Repriming of delayed potassium conductance in frog skeletal muscle

A dissection of the semitendinosus muscle from Rana pipiens was developed for three-microelectrode voltage-clamp studies of the delayed potassium-selective conductance system. The delayed conductance inactivates in muscles bathed in high concentrations of potassium or rubidium, but can be reprimed by hyperpolarizing voltage pulses to membrane potentials beyond -80 mV. The repriming time-course was studied by measuring the delayed conductance that coulde be activated following hyperpolarizing pulses of varying duration. Responses following 20-100 s pulses to potentials between -90 and -140 mV could not be reconciled with an exponential approach to the conductance present in normally polarized fibres. The sigmoid appearance of the early (< 25 s) time course was exaggerated by cooling from 20 to 10 degrees C. This effect was described by a sequential model invoking two inactivated states with different temperature dependences. An explanation is suggested for differences in the kinetics and voltage dependence of repriming between briefly and chronically depolarized muscle cells.