Intrinsic transient tracheal occlusion training and myogenic remodeling of rodent parasternal intercostal fibers.

INTRODUCTION The diaphragm contracts the earliest among the muscles of inspiration [1], and its displacement accounts for approximately 70 percent of the change in resting tidal volume [2]. Agonist intercostals in the parasternal region and lower external intercostals also have a large inspiratory mechanical advantage [3-4], and they contract in a predictable manner at rest [1,5]. Consequently, clinical weakness of either the diaphragm or intercostals results in paradoxical movements between the thorax and the abdomen [6] and impedes compensatory increases in ventilation during periods of elevated drive [7]. Rehabilitation with inspiratory muscle strength training (IMST) may counteract respiratory muscle insufficiency by improving maximal inspiratory pressure [8-9] and enhancing flow and volume compensations to loaded breathing [10]. However, the cellular remodeling responses of the inspiratory muscles to respiratory overloads are less understood, and their timing and magnitude could differ from those of the limb muscles. It has been noted that the inspiratory intercostals appear to remodel with chronic changes in motor demands. Of the intercostal segments, the midthoracic external intercostals are best understood, and they have been found to atrophy [11] and hypertrophy [12] in clinical diseases and conditions that often occur in conjunction with chronic changes in inspiratory motor activity. Single airway occlusions in anesthetized animals also induce respiratory load compensation responses of the diaphragm and parasternal intercostals that include a prolonged inspiratory time and increased peak electromyography (EMG) [13]. Our laboratory found that acute bouts of intrinsic transient tracheal occlusion (ITTO) in anesthetized animals appeared to induce progressive load compensation responses throughout the respiratory pump, as measured by elevated diaphragm EMG activity, increased inspiratory time, and increased esophageal pressure [14]. When repeated for several days in conscious animals, ITTO load compensation responses also promote neural plasticity [15] and diaphragmatic fast fiber hypertrophy [16]. It is not known whether the plasticity to ITTO observed in diaphragmatic muscle fibers can be similarly achieved in the parasternal intercostals. The model of ITTO developed in our laboratory [14,17] provides a reversible, inescapable load to the respiratory muscles that is high intensity with short trial durations and resembles the initial occlusion-load phase of clinical IMST [9]. ITTO is advantageous, because it eliminates the need for permanent tracheostomies or tracheal banding techniques [15]. We recently reported the ITTO training model and its resultant hypertrophy in diaphragmatic type IIx/b fibers [16]. Mechanical strain promotes muscle fiber hypertrophy by facilitating protein synthesis pathways [18], and it also can activate satellite cells, muscle progenitors anatomically and functionally distinct from other myonuclei [19-20]. Pax7 is a transcription factor expressed by quiescent, activated, and proliferating satellite cells [21]. Satellite cell proliferation and differentiation facilitates myofiber growth and repair and supplies new myonuclei to the tissue. Developmental myosin heavy chain (MHC) isoforms, which are typically absent in mature skeletal muscle, reappear transiently in regenerating myotubes [22]. Since intramuscular tension is particularly high within intercostal muscle segments [23], we reasoned that these markers of myogenic activity could be present following ITTO. In the current study, we investigated myogenic activity and morphological remodeling of parasternal muscle fibers in conscious rats after ITTO. The purpose of the study was twofold: to examine whether ITTO facilitated muscle fiber hypertrophy and myogenic activation in the parasternal muscles, and then to contrast the myogenic responses of the parasternals to the diaphragm. We hypothesized that respiratory loads generated during ITTO would result in significant hypertrophy and myogenic activity in the parasternal intercostals when compared with a surgical control group. …