Introduction
The prevalence of obesity is associated with an increased risk of metabolic syndrome, type 2 diabetes, and insulin resistance, so control and regulation of adipose tissue can prevent diseases associated with this tissue [1]. Adipose tissue plays an important role in energy balance management by secreting some adipokines including chemerin. Chemerin is secreted from visceral adipose tissue and liver [2] and plays an improtant role in macrophage recruitment to adipose tissue, development of inflammation and insulin resistance, leading to insulin stimulation and increased glucose uptake in adipocytes [3]. According to the observations, serum levels of chemerin are elevated in obese patients and are associated with various aspects of metabolic syndrome [4]. In addition, high concentrations of chemerin serum make it possible for pre-diabetic conditions which affects glucose homeostasis [5] and induce insulin resistance [7].
Regular exercise is a good strategy for treating many metabolic disorders, including insulin resistance and obesity, by improving insulin sensitivity via increasing the density and sensitivity of glucose transporter GLUT4 in skeletal muscle sarcoma [8] and affecting adipose tissue [9]. Twelve weeks of aerobic [4] and strength [3]exercises decreases insulin sensitivity in obese men by lowering chemerin serum level. There is also a reported significant decrease in chemerin plasma level after high-intensity intermittent training [15] and 12 weeks of aerobic interval training in 24 inactive students [6]; However, 8 weeks of sprint exercise have no significant effect on chemerin serum levels of female Sprague-Dawley rats [16]. Given that the effect of exercise on chemerin serum levels is somewhat contradictory [4], and the changes in its level are dependent on the type and intensity of the exercise, the present study aimed to investigate the effect of endurance exercise and high-intensity (HIIT) intermittent exercise on chemerin plasma level and insulin resistance in obese male rats.
Materials and Methods
Study samples were 27 male Wistar rats aged 35-45 days with a mean weight of 110±10 g. They were randomly divided into 4 groups of sedentary control (n=6), endurance exercise (n=8), HIIT intermittent exercise (n=8) and untreated obese control (n=5). Both endurance and HIIT intermittent exercise groups performed exercises on a treadmill for 8 weeks. For the HIIT group, exercise protocol consisted of training sets with an intensity of 90% VO2max for 15-30 seconds with a 1-min active resting interval. It reached from 5 sets in the first week to 12 sets in the last week. Total exercise activity was matched between the two training groups such that the intensity of exercise in the endurance group was 50-70% VO2max. Accordingly, endurance group activity in the first week reached from 25 minutes at a speed of 15 m/min and with intensity of 50-70% VO2max to 60 min at 22 m/min speed. Forty-eight hours after the last training session, the rats of all three groups were anesthetized by intraperitoneal injection of ketamine and xylazine. Blood samples was collected from the heart of Rats directly. Plasma and insulin levels of chemerin were calculated by ELISA method, while plasma level of glucose was measured by enzymatic glucose oxidase, and insulin resistance index was calculated by HOMA-IR formula. After testing the normality of data distribution by Smirnov-Kolmogorov test and confirmation of the equality of variance assumption using Levene’s test, ANOVA test was used for analyzing data and comparing groups. Scheffe’s test was used for determining the location of differences between groups regarding chemerin level. For analyzing data related to insulin resistance, Welch's ANOVA test and Dant3 post hoc test were used.
Results
The chemerin level was significantly reduced in the endurance group compared to the untreated obese control group (P=0.006), but it had no significant change compared to the sedentary control group (p=0.590). Moreover, the plasma level of chemerin in the HIIT intermittent group was not significantly different from that of untreated obese control group (p=0.781) and sedentary control group (P=0.421) and chemerin level reduction in the endurance group was significant compared to the HIIT intermittent group (P=0.035). Furthermore, there was no significant difference in the insulin resistance reported in HIIT intermittent group compared to that of endurance group (P=0.825), but its difference in HIIT intermittent and endurance training groups was significant compared to that of untreated obese control and sedentary control groups (P<0.05).
Conclusion
Both endurance and HIIT intermittent exercises can decrease plasma levels of chemerin and insulin resistance. However, the reduction is partly dependent on the type of exercise, such that plasma levels of chemerin significantly decreased after a period of endurance exercise compared to HIIT intermittent exercise.
Ethical Considerations
Compliance with ethical guidelines
This study obtained its ethical approval form the Research Ethics Committee of Sport Sciences Research Institute (code: IR.SSRI.REC.1398.55) and approved by Faculty of Physical Education and Sport Sciences, University of Tehran.
Funding
This study received no financial support from any organization.
Authors' contributions
All authors contributed equally to the writing of this article.
Conflicts of interest
The authors declare no conflict of interest.

References