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  • Author or Editor: N. Edward Robinson x
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Abstract

Objective—To determine the effect of histamine on the contractile elements of the respiratory tract in neonatal calves and young adult cattle.

Sample Population—Samples of trachealis muscle, bronchi, and intrapulmonary arteries and veins dissected from the respiratory tracts of healthy bovids (2 to 8 days and 16 to 20 months old).

Procedure—Histamine cumulative concentrationeffect curves (10–8 to 10–3M) were constructed in duplicate smooth muscle samples mounted in organ baths. Contractile responses to histamine were compared with reference contractions elicited by methacholine (10–5M) for airways or KCl (127mM) for vessels.

Results—In young adult cattle, trachealis muscle had a substantial contractile response to histamine (84% of methacholine-induced contraction), whereas bronchi reacted slightly (15 and 20% for large and small bronchi, respectively). Although contractile responses to KCl were comparable in arteries and veins, histamine-induced contractions were greater for intrapulmonary veins than for arteries (202 vs 48% of KCl-induced contraction). In neonatal calves, histamine- induced contraction of veins also exceeded that of arteries (230 vs 54% of KCl-induced contraction); however, unlike in young adult cattle, histamine produced notable contraction of large and small bronchi (48 and 60% of methacholine-induced contraction, respectively).

Conclusions and Clinical Relevance—Compared with intrapulmonary arteries, intrapulmonary veins have greater contractile responses to histamine in neonatal and young adult cattle. Data suggest loss of histamine responsiveness in bronchial smooth muscle as neonatal calves grow to young adults. Venodilation may be useful in treatment of lung edema in cattle. (Am J Vet Res 2003;64:819–822)

Full access
in American Journal of Veterinary Research

Abstract

Objective—To determine whether the hyoepiglotticus muscle has respiratory-related electromyographic activity and whether electrical stimulation of this muscle changes the position and conformation of the epiglottis, thereby altering dimensions of the aditus laryngis.

Animals—6 Standardbred horses.

Procedure—Horses were anesthetized, and a bipolar fine-wire electrode was placed in the hyoepiglotticus muscle of each horse. Endoscopic images of the nasopharynx and larynx were recorded during electrical stimulation of the hyoepiglotticus muscle in standing, unsedated horses. Dorsoventral length and area of the aditus laryngis were measured on images obtained before and during electrical stimulation. Electromyographic activity of the hyoepiglotticus muscle and nasopharyngeal pressures were measured while horses exercised on a treadmill at 50, 75, 90, and 100% of the speed that produced maximum heart rate.

Results—Electrical stimulation of the hyoepiglotticus muscle changed the shape of the epiglottis, displaced it ventrally, and significantly increased the dorsoventral length and area of the aditus laryngis. The hyoepiglotticus muscle had inspiratory activity that increased significantly with treadmill speed as a result of an increase in phasic and tonic activity. Expiratory activity of the hyoepiglotticus muscle did not change with treadmill speed in 4 of 6 horses.

Conclusions and Clinical Relevance—Findings reported here suggest that contraction of the hyoepiglotticus muscle increases dimensions of the airway in horses by depressing the epiglottis ventrally during intense breathing efforts. The hyoepiglotticus muscle may be an important muscle for dilating the airway in horses, and contraction of the hyoepiglotticus muscle may induce conformational changes in the epiglottis. (Am J Vet Res 2002;63:1617–1621)

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in American Journal of Veterinary Research

Abstract

Objective—To evaluate leukotriene (LT) biosynthetic capacity in lung tissue from healthy horses and horses with recurrent airway obstruction (RAO).

Sample Population—Lung parenchyma and airway specimens from 8 RAO-affected and 5 healthy horses.

Procedure—Horses were stabled for ≥ 72 hours. Blood was drawn before euthanasia, after which lung specimens were collected. Tissue strips from small airways and parenchyma were incubated in organ baths with the precursor LTA4 or stimulated with calcium ionophore A23187 or the tripeptide N-formyl- Met-Leu-Phe (fMLP), with or without exogenous arachidonic acid, in the presence of isolated blood neutrophils.

