatases, and iberiotoxin, an inhibitor of Ca2+-activated K+ channels. Furthermore, GEA 3175 had a relaxation potency similar to that of the commonly used NO-donor, S-nitroso-N-acetyl-penicillamine. On the contractile response provoked by electrical field stimulation, GEA 3175 induced a long-lasting relaxation which persisted even after repeated washing. The relaxing effect of GEA 3175 was associated with rises in guanosine 3':5'-cyclic monophosphate (cGMP). In time course studies, cGMP continued to increase with incubation time after stimulation with GEA 3175 and there was a significant elevation of cGMP even after washing. In contrast, incubation with S-nitroso-N-acetyl-penicillamine caused a transient rise in cGMP. The present investigation showed that GEA 3175 evokes long-lasting effects on contractile responses and cGMP levels in guinea-pig trachea. Our results indicate that the relaxing effect of GEA 3175 occurs through a mechanism involving phosphatases and iberiotoxin-sensitive K+ channels.

Phosphodiesterase inhibitors and endothelin as modulators of respiratory neurotransmission

1. Airway smooth muscle receives cholinergic, adrenergic and non-adrenergic, non-cholinergic (NANC) neural input. In guinea-pig airways cholinergic and NANC nerves provide contractile innervation, while adrenergic and NANC nerves provide relaxant pathways. In contrast, the major relaxant innervation in human airways is NANC in nature. 2. The present review describes the effects of selective phosphodiesterase (PDE) inhibitors on NANC relaxant and contractile responses in guinea-pig trachea as well as on NANC relaxations in human bronchus. 3. The effects of endothelin-1 on cholinergic contractile responses obtained in a variety of species are also assessed.

NANC neural control of airway smooth muscle tone

1. This review addresses the functional role of the nonadrenergic, noncholinergic (NANC) neural response in the control of airway smooth muscle tone. 2. Functional data from guinea pig airways in vitro indicate that the level of basal smooth muscle tone determines the direction and magnitude of the NANC neural response such that it can stabilise tone. 3. The NANC stabilising effect on tone is adjustable through variation in impulse frequency and the NANC stabilising effect is also powerful; it can abolish near-maximum differences in tone. 4. Cholinergic activation increases the level towards which the NANC responses tend to adjust tone. 5. Adrenergic activation reduces the level towards which the NANC responses tend to adjust tone via beta-adrenoceptors. 6. NANC neural activation, with or without simultaneous adrenergic or cholinergic activation, can stabilise tone at low, intermediate or high levels with a high degree of accuracy. 7. Evidence from other investigators on effects of putative NANC neurotransmitters supports the idea of functional interactions within the NANC system in the airways. 8. It remains to be confirmed whether or not NANC responses play a stabilising role in the control of airway smooth muscle tone in vivo and in higher mammals.

Effects of vagal stimulation on slowly adapting pulmonary stretch receptors and lung mechanics in anesthetized rabbits

An in vivo preparation was designed to investigate the effect of vagus nerve stimulation-induced bronchoconstriction on the relationship of slowly adapting pulmonary stretch receptor (SAR) activity and lung mechanics. SAR activities were recorded from the left vagus nerve. The responses of SARs, total lung resistance (RL), and dynamic lung compliance (Cdyn) to electrical stimulation of the peripheral end of the cut right vagus nerve (10-15 V, 5-30 Hz, 0.2 ms) were examined before atropine and 5 and 10 min after atropine (2 mg/kg) in anesthetized, artificially ventilated, bilaterally vagotomized rabbits. In the time course profile during vagal stimulation, an increase in RL and a decrease in Cdyn occurred simultaneously, and these opposite changes were frequency dependent. The average responses of SAR activity, RL, and Cdyn to vagal stimulation became more pronounced as the frequencies of the stimulation were increased. The responses obtained during vagal stimulation (5-30 Hz) were blocked or diminished greatly by the administration of atropine. Repeated vagus nerve stimulation in the presence of atropine did not show any significant change in SAR activity and lung mechanics. These results suggest that changes of SAR activity, RL, and Cdyn induced by vagal stimulation occur as a result of smooth muscle contraction in the airways, which is mediated mainly by muscarinic receptor activation and which is not involved in the release of neurotransmitters to relax airway smooth muscle.

Neural mechanisms and axon reflexes in asthma. Where are we?

