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Myosin phosphorylation decreases the ATPase activity of cardiac myofibrils
Abstract:
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-gamma s or ATP, for 15 minutes at 25 degrees 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 degrees 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.
Insights
Myosin phosphorylation reduces ATPase activity in fixed skeletal and cardiac muscle myofibrils. This effect, crucial for ordered myofilament structure, suggests reduced cross-bridge cycling and energy expenditure in muscle contraction.
Area of Science:
- Muscle physiology
- Biochemistry
- Molecular biology
Background:
- Previous studies indicated myosin phosphorylation decreases skeletal muscle myofibril ATPase activity.
- Glutaraldehyde fixation preserves myofibril structure, preventing ATP-induced shortening and array destruction.
Purpose of the Study:
- To investigate the effect of myosin phosphorylation on ATPase activity in cardiac muscle myofibrils.
- To determine if the observed effects in skeletal muscle are conserved in cardiac muscle across different species.
Main Methods:
- Myofibrils from rabbit, dog, and rat hearts were phosphorylated using myosin light chain kinase, calmodulin, and ATP-gamma S or ATP.
- ATPase activity was measured in both fixed (0.01% glutaraldehyde) and unfixed myofibrils.
- Myosin light chain phosphorylation levels were assessed, reaching 50% to 80%.
Main Results:
- Phosphorylation did not alter ATPase activity in unfixed myofibrils.
- Glutaraldehyde fixation alone did not affect ATPase activity.
- Phosphorylation decreased ATPase activity by 50% in fixed myofibrils, consistent across species.
- The effect of phosphorylation on ATPase activity was dependent on an ordered myofilament structure.
Conclusions:
- Myosin phosphorylation decreases cross-bridge cycling rate and energy expenditure in both skeletal and cardiac muscle.
- The influence of myosin light chain phosphorylation on ATPase activity necessitates an intact, ordered myofilament structure.