Effect of catecholamines on the smooth muscles of the esophageal groove of calves

The effect of noradrenaline and adrenaline on isolated smooth muscle from the reticular groove of calves was studied. Both catecholamines caused a concentration-dependent (1.1.10(-6) mol/l) contraction of the transversal muscle strips from the floor of the reticular groove. This excitatory effect was antagonized by prazosin (10(-7)) mol/l), and by high concentrations of yohimbine (10(-6) mol/l) and atropine (10(-5)) mol/l). Tetrodotoxin (3.10(-6) mol/l), an inhibitor of nerve conduction, did not change the contraction induced by catecholamines (55.10(-6)) mol/l). Catecholamines did not produce a contraction of the longitudinal muscle from the lips of the reticular groove. The beta-adrenergic agonist isoprenaline (55.10(-6) mol/l) even elicited a reduction of acetylcholine (55.10(-6)) mol/l) induced contraction of both the transversal and the longitudinal muscle from the reticular groove. The relaxing effect of isoprenaline was antagonized by propranolol (55.10(-6)) mol/l). According to these results the excitatory effect of catecholamines on the smooth muscle cells occurs through alpha-adrenergic receptors, whereas the relaxing effect is mediated by beta-adrenergic receptors of the muscle cell. The excitatory effect of catecholamines on the transversal muscle appears to be predominant. Atropine appears to be an unspecific blocking agent of alpha-adrenergic receptors.     

外源性硫化氢对骨骼肌疲劳的影响简

试验旨在探讨硫化氢供体硫氢化钠改善疲劳状态下肌力减退的作用。采用脉冲式电流直接刺激腓肠肌作为疲劳模型,对比硫氢化钠和任氏液分别对蟾蜍腓肠肌疲劳的影响。结果表明,硫氢化钠在浓度为5×10^-4、1×10^-3、2×10^-3 mol/L时,离体腓肠肌达到最大收缩幅度90%、50%和10%的时间极显著长于在任氏液作用下的离体腓肠肌(P<0.01);5.6 mg/kg硫氢化钠也极显著延长了在体腓肠肌达到最大收缩幅度90%、50%和10%的时间(P <0.01)。表明外源性硫化氢具有延缓肌肉疲劳的作用。     

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.     

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 evaluation of the effect of the opioid antagonist N-methylnaltrexone on motility of the equine jejunum and pelvic flexure

REASONS FOR PERFORMING STUDY: Although potent analgesics, opioids decrease intestinal activity, leading to ileus in many species. N-methylnaltrexone (MNTX), an opioid antagonist which does not cross the blood-brain barrier and antagonises the morphine effect on the intestine, directly stimulates motility and restores function without affecting analgesic properties. While its use has been reported in human subjects, there is no information with regard to its usage in the horse. OBJECTIVES: To determine whether MNTX has an effect on contractile activity of the equine jejunum and pelvic flexure. METHODS: Using circular smooth muscle strips obtained from 8 mature horses, increasing concentrations of MNTX were added to tissue baths in the range of 1 x 10(-9) to 1 x 10(-5) mol/l, and contractile responses were recorded for 3 mins. Data were analysed using a repeated measures ANOVA to determine whether there was a significant drug effect compared to baseline activity. Data were analysed between the jejunum and pelvic flexure using a Mann-Whitney U test. Statistical significance was established as P < 0.05. RESULTS: The administration of MNTX significantly increased the contractile frequency and amplitude at all concentrations relative to baseline (P < 0.0001) for the jejunum. The response was greatest at 1 x 10(-7) mol/l (P = 0.0005), with a mean difference from baseline of 115.12 g/cm2. The highest concentration evaluated (1 x 10(-5) mol/l) had a mean contractile strength of 69.76 g/cm2, which was significantly greater than baseline activity (P = 0.04). A significant increase in contractile activity for the colon was detected at 3 x 10(-7) mol/l and all subsequent concentrations (P < 0.04). Unlike the jejunum, the contractile activity of the pelvic flexure increased progressively with the addition of each subsequent concentration. CONCLUSIONS: N-methylnaltrexone has a direct effect on circular smooth muscle of the equine jejunum and pelvic flexure resulting in an increase in contractile activity. POTENTIAL RELEVANCE: N-methylnaltrexone could potentially be used in conjunction with morphine to provide potent and effective analgesia without compromising intestinal function. Further in vivo investigations are required to determine whether this agent antagonises morphine's effect on motility.     