For many years, asthma has been classified as a "neural" disease, with an imbalance between constrictor and dilator nerves being responsible for the symptomatology. Although, nowadays, asthma is recognized as an inflammatory disorder of the airways, neural mechanisms remain very important; axon reflexes, in particular, have received a lot of attention in recent years. In this commentary, an overview is given on the innervation of the airways and its relevance in asthma, and potential new insights in airways innervation are discussed. In a second part, the role of axon reflexes is highlighted. Although neuropeptides such as substance P and neurokinin A are present in human airways, where they produce many of the features characteristic of asthma, and although there is an elevation of their content in induced sputum from asthmatics, there is still no clear direct evidence for the existence of operational axon reflexes in human airways. Most of the research focused on this subject is performed in guinea pigs, where such an axon reflex clearly operates in the airways. In these animals, different receptors have been identified on C-fiber endings, which, upon stimulation, cause inhibition of neuropeptide release. Some of these receptors have also been identified on human airway nerves. Therefore, it has been suggested that modulation of axon reflexes could be of potential benefit in asthma treatment. Indeed, some drugs (e.g. sodium cromoglycate, nedocromil sodium, and ketotifen), which have been demonstrated to partially inhibit neuropeptide release in guinea pig airways, have anti-inflammatory effects on neuropeptide release in guinea pig airways, do not seem to have any anti-inflammatory effects in human asthma. Other drugs, however, such as beta2-mimetics, which have a much more pronounced inhibitory effect in asthma. In conclusion, although there is a lot of indirect evidence for the existence of axon reflex mechanisms in human airways, most of the data now available are derived from animal studies. The key question of whether axon reflexes are operational in human airways remains unanswered. Hopefully, the near future will bring a solution to this enigma with the introduction of very potent tachykinin antagonists for the treatment of human asthma.

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.

KW-4679-induced inhibition of tachykininergic contraction in the guinea-pig bronchi by prejunctional inhibition of peripheral sensory nerves

1. Sensory mechanisms play an important role in the vagal regulation of tracheobronchial smooth muscle tone. We examined the effect of KW-4679, an anti-allergic drug, on guinea-pig tachykinin-mediated contractile responses induced by electrical field stimulation (EFS) in guinea-pig bronchial muscles. 2. EFS (8 Hz, 0.5 ms, 15 V, for 15 s) evoked biphasic contractile responses in the guinea-pig isolated main bronchus in the presence of 5 microM indomethacin. The contractions consisted of a fast phase of an atropine-sensitive transient contraction and a slow phase of a sustained contraction which was inhibited by a combination of the tachykinin NK1 receptor antagonist, (+/-)-CP-96,345 (1 microM) and the NK2 receptor antagonist, SR 48969 (0.1 microM). 3. KW-4679 preferentially inhibited the slow phase in a concentration-dependent manner by 43.2 +/- 7.7% at 10 microM, whereas the drug had no effect on the fast phase at concentrations up to 10 microM. KW-4679, at a concentration of 100 microM, inhibited not only the slow phase by 49.2 +/- 11.4%, but also the fast phase by 36.8 +/- 9.3% [corrected]. 4. KW-4679 (10 microM and 100 microM) did not affect the substance P-induced or neurokinin A-induced contraction. Against the acetylcholine-induced contractile responses, 100 microM KW-4679 had a marked effect producing a 10.2 fold shift to the right in the curve. 5. The inhibitory effect of KW-4679 (10 microM) on the slow phase contraction was not influenced by treatment with naloxone (100 nM), propranolol (1 microM), thioperamide (1 microM), saclofen (50 microM), yohimbine (1 microM), methiothepin (1 microM) or methysergide (1 microM). 6. The inhibitory effect of KW-4679 (10 microM) on the slow phase contraction was not influenced by treatment with intermediate or large conductance Ca(2+)-activated K+ channel blockers (charybdotoxin (10 nM) or iberiotoxin (10 nM)), but suppressed by treatment with small conductance Ca(2+)-activated K+ channel blockers, apamin (500 nM) or scyllatoxin (300 nM). Apamin or scyllatoxin per se did not influence the slow phase contractions. 7. The results suggest that KW-4679 preferentially inhibits the release of tachykinins from the bronchial sensory nerves through activation of small conductance Ca(2+)-activated K+ channels.

Evidence for serotonin involvement in the NANC excitatory neurotransmission in the catfish intestine

