Cadmium withdrawal contractures in rat soleus muscle fibres

Transient "Cd2+ withdrawal" contractures, with amplitudes of < or =60% peak tetanic tension, were seen when > or =3 mM Cd2+ was removed, after exposures lasting > or =5 min, from solutions bathing rat soleus fibres at 22 degrees C with Cl- as the major external anion. The minimum free [Cd2+] for withdrawal contractures was reduced twofold when the external anion was SO4(2-). Withdrawal contractures were not seen after removal of 3 mM Co2+, Zn2+ or La3+ and were not observed in rat extensor digitorum longus fibres. The contractures were not due to depolarization (membrane potential, Vm, did not change during Cd2+ removal) or to an influx of external Ca2+ (the transient tension increase was recorded when solutions either lacked Ca2+, or contained 2 mM Co2+). Cd2+ withdrawal contractures were abolished by inactivation of excitation-contraction coupling (ECC) following depolarization in 40 mM K+ for 20 min, and recovered from inactivation at the same time as twitch and tetanic contractions with repriming of ECC. Withdrawal contractures were depressed by agents that depress ECC, i.e. low [Ca2+], 2 mM Co2+, 30 mM Ca2+, 30 mM Mg2+ and 50 microM nifedipine. The results support a hypothesis in which withdrawing Cd2+ from the external solution induces a contracture by activating the voltage sensor for ECC.