A NOTE ON THE ELECTRICALLY TRANSMURALLY STIMULATED ISOLATED TRACHEA OF THE GUINEA-PIG

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.

Relaxant innervation of the guinea-pig trachealis: demonstration of capsaicin-sensitive and -insensitive vagal pathways

1. The guinea-pig trachea was isolated with its extrinsic innervation intact and placed in a water-jacketed dissecting dish containing warmed, oxygenated Krebs solution. The trachea was not separated from the oesophagus. Two adjacent cartilage rings of the rostral portion of the trachea were cut open opposite the trachealis and prepared for isometric tension measurements. 2. Following the addition of atropine and contraction of the trachealis with prostaglandin F2 alpha (PGF2 alpha), stimulation of the cervical sympathetic trunks elicited relaxations that were abolished by propranolol or hexamethonium. Stimulation of the vagus nerves caudal to the nodose ganglia also elicited relaxations. These vagally mediated relaxations were unaffected by propranolol but were abolished by hexamethonium or by cutting the recurrent laryngeal nerves. 3. After cutting the vagi caudal to the nodose ganglia, stimulation of the vagi rostral to the nodose ganglia elicited relaxations of the trachealis that were not significantly affected by either propranolol or hexamethonium but were abolished by cutting the superior laryngeal nerves. Stimulation of right vagi which had undergone supranodose vagotomy 14 days prior to experimentation was without effect on the smooth muscle of the guinea-pig trachea while the response to stimulation of the left vagus was unchanged. 4. Acute capsaicin desensitization abolished relaxations of the guinea-pig trachealis elicited by stimulation of the vagal fibres carried by the superior laryngeal nerves. In contrast, capsaicin desensitization only modestly inhibited relaxations elicited by stimulation of the preganglionic parasympathetic fibres carried by the recurrent laryngeal nerves and had no effect on sympathetic nerve-induced relaxations. 5. Removing the oesophagus selectively abolished relaxations elicited by stimulation of both vagal pathways of non-adrenergic relaxant innervation. Non-adrenergic relaxations of the trachealis elicited by electrical field stimulation were unaffected by removing the oesophagus. Oesophagus removal also had no effect on the parasympathetic-cholinergic contractile innervation or the sympathetic relaxant innervation of the trachealis. 6. The results indicate that the guinea-pig trachealis receives non-adrenergic relaxant innervation from both parasympathetic and capsaicin-sensitive vagal pathways. The results also suggest that the neurones mediating non-adrenergic relaxations of the trachea are sensitive to oesophagus removal. The observation that oesophagus removal abolishes parasympathetic relaxations of the trachealis while having no effect on parasympathetic contractions supports the hypothesis that the guinea-pig trachealis receives excitatory and inhibitory innervation from distinct vagal parasympathetic pathways.

Prostanoids inhibit release of endogenous norepinephrine from rat isolated trachea

Prostanoids, of epithelial origin, are known as modulators of several processes in the airways. The present study examined whether prostanoids are involved in the local control of sympathetic neurotransmission. The release of endogenous norepinephrine from rat isolated tracheae was evoked by electrical field stimulation (3 Hz, 540 pulses) in the presence of yohimbine, desipramine, and tyrosine. In different series of experiments, indomethacin (3 mumol/L) increased the evoked release of endogenous norepinephrine by 70 to 80%. In the presence of indomethacin, prostaglandin E2 (PGE2) and several prostanoid receptor agonists inhibited the evoked release of norepinephrine in a concentration-dependent manner, maximally by 60 to 70%. According to the concentration producing 35% inhibition of norepinephrine release (half-maximal effect), the following rank order of potencies was observed (EC35): nocloprost (8 nmol/L), sulprostone (30 nmol/L), PGE2 (308 nmol/L), iloprost (2 mumol/L), and U46619 (> 10 mumol/L). The EP1 receptor antagonist AH 6809 (3 mumol/L) had no effect on the evoked norepinephrine release and did not affect the inhibitory effect of 1 mumol/L of sulprostone. In the absence of indomethacin, the inhibitory effect of PGE2 was similar to that observed in the presence of indomethacin. After removal of the epithelium, the evoked norepinephrine release was markedly reduced. However, no significant effect of indomethacin was observed in epithelium-denuded tracheae. In conclusion, norepinephrine release in the rat trachea is inhibited via prostaglandin receptors that have the pharmacologic characteristics of the EP3 subtype. Endogenous eicosanoids, most likely of epithelial origin, are involved in the local control of the release of norepinephrine.

