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Unchanged myosin kinase activity in hypertrophied rat heart
Insights
Cardiac overload leads to decreased myosin ATPase activity. This study found that myosin kinase activity remains unchanged in hypertrophied rat hearts, suggesting no direct link to myosin function during chronic cardiac hypertrophy.
Area of Science:
- Biochemistry
- Cardiology
- Molecular Biology
Background:
- Cardiac hypertrophy, often induced by cardiac overload, is associated with reduced myosin ATPase activity.
- This reduction has been linked to changes in myosin isoenzymes.
- The role of light chain phosphorylation in regulating myosin ATPase activity in hypertrophy requires further investigation.
Purpose of the Study:
- To investigate the hypothesis that myosin light chain phosphorylation regulates myosin ATPase activity.
- To measure and compare myosin kinase activity in normal and hypertrophied rat hearts.
Main Methods:
- Myosin kinase was purified from sham-operated and hypertrophied rat hearts using ion exchange and calmodulin-affinity chromatography.
- Proteolytic activity and calmodulin-dependency of myosin kinase were assessed.
- Calmodulin-dependent myosin kinase activity was measured under specific calcium and calmodulin concentrations.
Main Results:
- Myosin kinase was purified with a 600-fold enrichment and 6% yield.
- Significant proteolytic activity partially reduced myosin kinase's calmodulin-dependency.
- The major purified component was a 63 kDa protein, with identical protein content in both groups.
- Calmodulin-dependent myosin kinase activity was unchanged in hypertrophied hearts compared to sham-operated controls.
Conclusions:
- Myosin kinase specific activity is not altered in the chronic phase of rat heart hypertrophy under the tested conditions.
- These findings suggest that myosin kinase does not directly influence myosin enzymatic properties during chronic cardiac hypertrophy.
Abstract:
The decrease in myosin ATPase activity observed in cardiac hypertrophy induced by cardiac overload has been related to an isoenzymic redistribution of myosin. To test the hypothesis of an additional regulation of myosin ATPase through light chain phosphorylation, we measured the myosin kinase activity together in sham-operated and 50% to 100% hypertrophied rat hearts. The myosin kinase were purified approximately 600 fold with 6% yield by ion exchange chromatography and calmodulin-affinity chromatography. The presence of very important levels of proteolytic activity in the rat heart resulted in a partial loss of the myosin kinase calmodulin-dependency. The major component from both myosin kinase purified fractions was a 63 kdaltons protein. The protein content was identical in myosin kinase purified fractions from sham-operated and hypertrophied hearts. The calmodulin-dependent activity of myosin kinase, assayed in the presence of 0.1 mM Ca2+ and 10(-6) M calmodulin (about 6.6 nmol P X min-1 X mg-1), was identical in sham-operated and 50% to 100% hypertrophied hearts. Thus, myosin kinase specific activity, in these conditions, was unchanged in rat heart chronic hypertrophy. This result suggests that no direct functional relationship exists between the enzymatic properties of myosin and myosin kinase during the chronic phase of cardiac hypertrophy.