Calmodulin Regulation of Excitation-Contraction Coupling in Cardiac Myocytes

ABSTRACT—Calmodulin (CaM) as a ubiquitous Ca sensor interacts with multiple key molecules involved in excitation-contraction (EC) coupling. In the present study, we report that adenoviral expression of a mutant CaM lacking all of its four Ca-binding sites, CaM(1-4), at a level 6.5-fold over endogenous CaM markedly increases the amplitude and abbreviates the decay time of Ca transients and contraction in cultured rat ventricular myocytes. To determine the underlying mechanisms, we examined the properties of L-type Ca channels, Ca/CaM-dependent protein kinase II (CaMKII), and phospholamban (PLB) in the sarcoplasmic reticulum (SR). We found that CaM(1-4) expression markedly augmented L-type Ca current amplitude and slowed its inactivation. Surprisingly, overexpression of CaM(1-4) increased CaMKII activity and phosphorylation of PLB-Thr-17. Moreover, CaM(1-4) elevated diastolic Ca and caffeine-labile Ca content of the SR. Inhibition of CaMKII by KN-93 or a myristoylated autocamtide-2 related inhibitory peptide prevented the aforementioned PLB phosphorylation and reversed the positive inotropic and relaxant effects, indicating that CaMKII is essential to CaM(1-4) actions. These results demonstrate that CaM modulates Ca influx, SR Ca release, and Ca recycling during cardiac EC coupling. A novel finding of this study is that expression of a Ca-insensitive CaM mutant can lead to activation of CaMKII in cardiac myocytes.