Cytoplasmic Ca2+ is a primary determinant for myosin phosphorylation in smooth muscle cells.
Initiation of smooth muscle contraction is associated with Ca2+/calmodulin activation of myosin light chain kinase which catalyzes the phosphorylation of the 20-kDa light chain of myosin. In tracheal smooth muscle cells in culture, the extent of myosin light chain phosphorylation is less than 10% at basal cytosolic free Ca2+ concentrations of 150 nM. Stimulation of these cells with serotonin, histamine, carbachol, or the Ca2+ ionophore, ionomycin, increases free cytosolic Ca2+ concentrations and the extent of myosin light chain phosphorylation. Light chain phosphorylation reaches a maximal value of 67% at Ca2+ concentrations below 1 microM. The relationship between the extent of light chain phosphorylation and cytosolic free Ca2+ concentration is apparently independent of the source of free intracellular Ca2+ or the agent used to stimulate the cells and is not altered by pre-exposure of the contractile apparatus to high concentrations of free Ca2+. Pretreatment of cells with 8-bromo-cyclic GMP or forskolin decreases free cytosolic Ca2+ concentrations and the extent of myosin light chain phosphorylation in response to histamine or ionomycin. Pretreatment with 8-bromo-cyclic GMP also decreases the maximal extent of light chain phosphorylation. These results indicate that cytosolic free Ca2+ concentration, per se, is a primary determinant for myosin light chain phosphorylation in tracheal smooth muscle cells.
Taylor
D A
DA
Department of Physiology, University of Texas Southwestern Medical Center, Dallas 75235.
Bowman
B F
BF
Stull
J T
JT
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002417:Cattle; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000502:physiology; D006152:Cyclic GMP / Q000502:physiology; D003593:Cytoplasm / Q000378:metabolism; D004987:Ethers / Q000494:pharmacology; D006632:Histamine / Q000494:pharmacology; D066298:In Vitro Techniques; D015759:Ionomycin; D009130:Muscle, Smooth / Q000378:metabolism*; D009218:Myosins / Q000378:metabolism*; D010766:Phosphorylation; D012701:Serotonin / Q000494:pharmacology; D014132:Trachea
D004987:Ethers; D005576:Colforsin; D012701:Serotonin; D015759:Ionomycin; D006632:Histamine; D000242:Cyclic AMP; D009218:Myosins; D006152:Cyclic GMP; D002118:Calcium