Cholinergic Neuroeffector Transmission Research Articles

Low-dose salbutamol suppresses airway responsiveness to histamine but not methacholine in subjects with asthma

Airway hyperresponsiveness is a cardinal feature of asthma. Although the modulation of cholinergic neuroeffector transmission may play a role in airway responsiveness, in vivo evidence remains scarce. It is well known that histamine causes bronchoconstriction partly via vagal reflex, whereas methacholine does not. To investigate the significance of modulating neuroeffector transmission, we compared the effect of low-dose salbutamol-a β2-adrenoceptor agonist-on airway responsiveness to histamine with that to methacholine. We enrolled 12 subjects with stable asthma. After screening confirmed that inhalation of low-dose salbutamol (1μg) did not change their basic pulmonary function, subjects underwent measurement of airway responsiveness to inhaled histamine and methacholine with or without pretreatment with low-dose salbutamol, in a randomized, crossover fashion. Airway responsiveness was measured by an astograph by which respiratory conductance (Grs) was assessed by the forced oscillation method during continuous inhalation of histamine or methacholine in stepwise incremental concentrations. Airway responsiveness was calculated as the cumulative dose of bronchoconstrictors that induced a decrease of 35% in Grs. Inhalation of 1μg of salbutamol significantly attenuated airway responsiveness to histamine but not methacholine. This selective attenuation was observed irrespective of disease severity or phenotype, namely atopy or non-atopy. Low-dose salbutamol suppresses airway responsiveness to histamine but not methacholine in subjects with asthma. The present study may provide a novel insight into the bronchoprotective mechanism of β2-adorenoceptor agonist in clinical settings.

Biomechanical properties and innervation of the female caveolin-1-deficient detrusor

Caveolin-1-deficiency is associated with substantial urogenital alterations. Here, a mechanical, histological and biochemical characterization of female detrusors from wild-type and caveolin-1-deficient (KO) mice was made to increase the understanding of detrusor changes caused by lack of caveolae. Length-tension relationships were generated, and we recorded responses to electrical field stimulation, the muscarinic receptor agonist carbachol and the purinoceptor agonist ATP. Tyrosine nitration and the contents of caveolin-1, cavin-1, muscarinic M₃ receptors, phospholipase C(β1), muscle-specific kinase (MuSK) and L-type Ca(2+) channels were determined by immunoblotting. Innervation was assessed by immunohistochemistry. Bladder to body weight ratio was not changed, nor was there any change in the optimum circumference for force development. Depolarization- and ATP-induced stress was reduced, as was carbachol-induced stress between 0.1 and 3 µM, but the supramaximal relative (% K(+)) response to carbachol was increased, as was M₃ expression. The scopolamine-sensitive component of the electrical field stimulation response was impaired, and yet bladder nerves contained little caveolin-1. The density of cholinergic nerves was unchanged, whereas CART- and CGRP-positive nerves were reduced. Immunoblotting revealed loss of MuSK. Ablation of caveolae in the female detrusor leads to generalized impairment of contractility, ruling out prostate hypertrophy as a contributing factor. Cholinergic neuroeffector transmission is impaired without conspicuous changes in the density of cholinergic nerves or morphology of their terminals, but correlating with reduced expression of MuSK.

Effects of a selective neuropeptide Y Y2 receptor antagonist, BIIE0246, on Y2 receptors at peripheral neuroeffector junctions.

1. This study investigated the effects of BIIE0246, a novel neuropeptide Y (NPY) Y2 receptor antagonist, on the inhibition of cholinergic neuroeffector transmission in rat heart and guinea-pig trachea and purinergic neuroeffector transmission in guinea-pig vas deferens produced by the NPY Y2 receptor agonist, N-acetyl [Leu28,31] NPY 24-36. 2. In pentobarbitone anaesthetized rats, supramaximal stimulation every 30 s, of the vagus nerve innervating the heart, increased pulse interval by approximately 100 ms. This response was attenuated by intravenous administration of N-acetyl [Leu28,31] NPY 24-36 (10 nmol x kg(-1)). 3. Transmural stimulation of segments of guinea-pig trachea at 1 min intervals with 5 s trains of stimuli at 0.5, 5, 10, 20 and 40 Hz evoked contractions which were reduced in force by N-acetyl [Leu28,31] NPY 24-36 (2 microM). 4. In guinea-pig vasa deferentia, the amplitude of excitatory junction potentials evoked by trains of 20 stimuli at 1 Hz was reduced in the presence of N-acetyl [Leu28,31] NPY 24-36 (1 microM). 5. In all preparations BIIE0246 attenuated the inhibitory effect of N-acetyl [Leu28,31] NPY 24-36 but had no effect when applied alone. 6. The findings support the view that the nerve terminals of postganglionic parasympathetic and sympathetic neurones possess neuropeptide Y Y2 receptors which, when activated, reduce neurotransmitter release.

