Effects of increased ambient temperature on skin sympathetic nerve activity and core temperature in humans

The strength of sympathetic vasoconstrictor nerve traffic to the skin has an important role in human thermoregulation since it controls heat loss from the skin by constricting or dilating cutaneous blood vessels. This study sought to clarify the time relationship between a reduction of the vasoconstrictor activity induced by elevating the ambient temperature (Ta), and subsequent change of core temperature (Tty). For this purpose, we recorded peroneal skin sympathetic nerve activity (SSNA), laser Doppler skin blood flow, skin and core (tympanic) temperatures in 11 subjects while increasing Ta from 15 to 30 degrees C during approximately 30 min. We observed a significant suppression of SSNA 7.7 min after Ta rise with marked interindividual variations. Tty displayed an increase with a peak after 8.2 min followed by a successive decrease, which became significant 14 min after the Ta rise. The rate of decrease of vasoconstrictor SSNA correlated both with the rate of decrease of Tty (P<0.01) and the magnitude of the Tty decrease (P<0.0005). A cross-correlogram between SSNA and Tty showed a peak at 7 min (r=0.52). We conclude that a Ta rise-induced reduction of skin vasoconstrictor nerve traffic leads to a core temperature decrease after 7-8 min.