Plasma epinephrine and norepinephrine responses to extreme heat exposures in young and older adults.

Abstract

Hyperthermia is known as a hyperadrenergic state, yet there is a lack of data on the sympathetic responses to ambient heat stress in humans. Therefore, we investigated the plasma epinephrine and norepinephrine concentrations of healthy young and older adults exposed to 3-hours of very-hot and dry and hot-humid heat, both with accompanying activities of daily living. We hypothesized that older adults, compared to young adults, would have augmented increases in epinephrine and norepinephrine concentrations secondary to increased thermal strain. Young (n=20) and older (n=18) participants underwent two 3-hour heat exposures on different days: very hot and dry (47°C and 15% RH) and hot and humid (41°C and 40% RH). To mimic heat generation comparable to activities of daily living, participants performed seven 5-minute bouts of light cycling (~3 METS) dispersed throughout the heat exposure. We measured plasma concentrations of epinephrine and norepinephrine at baseline, end, and 2hrs post-heat exposure. There was a group-wide increase in epinephrine from baseline to the end of the heat exposure (Δ19±27 pg/mL; p<0.001) in the hot and humid but not the very hot and dry condition (Δ6±19 pg/mL; p=0. 10 ). There were group-wide decreases in norepinephrine concentrations from baseline to the end of the heat exposure in both the very hot and dry (Δ-131±169 pg/mL; p<0.001) and the hot and humid (Δ-138±157 pg/mL; p<0.001) conditions, with both returning to near baseline at 2hrs post-exposure. These data suggest that ambient heating with accompanying bouts of light intermittent exercise may lead to decreases in circulating concentrations of norepinephrine.