奶牛输卵管平滑肌前列腺素类受体的分析研究

本研究选用了前列腺素类化合物(Prostaglandin E2（PGE2）、Prostaglandin F2α（PGF2α）、Prostaglandin D2（PGD2)）和受体选择性激动剂（Butaprost、U-46619）,用多道生理信号采集系统测定其对体外分离的奶牛输卵管峡部及壶腹部平滑肌收缩的影响,以此初步揭示受体的种类。结果发现,PGE2和Butaprost均抑制了奶牛输卵管峡部、壶腹部的自发性收缩运动,呈现明显的浓度依存性舒张反应;PGF2α在高浓度(1～10 μmol/L)下,使峡部和壶腹部的自发性收缩运动有了显著增强;PGD2在低浓度时轻微抑制了壶腹部平滑肌的收缩,但在高浓度(1～10 μmol/L)时加强了峡部和壶腹部平滑肌的收缩;U-46619从低浓度便引起了输卵管峡部和壶腹部平滑肌的强烈收缩。结果表明,EP4、EP2、FP、DP和TP受体同时存在于奶牛输卵管平滑肌上,能与内源性前列腺素类结合并参与调节输卵管平滑肌的自发性收缩。     

Spasmogenic action of endothelin-1 on isolated equine pulmonary artery and bronchus

REASONS FOR PERFORMING STUDY: There is currently little published information about the effects of endothelin-1 (ET-1), a potent endogenous spasmogen of vascular and airway smooth muscle, on pulmonary vasculature and airways or which ET receptor subtypes mediate ET-1-induced vasoconstrictive and bronchoconstrictive action in the horse. OBJECTIVES: To investigate the effect of endothelin-1 (ET-1) on smooth muscle from isolated equine pulmonary artery and bronchus. In addition, the roles of ETA and ETB receptors in ET-1 mediated contraction in these tissues were assessed. METHODS: The force generation of ring segments from pulmonary arteries or third-generation airways (obtained from horses subjected to euthanasia for orthopaedic reasons) were studied in an organ bath at 37 degrees C in response to exogenous endothelin and selective endothelin A (BQ123) or B receptor (BQ788) antagonists. RESULTS: ET-1 produced concentration-dependent contractions of the equine pulmonary artery and bronchus. The threshold for contraction was 10(-10) and 10(-9) mol/l ET-1 for pulmonary artery and bronchus, respectively. The maximal contraction induced by the highest ET-1 concentration (10(-7) mol/l) was 173 and 194% of the contraction obtained with 100 mmol/l KCl in pulmonary artery and bronchus, respectively. ET-1 potency was 25 times greater in equine pulmonary artery than in equine bronchus (concentration of ET-1 producing 50% of maximal contraction [EC50] = 5.6 10(-9) mol/l and 2.2 10(-8) mol/l, respectively). In pulmonary artery, ET-1 induced contractions were significantly inhibited by the ETA receptor antagonist BQ123 (1 micromol/l; dose-response curve to ET-1 was shifted to the right by 5.4-fold), but not by the ETB antagonist BQ788. In bronchus, dose-responses curves to ET-1 were shifted to the right by BQ123 (1 micromol/l; 2.5-fold), but not by BQ788 (1 micromol/l). In the presence of both antagonists, the dose-response curve to ET-1 was shifted to the right by 4.5-fold. CONCLUSIONS: These functional studies demonstrate that ET-1 is a potent spasmogen of equine third generation pulmonary artery and bronchus, and that contractions are mediated via ETA receptors in the former and both ETA and ETB receptors in the latter. POTENTIAL CLINICAL RELEVANCE: Endothelin receptor antagonists may have potential for treating equine pulmonary hypertension or bronchoconstriction.     

