Similarity of ATP-dependent K+ channels in skeletal muscle fibres from normal and mutant mdx mice

1. ATP-dependent K+ (K-ATP) channels were studied in fibres isolated from flexor digitorum brevis and interosseal skeletal muscles of normal and mutant mds mice using the patch clamp technique in the presence of asymmetrical K+ concentrations (5 mM K+ in the pipette and in vivo intracellular [K+] or 145 mM K+ at the cytoplasmic face). 2. In cell-attached patches from mdx muscle fibres bathed in K+-rich solution, cell poisoning with fluorodinitrobenzene induced partially reversible opening of channels carrying an outward current of an amplitude of 1.2 pA at 0 mV. Exposure of fibres to the K+ channel opener cromakalim led to opening of the same type of channel. These channels were assumed to be K-ATP channels. 3. On excision of inside-out patches from mdx muscle fibres, in the absence of intracellular ATP, K-ATP channels were active: they carried a unitary outward current of 1.6 pA at 0 mV and were inhibited by intracellular ATP and glibenclamide. The number of K-ATP channels per patch was not significantly different in muscles from normal and mdx mice. 4. In inside-out patches, in the presence of 1 mM intracellular Mg2+, slope conductances of 21 and 20.3 pS were found for K-ATP channels in normal and mdx muscle, respectively. In the absence of Mg2+, slope conductances of K-ATP channels were 31.3 and 32 pS in normal and mdx muscle, respectively and K-ATP channel activity was augmented in mdx muscle in the same way as in normal muscle. Activity of the same K-ATP channel was observed in extensor digitorum longus muscle from normal and mdx mice. 5. In inside-out patches held at 0 mV, the relationship between K-ATP channel activity and intracellular ATP was described by a Hill equation: K-i values mere 23 and 21 mu M and Hill coefficients were 1.8 and 1.9 in normal and mdx muscle, respectively. 6. These results indicate that the distribution, the conductance properties and ATP sensitivity of K-ATP channels do not differ in normal and in mdx mouse skeletal muscle.