Abstract 598: Irisin, a Novel Hormone, and Its Response to Increasing Exercise Intensity in Young Healthy Subjects

Introduction: Irisin, a recently discovered hormone, has been shown to induce white adipose tissue browning, enhancing energy expenditure and possibly mediating some of the beneficial effects of exercise. We aimed to estimate a) the timeframe of changes in plasma irisin levels after acute maximal exercise, b) the effect of different exercise intensities on irisin levels immediately post-exercise, and c) the effect of smoking (chronic and acute) on exercise-induced irisin release. Methods: a) 4 healthy subjects (22.5±1.73 years) underwent maximal exercise to attain maximal oxygen consumption (VO 2max ). Blood was drawn at pre-specified intervals to define the time curve of irisin changes over a 24-hr period; b) 35 healthy, non-smoking (23.0±3.3 years) men and women (n=20/15) underwent three exercise protocols ≥48 hrs apart, in random order: 1) maximal intensity (VO 2max ), 2) relative intensity (70% VO 2max , 10 mins); and 3) absolute intensity (75W output, 10 mins). Blood was drawn immediately pre- and 3 mins post-exercise; c) In 20 smokers irisin levels were measured pre- and 3 mins post-maximal exercise after 12 hrs abstinence from smoking on two separate days 1) continued abstinence (chronic smoking) and 2) after smoking one cigarette pre-exercise (acute smoking), compared to 10 non-smokers. Results: a) Irisin levels increased by 35% 3 mins post-exercise, then dropped and remained constant; b) Irisin levels post-exercise were significantly higher than pre-exercise after all intensities (all, P<.001). Post-pre exercise differences in irisin levels were significantly different between exercise intensities (P=.001), with the greatest increase after maximal intensity (P=.004 vs. relative, and absolute). Higher VO 2max correlated with a greater post-pre exercise difference in irisin levels after maximal exercise; c) Smoking study analyses are underway. Conclusions: Plasma irisin levels were acutely elevated in response to exercise, with a greater increase after maximal exercise. Higher VO 2max was associated with greater irisin release. These findings suggest that irisin release could be a function of muscle energy demand. Future studies need to determine the underlying mechanisms of irisin release and explore irisin’s therapeutic potential.