1. Four putative neurotransmitters (serotonin, substance P, ATP (alpha-beta-methylene-ATP), and vasoactive intestinal peptide, VIP) of the non-adrenergic non-cholinergic (NANC) innervation were examined for their role in the NANC excitatory neurotransmission in channel catfish intestine after adrenergic and cholinergic blockade. 2. VIP at concentrations ranging from 10(-12)M to 10(-4)M did not produce either a relaxant or a contractile response in these segments. 3. Serotonin, substance P and alpha-beta-methyl-ATP produced contractile responses in a dose-dependent manner. Their EC50 values were 5 x 10(-7)M, 5 x 10(-9)M and 5 x 10(-9)M and 5 x 10(-6)M, respectively. 4. Electrical field stimulation of the intestinal segments produced a predominant excitatory response after complete blockade of adrenergic and cholinergic divisions, suggesting a predominant NANC excitatory innervation. 5. Three types of serotonin receptor antagonists, namely methiothepin (predominantly a 5-HT1 antagonist), ketanserin (a selective 5-HT2 antagonist), methysergide and cyproheptadine (predominantly 5-HT2 blockers) and metoclopramide (a selective 5-HT3 blocker) were tested for their effectiveness against serotonin and EFS-induced contractions. Methiothepin, methysergide, cyproheptadine and metoclopramide produced significant blockade of the response to serotonin, whereas only methiothepin and cyproheptadine produced blockade of EFS-induced response. 6. Three agents tested for substance P blockade, namely spantide, 4-11 fragment of substance P, and methysergide (also a serotonin blocker), did not produce significant inhibition of the response to either substance P or EFS. 7. Suramin at a dose that blocked the ED50 concentration of ATP did not produce a significant blockade of the response to EFS suggesting that ATP-involvement in the NANC-e neurotransmission is unlikely. 8. This study confirmed the involvement of serotonin in the expression of non-adrenergic non-cholinergic excitatory response of the channel catfish intestine.

Influence of tolfenamic acid on airway contractility in guinea pigs

The effect of the NSAIDs tolfenamic acid and indomethacin was tested on acetylcholine-induced and antigen-induced contractions in guinea pig airways. Indomethacin potentiated antigen-induced contractions while tolfenamic acid showed dilatory properties. The effects of the drugs on acetylcholine-induced contractions showed no significant differences; indomethacin had a slightly increasing tendency while tolfenamic acid reduced the contractile response. These results indicate that tolfenamic acid does not have bronchoconstrictive properties, which is a common side effect of other NSAIDs in asthmatics.

Non-adrenergic non-cholinergic (NANC) excitatory response of the channel catfish intestine

1. Optimal parameters for electrical field stimulation (EFS) of catfish pyloric and middle intestinal segments were determined (15 Hz, 60 V) from a range of frequencies (5-45 Hz) and voltages (40-120 V) using a modified Magnus' method. Contractile responses were produced by EFS which were reproducible and showed no significant difference between the tissues. 2. The contractile cholinergic responses of the tissues to carbachol and acetylcholine (ACh) were blocked by atropine on an equimolar concentration, whereas, these responses were enhanced in the presence of neostigmine, and acetylcholinesterase inhibitor. 3. Adrenergic responses were examined with noradrenaline (NA). NA produced contraction of the segments only, at a concentration of 10(-4) M. Among the various adrenoceptors, beta-adrenoceptor stimulation produced a weak relaxation whereas, both alpha 1- and alpha 2-adrenoceptor stimulation produced contractions, of which alpha 2-induced contraction was of greater magnitude. The beta, alpha 1 and alpha 2 responses were blocked by their respective blocking agents propranolol, prazosin and yohimbine. 4. The autonomic components of the response to EFS were determined by using selected cholinergic and adrenergic antagonists separately or collectively. Cholinergic blockade with atropine did not produce a significant blockade of the EFS-induced response. Similarly, blockade of beta-adrenoceptors with propranolol did not modulate the contractile response to EFS to any significant level. Blockade by prazosin or yohimbine did not significantly change the contractile response to EFS. After a complete blockade of the adrenergic and cholinergic divisions, the intestinal segments still showed a contractile response to EFS which was not significantly different from the control response. This indicated the presence of a non-adrenergic non-cholinergic (NANC) response. 5. Tetrodotoxin, at 10(-6) M, significantly blocked the EFS-induced NANC response suggesting a neurogenic origin for the response. 6. The present study indicated that the EFS-induced response of the catfish intestinal segments is predominantly NANC-e in nature suggesting an important role for it in the regulation of intestinal motility.

Inhibition of excitatory non-adrenergic non-cholinergic bronchoconstriction in guinea-pig airways in vitro by activation of an atypical 5-HT receptor

