Chloride-induced contraction in a skinned fiber of toad striated muscle

Cl- -induced contraction was studied in a skinned fiber dissected from toad sartorius muscle. When K-methanesulphonate in a relaxing solution was replaced isotonically with KCl, the skinned fiber contracted due to 'depolarization'-induced Ca++ release on the SR. Contraction was induced by superfusing the fiber with solutions containing different concentrations of Cl-. In the presence of 2 mM EGTA, the Cl- -induced contraction developed when the relaxing solution was replaced by a Cl- solution which contained up to 20 mM Cl-, while contraction started at 15 mM-Cl- in the presence of 0.5 mM EGTA. The peak tension of the Cl- -induced contraction depended on Cl- concentration without obeying the all-or-none law. When the fiber was treated with 1.5 mM caffeine, the threshold for the Cl- -induced contraction was significantly lowered. In spite of previous application of Cl- at different concentrations below 80 mM, the skinned fiber contracted by sudden increase in Cl- concentration, and the tension versus Cl- concentration curve revealed almost the same shape as that in a fiber equilibrated with Cl- -free solution. This suggests that the SR membrane is repolarized after transient depolarization caused by elevation of Cl- concentration.