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

1 Department of Surgery, Vanderbilt University, and the Nashville VA Medical Center; 2 Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, Tennessee 37232; and 3 Division of Endocrinology, Department of Medicine, Albert Einstein College of Medicine, New York, New York 10461 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 [ 14 C]phospho enol pyruvate-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 µg · 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. fasting; cortisol