Summary
Utilization of carbohydrates and free fatty acids (FFA) has been investigated in gastrocnemii of dogs during long lasting isotonic rhythmical exercise induced by supramaximal stimulation of the sciatic nerve. Uptake or output of gases and substrates was determined according to the Fick principle. The first measurements were done at about 2 min after the beginning of work when blood flow has reached a steady state, and the latest at about 100 min after the beginning of exercise.
During the first 7 min when the work performed exceeded 5 kg/100g×min and O2 consumption exceeded 11 ml/100g×min, uptake of arterial glucose and FFA was low, accounting for less than 40% of the total O2 consumption. Since the RQ values at the same time were about 1.0, glycogen must have been oxidized as the major aerobic energy source.
About 13 min after the beginning of exercise, the work the muscles could perform declined to about half of the initial value and remained so for the following 90 min. During this time the oxygen extraction ratio of FFA was about 50% and of arterial glucose was 40–50%, while the RQ value was about 0.8.
During initial strong exercise an output of lactic acid (LA) of about 10 mg/100 g×min was measured. With the decrease of work as a consequence of fatigue, LA output became negligible, and in many experiments small amounts of LA were taken up by the working gastrocnemii.
It is concluded that glycogen is the major aerobic energy source for strong muscular exercise which cannot be substituted for by the oxidation of arterial glucose or FFA.
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Hirche, H., Grün, D. & Waller, W. Utilization of carbohydrates and free fatty acids by the gastrocnemius of the dog during long lasting rhythmical exercise. Pflugers Arch. 321, 121–132 (1970). https://doi.org/10.1007/BF00586367
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DOI: https://doi.org/10.1007/BF00586367