Relaxation by sevoflurane, desflurane and halothane in the isolated guinea-pig trachea via inhibition of cholinergic neurotransmission

We have studied relaxation of airway smooth muscle by sevoflurane, desflurane and halothane in the isolated guinea-pig trachea. Ring preparations were mounted in tissue baths filled with physiological salt solution (PSS), aerated continuously with 5% carbon dioxide in oxygen. Electrical field stimulation (EFS) elicited cholinergic contractions that were abolished by tetrodotoxin, indicating nerve-mediated responses. Anaesthetics were added to the gas aerating the tissue baths. Halothane, sevoflurane and desflurane at 0.5-1.0 MAC markedly attenuated cholinergic contractions to EFS. Initiation of contractile responses to acetylcholine (ACh) were not affected by volatile anaesthetics, suggesting prejunctional inhibition (i.e. inhibition of acetylcholine release). When added to a maintained submaximal contraction to ACh, volatile anaesthetics induced relaxation, indicating postjunctional inhibition. We conclude that sevoflurane, desflurane and halothane inhibited postganglionic cholinergic neuroeffector transmission in the trachea. The effect was probably exerted via pre- and postjunctional mechanisms (i.e. inhibition of acetylcholine release and direct muscle actions). Sevoflurane and desflurane were more potent than halothane both pre- and postjunctionally.

Effects of epinastine hydrochloride on cholinergic neuro-effector transmission in canine tracheal smooth muscle

We determined the effects of epinastine hydrochloride, an anti-asthmatic drug, on cholinergic neuro-effector transmission in canine trachea. Isometric tension of tracheal strips was measured in the presence of indomethacin and propranolol. Epinastine (10 −6 M) significantly suppressed the contraction evoked by electrical field stimulation, but had no effect on the acetylcholine-evoked contraction. An L-type Ca 2+ channel blocker, nicardipine, did not suppress the electrical field stimulation-induced smooth muscle contraction and did not alter the inhibitory effect of epinastine. An N-type Ca 2+ channel blocker, ω-conotoxin, suppressed the electrical field stimulation-induced contraction in a dose-dependent manner, and in a subthreshold/intermediate concentration abolished the inhibitory effect of epinastine. These findings indicate that epinastine exerts prejunctional inhibitory effects on airway smooth muscle of dogs, presumably by inhibiting acetylcholine release from vagal nerve terminals, and suggest that this effect is mediated by N-type Ca 2+ channels.

Pituitary adenylate cyclase activating peptide regulates neurally mediated airway responses.

To clarify the protective effects of pituitary adenylate cyclase activating peptide (PACAP) on airway narrowing, we examined the effects of PACAP on smooth muscle contraction and plasma extravasation in guinea-pig airways. Smooth muscle contraction evoked by electrical field stimulation (EFS) or exogenously applied acetylcholine (ACh) or substance P (SP) was measured before and after PACAP in vitro. The effect of PACAP on airway plasma extravasation was also measured in vivo. In trachea, PACAP (10(-9) - 10(-7) M) significantly suppressed smooth muscle contraction evoked by EFS without affecting ACh sensitivity, suggesting that PACAP inhibits cholinergic neuroeffector transmission. In the main bronchi, PACAP (10(-9) - 10(-8) M) significantly suppressed the contraction evoked by EFS without affecting SP sensitivity in the presence of atropine, suggesting that PACAP inhibits SP release from excitatory nonadrenergic noncholinergic (eNANC) nerves. In animals treated with atropine and propranolol, PACAP attenuated the increase in plasma extravasation induced by electrical vagus stimulation or by SP. These results suggest that pituitary adenylate cyclase activating peptide may play a role in modulation of airway responses through inhibition of cholinergic and noncholinergic mechanisms.

