Effect of External Potassium Concentration on Caffeine Contractures in Frog Toe Muscle

Increased [K]0 sufficient to produce submaximum depolarization contractures of frog toe muscle enhanced the tension produced during subsequent caffeine contractures, but caffeine-contracture tension was depressed after an increase in [K]0 sufficient to produce maximum or near-maximum depolarization contractures.Reduced external concentration of divalent cations often augmented caffeine-contracture tension at normal [K]0 but did not do so after [K]0 was increased to levels which produce maximum K contractures. Increased [Mg]0 or [Ca]0 had little direct effect on caffeine contractures but shifted the effect of [K]0 on caffeine contractures toward higher [K]0 in accordance with the shift in [K]0 required to produce depolarization contractures. Replacement of external chloride with acetate, which shifts the relation between [K]0 and depolarization-contracture tension toward lower [K]0, produced a similar shift in the effect of [K]0 on caffeine contractures. Maximum depolarization contractures relaxed rapidly ...