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One session of exercise or endurance training does not influence serum levels of irisin in rats.

Irisin induces the browning of adipose tissue. The goal of this study was to investigate the influence of acute exercise in untrained and trained rats and endurance training on FNDC5 mRNA and irisin levels in white and red skeletal muscle and serum. Rats (n=60) were randomly divided into two groups: untrained and trained (subjected to 6-week endurance training with increasing load). Subgroups of rats from each group were sacrificed before (controls), immediately after, or 3 hours following acute exercise with the same work load. Muscle samples (red and white) and serum were collected. FNDC5 mRNA was evaluated using RT-PCR. Irisin levels were measured using an immunoenzymatic method. Muscle FNDC5 mRNA decreased immediately after acute exercise compared with baseline levels, but not in red muscle in trained rats. Atrend toward a return to baseline appeared 3 hours after the exercise, but only in white muscle in untrained group. Irisin protein levels increased after acute exercise in red muscle 3 hours post-exercise compared with samples taken immediately after exercise, and decreased 3 hours post-exercise compared to pre-exercise level in white muscles. FNDC5 mRNA did not change following training, whereas irisin protein levels increased in red muscle and decreased in white muscle. Serum irisin levels remained unchanged following acute exercise and training. We concluded that changes in irisin mRNA and protein levels in rat muscle after acute exercise are limited and depend on training status and the muscle type. Irisin serum levels remained stable after acute exercise or endurance training.