Tamoxifen induces vasorelaxation via inhibition of mitogen-activated protein kinase in rat aortic smooth muscle

The contribution of the mitogen-activated protein kinase (MAPK) pathway to the relaxation induced by tamoxifen, a synthetic non-steroidal anti-estrogen, was examined in rat vascular smooth muscle. Tamoxifen (0.1-300 microM) inhibited the contraction induced by endothelin-1 (ET-1, 3 nM) in aortic smooth muscle in a concentration-dependent manner. The inhibitory effect of tamoxifen was not attenuated by 10 microM ICI 182,780, a selective antagonist of estrogen receptors. In the Ca(2+) channel inhibitor verapamil (1 microM)-pretreated strips, tamoxifen also inhibited the contraction induced by ET-1. Both PD098059 and SB203580, inhibitors of MAPK/extracellular signal-regulated kinase (ERK) kinase and p38 MAPK, respectively, inhibited ET-1-induced contraction in aortic smooth muscle. In Western blot analysis with anti-phosphorylated MAPK antibodies, ET-1 (3 nM) enhanced activities of both ERK1/2 and p38 MAPK in aortic muscle strips, which were not attenuated by the treatment with 4 mM EGTA. Tamoxifen (100 microM) inhibited the activities of ERK1/2 and p38 MAPK induced by ET-1 without significant changes in the expression of these kinases. These results suggest that tamoxifen induces relaxation of rat vascular smooth muscle, and that this is, at least in part, mediated by the inhibition of the Ca(2+)-independent MAPK pathway.     

In vitro effects of lipopolysaccharides on bovine uterine contractility

Metritis is an important disorder in dairy cows during the early postpartum period. Myometrial contractility is a prerequisite for uterine involution; however, very scanty literature is available about the effect of metritis on this process and endocrine responsiveness. This study was aimed to evaluate the effect of inflammation on uterine contractility in vitro, and the inflammation was induced by incubating myometrial strips with lipopolysaccharides (LPS). Myometrial samples were collected from 17 healthy Holstein Friesian cows during caesarean section. Eight longitudinal strips from each cow were incubated in organ baths with LPS concentrations of 0 (LPS₀), 0.1 (LPS_0.1), 1 (LPS₁) and 10 µg/ml (LPS₁₀). Spontaneous contractility and contractility induced by increasing concentrations of oxytocin (10^–10 – 10^–7 mol/L) were recorded during nine 30-min intervals (T1 to T9). The minimum amplitude (minA), maximum amplitude (maxA), mean amplitude (meanA) and area under the curve (AUC) were calculated for each time interval. LPS had an effect (p ≤ .05) on maxA, meanA and AUC. In T1, myometrial strips incubated with LPS_0.1 and LPS₁ had higher (p ≤ .05) maxA, meanA and AUC than the strips incubated with LPS₀. In T9 without oxytocin, LPS₀ led to higher (p ≤ .05) maxA, meanA and AUC than LPS_0.1 and LPS₁. In T8 and T9 with oxytocin, LPS₁ had lower (p ≤ .05) maxA, meanA and AUC than the other LPS concentrations. Interestingly, the results show that LPS has a transient positive effect on myometrial contractility in vitro and that this effect is dependent on LPS concentration and duration of incubation.     

Effect of short-chain fatty acids on contraction of smooth muscle in the canine colon

OBJECTIVE: To determine effects of short-chain fatty acids (SCFA) on canine colonic smooth muscle. SAMPLE POPULATION: Colonic tissue obtained from 14 healthy dogs. PROCEDURE: Short-chain fatty acid (SCFA; acetate, propionate, and butyrate; 1 to 100 mmol/L)-induced contractions were compared with responses obtained with acetylmethylcholine (AMCh; 10(-4) mol/L). Roles of enteric neurons, cholinergic receptors, calcium stores in the sarcoplasmic reticulum, and extracellular calcium in the SCFA-induced responses were investigated by incubating muscle strips with tetrodotoxin (1 micromol/L), atropine (1 micromol/L), ryanodine (10 micromol/L), nifedipine (1 micromol/L), ethylene glycol-bis (beta-aminoethylether)-N,N,N',N'-tetra-acetate (EGTA; 0.1 mmol/L), or an extracellular calcium-depleted (zero extracellular calcium) solution prior to the addition of propionate or butyrate. RESULTS: Incubation with SCFA elicited isometric stress responses (0.25 to 2.15 x 10(4) N/m2) in colonic longitudinal smooth muscle. Maximal responses to butyrate and propionate (50 mmol/L) were 37 and 23%, respectively, of the maximal AMCh response. Acetate was least effective in stimulating contractile responses. Tetrodotoxin and atropine did not affect SCFA-induced contractions. Nifedipine and zero extracellular calcium solution abolished responses to butyrate and propionate, whereas EGTA attenuated (> 60%) but did not abolish those responses. Ryanodine did not affect SCFA-induced contractile responses. The SCFA did not affect colonic circular smooth muscle. CONCLUSIONS AND CLINICAL RESPONSE: The SCFA stimulate longitudinal but not circular colonic smooth muscle contractions via a direct effect on smooth muscle. The mechanism of the SCFA effect appears to involve the influx of extracellular calcium. These findings may account for some of the effects of fiber on canine colonic motility [corrected].     

