Introduction
ß2-agonists have been widely used by asthmatic subjects to relieve obstructive symptoms. The most frequently used ß2-agonists are rapid-acting and include salbutamol, terbutaline and fenoterol. These drugs act by binding to ß2- adrenergic receptors and this interaction stimulates adenyl cyclase and promotes an increase in intracellular cAMP. This reduces intracellular calcium, promotes activation of phosphokinase A and phosphorylation of some proteins such as myosin light chain, resulting in relaxation of airway smooth muscle (1).
Many physicians are concerned about the misuse of these bronchodilators because of their paradoxical effects. It has been suggested that regular use of ß2-agonists increases airway responsiveness to allergen challenge and promotes tolerance to their bronchodilator effects, resulting in a loss of bronchial protection against allergens, possibly exacerbating asthma symptoms (2-8).
However, there is evidence of some ß2-agonist anti-inflammatory activity. Clinical and experimental studies have demonstrated a reduction in inflammatory cells obtained in bronchoalveolar lavage (9) and inhibition of allergy-like effects (10). This anti-inflammatory response is secondary to a negative feedback on T lymphocytes, eosinophils and neutrophils (11). Moreover, the association of ß2-agonists and corticosteroids reduces the inflammatory cell influx in the bronchoalveolar lavage (12), improving asthma symptoms.
In some clinical studies a regular regimen of high doses of ß2-agonists resulted in a fall in forced expiratory volume in 1 s with an increase in eosinophilic cationic protein levels and eosinophil recruitment (13,14). This increase in airway inflammation may contribute to the worsening of pulmonary function and asthma severity. These side effects were observed four weeks after a regular salbutamol regimen and were reversed by corticosteroid treatment (15). In addition, some investigators have observed tolerance to bronchodilator effects promoted by the use of short-acting ß2-agonists (4-8).
In view of conflicting evidence suggesting that regular use of ß2-agonists can either improve or worsen airway inflammation and hyperresponsiveness, we compared two regimens of ß2-agonist administration in an experimental model of chronic allergic airway inflammation in guinea pigs. We determined the effects of two regimens of salbutamol administration (twice and five times a week) on the acute response to the allergen, the pulmonary responsiveness to methacholine and the development of specific anaphylactic antibodies.