## 4 Discussion
MyoD expression is associated with satellite cell activation and proliferation, whereas myogenin reflects myoblast differentiation; a study examining skeletal muscle from Cy/+ rats reported that both MyoD and myogenin were upregulated [7], a result different from the present study. The increased expression of myogenin observed in the present study may reflect the process of muscle regeneration in an adenine-induced CKD rat model. Myogenin is involved in myoblast differentiation and acts to regulate myofiber maturation and size [28]. Among its functions, it has been suggested that it induces the fusion of skeletal muscle fibers, mainly affecting the formation of fast-twitch muscle fibers and muscle hypertrophy [29]. In the present study, myogenin expression was increased in 12-week-old CKD model rats. This may be a compensatory mechanism for the muscle wasting that occurs in the CKD rat model. In contrast, no significant difference was observed for MyoD, a myogenic marker. It has been suggested that uremic toxin (UT) is also involved in muscle atrophy and weakness in CKD [30; 31]. One in vitro study reported that different doses of uremic substances affect the myogenic process differently. Low doses of UT impair normal myogenic differentiation by promoting fibrotic and adipogenic differentiation of myoblasts, whereas high doses of UT impair myoblasts by inducing cell cycle arrest, disrupting their proliferation, and causing apoptosis [32]. MyoD is expressed at the muscle differentiation stage when satellite cells are activated before myoblast differentiation. Therefore, the difference in the expression of myogenic markers in the present study suggests that the model may have been exposed to high concentrations of uremic substances. In the adenine-induced CKD rat model, SDH staining intensity was decreased only at 12 weeks. Furthermore, the percentage of SDH-positive fibers was significantly lower only in CKD rats at 12 weeks. This result suggests that mitochondrial activity was reduced early after the model was created. It has been reported that mitochondrial dysfunction is associated with muscle weakness in CKD [33]. It has also been reported that CKD is preceded by a decrease in muscle strength and muscle mass, and is associated with a decrease in muscle mitochondrial function [34]. In addition, type I muscle fibers that showed reduced CSA in this study were rich in mitochondria [35], and the reduction in type I muscle CSA early after model creation may be related to reduced mitochondrial activity. The present study examined the changes in skeletal muscle atrophy over time in a rat model of adenine-induced CKD. This study has several limitations. First, it was not possible to confirm serum creatinine levels and other data in each group; thus, the relationship between blood data and muscle atrophy could not be evaluated. However, since the method used to create the model was the same as that in our previous study, which confirmed that creatinine levels worsened over time, we assumed that the early and late stages of CKD onset were compared in the present study. Second, nutritional status was not assessed. There is a trend toward weight loss when adenine is administered [36]. In our previous report of adenine-induced CKD rat models generated using the same protocol as in the present study [37], body weight was also decreased, suggesting that adenine administration may have contributed to the weight loss and associated deterioration in nutritional status. Third, the evaluation was conducted on the back muscles and the TA muscles, and a comprehensive evaluation of the entire model is lacking. To evaluate muscle change in this model, it is necessary to consider muscles in other regions, such as the soleus muscle or quadriceps muscle.