Effects of bethanechol on canine urinary bladder smooth muscle function

We studied whether the effects of bethanechol are mediated via a muscarinic receptor, the role of extracellular calcium on bladder contraction, and down-regulation of bladder contraction by bethanechol after activation with potassium chloride (KCl) and acetylcholine (Ach). Smooth muscle strips of normal urinary bladder were studied with standard methods to measure isometric force. Bethanechol caused a dose-dependent increase in bladder contraction. The potency of bethanechol is higher than Ach, as shown by higher peak active isometric stress (P(max)) and lower half-maximal contraction (ED(50)) (P< 0.01). The contractile responses to bethanechol were diminished in the presence of atropine, nifedipine and in calcium-free medium as shown by P(max) decreased by 58%, 87% and 65% and ED(50) increased by 314-, 24- and 16-fold, respectively. When bladder strips were stimulated with KCl and Ach, pre-treatment with bethanechol reduced the responses to KCl by 116-242% (P<0.05), while the contractile responses to Ach were unaltered. Thus, bethanechol induces bladder contraction via muscarinic receptor activation while both intracellular and extracellular calcium play a crucial role on bladder smooth muscle contraction. The mechanisms of down-regulation by bethanechol may be related to interference with calcium influx into the smooth muscle cells, rather than the desensitisation of muscarinic receptors or post-receptor steps of signal transduction following bethanechol binding to the receptor.     

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.     

Responses of goat ruminal musculature to substance P in vitro and in vivo

A concentration-response curve obtained for contraction of goat rumen strips indicated that substance P (SP) is a very potent spasmogenic agent. In vivo, SP caused a dose-dependent inhibition of the extrinsic rumen contractions possibly due to an increase in ruminal smooth muscle tone, resulting in reflex inhibition. SP did not induce significant changes in heart rate. Further studies are necessary to elucidate a possible physiological role of SP in the modulation of forestomach motility.     

Mediation of acetylcholine and substance P induced contractions by myosin light chain phosphorylation in feline colonic smooth muscle

OBJECTIVES: To determine the role of myosin light chain phosphorylation in feline colonic smooth muscle contraction. SAMPLE POPULATION: Colonic tissue was obtained from eight 12- to 24-month-old cats. PROCEDURE: Colonic longitudinal smooth muscle strips were attached to isometric force transducers for measurements of isometric stress. Myosin light chain phosphorylation was determined by isoelectric focusing and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Stress and phosphorylation were determined following stimulation with ACh or SP, in the absence or presence of a calmodulin antagonist (W-7; 0.1 to 1.0 mM), myosin light chain kinase inhibitor (ML-9; 1 to 10 microM), or extracellular calcium free solutions. RESULTS: Unstimulated longitudinal colonic smooth muscle contained low amounts (6.9+/-3.2%) of phosphorylated myosin light chain. Phosphorylation of the myosin light chains was dose and time dependent with maximal values of 58.5% at 30 seconds of stimulation with 100 microM Ach and 60.2% at 45 seconds of stimulation with 100 nM SP Active isometric stress development closely paralleled phosphorylation of the myosin light chains in ACh- or SP-stimulated muscle. W-7 and ML-9 dose dependently inhibited myosin light chain phosphorylation and isometric stress development associated with ACh or SP stimulation. Removal of extracellular calcium inhibited myosin light chain phosphorylation and isometric stress development in ACh-stimulated smooth muscle. CONCLUSIONS AND CLINICAL RELEVANCE: Feline longitudinal colonic smooth muscle contraction is calcium-, calmodulin-, and myosin light chain kinase-dependent. Myosin light chain phosphorylation is necessary for the initiation of contraction in feline longitudinal colonic smooth muscle. These findings may prove useful in determining the biochemical and molecular defects that accompany feline colonic motility disorders.     

