Airway smooth muscle cell (ASMC) contraction is regulated by myosin phosphorylation to control actin-myosin cross-bridge activity. Myosin phosphorylation is determined by the antagonistic activity of myosin light chain (MLC) kinase (MLCK) and phosphatase (MLCP). MLCK activity is increased by increases in intracellular Ca²⁺ concentration ([Ca²⁺]_i) associated with Ca²⁺ oscillations. MLCP activity is decreased by phosphorylation of MLCP or accessory proteins by kinases, including Rho-kinase or protein kinase C. During agonist-induced ASMC contraction, these 2 pathways are simultaneously activated. Because MLCP activity is often independent of [Ca²⁺]_i, changes in MLCP activity can alter ASMC tone at a constant [Ca²⁺]_i; a behavior termed Ca²⁺ sensitivity. In asthma, airway hyperresponsiveness (AHR) may result from an increase in the Ca²⁺-dependent contractile mechanisms and/or the Ca²⁺ sensitivity of ASMCs. Conversely, inhalation of beta2-adrenergic agonists induce airway relaxation by simultaneously slowing the Ca²⁺ oscillations and reducing the Ca²⁺ sensitivity of ASMCs. However, the action of beta2-adrenergic agonists varies with species. Consequently, the development of beta2-adrenergic agonists requires a characterization of their action in human airways.