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  • Author or Editor: Patrick H. LeBlanc x
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Summary

In horses with noninduced, reversible airway obstruction (heaves), pulmonary function is improved after sedation with the α2-adrenergic agonist xylazine. The mechanism of this effect is undetermined. Because the predominant excitatory innervation of equine airways is cholinergic, the influence of α2-adrenergic receptor stimulation on the response of isolated distal airways to cholinergic stimulation was determined. Distal bronchial segments from 22 healthy horses were suspended in isolated organ baths where their mechanical responses to various stimuli could be studied. Each tissue was incubated with one of several concentrations of clonidine, clonidine vehicle, or clonidine plus tolazoline. Then, the contractile response of the tissues to either cumulative acetylcholine (ACh) addition or cumulative electrical field stimulation (efs) was recorded. All contractile responses evoked by efs were mediated through stimulation of cholinergic airway nerves. Clonidine had no effect on the contractile response of distal airway segments to exogenous ACh. However, clonidine (at concentrations > 10-5 M) significantly (P < 0.05) diminished the contractile response of the distal airway segments to efs. This inhibitory effect of clonidine was not observed in the presence of tolazoline. Similar results were observed when the less-selective α2-adrenergic agonist xylazine was exposed to the isolated segments instead of clonidine. Because efs-but not exogenous ACh-induced contractions were inhibited, α2-adrenergic receptor stimulation apparently causes presynaptic inhibition of the cholinergic nerves innervating distal portions of the bronchi of horses.

Free access
in American Journal of Veterinary Research

Summary

Respiratory syncytial virus (rsv) infection causes severe lower respiratory tract disease in infants and calves. Neonatal respiratory tract infection in children often produces persistent changes in lung function. The specific objective of this study was to determine whether neonatal calves have transient or persistent alterations in pulmonary function and airway reactivity following rsv infection. Six 2- to 3-day-old Holstein bull calves were inoculated with 10 ml of bovine respiratory syncytial virus (brsv) inoculum (102.7 to 103.8 cell culture infective doses/ml) intranasally and 10 ml of brsv inoculum (104.8 to 105.9 cell culture infective doses/ml) intratracheally for 4 consecutive days, and 5 other calves were sham-inoculated. Prior to inoculation (day 0) and on days 4, 14, and 30 after the last inoculation, body weight (kg), dynamic compliance (Cdyn), pulmonary resistance (RL), and 2 indices of airway reactivity (effective dose [ed] 65Cdyn and ed 200RL) were measured. Control calves gained weight progressively throughout the study, whereas rsv-inoculated calves failed to gain weight for 14 days, but equaled control calf weight by 30 days after inoculation. The Cdyn of control calves increased significantly by 30 days, but did not in the rsv-infected calves. Pulmonary resistance was increased significantly at 4, 14, and 30 days, but was unaffected by sham inoculation. The ed 65Cdyn and ed 200RL indicated an age-dependent increase in reactivity to histamine and an increase in responsiveness in the infected group beginning at 14 days and persisting until the end of the study. The data indicate that brsv causes airway obstruction and hyperreactivity in neonatal calves, which persists for at least 30 days following viral exposure.

Free access
in American Journal of Veterinary Research