Glyceride–glycerol release and the interconversion of glucose and glycerol in normal and fasted rats

Abstract

Specific activities of plasma glycerol, plasma glucose, and respiratory CO₂ were determined during continuous intra-arterial infusion of trace amounts of glycerol uniformly labelled with ¹⁴C into normal and 24-h fasted white rats. The same measurements were made in other rats during primed continuous infusion of glucose uniformly labelled with ¹⁴C. The data were analyzed by means of a previously described two-pool model in material and isotopic steady state. The two major precursors of plasma glycerol were considered to be glycerol from triglyceride hydrolysis and glycerol derived from glycolytic intermediates formed during the metabolism of glucose or glucogenic amino acids. The model allowed quantitative assessment of each source. In normal adult rats the rate of total net formation of plasma glycerol was 0.27 mg carbon per min per rat of which about 30% was ascribed to glyceride–glycerol release and 70% to net formation of glycerol from glycolytic intermediates. In fasted rats, the rate of total net formation of glycerol was 0.23 mg glycerol carbon per min of which 83% was ascribed to glyceride–glycerol release and 17% to formation from glycolytic intermediates. Only 4% of the rate of total net formation of glucose could be attributed to net formation of glucose from glyceride–glycerol in normal rats whereas this increased to 18% in the fasted rat. Contrary to previous reports these results indicate that glyceride–glycerol is not the major gluconeogenic substance in the fasted rat.