Effect of sodium on voltage-current relationships in rat muscles.

(1) Replacement of Tris by Na in propionate solution causes depolarizations (3-10 or more than 30 mV) in rat muscles. As a result, the resting potentials are distributed in two groups, one at about -70 mV and the other at about -40 mV. Small inward or outward currents are often sufficient for the membrane potential to switch from one level to the other. The change from the low (more positive) to the high (more negative) resting potential can also be provoked by small increases in [K] and vice versa. (2) High frequency, low-amplitude oscillations are produced by gradually repolarizing the membrane at the low resting potential level. The frequency decreases (from a high 2/sec to 5/min or less) and the amplitude increases (up to 30 mV) with further repolarization. Low amplitude oscillations are sinusoidal, high amplitude oscillations resemble pacemaker potentials in other tissues. (3) The voltage-current relationship in Na propionate solutions containing 2 mM K frequently displays pronounced hysteresis presumably covering a negative conductance region. Hysteresis is about the same in Na and Tris containing solutions at high (more than 20 mM) [K]. The results are discussed in terms of an interaction between depolarizing K inactivation and gNa activation, possibly in a channel not involved in spike production.