Control of Lipogenesis in Adipose Tissue of Fasted and Fed Meal-eating Rats

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Abstract
The capacity of adipose tissue from meal-eating rats (animals having access to food for a single daily 2-hour period) to incorporate acetate-1-14C into fatty acids in vitro was markedly stimulated by ingestion of the daily meal. Adipose tissue of fed meal-eating rats (killed after the daily meal) was found to contain more glycogen and less free fatty acids than tissue from animals killed before the daily meal (fasted). Tissue from fed animals also had a higher in vitro capacity to convert acetate-14C to fatty acids, and the glycogen content of tissue from fed animals decreased by 75% during a 2-hour incubation period. These data suggest that glucose derived from glycogen was responsible for the greater lipogenic capacity of adipose tissue from fed meal-eating rats. Adipose tissue from fasted animals converted glucose-14C to glyceride-glycerol at over 3 times the rate of tissue from fed meal-eaters during the first 30 minutes of incubation in vitro, whereas the conversion of glucose to fatty acids was at less than half the rate observed in tissue of fed animals. In comparison with this initial 30-minute period of incubation, the rate of glucose incorporation into glyceride-glycerol by tissue of fasted meal-eating rats decreased by about 40%, and the incorporation into fatty acids increased by approximately 270% during a subsequent 60-minute incubation period. These data suggest that lipogenesis is inhibited in adipose tissue of fasted meal-eating rats by the elevated level of free fatty acids (or fatty acyl CoA) and that glucose (supplied by ingestion of the daily meal or by addition to the incubation medium in vitro) stimulates fatty acid esterification by conversion to α-glycerophosphate, thereby lowering the level of free fatty acids and removing their inhibitory effects. In accord with this mechanism it was shown that pyruvate can stimulate in vitro fatty acid synthesis in adipose tissue of fasted meal-eating rats and that this stimulatory effect is related to the conversion of pyruvate to α-glycerophosphate. Data are also presented demonstrating that glucose is a more effective precursor of α-glycerophosphate in rat adipose tissue than is pyruvate and that glucose is apparently utilized preferentially when both substrates are available. The possible physiological significance of these observations in the meal-fed rat is discussed.

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