Effects of external cadmium ions on excitation-contraction coupling in rat soleus fibres

The actions of external Cd²⁺ on the twitch and tetanic contractions, action potentials and potassium (K⁺) contractures of rat soleus muscle fibre bundles have been investigated. Cd²⁺ at 1-1.5 mM did not significantly alter tetanic tension, but increased twitch tension and increased the duration and overshoot of action potentials. At ≥3 mM, Cd²⁺ (1) depressed tetanic contractions and initially potentiated but later depressed twitches, (2) abolished the action potential overshoot, and (3) shifted peak K⁺ contracture tension to more positive membrane potentials. Twitch and tetanic contractions, and action potentials remained depressed when Cd²⁺ was washed out of the bath. The effects of Cd²⁺ on the twitch, tetanus and action potential were mimicked by Zn²⁺, while La³⁺ and Co²⁺ at 3 mM - or Mg²⁺ and Ca²⁺ at 30 mM - depressed peak twitch and tetanic tension, but did not potentiate twitches. The results suggest that: (1) Cd²⁺ and Zn²⁺ potentiate twitch tension by prolonging action potential depolarisation; (2) Cd²⁺ depresses twitch and tetanic tension by reducing the action potential overshoot, and causing a positive shift in the voltage dependence of contraction; and (3) the irreversible depression of action potential amplitude in rat soleus muscle is a specific property of Cd²⁺ and Zn²⁺ that is not shared by Co²⁺, Mg²⁺ or Ca²⁺.