Characterization of endogenous noradrenaline release from intact and epithelium-denuded rat isolated trachea

1. Overflow of endogenous noradrenaline (NA) from the in vitro incubated rat trachea evoked by two periods of electrical field stimulation (S1, S2 at 3 or 15 Hz) or by high potassium (60 mM) was determined by high performance liquid chromatography (h.p.l.c.) with electrochemical detection. 2. In the presence of the neuronal uptake inhibitor desipramine, the alpha 2-adrenoceptor antagonist, yohimbine, enhanced the overflow of NA evoked by stimulation at 3 Hz by about 100% suggesting the presence of presynaptic inhibitory autoreceptors on the sympathetic nerves innervating the trachea. 3. When desipramine and yohimbine were present throughout the experiments, the overflow of NA evoked by the second period of electrical stimulation (S2) was significantly smaller than that evoked by the first (S1). This decline of overflow was prevented when the NA precursor, tyrosine, was additionally present throughout the experiments. 4. After removal of the epithelium, the tissue content of NA was reduced by about 30%, suggesting that part of the NA may be present and released within the epithelium. However, the overflow of NA evoked by stimulation at 3 Hz or 15 Hz was reduced by 70-80%, indicating that the epithelium may additionally exert a permissive role on the release of NA within the airways, possibly by suppressing inhibitory factors. 5. Stimulation by high potassium (60 mM for 10 min) caused a large overflow of NA (about 45% of the tissue NA), both from epithelium-free and epithelium-denuded tracheae. Thus the 'endogenous inhibition' of NA release after removal of the epithelium is surmountable when a high potassium stimulus is applied.

Neuronally released (-)-noradrenaline relaxes smooth muscle of calf trachea mainly through beta 1-adrenoceptors: comparison with (-)-adrenaline and relation to adenylate cyclase stimulation

The nature of the receptors that mediate the relaxation of smooth muscle by field stimulation, (-)-noradrenaline and (-)-adrenaline was investigated in calf tracheal smooth muscle. The relation between relaxation, stimulation of the adenylate cyclase and density of beta-adrenoceptor subtypes was studied with the help of antagonists of beta 1- and beta 2-adrenoceptors. The question of the existence of catecholamine-containing nerves was also investigated. (1) Nerves with varicosities exhibiting catecholaminergic fluorescence were observed between bundles of smooth muscle cells. (2) Consistent with the existence of adrenergic nerves (-)-noradrenaline was also found. The content of (-)-noradrenaline (1 microgram.g-1 w.w.) was the same in smooth muscle strips from the sublaryngeal region and from the region close to the bifurcation of the calf trachea. (-)-Adrenaline was not detected. (3) Smooth muscle relaxation by low (-)-noradrenaline concentration (0.6-2 nmol/l) was mediated through beta 1-adrenoceptors. Low concentrations of (-)-adrenaline (0.06-1 nmol/l) relaxed through beta 2-adrenoceptors. High concentrations of (-)-noradrenaline and (-)-adrenaline also caused relaxation through beta 2- and beta 1-adrenoceptors respectively. (4) Field stimulation caused relaxation which was half maximal at 0.2-0.8 Hz. Blockade of beta 1-adrenoceptors strongly attenuated the relaxant response to field stimulation and shifted the frequency-relaxation curves to 4 times higher frequencies. These results are consistent with a beta 1-adrenoceptor-mediated relaxation caused by (-)-noradrenaline released from sympathetic nerve endings at low stimulation frequencies. (5) Blockade of beta 2-adrenoceptors failed to reduce smooth muscle relaxation caused by field stimulation at low stimulation frequencies (0.1-1 Hz). However, after beta 1-adrenoceptor blockade, additional blockade of beta 2-adrenoceptors reduced the relaxant effects observed at high frequencies (2-400 Hz). The results suggest that high concentrations of endogenous (-)-noradrenaline cause relaxation through beta 2-adrenoceptors. (6) Binding experiments with 3H-(-)-bupranolol and 3H-ICI 118,551 revealed between 10,000 and 20,000 beta-adrenoceptors per smooth muscle cell of which 3/4 were beta 2 and 1/4 beta 1. The equilibrium dissociation constant of (-)-adrenaline for both beta 1- and beta 2-adrenoceptors and of (-)-noradrenaline for beta 1-adrenoceptors was 1 mumol/l. The affinity of (-)-noradrenaline for beta 2-adrenoceptors was 10 to 20 times lower than for beta 1-adrenoceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Innervation of airway smooth muscle. Efferent mechanisms