Responses of goat ruminal musculature to substance P in vitro and in vivo

A concentration-response curve obtained for contraction of goat rumen strips indicated that substance P (SP) is a very potent spasmogenic agent. In vivo, SP caused a dose-dependent inhibition of the extrinsic rumen contractions possibly due to an increase in ruminal smooth muscle tone, resulting in reflex inhibition. SP did not induce significant changes in heart rate. Further studies are necessary to elucidate a possible physiological role of SP in the modulation of forestomach motility.     

Evaluation of substance P as a neurotransmitter in equine jejunum

OBJECTIVE: To determine whether substance P (SP) functions as a neurotransmitter in equine jejunum. SAMPLE POPULATION: Samples of jejunum obtained from horses that did not have lesions in the gastrointestinal tract. PROCEDURE: Jejunal smooth muscle strips, oriented in the plane of the circular or longitudinal muscle, were suspended isometrically in muscle baths. Neurotransmitter release was induced by electrical field stimulation (EFS) delivered at 2 intensities (30 and 70 V) and various frequencies on muscle strips that were maintained at low tension or were under contraction. A neurokinin-1 receptor blocker (CP-96,345) was added to baths prior to EFS to interrupt SP neurotransmission. Additionally, direct effects of SP on muscle strips were evaluated, and SP-like immunoreactivity was localized in intestinal tissues, using indirect immunofluorescence testing. RESULTS: Substance P contracted circularly and longitudinally oriented muscle strips. Prior treatment with CP-96,345 altered muscle responses to SP and EFS, suggesting that SP was released from depolarized myenteric neurons. Depending on orientation of muscle strips and stimulation variables used, CP-96,345 increased or decreased the contractile response to EFS. Substance P-like immunoreactivity was detected in the myenteric plexus and circular muscle layers. CONCLUSIONS AND CLINICAL RELEVANCE: Substance P appears to function as a neurotransmitter in equine jejunum. It apparently modulates smooth muscle contractility, depending on preexisting conditions. Effects of SP may be altered in some forms of intestinal dysfunction. Altering SP neurotransmission in the jejunum may provide a therapeutic option for motility disorders of horses that are unresponsive to adrenergic and cholinergic drugs.     

5-Hydroxytryptamine potentiates contraction mediated by the intramural cholinergic nerve in the longitudinal smooth muscle of the ruminant forestomach

The effects of 5-hydroxytryptamine (5-HT) on the longitudinal smooth muscle from the rumen and reticulum of the bovine forestomach were investigated. 5-HT (0.25–490 μM) caused a contraction and a relaxation of the ruminal strips while it produced only an excitatory effect on the reticular strips. These effects were not affected by tetrodotoxin, hexamethonium, atropine or morphine, but were blocked by methysergide, LSD-25 or phenoxybenzamine. 5-HT potentiated the contraction evoked by stimulation of the intramural cholinergic nerves but did not show any effect on the relaxation produced by the non-adrenergic inhibitory nerves' excitation. The 5-HT-induced potentiation was not affected by morphine, LSD-25, methysergide and hexamethonium or high concentration of nicotine. Nicotine and dimethylphenylpiperazinium also caused a transient augmentation of the nerve-mediated contraction, but these effects were abolished by the competitive ganglionic blockers. The evoked contraction was depressed in high-Mg²⁺ solution, but this depression was antagonized partly by 5-HT. The affinity of the cholinomimetics to post-synaptic muscarinic receptor was not affected by 5-HT. It is concluded that contractions or relaxations of bovine forestomach strips induced by 5-HT are mediated through activation of D-receptors in the smooth muscle, and the 5-HT-induced potentiation of the evoked contraction may be elicited through presynaptic neural effects of 5-HT on the cholinergic nerves.     

