Characterization of muscarinic receptor subtype mediating contraction and relaxation in equine coronary artery in vitro

In coronary arterial rings isolated from horses, 10-^-8-10^-6 mol/l acetylcholine (ACh) induced concentration-dependent contractions which were potentiated by the removal of endothelium and by pretreatment with I,-nitro-arginine (LNAG) or methylene blue (MB). Relatively lower concentrations of Ach 10-¹⁴-10-⁸ mol/l) induced relaxation when the coronary rings were contracted by phenylephrine (PE). ACh-induced contractions in the coronary rings without endothelium were competitively inhibited by each muscarinic subtype selective antagonist in the following order of potency: 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) > pirenzepine ≥ parafluoro-hexahydrosiladiphenidol (pFHHSiD) > methoctramine. ACh-induced relaxation in the rings with endothelium was inhibited by LNAG or MB, and by each selective antagonist in the following order of potency: 4-DAMP < pFHHSiD ≥ pirenzepine ≥ methoctramine. These results suggest that the ACh-induced contraction and relaxation in equine coronary arteries are mediated mainly by an M₃-receptor located on the smooth muscle cells and endothelial cells, respectively, and that the stimulation of the M₃-receptor on the endothelial cells liberates nitric oxide.     

In vitro effects of bethanechol on equine gastrointestinal contractility and functional characterization of involved muscarinic receptor subtypes

The goal of this study was to investigate the effect of bethanechol (BeCh) on contractility patterns of smooth muscle preparations of equine duodenum descendens, jejunum, caecum and pelvic flexure in vitro. Concentration-response relationships were developed for BeCh using in vitro assays with and without preincubation of muscarinic (M) receptor antagonists for M2 and M3 receptors. BeCh induced a significant, concentration-dependent increase in contractile response in equine intestine in specimens with circular orientation. The maximal effect was largest for jejunal specimens with no difference in EC50 within the different locations investigated. The M2 antagonist, AF-DX 116, caused a rightward shift of the concentration-response curve and the M3 antagonist, 4-DAMP (1,1-dimethyl-4-diphenylacetoxypiperidinium iodide), almost completely inhibited the effect of BeCh over the entire concentration-response curve. These data provide evidence that, although the effect of BeCh is predominantly mediated by M3 receptors, M2 muscarinic receptors also play a role in BeCh-induced contraction in specimens of equine intestine. The involvement of other muscarinic receptor subtypes cannot be excluded. Further studies are necessary to understand the effect of BeCh in vivo including diseased animals.     

Acetylcholine-induced contractions in the porcine internal mammary artery: possible role of muscarinic receptors

The effect of acetylcholine on the isolated, non-precontracted, porcine internal mammary artery (IMA) was investigated. Acetylcholine induced concentration-dependent contractions of non-precontracted IMA rings with denuded endothelium (pEC50 = 5.80 +/- 0.04) and was without effect on arterial segments with intact endothelium. The muscarinic receptor antagonists atropine, pirenzepine, methoctramine and p-fluoro-hexahydro-sila-diphenidol (pFHHSiD) antagonized the response to acetylcholine. The constrained pA2 values were 10.14, 7.74, 7.34 and 10.5, respectively. It is concluded that acetylcholine induces concentration-dependent contractions of porcine internal mammary artery rings on basal tone and that this contractile effect is probably due to direct cholinergic stimulation of smooth muscle cells, maybe including activation of muscarinic M1 receptors.     