Results—Stabling induced typical clinical signs of airway obstruction in RAO-affected horses but not control horses. When lung parenchyma or airway specimens from both groups of horses were incubated with calcium ionophore, with or without arachidonic acid, they did not form LT. In contrast, addition of LTA4 to both tissues resulted in conversion to LTB4, although concentrations of LTC4 were negligible in airways and parenchymal strips from healthy and RAOaffected horses. Incubation of airway and parenchymal strips with suspensions of autologous neutrophils did not influence formation of LT stimulated by calcium ionophore or fMLP, with or without exogenous arachidonic acid.

Conclusion and Clinical Relevance—Results suggest that lung parenchyma and airway tissues themselves are not of substantial importance for LT formation in the lungs, although these tissues possessed some LTA4 hydrolase activity, enabling LTB4 formation. It may be speculated that LTB4 originates primarily from neutrophils and may play a role in the inflammatory events of RAO. (Am J Vet Res 2002; 63:794–798)

Full access
in American Journal of Veterinary Research

Abstract

Objective—To use noninvasive respiratory inductance plethysmography (RIP) to investigate differences in breathing patterns between horses with and without recurrent airway obstruction (RAO) during the onset of airway obstruction induced through confinement to stables.

Animals—12 horses with no history or clinical signs of respiratory disease (control horses) and 7 RAO-affected horses.

Procedures—The study involved 2 phases. In phase 1, the optimal position of RIP bands for recording pulmonary function was investigated in 12 control horses. In phase 2, 7 RAO-affected and 7 control horses were confined to stables. Respiratory inductance plethysmography bands were applied to horses for 24 h/d to record respiratory rate and total displacement in 4-hour periods for 7 days or until RAO-affected horses developed signs of severe RAO that persisted for 2 consecutive days. Lung function was measured once daily.

Results—In phase 1, thoracic and abdominal cavity displacements were best represented by RIP bands positioned at intercostal spaces 6 and 17, respectively. In phase 2, pulmonary function indicated airway obstruction in the RAO-affected group on the final 2 days of stable confinement. Respiratory rate and total degree of respiratory displacement measured by RIP did not differ between the RAO-affected and control groups, but the SDs of these decreased significantly within 8 hours after stable confinement began in RAO-affected horses. Respiratory inductance plethysmography and pulmonary function findings became highly correlated as severity of disease progressed.

Conclusions and Clinical Relevance—The decrease in the SDs of RIP measurements indicated a lower degree of variability in breathing patterns of RAO-affected horses. This loss of variability may provide an early indicator of airway inflammation.

Full access
in American Journal of Veterinary Research

SUMMARY

During acute bouts of recurrent airway obstruction (heaves) in horses, neutrophils that are capable of increased production of reactive oxygen species accumulate in the airways. In the study reported here, the effect of hydrogen peroxide (H2O2; 1 µM to 0.1M), one of these reactive oxygen species products, on the responses of isolated trachealis muscle of horses was determined. Before and after incubation with H2O2, contractile responses to acetylcholine, electrical field stimulation (efs), 127 mM KCl, and relaxation responses to isoproterenol and activation of the nonadrenergic noncholinergic inhibitory response (iNANC) were evaluated. Beginning at 1 mM, H2O2 contracted trachealis muscle in a concentration-dependent manner. This contraction was unaffected by atropine (1 µM), tetrodotoxin (1 µM), or 1 µM meclofenamate. Contraction of trachealis muscle in response to H2O2 is, therefore, not attributable to release of prostaglandins, acetylcholine, or other neurotransmitters. Above a concentration of 0.1 mM, H2O2 depressed the responses to efs, acetylcholine, and KCl in a concentration-dependent manner. At 0.1M, H2O2 decreased the maximal responses to efs, acetylcholine, and KCl by 62.7 ± 7.2, 60.58 ± 6.12, and 37.8 ± 9.54%, respectively. In the presence of meclofenamate (1 µM), partial but significant protection against 1 to 100 mM H2O2 was observed. In tracheal strips contracted with 0.3 µM methacholine, H2O2 had no effect on the isoproterenol concentration-response curve. Up to a concentration of 100 mM, H2O2 had no effect on iNANC response. However, in the presence of 100 mM H2O2, this response was abolished in 2 of 4 horses. We conclude that high concentrations of H2O2 affected the responses of airway smooth muscle by actions on neurotransmission, muscarinic receptors, and downstream from receptors; some of the H2O2 effects were in part mediated by cyclooxygenase products; and H2O2 had no effect on β-adrenergic or iNANC-induced relaxation.

