Pharmacological properties of mechanical responses of the rat oesophageal muscularis mucosae to vagal and field stimulation.

Electrical stimulation applied to vagal oesophageal branches of the isolated curarized oesophagus, or via field electrodes to the isolated tunica muscularis mucosae (TMM), increased longitudinal tension and intraluminal pressure in a frequency-dependent manner. Differences between cervical and distal TMM segments were noted in frequency-response relationships, as well as in the pulse-width dependence of contractions to field stimulation. Vagally- and field-stimulated contractions were eliminated by tetrodotoxin or hyoscine, indicating their mediation by cholinergic neurones. The field-stimulated postganglionic responses were resistant to hexamethonium or (+)-tubocurarine and weakly inhibited by morphine. Vagally- and field-stimulated TMM contractions were mimicked by muscarinic agonists, augmented by acetylcholinesterase inhibitors, and inhibited more effectively by beta-than by alpha 1-or alpha 2-adrenoceptor agonists. 5-Hydroxytryptamine (5-HT) exerted excitatory and/or inhibitory effects: TMM in situ responded with a hexamethonium-resistant, ketanserin-sensitive transient increase in tension comparable to that produced by field stimulation. In the isolated TMM, moderate excitatory responses were limited to the distal portion with inhibition predominating in the remaining proximal portion. 5-HT-induced inhibitory effects on field-stimulated tension responses were paralleled by relaxant effects on muscarinic agonist-induced tonic contractile responses, both of which were resistant to 5-HT-receptor antagonists including ketanserin, lysergic acid diethylamide (LSD), methysergide or methergoline. Field stimulation at a low frequency and pulse durations greater than 1.0 ms produced a relaxation response in preparations exposed to tetrodotoxin or hyoscine, provided that active muscle tonus was present. The relaxation in response to field stimulation was insensitive to antagonists of 5-HT, catecholamines, histamine, or indomethacin, suggesting a non-neurogenic origin. Histochemical examination of the isolated TMM preparation for cholinesterases revealed the presence of an extensive submucosal ganglionic plexus. It is concluded that: (i) intrinsic cholinergic neurones of the submucosal plexus form the final common pathway for extrinsic vagal and local (myenteric) projections to the TMM; (ii) the neural basis, if any, of non-cholinergic non-adrenergic inhibitory mechanisms remains to be established; (iii) the TMM may assist in generating propulsive oesophageal motility.     

Pharmacological characterization of endothelin-induced contraction in the guinea-pig oesophageal muscularis mucosae

1. In the oesophageal muscularis mucosae, we examined the effects of endothelin-1 (ET-1), endothelin-2 (ET-2), endothelin-3 (ET-3) and sarafotoxin S6c (SX6c) as agonists, and {"type":"entrez-nucleotide","attrs":{"text":"FR139317","term_id":"258103156","term_text":"FR139317"}}FR139317, BQ-123 and RES-701-1 as endothelin receptor antagonists.
2. All of the endothelins produced tonic contractions which were frequently superimposed on rhythmic motility in a concentration-dependent manner. The order of potency (−log EC₅₀) was ET-1 (8.61)=SX6c (8.65)>ET-2 (8.40)>ET-3 (8.18).
3. {"type":"entrez-nucleotide","attrs":{"text":"FR139317","term_id":"258103156","term_text":"FR139317"}}FR139317 (1–3 μM) and BQ-123 (1 μM) caused parallel rightward shifts of the concentration-response curve to ET-1, but at higher concentrations caused no further shift. RES-701-1 (3 μM) caused a rightward shift of the concentration-response curve to ET-1, while RES-701-1 (10 μM) had no additional effect. RES-701-1 (0.1–1 μM) concentration-dependently caused a rightward shift of the concentration-response curve to SX6c. The contraction to ET-1 (10 nM) in preparations desensitized to the actions of SX6c was greatly inhibited by pretreatment with {"type":"entrez-nucleotide","attrs":{"text":"FR139317","term_id":"258103156","term_text":"FR139317"}}FR139317 (10 μM).
4. Modulation of the Ca²⁺ concentration in the Krebs solution caused the concentration-response curve to ET-1 or SX6c to shift to the right and downward as external Ca²⁺ concentrations decreased. Verapamil (30 μM) abolished rhythmic motility induced by ET-1 or SX6c. Ni²⁺ (0.1 mM) weakly inhibited ET-1- or SX6c-induced tonic contraction. SK&F 96365 (60 μM) completely inhibited ET-1-induced contractions.
5. We conclude that there are two types of ET-receptors, excitatory ET_A- and ET_B-receptors in the oesophageal muscularis mucosae. These receptors mediate tonic contractions predominantly by opening receptor-operated Ca²⁺ channels (ROCs) and partly by opening T-type Ca²⁺ channels, and mediate rhythmic motility by opening L-type Ca²⁺ channels.