Endothelin receptor mediating contraction of isolated bovine coronary artery

We studied endothelin (ET) receptors and their subtypes on isolated bovine coronary arteries. Endothelin receptors that mediated contraction of isolated bovine coronary artery were characterized by the use of antagonists and agonists. Contractions induced by the nonselective agonist ET-1 (10-10-10-7 M) were not affected by the removal of the endothelium (pEC50: 8.52, maximal contraction: 105% of that induced by 60 mM KCl). BQ-123 (3 x 10-7 M) antagonized contractions of endothelium-denuded coronary rings induced by low concentrations of ET-1 (10-10 or 10-9 M), but potentiated the contractions induced by higher concentrations of ET-1 (3 x 10-8 and 10-7 M). BQ-788 (10-6 M) potentiated contractions induced by ET-1 (3 x 10-10 and 10-7 M). In the presence of BQ-788 (10-6 M), BQ-123 (3 x 10-8-3 x 10-6 M) concentration - dependently inhibited contractions induced by ET-1 (3 x 10-10 and 10-7 M) (pA2: 6.61). Sarafotoxin S6b (10-9-3 x 10-7 M) evoked contractions in the denuded coronary artery (pEC50: 8.49, maximal contraction: 139% of 60 mM KCl). The BQ-123 caused a concentration-dependent rightward shift of contractions induced by sarafotoxin S6b (pA2: 7.89). The present study indicates that ET-1 and sarafotoxin S6b contract the isolated bovine coronary artery by stimulating ETA receptors on smooth muscle cells, and that ETB receptors might suppress the ET-1-induced contractions.     

In vitro effects of bethanechol on specimens of intestinal smooth muscle obtained from the duodenum and jejunum of healthy dairy cows

OBJECTIVE: To describe the in vitro effects of bethanechol on contractility of smooth muscle preparations from the small intestines of healthy cows and define the muscarinic receptor subtypes involved in mediating contraction. SAMPLE POPULATION: Tissue samples from the duodenum and jejunum collected immediately after slaughter of 40 healthy cows. PROCEDURES: Cumulative concentration-response curves were determined for the muscarinic receptor agonist bethanechol with or without prior incubation with subtype-specific receptor antagonists in an organ bath. Effects of bethanechol and antagonists and the influence of intestinal location on basal tone, maximal amplitude (A(max)), and area under the curve (AUC) were evaluated. RESULTS: Bethanechol induced a significant, concentration-dependent increase in all preparations and variables. The effect of bethanechol was more pronounced in jejunal than in duodenal samples and in circular than in longitudinal preparations. Significant inhibition of the effects of bethanechol was observed after prior incubation with muscarinic receptor subtype M(3) antagonists (more commonly for basal tone than for A(max) and AUC). The M(2) receptor antagonists partly inhibited the response to bethanechol, especially for basal tone. The M(3) receptor antagonists were generally more potent than the M(2) receptor antagonists. In a protection experiment, an M(3) receptor antagonist was less potent than when used in combination with an M(2) receptor antagonist. Receptor antagonists for M(1) and M(4) did not affect contractility variables. CONCLUSIONS AND CLINICAL RELEVANCE: Bethanechol acting on muscarinic receptor sub-types M(2) and M(3) may be of clinical use as a prokinetic drug for motility disorders of the duodenum and jejunum in dairy cows.     

In vitro pharmacologic effect of two endothelin-1 antagonists on equine colonic arteries and veins

OBJECTIVE: To evaluate the effectiveness of 2 potential endothelin (ET)-1 antagonists in blocking the contractile responses of equine colonic vessels to increasing concentrations of ET-1. SAMPLE POPULATION: Mesenteric vessels from 6 clinically healthy horses. PROCEDURE: Colonic vessels (arterial and venous rings) were placed in organ baths with oxygenated Tyrode solution at 37 C. Each was attached to a force transducer interfaced with a polygraph, and 2 g of tension was applied and equilibrated for 45 minutes. Then, B-1 (PD 142893) and B-2 (PD 145065) ET-1 antagonists were tested. One ring from each vessel type was used as a control for determining concentration-response relationships of ET-1 (10(-10) to 10(-6)M). Three rings of each vessel type were incubated with 3 concentrations of each antagonist (10(-7), 10(-6), and 10(-5) M) for 30 minutes before ET induced contractions were determined. The maximum contractile response and pA2 values were determined. RESULTS: Vessels contracted in a concentration-dependent manner to ET-1. Arteries responded slowly but reached greater contractions. Veins responded immediately with sustained contractions. Both antagonists inhibited contractions in a concentration-dependent manner with significant differences at 10(-6) and 10(-5)M for arteries and 10(-5) M for veins. Complete blockade of contractions was observed with B-2 (10(-5)M). The pA2 values for B-1 were 8.26 and 6.82 for arteries and veins, respectively, whereas they were 8.25 and 7.21 for B-2. CONCLUSION AND CLINICAL RELEVANCE: Both antagonists effectively blocked ET-1-induced contractions of equine colonic vessels. Because B-2 is water soluble and caused complete blockade at 10(-5) M, it appears to be the preferred antagonist.     

