A Quantitative Study of Potassium Channel Kinetics in Rat Skeletal Muscle from 1 to 37 ' C

Potassium currents were measured using the three-microelectrode voltage-clamp technique in rat omohyoid muscle at temperatures from 1 to 37°C . The currents were fitted according to the Hodgkin-Huxley equations as modified for K currents in frog skeletal muscle (Adrian et al ., 1970x) . The equations provided an approximate description of the time course of activation, the voltage dependence of the time constant of activation (T,,), and the voltage dependence of gxm . At higher temperatures the relationship between gKm and voltage was shifted in the hyperpolarizing direction . The effect of temperature on Tn was much greater in the cold than in the warm: Tn had a QIo of nearly 6 at temperatures below 10°C, but a Qo of only -r2 over the range of 30-38°C. The decreasing dependence of T on temperature was gradual and the Arrhenius Plot of Tn revealed no obvious break-points. In addition to its quantitative effect on activation kinetics, temperature also had a qualitative effect . Near physiological temperatures (above -25 °C), the current was well described by n4 kinetics . At intermediate temperatures (^" 15-25 °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 ^"15°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.