Ca²⁺ entry units in a superfast fish muscle

Area of Science:

• Muscle physiology
• Cellular biology
• Comparative physiology

Background:

• Store-operated Ca2+ entry (SOCE) sustains skeletal muscle contractility during fatigue.
• Ca2+ entry units (CEUs) are intracellular junctions linked to SOCE and muscle function in mice.
• The conserved role of CEUs across vertebrates remains largely unexplored.

Purpose of the Study:

• To investigate the presence and relevance of CEUs in vertebrate skeletal muscle.
• To determine if CEU formation is use-dependent in a non-mammalian model.
• To explore the adaptive mechanisms of superfast muscles in plainfin midshipman fish.

Main Methods:

• Examined superfast sonic muscles of the plainfin midshipman (Porichthys notatus).
• Compared muscle structure in actively vocalizing (humming) territorial males versus inactive territorial males.
• Utilized electron microscopy to identify and quantify CEU structures.

Main Results:

• Actively humming midshipman had a significantly higher incidence of CEU SR stacks compared to inactive males.
• CEU formation is dynamic and use-dependent, mirroring findings in exercised mouse muscle.
• This suggests CEUs are a conserved adaptation for high-frequency muscle activity.

Conclusions:

• CEUs represent a conserved mechanism across vertebrates for supporting repetitive muscle activity.
• CEU assembly is dynamic and influenced by muscle usage.
• These findings offer insights into the unique adaptations of superfast sonic muscles.