Calcium transients in developing mouse skeletal muscle fibres

Area of Science:

• Muscle physiology
• Developmental biology
• Calcium signaling

Background:

• Excitation-contraction coupling in muscle relies on precise calcium (Ca2+) handling.
• Age-related changes in muscle fiber Ca2+ dynamics are not fully understood.
• Investigating developmental shifts in Ca2+ transients is crucial for understanding muscle maturation.

Purpose of the Study:

• To characterize age-dependent alterations in Ca2+ transients during muscle excitation-contraction coupling.
• To explore the roles of Ca2+ influx and sarcoplasmic reticulum function in developing muscle fibers.
• To identify developmental changes in the expression or function of Ca2+ handling proteins.

Main Methods:

• Measurement of Ca2+ transients using MagFluo-4 in enzymatically dissociated mouse muscle fibers from different age groups (7, 10, 15, 42 days).
• Analysis of Ca2+ transient kinetics, including rise time and double exponential decay.
• Application of twin pulses to assess fractional recovery and Ca2+ release inactivation.
• Treatment with Ca2+-free solutions and cyclopiazonic acid (a sarcoplasmic reticulum Ca2+-ATPase inhibitor) to probe Ca2+ influx and SR pump activity.

Main Results:

• Ca2+ transient rise time decreased significantly with age (2.4 ms in 7-day-old vs. 1.1 ms in 42-day-old mice).
• Ca2+ transient decay was markedly slower in younger animals (time constants 4.6 & 105 ms) compared to adults (1.8 & 16.4 ms).
• Younger fibers showed greater sensitivity to Ca2+-free solutions and complete abolition of decay with cyclopiazonic acid, indicating higher reliance on SR Ca2+ pump and potentially Ca2+ influx.

Conclusions:

• Muscle fiber Ca2+ handling undergoes significant developmental remodeling from neonatal to adult stages.
• The sarcoplasmic reticulum Ca2+ pump plays a more dominant role in Ca2+ removal in young muscle fibers.
• Lower expression of myoplasmic Ca2+ buffers and potentially increased Ca2+ influx contribute to altered Ca2+ transient dynamics in developing muscle.