BACKGROUND: Irisin is a newly identified hormone secreted mainly by skeletal muscles. It has different effects on the cardiovascular system and blood vessels. The present study investigated the possible effects of irisin on the microscopic structure of the perivascular adipose tissue in the rat thoracic aorta. MATERIALS AND METHODS: Twenty rats were randomly divided into two groups: a non-injected control group (n=4) and four injected experimental groups (each n=4). The experimental rats were injected intraperitoneally with one of the following concentrations of irisin
250, 500, 1000, and 2000ng/mL, twice a week for 4 weeks. After that, all experimental rats' descending thoracic aortas were removed, and imaging was performed. ANOVA and Bonferroni's Multiple Comparison Test were used to achieve statistical comparisons. RESULTS: A trend of a dose-dependent increase in the number of brown adipocytes in all irisin-injected groups reached statistical significance at a dose of 2000ng/mL, compared to that of the control group (from 7.9±1 control to 67±6.1 in 2000ng/mL of irisin). A dose-dependent decrease in the number of white adipocytes compared to that of the control group (from 40±4.8 control to 3±0.9 in 2000ng/mL of irisin). CONCLUSION: The present study has for the first time demonstrated that irisin has significantly increased the number of brown adipocytes and decreased the number of white adipocytes in the perivascular adipose tissue in rat thoracic aorta.