CAMKII IS RESPONSIBLE FOR ACTIVITY-DEPENDENT ACCELERATION OF RELAXATION IN RAT VENTRICULAR MYOCYTES

We investigated the role of Ca/calmodulin-dependent protein kinase (CaMKII) in relaxation and cytosolic free [Ca] ([Ca](i)) decline during steady-state (SS) and postrest (PR) twitches in intact rat ventricular myocytes. Half-time of mechanical relaxation and time constant of [Ca](i) decline (tau) were twofold greater during PR than with SS at 1 Hz. This difference was 1) abolished by inhibition of sarcoplasmic reticulum (SR) Ca accumulation by thapsigargin or caffeine; 2) greater at higher stimulation frequency and extracellular [Ca], which affected only SS tau; 3) abolished by the protein phosphatase inhibitor okadaic acid (10 mu M, which selectively accelerated [Ca]i decline during PR); 4) still present during stimulation or inhibition of adenosine 3',5'-cyclic monophosphate-dependent protein kinase (PKA) by 10 mu M forskolin or 1 mu M H-89, respectively (SS and PR tau values were abbreviated and prolonged, respectively); and 5) suppressed by 10 mu M KN-62, a selective inhibitor of CaMKII, which selectively prolonged [Ca](i) decline during SS twitches. Both protein kinase inhibitors were also shown to decrease the SR Ca-uptake rate in digitonin-permeabilized rat myocytes. We conclude that CaMKII plays a major role in modulation of relaxation in rat ventricular myocytes, enhancing SR Ca uptake in a activity-dependent fashion. Our results are also compatible with a background, activity-independent stimulation of SR Ca uptake by PKA in intact rat myocytes.