Whole‐Body Heat Stress Alters the Exercise Pressor Reflex in Humans

Abstract

The effects of whole-body heat stress on sympathetic and cardiovascular responses to stimulation of muscle metaboreceptors and mechanoreceptors remain unclear. We examined the muscle sympathetic nerve activity (MSNA), renal blood velocity, blood pressure and heart rate in 14 young healthy subjects during fatiguing isometric handgrip exercise, post exercise circulatory occlusion (PECO), and passive muscle stretch during PECO. The protocol was performed under normothermic and whole-body heat stress (increase internal temperature ~0.6 °C via a heating suit) conditions. Heat stress increased the resting MSNA and heart rate. Heat stress did not alter the responses to the fatiguing exercise in mean blood pressure (MAP), heart rate and MSNA, but augmented the response in renal vascular resistance (Δ45± 6 to Δ60±8 %, P=0.04). During PECO, the increase in MAP was lower during heat stress condition than during normothemic condition (Δ20±2 vs. Δ12±1 mmHg, P<0.001), while the increases in MSNA were not different between the thermal conditions (Δ16.4±2.8 vs. Δ17.3±3.8 bursts/min, P=0.74). Under normothermic conditions, the passive stretch during PECO evoked significant increases in MAP and MSNA, while these intervention did not alter MSNA or MAP in the presence of heat stress. These data show that the pressor response to metaboreceptor stimulation is attenuated, and the MSNA response to mechanoreceptor stimulation is also attenuated in heat stress condition. These observations suggest that whole body heat stress alters exercise pressor reflex in humans.