     

Pharmacological characterization of the histamine receptor in the isolated muscularis mucosae of the guinea-pig oesophagus.

To characterize the histamine receptors in the muscularis mucosae, the isotonic responsiveness of the isolated muscularis mucosae of the guinea-pig oesophagus to histamine receptor agonists and antagonists was examined in vitro. Histamine (0.1-100 microM) produced a concentration-dependent contraction of the muscularis mucosae (EC50 = 1.6 +/- 0.2 microM). The contractions were rapid in onset, sustained, reversible by washing and the preparation did not show tachyphylaxis. 2-Methylhistamine (2-MH), 2-pyridylethylamine (PEA) and 4-methylhistamine (4-MH) produced similar sustained contractions of the muscularis mucosae. The order of sensitivity was histamine greater than 2-MH greater than PEA greater than 4-MH. Impromidine (10-300 microM) and dimaprit (10-300 microM) caused no response in this tissue. The contractile responses to histamine, 2-MH, and PEA were competitively antagonized by diphenhydramine, and the pA2 values were almost the same (approximately 8.1). Cimetidine (100 microM) could not modify the contractile response to these agonists. The contractile response to histamine was slightly inhibited by tetrodotoxin (0.3 microM), atropine (1 microM), indomethacin (0.1-3 microM) or aspirin (30-300 microM), and the EC50 value was increased about 2-6 times by these drugs. When the preparation was incubated in Tyrode solution containing various calcium concentrations (0, 0.45, 0.9 and 1.8 mM), the concentration-response curve to histamine was shifted to the right and downward; the effect was inversely dependent on the calcium concentration, and in a calcium-free medium the response to histamine was abolished. Verapamil (1-10 microM) partially inhibited the contractile response to histamine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Adrenergic responses of rat colonic muscularis mucosae

We have investigated adrenoceptor-mediated responses of muscularis mucosae from the proximal, mid and distal regions of the rat colon. Noradrenaline-induced relaxations of the muscularis mucosae in each region were unaffected by atenolol, butoxamine or propranolol, but they were attenuated by the selective beta3-adrenoceptor antagonist cyanopindolol. The beta3-adrenoceptor agonist CL216343 elicited concentration-dependent relaxation of the muscularis mucosae in all regions of the colon. Isoprenaline, a non-selective beta-adrenoceptor agonist, evoked concentration-dependent relaxations of the muscularis mucosae in all regions, but only in the proximal colon were these significantly larger than the maximum noradrenaline-induced relaxation. The alpha1-adrenoceptor agonist methoxamine caused large contractions of the proximal colonic muscularis mucosae. When proximal tissue was pretreated with phentolamine, an alpha1-adrenoceptor antagonist, maximal noradrenaline- and isoprenaline-induced relaxations did not differ significantly. Although the mid colonic muscularis mucosae was also found to possess excitatory alpha1-adrenoceptors, these were associated with small contractions and did not modify the muscle's inhibitory responses to noradrenaline. Distal colonic muscularis mucosae lacked excitatory adrenoceptors and only responded to noradrenaline with beta3-adrenoceptor-mediated relaxations. No evidence was obtained for functional alpha2-adrenoceptors on the muscularis mucosae in any region of the rat colon. These data demonstrated that noradrenaline-induced relaxation of the rat colonic muscularis mucosae was mediated via beta3-adrenoceptors throughout, but in the proximal region this was modified by concurrent excitatory alpha1-adrenoceptor activation. Based upon these observations it appeared unlikely that noradrenaline-induced relaxation of rat colonic muscularis mucosae would be functionally linked to the secretory responses of the corresponding mucosa during periods of increased sympathetic activity.     

