Heat-stress-induced dehydration is associated with extracellular hyperosmolality. To counteract the associated stress, cells employ cytoprotective mechanisms, including autophagy; however, the autophagic response to hyperosmotic stress has yet to be evaluated in humans. Thus, we investigated autophagy and associated cellular stress pathways [the heat shock response (HSR), apoptosis, and the acute inflammatory response] to isosmotic and hyperosmotic conditions with and without hyperthermia in 12 young men (mean [SD]; 25 [5] yr). Participants received a 90-min intravenous infusion of either isosmotic (ISO; 0.9% NaCl; serum osmolality of 293 [4] mosmol/kgH2O) or hyperosmotic (HYP; 3.0% NaCl; 300 [6] mosmol/kgH2O) saline, followed by passive whole body heating using water perfused suit to increase esophageal temperature by ∼0.8°C. Peripheral blood mononuclear cells were harvested at baseline (preinfusion), postinfusion, and after heating, and changes in protein content were analyzed via Western blotting. Post infusion, the LC3-II/I ratio was higher in HYP compared with ISO infusion (P < 0.001), although no other protein changes were observed (all P > 0.050). Following passive heating, autophagy increased in HYP, as demonstrated by an increase in LC3-II from baseline (P = 0.004) and an elevated LC3-II/I ratio compared with ISO (P = 0.035), and a decrease in p62 when compared with the ISO condition (P = 0.019). This was accompanied by an elevation in cleaved caspase-3 following heating in the HYP condition (P < 0.010); however, the HSR and acute inflammatory response did not change under any condition (all P > 0.050). Taken together, our findings indicate that serum hyperosmolality induces autophagy and apoptotic signaling during mild hyperthermia with minimal autophagic activation during normothermia.NEW & NOTEWORTHY We demonstrate that a physiologically relevant increase in serum osmolality causes minimal activation of the autophagic response. However, the combined stressors of serum hyperosmolality and mild hyperthermia causes activation of both autophagy and apoptotic signaling. Thus, changes in osmotic homeostasis appear to influence the cell's cytoprotective ability during periods of heat stress, highlighting the importance of considering osmotic status when examining autophagic responses in vivo.
Keywords: apoptosis; autophagy; heat shock response; heat stress; osmotic stress.