The sweat glands’ maximum ion reabsorption rates following heat acclimation in healthy older adults

Abstract

We examined whether the eccrine sweat glands' ion reabsorption rates improved following heat acclimation (HA) in older individuals. Ten healthy older adults (>65 years) completed a controlled hyperthermic (+0.9 • C rectal temperature, T re) HA protocol for nine non-consecutive days. Participants completed a passive heat stress test (lower leg 42 • C water submersion) pre-HA and post-HA to assess physiological regulation of sweat gland ion reabsorption at the chest, forearm and thigh. The maximum ion reabsorption rate was defined as the inflection point in the slope of the relation between galvanic skin conductance and sweat rate (SR). We explored the responses again after a 7-day decay. During passive heating, the T b thresholds for sweat onset on the chest and forearm were lowered after HA (P < 0.05). However, sweat sensitivity (i.e. the slope), the SR at a given T re and gross sweat loss did not improve after HA (P > 0.05). Any changes observed were lost during the decay. Pilocarpine-induced sudomotor responses to iontophoresis did not change after HA (P ≥ 0.801). Maximum ion reabsorption rate was only enhanced at the chest (P = 0.001) despite unaltered aldosterone concentration after HA. The data suggest that this adaptation is lost after 7 days' decay. The HA protocol employed in the present study induced partial adaptive sudomotor responses. Eccrine sweat gland ion reabsorption rates improved heterogeneously across the skin sites. It is likely that aldosterone secretion did not alter the chest sweat ion reabsorption rates observed in the older adults.

Exercise heat acclimation and post-exercise hot water immersion improve resting and exercise responses to heat stress in the elderly

“…It would also appear that peripheral adaptations to the sweat gland are limited in older individuals undertaking exercise HA (targeting a + 0.9°C increase in core temperature) for nine days. 15 That study reported sudomotor responses to iontophoresis during passive heating as unchanged after HA, and whilst maximal ion absorption improved, this decayed within seven days. 15 The success of the longer duration protocols i.e.…”

“…15 That study reported sudomotor responses to iontophoresis during passive heating as unchanged after HA, and whilst maximal ion absorption improved, this decayed within seven days. 15 The success of the longer duration protocols i.e. six to nine sessions [12][13][14][15] , over shorter protocols i.e.…”

“…6 Despite HA being a prevalent strategy implemented in athletes and occupational/military personnel to mitigate heat stress, [7][8][9] and evidence that the elderly can adapt to seasonal temperature change, 10 elderly HA research remains limited. [11][12][13][14][15] An experiment examining the benefits of three, 1 h daily fixed intensity exercise-heat HA sessions in untrained elderly females deemed this approach insufficient (in dose) to induce adaptation. 11 A greater HA dose, specifically nine 90-120 min fixed intensity exercise-heat sessions, demonstrated that trained old participants made comparable adaptations to younger participants during passive heat stress.…”

“…15 The success of the longer duration protocols i.e. six to nine sessions [12][13][14][15] , over shorter protocols i.e. three sessions 11 , points to the need to investigate the minimum dose required for adaptation.…”

Experimental Physiology special issue: Extreme environmental physiology

“…Five papers deal with this area and investigate the impact of passive acclimation on kidney function in the heat (Ravanelli, Barry, Schlader, & Gagnon, 2021a); the impact of 5 weeks of heat training on haemoglobin mass in elite cyclists (Rønnestad et al., 2021); the relative roles of heat acclimation and increases in fitness in exercise training‐induced thermoregulatory adaptations (Ravanelli, Gagnon, Imbeault, & Jay, 2021b); changes in aerobic capacity, heat shock protein 72, markers of mitochondrial biogenesis and oxidative phosphorylation in skeletal muscle with heat acclimation and low intensity exercise (Mang et al. 2021); and the induction and decay of sweat gland maximum ion reabsorption rates following heat acclimation in healthy older adults (Gerrett, Amano, Inoue, & Kondo, 2021).…”