Abstract
Orthostatic tests performed in hot environment reduced orthostatic tolerance, while the same test in cold environment improved it. To clarify the mechanisms underlying the differences, we assessed the autonomic response to ambient temperature changes by analyzing the variations of the muscle and skin sympathetic nerve activities (MSNA, SSNA), as well as the heart rate variability during tilt-tests performed at 27°C, 15°C and 35°C in healthy human subjects. MSNA and SSNA were simultaneously recorded from bilateral tibial nerves by a double recording technique of microneurography. Skin blood flow, used to calculate the vascular conductance, was recorded by laser Doppler flowmetry in a region of the toe innervated by the impaled skin nerve fascicle. While standing, the highest increase in heart rate was observed at 35°C (+ 41.6%) associated with the lowest LF/HF ratio: 3.6. Upright MSNA and SSNA burst rates increased with the temperature. However compared to the supine position, MSNA response to the tilt decreased with the temperature while SSNA response increased at 15°C and decreased at 27 and 35°C. Toe vascular conductance followed the same pattern of variation as the SSNA reflecting dissociation between the sympathetic and vascular response to sympathetic stimulation. To conclude this study not only showed that MSNA response to orthostasis is negatively correlated to the temperature but also that MSNA and SSNA are dissociated in normo and hyperthermia while coordinated in hypothermia. [Jpn J Physiol 55 Suppl:S239 (2005)]
