CUTANEOUS VASODILATATION RESPONSES SYNCHRONIZE WITH SWEAT EXPULSIONS

To examine whether cutaneous active vasodilatation is mediated by sudo motor nerve fibres we recorded cutaneous blood flow and sweat rates co ntinuously with laser-Doppler flowmetry and capacitance hygrometry, re spectively, from the dorsal and plantar aspects of the foot in 11 male subjects at varying ambient temperatures (T-a) between 22 and 40 degr ees C (relative humidity 40%). In a warmer environment (T-a 29-40 degr ees C.), predominant responses of the blood flow curve from the sole o f the foot were transient depressions (negative blood flow responses, NBR), whereas those from the dorsal foot were transient increases (pos itive blood flow responses, PBR). The PBR on the dorsal foot occurred spontaneously or in response to mental or sensory stimuli, and when PB R did not fuse with each other the rate of PBR was linearly related to tympanic temperature. When dorsal foot sweating was continuous, PBR o n the dorsal foot almost entirely synchronized with sweat expulsion. W hen dorsal foot sweating was intermittent PBR sometimes occurred on th e dorsal foot without corresponding sweat expulsions, but these PBR sh owed a complete correspondence with subthreshold sweat expulsion seen on a methacholine-treated area. The amplitude and the duration of PBR showed a significant linear relationship with the amplitude and the du ration of the corresponding sweat expulsion. In a thermoneutral or coo ler environment (T-a 22-29 degrees C), PBR occurred on the sole of the foot when mental or sensory stimuli elicited sweating in that area. T hus, PBR occurred when and where sweating appeared. Atropine failed to abolish PBR on the dorsal foot. Blockade of the peroneal nerve elimin ated both PBR and NBR on the dorsal foot. The results indicate that an active vasodilatation mechanism is present on the sole of the foot as well as on the dorsal foot, and thus suggest that active vasodilatati on is closely related to sudomotor nerve activation.