When skeletal muscle is subjected to stretch it undergoes a rapid increase in muscle mass. However, the effect of stretch on the native myosin isozyme content of muscle has received attention only recently. Using the Japanese quail to investigate stretch-induced hypertrophy, we demonstrated an increase in the expression of fast myosin in the predominantly slow anterior latissimus dorsi muscle (ALD). The fast myosin content of the control quail ALD is not sufficient to be quantified on native myosin pyrophosphate gels. After 33 days of stretch, the fast myosin content (N = 10) averaged 16 +/- 11% in the stretched muscles and reached a maximum of 40%. Mean hypertrophy in the stretched muscle, as indicated by muscle weight, was 247 +/- 91% (range, 168-378%). Fast myosin was consistently expressed in muscles with hypertrophy greater than 250%. Muscle fiber size from the stretched muscles contained a greater number of fibers with small cross-sectional areas than was observed in controls. These results indicate that substantial remodeling occurs in the stretched ALD muscle of the Japanese quail.
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