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.     

Muscarinic Receptors in Equine Airways

The distribution of muscarinic receptors in equine airways was investigated using autoradiography. Frozen sections of tissue from six different levels in the bronchial tree, from the trachea to the distal bronchioles, were incubated in vitro with 1.5 nmol/L of the muscarinic receptor antagonist 1-[N-methyl-³H]scopolamine methyl chloride (³H-NMS). In addition, the subtype pattern of muscarinic receptors was investigated in equine tracheal smooth muscle using radioligand binding with methoctramine, tripinamide, 4-DAMP-methiodide and pirenzipine as competitors against the binding of 1.3 nmol/L ³H-NMS. The autoradiograms showed specific labelling indicating a high density of muscarinic receptors in smooth-muscle tissue in all levels of the airway tree investigated. Besides muscle tissue, subepithelial glands were the only structures specifically labelled. The dominating subtypes in tracheal smooth muscle investigated with radioligand binding studies were found to be M₂ and M₄, as both methoctramine (pK _d = 8.5) and tripinamide (pK _d = 8.6 and 6.7 for two different sites) showed high affinity. The density of the M₃-muscarinic receptor subtype was low, but this subtype could be detected with statistical significance when methoctramine was used as the competitor against ³H-NMS binding.     

Penetrating injury at the thoracic inlet in a Paint‐Arab mare

A 12‐year‐old Paint‐Arab mare was admitted for evaluation of a penetrating chest laceration at the thoracic inlet. The left brachiocephalic muscle was transected and the recurrent laryngeal nerve was traumatised. Subsequent to the injury, the horse developed Horner's syndrome on the left side of the neck and face, Grade IV left laryngeal hemiplegia, dysphagia, cough and subcutaneous emphysema. The defect was closed in multiple layers. Antimicrobial and antiinflammatory therapy was instituted along with local wound care. The mare remained bright and responsive and the wound healed normally. The mare showed no signs of respiratory distress. Dysphagia and ptosis persisted at 30 days post trauma.     

Assessment of computed tomography derived cricoid cartilage and tracheal dimensions to evaluate degree of cricoid narrowing in brachycephalic dogs

The aims of this observational, analytical, retrospective study were to (i) obtain computed tomographic (CT) cricoid dimensions (height, width, and transverse‐sectional area), (ii) compare the cricoid dimensions between brachycephalic and mesaticephalic breeds, and (iii) compare cricoid cartilage dimensions between dogs without and affected with brachycephalic airway syndrome. The study is important to help to further evaluate and understand the anatomical components of brachycephalic airway syndrome. Measurements were performed in 147 brachycephalic and 59 mesaticephalic dogs. The cricoid cartilage was found to be significantly more oval in Pugs and French Bulldogs compared to mesaticephalic breeds. The cricoid cartilage transverse‐sectional area was smallest for the Pug and, after adjusting for weight, significantly smaller for Pugs (P < 0.001), Boston Terriers (P = 0.001), and French Bulldogs (P < 0.001) compared to Jack Russell Terriers. The tracheal transverse‐sectional area at C4 of English Bulldogs was significantly smaller than for Jack Russell Terriers (P = 0.005) and Labradors (P < 0.001). The cricoid cartilage transverse‐sectional area:weight ratio was significantly lower in brachycephalic breeds compared to mesaticephalic breeds (P < 0.001). The cricoid cartilage:trachea at C4 transverse‐sectional area for brachycephalic dogs was significantly larger than for mesaticephalic dogs (<0.001), demonstrating that the trachea was the narrowest part of the airway. No significant differences were found for cricoid dimensions between dogs affected with and without brachycephalic airway syndrome. However, large individual variation was found among the brachycephalic breeds and further studies investigating the relationship between cricoid cartilage size, laryngeal collapse, concurrent tracheal hypoplasia, and/or severity of brachycephalic airway syndrome are warranted.     

传染性支气管炎病毒的鉴定

试验采用鸡胚接种和气管环培养相结合的方法,对IBV毒株进行培养鉴定,IBV经鸡胚传一代后再上鸡胚气管环培养可引起气管环纤毛运动停止,应用此法可以快速地进行IBV检测。     

Treatment of traumatic tracheal collapse with extraluminal titanium mesh screens

A 12‐year‐old Thoroughbred gelding presented with a history of exercise intolerance and inspiratory stridor. Physical examination revealed a blunt crush injury to the ventral neck and underlying trachea of unknown duration. Tracheoscopy revealed a grade 3 tracheal collapse. Surgical management of the case involved placement of extraluminal titanium mesh screens circumferentially around the trachea for complete support. During the period of hospitalisation, the only post operative complication was seroma formation, which resolved with routine care. Follow‐up after 5 years revealed no further evidence of exercise intolerance, inspiratory dyspnoea or stridor. The horse returned to his previous level of work as a show hunter.     

Tracheal rupture following general anaesthesia in a horse

A Dutch Warmblood, with no other underlying clinical disease, presented for surgical excision of a sarcoid tumour on the distal right pinna under general anaesthesia. At the end of the procedure, immediately before being moved to recovery, the horse became light and made repeated attempts to move whilst attached to the hoist. Anaesthesia was deepened with intravenous thiopental sodium (Thiopentone)¹ and the horse was moved into the recovery room. The trachea was extubated with the cuff of the endotracheal tube inadvertently left partially inflated. Recovery was smooth and the horse stood uneventfully. The following day subcutaneous emphysema was noted along the neck and tracheoscopy revealed an abnormal dorsoventrally flattened trachea and a 5 cm tear in the dorsal aspect of the trachea. Symptomatic treatment resulted in progressive healing of the lesion and the horse recovered fully with no evidence of respiratory complications.