A REDUCED LACTATE MASS EXPLAINS MUCH OF THE GLYCOGEN SPARING ASSOCIATED WITH TRAINING

Endurance training is associated with glycogen (Gly) sparing, generall y attributed to less carbohydrate (CHO) oxidation. However, untrained individuals commit a greater fraction of CHO to lactate (La), accounti ng for a portion of the Gly ''spared.'' We examined the effects of tra ining (running 1 h/day at 30 m/min up an 8 degrees grade) on whole bod y CHO distribution and oxidation. Female Long Evans rats (n = 27) were assigned to control (Untr) and trained (Tr) groups. Two days before t he experiment, animals were chronically catheterized. On tile day of t he experiment, animals ran for 20 min at a speed of 28 m/min and were killed with an overdose of pentobarbital sodium injection while runnin g. Whole carcasses were then promptly freeze-clamped with a liquid N-2 -cooled press. Whole body carcass powder was assayed for La, Gly, and glucose. Resting whole body values were not different between groups ( La = 0.78 +/- 0.06 vs. 0.83 +/- 0.07, Gly = 4.46 +/- 0.62 vs. 3.77 +/- 0.35, glucose = 0.19 +/- 0.07 vs. 0.23 +/- 0.09 mmol/body for Tr and Untr rats, respectively). However, postexercise La was higher in Untr vs. Tr group (2.01 = 0.28 vs. 1.13 +/- 0.09 mmol/body), and Gly was lo wer in the Untr vs. Tr rats (1.58 +/- 0.25 vs. 3.42 +/- 0.43 mmol/body ). Similarly, Untr animals displayed higher epinephrine levels than Tr at the end of the exercise bout (4.9 +/- 1.0 vs. 1.7 +/- 0.4 ng/ml). Differences between groups in La and glucose masses (postexercise minu s rest data) accounted for 60% of the Gly differences. Gly spared from oxidation and replaced by increased fat oxidation only accounted for 40% of the differences in Gly levels between Tr and Untr animals. We c onclude that untrained mammals commit a significant portion of their C HO pool to La, which accounts far almost one-half of tile apparent Gly spared during moderate-intensity exercise in the trained state.