Adrenergic receptor function in fat cells.

All classical adrenoceptor subtypes are functionally expressed in fat cells. However, only beta 1 adrenoceptors appear to be present in all types of fat cells. There is a substantial adrenoceptor reserve in fat cells; approximately 50% of beta and alpha 2 adrenoceptors are spare receptors. Beta adrenoceptors are subject to intensive regulation. They are regulated by insulin, estrogens, and androgens as well as by thyroid hormones and are altered by nutritional factors, diabetes, autonomic neuropathy, and beta-blocking treatment. Alpha receptors are less sensitive to changes except during infancy, when there are marked developmental alterations in the function of alpha 2 adrenoceptors, and during fasting, when there is a decrease in receptor expression. In addition, beta adrenoceptors but not alpha 2 adrenoceptors are sensitive to homologous desensitization after exposure to agonists. Site variations in the expression and function of beta and alpha 2 adrenoceptors, which in part are situated at the level of gene transcription, may be involved in the development of regional obesity.