The use of in vivo—in vitro labeling techniques to study phospholipid fatty acid turnover and fatty acid esterification into triglycerides in adipose tissue of aging mice

Abstract

We are interested in membrane phospholipid and triglyceride synthesis and turnover in aging cells. As a preliminary, short-term feasibility study we have used an established in vivo-in vitro technique to estimate the initial rates of [1- 14C] palmitate (complexed to albumin) esterification to triglycerides and phospholipids in adipocytes and non-adipocytes in the epididymal fat pads of aging mice (8–92 weeks). We have expressed our data in terms of unit cell, unit triglyceride mass and unit (membrane) phospholipid mass. Fat pad and adipocyte size, cell surface area, and adipocyte volume changes were measured and found to follow the same relations as reported in the literature, with some exceptions in very old mice (retired breeders). Rates of fatty acid esterification to triglycerides were about 100 times faster than those to phospholipids in adipocytes. Aging caused a marked fall in the rates of triglyceride fatty acid formation from added palmitate; thus, the rate of fatty acid esterification to triglycerides fell from 0.75 to 0.13 nequiv. fatty acid per min per fat pad (youngest most active group, cf. oldest group). Esterification of fatty acids into phospholipids in adipocytes of the oldest mice was significantly lower than in those of the young and middle-aged groups. Contamination of adipocytes by non-adipocytes was observed in fat pads from old, but not from young, mice. The non-adipocytes accounted for about half of the phospholipid fatty acid esterification. The rate of phospholipid esterification was so slow in adipocytes (all ages) and so relatively fast in non-adipocytes that further studies of phospholipid fatty acid turnover in adipocytes using this system are not considered feasible, especially as a means for studying removal rates of autoxidized fatty acids from membrane phospholipids in vivo during aging.