Effects of fasting and glucocorticoids on hepatic gluconeogenesis assessed using two independent methods in vivo

The purpose of this study was to compare the assessment of gluconeogenesis (GNG) in the overnight- and prolonged-fasted states and during chronic hypercortisolemia using the arteriovenous difference and [14C]phosphoenolpyruvate-liver biopsy techniques as well as a combination of the two. Two weeks before a study, catheters and flow probes were implanted in the hepatic and portal veins and femoral artery of dogs. Animals were studied after an 18-h fast (n = 8), a 42- or 66-h fast (n = 7), and an 18-h fast plus a continuous infusion of cortisol (3.0 microg. kg(-1). min(-1)) for 72 h (n = 7). Each experiment consisted of an 80-min tracer ([3-(3)H]glucose and [U-(14)C]alanine) and dye equilibration period (-80 to 0 min) and a 45-min sampling period. In the cortisol-treated group, plasma cortisol increased fivefold. In the overnight-fasted group, total GNG flux rate (GNG(flux)), conversion of glucose 6-phosphate to glucose (GNG(G-6-P-->Glc)), glucose cycling, and maximal GNG flux rate (GNG(max)) were 0.95 +/- 0.14, 0.65 +/- 0.06, 0.62 +/- 0.06, and 0.70 +/- 0.09 mg. kg(-1). min(-1), respectively. In the prolonged-fasted group, they were 1.50 +/- 0.18, 1.18 +/- 0.13, 0.40 +/- 0.07, and 1.28 +/- 0.10 mg. kg(-1). min(-1), whereas in the cortisol-treated group they were 1.64 +/- 0.33, 0.99 +/- 0.29, 1.32 +/- 0.24, and 0.91 +/- 0.13 mg. kg(-1). min(-1). These results demonstrate that GNG(G-6-P-->Glc) and GNG(max) were almost identical. However, these rates were 15-38% lower than GNG(flux) generated by a combination of the two methods. This difference was most apparent in the steroid-treated group, where the combination of the two methods (GNG(flux)) detected a significant increase in gluconeogenic flux.