In vitro effects of bethanechol on smooth muscle preparations from abomasal fundus, corpus, and antrum of dairy cows

Abomasal displacement has been associated with gastric hypomotility. The supply of prokinetic drugs available to address this problem is insufficient. The goal of the study was to investigate the effect of the muscarinic agonist bethanechol (BeCh) on contractility parameters of smooth muscle preparations from several regions of the bovine abomasum (fundus, corpus, and antrum). Cumulative concentration-response curves were constructed using BeCh in vitro with and without pre-incubation with antagonists targeted at M(2) and M(3) muscarinic acetylcholine receptor (mAChR) subtypes. In all preparations investigated, BeCh induced a significant and concentration-dependent increase in all contractility parameters investigated. The maximal attainable effect (V(max)) was more pronounced in circular specimens, and V(max) of antral specimens in circular orientation were significantly lower when compared to the other preparations. Both antagonists caused a rightward shift of the concentration-response curve, suggesting that the effect of BeCh is mediated at least partly by M(2) and M(3) AChRs.     

In vitro effects of oxytocin,acepromazine, detomidine,xylazine, butorphanol,terbutaline, isoproterenol,and dantrolene on smooth and skeletal muscles of the equine esophagus

OBJECTIVE: To characterize the in vitro effects of oxytocin, acepromazine, xylazine, butorphanol, detomidine, dantrolene, isoproterenol, and terbutaline on skeletal and smooth muscle from the equine esophagus. ANIMALS: 14 adult horses without digestive tract disease. PROCEDURE: Circular and longitudinal strips from the skeletal and smooth muscle of the esophagus were suspended in tissue baths, connected to force-displacement transducers interfaced with a physiograph, and electrical field stimulation was applied. Cumulative concentration-response curves were generated for oxytocin, acepromazine, xylazine, detomidine, butorphanol, isoproterenol, terbutaline, and dantrolene. Mean maximum twitch amplitude for 3 contractions/min was recorded and compared with predrug-vehicle values for the skeletal muscle segments, and area under the curve (AUC) for 3 contractions/min was compared with predrug-vehicle values for the smooth muscle segments. RESULTS: No drugs caused a significant change in skeletal muscle response. In smooth muscle, isoproterenol, terbutaline, and oxytocin significantly reduced AUC in a concentration-dependent manner. Maximum reduction in AUC was 69% at 10(-4) M for isoproterenol, 63% at 10(-6) M for terbutaline, and 64% at 10(-4) M for oxytocin. CONCLUSIONS AND CLINICAL RELEVANCE: Isoproterenol, terbutaline, and oxytocin cause relaxation of the smooth muscle portion of the esophagus. The clinical relaxant effects on the proximal portion of the esophagus reported of drugs such as oxytocin, detomidine, and acepromazine may be the result of centrally mediated mechanisms.     

