Expression of small-conductance calcium-activated potassium channels (SK3) in skeletal muscle: regulation by muscle activity.

The type 3 small conductance calcium-activated potassium channel (SK3) is expressed in embryonic and adult denervated skeletal muscles where it contributes to hyperexcitability. This study aimed at determining the role of muscle activity in regulating SK3 channels. Soleus muscles of adult rats were denervated by cutting the sciatic nerve. In reinnervation studies, the soleus nerve was crushed: in one group, muscles were reinnervated with electrically silent axons, by chronic sciatic nerve perfusion with tetrodotoxin. Several groups of denervated muscles were subjected to chronic direct electrical stimulation, using either fast (100 Hz) or slower patterns (20 or 30 Hz). The SK3 mRNA and protein levels in soleus muscle were determined by reverse transcriptional-PCR, Western blot and immunofluorescence. Both denervated and reinnervated-paralysed soleus muscles displayed similar up-regulation of SK3 mRNA and protein. Reinnervation with electrically active axons instead inhibited SK3 up-regulation. Chronic muscle direct stimulation in vivo, irrespective of the pattern used, reversed the denervation-induced up-regulation of SK3 expression or prevented it when initiated at the time of denervation. Chronic electrical stimulation of denervated muscles also completely prevented the development of the after-hyperpolarization (AHP) following the action potential, normally induced in the muscle fibres by denervation. We conclude that action potential activity evoked by motor neurones in muscle fibres is both necessary and sufficient to account for the physiological down-regulation of SK3 channels in the non-junctional membrane of skeletal muscle.