Zinc‐Induced Relaxation of Muscle Fibres

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Abstract
The extractability of zinc from rabbit psoas muscle by glycerol‐extraction and by further treatment with EDTA and ATP has been determined as has the uptake of zinc by glycerol‐extracted fibres at bath concentrations of zinc corresponding to the free zinc ion concentration needed for induction of complete isometric relaxation of the fibres in the presence of ATP. The functionally relevant uptake of zinc needed for relaxation of extracted fibres could be covered by the total amount of zinc available in fresh muscle. Fully repeatable contraction‐relaxation cycles with glycerol‐extracted fibres in a highly plasticized state were produced with 0.6 mM total zinc simply by alternating the ATP concentration between 3 mM (relaxation) and 7 mM (contraction). The result was similar if, at constant total zinc (0.6 mM) and ATP (3 mM), 1.25 mM EDTA was alternately added (contraction) and removed (relaxation). Based upon these findings a possible mechanism for the action of zinc as a relaxing factor in vivo is presented: Relaxation is caused by inhibition of contractility by zinc; contraction is induced by release into the appropriate compartment of the fibre of a zinc‐complexing substance — ATP or a similar compound — which, after breakdown, again returns the complexed zinc with consequent re‐inhibition of contractility and, thus, relaxation.