Depressant effects of hypoxia and hypoglycaemia on neuro-effector transmission of guinea-pig intestine studied in vitro with a pharmacological model.

1. Since intermittent ischaemia may play an important role in the aetiology of Inflammatory Bowel Disease, particularly Crohn's Disease, a pharmacological model of neuronal ischaemia was applied to guinea-pig isolated intestinal preparations to mimic the acute effects of reduced blood flow on intestinal motility. 2. Neuro-effector transmission and smooth muscle performance were examined in myenteric plexus-longitudinal muscle preparations of guinea-pig ileum exposed to sodium cyanide (NaCN), in order to inhibit oxidative phosphorylation, or to iodoacetic acid (IAA), to block glycolysis. Comparisons were made with the effects due to simple deprivation of oxygen or glucose. 3. Depression of cholinergic neuro-effector transmission induced by hypoxia or NaCN (effective concentration range 0.1-3 mM), given as separate treatments, singly or repetitively over 60-90 min, were apparent within 30 s and were reversible. The maximum inhibition was 90% and the IC50 for NaCN was 0.3 mM. A conspicuous component of these inhibitions was prejunctional. 4. Non-cholinergic neuro-effector contractions were inhibited by up to 90% by anoxia or NaCN but recovery was incomplete and slower than with cholinergic contractions. 5. Glucose-free solutions also caused a reversible failure of cholinergic neuro-effector transmission but of slower onset. In contrast, IAA (0.06-1 mM) abolished contractions irreversibly, apparently by a direct depressant effect on smooth muscle contraction. Unlike NaCN, IAA caused an initial potentiation of electrically-induced contractions, partly by a prejunctional potentiation of cholinergic neuro-effector transmission. 6. It is concluded that a disruption of intestinal activity in pathological conditions associated with intestinal ischaemia may result from disturbances in the function of enteric neurones.

Effects of macrolide antibiotics on neurally mediated contraction of human isolated bronchus

Background: Long-term administration of macrolide antibiotic substances is an alternative therapy used in the treatment of asthma and airway hyperresponsiveness, but neither its mechanism of action nor whether this substance exerts an immediate action in the airways is known. Methods: Contractile responses of human isolated bronchial strips to electrical field stimulation (EFS) and acetylcholine were assessed under isometric conditions in the absence and presence of erythromycin, roxithromycin, or clarithromycin. Results: Incubation of tissues with erythromycin (3 × 10 -5 mol/L) attenuated the contractile responses to EFS so that the stimulus frequency required to produce 50% of the maximal contraction increased from 4.1 ± 0.5 to 10.1 ± 0.7 Hz (mean ± SE; p < 0.001). In contrast, contractile responses to acetylcholine were not changed. Erythromycin reduced the EFS-induced contraction in a concentration-dependent fashion; the maximal decrease from the baseline response was 92.8% ± 3.6% ( p < 0.001). This inhibitory effect was not altered by propranolol, indomethacin, ouabain, charybdotoxin, or mechanical removal of the epithelium. Roxithromycin and clarithromycin likewise inhibited neurally mediated contraction. Conclusions: These results suggest that macrolides may inhibit cholinergic neuroeffector transmission in the human airway smooth muscle, probably by reducing exocytotic release of acetylcholine from the nerve terminals. (J A LLERGY C LIN I MMUNOL 1995;95:853-9.)

Regulation of airway cholinergic neurotransmission by Ca(2+)-activated K+ channel and Na(+)-K+ adenosinetriphosphatase.

