Regionalized adaptations and muscle fiber proliferation in stretch-induced enlargement

Abstract

The relative contribution of increases in fiber area to stretch-induced muscle enlargement was evaluated in the slow tonic fibers of the anterior latissimus dorsi of adult Japanese quails. A weight corresponding to 10% of the bird's body mass was attached to one wing. Thirty days of stretch in 34 birds averaged 171.8 +/- 13.5% increase in muscle mass and 23.5 +/- 0.8% increase in muscle fiber length. The volume density of noncontractile tissue increased in middle and distal regions of stretch-enlarged muscles. Mean fiber cross-sectional area increased 56.7 +/- 12.3% in the midregion of stretched muscles. Further analysis indicated slow beta-fiber hypertrophy occurred in proximal, middle, and distal regions; however, fast alpha-type fiber hypertrophy was limited to middle regions of stretched muscles. Stretched muscles had a significant increase in the frequency of slow beta-fibers that were less than 500 microns 2 in all regions and fast alpha-type fibers in middle and distal regions. Total fiber number was determined after nitric acid digestion of connective tissue in 10 birds. Fiber number increased 51.8 +/- 19.4% in stretched muscle. These results are the first to clearly show that muscle fiber proliferation contributes substantially to adult skeletal muscle stretch-induced enlargement, although we do not know whether the responses of the slow tonic anterior latissimus dorsi might be similar or different from mammalian twitch muscle.

Animal Models for Inducing Muscle Hypertrophy: Are They Relevant for Clinical Applications in Humans?

“…Birds and small mammals have been studied the most extensively. In general, hypertrophic results from the stretch of chicken and quail wing muscles 5,7,16,18,45,59 have been the same as those found in rabbit 21,30,75 and rat 27 hindlimb muscle stretch models. The functional and hypertrophic outcomes of the stretch model are well documented as described below but the mechanism underlying how stretch induces hypertrophy is not known (although it is likely a mechanoreceptor type of response).…”

“…5,7,16,18,45,59 This unilateral model is advantageous because any systemic alteration (eg, hormonal changes) due to the experimental manipulation are common in the control and stretched muscles, so the effects of the mechanical overload can be distinguished. The use of young chickens is slightly disadvantageous because they continue to grow throughout the experimental period.…”

Role of satellite cells in altering myosin expression during avian skeletal muscle hypertrophy

“…By 1 week, myofiber cross-sectional area in the weighted wing was not significantly different from contralateral control. These later increases in myofiber size are concurrent with increases in myofibrillar and sarcoplasmic protein content (Laurent et al, 1978;Alway et al, 1989;Matthews et al, 1990). …”

Interleukin-15 responses to aging and unloading-induced skeletal muscle atrophy

“…The PAT muscle is flexed with the wing on the birds back at rest, but it is stretched when the wing is extended. In our experimental stretchoverloading model, a tube containing 10 -12% of the bird's body weight was placed over the left humeral-ulnar joint (4). This maintains the joint in extension throughout the period of stretch and induces stretch at the origin of the PAT muscle.…”

“…Previous studies have shown this stretch-overloading protocol results in moderate hypertrophy of the PAT muscles (i.e., 14-day stretch-loading induces ϳ35% and ϳ15% increases in muscle mass of young adult and aged birds, respectively; see Ref. 4). Following 14 days of stretch overload of the left wing, eight young and eight aged birds were killed with an overdose of xylazine.…”