Effects of activity on contractions of normal and dystrophic mouse muscles

¹ Department of Biology, Washington Square College of Arts and Science, New York University; and the Institute for Muscle Disease, New York City

Excised gastrocnemii of normal and dystrophic mice, directly stimulated in long series of isometric twitches or tetani, show different fatigue patterns. During a series of 1,200 maximal twitches, developed tension drops much more slowly in dystrophic responses so that final output relative to initial is: dystrophic, 70%, normal, 10%. A similar but smaller difference holds for a tetanus series. Pronounced quantitative and qualitative distinctions between the two muscle types also appear in the changes of other twitch characteristics: relaxation time and rate, and contraction rate but not time. The qualitatively distinct fatigue pattern of the dystrophic muscle is dependent not on diffusional differences stemming from its smaller bulk, but on some basic dissimilarity inherent in the dystrophic myopathy. The possible nature of this dissimilarity is discussed in relation to general functional features of fatigue and to special aspects of dystrophic muscle concerned with metabolism, intensity property of the active state, and role of the relaxing factor.