5-Hydroxytryptamine4 receptors mediate relaxation of the rat oesophageal tunica muscularis mucosae

Summary The present study was designed to characterize an atypical 5-hydroxytryptamine (5-HT) receptor mediating relaxation of the rat oesophageal tunica muscularis mucosae. All experiments were performed under equilibrium conditions, using pargyline to inhibit the oxidative deamination of indoleamines, and cocaine and corticosterone to inhibit neuronal and extraneuronal uptake. Under these conditions 5-HT (0.3–1000 nmol/l) produced a concentration-dependent relaxation of carbachol-induced tension. The concentration-effect curve to 5-HT was unaffected by potent antagonists for 5-HT₁, 5-HT₂, 5-HT₃ and so called 5-HT_1P receptors (metergoline, methysergide, ketanserin, ondansetron, N-acetyl-5-hydroxytryptophyl-5-hydroxytryptophan amide), but was antagonized competitively by ICS 205–930 (pA₂ = 6.7). Responses to 5-HT were mimicked by other indoleamines and substituted benzamides with the following order of potency: 5-HT 5-methoxytryptamine > cisapride = -methyl-5-HT = (S)-zacopride = renzapride > (RS)-zacopride > 5-carboxamido-tryptamine = metoclopramide = (R)-zacopride > tryptamine > 2-methyl-5-HT. ICS 205–930 afforded similar pA₂ values (6.0–6.7) against each agonist, indicating a common site of action. Concentration-effect curves to 5-HT were not affected by tetrodotoxin or indomethacin, sugesting that 5-HT-induced relaxation of the tunica muscularis mucosae was mediated via a postjunctional receptor, independent of endogenous prostanoids. The pharmacological profile of the 5-HT receptor in the rat oesophageal tunica muscularis mucosae correlates well with the 5-HT₄ receptor characterized recently in both the CNS and gastro-intestinal tract. Send offprint requests to D. E. Clarke at the above address     

Noradrenaline-induced relaxation of rat oesophageal muscularis mucosae: mediation solely by innervated beta 3-adrenoceptors.

We investigated the effects of cocaine and corticosterone on the noradrenaline-induced relaxation of rat oesophageal smooth muscle in the absence and presence of the selective beta2-antagonist, ICI 111,551. It was found that the concentration-response curve (CRC) of noradrenaline was not shifted by ICI 118,551 at 1 microM, whereas a clear shift to the right was observed at 100 microM of the antagonist. In the presence of corticosterone (10 microM), CRC's were clearly shifted to the left; with cocaine (10 microM) additionally present, a further leftward shift was observed, indicating the involvement of both neuronal and extraneuronal uptake sites. It was concluded that the relaxation of rat oesophageal muscularis mucosae by noradrenaline is solely mediated by beta3-adrenoceptors which are sympathetically innervated.     

Pharmacological characterization of the adrenoceptors in the muscularis mucosae of the guinea-pig esophagus

