The role of adrenergic mechanisms in thermoregulatory control of blood flow through capillaries and arteriovenous anastomoses in the sheep hind limb.

The possible role of adrenergic mechanisms in thermoregulatory changes in the partition of femoral blood flow between nutrient (capillary) and non-nutrient (arteriovenous anastomoses, AVA) circuits in the hind limb of conscious sheep has been investigated employing radioactive microsphere and electromagnetic blood flow measurement techniques. Constriction of AVAs, normally induced by spinal cooling, could be inhibited by phentolamine, whereas dilatation of AVAs, normally induced by spinal heating, could be inhibited by noradrenaline or methoxamine. AVA constriction could be induced by noradrenaline or methoxamine, or dilatation by phentolamine. Isoprenaline had a small dilator and propranolol a small constrictor effect on AVAs. It is concluded that adrenergic pathways involving predominantly alpha-receptors play a role in thermoregulatory changes in skin blood flow (through AVAs) elicited by manipulation of CNS temperature; under these conditions, beta-receptors do not play any role, although manipulation of their activity will influence AVAs under non-thermoregulatory conditions. Capillary blood flows in skin, bone and fat were sensitive, at different ambient temperatures and to varying degrees, to some alpha- and beta-adrenergic agents.