Free access
in American Journal of Veterinary Research

Abstract

Objective

To determine the effect of bilateral blockade of the pharyngeal branch of the vagus nerve on soft palate function in horses.

Animals

5 Standardbreds.

Procedure

Peak tracheal inspiratory and expiratory pressures and airflow were measured while horses exercised at the speeds corresponding to 75 and 100% of the speed that resulted in maximal heart rate, with and without pharyngeal branch of the vagus nerve blockade. Respiratory frequency-to-stride frequency coupling ratio was measured by correlating foot fall measurements with respiratory frequency. The pharyngeal branch of the vagus nerve was blocked bilaterally as the nerve coursed through the auditory tube diverticulum (guttural pouch) across the longus capitus muscle.

Results

Persistent, reversible dorsal displacement of the soft palate (DDSP) occurred in all horses after nerve blockade, and lasted from 1 to 3 hours; normal nasopharyngeal function returned within 3 hours. Compared with control values, peak expiratory tracheal pressure increased (P = 0.001), expiratory impedance increased (P = 0.007), and minute ventilation decreased (P = 0.04). Respiratory frequency-to-stride frequency coupling ratio decreased (P = 0.009) so that horses took 1 breath/stride without the nerve block and, approximately, 1 breath/2 strides with the block.

Conclusion

DDSP creates flow-limiting expiratory obstruction and may be caused by neuromuscular dysfunction involving the pharyngeal branch of the vagus nerve. It may alter performance by causing expiratory obstruction and by altering breathing strategy in horses.

Clinical Relevance

A repeatable, reversible model of DDSP exists that allows further study of the disease. Dysfunction of the neuromuscular group, pharyngeal branch of the vagus nerve and palatinus and palatopharyngeus muscles, may be implicated in the pathogenesis of clinical DDSP. (Am J Vet Res 1998;59:504–508)

Free access
in American Journal of Veterinary Research

Abstract

Objective

To determine the effect of bilateral hypoglossal and glossopharyngeal nerve block on epiglottic and soft palate position and tracheal and pharyngeal pressures in exercising horses.

Animals

5 Standardbreds.

Procedure

Tracheal and pharyngeal pressures were measured in 5 Standardbreds exercising at the speed at which the horses achieved 50, 75, and 100% of maximal heart rate after bilateral hypoglossal and glossopharyngeal nerve block and without nerve block. Nerve block was achieved by injection of 1 to 2 ml of 2% mepivicaine hydrochloride between the glossopharyngeal and hypoglossal nerves, as they coursed through the medial compartment of the diverticulum of the auditory tube (guttural pouch), using videoendoscopic guidance and an injection apparatus.

Results

Compared with control values, peak inspiratory tracheal pressure was significantly (P = 0.02) more negative, and peak pharyngeal inspiratory pressure was less negative (P = 0.004) after bilateral hypoglossal and glossopharyngeal nerve block. Respiratory frequency was significantly (P = 0.024) lower after nerve block, compared with control values. The epiglottis was unstable and retroflexed through the rima glottis during inspiration after bilateral hypoglossal and glossopharyngeal nerve block. Despite loss of contact between the epiglottis and the caudal free margin of the soft palate, dorsal displacement of the soft palate did not occur.