The classical view, with one excitatory (cholinergic) and one inhibitory (noradrenergic) component, of the innervation of airway smooth muscle is incomplete and at least two other, possibly peptidergic, types of innervation must be included when the innervation of airways is considered. A summary of these neuronal components is given in Fig. 1 and their possible origin is outlined. Besides the inhibitory noradrenergic innervation of the airways observed in some species, an inhibitory NANC (i-NANC) innervation has been demonstrated. The polypeptide, VIP, seems to be the most likely candidate for the neurotransmitter in the i-NANC innervation of the airways. The excitatory cholinergic innervation is present in the airways from the trachea down to the peripheral bronchi. In the guinea-pig bronchi an excitatory NANC (e-NANC) innervation has been demonstrated as well. The e-NANC nerves may correspond to chemosensitive primary afferent nerves with substance P or a related tachykinin as transmitter. When the innervation of airway smooth muscle of different mammalian species is compared it is evident that all nerve components except the cholinergic, show a considerable variability among species. The cholinergic innervation seems to be present in all mammalian species whereas the other components may be completely absent from some species. Distinct regional variations in the innervation of the airways may occur, which is exemplified by the distribution of the autonomic innervation in the guinea-pig tracheo-bronchial tree. Cholinergic neurotransmission in for example the guinea-pig and human airways can be modulated by NA via prejunctional inhibitory alpha 2-adrenoceptors. Furthermore, the e-NANC neurotransmission in the guinea-pig airways may be modulated by NA or by selective alpha 2-adrenoceptor agonists, acting via prejunctional inhibitory alpha 2-adrenoceptors. The clinical importance of the NANC innervation in relation to asthma is discussed. The i-NANC nerves may exert a modulating effect on bronchoconstriction, and a functional defect would presumably lead to an exaggerated response to constrictor stimuli. The e-NANC nerves in the airways may also be clinically relevant since the transmitter (tachykinins) from these nerves can produce bronchoconstriction and promote inflammation of the airway epithelium, either by direct mechanisms or indirectly by activation of mast cells, and thus contribute to the features of asthma.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation of bronchoconstrictor responses to histamine in pithed guinea-pigs by sympathetic nerve stimulation

1 Electrical stimulation (40V, 0.5-8 Hz, pulse width 0.5 ms) of the thoracic spinal outflow for between 10 and 120 s inhibited histamine-induced bronchoconstriction in pithed guinea-pigs. 2 The degree of this bronchodilatation varied with the position of the stimulating electrode within the spinal canal. Two maxima were identified. The first, at the level of the 9th and 10th thoracic vertebrae, was abolished by adrenalectomy. The second, at the level of the 3rd and 4th thoracic vertebrae, was associated with tachycardia and was unchanged by adrenalectomy. 3 The magnitude of this second bronchodilator effect varied with the frequency of stimulation. It was abolished by pretreatment with reserpine (5 mg/kg i.p. 48 and 24 h beforehand) and was competitively blocked by propranolol (0.01-1.0 mg/kg). 4 These observations are consistent with the view that bronchodilator tone is derived from neuronally-released noradrenaline within the lung. The noradrenaline probably overflows from well-innervated vasculature adjacent to sparsely innervated airways.

