<prompt> Calmodulin acts by -Opening of Na channelsPhosphorylation of myosine by protein kinaseActivation of Na-K pumpDephosphoiylation by protein kinase  <response>: Ans. is b i.e., Phosphorylation of myosine by protein kinaseo In smooth muscle there is no troponin.o Therefore calcium initiates contraction through a mechanism different from that employed by skeletal muscle.o Smooth muscle contains a calcium binding protein called calmodulin.o Increase in cytoplasmic sarcoplasmic calcium leads to its binding to calmodulin.o The calcium-calmodulin complex activates myosin kinase, also called myosin light chain kinaseMLCK.o MLCK is a phosphorylase whichphosphorylates a light chain belonging to myosin chain, often called cross-bridge phosphorylation.o The phosphorylated myosin head interact with actin, i.e., cross-bridging of myosin with actin.o The cross-bridging leads to contraction.Binding of acetylcholine to muscarinic receptorsIncreased influx of Ca2 into the cellActivation of calmodulin-dependent myosin light chain kinasePhosphorylation of myosinIncreased myosin ATPase activity and binding of myosin to actinContractionDephosphorylation of myosin by myosin light chain phosphataseRelaxation, or sustained contraction due to latch bridge and other mechanisms 