Potassium currents were measured using the three-microelectrode voltage-clamp technique in rat omohyoid muscle at temperatures from 1 to 37 degrees C. The currents were fitted according to the Hodgkin-Huxley equations as modified for K currents in frog skeletal muscle (Adrian et al., 1970a). The equations provided an approximate description of the time course of activation, the voltage dependence of the time constant of activation (tau n), and the voltage dependence of gK infinity. At higher temperatures the relationship between gK infinity and voltage was shifted in the hyperpolarizing direction. The effect of temperature on tau n was much greater in the cold than in the warm: tau n had a Q10 of nearly 6 at temperatures below 10 degrees C, but a Q10 of only approximately 2 over the range of 30-38 degrees C. The decreasing dependence of tau n on temperature was gradual and the Arrhenius plot of tau n revealed no obvious break-points. In addition to its quantitative effect on activation kinetics, temperature also had a qualitative effect. Near physiological temperatures (above approximately 25 degrees C), the current was well described by n4 kinetics. At intermediate temperatures (approximately 15-25 degrees C), the current was well described by n4 kinetics, but only if the n4 curve was translated rightward along the time axis (i.e., the current had a greater delay than could be accounted for by simple n4 kinetics). At low temperatures (below approximately 15 degrees C), n4 kinetics provided only an approximate fit whether or not the theoretical curve was translated along the time axis. In particular, currents in the cold displayed an initial rapid phase of activation followed by a much slower one. Thus, low temperatures appear to reveal steps in the gating process which are kinetically "hidden" at higher temperatures. Taken together, the effects of temperature on potassium currents in rat skeletal muscle demonstrate that the behavior of potassium channels at physiological temperatures cannot be extrapolated, either quantitatively or qualitatively, from experiments carried out in the cold.
Voltage dependence and stability of the gating kinetics of the fast chloride channel from rat skeletal muscle.