Salbutamol Inhalation Does not Modify Airway Hyper-Reactivity in the Horse

Chronic (10 days) treatment with inhaled salbutamol (albuterol) did not increase airway reactivity in horses with pre-existent airway hyper-responsiveness to aerosolized histamine. Additionally, there was no clinical evidence of recurrent airway obstruction.

1. Introduction

Up to 80% of stabled horses experience inflammation of the lower airways [1,2]. This results in inflammatory airway disease (IAD), which is characterized by inflammation of the lower airways as well as a pronounced airway reactivity when exposed to non-specific stimuli such as histamine [3]. Horses with IAD are often treated with an aerosolized beta-2 adrenergic (B2-AR) agonist like salbutamol (albuterol) to elicit bronchodilation [4]. The regular administration of B2-ARs to human asthmatics results in loss of protection against bronchoconstrictor challenges such as histamine, and a truncated study of three horses with heaves suggested that chronic administration of albuterol may lead to tolerance to the bronchodilator effects of the drug [5]. Loss of bronchoprotection with chronic use of a B2-AR would result in a diminished ability to use salbutamol as a rescue drug in the face of a bronchoconstrictive event; therefore, the objective of this study was to determine if the chronic (i.e., 10 day) use of salbutamol resulted in loss of protection against bronchoprovocation in horses with IAD.

2. Materials and Methods

Subjects

Study subjects used were seven currently athletic horses with established moderate airway hyper-responsiveness to inhaled histamine, as evidenced by a provocative concentration causing a 35% increase in delta flow (PC35delta flow; an index of airway obstruction) at a concentration ≤6 mg/ml histamine.

Study Design

This was a randomized, blinded, placebo-controlled cross-over study carried out at the Equine Facility at the University of New Hampshire in Durham, NH. All procedures were approved by the Institutional Animal Care and Use Committees of both Tufts University and the University of New Hampshire. Hyper-responsiveness to histamine and response to inhaled salbutamol were established during two screening periods that took place at least 2 wk before the study. No B2-AR agonists, corticosteroids, or other bronchodilator or anti-inflammatory drugs were given during the study period. Inhalers with 90 μg salbutamol per metered dose or placebo were used [a].Ten metered doses of salbutamol sulfate [b] (900 μg) or placebo were given twice daily for 10 days using a proprietary mask affixed over one nostril. Twenty-four hours after the last inhalation, a baseline measurement of delta flow was made, and 900 μg of salbutamol was administered through a metered dose inhaler. After 10 min, a second baseline measurement of airway obstruction was made, and subsequently, a histamine bronchoprovocation test was performed. There was a 1-wk wash-out period between salbutamol and placebo treatments.

Lung Function Testing

Baseline lung-function testing was performed using the flowmetric pneumotachograph and inductance plethysmography system as previously described [6]: one sensor measures the flow at the nose through the face mask and another sensor measures the body surface through elastic bands that conduct a signal proportion to changes in a cross-sectional area of the chest and abdomen. The displacement of volume at the nose and chest/abdomen are compared, and their differences are recorded as a signal that is proportional to airflow obstruction (delta flow). Nasal airflow and volume changes are recorded and analyzed using commercial data acquisition software customized for this purpose [c].

Histamine Bronchoprovocation

Histamine bronchoprovocation was performed as described previously [7]. Briefly, increasing concentrations of histamine diphosphate in saline aerosol [d] (1, 2, 4, 8, 16, up to 32 mg/dl) were administered for 2 min each through a nebulizer [e] powered by a high-flow (10 l/min) compressor [f]. The test was terminated when the index of airway obstruction for the horse increased by 50% of the baseline.

Statistical Analysis

Dose-response curves were generated using a log-linear plot, and the concentration of histamine necessary to elicit a 35% increase in delta flow was found by linear interpolation. Provocative doses were then transformed by taking the inverse log such that the PC35delta flow is expressed as mg/ml histamine. Data are expressed as mean ± SD throughout. A univariate general linear model was used to look for differences between before and after treatment PC35delta flow and between PC35delta flow after treatment with placebo or salbutamol, and a general linear model repeated measures analysis was used to determine if PC35delta flow before treatment differed over time.

3. Results

There was no effect of time on post-salbutamol PC35delta flow at the screening period (4.27 ± 2.52 mg/ml histamine) before treatment with placebo (4.31 ± 2.38 mg/ml histamine) or before chronic treatment with salbutamol (3.83 ± 1.86 mg/ml histamine). There was no significant difference between post-salbutamol PC35delta flow after chronic treatment with salbutamol or placebo (4.87 ± 2.52 and 3.01 ± 1.78 mg/ml histamine, respectively). Although placebo treatment was associated with a slightly lower PC35delta flow, there was no significant difference between post-salbutamol PC35delta flow before and after placebo treatment (4.31 ± 2.38 and 3.01 ± 1.78 mg/ml histamine, respectively) or before and after chronic salbutamol treatment (3.58 ± .67 and 4.87 ± 2.50 mg/ml histamine, respectively).

4. Discussion

Inhaled salbutamol (albuterol) is used frequently for the treatment of IAD. Albuterol is also commonly used in the treatment of the similar human disease asthma. Studies in humans have shown contradicting data concerning the effects of B2-ARs on lung function: B2-ARs have been shown to have a bronchoprotective effect in the face of methacholine challenge [8,9] and have also been shown to induce bronchodilator tolerance [10]. A single report in horses with recurrent airway obstruction suggested that a bronchodilator tolerance may develop to inhaled albuterol after 1 wk of twice daily administration [5].

In this study, we found that airway reactivity, measured as PC35delta flow, did not change with time over a 3-mo period. We also found that salbutamol maintained its bronchoprotective effect in that PC35delta flow did not decrease after chronic administration of salbutamol (i.e., it took a similar dose of histamine to cause bronchoconstriction both before and after treatment with salbutamol for 10 days). Finally, there was no difference between PC35delta flow after treatment with placebo versus chronic treatment with salbutamol.

This study shows that chronic (10 days) treatment with salbutamol does not result in a decreased sensitivity to acute treatment with the same drug in horses with IAD. This may be because of the high turnover of B2-ARs on airway smooth muscle [10], or it may also reflect a strong innate bronchoprotective effect in the airways of the horse. In humans with mild, stable asthma, the bronchodilator effect of albuterol, despite chronic use, is remarkably preserved; however, B2-ARs may fail during acute severe asthma [11]. With this in mind, it is important to avoid extrapolating data from horses with moderate IAD to horses with the more severe airway disease of heaves.

This study was funded by the Overly Foundation, Tufts University, Grafton, MA. The Ivax Company kindly supplied the equine breath-operated inhaler mask, salbutamol sulfate, and placebo.

Footnotes

[a] Ivax, Norton Healthcare Ltd., T/A IVAX Pharmaceuticals UK, Albert Basin, Royal Docks, London, E16 20J UK.
[b] Salamol Easi-Breath, Norton Healthcare Ltd., T/A IVAX Pharmaceuticals UK, Albert Basin, Royal Docks, London, E16 20J UK.
[c] XA Biosystems V 2.7, Buxco Electronics, Sharon, CT 06069.
[d] Histamine Di-phosphate, Sigma-Aldrich Canada, Ltd., 2149 Winston Park Dr., Oakville, Ontario L6H 6J8.
[e] Pari LC Plus, Pari Respiratory Equipment, Inc., Monterey, CA 93940.
[f] ProNeb Turbo, Pari Respiratory Equipment, Inc., Monterey, CA 93940.