Cerebral metabolic response to submaximal exercise

Abstract

We studied cerebral oxygenation and metabolism during submaximal cycling in 12 subjects. At two work rates, middle cerebral artery blood velocity increased from 62 ± 3 to 63 ± 3 and 70 ± 5 cm/s as did cerebral oxygenation determined by near-infrared spectroscopy. Oxyhemoglobin increased by 10 ± 3 and 25 ± 3 μmol/l (P < 0.01), and there was no significant change in brain norepinephrine spillover. The arterial-to-internal-jugular-venous (a-v) difference for O₂ decreased at low-intensity exercise (from 3.1 ± 0.1 to 2.9 ± 0.1 mmol/l;P < 0.05) and recovered at moderate exercise (to 3.3 ± 0.1 mmol/l). The profile for glucose was similar: its a-v difference tended to decrease at low-intensity exercise (from 0.55 ± 0.05 to 0.50 ± 0.02 mmol/l) and increased during moderate exercise (to 0.64 ± 0.04 mmol/l;P < 0.05). Thus the molar ratio (a-v difference, O₂ to glucose) did not change significantly. However, when the a-v difference for lactate (0.02 ± 0.03 to 0.18 ± 0.04 mmol/l) was taken into account, the O₂-to-carbohydrate ratio decreased (from 6.1 ± 0.4 to 4.7 ± 0.3;P < 0.05). The enhanced cerebral oxygenation suggests that, during exercise, cerebral blood flow increases in excess of the O₂demand. Yet it seems that during exercise not all carbohydrate taken up by the brain is oxidized, as brain lactate metabolism appears to lower the balance of O₂-to-carbohydrate uptake.