Nanomolar concentrations of prostaglandin F2 alpha potentiate cholinergic contractions of rabbit isolated tracheal muscle

The effects of prostaglandin F2 alpha (PGF2 alpha) on stimulation- or acetylcholine-evoked contractions were studied in isolated airway muscle preparations from rabbits, guinea-pigs and humans. Low concentrations of PGF2 alpha (10(-9) to 9 X 10(-8) mol/1) produced a dose-related (10-300%) increase in the contractile responses of the rabbit trachealis muscle to electrical stimulation at 2 Hz. This effect was inversely related to the rate of stimulation. In seven out of forty two preparations the resting muscle tone was also increased by 1.1 X 10(-8) mol/1 or higher concentrations of PGF2 alpha. This substance enhanced the contractile responses to acetylcholine (1-2.7 X 10(-8) mol/1) to the same extent as those to electrical stimulation. The potentiation produced by PGF2 alpha was not affected by indomethacin, mepyramine, methysergide or phenoxybenzamine. Electrically evoked contractions of isolated tracheal strips of guinea-pig or segments of human bronchial muscles were not changed significantly in the presence of 0.1-5 X 10(-8) mol/1 of PGF2 alpha. These results suggest that PGF2 alpha may modulate airway muscle tone by enhancing the postsynaptic stimulatory effect of acetylcholine released from the pulmonary cholinergic nerve endings. This modulation seems to be species-dependent.

The inhibitory innervation of the guinea-pig trachea: a study of its adrenergic and non-adrenergic components

(1) Inhibitory responses to field stimulation have been determined in strip preparations from the thoracic, middle and cervical regions of the trachea and in the tracheal tube preparation. (2) The adrenergic neurone blocking drug guanethidine was found to cause a partial reduction of the amplitudes of the responses in all preparations. (3) The guanethidine-resistant inhibitory responses were resistant to pentolinium but were abolished by tetrodotoxin. The data favours the existence of non-adrenergic inhibitory intramural nerves in the guinea-pig trachea. (4) Inhibitory responses have been determined in the presence and absence of guanethidine at frequencies ranging from 2 to 60 Hz. This has enabled the combined responses to stimulation of both adrenergic and non-adrenergic nerves to be compared with the response to stimulation of non-adrenergic nerves alone at each frequency and in each preparation. (5) The contribution of each innervation to the combined inhibitory response was frequency dependent. The adrenergic innervation was more effective at lower frequencies and the non-adrenaration is discussed. Its origin is considered.

Isolated lung strips of guinea pigs: responses to beta-adrenergic agonists and antagonists

Isolated lung strips of guinea pigs were examined as an in vitro model for assessing the direct effect of beta-adrenergic drugs at the level of peripheral airways. Changes in intrinsic tone of thin strips of lung parenchyma were measured with an isometric force transducer. Isoproterenol, a nonselective beta-adrenergic agonist, and several beta-adrenergic agonists, soterenol, salbutamol, metaproterenol and ritodrine elicited a dose-related relaxation of lung strip. Responses to isoproterenol were antagonized by propranolol and the selective beta blocking agents butoxamine (beta2) and practolol (beta1). These results were compared to data obtained with the same compounds on isolated guinea pig atria. All agonists except ritodrine were full agonists in the lung strip whereas isoproterenol and metaproterenol were the only full agonists in the atrial preparation. In the atria, practolol was a more effective blocker of isoproterenol responses than butoxamine, and the reverse was true for the lung strip.

Pathways for excitatory and inhibitory innervation to the guinea-pig tracheal smooth muscle

The trachea receives excitatory cholinergic innervation from the vagus nerve and the stellate ganglion. Inhibitory adrenergic fibres have the same sources. Those in the vagus nerve probably derive from high vagosympathetic anastomoses. Nonadrenergic inhibitory fibres have a preganglionic vagal supply.

Release of prostaglandin-like material from isolated cat tracheal muscle by electrical and mechanical stimulation

Release of PGE-like material has been studied on the isolated continuously-superfused cat tracheal muscle using dynamic bioassay methods. The effluent of transmural electrically-stimulated cat tracheal muscle induced a contraction when superfused over the rat stomach fundus strip. This response did not alter with atropine, methysergide, phentolamine and propranolol but was inhibited by aspirin and Sc 19220. The same myotropic activity in the effluent was found when trachea was mechanically stimulated by an additional increase in tension. The effluent from mechanically- and electrically-stimulated tracheal muscle caused a definite relaxation when superfused over a second cat tracheal muscle contracted by serotonin and pretreated with propranolol. Electrically-stimulated cat trachea itself gave a relaxant response which was blocked by propranolol but potentiated by aspirin. From these results it was concluded that both electrical and mechanical stimulation can elicit a release of PGE-like material from isolated cat tracheal muscle.

