Adrenoceptor and local modulator control of cutaneous blood flow in thermal stress

Blood flow to the skin is controlled by body temperatures in two ways: core and mean skin temperature combine in the central nervous system to form a reflex mechanism that controls the frequency of activity in sympathetic nerves to the cutaneous blood vessels; and local mechanisms independent of reflex effects control contractile response to the sympathetic transmitter norepinephrine (NE) at different temperatures. Cutaneous vessels differ in responsiveness to NE across temperatures: in limbs and tails, the superficial vessels constrict more strongly to NE when cooled, while the deep vessels show weaker responses to NE when cooled. This allows the limb to dissipate heat when warm and to conserve heat when cool. The mechanism for this difference in thermal response of deep and superficial vessels is not completely known, but may relate to differences in the adrenoceptors on which NE acts, and/or to the actions of locally produced substances that modulate the responses to NE in different ways at different temperatures. This paper discusses the alpha1- and alpha2-adrenoceptors involved in contraction of deep and superficial cutaneous vessels and also describes the roles of the local modulator nitric oxide, which interacts with adrenoceptors to affect cutaneous blood flow.