Effects of Zinc on Responses of Skeletal Muscle

Zn⁺⁺ potentiates the twitch tension of the frog's sartorius muscle by as much as two to three times, and prolongs twitch time parameters. Tetanus tension is unchanged, but fusion frequency is reduced. Thus, the basic mechanical effect of Zn⁺⁺ is prolongation, but not intensification, of the active state. Threshold effects appear at about 0.005 mM Zn⁺⁺, and maximal changes at 0.05 mM. In 0.05 mM, potentiation begins after a delay of about 1 min. and develops with half-time of 6 min.; full changes reverse in pure Ringer's with halftime of 60 min. Diffusion theory analysis of these kinetics indicates that the potentiating action of zinc involves special features not found in potentiation by anions: a primary membrane action is not excluded, but Zn⁺⁺ may have to penetrate to and act at sarcoplasmic reticular or myoplasmic sites. Zn⁺⁺ does induce excitatory membrane effects: the fall of the action potential is slowed and the refractory period is increased about four times; but excitability as such is not changed. Unique chemical features of Zn⁺⁺ are discussed in relation to possible mechanisms of its action on muscle fibers.