Stretch induces non‐uniform isomyosin expression in the quail anterior latissimus dorsi muscle

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Abstract
Stretch‐overload has been shown to elevate muscle mass in the avian anterior latissimus dorsi (ALD) by increasing both fiber size and fiber number; however, it is not known if these adaptations result in uniform regulation of myosin isoform expression along the length of the fibers in this slow tonic muscle. When a weight was added to the left wing of 20 adult quail for 30 days, ALD mass was increased by 161%. ALDs were divided into proximal, middle, and distal portions. Tissue cross‐sections from each muscle portion were reacted against monoclonal antibodies for either fast (FM) or slow myosin (SM), or processed for identification of native myosin isoforms. The number of fibers expressing FM increased by 190% in the distal region after stretch; however, most of these were small fibers. Electrophoretic analyses of stretched muscles demonstrated an upregulation of SM2 in all regions of the ALD. SM1 was only down‐regulated in the proximal region of the ALD. FM isoforms had greater increases in the proximal region than other regions of the overloaded ALD. These data indicate that stretch‐induced hypertrophy induces a non‐uniform increase in fast myosin isoforms and SM1 along the length of the fibers of the ALD.