Pharmacological characteristics of the post-junctional adrenoceptors in the muscularis mucosae of the guinea-pig esophagus were examined in vitro. In the presence of propranolol (3 microM), all catecholamines, phenylephrine and methoxamine produced a weak contraction of the isolated muscularis mucosae. The order of potency of the contractile response was adrenaline greater than noradrenaline greater than methoxamine greater than phenylephrine and much weaker isoproterenol, dopamine and clonidine. The contractile responses to noradrenaline (10 microM) and adrenaline (10 microM) were markedly inhibited by phentolamine (3 microM), prazosin (3 microM), indomethacin (1 microM), aspirin (100 microM) and polyphloretin (30 microM), and slightly by yohimbine (3 microM). On the other hand, all catecholamines and terbutaline produced relaxation of the isolated muscularis mucosae which was maximally contracted with carbachol (3 microM) in the presence of phentolamine (10 microM), in a concentration-dependent manner. The order of potency of the inhibitory response was isoproterenol greater than noradrenaline greater than adrenaline and much weaker terbutaline and dopamine. The inhibitory responses to catecholamines were competitively antagonized by three beta blockers, being in the following order of potency as estimated by their pA2 values: propranolol greater than atenolol much greater than butoxamine. The pretreatment of the tissue with indomethacin (1 microM) did not modify the inhibitory responses to catecholamines. These results suggest that the guinea-pig esophageal muscularis mucosae has excitatory alpha 1 adrenoceptors and inhibitory beta 1 adrenoceptors, and the alpha 1, but not beta 1, adrenoceptors may link with the endogenous arachidonic acid cascade.     

Pharmacological characterizations of the in vitro anaphylactic contraction of the guinea-pig esophageal muscularis mucosae

Characteristics of the antigen-induced contraction of the isolated esophageal muscularis mucosae of actively sensitized guinea-pigs to ovalbumin (OA) were examined in vitro, and they were compared with those of compound 48/80- or polymyxin B-induced contraction. OA, above 0.01 microgram/ml, produced a sustained contraction of the sensitized esophageal muscularis mucosae, the amplitude of which was about 80-100% of the maximum contraction induced by carbachol (10 microM), while compound 48/80 and polymyxin B (10-300 micrograms/ml) produced less potent contractions of the non-sensitized esophageal muscularis mucosae. Contractions to OA or compound 48/80, but not to polymyxin B, were diminished by their repetitive applications. The contractile responses to OA, compound 48/80 and polymyxin B depended on the external calcium concentrations, and were abolished in the calcium-free medium. Pretreatment with tetrodotoxin (0.3 microM), atropine (0.3 microM), diphenhydramine (30 microM) or DSCG (300 microM) did not modify any of these contractions, whereas BW755C (100 microM) and quercetin (10 microM) significantly inhibited them. Indomethacin (10 microM) largely prevented only the polymyxin B-induced contraction, while FPL55712 (10 microM) inhibited both contractions to OA and compound 48/80. These findings indicate that the OA-induced anaphylactic contraction of the esophageal muscularis mucosae taken from the OA-sensitized guinea-pig may be an indirect action via the stimulation of releases of some mast cell-derived spasmogens. The spasmogens may involve the lipoxygenase products of arachidonic acid in part, but not histamine, acetylcholine or the cyclooxygenase products.     

Indirect action of 5-hydroxytryptamine on the isolated muscularis mucosae of the guinea-pig oesophagus

1 The site of action of 5-hydroxytryptamine (5-HT) was examined on the isolated muscularis mucosae attached to the submucous plexus of the guinea-pig oesophagus. Isotonic responses of the longitudinal muscularis mucosae were recorded.

2 5-HT produced a transient contraction of the muscularis mucosae at concentrations higher than 3 μM. The contraction was rapid in onset, reaching a peak in about 15 s or less, and was restored to the basal level after 20 to 30 s without washing out 5-HT. When the 5-HT-induced contraction faded to the basal tone, successive applications of 5-HT no longer produced any contracture.

3 Nicotine (Nic), at concentrations higher than 10 μM, also produced a transient contraction which had a very similar pattern to that induced by 5-HT. Again, the successive application of Nic no longer produced any contracture following prior treatment with Nic itself. However, the 5-HT-induced contraction was not modified by the presence of Nic.

4 Exogenously applied acetylcholine (ACh) produced a concentration-dependent contraction of the muscularis mucoase, the 50% effective concentration (EC₅₀) was 69 ± 5.6 nM. The contraction was sustained during incubation with ACh, and was not modified by prior treatment with 5-HT or Nic.

