Potentiation of the isometric twitch and mechanism of tension recruitment in mammalian skeletal muscle.

Changes produced by bromide, nitrate, and iodide in the isometric myogram of the fast-twitch extensor digitorum longus and the slow-twitch soleus muscles of the rat were studied in vitro at 35°C. Replacement of chloride with bromide, nitrate, or iodide produced only small changes in the isometric tetanic tension, but it produced a reversible potentiation of the isometric twitch tension. The order of the potentiating influence was iodide > nitrate > bromide. The tension potentiation was associated with increases in rate of tension rise and the time to peak of the isometric twitch. The results from the two muscles were essentially similar and they were qualitatively similar to those reported from frog twitch muscle. The form of tension enhancement is qualitatively similar to that obtained during summation of mechanical responses, and the results predict with sufficient accuracy the characteristics of the fully summated two-stimuli mechanical response. We suggest that changes in excitation-contraction coupling, similar to those produced by these anions, occur in mammalian skeletal muscle fiber at 35 to 38°C during summation of isometric mechanical responses.