Transport and metabolism of extracellular free fatty acids in adipose tissue of fed and fasted mice.

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.