Membrane potential and mechanical tension in white and red muscles of the rat.

LORKOVI~, H. Membrane potential and mechanical tension in white and red muscles of the rat. Am. J. Physiol. 221(4) : 1044-1050. 197 1. -Threshold membrane potential for mechanical activity, determined by the current pulse technique, is between -30 and -36 mv for white and red muscles. Potentiation of K contractures by substituting CH$Oafor Clis accounted for by differences in the degree of depolarization. The effect of KC1 on membrane potential is slightly stronger in white than in red muscles. The concentration of KCH&Os at which half-maximum tension is provoked (KT,2) is about 40 mM in red muscles and between 40 and 130 mu in white muscles. The slope of the tension-[K], relationship is less steep for most white than for red muscles. Recovery from exposure to high K is faster and occurs at lower [K] o in (red) toe than in red and white calf muscles. KT,2 is increased about 1.5 times by raising [Cal, from 2 to 10 mM, but it is little changed by lowering [Cal, from 2.0 to 0.2 ITEM in Tris CH$Os solutions. KT,z is reduced 4 times by lowering [Cal, from 2.0 to 0.0 mu in sucrose solutions. It is concluded that the differences between the contracture patterns of white and red muscles reside in the contraction-activation mechanism.