Stimulation of Ca(2+)-activated K+ channel and Na(+)-K(+)-ATPase may play an important role in the relaxant responses of airway smooth muscle to certain bronchodilators. To test whether cholinergic neuroeffector transmission can be modulated by Ca(2+)-activated K+ channel and Na(+)-K(+)-ATPase, canine airway smooth muscle was studied under isometric conditions in vitro. Addition of charybdotoxin (10(-7) M) did not alter the contractile responses to acetylcholine but augmented electrical field stimulation-induced contractions at 1-10 Hz (p < .01), whereas apamin and glibenclamide were without effect. This effect of charybdotoxin was dose dependent, with the maximal increase being 36.8 +/- 5.3% (p < .001). Ouabain (10(-7) M) increased contractions induced by both electrical field stimulation and acetylcholine. The magnitude of the increase in contractile responses to electrical field stimulation was similar to that of acetylcholine at an ouabain concentration of up to 3 x 10(-7) M, but the former was significantly greater at 10(-6) M ouabain (p < .05). These results suggest that both Ca(2+)-activated K+ channel and Na(+)-K(+)-ATPase may be operative in the regulation of cholinergic neurotransmission by inhibiting the exocytotic release of acetylcholine from the vagal nerve terminals.

Effect of ciprofloxacin on cholinergic neuro-effector transmission in airway smooth muscle.

Effect of ciprofloxacin on cholinergic neuro-effector transmission in airway smooth muscle.

Effect of ciprofloxacin on contractile responses of canine airway smooth muscle

To elucidate whether the new quinolone ciprofloxacin affects cholinergic neuto-effector transmission in the airway, we studied canine isolated bronchial segments under isometric conditions in vitro. Intrapulmonary lobar or segmental bronchi were suspended in the organ chambers filled with Krebs-Henseleit solution, and the isometric tension was continuously recorded by a force-displacement transducer. Addition of ciprofloxacin (2 x 10(-3) M) attenuated the contractile responses to electrical field stimulation (EFS), so that the stimulus frequency required to produce a half-maximal contraction (ES50) increased from 1.2 +/- 0.2 to 1.5 +/- 0.2 Hz (p < 0.05), whereas it was without effect on those to exogenously administered acetylcholine. The decrease in the EFS-induced contraction produced by ciprofloxacin was concentration-dependent and was not influenced by propranolol or tetraethylammonium, but partially inhibited by ouabain. These results suggest that ciprofloxacin may inhibit cholinergic neuro-effector transmission in the airway smooth muscle by inhibiting the exocytotic release of acetylcholine, probably involving the stimulation of Na(+)-K(+)-ATPase and concomitant repolarization/hyperpolarization of cholinergic nerve terminals.

VIP antagonists enhance excitatory cholinergic neurotransmission in the human airway.

It has been reported that a low concentration of exogenously applied vasoactive intestinal peptide (VIP) suppresses the release of acetylcholine (ACh) from vagus nerve terminals in the ferret and feline trachea. There has been, however, no documentation of the prejunctional action of VIP in the human airway. We observed the effects of VIP and VIP antagonists on cholinergic excitatory neuro-effector transmission in the human bronchus to study the possible role of endogenous VIP on excitatory neurotransmission. In the human bronchus, VIP (10(-10) to 10(-7) M) showed no effect on either the contractions evoked by electrical field stimulation (EPS) or those evoked by ACh. To investigate the possible role of endogenous VIP on the human bronchus, we observed the effects of the VIP antagonists [4-Cl-D-Phe6,Leu17]-VIP and [Ac-Tyr1,D-Phe2]-GRF(1-29)-NH2 on excitatory neuro-effector transmission. Both VIP antagonists (10(-8) M) significantly enhances the contractions evoked by EFS without affecting the ACh sensitivity of smooth muscle cells. These results indicate that VIP antagonists have a prejunctional action that enhances excitatory neurotransmission. This study suggests that endogenous VIP may suppresses ACh release from the vagus nerve terminals in the human airway. It is also suggested that exogenously applied VIP may be inactivated by some mechanism in the human airway.

Effects of BRL 38227 on neurally-mediated responses in the guinea-pig isolated bronchus.

