Alanine metabolism in skeletal muscle in tissue culture

Lactate and Energy Metabolism During Exercise in Patients With Blocked Glycogenolysis (McArdle Disease)

CONTEXT

Patients with blocked muscle glycogen breakdown (McArdle disease) have severely reduced exercise capacity compared to healthy individuals and are not assumed to produce lactate during exercise.

OBJECTIVES

The objectives were: 1) to quantify systemic and muscle lactate kinetics and oxidation rates and muscle energy utilization during exercise in patients with McArdle disease; and 2) to elucidate the role of lactate formation in muscle energy production.

DESIGN AND SETTING

This was a single trial in a hospital.

PARTICIPANTS

Participants were four patients with McArdle disease and seven healthy subjects.

INTERVENTION

Patients and healthy controls were studied at rest, which was followed by 40 minutes of cycle-ergometer exercise at 60% of the patients' maximal oxygen uptake (∼35 W).

MAIN OUTCOME MEASURES

Main outcome measures were systemic and leg skeletal muscle lactate, alanine, fatty acid, and glucose kinetics.

RESULTS

McArdle patients had a marked decrease in plasma lactate concentration at the onset of exercise, and the concentration remained suppressed during exercise. A substantial leg net lactate uptake and subsequent oxidation occurred over the entire exercise period in patients, in contrast to a net lactate release or no exchange in the healthy controls. Despite a net lactate uptake by the active leg, a simultaneous unidirectional lactate release was observed in McArdle patients at rates that were similar to the healthy controls.

CONCLUSION

Lactate is an important energy source for contracting skeletal muscle in patients with myophosphorylase deficiency. Although McArdle patients had leg net lactate consumption, a simultaneous release of lactate was observed at rates similar to that found in healthy individuals exercising at the same very low workload, suggesting that lactate formation is mandatory for muscle energy generation during exercise.

Effect of endurance training on muscle TCA cycle metabolism during exercise in humans

We tested the theory that links the capacity to perform prolonged exercise with the size of the muscle tricarboxylic acid (TCA) cycle intermediate (TCAI) pool. We hypothesized that endurance training would attenuate the exercise-induced increase in TCAI concentration ([TCAI]); however, the lower [TCAI] would not compromise cycle endurance capacity. Eight men (22 +/- 1 yr) cycled at approximately 80% of initial peak oxygen uptake before and after 7 wk of training (1 h/day, 5 days/wk). Biopsies (vastus lateralis) were obtained during both trials at rest, after 5 min, and at the point of exhaustion during the pretraining trial (42 +/- 6 min). A biopsy was also obtained at the end of exercise during the posttraining trial (91 +/- 6 min). In addition to improved performance, training increased (P < 0.05) peak oxygen uptake and citrate synthase maximal activity. The sum of four measured TCAI was similar between trials at rest but lower after 5 min of exercise posttraining [2.7 +/- 0.2 vs. 4.3 +/- 0.2 mmol/kg dry wt (P < 0.05)]. There was a clear dissociation between [TCAI] and endurance capacity because the [TCAI] at the point of exhaustion during the pretraining trial was not different between trials (posttraining: 2.9 +/- 0.2 vs. pretraining: 3.5 +/- 0.2 mmol/kg dry wt), and yet cycle endurance time more than doubled in the posttraining trial. Training also attenuated the exercise-induced decrease in glutamate concentration (posttraining: 4.5 +/- 0.7 vs. pretraining: 7.7 +/- 0.6 mmol/kg dry wt) and increase in alanine concentration (posttraining: 3.3 +/- 0.2 vs. pretraining: 5.6 +/- 0.3 mmol/kg dry wt; P < 0.05), which is consistent with reduced carbon flux through alanine aminotransferase. We conclude that, after aerobic training, cycle endurance capacity is not limited by a decrease in muscle [TCAI].

Exercise with low muscle glycogen augments TCA cycle anaplerosis but impairs oxidative energy provision in humans

