Isometric contractile properties of the posterior latissimus dorsi muscle in normal and genetically dystrophic chickens

Abstract

Various isometric contractile properties of posterior latissimus dorsi muscles from normal and genetically dystrophic New Hampshire chickens were examined. Compared to normal, the dystrophic muscles were lighter, the indirectly elicited twitch was weaker, and the twitch-to-tetanus ratio was reduced. The time course of the twitch contraction was not significantly different from normal. However, fusion frequency was higher in dystrophic muscle and the entire frequency-tension relationship was shifted to higher frequencies. The maximal rate of rise of tension was significantly reduced during both twitch and tetanus in dystrophic muscle. There was no difference in the decrement of the gross electromyogram during high-frequency stimulation. A significantly greater post-tetanic potentiation of the twitch contraction was observed in dystrophic muscle. We suggest that the modification of contractile properties observed in dystrophic chicken muscle represents a shift toward slow muscle characteristics. The paradoxical observation of an unchanged twitch time course in the presence of a reduced maximal rate of rise of tension is discussed in relation to an apparent reduction in duration of the plateau of the active state.