5 The 5-HT (100 μM)-induced contraction was completely abolished by tetrodotoxin (0.2 μM) and atropine (0.2 μM). This means that the action is mediated by stimulating cholinergic nerves in the submucous plexus attached to muscularis mucosae. Moreover, the stimulating action of 5-HT does not involve nicotinic receptors, since the action was not blocked by hexamethonium (100 μM).

6 Among several 5-MT antagonists examined, methysergide (1 μM), ketanserin (1 μM) and morphine (100 μM) failed to modify the 5-HT (100 μM)-induced contraction significantly. Cinanserin (0.1-3 μM), cyproheptadine (3-100 nM) and phenoxybenzamine (0.1-3 μM) inhibited the 5-HT-induced contraction, in a concentration-dependent manner, and each highest concentration abolished the response. However, none of these antagonists was specific for 5-HT, but the Nic (100 μM) or ACh (0.1 μM)-induced contractions were also inhibited by them.

7 The present results indicate that 5-HT contracts the muscularis mucosae of the guinea-pig oesophagus indirectly by stimulating cholinergic nerves in the submucous plexus, and has no direct action on the muscularis mucosae. In addition, the type of 5-HT receptors responsible for the stimulant action may be different from those in other parts of the gastrointestinal tract, blood vessels or brain, because of the different effects of 5-HT antagonists.

     

Similarity of relaxations evoked by BRL 34915, pinacidil and field-stimulation in rat oesophageal tunica muscularis mucosae.

1. In the rat oesophageal tunica muscularis mucosae (TMM) the potassium channel openers, BRL 34915 and pinacidil, raised the threshold for concentration-dependent K+ contractions, suppressed contractions evoked by field stimulation of the TMM in the presence of tetrodotoxin (TTX) and tetraethylammonium (TEA), and relaxed tonic contractions resulting from muscarinic cholinoceptor stimulation. 2. BRL 34915 and both (+)- and (--)-pinacidil increased 86Rb efflux from tracer-loaded tissues; nifedipine abolished this effect. 3. Relaxations produced by potassium channel openers were inhibited by a temperature drop from 37 degrees C to 26.5 degrees C, an increase in extracellular K+ concentration to 64 mM, and treatment with the calcium channel antagonist, nifedipine. The same treatments also blocked field stimulation-evoked TTX-insensitive relaxations. 4. It is concluded that field stimulation of rat oesophageal smooth muscle in the presence of cholinoceptor-induced tone results in an increase in K+ permeability that is directly or indirectly coupled to Ca2+ influx through potential-operated channels.     

Effects of drugs and of electrical stimulation on the muscularis mucosae of rabbit large intestine

1. The muscularis mucosae of the rabbit large intestine shows spontaneous rhythmic contractions with an average frequency of about 11/minute.

2. The muscularis mucosae responds to acetylcholine, adrenaline and noradrenaline in a manner similar to that of whole segments of terminal large intestine.

3. The muscularis mucosae would seem to have a cholinergic motor innervation.

     

Classification of tachykinin receptors in muscularis mucosae of opossum oesophagus.

1. Muscularis mucosae of the distal oesophagus of the opossum contracts in response to substance P and to a variety of tachykinins. To delineate the nature of the receptors present in this tissue, we evaluated contractile responses to substance P, neurokinin A, neurokinin B, eledoisin and analogues believed to be highly selective for NK-1, NK-2 and NK-3 receptors. In addition, the effects of prolonged exposure to each of these agents (10(-6) M or 10(-5) M) on contractile responses to substance P and to itself were evaluated. Similarly effects of prolonged exposure to the various tachykinins and their analogues on the field-stimulated responses of this muscle were studied. 2. All naturally occurring tachykinins were full agonists and differed in potency (comparing ED50 values) by less than ten fold. In nearly all cases there was cross tachyphylaxis between substance P and the other tachykinins and each reduced tonic responses to field stimulation, a response previously shown to be mediated by a substance P like agent. Eledoisin failed to cause tachyphylaxis under the conditions of these experiments. 3. When highly selective tachykinin analogues were used, only that believed to activate NK-1 receptors was a full agonist. [beta-Ala4,Sar9,MetO2(11)]SP(4-11) was also only slightly less potent than substance P. In contrast, an agonist selective for NK-2 (NK-A) receptors, [Nle10]NKA(4-10), and one selective for NK-3 (NK-B) receptors, [beta-Asp4, MePhe7]NKB(4-10) were unable to produce a response equal to 50% of the maximum even at 10(-5) M. However, all three selective tachykinin analogues reduced responses to substance P but not to carbachol.(ABSTRACT TRUNCATED AT 250 WORDS)     

Electrical and mechanical responses of guinea-pig bladder muscle to nerve stimulation.

