Selective Inhibition of Potassium Contracture in Presence of Intact Twitch

If a sartorius muscle of frog is pretreated for 30 min. with a hyposmotic Ringer's (called "hyposmotic potassium chloride") in which all sodium chloride is replaced by 23 mM potassium chloride, and then is immersed in normal Ringer's for about 40 min., a twitch of normal size can be elicited although the potassium contracture is almost completely abolished. During the time that this selective inhibition of potassium contracture occurs, the action potential and both the size and the speed of depolarization due to potassium are almost the same as in a control muscle that is pretreated for 30 min. with 23 mM potassium chloride under isosmotic conditions and that can give a normal potassium contracture. If a muscle is pretreated not with hyposmotic potassium chloride but with a hyposmotic Ringer's in which the concentration of sodium chloride is reduced to 22 mM, the evoked potassium contracture is inhibited, but to a much smaller degree. The partial replacement of chloride by nitrate in hyposmotic potassium chloride has no influence on the degree of the selective inhibition. If after the pretreatment with hyposmotic potassium chloride the muscle is left in normal Ringer's for about 3 hours, there is considerable recovery of ability to undergo contracture. The mechanism of the selective inhibition is briefly discussed.