Critical illness myopathy (CIM) is a debilitating common consequence of modern intensive care, characterized by severe muscle wasting, weakness and a decreased myosin/actin (M/A) ratio. Limb/trunk muscles are primarily affected by this myopathy while cranial nerve innervated muscles are spared or less affected, but the mechanisms underlying these muscle-specific differences remain unknown. In this time-resolved study, the cranial nerve innervated masseter muscle was studied in a unique experimental rat intensive care unit (ICU) model, where animals were exposed to sedation, neuromuscular blockade (NMB), mechanical ventilation, and immobilization for durations varying between 6 h and 14d. Gel electrophoresis, immunoblotting, RT-PCR and morphological staining techniques were used to analyze M/A ratios, myofiber size, synthesis and degradation of myofibrillar proteins, and levels of heat shock proteins (HSPs). Results obtained in the masseter muscle were compared with previous observations in experimental and clinical studies of limb muscles. Significant muscle-specific differences were observed, i.e., in the masseter, the decline in M/A ratio and muscle fiber size was small and delayed. Furthermore, transcriptional regulation of myosin and actin synthesis was maintained, and Akt phosphorylation was only briefly reduced. In studied degradation pathways, only mRNA, but not protein levels of MuRF1, atrogin-1 and the autophagy marker LC3b were activated by the ICU condition. The matrix metalloproteinase MMP-2 was inhibited and protective HSPs were up-regulated early. These results confirm that the cranial nerve innervated masticatory muscles is less affected by the ICU-stress response than limb muscles, in accordance with clinical observation in ICU patients with CIM, supporting the model' credibility as a valid CIM model.

Severe muscle wasting is a debilitating condition in critically ill intensive care unit (ICU) patients, characterized by general muscle weakness and dysfunction, resulting in a prolonged mobilization, delayed weaning from the ventilator and a decreased quality of life post-ICU. The mechanisms underlying limbmuscle weakness in ICU patients are complex and involve the impact of primary disease, but also factors common to critically ill ICU patients such as sepsis, mechanical ventilation (MV), immobilization and systemic administration of corticosteroids (CS).  These factors may have additive negative effects on skeletal muscle structure and function, but their respective role alone remain unknown. The primary aim of this study was to examine how CS administration potentiates ventilator and immobilization-related limb muscle dysfunction at the gene level. Comparing biceps femoris gene expression in pigs exposed to MV and CS for five days with only MV pigs for the same duration of time showed a distinct deregulation of 186 genes using microarray. Surprisingly, the decreased force-generation capacity at the single muscle fiber reported in response to the addition of CS administration in mechanically ventilated and immobilized pigs was not associated with an additional up-regulation of proteolytic pathways. On the other hand, an altered expression of genes regulating kinase activity, cell cycle, transcription, channel regulation, oxidative stress response , cytoskeletal, sarcomeric and heat shock protein as well as protein synthesis at the translational level appear to play an additive deleterious role for the  limb muscle weakness in immobilized ICU patients.