Arteriovenous anastomoses and the thermoregulatory shift between cutaneous vasoconstrictor and vasodilator reflexes.

The reflex changes in skin blood flow which occur in response to various non-thermal stimuli (e.g., deep inspiratory gasps, arousing or painful stimuli, emotional stress) are profoundly influenced by the thermoregulatory state. The aim of the present study was to evaluate the involvement of arteriovenous anastomoses in the thermoregulatory modulation of skin vasomotor reflexes elicited by painful intraneural electrical stimulation and emotional stress (forced arithmetics), respectively. Vasomotor responses were recorded with laser Doppler flowmeters (LDF) placed on glabrous skin containing arteriovenous anastomoses (3rd finger and thenar eminence) and hairy skin which lack them (dorsal side of the first metacarpal bone). In some experiments, a laser Doppler flowmeter emitting laser light of two different wavelengths (infrared and green light) into the same skin site was used to record skin perfusion at different depths of glabrous skin on the thenar eminence. 40 subjects were investigated, both in the cold state (finger skin temperatures below 25 degrees C) and after subsequent warming (finger skin temperatures above 30 degrees C). Thermoregulatory modulation of electrical stimulation- or stress-induced vasomotor reflexes occurred both in glabrous and hairy skin, but hairy skin differed from glabrous skin by showing no significant vasoconstrictions. Relative perfusion changes were most marked in laser Doppler flowmeter recordings using the deeper penetrating infrared light. The results suggest that arteriovenous anastomoses are major contributors to the vasoconstrictor component of vasomotor reflexes in glabrous skin of warm subjects. The reflex increase in perfusion, on the other hand, which occurs in both glabrous and hairy skin of cold subjects may be mediated by resistance vessels.