KATP channel deficiency in mouse flexor digitorum brevis causes fibre damage and impairs Ca2+ release and force development during fatigue in vitro

Activation of the K-ATP channels results in faster fatigue rates as the channels depress action potential amplitude, whereas abolishing the channel activity has no effect in whole extensor digitorum longus (EDL) and soleus muscles. In this study, we examined the effects of abolished K-ATP channel activity during fatigue at 37 degrees C on free intracellular Ca2+ (Ca-i(2+)) and tetanic force using single muscle fibres and small muscle bundles from the flexor digitorum brevis (FDB). K-ATP channel deficient muscle fibres were obtained (i) pharmacologically by exposing wild-type fibres to glibenclamide, and (ii) genetically using null mice for the Kir6.2 gene (Kir6.2(-/-) mice). Fatigue was elicited using 200 ms tetanic contractions every second for 3 min. This study demonstrated for the first time that abolishing K-ATP channel activity at 37 degrees C resulted in faster fatigue rates, where decreases in peak Ca-i(2+) and tetanic force were faster in K-ATP channel deficient fibres than in control wild-type fibres. Furthermore, several contractile dysfunctions were also observed in K-ATP channel deficient muscle fibre. They included partially or completely supercontracted single muscle fibres, greater increases in unstimulated Ca-i(2+) and unstimulated force, and lower force recovery. We propose that the observed faster rate of fatigue in K-ATP channel deficient fibres is because the decreases in peak Ca-i(2+) and force caused by contractile dysfunctions prevail over the expected slower decreases when the channels do not depress action potential amplitude.