Gluconeogenesis from glycerol at rest and during exercise in normal, diabetic, and methylprednisolone-treated dogs

Glucose turnover, glycerol turnover, and the rate of incorporation of glycerol carbon into glucose were measured with the tracer technique (primed constant rate infusion) using 2-3H-glucose and 14C-glycerol, at rest and during exercise (treadmill run) in normal (N), alloxan-diabetic (D), and methylprednisolone treated diabetic (MPD) dogs. At rest only 2%-3% of the hepatic glucose output arose from glycerol. Exercise increased gluconeogenesis about ninefold in N dogs and about fourfold in D and MPD animals, yet less than 9% of the elevated glucose turnover was derived from glycerol. There was a direct linear correlation between the rates of glycerol turnover and gluconeogenesis from glycerol at rest and during exercise in all three groups. The slope constants were however significantly different: 0.45, 0.51, and 0.67 for N, D, and MPD dogs, respectively. In vivo the major factor controlling the rate of gluconeogenesis from glycerol seems to be the glycerol supply on which the specific effects of insulin deficiency and glucocorticoid treatment are superimposed. They appear to be of minor importance. A comparison of the glucose turnover measured by 2-3H-glucose with that measured by 6-3H-glucose showed that the activity of the glucose in equilibrium glucose-6-P cycle was threefold higher in D dogs and elevated by 15-fold in MPD animals.