Evidence that cutaneous vasomotor adaptation may be influenced by the nature of the heat‐acclimation method

Abstract

It is widely accepted that repeated heat exposure enhances thermoregulatory function, yet few investigators have observed improvements in cutaneous vasomotor responses during heat acclimation. This may be related to the nature of the heat‐acclimation method employed, with most studies using a constant‐load model involving several days of exercise at the same absolute work rate in the heat. Since the rise in body temperature in response to a given absolute work rate is lowered as adaptation progresses, that design elicits only partial acclimation, and also reduces thermoafferent flow across successive treatments. To reveal vasomotor adaptations during acclimation, it may be necessary to examine those responses under conditions eliciting equivalent (clamped) deep‐body temperatures. That design optimises adaptation by providing a constant stimulus for physiological change throughout the protocol, and it ensures deep‐body thermoafferent flow is matched before and after acclimation. Accordingly, this experiment was aimed at determining whether or not convective heat delivery to the skin might increase following this isothermal acclimation procedure.Cutaneous vasomotor responses were examined in twenty healthy, physically‐active, but not heat‐ adapted, males of similar age, adiposity and endurance fitness during heat stress tests performed before (day 1) and after (day 10) eight consecutive days of heat acclimation (days 2–9). All trials were performed in hot, dry conditions (40.0°C, 37% relative humidity). Heat acclimation consisted of intermittent rest and cycle exercise (90 min.day−1) to elevate and clamp deep‐body temperature (38.5°C). Each heat stress test involved seated rest (30 min) followed by steady‐state cycling (45 min), eliciting two matched and clamped deep‐body temperatures (mild hyperthermia: 37.5°C, moderate hyperthermia: 38.5°C). Deep‐body temperature was measured throughout, while cutaneous blood flow and vascular conductance (forearm plethysmography) were determined within the final 5 min of exercise.Deep‐body temperature averaged 38.5°C (SD 0.1) over the final 60 min of each exposure during heat acclimation (days 2–9). Similarly, deep‐body temperatures were matched before and after acclimation during mild (day 1: 37.5°C [SD 0.1], day 10: 37.4°C [SD 0.0]; P=0.08) and moderate hyperthermia (day 1: 38.5°C [SD 0.3], day 10: 38.4°C [SD 0.2]; P=0.88). Following acclimation, cutaneous blood flow and vascular conductance increased by 24% and 21% (respectively) relative to day 1 during mild hyperthermia, and by 24% and 25% during moderate hyperthermia (all P<0.05).These outcomes have confirmed, perhaps for the first time, that vasomotor function improves significantly following heat acclimation, provided those responses are examined under experimental conditions eliciting matched and clamped body temperatures. Nevertheless, the mechanism(s) facilitating those adaptations remains unclear.Support or Funding InformationSean R. Notley was supported by a University Postgraduate Award (University of Wollongong, Australia).