β-adrenergic receptor agonists

Selective β₂-receptor agonists relax bronchioles and uterine smooth muscle without affecting the heart via β₁-receptor stimulation. These drugs activate adenyl cyclase, which converts adenosine triphosphate (ATP) to cyclic adenosine 3´-5´-monophosphate (cAMP), which in turn causes relaxation of smooth muscle, resulting in bronchodilation. Nonselective β-receptor agonists used for bronchodilation include epinephrine, isoproterenol, and isoetharine. Selective β₂-receptor agonists include albuterol, terbutaline, metaproterenol, and others (Table 94-1). Side effects associated with the use of nonselective medications include increased heart rate, contractility, and myocardial O₂ consumption. Selective β₂-receptor agonists may also produce some cardiac effects, especially if administered subcutaneously or intravenously. Hypokalemia and hyperglycemia may also occur. Chronic use can be associated with tachyphylaxis.