Abstract
We used a new tracer technique, direct tracer injection of [1-14C]palmitate-serum albumin into extracellular fluid (ECF) of epididymal fat pads, to study relative transport rates of ECF-free fatty acids (FFA) to cell-FFA and subsequent esterification to diglyceride fatty acid (DGFA) and triglyceride fatty acid (TGFA) in adipose tissue versus movement of ECF- and cell-FFA into the circulation of mice fed ad libitum or fasted 48 hr. Radioactivity was measured in the following fractions at varying times (for 1 hr): ECF-FFA, cell-FFA, cell-DGFA, cell-TGFA, plasma-FFA (total lipids), and breath CO2. Pool sizes of ECF-FFA, cell-FFA, cell-TGFA, and plasma-FFA were determined. Analysis by multicompartmental methods (SAAM) indicates that the ECF-FFA compartment of epididymal fat pads is in a relatively rapid exchange with a cellular-FFA compartment, but neither is in direct, nor appreciably rapid, communication with circulating FFA. FFA is rapidly esterified in adipocytes of fed mice, but esterification is significantly inhibited in mice fasted for 48 hr. In both dietary states, essentially all labeled FFA appearing in the circulation was derived from ECF-FFA that were first transferred to the cell, esterified to TGFA, then hydrolyzed to FFA before being transported to the circulation.
Highlights
We used a new tracer technique, direct tracer injection of [1-”Clpalmitate-serum albumin into extracellular fluid (ECF) of epididymal fat pads, to study relative transport rates of ECF-free fatty acids (FFA) to cell-FFA and subsequent esterification to diglyceride fatty acid (DGFA) and triglyceride fatty acid (TGFA) in adipose tissue versus movement of ECFand cell-FFA into the circulation of mice fed ad libitum or fasted 48 hr
I n addition to demonstrating numerous significant changes in various rates of FFA transport into and out of epididymal adipocytes induced by fasting, our analysis demonstrates rather conclusively that FFA molecules in the extracellular compartment into which we injected our Abbreviations: FA, fatty acid(s); FFA, free fatty acid(s); DGFA, diglyceride fatty acid(s); TGFA, triglyceride fatty acid(s); TLFA, total lipid fatty acid(s); ECF, extracellular fluid; EFP, epididymal fat pad(s)
We have recently described a new approach for studying the esterification of labeled FFA following direct injection of the tracer into the extracellular fluid of selected fat pads [6]
Summary
We used a new tracer technique, direct tracer injection of [1-”Clpalmitate-serum albumin into extracellular fluid (ECF) of epididymal fat pads, to study relative transport rates of ECF-free fatty acids (FFA) to cell-FFA and subsequent esterification to diglyceride fatty acid (DGFA) and triglyceride fatty acid (TGFA) in adipose tissue versus movement of ECFand cell-FFA into the circulation of mice fed ad libitum or fasted 48 hr. From cell-FFA [“cell-associated-FFA” [5]] We carried out this separation, coupled with analyses of radioactivity and mass, in various lipid fractions in the cellular and extracellular compartments, including simultaneous measurement of radioactivity in plasma-FFA or -TLFA and in breath COP.it seemed theoretically possible, even in a short-term study, to obtain a great deal of information about the rates of transport and possible pathways of FFA movement into and out of adipocytes in the heterogeneous system that exists in a fat pad. We followed the appearance of tracer in plasma a n d in breath l4CO, a n d used these data, together with the measured pool sizes of the total adipose tissue-TGFA compartment, to develop a model of FFA transport and metabolism in which rates of transport in each condition (fed and 48 hr-fasted) were estimated by multicompartmental analysis
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