1. The effect of 5-hydroxytryptamine (5-HT) was studied on excitatory neurally mediated non-adrenergic non-cholinergic (NANC) contractions evoked by electrical field stimulation (EFS) in guinea-pig isolated bronchi. 2. 5-HT (0.1-100 microM) produced a concentration-dependent inhibition of the excitatory NANC response with 50.9 +/- 5.0% (n = 5, P < 0.01) inhibition at 100 microM. This inhibition was not significantly affected by the 5-HT2 antagonist, ketanserin (1 microM) when inhibitions (+/- ketanserin) at each concentration of 5-HT were compared by unpaired t tests; however, this concentration appeared to produce a leftward shift (approximately 10 fold) of the 5-HT concentration-inhibition curve. Ketanserin (1 microM) was effective in blocking bronchoconstriction evoked by activation of 5-HT2A receptors on airway smooth muscle. In the presence of ketanserin (1 microM) 5-HT (100 microM) evoked an inhibition of 57.4 +/- 5.9% (n = 5, P < 0.01) with an EC50 of 0.57 microM. 3. Inhibition evoked by 5-HT (0.1-100 microM) was unaffected by the alpha-adrenoceptor antagonist phentolamine (1 microM), the beta 2-adrenoceptor antagonist, ICI 118551 (0.1 microM), the 5-HT1A/B antagonist, cyanopindolol (1 microM) or the 5-HT3/4 antagonist, ICS 205-930 (1 microM). 4. Methiothepin (0.1 microM) produced an insurmountable inhibition of the effect of 5-HT (0.1-100 microM), reducing the maximum inhibition produced by 5-HT (100 microM) to 30.2 +/- 5.0% (n = 5, P < 0.001) and suggesting a non-competitive antagonism. Methiothepin inhibited the effect of 5-HT (10 microM) in a concentration-dependent manner with an IC50 of 81 nM. 5. Selective 5-HT receptor agonists were also tested on excitatory NANC responses. 5-Carboxamidotryptamine (5-CT, 0.1-100 MicroM) was the most potent, producing a concentration-dependent inhibition with an EC50 of 0.13 MicroM. Calculation of approximate IC25 values (concentration of the agonist required to give a 25% inhibition of the excitatory NANC response) gave a rank order of potency 5-CT > 5-HT> > 8-hydroxy-dipropylaminotetralin (8-OH-DPAT) >alpha-methyl-5-hydroxytryptamine (alpha-Me-5HT). Sumatriptan, 5-methoxytryptamine (5-MeOT) and 2-methyl-5-hydroxytryptamine (2-Me-5HT) were essentially inactive with IC25> 100 MicroM.6. 5-HT (10 microM) did not significantly affect contractile responses to exogenously applied substance P(1 nM-10 Microm).7. The effect of 5-HT was unchanged after incubation with the nitric oxide (NO) synthase inhibitor L-NG-nitroarginine methyl ester (L-NAME, 100 Microm). However, pretreatment with charybdotoxin (ChTX,0.1-30 nM), a blocker of the large conductance Ca2+-activated K+channel (K+ca), produced a concentration-dependent inhibition of the effect of 5-HT (10 MicroM).8. 5-HT evokes a concentration-dependent inhibition of e-NANC bronchoconstriction in guinea-pig isolated bronchi but does not affect cumulative concentration-dependent contractile responses to substance P, suggesting that inhibition is via a prejunctional receptor. Effects of selective antagonists and agonists suggest that an atypical 5-HT receptor mediates this inhibition. The inhibitory effect of 5-HT does not involve the production of NO, but may involve the opening a ChTX-sensitive K+ca channel.These data suggest that an atypical 5-HT receptor inhibits the release of neuropeptides from sensory C fibres and may act as other inhibitory neuromodulators via the opening of a common K'channel.

Effect of an histaminergic H3 agonist on the non-adrenergic non-cholinergic contraction in guinea-pig perfused bronchioles

From the bronchioles of guinea-pigs, preparations were isolated for registration of perfused pressure on electrical field stimulation (EFS) and by application of drugs. The perfused bronchioles contracted when EFS was applied in the presence of atropine and phentolamine suggesting a non-adrenergic non-cholinergic (NANC) response. (R)-alpha-Methylhistamine (methylhistamine), a selective H3 agonist, reduced the NANC bronchoconstrictor response in a concentration-dependent manner. beta-Adrenoceptors, muscarinic and histamine (H1 and H2 receptor) antagonists, epithelial removal and cyclooxygenase inhibition had no effect on this inhibitory action of methylhistamine whereas the H3 antagonist, thioperamide, reduced the inhibitory effect of methylhistamine with a Ki value of 2.98 x 10(-9) M. Methylhistamine had no effect on the concentration-dependent contraction induced by exogenous substance P and neurokinin A, demonstrating that an H3 receptor might inhibit the release of transmitter from NANC nerves on guinea-pig perfused bronchioles in-vitro.

