Physiological approach to maturation of brown adipocytes in primary cell culture.

Molecular and metabolic aspects of differentiation of brown adipocytes of the Djungarian hamster (Phodopus sungorus) were studied in primary culture. Expression of uncoupling protein and lipoprotein lipase were investigated by Western and Northern blotting and indirect immuno-fluorescence microscopy. The activity of 5'-deiodinase type II was determined by a radioactive enzyme assay. Activity of cytochrome-c-oxidase and cell respiration rates were measured with a Clark electrode. We evaluated functional differences of developmental stages by measuring the reaction to beta-adrenergic stimulation throughout the differentiation process. The results show that differentiation of hamster brown adipocytes is an at least two-step development with physiologically discriminable cell types. Generation of triiodothyronine (T3) from thyroxine by activation of the 5'deiodinase occurs in immature brown adipocytes and is mediated primarily by beta1- rather than beta3-adrenergic receptors. The thermogenic capacity is subsequently increased in mature brown adipocytes. beta-Adrenergic receptor stimulation increases UCP expression of mature adipocytes but is not able to recruit new brown adipocytes.