Fatty acid metabolism in adipose tissue of aging mice after direct tracer injection into fat pads

Abstract

We have examined previously reported age-related defects in triglyceride synthesis from [1- 14C]palmitate in adipose tissue of mice. Three techniques were used: in vitro, using adipocytes isolated from epididymal fat pads of young and old mice; and in vivo, using two new methods to measure free fatty acid (FFA) esterification by adipose tissue (direct injection of labeled palmitate-albumin complexes in large or small volumes into the extracellular spaces of the epididymal or inguinal fat pads of young and old mice). When the entire fat pad was filled with tracer we no longer observed heterogeneous labeling of adipocytes in epididymal fat pads that occurred in an earlier study in which an in vivo-in vitro method has been used. Free fatty acids were converted to triacylglycerol faster by adipocytes of large cells from older animal than by those of small cells from young mice; when the cell sizes of young and old mice were approximately equal, then the rates of FFA esterification were the same in young and old adipocytes. When FFA was injected as a small bolus the fractional rates of FFA disappearance and of FFA incorporation into triacylglycerol in the different fat pads, observed during a 60-min period, were the same (about 5 min or less) regardless of the region of the fat pad studied (distal or proximal epididymal fat pad), the type of fat pad (epididymal or inguinal), or the age of the mice (12–92 weeks). Other potential applications of the direct injection technique for studying FFA metabolism and structure-function in adipose tissue in vivo are discussed. Our findings, coupled with the earlier study in which labeled FFA was added to the outside of fat pads, indicate that, in adipose tissue of old mice, there exist barriers comprising mesothelial cells, collagenous structures, and/or the outer layer of adipocytes in fat pads, that interfere in the transport of FFA to the interior adipocytes when FFA is added outside the fat pad. This age-related defect may be circumvented by injecting tracer directly into the interstitial fluid compartment.