Effects of adenosine A2 receptor agonists on the excitation of capsaicin-sensitive afferent sensory nerves in airway tissues

We examined the effects of adenosine analogues on the asthmatic reactions induced by the stimulation of capsaicin-sensitive afferent sensory nerves. Intravenous (i.v.) injection of adenosine A2 receptor agonists, 5'-(N-ethylcarboxamido)-adenosine (NECA) and 2-[p-(carboxyethyl)-phenylethylamino]-5'-N-ethylcarboxamido-adenos ine (CGS 21,680), dose dependently inhibited capsaicin-induced guinea-pig bronchoconstriction (1-1000 nmol kg-1), whereas i.v. administration of the adenosine A1 receptor agonist, N6-cyclo-hexyladenosine (CHA), did not affect it (1000 nmol kg-1). Intratracheal injection of NECA (0.05-5 nmol site-1) and CGS 21,680 (0.05-5 nmol site-1) also reduced capsaicin-induced constriction in a dose-dependent manner. However, NECA (1000 nmol kg-1) failed to inhibit substance P-induced guinea-pig bronchoconstriction. NECA (1-1000 nmol kg-1) dose-dependently inhibited cigarette smoke-induced rat tracheal plasma extravasation, but not substance P-induced reaction. NECA (0.1-10 microM) and CGS 21,680 (10 microM) significantly blocked the capsaicin-induced release of substance P-like immunoreactivity from guinea-pig lung, whereas CHA (10 microM) had no effect. This evidence suggests that adenosine A2 receptors modulate negatively the excitation of capsaicin-sensitive afferent sensory nerves and substance P release from their endings in airway tissues.

Non-adrenergic, non-cholinergic neural activation in guinea-pig bronchi: powerful and frequency-dependent stabilizing effect on tone

1. We examined non-adrenergic, non-cholinergic (NANC) stimulation for its stabilizing effect on bronchial smooth-muscle tone with respect to its regulatory power and the effect of variations in neural impulse frequency. 2. The guinea-pig isolated main bronchus (n = 4-12) was pretreated with indomethacin (10 microM) and incubated with atropine (1 microM) and guanethidine (10 microM). Electrical field stimulation (EFS: 1200 mA, 0.5 ms, 240 s) was applied at various levels of tone prior to EFS: first without tone, then at a moderate tone induced by histamine (0.3 microM) and, finally, at a high tone induced by histamine (6 microM). Three different stimulation frequencies (1, 3 or 10 Hz) were used in order to produce moderate to near-maximum contractile and relaxant NANC neural responses. Both the contractile and the relaxant NANC responses were tetrodotoxin-sensitive in the guinea-pig isolated main bronchus (3 Hz). 3. Without tone prior to EFS, NANC activation (1, 3 or 10 Hz) induced a pronounced contractile response. At a moderate level of tone prior to EFS, NANC activation induced a less pronounced contractile response. At the highest level of tone prior to EFS, NANC activation induced a relaxant response. All these NANC responses adjusted the tone towards a similar level and this 'stabilization level' was 56(6)% at 1 Hz, 65(3)% at 3 Hz and 56(5)% at 10 Hz, expressed as a percentage of the maximum histamine-induced (0.1 mM) tone in each airway preparation. 4. There was a difference of approximately 90% of maximum between the highest and the lowest tone level prior to NANC activation. This difference was reduced by the converging contractile and relaxantNANC responses and the magnitude of this 'convergence effect' was 40(8)% at 1 Hz, 72(4)% at 3 Hz and 90(2)% at 10 Hz.5. These findings indicate that NANC neural activation stabilizes bronchial smooth-muscle tone via a contraction when the tone is low prior to activation and via a relaxation when the tone is high prior to activation. The NANC stabilizing effect on tone appears to be powerful and its magnitude can be controlled by the neural impulse frequency. The level of tone towards which the NANC responses converge does not appear to be markedly altered by variations in the impulse frequency. Our findings are consistent with a regulatory role for NANC responses in the control of bronchial smooth-muscle tone.

Tachykinin control of ferret airways: mucus secretion, bronchoconstriction and receptor mapping

The effects of synthetic tachykinin receptor agonists on mucus secretion by ferret trachea was determined in vitro in Ussing chambers using 35SO4 as a mucus marker and the synthetic peptides [Sar9,Met(O2)11]substance P (SarSP), [beta Ala8]neurokinin A-(4-10) and [MePhe7] neurokinin B which are selective for NK1, NK2 and NK3 tachykinin-receptors respectively. The bronchomotor effects of the same agonists were also studied in vitro and tachykinin receptors were localized by autoradiographic mapping. SarSP was the only synthetic agonist able to elicit a concentration-dependent increase in mucus secretion and was much more potent than SP. The EC50 for SarSP was 1.7 x 10(-6) M. Moreover, the maximal increase in release of 35SO4 produced by SarSP 10(-5) M was 95% of the increase produced by methacholine 10(-4) M indicating that this concentration of SarSP induced a near maximal secretory response. There was no significant difference in the secretory action of SP administered from the luminal or the submucosal side of the tissue. Only the NK2 agonist was able to produce a concentration-dependent contractility of bronchial ring preparations and its effect was relatively weak (EC50 6.4 x 10(-6) M). Capsaicin (10(-5) M) produced only a slight increase in tracheal mucus secretion (28 +/- 5%; n = 6) and was completely ineffective in inducing bronchoconstriction. Binding sites for [125I]-Bolton Hunter SP were more evident than sites for [125I]-NKA on submucosal glands and epithelium. In contrast, only binding sites to NKA could be observed over the smooth muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

