Influence of divalent cations on potassium contracture duration in frog muscle fibres

Abstract

Single fibres or small fibre bundles were dissected from twitch muscles of frogs and washed in low Cl-solutions. Contractures were provoked by 122.5 mM K⁺. At room temperature (17–20° C) the contracture duration was about 1.5 s in the absence of divalent cations and about 3 s in the presence of 2 mM Ca²⁺. Contractures were prolonged when Ca²⁺ was replaced by Ni²⁺ showing that inward Ca current was not the factor responsible for the contracture prolongation. K contracture duration was prolonged in the cold (3° C) by a factor of about 4 in the presence of non-permeating divalent cations (Ni, Co), when 0.1 mM La³⁺ was applied together with 2 mM Ca²⁺, and in the virtual absence of divalent cations. The contractures were prolonged in the cold by a factor of 6 or more in the presence of permeant divalent cations (Ca, Sr, Ba, Mg, and Mn at 8 mM). Diffusion of divalent cations in the transverse tubules of the muscle fibres was shown to have a Q₁₀ similar to that in free solution. It was concluded that inward current of divalent cations may shorten contracture duration by causing ionic depletion of the transverse tubules.