Fat mobilization in vitro and in vivo in the genetically obese Zucker rat "fatty".

Fat mobilization was studied in vitro with epididymal fat pad tissue and also with cell suspensions from epididymal, retroperitoneal, and subcutaneous fat from the obese mutant "fatty" (fafa) and control rats of four different ages. Fat mobilization per cell in response to epinephrine was well above normal in young "fatties"; in older "fatties" the output per cell fell to near normal, but the much greater number of fat cells per rat indicated that the fat mobilizing capacity of the older "fatty" is well above normal. The "fatty" showed normal reactions to epinephrine in vivo: hyperglycemia, glycogenolysis, lipolysis with elevated free fatty acids and glycerol, and increased entry of free fatty acids into muscle and liver. Response was at least as great in "fatty" as in control animals. Metabolic indices-levels of circulating free fatty acids, glycerol, and in some cases glucose and lipid-determined at various ages in fed "fatties" and controls, and at intervals during prolonged fasting (70 days), were consistent with a picture of excessive adipose tissue lipolysis, excessive reesterification in the adipose tissue, fat mobilization in excess of need, and return of the excess to the adipose tissue via lipoproteins.