No abstract

To detect cation channels, the expression of which is dependent on the physiological state of muscle, single-channel activities of dissociated fibres of the mouse interosseus muscle were recorded using the patch-clamp technique in the cell-attached mode. Fibres were prepared from juvenile and adult wild-type (WT), from chloride channel-deficient myotonic and from denervated adult WT muscles. In all cases delayed-rectifier K + channels (K DR ) with a unitary conductance of 11 pS were recorded in more than 95% of sarcolemmal patches, but with a low, steady-state open probability. Inwards-rectifying K + channels (K IR ) with a conductance of 31 pS in 140 mM [K + ] o were active in about 50% of the membrane patches from WT and in more than 90% of those from myotonic fibres. A hitherto undescribed, inwards-rectifying, cation channel, provisionally termed C IR , with fast kinetics and a unitary conductance of 36 pS, was active in nearly every membrane patch from juvenile mice, both WT and myotonic. The abundance of C IR decreased during development, but was not changed 7 days after denervation of adult WT muscle. Ca 2+ -dependent K + channels were seen sporadically. Channels with the characteristics of adenosine 5'-triphosphate (ATP)-sensitive K + channels were recorded frequently upon excision of membrane patches, but remained inactive in most cell-attached recordings. In conclusion, of the investigated ion channels, only K IR was responsive to the activity pattern of adult muscle, whereas C IR was down-regulated during muscle maturation.