Abstract
beta 3-adrenoceptors have been identified in a variety of tissues from humans and animals: adipose tissue, gastrointestinal smooth muscle, rat skeletal muscle, bovine skeletal muscle, and human and canine heart. In the airways, the investigation of the beta 3-adrenoceptors came from studies with a series of novel selective agonists. Stimulation of the "atypical" beta-adrenoceptor increases the active transport of albumin across the ferret tracheal epithelium and the ciliary beat frequency of canine bronchial epithelium. Furthermore, it has been demonstrated that beta 3-adrenoceptors agonists selectively inhibited nonadrenergic noncholinergic contractions of guinea-pig bronchi induced by electrical field stimulation or capsaicin. The presence of functional beta 3-adrenoceptors in the bronchial smooth muscle is disputed and seems to be species-related. In isolated canine bronchi, selective agonists induced a relaxation whereas they had no or slight effect in isolated human, guinea-pig and sheep bronchi. Likewise in man, a fall in airway resistance measured by plethysmography, was mediated by beta 2-adrenoceptors, but not beta 3-adrenoceptors. To conclude, an "atypical" or beta 3-adrenoceptor-mediated modulation of bronchomotricity exists, nevertheless strong species specific differences have been reported.
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