Orthostatic challenge does not alter skin sympathetic nerve activity in heat-stressed humans

Perturbations that load or unload baroreceptors do not alter skin sympathetic nerve activity (SSNA) in normothermic individuals. However, in pronounced heat-stressed individuals, when a significant component of the SSNA signal is sudomotor and possibly vasodilator in origin, the effects of baroreceptor unloading via an orthostatic stress on SSNA remain unclear. The purpose of the present study was to test the hypothesis that low and moderate levels of orthostatic stress via lower body negative pressure (LBNP) alter SSNA in pronounced heat-stressed individuals. In both normothermic and heat-stressed conditions, progressive LBNP at -3, -6, -9, -12, -15, -18, -21 and -40 mm Hg were applied to 11 subjects for 2 min per stage. Whole-body heating increased sublingual temperature by 0.7±0.1°C, heart rate by 28±2.1 bpm, SSNA by 259±76 %, forearm skin blood flow by 631±142% and forearm sweat rate to 0.68±0.14 mg/cm ²/min (all p<0.005), but did not change mean arterial blood pressure (MAP) (p>0.05). LBNP did not change total SSNA in normothermic or heat-stressed conditions (both p>0.05), although skin blood flow and sweat rate decreased during moderate levels of LBNP while heat stressed. These data suggest that in pronounced heat-stressed individuals, when a significant component of the SSNA signal contains sudomotor and possibly cutaneous active vasodilator activities, low and moderate levels of baroreceptor unloading via LBNP do not alter total SSNA. This observation, coupled with reductions in skin blood flow and sweating during moderate levels of LBNP, suggests that integrated SSNA should not be used as an indicator of baroreflex modulation of the cutaneous vasculature or sweat rate in heat-stressed subjects.