1. In guinea-pig isolated bronchus treated with indomethacin (2.8 microM), electrical field stimulation (EFS; 10 Hz, 0.5 ms, 60-70 V, for 10 s) evoked a tetrodotoxin (3 microM)-sensitive, biphasic contraction comprising a rapid, atropine (1 microM)-sensitive cholinergic response succeeded by a slowly developing, capsaicin (10 microM)-sensitive, non-adrenergic, non-cholinergic excitatory (NANCe) response. 2. BRL 38227 (0.3-3 microM), salmeterol (0.003-3 microM) and ketotifen (1.0-300 microM) each produced concentration-dependent inhibition of both NANCe and cholinergic responses to EFS in guinea-pig isolated bronchus. 3. Substance P (SP; 1 microM) and neurokinin A (NKA; 0.07 microM) produced contractions equivalent in magnitude to the NANCe response to EFS, which were inhibited by salmeterol (1 microM), but not by BRL 38227 (3 microM) or ketotifen (100 microM). 4. Acetylcholine (ACh; 6 microM) was equi-effective with the electrical activation of cholinergic neurones. BRL 38227 (3 microM) slightly inhibited responses to ACh (6 microM). Salmeterol (1 microM) and ketotifen (100 microM) markedly inhibited responses to ACh (6 microM). 5. In bronchial rings pre-contracted with ACh (100 microM), BRL 38227 (0.1-30 microM), salmeterol (0.001-3 microM) and ketotifen (0.1-100 microM) each produced concentration-dependent relaxation. Unlike ketotifen, BRL 38227 and salmeterol only partially (18.8 +/- 2.1% and 51.8 +/- 3.9% respectively) reversed the ACh-induced contraction. 6. The (+)-analogue of BRL 38227, BRL 38226 (0.3-100 microM), was without effect on responses to EFS and had no effect on the inhibition caused by BRL 38227. The K+-channel activators pinacidil (3.0-30 microM) and RP 52891 (3.0-30 microM) exerted similar inhibitory actions on responses to EFS as BRL 38227, but were less potent. Glibenclamide (0.1-1.O microM) and phentolamine (3 microM) antagonized the inhibitory effects of BRL 38227 on responses to EFS.7. It is concluded that BRL 38227 and ketotifen can inhibit NANCe neuroeffector transmission at concentrations exerting little or no inhibitory effects on responses to exogenously applied tachykinins.By contrast, in addition to suppressing NANCe responses to EFS, salmeterol also markedly inhibits responses to SP and NKA. At concentrations markedly suppressing cholinergic neuroeffector transmission, BRL 38227 has only minor effects on responses to exogenously-applied ACh. Salmeterol and ketotifen both depress responses to ACh within the concentration-range over which they inhibit cholinergic responses to EFS. The inhibitory effects of BRL 38227 on responses to EFS exhibit stereo-specificity and may involve the opening of a neuronal K+-channel. This K+-channel is glibenclamide-and phentolamine-sensitive and appears similar to the smooth muscle K+-channel which is modulated by BRL 38227.

Cholinergic neuroeffector transmission in the longitudinal muscle of guinea-pig ileum

Cholinergic neuroeffector transmission in the longitudinal muscle of guinea-pig ileum

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

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

Characteristics of cholinergic neuroeffector transmission of ganglionic and aganglionic colon in Hirschsprung's disease.

Differences in the release and content of acetylcholine and the alpha 2 adrenoceptor mediated interaction between noradrenergic and cholinergic neurons were investigated by neurochemical and pharmacological methods in aganglionic and ganglionic segments of isolated human colon taken from children suffering from Hirschsprung's disease. Both at rest and during transmural stimulation the release of acetylcholine was significantly higher in the spastic (aganglionic) segment than in the proximal dilated bowel. Significant differences were found in the tissue concentration of acetylcholine between ganglionic and aganglionic specimens. The pattern of response to transmural stimulation was also different in the spastic and dilated bowel. Transmural stimulation induced relaxation and contraction in ganglionic specimens but only contractions in aganglionic specimens. The sensitivity of the smooth muscle in the aganglionic portion to exogenous acetylcholine and to field stimulation was found to be higher than in the ganglionic portion. While noradrenaline added to the organ bath reduced the stimulation-evoked release of acetylcholine from spastic segments, via an alpha 2 adrenoceptor mediated process, yohimbine did not enhance the release. It is suggested that in Hirschsprung's disease the increased acetylcholine release, the enhanced sensitivity of smooth muscle cells to acetylcholine, and the lack of alpha 2 adrenoceptor mediated noradrenergic modulation of acetylcholine release from cholinergic interneurons might be responsible for the spasm of aganglionic segments.

