Introduction: Myostatin (MSTN), growth and differentiation factor 8 (GDF8), is a member of the large family of secretory growth factors TGF-β, and inhibition of this protein has a significant effect on muscle growth. DNAi is an advanced and novel technology for regulating gene expression. The aim of the present study was to inhibit MSTN gene expression in muscle tissue using intraperitoneal injection of DNAi, an inhibitor of MSTN gene expression, in rats and intramuscular injection of the same molecule in mice. Materials and Methods: In this study, 12 mice were used in the first experiment and 12 male Wistar rats in the second experiment in a completely randomized design with 2 treatments and 6 replications. The groups included: the injection group at a dose of 10 ????????/???????? body weight three times a week and the control group without injection. The experiment lasted for 5 weeks and the weight of the animals was measured every week. Then, the effect of DNAi molecules on the weight of the heart and leg muscle was examined. Histological slides were prepared from the gastrocnemius muscle for histological examination. Finally, the collected data were analyzed using the one-way analysis of variance and JMP PRO software version 17.0. Results and discussion: The results showed that the weight of the calf muscle increased significantly, but DNAi injection had no significant effect on weekly weight gain or heart weight (P>0.05). Also, histological examination indicated the occurrence of hypertrophy in the leg muscle tissue in mice, which is evidence of a decrease in muscle density per unit area and an increase in muscle tissue mass, which was not seen in rats. Considering the effect of reducing MSTN gene expression on adipose tissue, the lack of a significant increase in body and heart weight may be due to the loss of interstitial fat in the muscles or heart. Comparing the results of intraperitoneal and intramuscular injection also indicated that the intramuscular injection method was more effective. This may be because the molecule was injected into the target tissue. Conclusion: DNAi-based gene expression inhibition is a new method that has not yet been widely used. The results of current study showed that injection of DNAi molecules can inhibit MSTN gene transcription. The results of this study could be further investigated by measuring gene expression as well as blood parameters.

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