Adrenergic Receptors and Fat Cells: Differential Recruitment by Physiological Amines and Homologous Regulation

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Abstract
The control of fat cell lipolysis by the catecholamines involves at least four different adrenoceptor subtypes; three β (β1‐, β2‐, and β3‐ARs) and one α2‐adrenoceptor(α2‐AR). The physiological importance of the β‐ and α2A‐ARs varies according to the species, the sex, the age, the anatomical location of fat deposits and the degree of obesity in humans and animals. The physiological amines operate through differential recruitment of these sites on the basis of their relative affinities. This point has been assessed by in vitro studies and has partly been confirmed in in vivo experiments using selected a/β‐AR antagonists and in situ microdialysis. The affinity of the β3‐AR for catecholamines is less than that of the classical β1‐ and β2‐ARs in the various species investigated. Conversely, it is the α2‐AR which exhibit the highest affinity for the physiological amines in all fat cells. The relative order of affinity of the various fat cell ARs for the physiological amines defined in binding studies and in vitro ass ays is α2 > β1 > β2 > β3 for norepinephrine and α2 >β2 > β1> β3 for epinephrine. When considering differential β‐AR recruitment by catecholamines, it is the β1‐AR which is always activated at the lowest norepinephrine levels, whatever the species, while the activation of the β3‐AR requires higher norepinephrine levels. In addition to the differential recruitment, differential regulation by hormones could also occur for each fat cell AR subtype. The α2‐and β3‐ARs are less prone to desensitization and down‐regulation by comparison with the β1‐ and β2‐AR.