Effects of external cations and calcium-channel blockers on depolarization-contraction coupling in denervated rat twitch skeletal muscles

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Abstract
In the double mannitol gap arrangement the contraction was estimated in single fibres isolated from rat extensor digitorum longus (e.d.l.) muscles that had been denervated for 2–48 days. Denervation induced large changes in the characteristics of the action potential and of the twitch. Up to 15–20 days after denervation the contraction-depolarization curve was sigmoidal and the maximum amplitude of the contraction was not modified by variation of [Ca]o or [Na]o. After 15–20 days of denervation a bell-shaped curve described the relation between contraction and potential. The maximum amplitude was dependent upon the [Ca]o. In Ca-free solution no contractile response was obtained. In Na-free, Ca-containing solution the relationship between contraction and potential was not modified by the addition of divalent cations Co2+, Cd2+, Mg2+, Mg2+, Ni2+, or Ba2+. The contraction, which appeared in Ca-free solution, was restored by adding Sr2+. D600, verapamil and bepridil failed to change the amplitude of the contraction while a marked reduction was found with dihydropyridines. The reduction was overcome by increasing [Ca]o. The present results suggest that the slow calcium current is not involved in the generation of the contractile responses developed by denervated muscles in Na-free (TEA) solution.