The postprandial rates of glycogen and lipid synthesis of lean and obese female Zucker rats

OBJECTIVE: To determine the relative rates of glycogenesis and lipogenesis following administration of a test meal in lean and obese Zucker rats. PROTOCOL: Nine-week-old lean and obese Zucker rats were fasted overnight th en tube-fed a test meal of balanced composition amounting to 16 kJ (lean ra ts and one group of obese rats) or 24 kJ (one group of obese rats) and cont aining 200 mg 1-C-13 glucose. Immediately after the meal the rats were inje cted intraperitoneally with 5 mCi of (H2O)-H-3 and killed 1 h later. METHODS: Glycogenesis was calculated from the incorporation of H-3 into liv er glycogen divided by the specific activity of plasma water. Lipogenesis w as calculated similarly from the incorporation of H-3 into saponifiable lip ids in liver and perirenal adipose tissue. The proportion of glycogen synth esized by the indirect pathway via pyruvate was determined from the ratio o f H-3 labelling at positions C6 and C2 in the glycogen glucose residues. Gl ycogen synthesis from glucose was determined from the ratio of C-13 enrichm ent in liver glycogen to that in plasma glucose. RESULTS: The rate of synthesis of glycogen was considerably lower in the li vers of obese rats than those of lean controls, with the larger meal causin g a small but significant increase in glycogenesis. The proportion of glyco gen synthesized via pyruvate showed a non-significant increase in the obese rats, while the amount of glycogen synthesized from glucose was significan tly decreased. Hepatic lipogenic rates were about five times higher in both groups of obese rats than the lean controls, In adipose tissue, lipogenesi s per g tissue was slightly reduced in the obese rats, although there was c learly an increase in adipose tissue lipogenic activity per whole animal. T he larger meal caused a greater rise in plasma glucose and insulin concentr ations but did not affect lipogenic rates, although it did cause a greater suppression of lipolysis, as indicated by a lower plasma glycerol concentra tion. CONCLUSION: Ingested carbohydrate is partitioned predominantly into hepatic fatty acid synthesis in obese Zucker rats. Hepatic glycogen synthesis is s uppressed and comes mainly from precursors other than glucose. The suppress ion of hepatic glycogen synthesis may contribute to the increased energetic efficiency of obese Zucker rats.