Development of Cholinergic Neuroeffector Transmission in the Avian Heart

A brief outline of the ontogenesis of cholinergic neuroeffector transmission in the embryonic chick heart is given. It is important to define the development of neuroeffector transmission to evaluate the possible effects on postjunctional membrane properties. The membrane response of the aneural chick heart undergoes a qualitative change to muscarinic agonist at a time well before cholinergic innervation. This suppressed response can be revealed later in life, even in adults, when provision is made to exclude Gi/Go and or K channel activity.

Effect of cromakalim on bronchoconstriction evoked by cholinergic nerve stimulation in guinea-pig isolated trachea.

1. Cromakalim reduced intraluminal pressure in the guinea-pig isolated, innervated trachea. 2. Preganglionic stimulation of the cervical vagus nerve elicited a frequency-dependent increase in intraluminal pressure. Cromakalim attenuated responses to vagal stimulation in a concentration-dependent manner at all frequencies tested. 3. Field stimulation caused a frequency-dependent increase in intraluminal pressure mediated by muscarinic cholinoceptors. Cromakalim did not affect the amplitude of responses at any frequency of stimulation, even at high concentrations. 4. Acetylcholine, added to the Krebs solution bathing the adventitial surface of the trachea, evoked a concentration-dependent increase in intraluminal pressure. The concentration-effect curve for acetylcholine was unaltered in the presence of cromakalim. 5. It is concluded that cromakalim modulates cholinergic neuroeffector transmission in the trachea chiefly by a prejunctional mechanism. However, cromakalim probably does not interfere with acetylcholine release from postganglionic cholinergic neurones.

Cholinergic neuromodulation by endothelin in guinea pig ileum

The effect of endothelin on cholinergic neuroeffector transmission in guinea pig ileum was investigated. Endothelin was shown to inhibit the nerve-induced contractions and concomitantly to increase the basal muscle tone. Furthermore, endothelin inhibited the nerve-induced release of [ 3H]acetylcholine whereas the contractile response to exogenous acetylcholine was enhanced. In conclusion, our findings suggest that endothelin is a modulator of cholinergic neuroeffector transmission in guinea pig ileum with possible action via both inhibitory prejunctional and stimulatory postjunctional mechanisms.

On the nature of endogenous purines modulating cholinergic neurotransmission in the guinea-pig ileum.

By means of agonist and enzyme experiments, the relative importance of endogenous adenosine, adenine nucleotides or other purines as modulators of cholinergic neuroeffector transmission in preparations of guinea-pig ileum muscle has been examined. Adenosine, 2-chloroadenosine, AMP, ADP, ATP and AMPPNP reversibly inhibited contractile responses to transmural stimulation of the guinea-pig ileum longitudinal muscle. 5'-adenylate deaminase dose-dependently antagonized the inhibitory effect of adenosine, AMP, ADP, ATP and AMPPNP, but not that of 2-chloroadenosine. 8-p-sulphophenyltheophylline, adenosine deaminase and 5'-adenylate deaminase enhanced contractile responses to transmural nerve stimulation. Adenosine deaminase and 5'-adenylate deaminase were virtually equiactive whereas 8-p-sulphophenyltheophylline was much more effective, and the theophylline derivative also enhanced contractile responses in preparations pretreated with adenosine deaminase or 5'-adenylate deaminase. Moreover, 8-p-sulphophenyltheophylline abolished the inhibition by dipyridamole, whereas adenosine deaminase and 5'-adenylate deaminase only partly antagonized the inhibitory effect of dipyridamole. Application of 5'-adenylate deaminase did not enhance the nerve-induced contractions in preparations pretreated with adenosine deaminase or a combination of dipyridamole and adenosine deaminase. In conclusion, adenosine deaminase and 5'-adenylate deaminase enhanced the nerve-induced contractions in the ileum, and, since 5'-adenylate deaminase was inactive after pretreatment with adenosine deaminase, this suggests that endogenous adenosine rather than 5'-adenine nucleotides modulated cholinergic neurotransmission in the ileum.(ABSTRACT TRUNCATED AT 250 WORDS)