Delayed contractures induced by external cadmium ions in rat soleus muscle fibres

Cd²⁺-induced contractures began with a delay of ≃4 min after adding 3 mM Cd²⁺ to external solutions that contained Cl^- as the major anion. Tension increased to ≃20% of peak tetanic tension after 30 min and was maintained after Cd²⁺ washout. Tension developed more rapidly at higher [Cd²⁺] (up to 10 mM). There was a lack of correlation between the delay before the contracture and contracture tension: (1) tension was reduced by 2 mM Co²⁺ or 50 μM nifedipine, although the delay remained at ≃4 min, and (2) the delay fell to seconds when Cd²⁺ was added in SO₄^2- solutions, although tension was the same as in Cl⁺ solutions. Since SO₄^2- solutions swell T-tubules, Cd²⁺ may enter the T-system before inducing contractures. Cd²⁺-induced contractures depended on external [Ca²⁺] since they were reduced when Ca²⁺ was omitted from solutions. The contractures did not depend on activation of excitation-contraction coupling, since tension was not altered when the voltage sensor was inactivated by depolarization in 40 mM K⁺. A small contracture developed with 3 mM Zn²⁺, but not 3 mM Co²⁺ or La³⁺. Both Cd²⁺ and Zn²⁺ activated the contractile proteins in skinned fibres. Cd²⁺-induced contractures may depend on external Cd²⁺ releasing Ca²⁺ from the sarcoplasmic reticulum (SR), or on Cd²⁺ entering the fibre, releasing Ca²⁺ from the SR and/or directly activating the contractile proteins.