Modulation of muscle metaboreceptor activation upon sweating and cutaneous 1 vascular responses to rising core temperature in humans 2 3 4

28 The present study investigated the role of muscle metaboreceptor activation on human 29 thermoregulation by measuring core temperature thresholds and slopes for sweating and 30 cutaneous vascular responses during passive heating associated with central and peripheral 31 mechanisms. Six male and eight female subjects inserted their lower legs into hot water (43°C) 32 while wearing a water perfusion suit on the upper body (34°C). One minute after immersion, an 33 isometric handgrip exercise 40% of maximum voluntary contraction was conducted for 1.5 34 min in both control and experimental conditions, while post exercise occlusion was performed 35 in the experimental condition only for 9 min. The post exercise forearm occlusion during 36 passive heating consistently stimulated muscle metaboreceptors, as implicated by significantly 37 elevated mean arterial blood pressure throughout the experimental period (P <0.05). 38 Stimulation of the forearm muscle metaboreceptors increased sweating and cutaneous vascular 39 responses during passive heating, and was associated with significant reductions in esophageal 40 temperature threshold of sweating and cutaneous vasodilation (Δthreshold, sweating: 0.33 ± 41 0.05 and 0.16 ± 0.04°C, cutaneous vascular conductance: 0.38 ± 0.08 and 0.16 ± 0.05°C for 42 control and experimental, respectively, P <0.05). The slopes of these responses were not 43 different between the conditions. These results suggest that muscle metaboreceptor activation 44 in the forearm accelerates sweating and cutaneous vasodilation during passive heating 45 associated with a reduction in core temperature thresholds, and may be related to central 46 mechanisms controlling heat loss responses. 47 48