Skeletal muscle Ca(2+)-independent kinase activity increases during either hypertrophy or running

Area of Science:

• Muscle physiology
• Molecular signaling
• Exercise science

Background:

• Calcium ions (Ca2+) trigger skeletal muscle contractions.
• Serum response factor (SRF) is a transcription factor regulated by Ca2+ signaling in non-muscle cells.
• The role of Ca2+ signaling in activating transcription factors in skeletal muscle is not well understood.

Purpose of the Study:

• Investigate Ca2+-dependent and Ca2+-independent SRF kinase activity in skeletal muscle.
• Determine if SRF kinase activity changes with muscle hypertrophy and exercise.
• Explore the potential involvement of Ca2+/calmodulin-dependent protein kinase II (CaMKII) in SRF phosphorylation.

Main Methods:

• Measured SRF kinase activity in hypertrophied rooster anterior latissimus dorsi muscles.
• Assessed Ca2+/calmodulin-dependent SRF kinase activity in rat vastus lateralis muscles after running.
• Utilized autocamtide-3 to evaluate CaMKII activity.

Main Results:

• Ca(2+)-independent SRF kinase activity was elevated in hypertrophied rooster muscle.
• No significant changes in Ca(2+)/CaM-dependent SRF kinase activity were observed in either hypertrophied or exercised muscle.
• Ca(2+)-independent CaMKII increased in hypertrophied muscle, suggesting its potential role in SRF phosphorylation.
• Ca(2+)-independent SRF kinase activity increased in exercised rat muscle, but Ca(2+)/CaM-dependent activity did not.

Conclusions:

• Skeletal muscle adaptations to exercise involve alterations in Ca(2+)-independent SRF kinase activity.
• The signaling pathways downstream of Ca(2+)-independent SRF kinase appear to be specific to the type of exercise stimulus (hypertrophy vs. endurance).
• CaMKII may contribute to SRF phosphorylation in skeletal muscle, particularly under conditions of increased Ca(2+)-independent activity.