Myosin phosphorylation decreases the ATPase activity of cardiac myofibrils

Our previous work showed that myosin phosphorylation decreased the ATPase activity of skeletal muscle myofibrils that were lightly fixed with glutaraldehyde. The fixation process prevented sarcomere shortening and destruction of the ordered filament array upon the addition of ATP. We have now extended these results to myofibrils prepared from hearts of rabbits, dogs and rats. Myofibrils were phosphorylated by incubation with myosin light chain kinase, calmodulin and either ATP-γs or ATP, for 15 minutes at 25°C. The extent of myosin light chain phosphorylation was 50% to 80%. The ATPase activity of unphosphorylated myofibrils was not altered by reaction with 0.01% glutaraldehyde for 5 minutes at 0°C, and the ATPase activity of unfixed myofibrils was not changed by phosphorylation. However, phosphorylation decreased the ATPase activity of fixed myofibrils by 50%. The effect on myocardial myofibrillar ATPase activity of phosphorylation was similar in the three animal species. These results suggest that in both skeletal and cardiac muscle, myosin phosphorylation decreases the rate of cross-bridge cycling resulting in decreased energy expenditure. It also appears that the effect of myosin light chain phosphorylation on ATPase activity requires an ordered myofilament structure.