1. Arterial concentrations and leg exchange of carbohydrate substrates and non-esterified fatty acids (NEFA) were examined together with concentrations of intramuscular metabolites at rest and during exercise in six patients with Parkinson's disease and in a group of five healthy control subjects.

2. Heart rate, pulmonary oxygen uptake and ventilatory exchange ratio were all significantly higher in the group of patients during exercise. The release of lactate by the exercising leg in the patients exceeded that of the control subjects and was associated with a significantly elevated arterial lactate concentration. Glucose uptake by the leg was greater in the patients during exercise. Arterial NEFA was higher and fractional NEFA turnover lower in the patients both at rest and during exercise, whereas oxidation of NEFA by the leg muscles during exercise was similar in the two groups.

3. ATP, creatine phosphate and glycogen content of muscle were significantly reduced in the basal state in the patients. During exercise, ATP fell markedly but the lactate content of muscle was within the normal range. Muscle glycogen utilization during exercise exceeded that of the controls. As in normal subjects, the reduction of glycogen content was most marked in type I (presumably slow-twitch) muscle fibres.

4. It is concluded that, in relation to controls, muscle metabolism in patients with Parkinson's disease (a) is associated with lower content of phosphagens, (b) is altered towards greater utilization of carbohydrate substrates, and (c) is associated with a low efficiency in the peripheral utilization of NEFA.

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