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

Cd2+-induced contractures began with a delay of congruent to 4 min after adding 3 mM Cd2+ to external solutions that contained Cl- as the major anion. Tension increased to congruent 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 mu M nifedipine, although the delay remained at congruent to 4 min, and (2) the delay fell to seconds when Cd2+ was added in SO42- solutions, although tension was the same as in Cl- solutions. Since SO42- 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.