Conclusions and Clinical Relevance

Loss of contact of the epiglottis with the soft palate did not affect soft palate position, suggesting that when the soft palate is normal, the epiglottis does not function as a support, holding the soft palate in a ventral position. Therefore, epiglottic dysfunction is not solely responsible for intermittent dorsal displacement of the soft palate in horses, and neuromuscular dysfunction involving the hyoepiglotticus muscle, geniohyoideus muscle, or the hypoglossal nerve may cause epiglottic retroflexion in horses. (Am J Vet Res 1997;58:1022–1026)

Free access
in American Journal of Veterinary Research

Abstract

Objective

To determine the effect of bilateral tenectomy of the tensor veli palatini muscle on soft palate and nasopharyngeal function in exercising horses.

Animals

5 Standardbreds.

Procedure

Treadmill videoendoscopy was performed on 5 Standardbreds exercising at 50, 75, and 100% of the speed that produced maximum heart rate; tracheal and pharyngeal pressures were measured before and after surgery, Tenectomy of the tensor veli palatini muscle was performed bilaterally on each horse while under general anesthesia, using a transoral approach.

Results

Peak inspiratory tracheal pressures were significantly (P = 0.016) more negative and there was a trend (P = 0.06) for peak pharyngeal inspiratory pressure to be less negative following bilateral tenectomy of the tensor veli palatini muscle, compared with preoperative values. The rostral half of the soft palate was unstable and collapsed dorsally into the nasopharynx during inspiration, causing partial obstruction of the nasopharynx. The caudal free margin of the soft palate remained ventral to the epiglottis, and dorsal displacement of the soft palate did not occur in any horse.

Conclusions

Bilateral tenectomy of the tensor veli palatini muscle did not cause dorsal displacement of the soft palate in horses while exercising at maximum heart rate, but resulted in collapse of the nasopharynx during inspiration.

Clinical Relevance

Results of our study indicate that the tensor veli palatini muscle functions to support and dilate the nasopharynx during intense inspiratory efforts in horses by tensing the palatine aponeurosis. (Am J Vet Res 1997;58:317–321)

Free access
in American Journal of Veterinary Research

Summary

Tidal breathing flow-volume (tbfv) loops were determined in a group of control horses and in horses affected with recurrent airway obstruction (heaves). The latter group was studied when the condition was in remission and under increasing amounts of airway obstruction as reflected by measurements of change in pleural pressure, pulmonary resistance, and dynamic compliance. The tbfv loops of control horses had biphasic inspiratory and expiratory patterns; peak inspiratory and peak expiratory flows were detected early in inspiration and expiration, respectively. Tidal volume was unaffected by heaves, but at all stages of heaves, respiratory frequency was increased principally because of shorter inspiratory time, and therefore, inspiratory flow rate was increased. In horses with heaves, tbfv loops did not have a biphasic pattern; peak inspiratory flow was observed late in inspiration, and peak expiratory flow was observed early in expiration. As airway obstruction became more severe, peak expiratory flow increased as pulmonary resistance increased so that, during severe airway obstruction, tbfv loops had a characteristic appearance with high peak expiratory flow early in expiration followed by low flow rate.

Free access
in American Journal of Veterinary Research

SUMMARY

The effect of iv administration of the α2-adrenoceptor agonist xylazine hydrochloride (0.5 mg/kg of body weight) was examined in ponies with recurrent obstructive pulmonary disease, commonly called heaves. Six ponies with the disease (principals) were studied during clinical remission and during an acute attack of airway obstruction precipitated by stabling and feeding of dusty hay. Six control ponies were also studied. In principal ponies with airway obstruction, xylazine administration significantly (P < 0.05) decreased pulmonary resistance and increased dynamic compliance, but did not affect Pa o 2 or Pa co 2 . The α2-antagonist yohimbine blocked the pulmonary effects of xylazine. Administration of saline solution was without effect in both groups of ponies at all periods and xylazine did not have effect in controls or in principals in clinical remission.

Free access
in American Journal of Veterinary Research