1 The electrical and mechanical responses to transmural stimulation of intrinsic nerves have been recorded from smooth muscle strips dissected from the dome of the guinea-pig bladder, by use of intracellular microelectrodes, and conventional tension recording techniques. 2 Stimulation of intrinsic nerves evoked action potentials in all cells studied. Hyperpolarization of the cells by extracellular current injection revealed subthreshold excitatory junction potentials (e.j.ps) in about a quarter of the cells studied. 3 Action potentials could still be evoked in the presence of atropine and neostigmine, but were abolished after desensitization of the cells to alpha, beta-methylene ATP, a stable analogue of ATP. 4 In the presence of neostigmine, the evoked action potential was followed by a slow depolarization of the membrane. The mechanical response increased in amplitude and duration. 5 The contractile response to transmural nerve stimulation was reduced but not abolished in the presence of either atropine or desensitizing doses of alpha, beta-methylene ATP. Atropine was more effective at high frequencies of stimulation (greater than or equal to 30 Hz), and alpha, beta-methylene ATP at low frequencies (less than or equal to 15 Hz). In combination the drugs abolished the response. 6 The results suggest that the mechanical response to excitatory nerve stimulation is biphasic. The early transient response is elicited by e.j.ps and evoked spikes, is resistant to atropine, but sensitive to desensitization of purinoceptors. The late response is mediated through muscarinic receptors, involves little membrane depolarization, and is unaffected by desensitization of purinoceptors. These responses are analogous to the responses seen in rabbit bladder, and in the sympathetically innervated rat tail artery and guinea-pig vas deferens.     

Effect of bupranolol for BRL37344 and noradrenaline-induced relaxations mediating atypical beta/beta3-adrenoceptor in rat oesophageal muscularis mucosae.

We previously suggested that the existence of atypical beta/beta3-adrenoceptor with pA2-values for bupranolol, a non-selective beta-adrenoceptor antagonist, against BRL37344 and noradrenaline were 5.79 and 5.53 in guinea pig taenia caecum, respectively. We furthermore determined the affinity of bupranolol to subclassify atypical beta/beta3-adrenoceptor in rat oesophageal muscularis mucosae, because it is rich in atypical beta/beta3-adrenoceptor. BRL37344 and noradrenaline produced a concentration-dependent relaxation of rat oesophageal muscularis mucosae. The responses to BRL37344 and noradrenaline were resistant to 3x10(-6) M propranolol, 10(-4) M atenolol, and 10(-4) M butoxamine. However, bupranolol antagonized the responses to BRL37344 and noradrenaline in a concentration-dependent manner. Schild plot analyses of bupranolol against BRL37344 and noradrenaline gave pA2-values of 7.06 and 6.96, respectively. These results suggest that bupranolol can distinguish the difference in affinity between atypical beta/beta3-adrenoceptors in rat oesophageal muscularis mucosae and guinea pig taenia caecum. The difference in behavior of bupranolol confirms the existence of some atypical beta/beta3-adrenoceptors subtypes.     

A pharmacological study of oesophageal muscularis mucosae from the cat, dog and American opossum (Didelphis virginiana).

