Regulation of the Ca2+ dependence of smooth muscle contraction. |
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Authors: | D C Tang J T Stull Y Kubota K E Kamm |
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Institution: | Department of Physiology, University of Texas Southwestern Medical Center, Dallas 75235. |
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Abstract: | Cellular mechanisms for the regulation of Ca(2+)-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. KCaM, the concentration of Ca2+/calmodulin required for half-maximal activation of myosin light chain kinase, also increased proportionally, presumably due to Ca(2+)-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, KCaM, light chain phosphorylation, and force. However, the Ca2+]i required to obtain a given value of either light chain phosphorylation or KCaM 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 KCaM. 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. |
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