We tested the hypotheses that: (i) exercise with low muscle glycogen would reduce pyruvate flux through the alanine aminotransferase (AAT) reaction and attenuate the increase in tricarboxylic acid (TCA) cycle intermediates, and (ii) attenuation of tricarboxylic acid cycle intermediate (TCAI) pool expansion would limit TCA cycle flux, thereby accelerating phosphocreatine (PCr) degradation. Eight men cycled for 10 min at 70 % of their (VO(2,max) on two occasions: (i) following their normal diet (CON) and (ii) after cycling to exhaustion and consuming a low carbohydrate diet for approximately 2 days (LG). Biopsies (m. vastus lateralis) confirmed that [glycogen] was lower in LG vs. CON at rest (257 +/- 18 vs. 611 +/- 54 mmol (kg dry mass)(-1); P 0.05); however, net glycogenolysis was not different after 1 or 10 min of exercise. PCr degradation from rest to 1 min was approximately 26 % higher in LG vs. CON (38 +/- 4 vs. 28 +/- 4 mmol (kg dry mass)(-1); P< or =0.05). The sum of five measured TCAIs (approximately 90 % of total pool) was not different between trials at rest and after 1 min, but was higher after 10 min in LG vs. CON (5.51 +/- 0.43 vs. 4.45 +/- 0.49 mmol (kg dry mass)(-1); P 0.05). Pyruvate dehydrogenase complex (PDC) activity was lower during exercise in LG vs. CON (2.2 +/- 0.2 vs. 1.4 +/- 0.2 mmol min(-1) (kg wet weight)(-1) after 10 min; P< or =0.05), and acetylcarnitine was approximately threefold less, implying increased pyruvate availability for flux through AAT. Resting muscle [glutamate] was higher in LG vs. CON (16.1 +/- 0.8 vs. 11.8 +/- 0.4 mmol (kg dry mass)(-1); P< or =0.05) and the net decrease in [glutamate] during exercise was approximately 30 % greater in LG vs. CON. These findings suggest that: (i) contrary to our hypotheses, LG increased anaplerosis by decreasing PDC flux and/or increasing the conversion of glutamate carbon to TCAIs, and (ii) accelerating the rate of muscle TCAI expansion did not affect oxidative energy provision during the initial phase of contraction, since changes in [TCAI] were not temporally related to PCr degradation.

Role of cell type in net lactate removal by skeletal muscle

Net lactate uptake and subsequent pathways for removal were studied in three rabbit skeletal muscle preparations of distinct fiber type composition, i.e., glycolytic (99.1 +/- 0.2% type IIb fibers), oxidative (97.5 +/- 0.6% type I fibers), and mixed (type I, IIa, and IIb fibers). Single-pass perfusions were carried out for 3 h in the presence of glucose, lactate, and [U-14C]lactate. Lactate levels, initially set at either 1 mM (n = 4/prep) or 2 mM (n = 4/prep), were elevated twice during the perfusion at 60 and 120 min. Net lactate uptake (mumol.100 g-1.min-1) was first observed in the oxidative preparation, 1.4 +/- 0.2, at an arterial lactate concentration of approximately 2.5 mM, whereas net lactate uptake in the glycolytic, 0.7 +/- 0.2, and mixed preparations, 7.0 +/- 0.5, was first observed at 4 mM. Net lactate balance, [14C]lactate removal, and 14CO2 release demonstrated strong linear correlations (r = 0.94-0.98) with arterial lactate concentration. To quantify the fate of [14C]lactate, preparations were perfused at a single elevated lactate concentration (approximately 8 mM) for 2 h. Oxidation was the primary means of disposal in the oxidative and mixed preparations, whereas glyconeogenesis dominated removal in the glycolytic preparation. The arterial lactate concentration at which a given muscle switches from net production to net removal, the rate of removal, and subsequent pathway(s) for disposal are a function of that muscle's fiber type composition.

Branched chain amino acid oxidation in cultured rat skeletal muscle cells. Selective inhibition by clofibric acid

Leucine metabolism in skeletal muscle is linked to protein turnover. Since clofibrate is known both to cause myopathy and to decrease muscle protein content, the present investigations were designed to examine the effects of acute clofibrate treatment on leucine oxidation. Rat skeletal muscle cells in tissue culture were used in these studies because cultivated skeletal muscle cells, like muscle in vivo, have been shown to actively utilize branched chain amino acids and to produce alanine. The conversion of [1-(14)C]leucine to (14)CO(2) or to the [1-(14)C]keto-acid of leucine (alpha-keto-isocaproate) was linear for at least 2 h of incubation; the production of (14)CO(2) from [1-(14)C]leucine was saturable with a K(m) = 6.3 mM and a maximum oxidation rate (V(max)) = 31 nmol/mg protein per 120 min. Clofibric acid selectively inhibited the oxidation of [1-(14)C]leucine (K(i) = 0.85 mM) and [U-(14)C]isoleucine, but had no effect on the oxidation of [U-(14)C]glutamate, -alanine, -lactate, or -palmitate. The inhibition of [1-(14)C]leucine oxidation by clofibrate was also observed in the rat quarter-diaphragm preparation. Clofibrate primarily inhibited the production of (14)CO(2) and had relatively little effect on the production of [1-(14)C]keto-acid of leucine. A physiological concentration-3.0 g/100 ml-of albumin, which actively binds clofibric acid, inhibited but did not abolish the effects of a 2-mM concentration of clofibric acid on leucine oxidation. Clofibrate treatment stimulated the net consumption of pyruvate, and inhibited the net production of alanine. The drug also increased the cytosolic NADH/NAD(+) ratio as reflected by an increase in the lactate/pyruvate ratio, in association with a decrease in cell aspartate levels. The changes in pyruvate metabolism and cell redox state induced by the drug were delayed compared with the nearly immediate inhibition of leucine oxidation. These studies suggest that clofibric acid, in concentrations that approximate high therapeutic levels of the drug, selectively inhibits branched chain amino acid oxidation, possibly at the level of the branched chain keto-acid dehydrogenase.