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

Influence of lipolysis on the mobilization of 2,4,5,2'4',5'-hexachlorobiphenyl from adipocytes in vitro

Epididymal adipocytes, isolated from rats pretreated with [14C]-2,4,5,2',4',5'-hexachlorobiphenyl (6-CB), were utilized to examine the relationship between the mobilization of lipid and 6-CB and to determine whether 6-CB was differentially associated with subcellular organelles over time as has been demonstrated for newly synthesized lipid. Lipolysis, induced by the presence of 8 X 10(-7) M isoproterenol (ISO) for 50 min, depleted approximately 1% of total cellular triacylglycerols (TG) regardless of time from treatment with 6-CB. The percentage of cellular 6-CB released from adipocytes to incubation buffer infranatants was not correlated with the magnitude of lipolysis produced over the 50-min incubation period; nor was the percentage of 6-CB released to the buffers correlated with the length of the incubation period, regardless of the presence of ISO. Although adipocytes responded similarly to lipolytic stimuli independent of time (days) since 6-CB treatment, significant decreases were found in the percentage of 6-CB released from adipocytes over time. The in vitro labeling of this newly synthesized TG in fat cells with [U-14C]glucose or [1-14C]palmitate demonstrated that TG was differentially distributed among adipocyte organelles. Newly synthesized TG was also the first to be mobilized under lipolytic stimulus. 6-CB was not released in a similar fashion, since radioactivity associated with the chemical levels of [14C]-6-CB and glucose-derived 14C in buffers were not correlated over time. 6-CB was found to redistribute to all available lipid pools during the subcellular fractionation procedure and thus did not resemble TG. However, it is possible that 6-CB may exist in equilibrium among organelle fractions and that it becomes sequestered within the nonsoluble lipid compartment with time, thus decreasing its appearance in the soluble buffer infranatants over the experimental time course.

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

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 triacyglycerol 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.

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.