Endocrine Abstracts

Background: Heat acclimatisation (HA) describes phenotypic changes (decreased heart rate, HR; lower core body temperature, Tc) resulting from exposure to a hot environment. Heat tolerance with HA may reflect altered adrenocortical and autonomic nervous responses to heat stress, though evidence for sympathetic downregulation is lacking. Methodological limitations to further investigation (e.g. with direct catecholamine measurement) could be overcome by assaying the catecholamine metabolites, normetanephrine and metanephrine.

Aim: To assess whether nephrine concentrations reflect changes in physiological strain with HA using serial Heat Tolerance Assessment (HTA).

Methods: Military volunteers (n=24) were assessed at baseline in the UK and on Day 2, 6, 9 and 23 of HA in Cyprus. HTA consisted of 60 min relative-intensity stepping exercise in a temperature/humidity-controlled chamber. During exercise, HR and Tc were recorded every 5 min. A validated Physiological Strain Index (PSI) integrated Tc and HR responses on a scale of 0 to 10 (0=resting strain, 10=maximal strain). Resting blood samples were taken before and after HTA, for assay of plasma free nephrines and serum cortisol.

Results: From UK to Day 23 in Cyprus, PSI fell from 6.7±1.3 to 4.8±1.4 (F=18.2, P<0.0001). Main effects of HA on each biochemical analyte (P<0.0001) were evident over the same period, with significant reductions in post-HTA values (normetanephrine: 948±328 vs 461±132 pmol/l; metanephrine: 302±103 vs 230±91 pmol/l; cortisol 577±203 vs 314±120 nmol/l). Combined data from HTAs showed significant correlations (P<0.0001) between ΔPSI and Δnormetanephrine (r=0.68), Δmetanephrine (r=0.34) and Δcortisol (r=0.62).

Conclusions: The progressive reduction in nephrine concentrations post-HTA and strong association between Δnormetanephrine and PSI provide evidence for reduced sympathetic activation with HA. Like cortisol, nephrine responses could contribute to characterising occupational heat exposure and assessing heat tolerance in future clinical and research settings.