Characterization of norepinephrine-stimulated protein synthesis in rat brown adipocytes

Rat brown adipocytes were incubated for 24 h with or without norepinephrine (NE) in Dulbecco's modified Eagle's medium with albumin, calf serum, and antibiotics. Brown fat cells were viable as defined by unchanged cell morphology, ATP content, or basal and NE-stimulated respiration. However, a 24-h exposure to NE led to a decline in NE-stimulated respiration that was not due to loss of thermogenic capacity. Brown fat cells incubated with or without NE had similar protein, succinate dehydrogenase, and uncoupling protein (UCP) content. These results differ from those observed after food deprivation in rats where loss of mitochondrial proteins occurs within 24 h, suggesting that reduced exposure to NE is not the only factor responsible for brown fat atrophy. NE increased [35S]methionine incorporation into cellular proteins, mitochondrial proteins, and UCP. The effect of NE on cell protein synthesis was inhibited by propranolol but not by prazosin. It was also inhibited 95% by cycloheximide but only partially (50%) by actinomycin D in contrast to NE stimulation of UCP labeling, which required RNA transcription. Chloramphenicol-sensitive protein synthesis was stimulated by NE. These results indicate a trophic action of NE in brown adipocytes exerted both at the level of RNA transcription and translation.