Prior Heat or Hypoxic Acclimation Does Not Attenuate the Cytokine Response to Hypoxic Exercise

Abstract

BACKGROUND: Heat acclimation activates the cellular heat shock response (HSR), inhibiting NF-κB and reducing heat stress-mediated cytokine production and limiting stress-induced inflammation. Hypoxia-mediated intestinal ischemia activates the NF-κB pathway and stimulates pro-inflammatory responses linked to acute mountain sickness and pulmonary and cerebral oedema. The upregulation of the HSR via heat acclimation may therefore reduce inflammatory responses following subsequent hypoxic exercise. PURPOSE: To determine whether prior heat acclimation attenuates the cytokine response following hypoxic exercise. METHODS: Plasma TNF-α, IL-6, IL-10 and IL-1ra were determined at rest and following a 60 minute cycling normoxic stress test (NST) and hypoxic stress test (HST1; FiO2 = 0.14, 50% VO2peak) in 21 men (age 22 ± 5 years; stature 1.76 ± 0.07 m; mass 71.8 ± 7.9 kg; VO2 peak 51 ± 7 mL·kg−1·min−1). Participants formed 3 groups and completed 10 x 60 minute acclimation sessions (50% VO2peak) in control (n = 7; 18 °C, 35% RH), hypoxic (n = 7; FiO2 = 0.14, 18 °C, 35% RH), or hot (n = 7; 40 °C, 25% RH) conditions. A second HST (HST2) was completed 48 hours after the final acclimation session. Cytokine data are presented as mean change in ng·mL−1 with 95% confidence intervals and comparisons made using mixed ANOVA. RESULTS: Following the NST plasma IL-6 (+0.6, 95% CI 0.3 - 0.9 ng·mL-1), IL-10 (+1.1, 95% CI -2.8 - 5.0 ng·mL-1) and IL-1ra (-16.6, 95% CI -60.5 - 27.2 ng·mL-1) exhibited minimal change (p > 0.05). TNF-α was unaltered throughout the study. IL-6 (+3.9, 95% CI 2.8 - 4.9 ng·mL-1), IL-10 (+26.2, 95% CI 15.0 - 37.3 ng·mL-1) and IL-1ra (+1506, 95% CI 746 - 2266 ng·mL-1) were elevated following HST1 (p < 0.01), with no main effect for acclimation group (p > 0.05). A similar trend was observed after acclimation (HST2), with IL-6 (+3.1, 95% CI 2.5 - 3.7 ng·mL-1), IL-10 (+23.2, 95% CI 14.0 - 32.5 ng·mL-1) and IL-1ra (+1237, 95% CI 721 - 1753 ng·mL-1) increased (p < 0.01), and no main effect for trial or acclimation group observed (p > 0.05). CONCLUSIONS: Neither prior heat nor hypoxic acclimation attenuated the systemic cytokine response following acute exercise in hypoxia. Future work investigating the effectiveness of different acclimation modalities on improving inflammatory outcomes to hypoxic stress is recommended.