Three types of single voltage-dependent potassium channels in the sarcolemma of frog skeletal muscle.

Patch-clamp experiments in the sarcolemma of frog skeletal muscle evidenced the presence of three types of voltage-dependent single-channel K(+) currents. According to their unitary conductance at a membrane voltage of +40 mV, we classified them as 16-, 13-, and 7-pS K(+) channels. The 16-pS K(+) channels are active close to a membrane voltage of -80 mV and they do not become inactivated during voltage pulses of 100 ms. Within 10 min after beginning the recording, these channels developed rundown with an exponential time course. The 13-pS K(+) channels are active near -60 mV; upon a 100-ms depolarization, they exhibited inactivation with an approximate exponential time course. The 7-pS K(+) channels were recorded at voltages positive to 0 mV. In patches containing all three types of K(+) channels, the ensemble average currents resemble the kinetic properties of the macroscopic delayed rectifier K(+) currents recorded in skeletal muscle and other tissues. In conclusion, the biophysical properties of unitary K(+) currents suggest that these single-channel K(+) currents may underlie the macroscopic delayed K(+) currents in frog skeletal muscle fibers. In addition, since the 16- and 13-pS channels were more frequently recorded, both are the main contributors to the delayed K(+) currents.