A comparison of the effects of some metal ions on innervated and denervated mouse diaphragm preparations

A greater degree of contracture was produced in denervated as opposed to innervated mouse diaphragm preparations in response to acetylcholine, high K⁺ and caffeine. However, N-ethylmaleimide elicited a greater contracture in innervated than in denervated muscle. In innervated muscles metal ions (Cd²⁺, 0.5 μM; Zn²⁺, 0.5 μM and Cu²⁺, 0.1 mM) induced contractures and a decrease of ATP content. These effects were less marked in denervated muscle. In a comparative study, the mitochondrial uncoupler dinitrophenol (0.01 mM) decreased ATP content and also induced a contracture of the innervated mouse diaphragm but had only a feeble effect on the denervated muscle. In 0.25 mM Ca²⁺ Krebs solution, low concentration of Cd²⁺ and Zn²⁺ (both 0.1 mM) increased both the amplitude and duration of contractions to direct stimulation of innervated preparations. Both Cd²⁺ and Zn²⁺ induced repetitive action potential firing of muscle fibres upon injecting a depolarizing current intracellularly. In denervated muscle, most of these effects of the metal ions were attenuated. Thus, Cd²⁺ and Zn²⁺ (0.1 mM) only slightly augmented the amplitude and duration of single twitch and did not induce repetitive action potential firing. The denervated mouse diaphragm exhibited a lower oxygen consumption than that found in innervated muscle. However, the mitochondrial Mg²⁺-ATPase activity and muscle oxygen consumption of both innervated and denervated mouse diaphragms showed similar susceptibility to metal ions. These findings suggest that changes of surface membrane sulfhydryl groups and metabolic alterations induced by denervation may be responsible for the attenuation of the actions of drugs and metal ions.