Ca Activation of Smooth Muscle Contraction EVIDENCE FOR THE INVOLVEMENT OF CALMODULIN THAT IS BOUND TO THE TRITON-INSOLUBLE FRACTION EVEN IN THE ABSENCE OF Ca *
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چکیده
Smooth muscle contraction is activated by phosphorylation of the 20-kDa light chains of myosin catalyzed by Ca /calmodulin (CaM)-dependent myosin light chain kinase (MLCK). According to popular current theory, the CaM involved in MLCK regulation is Ca -free and dissociated from the kinase at resting cytosolic free Ca concentration ([Ca ]i). An increase in [Ca 2 ]i saturates the four Ca -binding sites of CaM, which then binds to and activates actin-bound MLCK. The results of this study indicate that this theory requires revision. Sufficient CaM was retained after skinning (demembranation) of rat tail arterial smooth muscle in the presence of EGTA to support Ca -evoked contraction, as observed previously with other smooth muscle tissues. This tightly bound CaM was released by the CaM antagonist trifluoperazine (TFP) in the presence of Ca . Following removal of the (Ca )4-CaM-TFP2 complex, Ca 2
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