Impact of passive heat stress and passive heat acclimation on circulating extracellular vesicles: An exploratory analysis.

New Findings: What is the central question of this study? How does passive heat stress and subsequent heat acclimation affect the circulating concentration of extracellular vesicles? What is the main finding and its importance? Passive heat stress increased the circulating concentration of total and platelet extracellular vesicles. Seven days of hot water immersion did not modify the change in circulating concentrations of extracellular vesicles during passive heat stress.

Abstract: This retrospective exploratory analysis aimed to improve our understanding of the effect of passive heat stress and subsequent heat acclimation on the circulating concentration of extracellular vesicles (EVs). Healthy young adults (four females and six males, 25 ± 4 years of age, 1.72 ± 0.08 m in height and weighing 71.6 ± 9.0 kg) were heated with a water-perfused suit before and after seven consecutive days of hot water immersion. Pre-acclimation, participants were heated until oesophageal temperature increased to ∼1.4°C above baseline values. Post-acclimation, participants were heated until oesophageal temperature reached the same absolute value as the pre-acclimation visit (∼38.2°C). Venous blood samples were obtained before and at the end of passive heating to quantify plasma concentrations of EVs from all cell types (CSFE ), all cell types except erythrocytes (CSFE MHCI ), platelets (CSFE MHCI CD41 ), endothelial cells (CSFE MHCI CD62e ), red blood cells (CSFE CD235a ) and leucocytes (CSFE MHCI CD45 ) via flow cytometry. Passive heat stress increased the concentration of CFSE EVs (46,150,000/ml [3,620,784, 88,679,216], P = 0.036), CFSE MHCI EVs (28,787,500/ml [9,851,127, 47,723,873], P = 0.021) and CSFE MHCI CD41 EVs (28,343,500/ml [9,637,432, 47,049,568], P = 0.008). The concentration of CSFE MHCI CD62e EVs (94,230/ml [-55,099, 243,559], P = 0.187), CSFE CD235a EVs (-1,414/ml [-15,709, 12,882], P = 0.403) or CSFE MHCI CD45 EVs (-192,915/ml [-690,166, 304,336], P = 0.828) did not differ during heat stress. The change in circulating EVs during passive heat stress did not differ after heat acclimation (thermal state × acclimation interactions, all P ≥ 0.180). These results demonstrate that passive heat stress increases the circulating concentration of total and platelet EVs and that passive heat acclimation does not alter this increase.