Serum Irisin Responses To Daylong Passive Heat Exposure In Young And Older Adults

We have recently demonstrated that the heat-induced cytoprotective function of autophagy declines with increasing age. When paired with marked elevations in circulating stress biomarkers including systemic inflammation (e.g., interleukin-6, c-reactive protein) and intestinal epithelial injury (e.g., intestinal fatty acid binding protein), reduced autophagy may contribute to age-related heat vulnerability. Irisin is a myokine and adipokine that has also been implicated in cryoprotection during exposure to acute physiological stress. However, the irisin response to a passive heat stress has yet to be evaluated and whether this is altered by aging remains unknown. PURPOSE: To assess serum irisin responses in young and older adults exposed to a daylong (9-h) heatwave simulation. METHODS: We evaluated serum irisin responses from 17 young (19-31 years; 10 female) and 37 older (19-31 years; 10 female) adults before and after a 9-h heat exposure [40.3 °C and 9.3% relative humidity] using enzyme-linked immunosorbent assays (ELISA). Core (rectal) temperature was measured throughout the exposure. Data were analyzed via a two-way repeated measures ANOVA with Bonferroni correction (α = 0.05). RESULTS: After 9-h of heat exposure, the elevation in core temperature from baseline was greater in the older (1.0 °C [SD 0.3]) as compared to young adults (0.6 °C [0.2]; p < 0.01). Serum irisin concentrations were significantly lower at baseline in older (364 pg/mL [335]) as compared to the younger cohort (1090 pg/mL [1138]; p < 0.01). This difference persisted at the end of the 9-h of heat exposure, wherein serum irisin was lower in older adults as compared to their younger counterparts (422 pg/mL [641] vs 1244 pg/mL [1229]; p < 0.01). When comparing responses from baseline, serum irisin responses were not significantly different at end-exposure in either group (all, p > 0.42). CONCLUSION: While older adults experienced significant elevations in core temperature as compared to their younger counterparts at the end of the daylong extreme heat exposure, changes in serum irisin concentrations from baseline were not different at end-exposure in either group. Despite this, young adults maintained greater serum irisin concentrations relative to older adults throughout exposure. Supported by: Canadian Institutes of Health Research, Health Canada