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Articles in PresS, published online ahead of print September 10, 2002
Am J Physiol Endocrinol Metab, 10.1152/ajpendo.00273.2002
Submitted on June 20, 2002
Accepted on September 5, 2002
1 The Copenhagen Muscle Research Centre, Copenhagen 0, Denmark
2 Department of Cardiology, Karolinska Institute, Stockholm, Sweden
3 Department of Physiology-Pharmacology, Karolinska Institute, Stockholm, Sweden
* To whom correspondence should be addressed. E-mail: gvhall{at}rh.dk.
To study the role of muscle mass and muscle activity on lactate and energy kinetics during exercise, six elite cross-country skiers performed roller-ski exercise for 40 min with the diagonal stride (Continuous Arm+Leg) at 76 ± 1 % VO2max, immediately followed by 10 min double pooling (Arm) at 72 ± 2 % VO2max and again 10 min with the diagonal stride (Arm+Leg). Lactate, glucose and fatty acid (FA) fluxes were determined across the leg and arm by femoral arterial and venous, and subclavian venous catheterization, and measurements of blood flow. In addition, constant infusions of [1-13C]lactate, [2H5]glycerol and [6,6-2H2]glucose were given to determine whole body rate of appearance/ disappearance (Ra/d) and limb lactate uptake and release. A high lactate Ra (184 ± 17 µmol.kg-1.min-1) but a low arterial lactate concentration (~2.5 mmol.l-1) was observed during Continuous Arm+Leg in spite of a substantial net lactate release by the arm of ~2.1 mmol.min-1, which was balanced by a similar net lactate uptake by the leg. Limb lactate uptake was highly correlated to the lactate delivery to the limb, whereas lactate release was not. Whole body and limb lactate oxidation was ~45% at rest and ~95% during Continuous Arm+Leg of Rd and limb lactate uptake, respectively. Lactate dehydrogenase (LDH) activity was lower in the vastus lateralis (189 ± 9 mmol.kg-1dry muscle.min-1) than in the deltoid muscle (340 ± 49), despite a very similar high heart type LDH isoform (~70%). Limb net lactate exchange and lactate uptake and release changed many-fold when changing exercise mode. Whole body glucose and glycerol turnover was unchanged during the different skiing modes, however, limb net glucose uptake changed several-fold. In conclusion: the arterial lactate concentration can be maintained at a relatively low level despite high lactate Ra during exercise with a large muscle mass due to the large capacity of active skeletal muscle to take up lactate, which is tightly correlated with lactate delivery. The limb lactate uptake during exercise is oxidized at rates far above resting oxygen consumption implying that lactate uptake and subsequent oxidation is also dependent on an elevated metabolic rate. The relative contribution of whole body and limb lactate oxidation are between 20-30% of total carbohydrate oxidation at rest and during exercise under the various conditions. Skeletal muscle can change its limb net glucose uptake several-fold within minutes, causing a redistributions of the available glucose since whole body glucose turnover was unchanged.
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