The effect of plasma osmolality and baroreceptor loading status on postexercise heat loss responses

Abstract

We examined the separate and combined effects of plasma osmolality and baroreceptor loading status on postexercise heat loss responses. Nine young males completed a 45-min treadmill exercise protocol at 58 ± 2% V̇o2 peak, followed by a 60-min recovery. On separate days, participants received 0.9% NaCl (ISO), 3.0% NaCl (HYP), or no infusion (natural recovery) throughout exercise. In two additional sessions (no infusion), lower-body negative (LBNP) or positive (LBPP) pressure was applied throughout the final 45 min of recovery. Local sweat rate (LSR; ventilated capsule: chest, forearm, upper back, forehead) and skin blood flow (SkBF; laser-Doppler flowmetry: forearm, upper back) were continuously measured. During HYP, upper back LSR was attenuated from end-exercise to 10 min of recovery by ∼0.35 ± 0.10 mg·min(-1)·cm(-2) and during the last 20 min of recovery by ∼0.13 ± 0.03 mg·min(-1)·cm(-2), while chest LSR was lower by 0.18 ± 0.06 mg·min(-1)·cm(-2) at 50 min of recovery compared with natural recovery (all P < 0.05). Forearm and forehead LSRs were not affected by plasma hyperosmolality during HYP (all P > 0.28), which suggests regional differences in the osmotic modulation of postexercise LSR. Furthermore, LBPP application attenuated LSR by ∼0.07-0.28 mg·min(-1)·cm(-2) during the last 30 min of recovery at all sites except the forehead compared with natural recovery (all P < 0.05). Relative to natural recovery, forearm and upper back SkBF were elevated during LBPP, ISO, and HYP by ∼6-10% by the end of recovery (all P < 0.05). We conclude that 1) hyperosmolality attenuates postexercise sweating heterogeneously among skin regions, and 2) baroreceptor loading modulates postexercise SkBF independently of changes in plasma osmolality without regional differences.

Effects of isomaltulose ingestion on postexercise hydration state and heat loss responses in young men

“…() showed that an elevated P osm following fructose ingestion impairs cutaneous vasodilatation and sweating as reflected by elevated body core temperature thresholds during a passive heat stress. In addition, a hyperosmolality (approximately 10 mosmol kg −1 ) induced by 3.0% NaCl infusion has been shown to attenuate postexercise sweating in the heat (Paull et al., ). However, in contrast to these previous observations, we showed no effect of 6.5% isomaltulose fluid ingestion on heat loss responses.…”

“…It is unclear why we did not observe a reduction in heat loss responses. It may be that the 30 min fluid ingestion period in the early stages of recovery induced only a slow transient change in P osm compared with the more rapid increases in P osm that occurred with direct infusion of NaCl (Paull et al., ). It is plausible therefore that the changes in P osm observed in the present study may be insufficient to modulate postexercise heat loss responses.…”

“…As noted above, changes in baroreceptor loading status have been show to modulate heat loss responses. For example, reversing the postexercise reduction in MAP, and therefore baroreceptor loading status, during the postexercise recovery period through the application of positive pressure to the lower limbs has been shown to enhance heat loss responses of skin blood flow (Journeay et al., ; McGinn et al., ; Paull et al., ) and sweating (Jackson & Kenny, ; Journeay et al., ) during recovery in the heat. However, as is the case with an elevation in P osm , we observed no effect on postexercise heat loss responses under any condition, including the ingestion of 2.0% and 6.5% isomaltulose solution, which caused an increase in MAP with a marginally greater increase measured in the 6.5% isomaltulose condition.…”

“…50% of their age‐predicted HR reserve) cycling to induce a further elevation in T co and enhance cutaneous perfusion and sweating. At the cessation of exercise, the participant rested in a semi‐recumbent position for 2 h during which time the temperature of the water circulating in the suit was adjusted (∼45°C) to maintain mean skin temperature ( T sk ) at ∼35°C (see results) as per previous studies assessing the regulation of postexercise heat loss responses (McGinn et al., ; Paull et al., ). We clamped skin temperature to minimize the confounding influences of changes in body temperature between conditions so that we could evaluate the potential influence of non‐thermal factors (e.g.…”

“…It has been demonstrated that following cessation of exercise in the heat, there is a sustained suppression of heat loss responses which is associated with a prolonged elevation in T co lasting up to 2 h (Thoden, Kenny, Reardon, Jetfe, & Livingstone, ; Wilkins, Minson, & Halliwill, ), which has been attributed to non‐thermal factors (Kenny & Journeay, ). Non‐thermal factors such as those associated with the activation of baroreceptors (blood pressure regulation) and osmoreceptors (elevated osmolality) have been shown to impinge upon the activation of skin blood flow and sweating during the postexercise recovery period thereby affecting T co recovery (Journeay, Reardon, Jean‐Gilles, Martin, & Kenny, ; McGinn, Paull, Meade, Fujii, & Kenny, ; Paull et al., ). Given that activity of these sensory receptors is directly stimulated by transient changes in PV and P osm , it is possible that ingesting beverages containing isomaltulose as compared to water only may modulate heat loss responses differently.…”