In the literature review, we found no published research that has evaluated the effect of WBV exercise applied at different environmental temperatures on irisin hormone distribution in muscle tissue by immunohistochemical methods. However, there are studies investigating the effect of different temperatures and different exercises on serum and plasma irisin levels.
Park et al. (2021) investigated the effect of hot application on irisin circulation in human subjects and found an increase in serum irisin levels after the application compared to room temperature in subjects who were immersed half of their bodies in 42-degree water and underwent hot application for a total of 30 minutes as 5 minutes immersion-5 minutes rest. In another study investigating the effects of heat and exercise on serum irisin levels, it was reported that being in a hot environment (47 ± 3°C for 45 min) increased serum irisin levels more than moderate exercise (continuous running of 7.3 km/h) (Aydin et al. 2013). It has been shown that resistance (85% incline, 135 cm high stair, 8 repetitions/day, 3 days/week, 8 weeks) and endurance (15% incline, 20 m/min, 5 days/week, 8 weeks) training applied both at room temperature (23 ± 2°C) and cold environment (16 ± 2°C) significantly increased serum irisin levels compared to the control group at both environment temperatures, but irisin levels were similar between exercise groups at both environment temperatures (Javadifar et al. 2021). Another study reported that the lowest irisin level was observed at the warm environment temperature and the highest irisin level was observed at the cold environment temperature after a one-time and continuous 60 min bicycle ergometer training at 60% of the maximum oxygen consumption applied at three different environment temperatures: cold (15–19°C), room temperature (24°C) and hot (34°C) (Tsuchiya et al. 2015).
In light of the above literature findings, it is seen that the effect of exercise type, duration, acute or chronic exercise and environment temperature on serum irisin levels has not yet been clarified.
In the literature, there are studies conducted to determine the presence and distribution of irisin in muscle tissues of different organs of some animal species. It has been reported that positive irisin reaction was observed in striated muscle cells in the tongue and oesophagus of dwarf hamsters (Gür et al. 2018), strong irisin immunoreactivity was found in cardiac muscle cells and tongue skeletal muscle cells of blind mice (Gur et al. 2017) and irisin positivity was detected in tongue skeletal muscle cells of the arrowed hedgehog (Gur et al. 2017).
It was also emphasised that a strong irisin reaction was found in the extraocular skeletal muscle fibrils of the arrowed hedgehog and that the positive reaction was more intense near the cell membrane than in the centre of the cell (Gençer Tarakçı et al. 2016). In our study, irisin immunoreactivity was detected not only in the cell membranes but also in the centre of the cell to varying degrees in the gastrocnemius tissue of all groups.
In one of the studies conducted to determine the localisation of irisin hormone in muscle tissue, Gür et al. (2018) examined heart and skeletal muscle tissues of dwarf and reported that strong irisin immunoreactivity was observed in heart and skeletal muscle cells. Contrary to Gür et al. (2018) in another study (Aydin et al. 2014) conducted in skeletal muscle of humans and heart and skeletal muscle of rats (2.5 months old), it was reported that irisin positivity was observed only in a few muscle fibrils in skeletal muscle tissue of rats, but strong irisin reaction was observed in heart muscle tissue. In contrast, in human skeletal muscle tissue, moderate irisin immunoreactivity has been reported in a few muscle fibrils and strong irisin immunoreactivity in the endomysium and perimysium. The same study also suggested that the main source of irisin may be perimysium and epimysium rather than skeletal muscle sarcoplasm. In our study, similar to the findings of Gür et al. (2018), strong and very strong irisin reaction was determined in the gastrocnemius tissues of the control group and the 3rd experimental group. However, unlike the findings obtained by Aydın et al. (Aydin et al. 2014) in rats, in our study, a moderate reaction was found in the control group, and 1st and 2nd experimental groups and a strong reaction was found in the 3rd experimental group in endomysium, the moderate reaction was found in the 1st experimental group, and the strong reaction was found in the control group and 2nd and 3rd experimental groups in perimysium. Therefore, contrary to the findings of Aydın et al. (2014), we think that muscle fibrils in the gastrocnemius tissue may be the source of the irisin.
In a study investigating the effect of swimming exercise on the distribution of irisin in skeletal muscle tissues in young and old male rats (Aydin et al. 2014), 12- and 24-month-old rats were subjected to swimming exercise in 24–26°C water for 10 min. It was reported that there was no irisin immunoreactivity in the skeletal muscle of young rats without swimming exercise, but moderate irisin positivity was observed after swimming exercise, especially in the perimysium. It has also been reported that irisin was not found in the skeletal muscles of old rats, whether they swim or not. Therefore, researchers have suggested that skeletal muscle is not the main source of irisin. In our study, unlike the researchers' findings in both experimental groups, irisin immunoreactivity was found in the gastrocnemius muscle fibrils of all groups. In addition, a moderate reaction was found in the perimysium in the muscle tissue of the 1st experimental group, which was subjected to vibration exercise in a cold environment, and a strong reaction was found in the perimysium in the muscle tissues of the other groups. According to the findings of our study, it can be concluded that both WBV application and rising environmental temperature increase the amount of irisin in muscle tissue.
In a study (Brenmoeh et al. 2014) in which the distribution of irisin in the rectus femoris tissue was investigated and running wheel (diameter = 33.4 cm) and progressive treadmill training (starting speed: 15 m/min, finishing speed: 38 m/min) were applied for 3 weeks, it was found that weak irisin immunoreactivity was observed in muscle fibrils and more intense irisin immunoreactivity was observed between fibrils in the treadmill training group. It was also reported that the reaction was similar but weaker in the control group and the running wheel group. The results of the study suggested that the density of irisin in muscle tissue in mice increased after acute exercise but not after repeated voluntary exercise. In our study, irisin immunoreactivity was weak in the 1st and 2nd experimental groups, moderate and strong in the control group, and strong and very strong in the 3rd experimental group in the muscle fibrils of gastrocnemius tissues. In addition, an intense distribution of irisin was observed in the connective tissues. In this case, it can be concluded that WBV exercise increases the amount of irisin hormone secreted from the muscles compared to aerobic-based exercises performed at room temperature in the above study.
In our study, since irisin positivity was detected at different intensities in the muscle tissue fibrils of all groups, and even strong and very strong irisin immunoreactivity was observed in the gastrocnemius muscle fibrils of the 3rd experimental group, it can be concluded that skeletal muscle tissue may constitute one of the main sources of irisin and irisin intensity increases in response to WBV exercise with increasing environment temperature.
When the effects of WBV exercise on the localisation and distribution of irisin hormone in M. gastrocnemius muscle tissue were examined, it was concluded that irisin hormone was secreted from both muscle fibrils and connective tissue in all groups, irisin hormone synthesised from muscle fibrils decreased after WBV exercise in cold and room temperature, but increased after WBV exercise in a hot environment.
With this study, it is thought that determining whether the distribution of irisin hormone in muscle tissue changes with WBV exercise applied at different environmental temperatures will contribute to the literature and other scientific research to be conducted.
We also think that further studies are needed to investigate the effects of WBV application on tissues and systems and to investigate the optimal vibration parameters.