Insulin target tissues and cells.

Rodent adipose tissue and cells represent the targets exhibiting the most prominent insulin sensitivity (i.e., lowest EC/IC50) and responsiveness (i.e., highest fold stimulation/inhibition above basal) of the relevant insulin signaling cascades (e.g., insulin receptor activation) and metabolic end effector systems (e.g., lipolysis) in comparison to liver (e.g., gluconeogenesis) and muscle cells (e.g., glucose transport). This might be based in part on technical advantages of the adipose tissue/adipocyte preparation in comparison to that of muscle/myocytes. But more likely, it reflects the exquisite physiological role of the adipose tissue in the regulation and coordination of glucose and lipid metabolism, i.e., insulin stimulation of lipid synthesis (lipogenesis) and insulin inhibition of lipolysis. On the basis of their relatively easy technical preparation, functional adipose tissue fragments (epididymal fat pads) and primary adipocytes (isolated epididymal adipocytes) from rats as well as adipocyte cell lines derived from mice (3T3-L1, F442A) are the first choice for the development of robust and reliable cell-/tissue-based assay systems for insulin-like activity