Increased workload fully activates the blunted IRS-1/PI3-kinase/Akt signaling pathway in atrophied uremic muscle

Muscle wasting in chronic renal failure is associated with increased morbidity and mortality; however, resistance exercise is effective at increasing muscle mass while improving muscle strength and function. To study the mechanism by which this occurs, we compared uremic and control rats where work overload was surgically induced unilaterally in the plantaris muscle. We found that work overload increases muscle insulin-like growth factor-1 and mechano-growth factor expression. This in addition to direct mechanical activation of signaling was likely the cause of the observed increased in the protein levels and phosphorylation of the mediators of these growth factors, the insulin receptor substrate-1/phosphoinositide 3-kinase/Akt pathway. The mechanical enhancement of signal transduction appeared to be mediated in part by increased signal protein levels and decreased SOCS2 mRNA expression (suppressor of cytokine signaling-2). Despite impaired basal signaling, the work-induced signaling response was similar to that observed in nonuremic rats and produced changes consistent with decreased muscle protein degradation, increased protein synthetic capacity, and an increased number of multinucleated muscle cells. Our studies suggest that these work-induced changes account for the improved uremic muscle mass reaching levels comparable to those seen in normal rats.