The modulatory effects of prostaglandins on both excitatory and inhibitory non-adrenergic non-cholinergic neurotransmission in guinea-pig airways

Guinea-pig trachea and bronchi were used to investigate the effects of indomethacin and prostaglandin E2 (PGE2) on non-adrenergic non-cholinergic excitatory (e-NANC) and inhibitory (i-NANC) neurotransmission evoked by electrical field stimulation. Indomethacin potentiated e-NANC responses in bronchi with intact epithelium but had no effect on epithelium-denuded preparations. Inhibitory NANC responses were increased by indomethacin independent of the epithelium. Both i-NANC and e-NANC neurotransmission were suppressed by PGE2 in a dose-dependent manner. These results indicate that endogenous prostaglandins (e.g. PGE2) generated from the epithelium have an inhibitory effect on i-NANC and e-NANC nerve responses in airways. The epithelium is presumably not the only source for generation of prostaglandins that are involved in i-NANC neurotransmission.

Calcium-activated potassium channels mediate prejunctional inhibition of peripheral sensory nerves

Activation of several receptors, including mu-opioid, alpha 2-adrenergic, and neuropeptide Y receptors, inhibits excitatory nonadrenergic noncholinergic (NANC) neural responses in airways, which were mediated by the release of peptides from capsaicin-sensitive sensory nerves. This raises the possibility of a common inhibitory mechanism, which may be related to an increase in K+ conductance in sensory nerves. To examine this hypothesis, we have studied whether K(+)-channel blockers inhibit the effects of neuromodulators of sensory nerves in guinea pig bronchi by using selective K(+)-channel blockers. Charybdotoxin (ChTX; 10 nM), which blocks large conductance Ca(2+)-activated K(+)-channel function, completely blocked and reversed the inhibitory effects of a mu-opioid agonist, neuropeptide Y, and an alpha 2-adrenoceptor agonist on excitatory NANC responses. Neither inhibitors of ATP-sensitive K+ channels (BRL 31660 or glibenclamide, both at 10 microM) nor an inhibitor of small conductance Ca(2+)-activated K+ channels (apamin; 0.1 microM) were effective. This suggests that ChTX-sensitive K(+)-channel activation may be a common mechanism for the prejunctional modulation of sensory nerves in airways. This may have important implications for the control of neurogenic inflammation.

Non-adrenergic, non-cholinergic neural activation stabilizes smooth-muscle tone independently of eicosanoid factors in guinea-pig isolated airways

1. We examined the effect of variations in resting smooth-muscle tone induced eicosanoid inhibitors on the direction and magnitude of the non-adrenergic, non-cholinergic (NANC) nervous response in guinea-pig isolated airways. 2. NANC responses (in the presence of 1 microM atropine, 10 microM guanethidine) to electrical field stimulation (1200 mA; 0.5 ms; 240 s; 3 Hz) were studied in guinea-pig isolated airway preparations (n = 4-7) taken from the proximal trachea, the distal trachea and the main bronchus. Tissues were treated with either the cyclo-oxygenase-blocker, indomethacin (10 microM), or the leukotriene receptor antagonist, FPL 55712 (11.5 microM), to modulate the resting tone. 3. Depending on the level of resting tone prior to electrical field stimulation, NANC activation induced either a contraction or a relaxation converging towards a similar level of tone, a 'tonus equilibrium'. This tonus equilibrium displayed an increasing level towards the periphery (3 +/- 3 (s.e.mean) % for the proximal trachea, 28 +/- 3% for the distal trachea and 54 +/- 4% for the main bronchus, in % of maximum active tension). After NANC activation, the tonus returned to a level similar to the resting tone. 4. We conclude that changes in resting smooth-muscle tone not only affect the magnitude but also the direction of the NANC response. It is suggested that the NANC nervous system is a stabilizing factor in the regulation of airway smooth-muscle tone, independent of eicosanoid factors. An increasing contractile component and a decreasing relaxant component of the NANC system towards the peripheral airways is indicated.

