Voltage-dependent K+ channels in the sarcolemma of mouse skeletal muscle.

The voltage-dependent K+ channels of the mammalian sarcolemma were studied with the patch-clamp technique in intact, enzymatically dissociated fibres from the toe muscle of the mouse. With a physiological solution (containing 2.5 mM K+) in the pipette, depolarizing pulses imposed on a cell-attached membrane patch activated K+ channels with a conductance of about 17 pS. No channel activity was observed when the pipette solution contained 2 mM tetraethylammonium (TEA), or 2 mM 4-aminopyridine (4-AP). Whole cell recordings from these very small muscle fibres showed the well-known delayed rectifier K+ outward current with a threshold of about -40 mV. The whole-cell current was completely blocked by 2 mM TEA in the bath, suggesting that the TEA-sensitive channels in the patch were also delayed rectifier channels. The inactivation properties of the channels were studied in the cell-attached mode. Averaged single-channel traces showed at least two types of channels discernible by their inactivation time course at a test potential of 60 mV. The fast type inactivated with a time constant of about 150 ms, the slow type with a time constant of about 400 ms. A little channel activity always remained during pulses lasting several minutes, indicating either the presence of a very slowly inactivating third type of K+ channel, or the tendency of the fast inactivating channels to re-open at constant voltage. No difference was seen in the single-channel amplitudes of the different types of K+ channels. The well characterized adenosine-5'-triphosphate-(ATP)-sensitive and Ca(2+)-dependent K+ channels, although present, were not active under the conditions used.(ABSTRACT TRUNCATED AT 250 WORDS)