Mammalian (including human) adipose tissue is composed mainly of white and brown adipocytes. White adipocytes store lipids in unilocular droplets, whereas brown adipocytes store lipids in multilocular droplets – this aspect reflects the different functions of these two cell types.
White adipocytes store lipids and release fatty acids in the intervals between meals to provide other tissues with a source of energy other than glucose. Brown adipocytes oxidize most of the stored intracellular lipid and release the energy as heat. The oxidation of lipids in brown adipocytes can be very rapid because it is uncoupled from the synthesis of ATP by the mitochondrial uncoupling protein-1 (UCP-1).
Lipolysis in white and brown adipocytes is activated by sympathetic stimulation of beta-adrenoceptors. In rodents the beta-3-adrenoceptor plays the dominant role in these cells. This partly explains why beta-3-adrenoceptor agonists are highly effective in causing weight loss and improving insulin sensitivity in animal models of obesity and type-II diabetes. Brown adipose tissue (together with skeletal muscle) is an important site of thermogenisis, while white adipose tissue provides much of the fuel for thermogenisis.