Nitric oxide as an endogenous modulator of cholinergic neurotransmission in guinea-pig airways

Guinea-pig trachea possesses an inhibitory (bronchodilator) non-adrenergic, non-cholinergic (i-NANC) innervation associated with cholinergic nerves, which may be mediated, at least in part, by nitric oxide (NO). L-NG-Nitro-arginine methyl ester (L-NAME), an inhibitor of endogenous NO synthesis, enhanced cholinergic responses to electrical field stimulation with no effect on contractile responses to exogenous acetylcholine or excitatory NANC (e-NANC) bronchoconstriction. The effects of L-NAME were reversed by L-arginine but not D-arginine. These results suggest that NO released by nerve stimulation modulates cholinergic neurotransmission in guinea-pig trachea.

Tachykinin receptors in the guinea-pig isolated bronchi

The aim of the study was to assess which tachykinin receptors mediate the contractile response in the guinea-pig isolated bronchi. Experiments with natural tachykinins and receptor-selective tachykinin agonists were performed in the absence or presence of peptidase inhibitors and in bronchi pretreated with phenoxybenzamine. Both NK-1 (substance P, substance P methylester and septide) and NK-2 (neurokinin A, [beta-Ala8]neurokinin A-(4-10) and MDL 28,564) receptor agonists produced concentration-dependent contraction. NK-3 agonists (senktide and [MePhe7]neurokinin B) were active only at high concentrations. Phenoxybenzamine pretreatment reduced the maximal response to NK-1 agonists and produced a rightward shift of the curve to NK-2 agonists, without depression of the maximum. Five tachykinin antagonists selective for the NK-1 (L 668,169) or the NK-2 (MEN 10,207, MEN 10,376, L 659,877 and R 396) receptor were tested against substance P methylester and [beta-Ala8]neurokinin A-(4-10). The results indicated that these receptor-selective antagonists maintain their characteristic even when tested in a multireceptor assay such as the guinea-pig bronchus. The rank order of potency of NK-2 antagonists against [beta-Ala8]neurokinin A-(4-10) was MEN 10,207 = MEN 10,376 greater than L 659,877 much greater than R 396. This pattern, with the observation of the full agonist activity of MDL 28,564, indicates that in addition to NK-1 receptors, NK-2 receptors also are present in the guinea-pig bronchi and belong to the same subtype (NK-2A) as present in the rabbit pulmonary artery.

Influence of some alpha 2-receptor agonists and antagonists on the excitatory non-adrenergic, non-cholinergic neurotransmission in the airways of guinea-pigs in vivo

Neural control of smooth muscle tension in the airways of guinea-pigs can be subdivided into at least four components: an excitatory cholinergic, an inhibitory noradrenergic, an excitatory non-adrenergic, non-cholinergic and an inhibitory non-adrenergic, non-cholinergic component. The existence of alpha 2-adrenoceptors that modulate the excitatory non-adrenergic, non-cholinergic nerve activity has also been demonstrated. The aim of the present study was to investigate the dose-dependence of the selective alpha 2-receptor agonists UK 14,304, dexmedetomidine and clonidine on these alpha 2-adrenoceptors in the airway of guinea-pigs in vivo. Electrical stimulation of the cervical vagus nerves in anaesthetized guinea-pigs resulted in a biphasic response in insufflation pressure: an immediate increase due to excitation of cholinergic nerve fibres and a slower, longer-lasting increase due to activation of the excitatory non-adrenergic, non-cholinergic fibres to be studied. The cholinergic compound was abolished by atropine, leaving the excitatory non-adrenergic, non-cholinergic component as the sole response recorded during vagal stimulation. UK 14,304 and dexmedetomidine attenuated the response in a dose-dependent manner, whereas the effect of the partial agonist clonidine was less dose-related. UK 14,304 and dexmedetomidine reduced the insufflation pressure about 60 and 53% respectively at the highest dose. In conclusion, the neurotransmission in the excitatory non-adrenergic, non-cholinergic nerves is attenuated by selective alpha 2-receptors in a dose-dependent manner. The present study adds support to earlier observations that the excitatory non-adrenergic, non-cholinergic neural system in the airways of guinea-pigs is modulated by alpha 2-adrenoceptors.

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.

Effects of ageing on nicotine-induced contraction and substance P-like materials release in guinea-pig bronchus

1. Effects of ageing on nicotine-induced contraction and release of substance P-like materials in the bronchial preparations from guinea-pigs of different ages were studied. 2. The pD2 value (potency) of nicotine decreased with age from 10 to 100 weeks. The pD2 value of substance P did not change with age suggesting that substance P receptor mechanisms do not alter with age. 3. The amount of substance P-like materials released by nicotine (10(-4) M) decreased with age from 10 to 100 weeks, supporting our previous findings that nicotine contracts the guinea-pig bronchus through the release of substance P-like materials. 4. These results suggest that the age-related decrease in the pD2 value (potency) of nicotine is due to the reduction in the amount of substance P-like materials released by nicotine.

