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a Substances producing changes in tone through the release of bronchoactive substances.

a Substances producing changes in tone through the release of bronchoactive substances.

the ^-adrenoceptor agonist albuterol demonstrate different biological activities and pharmacokinetics, with the R isomer possessing the beneficial therapeutic effects (i.e., bronchodilation) [30] and a shorter half-life in the body [31].

In the treatment of airway obstruction, as may occur in asthma, bronchitis, or emphysema, b2-adrenoceptor agonists may be used prophylactically (i.e., to prevent impending bronchoconstriction) or to reverse established bronchocon-striction. They represent the most effective bronchodilator drugs available. The longer-acting drugs, such as salmeterol, are used to prevent bronchoconstric-tion from occurring and are commonly used in combination with an inhaled

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Figure 2 Mechanisms of action of bronchodilator agents. Bronchodilation may be caused by administering one of the following types of drugs: (1) an agonist of b2-adrenoceptors (e.g., albuterol), which subserve relaxation of airway smooth muscle, (2) an antagonist (e.g., ipratropium), which blocks the muscarinic cholinoceptor (M)-mediated airway smooth muscle contractile activity of acetylcholine released from cholinergic nerves, (3) an antagonist of cysteinyl leukotriene receptors (cysLT), which blocks airway smooth muscle contractile activity of leukotrienes C4, D4, or E4 (LT) released from activated mast cells, or (4) an inhibitor of phosphodiesterase isozymes 3 (PDE3) and 4 (PDE4), which promotes airway smooth muscle relaxation by increasing intracellular levels of cyclic adenosine 3',5'-monophosphate (cAMP). The interactions between receptor activation, adenylate cyclase, cAMP, intracellular calcium ion concentrations, and the process of contraction are shown. Stimulation is indicated by" + " and inhibition by" - ."

airway smooth muscle cell

Figure 2 Mechanisms of action of bronchodilator agents. Bronchodilation may be caused by administering one of the following types of drugs: (1) an agonist of b2-adrenoceptors (e.g., albuterol), which subserve relaxation of airway smooth muscle, (2) an antagonist (e.g., ipratropium), which blocks the muscarinic cholinoceptor (M)-mediated airway smooth muscle contractile activity of acetylcholine released from cholinergic nerves, (3) an antagonist of cysteinyl leukotriene receptors (cysLT), which blocks airway smooth muscle contractile activity of leukotrienes C4, D4, or E4 (LT) released from activated mast cells, or (4) an inhibitor of phosphodiesterase isozymes 3 (PDE3) and 4 (PDE4), which promotes airway smooth muscle relaxation by increasing intracellular levels of cyclic adenosine 3',5'-monophosphate (cAMP). The interactions between receptor activation, adenylate cyclase, cAMP, intracellular calcium ion concentrations, and the process of contraction are shown. Stimulation is indicated by" + " and inhibition by" - ."

corticosteroid. The shorter-acting agents, such as albuterol, are the drug of choice for reversing established bronchoconstriction. Repetitive or continued activation of b2-adrenoceptors can lead to tolerance, wherein the extent of bronchodilation or inhibition of bronchoconstriction decreases with increased use of b2-adrenoceptor agonist. This issue is considered to be more problematic with the longer-acting agents [32].

The evolution of b-adrenoceptor agonists as bronchodilators demonstrates important concepts in drug development. The synthesis of drugs selective for adrenoceptors that mediate airway smooth muscle relaxation served to diminish the side effects associated with their administration. Removal of a-adrenoceptor activity (as exhibited by epinephrine) decreased the incidence of drug-induced increases in blood pressure and removal of ^-adrenoceptor activity attenuated drug-induced cardiostimulation. Nevertheless, specific b2-adrenoceptor agonists do induce side effects, including tachycardia (increased heart rate) and muscle tremor, which are physiological manifestations of the actions of these drugs on b2-adrenoceptors in cardiac and skeletal muscle. For these effects to manifest, the drug must be present in the circulation or blood supply to these tissues. Inhalation of an aerosol of b2-adrenoceptor agonist serves to deliver a high concentration of drug to the airway smooth muscle while minimizing the amount of drug available to the circulation.

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