Mechanisms of acetylcholine-induced vasorelaxation in high K+-stimulated rabbit renal arteries

To characterize the mechanisms of acetylcholine (ACh)-induced vasorelaxation in rabbit renal arteries precontracted with high K+ (100 mM), muscle tension and cytosolic free Ca2+ concentration ([Ca2+]i) were measured simultaneously in the fura-2-loaded arterial strips. In the artery with endothelium, high K+ increased both [Ca2+]i and muscle tension. Addition of ACh (10 microM) during high-K+ induced contraction significantly relaxed the muscle and induced additional increase in [Ca2+]i. In the presence of NG-nitro-L-arginine (L-NAME, 0.1 mM). ACh increased [Ca2+]i without relaxing the muscle. In the artery without endothelium, high K+ increased both [Ca2+]i and muscle tension although ACh was ineffective, suggesting that ACh acts selectively on endothelium to increase [Ca2+]i. 4-DAMP (10 nM) or atropine (0.1 microM) abolished the ACh-induced increase in [Ca2+]i and relaxation. However, pirenzepine (0.1 microM), AF-DX 116 (1 microM) and tropicamide (1 microM) were ineffective. The ACh-induced increase of [Ca2+li and vasorelaxation was significantly reduced by 3 microM gadolinium, 10 microM lanthanum or 10 microM SKF 96365. These results suggest that, in rabbit renal artery, ACh-evoked relaxation of 100 mM K+-induced contractions is mediated by the release of endothelial NO. ACh may stimulates the M3 subtype of muscarinic receptor in the endothelial cells, resulting in the opening of the nonselective cation channels followed by an increase of [Ca2+]i and stimulation of NO synthase.     

Contractile effects of 5-hydroxytryptamine (5-HT) in the equine jejunum circular muscle: functional and immunohistochemical identification of a 5-HT1A-like receptor

REASONS FOR PERFORMING STUDY: Prokinetic drugs used to treat gastrointestinal ileus in man have equivocal results in horses. In man, prokinetic drugs have 5-hydroxytryptamine4(5-HT4) receptors as their target, but little is known about the 5-HT-receptor subtypes in the equine small intestine. OBJECTIVE: Functional and immunohistochemical identification of the serotonin receptor subtype(s) responsible for the 5-HT induced contractile response in the equine circular jejunum. METHODS: Isometric organ-bath recordings were carried out to assess spontaneous and drug-evoked contractile activity of equine circular jejunum. Histological investigations by immunofluorescence analyses were performed to check for presence and localisation of this functionally identified 5-HT receptor subtype. RESULTS: Tonic contractions were induced by 5-HT in horse jejunal circular muscle. Tetrodotoxin, atropine and NG-nitro L-arginine did not modify this response. A set of 5-HT receptor subtype selective antagonists excluded interaction with 5-HT1B, 1D, 2A, 3, 4 and 7 receptors. The selective 5-HT1A receptor antagonists WAY 100635 and NAN 190 caused a clear rightward shift of the concentration-response curve to 5-HT. The contractile effect of 5-CT, that can interact with 5-HT1A, 1B, 1D, 5 and 7 receptors was also antagonised by WAY 100635, identifying the targeted 5-HT receptor as a 5-HT1A-like receptor. Immunohistology performed with rabbit polyclonal anti-5-HT1A receptor antibodies confirmed the presence of muscular 5-HT1A receptors in the muscularis mucosae, and both longitudinal and circular smooth muscle layers of the equine jejunum. CONCLUSIONS: Contractile responses in equine jejunal circular smooth muscle induced by 5-HT involves 5-HT1A-like receptors.     

