## 4 Discussion  
In the present study, changes over time in skeletal muscle atrophy were examined in a CKD model of male Wistar rats that were fed a 0.75% adenine diet for 4 weeks, starting at 8 weeks of age. At 20 weeks of age, only the type II muscle CSA was decreased. SDH staining, which reflects mitochondrial activity, was lower in the CKD group than in the control group at 12 weeks of age, but there was no significant difference at 20 weeks of age. Regarding the expression of genes related to muscle differentiation, increased expressions of myogenin, a myogenic regulatory factor, and of MuRF-1 and atrogin-1, muscle catabolic markers, were found only at 12 weeks of age, early after the establishment of CKD, but at 20 weeks of age, there were no differences in their expressions. In the present study, skeletal muscle atrophy in adenine-induced CKD model rats was found to change over time in different types of muscle fibers. Several previous studies involving CKD model rats have reported that skeletal muscle atrophy and changes in the molecular cellular mechanisms associated with muscle atrophy were different in adenine-induced CKD and 5/6 nephrectomy [7; 15]. However, no studies have focused on these changes over time. The present study showed differences in the type of skeletal muscle that atrophies at 12 and 20 weeks of age, differences in gene expression, and differences in mitochondrial activity. These differences in gene expression and mitochondrial activity may account for the differences in the characteristics of muscle atrophy. In the adenine-induced CKD rat model, the CSAs of both type I and type II skeletal muscles were significantly reduced compared to the controls at 12 weeks of age, but only the  
Figure 4: Gene expressions of TA muscle anabolic, muscle catabolic, and mitochondrial biogenesis markers of control rats and CKD rats (\(n=7\) rats per group). **(A,B)** Gene expression of MyoD shows no difference between the groups, whereas that of myogenin shows a significant increase in expression at 12 weeks in the CKD group. **(C,D)** Both atrogin-1 and MuRF-1, muscle catabolic markers, are significantly upregulated at 12 weeks in the CKD group. **(E)** There is no significant difference in the gene expression of PGC-1\(\alpha\) between the groups. b: \(p<0.01\) between control and CKD rats by the \(t\)-test. CSA of type II skeletal muscle was significantly lower at 20 weeks of age, with no significant difference observed in type I skeletal muscle. In general, type I muscle fibers are more susceptible to atrophy caused by inactivity and denervation, whereas type II muscle fibers are more susceptible to aging, diabetes mellitus, chronic heart failure, and cancer [22; 23]. It has been reported that fast-twitch muscle fibers tend to be decreased in CKD patients and CKD model rats [24]. Type II muscle fibers have a poorly distributed capillary network, which is thought to be due to the decreased hemoglobin concentration caused by worsening renal function due to CKD, resulting in tissue hypoxia [25]. Furthermore, it has been reported that a fast-to-slow, fiber-type shift is observed with muscle atrophy, such as aging [23]. These changes in muscle atrophy over time may indicate CKD-induced skeletal muscle atrophy, and compensatory improvement in the CSA of slow type I muscle fibers. In the adenine-induced CKD model rats, the mRNA expression levels of myogenin were significantly higher only at 12 weeks of age. However, the MyoD levels were not significantly different between the groups. MyoD and myogenin are important factors in muscle regeneration [26], and impaired skeletal muscle regeneration is involved in muscle atrophy. When a skeletal muscle is injured, satellite cells initiate proliferation and differentiation, and the expressions of MyoD and myogenin increase [27].