Non-adrenergic, non-cholinergic regulation of guinea-pig airway smooth muscle-indomethacin-induced changes and segmental differences

Several lines of evidence suggest a regulatory role for the non-adrenergic, non-cholinergic (NANC) nervous system in the airways of the guinea-pig and man. We examined NANC frequency-response characteristics (0.2-90 Hz) with respect to segmental differences and effects of cyclooxygenase-inhibition (indomethacin 10 microM). Furthermore, the neurogenic contribution to the contractile and the relaxatory NANC response was quantified with tetrodotoxin (1-10 microM). Frontally opened guinea-pig airway rings (n = 3-7) were used. NANC responses were obtained by electrical field stimulation (0.5 ms; 1200 mA; 240 s). A relaxatory NANC response was demonstrated in the proximal and the distal trachea. A contractile NANC response was demonstrated in the distal trachea and the main bronchus. Indomethacin lowered the baseline tension, decreased the relaxatory and increased the contractile NANC response (p less than 0.05; at 0.8 Hz; n = 4). A relationship between the baseline tension and the NANC response is suggested. The relaxatory NANC response was tetrodotoxin-sensitive at 3 Hz stimulation frequency (p less than 0.05; n = 3). At this frequency, the contractile NANC response was also mainly tetrodotoxin-sensitive (p less than 0.05; n = 6) whereas at 60 Hz a dominant, tetrodotoxin-resistant contraction was demonstrated. The pronounced frequency-response relationship as well as the tetrodotoxin sensitivity at very low frequencies (0.2-1.6 Hz) indicates that tetrodotoxin-sensitive NANC nerves are stimulated optimally at low impulse frequencies.

Electrical field stimulation of isolated guinea pig lung parenchyma strips: a method to study peripheral bronchi and peripheral blood vessels

The reaction of isolated guinea pig lung parenchyma strips to electrical field stimulation (EFS) has been characterized in vitro. Lung parenchyma strips reacted to EFS with a frequency-dependent initial twitch followed by a long-lasting contraction. The initial fast contraction was abolished by tetrodotoxin, whereas the tonic contraction was not influenced by this substance. The initial twitch was reduced but not abolished by atropine as well as by phentolamine. It was concluded that the reaction of guinea pig lung parenchyma strips to EFS consists of a cholinergic and an adrenergic component representing probably the responses of small bronchi and small blood vessels, respectively. The nature of the nonadrenergic-noncholinergic component of the initial twitch as well as of the nature of the nonneural tonic contraction are still unclear.

Neuropeptide Y modulates non-adrenergic, non-cholinergic neural bronchoconstriction in vivo and in vitro

Non-adrenergic, non-cholinergic (NANC) neural mechanisms regulate airway tone in guinea-pigs, and it is possible that they may be regulated by other nerves. We have investigated effects of neuropeptide Y (NPY), a co-transmitter of adrenergic nerves, on the excitatory (bronchoconstrictor) NANC (e-NANC) response elicited both in vivo (via bilateral vagal nerve stimulation) and in vitro (via electrical field stimulation [EFS]), and on inhibitory (bronchodilator) NANC (i-NANC) responses in upper trachea elicited by EFS. NPY inhibited the e-NANC to vagal stimulation in vivo in a dose-dependent manner (90.0 +/- 3.4% inhibition at 500 micrograms/kg), but failed to alter the bronchoconstrictor response to exogenous substance P (SP). NPY also inhibited the e-NANC response to EFS in main and hilar bronchi (57.2 +/- 8.6% in main and 46.34 +/- 7.5% in hilar bronchi at 10(-6) M), whilst having no effect on the contractile response to SP. In contrast NPY had no effect on the i-NANC response in vitro. Thus, NPY exerts a powerful inhibitory action on tachykinin release from peripheral endings of capsaicin-sensitive airway sensory nerves, but has no effect on the i-NANC neural mechanisms. This suggests that adrenergic nerves may modulate e-NANC responses in the airways.

Lack of local neurogenic involvement in the hyperreactivity induced by epithelium removal in guinea pig trachea

Airway epithelium removal produces tracheobronchial smooth muscle hyperreactivity to different constrictor agonists. Aside from the loss of an epithelium-derived relaxant factor, other mechanisms such as production of local axon reflex could hypothetically be involved. In the present work we investigated the effect of nerve activity inhibition or ganglionic transmission blockade in the epithelium removal-induced hyperreactivity in guinea pig trachea. Tetrodotoxin, tetracaine or hexamethonium did not modify the enhanced sensitivity of denuded tracheas to histamine, suggesting that, at least under these experimental conditions, local axon or ganglionic reflexes are not involved.