Muscarinic receptor subtypes in equine tracheal smooth muscle

Selective muscarinic receptor antagonists were used to identify muscarinic receptor subtypes in equine trachealis strips. The M₁ receptor antagonist pirenzepine (10^–7 mol/L to 3 × 10^–5 mol/L) and the M₃ receptor antagonist 4-diphenylacetoxy-N-methylpiperidine (4-DAMP, 10^–9 mol/L to 3 × 10^–7 mol/L³) dose dependently inhibited the contractile responses to electrical field stimulation (EFS) and exogenous acetylcholine (ACh). Schild plots yielded a pA₂ value for pirenzepine vs ACh of 6.75 ± 0.09, which is consistent with the affinity for M₂ or M₃ receptors, and a pA₂ value for 4-DAMP vs ACh of 8.47 ± 0.09, which is in agreement with the affinity for M₃ receptors. The M₂ receptor antagonist gallamine (10^–5 mol/L and 10^–4 mol/L) did not affect the response of trachealis to exogenous ACh and low-frequency EFS (0.1–2 Hz) but decreased the responses to high-frequency EFS (4–16 Hz). These results suggest that the muscarinic receptors mediating contractions induced by ACh in equine tracheal smooth muscle are of the M₃ subtype. The lack of an increase in the response to EFS following gallamine suggests that functional prejunctional inhibitory M₂ receptors are not present on the cholinergic nerves innervating equine tracheal smooth muscle.     

In vitro effects of erythromycin, lidocaine, and metoclopramide on smooth muscle from the pyloric antrum, proximal portion of the duodenum, and middle portion of the jejunum of horses

OBJECTIVE: To evaluate effects of erythromycin, lidocaine, and metoclopramide on smooth muscle of the pyloric antrum (PA), proximal portion of the duodenum (PD), and middle portion of the jejunum (MJ) of horses. Sample Population-Strips of smooth muscle from 7 horses. PROCEDURE: Isolated muscle strips were suspended in a bath and attached to isometric force transducers. Once stable spontaneous contractions were observed, agents were added. Isometric stress responses were compared with the amplitude of spontaneous contractions. RESULTS: A single dose of erythromycin to the PA increased contractile amplitude (CA) for the longitudinal smooth muscle (mean +/- SEM, 76+/-16 g/cm2) but decreased CA for circular smooth muscle (-79+/-23 g/cm2). The inhibitory effect was decreased by tetrodotoxin, N(G)-nitro-L-arginine methyl ester, and a vasoactive intestinal peptide antagonist. Erythromycin increased CA for the MJ, which was maximal at 10(-4)M (171+/-36 g/cm2). Lidocaine increased CA for the PD, which was maximal at 10(-4) M (60+/-5 g/cm2). Metoclopramide increased the CA, which was maximal at 10(-4) M for the PA (75+/-26 g/cm2), PD (279+/-33 g/cm2), and MJ (456+/-59 g/cm2). CONCLUSIONS: Regional differences in responses to erythromycin, lidocaine, and metoclopramide were evident in the gastrointestinal tract of horses. Metoclopramide increased CA in all tissues used, whereas erythromycin inhibited CA in circular smooth muscle but stimulated CA in longitudinal smooth muscle from the PA. Inhibition is caused by stimulation of inhibitory nerves and is mediated, in part, by nitric oxide and vasoactive intestinal peptide.     

Short chain fatty acids stimulate feline colonic smooth muscle contraction

The effect of short chain fatty acids (SCFA) on feline colonic smooth muscle contraction was evaluated in vitro. Colonic tissue was obtained from seven healthy male and female adult cats and seven healthy male and female kittens. Longitudinal and circular colonic smooth muscle strips from proximal and distal colon were incubated with SCFA (acetate, butyrate and propionate; 1-100mM). SCFA-induced contractions were compared to responses obtained using maximal concentrations (10(-4)M) of acetylcholine (ACh). The calcium dependence of the SCFA response was investigated by incubating with nifedipine (1 microM) or verapamil (1 microM). Acetate, butyrate and propionate elicited isometric stress responses (0.25-1.98 x 10(4)N/m(2)) in longitudinal, but not circular, smooth muscle from both the proximal and distal colon of adult cats. Maximal responses were attained at 50 and 100mM SCFA. Maximal butyrate and propionate responses were 29 and 19% of the maximal ACh response (10(-4)M), respectively. Acetate was least effective in stimulating contractile responses. Nifedipine and verapamil abolished all responses. Contractile responses in kittens were similar to those observed in adult cats, but were smaller in amplitude.Results of these studies have shown that SCFA stimulate longitudinal colonic smooth muscle contractions in kittens and adult cats in vitro. These SCFA-induced contractions involve activation of calcium influx. These in vitro findings may account for some of the effects of dietary fiber on feline colonic motility in vivo.     