Evidence that adrenergic nerves are responsible for the active uptake of noradrenaline in the guinea-pig isolated trachea

1 6-Hydroxydopamine (50 mg/kg, i.p.) was given to guinea-pigs to destroy the adrenergic nerve terminals in the trachea. 2 The destruction was demonstrated by fluorescence histochemistry, which showed a marked loss of beaded fluorescent terminal fibres and by electrical transmural stimulation of the isolated atropinized trachea, which showed a marked reduction of dilator responses. 3 Such tracheae showed greatly reduced uptake-with-retention of (minus)-[3H]-noradrenaline in incubation experiments and the efflux curve of radioactive material showed a selective but incomplete reduction in the volume of the slowly exchanging compartment. 4 It is concluded that much, but perhaps not all, of the uptake-with-retention occurs into adrenergic nerves.

A non-adrenergic inhibitory nervous pathway in guinea-pig trachea

1 Electrical stimulation of the guinea-pig isolated tracheal tube causes a biphasic response, initially excitatory and then inhibitory. The excitatory response was abolished by atropine leaving the inhibitory response unaffected.2 The inhibitory response was greatly reduced but not abolished by propranolol or guanethidine. A residual inhibitory response was still present in tracheas in which sympathetic nerve function had been abolished by pretreatment with syrosingopine or 6-hydroxydopamine. These results show that the inhibitory response is predominantly adrenergic but that a small non-adrenergic component is also present.3 The non-adrenergic inhibitory response was abolished by lignocaine and tetrodotoxin suggesting that it is nervous in origin.4 Optimal stimulation parameters for the predominantly adrenergic inhibitory response were a pulse width of 0.7-2 ms, a stimulation period of 7 s and a frequency of 20 Hz. For the non-adrenergic inhibitory response, optimal stimulation parameters were a pulse width of 2 ms, a stimulation period of 12 s and a frequency of 20 Hz.5 Evidence obtained with pharmacological antagonists, enzyme inhibitors and activators suggested that the transmitter mediating the non-adrenergic inhibitory nervous response is unlikely to be: acetylcholine, histamine, 5-hydroxytryptamine, cyclic 3',5'-adenosine monophosphate or a prostaglandin.6 The adenosine uptake blocking drugs dipyridamole, hexobendine and Dilazep potentiated the non-adrenergic inhibitory nervous response and unmasked inhibitory responses to adenosine and adenosine 5'-triphosphate.7 It is concluded that electrical stimulation of the guinea-pig trachea, in addition to activating cholinergic and adrenergic nervous pathways, may activate a separate and distinct inhibitory nervous pathway. This pathway has some features in common with the non-adrenergic non-cholinergic inhibitory pathways in gastro-intestinal muscle.

Histochemical localization of adrenergic nerves in the guinea-pig trachea

1. Specific catecholamine fluorescence was demonstrated in guinea-pig trachea in fine, varicose, nerve fibres running parallel to the tracheal smooth muscle fibres.2. The density of nerves in tracheal smooth muscle was greater at the laryngeal end than at the bronchial end of the trachea.3. The findings confirm pharmacological evidence for an adrenergic innervation of the guinea-pig isolated tracheal chain preparation.

A new preparation of the isolated intact trachea of the guinea-pig

A preparation is described in which the guinea-pig isolated intact trachea is subjected to repeated transmural electrical stimulation with alternating square wave pulses. The intraluminal pressure is continuously sensed by a pressure transducer. An increase in the intraluminal pressure is the predominant response to electrical stimulation. This response is prevented by low concentrations of atropine. After the initial rise a small fall in intraluminal pressure occurs; this is prevented by low concentrations of propranolol. The sensitivity of the preparation to various drugs is described.