Role of muscle fiber hypertrophy and hyperplasia in intermittently stretched avian muscle.

In the chronic stretch model, muscle fiber hyperplasia precedes fiber hypertrophy [Alway et al. Am. J. Physiol. 259 (Cell Physiol. 28): C92-C102, 1990]. This study was undertaken to determine if an intermittent stretch protocol would induce fiber hypertrophy without fiber hyperplasia. A weight equalt to 10% of the bird's mass was attached to the right wing of seven adult quail while the left wing served as the intra-animal control. The weight was attached to the wing for 24-h periods interspersed with a 48- to 72-hr rest interval. The actual stretch time was 5 days while the length of the treatment period was 15 days. Muscle mass and length increased significantly 53.1 +/- 9.0 and 26.1 +/- 7.3% in the stretched anterior latissimus dorsi. Fiber number, which was determined from a histological section in the midregion of the muscle, did not change (control 1,651.6 +/- 94.8; stretch 1,626.0 +/- 70.9). The slow tonic fiber areas increased significantly an average of 28.6 +/- 5.7%, whereas the fast fibers increased 18.5 +/- 8.4% when compared with control values. Mean fiber area (average of slow and fast fibers) increased significantly by 27.8 +/- 6.0% in the stretched anterior latissimus dorsi. There were no differences in the percentage of slow fibers or volume density of noncontractile tissue. These data indicate that muscle adapts differently to intermittent stretch than it does to chronic stretch despite an equivalent load and stretch duration. In contrast to chronic stretch, 5 days of intermittent stretch produces muscle fiber hypertrophy without fiber hyperplasia.