Skeletal muscle is a remarkably adaptive tissue able to optimize performance based on the demands of the organism. Adaptation involves regulating force output (sarcomeres in parallel), structural integrity (connective tissues, mysiums, tendons), endurance (fiber type, mitochondrial abundance), contractile velocity (actin filament density within the myofibril, myosin type), and length (sarcomeres in series). Of these parameters, length regulation is least well understood. To model chronic muscle unloading, which induces shortening maladaptation, a rat soleus tenotomy model was developed in which the distal tendon was cut. Stretch was applied through a ligature sutured to the distal tendon stump. Stretch is a time tested exercise thought to increase muscle length and joint flexibility and prevent injury; however, the efficacy of stretch on the regulation of muscle length is controversial. Central core breakdown of the contractile elements within slow muscle fibers appears by 4 d post tenotomy and provided a morphological marker for muscles undergoing shortening adaptation. By day 7 post-tenotomy, 70% of slow fibers had central cores. Solei passively stretched 20 min daily exhibited 50% fewer slow fibers with central cores; however, daily passive stretch did not prevent the shortening adaptation of the muscle fibers. Tenotomized muscle had 68% of the sham control number of sarcomeres per mm of muscle fiber. Passive stretch did not prevent sarcomere loss. Isometric muscle contraction was added to daily stretch via direct electrical stimulation of soleus. Solei stretched and stimulated daily had significantly more sarcomeres per mm of muscle than those with tenotomy alone or daily passive stretch. Stretch+stimulation is necessary to prevent shortening adaptation. Biomarkers known to be activated during exercise (Akt, p70S6K) and/or stretch (p38 MAPK, ERK) and linked to increased sarcomeric protein transcription and translation were analyzed for phosphorylation activation when the soleus was 1) electrically stimulated in a shortened position (plantarflexed), 2) stretched passively to the optimal muscle length (LO) (dorsiflexed), 3) stretched to LO then stimulated, 4) stretched passively 25% beyond LO, and 5) stretched 25% beyond LO then stimulated. Neither passive stretch nor stretch+contraction at any position resulted in the phosphorylation of Akt at T308, its primary activation site. Passive stretch of the soleus to LO (maximum physiological dorsiflexion) produced no activation of any markers. Stimulation at all positions significantly activated p70S6K at T389 and T421/S424, p38 MAPK, and ERK1/2. Passive stretch at LO+25% activated Akt at S473, p70S6K at T389 and T421/S424, p38 MAPK, and ERK 1/2. Stimulation at LO+25% significantly increased all biomarkers, including both Akt sites. The phosphorylation of S473 occurred at a length greater than the in vivo working muscle range. Muscles maladapted with LO near the midrange of motion, and those working across two joints would be expected to exhibit this unique stretch signaling. The proposed mechanism for sarcomere addition in contracting muscle is calcium release and signaling activation, although increased tension cannot be ruled out. Passive stretch beyond LO may open stretch-activated calcium channels and increase cytoplasmic calcium to activate calcium-dependent pathways. The identification of the phosphorylation of Akt S473 as a length-dependent biomarker has provided a first step in identifying a length-sensitive signaling mechanism of sarcomere number regulation.
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