Key Points
- To determine whether skeletal muscle myosin light chain kinase or calmodulin is the limiting factor for regulatory light chain phosphorylation and twitch force potentiation in skeletal muscle.
- Generated three transgenic mouse lines expressing an skMLCK calmodulin biosensor in fast-twitch extensor digitorum longus and slow-twitch soleus muscles.
- Measured rates of myosin regulatory light chain phosphorylation, calmodulin biosensor activation, and contractile force potentiation during low-frequency electrical stimulation.
- Transgenic lines displayed up to a 22-fold increase in skMLCK expression, accelerating regulatory light chain phosphorylation and twitch force potentiation in extensor digitorum longus but not soleus muscle.
- Calmodulin binding reached a maximal activation of 60%, demonstrating that skMLCK abundance rather than calmodulin availability normally limits regulatory light chain phosphorylation.
- Kinase activation occurred at 10.6 s⁻¹ while inactivation occurred at 2.8 s⁻¹ (12-fold slower than muscle relaxation), creating a biochemical memory that potentiates force in fast-twitch type IIb fibers.
Structured PICO
PPopulationTransgenic mice expressing an skMLCK CaM biosensor in skeletal muscle (three transgenic mouse lines)
IInterventionOverexpression of skMLCK (up to 22-fold increase in protein expression)
OOutcomeTwitch force potentiation and myosin regulatory light chain (RLC) phosphorylationsurrogate
In skeletal muscle, skMLCK rather than calmodulin is the limiting factor for RLC phosphorylation and twitch force potentiation, which plays a prominent role in fast-twitch type IIb fibers.