Aims/hypothesis: Our study was undertaken to examine directly the utilisation of glucose and alternative substrates, in particular amino acids, during hypoglycaemia.
Methods: Catheters were positioned in the jugular venous bulb and an artery in six healthy subjects in the overnight fasted state. Arterio-venous differences for glucose, amino acids, lactate and pyruvate were measured in the basal state, during hyperinsulinaemic euglycaemia and during hyperinsulinaemic hypoglycaemia. The subjects were studied on two different occasions, once during intravenous infusion of amino acids and once during infusion of saline.
Results: In the basal state the fractional extraction of glucose across the brain was 10 +/- 2%, glucose uptake accounted for 106 +/- 5% of the brain's oxidative metabolism. There was a small release of lactate and pyruvate. During hyperinsulinaemia glucose uptake continued to account for the entire fuel requirement of the brain. Hyperaminoacidaemia did not result in net amino acid uptake by the brain. During hypoglycaemia (2.4 +/- 0.2 mmol/l) fractional extraction of glucose by the brain increased (p < 0.01) and glucose uptake accounted for 90 +/- 15% of the brain's oxidative metabolism. Uptake of amino acids, lactate or pyruvate could not be detected.
Conclusion/interpretation: 1) Brain fractional extraction of glucose increases during hypoglycaemia, 2) hyperinsulinaemia does not change fractional extraction of glucose by the brain, 3) augmented availability of amino acids does not result in brain amino acid uptake during euglycaemia or hypoglycaemia and 4) under the present study conditions glucose remains the major substrate for cerebral metabolism during hypoglycaemia; lactate or pyruvate uptake by the brain can not be detected.