Measurement of gluconeogenesis in exercising men by mass isotopomer distribution analysis

Abstract

We evaluated the hypothesis that coordinated adjustments in absolute rates of gluconeogenesis (GNG(ab)) and hepatic glycogenolysis (Gly) would maintain euglycemia and match glucose production (GP) to peripheral utilization during rest and exercise. Specifically, we evaluated the extent to which gradations in exercise power output would affect the contribution of GNG(ab) to GP. For these purposes, we employed mass isotopomer distribution analysis (MIDA) and isotope-dilution techniques on eight postabsorptive (PA) endurance-trained men during 90 min of leg cycle ergometry at 45 and 65% peak O(2) consumption (VO(2 peak); moderate and hard intensities, respectively) and the preceding rest period. GP was constant in resting subjects, whereas the fraction from GNG (f(GNG)) increased over time during rest (22.3 +/- 0.9% at 11.25 h PA vs. 25.6 +/- 0.9% at 12.0 h PA, P < 0.05). In the transition from rest to exercise, GP increased in an intensity-dependent manner (rest, 2.0 +/- 0.1; 45%, 4.0 +/- 0.4; 65%, 5.84 +/- 0.64 mg. kg(-1). min(-1), P < 0.05), although glucose rate of disappearance exceeded rate of appearance during the last 30 min of exercise at 65% VO(2 peak). Compared with rest, increases in GP were sustained by 92 and 135% increments in GNG(ab) during moderate- and hard-intensity exercises, respectively. Correspondingly, Gly (calculated as the difference between GP and MIDA-measured GNG(ab)) increased 100 and 203% over rest during the two exercise intensities. During moderate-intensity exercise, f(GNG) was the same as at rest; however, during the harder exercise f(GNG) decreased significantly to account for only 21% of GP. The highest sustained GNG(ab) observed in these trials on PA men was 1.24 +/- 0.3 mg. kg(-1). min(-1). We conclude that, after an overnight fast, 1) absolute GNG rates increased with intensity of effort despite a reduced f(GNG) at 65% VO(2 peak), 2) during exercise Gly is more responsible than GNG(ab) for maintaining GP, and 3) in 12-h fasted men, neither increased Gly or GNG(ab) nor was their combination able to maintain euglycemia during prolonged hard (65% VO(2 peak)) exercise.

Gluconeogenesis in humans with induced hyperlactatemia during low-intensity exercise

“…We observed a significant decrease of blood glucose concentration during exercise from 5.0 to 4.2 mM after 4 h. A decrease of similar extent has been reported by others, albeit in humans performing exercise at higherintensity levels, up to 65% V O 2 max (15,28,29,44,45). In those studies, subjects were studied 7.5 h after their last meal, and exercise commenced 12 h following completion of their evening meal.…”

A Role for the Transcriptional Coactivator PGC-1α in Muscle Refueling

“…First, we found that, as predicted by the overexpression studies, muscle GLUT4 levels are reduced in the PGC-1␣ Ϫ/Ϫ mice. Second, it is known that glycogenolysis serves as the chief source of glucose export from liver during exercise (59,60). Whereas hepatic gluconeogenic capacity is reduced in PGC-1␣ Ϫ/Ϫ mice, glycogen stores and glycogenolysis rates are increased (58).…”

Adipose triglyceride lipase deletion from adipocytes, but not skeletal myocytes, impairs acute exercise performance in mice

“…Second, peak exercise does not require a significant contribution of fat metabolism for energy production. As exercise intensity increases, rates of both glycolysis and gluconeogenesis are adjusted to maintain adequate glucose availability (57). Thus, despite the loss of ATGL-mediated intramyocellular lipolysis, peak exercise could remain unaffected.…”