Effects of atropine in ponies with recurrent airway obstruction

Abstract

The effects of atropine on lung function and airway reactivity in two groups of ponies were measured. Principal ponies had a history of recurrent airway obstruction when housed in a barn and fed hay; control ponies had no history of airway obstruction. Principal and control ponies were paired, and measurements were made when principal ponies were in clinical remission (period A) and during an acute attack of airway obstruction (period B). Atropine did not alter pulmonary resistance (RL), dynamic compliance (Cdyn), or airway responsiveness in either group of ponies at period A or in the controls at period B. In principal ponies at period B, atropine did not alter Cdyn or the concentration of aerosol histamine required to decrease Cdyn to 65% of base line (ED65Cdyn) but reduced RL and the change in RL induced by 0.1 mg/ml histamine (delta RL0.1). It is likely that the latter observation was due to geometric changes in the airways, because the change in RL and in delta RL0.1 were significantly correlated. The results of this study show little resting bronchomotor tone in normal ponies, but a major portion of the increase in RL in principals at period B is mediated via muscarinic receptors. Little evidence exists for muscarinic receptor involvement in the response to aerosol histamine in either principal or control ponies.

Heaves, an asthma‐like disease of horses

“…As in other species, airway smooth muscle tone is controlled by the autonomic nervous system, both centrally and via local axonal reflex, through activation of receptors via circulating catecholamines and the non‐adrenergic non‐cholinergic (NANC) system. Bronchospasm in heaves is predominantly mediated via muscarinic (M) receptors 8 but there is no evidence of an exaggerated response of smooth muscle M2 or M3 receptors to acetylcholine stimulation, 180–183 which is similar to what is observed in asthma 184,185 . However, bronchospasm is possibly explained by an alteration of the prejunctional, inhibitory M2 receptors, which provide negative feedback to the release of Ach 186 .…”

“…1a) reveals increased pulmonary resistance and decreased dynamic compliance during exacerbation of heaves 84–86 . The former is caused mainly by bronchospasm, as shown by a rapid decline in resistance after bronchodilator administration; resistance values commonly decrease by 60–70% 7–9 but remain above those observed on pasture or of healthy controls, suggesting residual airway obstruction by mucus and inflammatory cells, and possibly airway wall remodelling 87 . The decrease in dynamic compliance is less consistently responsive to bronchodilator administration, possibly because of peripheral airways obstruction, uneven distribution of ventilation and parenchymal remodelling 7,8 .…”

“…The former is caused mainly by bronchospasm, as shown by a rapid decline in resistance after bronchodilator administration; resistance values commonly decrease by 60–70% 7–9 but remain above those observed on pasture or of healthy controls, suggesting residual airway obstruction by mucus and inflammatory cells, and possibly airway wall remodelling 87 . The decrease in dynamic compliance is less consistently responsive to bronchodilator administration, possibly because of peripheral airways obstruction, uneven distribution of ventilation and parenchymal remodelling 7,8 . Abnormal ventilation distribution has been shown by nitrogen washout, 88 helium‐dilution measurements, 89 radiolabelled aerosolized technetium 90 and, indirectly, by volumetric capnography 91 .…”

Effects of neuraminidase on equine isolated bronchi

“…that muscarinic autoreceptors could be involved in the control of cholinergic tone in horses (17). In fact, rather than being caused by some chronic changes in nerve terminals or smooth muscle itself, e.g., by upregulation of M3 muscarinic receptors on airway smooth muscle or decreased acetylcholinesterase activity, the increase in cholinergic airway tone of horses is most likely induced by factors, such as inflammatory mediators that, when present in the airways, facilitate either local release or the response of smooth muscle to acetylcholine released by the nerves (14,18,19).…”

Chronic obstructive pulmonary disease: a review

“…Horses affected by COPD generally have an increased airway resistance and decreased dynamic compliance, and a prolongation of nitrogen washout (Gillespie et al 1966;Muylle and Oyaert 1973;Willoughby and McDonnelll979;McPherson et al 1978;Thomson and McPherson 1984;Derksen et al 1985a;Armstrong et al 1986;Broadstone et al 1988). All of these changes are compatible with diffuse airway obstruction.…”