Adaptations in skeletal muscle exercise metabolism to a sustained session of heavy intermittent exercise
H. GREEN, R. TUPLING, B. ROY, D. O’TOOLE, M. BURNETT, AND S. GRANT Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
Green, H., R. Tupling, B. Roy, D. O’Toole, M. Burnett, and S. Grant. Adaptationsin skeletal muscle exercise metabolism to a sustained session of heavy intermittent exercise. Am. J. Physiol. Endocrinol. Metab. 278: E118–E126, 2000.—The purpose of this study was to investigate the hypothesis that a single, extended session of heavy exercise would be effective in inducing adaptations in energy metabolism during exercise in the absence of increases in oxidative potential. Tenhealthy males [maximal aerobic power ˙ (VO2 peak) 43.4 2.2 (SE) ml · kg 1 · min 1] participated in a 16-h training session involving cycling for 6 min each hour at 90% of maximal oxygen consumption. Measurements of metabolic changes were made on tissue extracted from the vastus lateralis during a two-stage standardized submaximal cycle protocol before (Pre) and 36–48 h after (Post) the trainingsession. At Pre, creatine phosphate (PCr) declined (P 0.05) by 32% from 0 to 3 min and then remained stable ˙ until 20 min of exercise at 60% VO2 peak before declining (P 0.05) by a further 35% during 20 min of exercise at 75% ˙ VO2 peak. Muscle lactate (mmol/kg dry wt) progressively increased (P 0.05) from 4.59 0.64 at 0 min to 17.8 2.7 and 30.9 5.3 at 3 and 40 min, respectively, whereas muscleglycogen (mmol glucosyl units/kg dry wt) declined (P 0.05) from a rest value of 360 24 to 276 31 and 178 36 at similar time points. During exercise after the training session, PCr and glycogen were not as depressed (P 0.05), and increases in muscle lactate were blunted (P 0.05). All of these changes occurred in the absence of increases in oxidative potential as measured by the maximal activities ofcitrate synthase and malate dehydrogenase. These ﬁndings are consistent with other studies, namely, that muscle metabolic adaptations to regular exercise are an early adaptive event that occurs before increases in oxidative potential. oxidative potential; enzymes; metabolites; metabolic control
IT IS BECOMING INCREASINGLY APPARENT that a variety of adaptations both in the skeletal muscle cell andin the vascular system occur soon after the onset of regular, contractile activity. In the muscle cell, as an example, mitochondrial oxidative potential is rapidly upregulated, resulting in increases in the capacity for both oxidative phosphorylation and -oxidation (6, 42). These adaptations are also accompanied by an increased expression of proteins involved in glucose transport and disposalwithin the cell, namely hexokinase (35, 44), the enzyme involved in glucose phosphorylation,
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and GLUT-4, the glucose transporterprotein (32, 34, 35). Rapid upregulation is also observed in other proteins such as the sarcolemmal Na -K -ATPase, the cation pump which is involved in Na and K transport (15, 22), and in the monocarboxylate transporter 1 (MCT1), which appears to function as a lactate transporter (3). One of the most conspicuous vascular adaptations is an increase in muscle capillarization, which also may occur withindays after the onset of training (41). Not unexpectedly, profound alterations also occur in muscle energy metabolism during moderate exercise early in training. These adaptations include a pronounced reduction in the rate of glycogen depletion and a lower lactate accumulation, two events which appear at least partly dependent on reductions in glycogenolysis and glycolysis (5, 6, 14, 34) and, in...