Heterogeneous labeling of adipocytes during in vivo-in vitro incubation of epididymal fat pads of aging mice with [1-14C] palmitate.

We have hypothesized that the in vivo-in vitro technique of Stein and Stein for studying free fatty acid incorporation into adipose tissue triglycerides and phospholipids may introduce artifacts due to diffusion barriers such as collagenous membranes, especially in fat pads of old animals. By using this technique in young and old mice and peeling the external cells, either physically or by collagenase treatment, we were able to show that the outer adipocytes are preferentially labeled. However, this pattern of heterogeneous labeling occurred in fat pads of both young (10-14 weeks) and old (80 weeks) mice. Fat pads are known to develop thicker, collagenous outer membranes during aging. Therefore, it seems likely to us that the marked decrease in free fatty acid esterification in fat pads of old mice, using the in vivo-in vitro method that we have described previously and confirmed here, could have been due to greater diffusion barriers in the tissues of the older mice.