Background: Ca(2+) influx through CaV1.1 is not required for skeletal muscle excitation-contraction coupling, but whether Ca(2+) permeation through CaV1.1 during sustained muscle activity plays a functional role in mammalian skeletal muscle has not been assessed.
Methods: We generated a mouse with a Ca(2+) binding and/or permeation defect in the voltage-dependent Ca(2+) channel, CaV1.1, and used Ca(2+) imaging, western blotting, immunohistochemistry, proximity ligation assays, SUnSET analysis of protein synthesis, and Ca(2+) imaging techniques to define pathways modulated by Ca(2+) binding and/or permeation of CaV1.1. We also assessed fiber type distributions, cross-sectional area, and force frequency and fatigue in isolated muscles.
Results: Using mice with a pore mutation in CaV1.1 required for Ca(2+) binding and/or permeation (E1014K, EK), we demonstrate that CaV1.1 opening is coupled to CaMKII activation and refilling of sarcoplasmic reticulum Ca(2+) stores during sustained activity. Decreases in these Ca(2+)-dependent enzyme activities alter downstream signaling pathways (Ras/Erk/mTORC1) that lead to decreased muscle protein synthesis. The physiological consequences of the permeation and/or Ca(2+) binding defect in CaV1.1 are increased fatigue, decreased fiber size, and increased Type IIb fibers.
Conclusions: While not essential for excitation-contraction coupling, Ca(2+) binding and/or permeation via the CaV1.1 pore plays an important modulatory role in muscle performance.
Keywords: CaM kinase II; CaV1.1; Fatigue; Fiber type; Protein synthesis and Skeletal muscle.