The changes in GM in active skeletal muscle fibers. Two microelectrodes were inserted into the same muscle fiber: One electrode was used to inject currents, whereas the other electrode recorded the membrane potential. Using this approach, short trains of action potentials (APs) can be repeatedly triggered in the fiber, and in between the trains, GM can be determined from the membrane potential response (ΔV) to the injection of a 50-ms constant current of small amplitude. (A) Typical recordings from a fast-twitch rat muscle fiber are shown. The dotted line in the first train indicates the resting membrane potential before action potential firing. The depolarized resting membrane potential during action potential firing reflects K+ accumulation in the t-system (Fraser et al., 2011). (B) Enlargements of the membrane potential response to the constant current injection are shown. It can be seen that with the onset of action potential firing ΔV became larger. This reflects a reduction in GM that is caused primarily by PKC-mediated inhibition of ClC-1 channels. With continued activity, ΔV decreased markedly. This reflects activation of both KATP and ClC-1 Cl− channels. This latter activation of ion channels was associated with clear declines in AP amplitude. (C) Average observations of the GM changes in fast- and slow-twitch muscle fibers are shown. It can be seen that the rise in GM with prolonged activity was only observed in fast-twitch muscle fibers. (D) The total GM in active fast-twitch muscle fibers under control conditions, reflecting the activities of both Cl− and K+ channels. Also shown are observations in the presence of 9-AC, which blocks ClC-1. GM with 9-AC therefore reflects the activity of K+ channels alone, and the difference between control GM and GM with 9-AC reflects ClC-1 function. Error bars represent SEM values, and to improve clarity of the figure, only every fifth error bar has been included.