Characterization of the in vitro responses of equine cecal longitudinal smooth muscle to endothelin-1

OBJECTIVE: To characterize the in vitro response of equine cecal longitudinal smooth muscle (CLSM) to endothelin (ET)-1 and assess the role of ETA and ETB receptors in those ET-1-induced responses. ANIMALS: 36 horses without gastrointestinal tract disease. PROCEDURE: To determine cumulative concentration-response relationships, CLSM strips were suspended in tissue baths containing graded concentrations of ET-1 (10(-9) to 10(-6)M) with or without BQ-123 (ETA receptor antagonist); with or without IRL-1038 (ETB receptor antagonist); or with both antagonists at concentrations of 10(-9), 10(-7), and 10(-5)M. To determine the percentage change in baseline tension of CLSM, the areas under the curve during the 3-minute periods before and after addition of each dose were compared. Also, the effects of ET-1 and a combination of selective ETA and ETB receptor antagonists on electrically evoked contractions were studied. RESULTS: ET-1 caused sustained increases in CLSM tension in a concentration-dependent manner. Contractile responses to ET-1 were not significantly inhibited by either BQ-123 or IRL-1038 alone at any concentration; however, responses were significantly inhibited by exposure to the antagonists together at a concentration of 10(-5)M. Electrical field stimulation did not change the spontaneous contractile activity of CLSM and did not significantly alter the tissue response to ET-1, BQ-123, or IRL-1038. CONCLUSIONS AND CLINICAL RELEVANCE: Results indicated that ET-1 has a contractile effect on equine CLSM that is mediated via ETA and ETB receptors. In vitro spontaneous contractions of equine CLSM apparently originate in the smooth muscle and not the enteric nervous system.     

Effects of Alcohol Extract of Aster tataricus L.f.on the Contraction of Guinea Pig Tracheal Smooth Muscle in vitro

The aim of the experiment was to study the effect of alcohol extract of Aster tataricus L.f.on the contraction of isolated guinea pig tracheal smooth muscle.The guinea pig tracheal smooth muscle was prepared by isolated tracheal thermostatic perfusion method.The effects of Aster tataricus L.f.on tracheal smooth muscle under the resting state and contraction stimulated by acetylcholine (Ach),histamine (His),CaCl₂,Ca²⁺ release and influx in cells without calcium were measured by BL-420F biological function experiment systems.The results showed that Aster tataricus L.f.could induce the contraction of isolated trachea at resting state at a low concentrations of 0.002 to 0.008 g/mL and exerted a relaxation on the isolated trachea when at the higher concentrations of 0.008 to 0.196 g/mL.All of the three concentrations of Aster tataricus L.f.could inhibit the tension stimulated by Ach,His and CaCl₂ which were antispasmodic agents,led to the non-parallelled rightward moving of cumulative concentration-response curve and the decline of maximal responses.The suppressive effect on His was the strongest,followed by Ach and CaCl₂,and the suppression could counteract the contraction induced by extracellular Ca²⁺influx significantly,and they were all expressed concentration-dependent manner.These results indicated the alcohol extract of Aster tataricus L.f.could relax tracheal smooth muscle by acting on M-receptor and histamine receptor and blocking Ca²⁺ passage,thus inhibiting extracellular Ca²⁺influx.     

