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

The effects of exercise and dietary restriction on the activities of hexokinase and phosphofructokinase in superficial, intra-abdominal and intermuscular adipose tissue of guinea-pigs

1. The activities per gram of adipose tissue of hexokinase and phosphofructokinase were higher in the centre two intermuscular adipose depots of guinea-pigs than in any of the seven superficial and intra-abdominal depots studied. 2. The capacity for glucose utilization was reduced following about 3 months of regular exercise in all adipose depots studied except in the popliteal depot. The proportion of lipid decreased, and that of protein increased, in this intermuscular depot following regular, strenuous exercise, but it did not enlarge disproportionately. 3. We suggest that certain intermuscular adipose depots may act as a local energy source for adjacent muscles. The anatomical relations of adipose tissue to muscle may be an integral part of its physiological function.

Contribution of intermuscular fat to lipogenesis from dietary glucose carbon in mice

We assessed the contribution of various tissues to the synthesis of fat from glucose carbon in mice during rapid lipogenic activation induced by a glucose test meal. Nibbling and gorging mice were maintained on a 58% glucose, fat-free diet. The mice were fasted 22 h and refed 5-10 muCi [U-14C]glucose (120 mg/20 g body weight) either by gastric intubation or as a test meal (58% glucose diet). The muscular carcass in both nibblers an gorgers contained more than 75% of the total radioactivity in the fatty acids derived from glucose; liver and epididymal fat pad accounted for only a small percentage. Mort than half the carcass activity was in the "muscular" tissue as neutral lipid acids. We could isolate a discrete fat body in the popliteal region of the leg muscle ("muscle fat", "intermuscular fat"). The popliteal fat converted glucose carbon to fatty acids an order of magnitude faster, per unit weight, than the epididymal fat pad or skeletal muscle. The fatty acid moiety of the triacylglycerols had the major portion of the label in the popliteal fat 2 and 6 h after ingestion of the glucose test meals. The diacylglycerol pool was active at 2 h and its activity faded at 6 h implicating its intermediary role in lipid metabolism similar to published findings in epididymal fat pad. These results indicate that fat cells associated with the muscular carcass may play a major role in the de novo synthesis of fat from dietary carbohydrate in mice.