Net changes in intermuscular fat before and during rapid lipogenic activation in mice.

We have attempted to measure net changes in lipid content in a discrete "intermuscular" fat pad during rapid lipogenic activation that occurs after a previously fasted mouse nibbles a glucose-rich test meal for several minutes. The popliteal fat pad was chosen for the study since it has been shown to be about an order of magnitude more active than the epididymal fat pad in the synthesis of fatty acids from glucose carbon in fasted-refed mice. We found a highly reproducible net loss in the popliteal fat pad's weight and lipid content during fasting. Net deposition of lipid occurred when 24-h fasted mice were allowed to eat a fat-free, 58% glucose diet for several minutes. In two out of three experiments lipid repletion was complete after one brief period of nibbling. Significant decreases in the net amounts of each majority fatty acid, 16:0, 16: 1, 18:1 and 18:2, were found to occur in the popliteal fat pad during a 24-h fast. After nibbling their test meal for several minutes, previously fasted mice restored their major essential fatty acid, linoleic acid, to the original fed level within 2 h, even though total lipid repletion was incomplete. Highly significant net increases in each major non-essential fatty acid were also found after brief ingestion of the test meal; however, in one experiment (incomplete repletion) only about half of the depleted fatty acids was restored. When two successive glucose test meals were eaten (2-h interval), popliteal fat converted glucose carbon to fatty acids more than twice as fast after the second feeding as after the first. However, no significant additional increment either in tissue weight or in total lipid fatty acids was found after the second test meal. Based on these findings, the possible significance of intermuscular fat in the utilization and deposition of fat is discussed.