紫菀乙醇提取物对豚鼠离体气管平滑肌收缩功能的影响

本试验旨在研究紫菀乙醇提物对豚鼠离体气管平滑肌收缩功能的影响。采用离体气管恒温灌流的方法,将豚鼠气管制成气管螺旋条,通过 BL-420F 生物机能实验系统测定其张力的变化,观察紫菀乙醇提物对离体气管平滑肌静息状态下的舒张作用,以及在氯化乙酰胆碱(Ach)、磷酸组胺(His)、CaCl₂条件下和无钙下Ach诱导细胞内钙释放和外钙内流所致两种收缩条件下对离体气管平滑肌张力变化的影响。试验结果显示,低浓度紫菀乙醇提取物(0.002~0.008 g/mL)对静息状态下豚鼠离体气管平滑肌具有一定的收缩作用,高浓度(0.008~0.196 g/mL)时具有舒张作用;低、中、高3个剂量组均可抑制Ach、His和CaCl₂引起的气管平滑肌收缩强度,使各致痉剂的量效曲线非平行右移并降低最大效应。紫菀乙醇提取物对His的抑制作用强度最大,其次为Ach,最后是CaCl₂,且均呈剂量依赖性。以上结果表明紫菀乙醇提物具有舒张气管平滑肌的作用,其机制可能与抑制豚鼠气管平滑肌M受体、H₁受体和阻断Ca²⁺通道从而抑制细胞Ca²⁺内流有关。     

Diazepam potentiates the positive inotropic effects of histamine and forskolin in guinea-pig papillary muscles

There have been diverse reports on the effects of diazepam on cardiac contractility. The purpose of this study was to examine whether diazepam modifies the inotropic response elicited by histamine on an isolated guinea-pig papillary muscle. The responses of electrically driven papillary muscle to histamine and cyclic AMP-related inotropic agents were recorded in the absence and in the presence of diazepam. Histamine and forskolin, which directly stimulate adenylate cyclase, significantly increased the contractile force in the papillary muscle in a concentration-dependent manner. A histaminergic H₂-receptor antagonist, cimetidine, but not a H₁-receptor antagonist, diphenhydramine, at 10 μ M produced a rightward shift in the concentration-response curve for histamine. Diazepam (10 μ M ) shifted the concentration-response curve for histamine and forskolin to the left by 1.8 and 1.6 times, respectively. Neither a central type (fulmazenil) nor a peripheral type (PK11195) of benzodiazepine receptor antagonist modified the effect of diazepam on the histaminergic-evoked contraction. Phosphodiesterase blockade by 3-isobutyl-1-methylxanthine shifted the concentration-dependent curve for histamine to the left. A combination of 3-isobutyl-1-methylxanthine also produced a leftward shift of the curve. However, there was no significant difference between the 3-isobutyl-1-methylxanthine only group and the combination group. These results indicate that diazepam potentiates the positive inotropic effect produced by histamine, probably mediated via an increase in cyclic AMP levels induced by histamine.     

Cisapride Stimulates Contraction of Idiopathic Megacolonic Smooth Muscle in Cats

We have previously shown that cisapride, a substituted piperidinyl benzamide, stimulates contraction of healthy feline colonic smooth muscle. The purpose of the present investigation was to determine the effect of cisapride on feline idiopathic megacolonic smooth muscle function. Longitudinal smooth muscle strips from ascending and descending colon were obtained from cats with idiopathic megacolon, suspended in a 1.5 mM Ca²⁺-HEPES buffer solution (37°C, 100% O₂, pH 7.4), attached to isometric force transducers, and stretched to optimal muscle length (L₀). Control responses were obtained at each muscle site with acetylcholine (10–⁸ to 10 ⁴ M), substance P (10–¹¹ to 10^-7 M), or potassium chloride (10 to 80 mM). Muscles were then stimulated with cumulative (10–⁹ to 10–⁶ M) doses of cisapride in the absence or presence of tetrodotoxin (10–⁶ M) and atropine (10–⁶ M), or in a 0 calcium HEPES buffer solution. In cats with idiopathic megacolon, cisapride stimulated contractions of longitudinal smooth muscle from both the ascending and the descending colon. Cisapride-induced contractions were similar in magnitude to those induced by substance P and acetylcholine in the ascending colon, but were less than those observed in the descending colon. Cisapride-induced contractions in megacolonic smooth muscle were only partially inhibited by tetrodotoxin and atropine, but were virtually abolished by removal of extracellular calcium. We concluded that cisapride-induced contractions of feline megacolonic smooth muscle are largely smooth muscle mediated and dependent on influx of extracellular calcium. Cisapride-induced contractions in megacolonic smooth muscle are only partially dependent on enteric cholinergic nerves. Thus, cisapride may be useful in the treatment of cats with idiopathic megacolon.     

