A schematic showing the activation of a brown adipose cell. (1) Agonists (norepinephrine and CL316,243) activate the β3-AR. (2) Production of cyclic adenosine monophosphate (cAMP) and protein kinase A (PKA) triggers lipase and hydrolysis of triglycerides. (3) The UCP-1 in the mitochondria increases the breakdown of fatty acids in the cell. (4) Increased metabolic activity in the mitochondria increases glucose and 18F-FDG uptake. (5) 18F-FDG-6-phosphate uptake in the cell increases and is measured by PET. (6) Outcome of this cascade of events is the formation of heat.