The impact of castration on physiological responses to exertional heat stroke in mice

Abstract

Introduction: The capability of male mice to exercise in hot environments without succumbing to exertional heat stroke (EHS) is markedly blunted compared to females. Epidemiological evidence in humans and other mammals also suggests some degree of greater vulnerability to heat stroke in males compared to females. The origins of these differences are unknown, but testosterone has previously been shown to induce faster elevations in core temperature during acute, passive heat exposure. In this study, we tested the hypothesis that loss of testosterone and related sex hormones through castration would improve the performance and heat tolerance of male mice during EHS exposure.

Methods: Twenty-four male mice were randomly divided into 3 groups, untreated EHS mice (SHAM-EHS), castrated EHS mice (CAS+EHS) and naïve exercise controls (NAIVE). Exercise performance and physiological responses in the heat were monitored during EHS and early recovery. Two weeks later, blood and tissues were collected and analyzed for biomarkers of cardiac damage and testosterone.

Results: Core temperature in CAS+EHS rose faster to 39.5°C in the early stages of the EHS trial (P<0.0001). However, both EHS groups ran similar distances, exhibited similar peak core temperatures and achieved similar exercise times in the heat, prior to symptom limitation (unconsciousness). CAS+EHS mice had ~10.5% lower body mass at the time of EHS, but this provided no apparent advantage in performance. There was no evidence of myocardial damage in any group, and testosterone levels were undetectable in CAS+EHS after gonadectomy.

Conclusions: The results of these experiments exclude the hypothesis that reduced performance of male mice during EHS trials is due to the effects of male sex hormones or intact gonads. However, the results are consistent with a role of male sex hormones or intact gonads in suppressing the early and rapid rise in core temperature during the early stages of exercise in the heat.

Fig 1

A) Timeline of changes in body mass throughout experiment. CAS+EHS exhibited lower weights after baseline compared to SHAM-EHS mice. Patterned box at week 4 represents the time within the protocol the mice were exposed to EHS. P values are Bonferroni corrected; Effect sizes: 0.83, 2.74, 3.19, respectively across time. ANOVA followed by orthogonal t-test comparisons between groups at each time point. B) Loss of body mass due to castration from baseline to pre EHS protocol. T- test, Effect size: 1.35. C). Loss of mass during the EHS protocol, not significant, t-test. Effect size: 1.1. All data, all data are means ± SD; n = 8 per group.

Fig 2

A) Core temperature-time profile during the EHS runs, comparing all CAS+EHS (black) and SHAM-EHS mice (red). B) Comparison of the time from the start of EHS protocol to Tc = 39.5°C, (Mann Whitney U test), Effect size: 1.71. C) Comparison of exercise time from 39.5°C to 41°C, between CAS+EHS or SHAM-EHS mice (t-test). Effect size: 0.96 D) Comparison of time from 41°C to Tc,max. Effect size: 0.34 Means ± SD; n = 8 per group. All data are means ± SD.

Fig 3

A) The total time taken for mice to reach the symptom-limited endpoint, T_c,max, of the EHS trial. Effect size: 0.77. B) The total distance run by the end of the EHS trial for CAS+EHS and SHAM-EHS mice. Effect size: 0.76. C) The cumulative ascending thermal area of EHS during the EHS trial (defined by the integration of Tc >39.5°C and time. Effect size: 0.89. All Data: means ± SD; (t-test comparisons), n = 8 per group.

Fig 4

A) Body surface area was calculated by using Meeh’s equation [17]. Effect size: 2.54. B). Body surface area to mass ratio. Effect size: 2.1 Means ± SD, n = 8, (t-test comparisons).

Fig 5

A) Comparison of Cardiac Troponin T. No differences were detected in plasma samples between NAIVE (wild type-no EHS), SHAM-EHS, and CAS+EHS mice; effect size: 0.007 (Kruskal-Wallis test). B) Comparison of Total Bile Acids, (Kruskal Wallis); effect size: -0.07. C) Comparison of testosterone levels (Mann Whitney U test); effect size:1.84. Samples were measured in plasma using the MAGPIX Luminex platform. Means±SD, n = 8 for Sham and CAS+EHS; n = 7 for SHAM-EHS.