Fat Cell α2-Adrenoceptors: The Regulation of Fat CellFunction and Lipolysis*

Abstract

THE fat cell is the functional entity of white adipose tissue and brown adipose tissue. Briefly, white adipose tissue is the main long-term energy store, whereas brown adipose tissue is the major site of compensatory thermogenesis for maintaining body temperature while also contributing to energy dissipation. The fat cell is of key significance to the physiologist investigating the mechanisms controlling lipid storage, lipid mobilization, and utilization as well as other specific functions of the adipose tissue. It also enables the pharmacologist to study the cellular signaling of the autonomic nervous system and to understand the impact of the physiological amines on cell function. Over the 10 last years, there has been a rapid accumulation of experimental data demonstrating the existence of multiple adrenoceptor subtypes (e.g. nine different subtypes) (1, 2). The control of fat cell function by the catecholamines involves at least five different adrenoceptor subtypes; three β- (e.g. β1-, β2-, and β3-), one α2-, and one α1B-adrenoceptors. The adrenergic receptors that activate adenylylcyclase (three j3-adrenoceptors) coexist on the same fat cell as those that inhibit (one α2-adrenoceptor) the plasma membrane enzyme. All of them share the same physiological agonists (norepinephrine and epinephrine) delivered at highly variable concentrations depending on the level of activation of the sympathetic nervous system (SNS). The functional importance of these receptors varies according to the species, the sex and the age of the animals, and the nature of the fat deposits. It is also highly variable within an individual tissue deposit, adapting to the physiological and/or pathological situations encountered.