The vascular responses of the isolated perfused bovine external ear to exogenous histamine and 5-HT (serotonin)

The responses of the vasculature of isolated, non-sensitized, bovine external ears to histamine and serotonin (5-HT) were evaluated while they were being perfused with Krebs-Henseleit solution, Histamine (10^–5 mol/L to 5×10^–3 mol/L) and 5-hydroxytryptamine (5-HT) (10^–9 mol/L to 10^–2 mol/L) caused increased vascular resistance. Mepyramine (10^–7 mol/L), cimetidine (10^–5 mol/L) and atropine (10^–6 mol/L) inhibited the responses to histamine. The responses to 5-HT were inhibited by methysergide (10^–9 mol/L) and potentiated by morphine (10^–5 mol/L). These results suggest the presence of H₁ and H₂ histamine, and 5-HT receptors in bovine auricular vessels, all of which cause net vasoconstriction.     

Vasomotor effects of histamine on bovine and equine basilar arteriesin vitro

The vasomotor effects of histamine on isolated bovine and equine basilar arteries were examined. Histamine induced contractions in both these preparations. The maximal response to and pEC₅₀ value for histamine of the equine artery were larger than those of bovine tissue. Similar results were obtained with endothelium-denuded basilar arteries. Diphenhydramine (H₁-receptor antagonist) inhibited histamine-induced contractions of the basilar arteries from both species in a concentration-dependent manner and its pA ₂ values (with 95% confidence limits) were 7.61 (7.39–7.83) and 8.15 (8.01–8.29) for the bovine and equine preparations, respectively. Cimetidine (H₂-receptor antagonist) slightly potentiated histamine-induced contractions of bovine, but not equine, basilar arteries. 2-Thiazolylethylamine (H₁-receptor agonist) induced contractions in both preparations, whereas impromidine (H₂-receptor agonist) induced weak relaxation of the bovine, but not the equine, tissue. These findings indicate that bovine basilar arterial smooth muscle cells possess H₁- and H₂-receptors. Stimulation of the former results in contraction, whereas stimulation of the latter results in weak relaxation. Equine basilar arterial smooth muscle cells possess H₁-receptors, stimulation of which results in contraction.Abbreviations CR concentration ratio - EC₅₀ concentration producing 50% maximal response - pA ₂ negative logarithm of the molar concentration of antagonist that produces a twofold rightward shift of a concentration-response curve - pEC₅₀ negative logarithm of EC₅₀ - PGF₂ prostaglandin F₂ - PGI₂ prostaglandin I₂     

Contractile responses of isolated equine digital arteries under hypoxic or hyperoxic conditions in vitro: role of reactive oxygen species and Rho kinase

The underlying pathophysiological triggers for equine acute laminitis are unknown, although digital vasoconstriction, ischaemia, hypoxia and reperfusion injury may be involved. The contractile responses of isolated equine digital arteries (EDAs), harvested from the hindlimbs of normal horses postmortem at an abattoir, were studied acutely (up to 3 h) under hyperoxic (95% oxygen, 5% CO₂) and hypoxic (95% nitrogen, 5% CO₂) conditions in organ baths. Phenylephrine (PHE; 10^?6 m ), 5‐hydroxytryptamine (5‐HT; 10^?7 m ) and high potassium (K⁺; 118 mm ) caused contraction in EDAs which was significantly (P < 0.0001) enhanced under hypoxic conditions. In contrast, contraction stimulated by 9,11‐dideoxy‐9α,11α‐epoxymethanoprostaglandin F_2α (U44069; 3 × 10^?8 m ) was not significantly enhanced by hypoxia (P = 0.75). Hypoxia‐enhanced contraction in response to K⁺ was greater (P < 0.03) in vessels with a functional endothelium than in vessels in which the endothelium was removed by rubbing. Fasudil (10^?6 to 10^?5 m ), a Rho kinase inhibitor, and apocynin (10^?3 to 3 × 10^?3 m ), an NADPH oxidase inhibitor, significantly (P ≤ 0.05) inhibited hypoxia‐enhanced contraction in response to PHE and 5‐HT. In conclusion, hypoxia‐enhanced contraction occurred in EDAs. This appears to be partially mediated by reactive oxygen species produced by NAPDH oxidase, which activate Rho kinase to increase calcium sensitisation and enhance smooth muscle contraction.