The mechanisms underlying the muscle metaboreflex modulation of sweating and cutaneous blood flow in passively heated humans

Abstract

Metaboreceptors can modulate cutaneous blood flow and sweating during heat stress but the mechanisms remain unknown. Fourteen participants (31 ± 13 years) performed 1‐min bout of isometric handgrip (IHG) exercise at 60% of their maximal voluntary contraction followed by a 3‐min occlusion (OCC), each separated by 10 min, initially under low (LHS, to activate sweating without changes in core temperature) and high (HHS, whole‐body heating to a core temperature increase of 1.0°C) heat stress conditions. Cutaneous vascular conductance (CVC) and sweat rate were measured continuously at four forearm skin sites perfused with 1) lactated Ringer's solution (Control), 2) 10 mmol L‐NAME [inhibits nitric oxide synthase (NOS)], 3) 10 mmol Ketorolac [inhibits cyclooxygenase (COX)], or 4) 4 mmol theophylline (THEO; inhibits adenosine receptors). Relative to pre‐IHG levels with Control, NOS inhibition attenuated the metaboreceptor‐mediated increase in sweating under LHS and HHS (P ≤ 0.05), albeit the attenuation was greater under LHS (P ≤ 0.05). In addition, a reduction from baseline was observed with THEO under LHS during OCC (P ≤ 0.05), but not HHS (both P > 0.05). In contrast, CVC was lower than Control with L‐NAME during OCC in HHS (P ≤ 0.05), but not LHS (P > 0.05). We show that metaboreceptor activation modulates CVC via the stimulation of NOS and adenosine receptors, whereas NOS, but not COX or adenosine receptors, contributes to sweating at all levels of heating.

Do nitric oxide synthase and cyclooxygenase contribute to sweating response during passive heating in endurance-trained athletes?

“…Consistent with the recent study under resting hyperthermic conditions in non‐endurance trained young males (i.e., exercised for no more than 2–3 times per week for 30 min duration) (Haqani et al. ), we observed that COX inhibition does not alter sweat rate during passive heating in untrained individuals and extend upon this observation by demonstrating a similar response in endurance‐trained individuals (Fig. ).…”

“…Inconsistent with the present observation, it was reported that NOS inhibition does not modulate sweating response at a similar elevations in core temperature (i.e., 1.0°C) during passive heating (Haqani et al. ). While the underlying reason(s) for this discrepancy is unknown, it is assumed that individual differences in the contribution of NOS to sweating may in part be a factor.…”

“…,b, , ; Haqani et al. ). For example, COX inhibition has been shown to attenuate sweat rate during exercise in the heat (Fujii et al.…”

“…) or hyperthermic (1.0°C elevation of rectal temperature) (Haqani et al. ) conditions. Consistent with the recent study under resting hyperthermic conditions in non‐endurance trained young males (i.e., exercised for no more than 2–3 times per week for 30 min duration) (Haqani et al.…”

“…, ; Haqani et al. ). While the underlying reasons for this disparity remain unclear, our recent study showed that NOS‐ (Fujii et al.…”