CHARACTERISTICS OF β-ADRENERGIC RECEPTORS IN BROWN ADIPOCYTES OF TEMPERATURE-ACCLIMATED RATS

Abstract

Characteristics of .beta.-adrenergic receptors in brown adipocytes of temperature-acclimated rats were investigated by the use of the adrenergic .beta.-antagonist (-)-[3H]dihydroalprenolol (DHA). DHA binding to brown adipocytes was rapid, reversible and saturable, displaying stereospecificity to .beta.-adrenergic agonists and antagonists. According to the potency order of .beta.-adrenergic (-)-agonists in a competition study, the adrenergic receptors of brown adipocytes proved to be the .beta.1-subtype. Both acute cold exposure and cold acclimation significantly decreased the number of DHA binding sites. Acute heat exposure did not affect the number of DHA binding sites, while heat acclimation significantly decreased it. In these experiments, the equilibrium dissociation constant of DHA was not influenced. .beta.-Adrenergic receptors of brown adipocytes respond to thermal stimuli with changes in number of binding sites, but not in affinity. Changes in .beta.-adrenergic receptors of cold-acclimated and cold-exposed brown adipocytes could not explain enhanced thermogenic response of brown fat to norepinephrine in cold acclimation, but such changes would appear to be adaptive to increased secretion of norepinephrine through down-regulation. A decreased number of .beta.-adrenergic receptors, probably due to hypothyroidism due to heat acclimation, could explain the depressed thermogenic response of brown fat to norepinephrine in heat acclimation.