Pharmacological characterization of alpha1-adrenoceptor subtypes in the bovine tail artery

Ioudina, M. V., Dyer, D. C. Pharmacological characterization of alpha1-adrenoceptor subtypes in the bovine tail artery. J. vet. Pharmacol. Therap. 25, 363-369. The purpose of this study was to identify the alpha1-adrenoreceptor subtypes present in the bovine tail artery which mediate contractions to adrenergic agonists. A61603, an alpha1A-selective agonist, was more potent compared with norepinephrine and phenylephrine. The pKA value of A61603 was 6.93 +/- 0.19 microM (n=6). Antagonists, BMY 7378, WB 4101 and 5-methylurapidil, caused a parallel shift to the right of the concentration-response curve to A61603 with pA2 values of 6.62, 9.27 and 8.86, respectively. Prazosin, BMY 7378 and WB 4101 inhibited phenylephrine induced contraction with pA2 values of 9.47, 7.17 and 9.73, respectively. The pA2 values obtained for 5-methylurapidil, WB 4101, BMY 7378 and prazosin against alpha1-adrenoceptor agonists were significantly correlated with pKi values (Zhu, Zhang & Han, 1997, Eur. J. Pharmacol.329, 55-61) for the cloned alpha1a-adrenoceptor but not with the cloned alpha1b- or alpha1d-adrenoceptor. BMY 7378, a selective alpha1D-adrenoceptor antagonist, was significantly more potent against the nonsubtype selective agonist phenylephrine than to A61603. Chloroethylclonidine (50 microM for 10 min) did not affect contractile responses to A61603, but caused a significant inhibition of contractile responses to phenylephrine. In conclusion, it appears that alpha1A- and alpha1D-adrenoceptors play a role in adrenergic mediated contraction in the bovine tail artery.     

Response of equine airway smooth muscle to acetylcholine and electrical stimulation in vitro

Smooth muscle strips from the midcervical portion of the trachea and bronchial smooth muscle strips from third-generation airways of horses were placed in tissue baths, and isometric contractile force was measured. Active force was measured in response to electrical stimulation and exogenous acetylcholine. Square-wave electrical stimuli were applied at various voltages (10, 12, 15, 18, 20, 25 V), frequencies (3, 5, 10, 15, 20, 25, 30 Hz), and pulse durations (0.2, 0.5, 1.0, 1.5, 2.0 ms). Isometric contractile force increased as voltage, frequency, and pulse duration increased. Maximal contractile response to electrical stimulation was obtained at 18 V, 25 Hz, and 0.5 ms. Atropine (10(-6)M) or tetrodotoxin (3 x 10(-6)M) blocked the contraction, indicating that the contractile response was attributable to the release of neurotransmitter from cholinergic nerves. Cumulative concentration-response curves to acetylcholine (10(-9)M through 10(-4)M) were determined. Isometric contractile force increased as acetylcholine concentration increased. There was a significant (P less than 0.05) difference in the 50% effective dose for acetylcholine in tracheal smooth muscle and bronchial smooth muscle. The mean (+/- SD) contractile response to maximal electrical stimulus was 89% (+/- 7.4%) of that in response to 10(-4)M acetylcholine in tracheal smooth muscle and was 68% (+/- 10.4%) of the response to 10(-4)M acetylcholine in bronchial smooth muscle.