sKATP channels protect skeletal muscle against fibre damage during fatigue by preventing excessive increases in [calcium 2+]i

Abstract

Opening sarcolemmal ATP-sensitive potassium channels (sK ATP channels) contributes to increasing the rate of fatigue in skeletal muscle by reducing action potential amplitude. However, in previous studies, when sKATP channel activity was abolished, the rate of fatigue was not decreased; rather, the muscles developed significant contractile dysfunctions including greater unstimulated force, reduced force recovery and fibre damage. The aim of this study was to determine how abolishing sKATP channel activity during fatigue results in a faster rate of fatigue compared to wild type muscle. Single fibres and small bundles from the flexor digitorum brevis (FDB) muscle were utilized. sKATP channel activity was abolished pharmacologically using glibenclamide and genetically using Kir6.2 -/- mice. This study demonstrated for the first time that abolishing sKATP channel activity at 37°C resulted in faster decreases in peak Ca2+i and tetanic force than in wild type fibres. Furthermore, several contractile dysfunctions were observed in sKATP channel deficient muscle fibres. They included partially or completely supercontracted single muscle fibres, large membrane depolarizations, greater increases in unstimulated Ca2+i and unstimulated force, and lower force recovery. The application of verapamil prevented the development of several of the contractile dysfunctions by reducing [Ca2+] influx. Furthermore, application of ROS scavengers in sKATP channel deficient muscle reduced the extent of the contractile dysfunctions, suggesting that the rise in [Ca2+]i can lead to deleterious ROS production. This study also demonstrated a novel phenomenon in which one fatigue bout at 37°C improves, within 30 min, fatigue resistance during a second fatigue bout. Furthermore, muscles no longer depended on sKATP channels to protect them against functional impairment and fibre damage during the second fatigue bout. The phenomenon was defined as 'fatigue pre-conditioning', and does not follow the same signaling pathway as ischemic preconditioning (IPC) observed in heart and skeletal muscle. Thus, it appears that sKATP channels are necessary for myoprotection during a single fatigue bout, and blocking channel activity leads to development of contractile dysfunctions and an apparent faster fatigue rate. However, during subsequent fatigue bouts separated by short time periods, the reliance on sKATP channels for myoprotection is abolished.