Voltage-current relationships in skeletal muscles of rats.

(1) Thin sheets of fibres from gastrocnemius and lumbricalis muscles of rats were washed in Tris-proponiate solutions containing 0.67 to 60 mM K. The voltage-current relationship was measured by the two microelectrode technique. (2) The V-C curve was S-shaped. The steep region, sometimes including a "forbidden" voltage zone, occurred between about -40 and -70 mV when the solution contained 2 mM K. In some fibres the steep region was found to occur at more positive currents and voltages in "upward" runs (steps of increasing depolarizing currents) then in "downward" runs. The V-C curves thus revealed hysteresis loops presumably covering a negative conductance region. (3) The voltage at which the steep region occurred was a function of [K]0. The mid-point of the steep region was 50 to 60 mV more positive than EK for a particular [K]0 was about 6 mM the steep region of the V-C relationship was not conspicuous. The steep V-C region is considered to reflect depolarizing K inactivation. The near disappearance of the phenomenon at 6 mM K is thought to result from an interference of delayed rectification and depolarizing K inactivation.