Strips of muscularis mucosae from the oesophagi of cat, dog and opossum have been studied to determine their responses to drugs to electrical field stimulation. All tissues were contracted by acetylcholine, histamine and, with the exception of strips of muscularis mucosae from the opossum proximal oesophagus, noradrenaline. The effects of acetylcholine and histamine were competitively antagonized by atropine (50 nM) and mepyramine (50 nM) and were abolished by atropine (1 microM) and mepyramine (1 microM) respectively. Contractile responses to noradrenaline were competitively antagonized by phentolamine (50 nM) but were converted to propranolol (50 nM)-sensitive relaxations by phentolamine (1 microM). Relaxations were abolished by propranolol (1 microM). Cholecystokinin octapeptide, gastrin 1 and vasoactive intestinal polypeptide were ineffective on any of the tissues examined. Substance P caused contractions in tissue from all three species. These effects were atropine and tetrodotoxin insensitive. All tissues gave atropine (50 nM)- and tetrodotoxin (100 nM)-sensitive contractions in response to electrical field stimulation. Contractions were not followed by relaxations and spontaneous mechanical activity was not suppressed between periods of stimulation. No evidence was obtained for the presence of non-adrenergic, non-cholinergic inhibitory innervation of the oesophageal muscularis mucosae in any species. During electrical field stimulation noradrenaline always reduced the amplitude of evoked contractions and, with the exception of tissue from proximal opossum oesophagus, increased resting tension. In opossum distal oesophageal muscularis mucosae, the effects of noradrenaline during electrical field stimulation were abolished by a 30 min pretreatment of the tissue with phentolamine (1 microM) and propranolol (1 microM). To Achieve this in all other tissues, it was also necessary to use yohimbine (1 microM).(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibitory actions of catecholamines on electrically induced contractions of the submucous plexus-longitudinal muscularis mucosae preparation of the guinea-pig oesophagus

1 The submucous plexus-longitudinal muscularis mucosae preparation of the guinea-pig oesophagus was used to study the actions of catecholamines on the twitch responses to electrical stimulation.

2 When the preparation was stimulated coaxially (0.1 Hz, 0.5 ms, supramaximal voltage), stable twitch-like contractions were obtained. These were abolished by tetrodotoxin (0.1 μM) and atropine (0.1 μM), potentiated by physostigmine (0.1 μM), and were mediated presumably by stimulation of intramural cholinergic nerves.

3 The twitch contractions of the muscularis mucosae were inhibited by catecholamines, in a concentration-dependent manner. The order of potency was isoprenaline > adrenaline > noradrenaline > dopamine.

4 The inhibitory actions of noradrenaline (1 μM) and adrenaline (1 μM) were partly reversed by phentolamine (1 μM) or by propranolol (1 μM), and completely abolished by both antagonists together. The inhibitory effect of dopamine (300 μM) was largely reversed by phentolamine (1 μM), but not by propranolol (1 μM), while the inhibitory action of isoprenaline was competitively antagonized only by propranolol (pA₂ of 7.6).

5 The contraction of the muscularis mucosae to exogenously applied acetylcholine (ACh, 20 nM) which was comparable in magnitude with that to electrical stimulation was also inhibited by isoprenaline (0.1 μM), adrenaline (1 μM) and noradrenaline (1 μM), but not by dopamine (300 μM). In the presence of propranolol (1 μM), noradrenaline, adrenaline and dopamine potentiated the ACh-induced contraction, while the effect of isoprenaline was mainly antagonized. The potentiating effects were antagonized by further treatment with phentolamine (1 μM).

6 Adrenaline, noradrenaline and dopamine but not isoprenaline, produced a weak contraction of the longitudinal muscularis mucosae in the presence of propranolol (3 μM). The contractile responses were completely inhibited by phentolamine (3 μM). Tone in the muscularis mucosae induced by carbachol (3 μM) in the presence of phentolamine (10 μM) was inhibited by catecholamines, in a concentration-dependent manner, an effect that was competitively antagonized by propranolol.

7 In the submucous plexus-longitudinal muscularis mucosae preparation of the guinea-pig oesophagus there are three types of adrenoceptor, inhibitory prejunctional α-adrenoceptors, excitatory postjunctional α-adrenoceptors and inhibitory postjunctional β-adrenoceptors, and cholinergic neurotransmission is inhibited by catecholamines acting at both prejunctional α- and postjunctional β-adrenoceptors.