Regulation of the Ca²⁺ dependence of smooth muscle contraction

Cellular mechanisms for the regulation of Ca²⁺-dependent myosin light chain phosphorylation were investigated in bovine tracheal smooth muscle. Increases in the free intracellular Ca²⁺ concentration ([Ca²⁺]_i), light chain phosphorylation, and force were proportional to carbachol concentration. K_CaM, the concentration of Ca²⁺/calmodulin required for half-maximal activation of myosin light chain kinase, also increased proportionally, presumably due to Ca²⁺-dependent phosphorylation of the kinase. Isoproterenol treatment inhibited agonist-induced contraction by decreasing [Ca²⁺]_i and thereby light chain phosphorylation. Depolarization by increasing concentrations of KCl also resulted in proportional increases in [Ca²⁺]_i, K_CaM, light chain phosphorylation, and force. However, the [Ca²⁺]_i required to obtain a given value of either light chain phosphorylation or K_CaM was greater in KCl-depolarized tissues compared to carbachol-treated tissues. In muscles contracted with KCl, isoproterenol treatment resulted in diminished light chain phosphorylation and force without alterations in [Ca²⁺]_i- or K_CaM. Thus, isoproterenol inhibition of KCl-induced contraction results from a cellular mechanism different from that found in agonist-induced contraction. In neither case does isoproterenol produce relaxation by altering the calmodulin activation properties of myosin light chain kinase.