Exogenous calmodulin increases Ca2+ sensitivity of isometric tension activation and myosin phosphorylation in skinned smooth muscle

Abstract

We have investigated the effect of exogenous calmodulin on chicken gizzard or rabbit ileum smooth muscle functionally skinned by mechanical grinding or exposure to Triton X-100 detergent. We found that a specific protein inhibitor, modulator binding protein, caused a loss of Ca²⁺-activated tension which was restored by subsequent treatment with calmodulin. Calmodulin at 5 μM increased 10-fold the speed of development of isometric tension while it had no significant effect on the rate of relaxation or on maximum tension at high Ca²⁺ concentrations. The Ca²⁺ sensitivity of steady state tension and LC₂₀ phosphorylation were also increased by 5 μM calmodulin. These results are consistent with a calmodulin-regulated light chain kinase/phosphatase system being responsible for activation of tension in smooth muscle.