β-Hydroxybutyrate Boosts Mitochondrial and Neuronal Metabolism but is not Preferred Over Glucose Under Activated Conditions

The ketone body, beta-hydroxybutyrate (beta OHB), is metabolised by the brain alongside the mandatory brain fuel glucose. To examine the extent and circumstances by which beta OHB can supplement glucose metabolism, we studied guinea pig cortical brain slices using increasing concentrations of [U-C-13]D-beta OHB in conjunction with [1-C-13]d-glucose under conditions of normo- and hypoglycaemia, as well as under high potassium (40 mmol/L K+) depolarization in normo- and hypoglycaemic conditions. The contribution of beta OHB to synthesis of GABA was also probed by inhibiting the synthesis of glutamine, a GABA precursor, with methionine sulfoximine (MSO). [U-C-13]D-beta OHB at lower concentrations (0.25 and 1.25 mmol/L) stimulated mitochondrial metabolism, producing greater total incorporation of label into glutamate and GABA but did not have a similar effect in the cytosolic compartment where labelling of glutamine was reduced at 1.25 mmol/L [U-C-13]D-beta OHB. At higher concentrations (2.5 mmol/L) [U-C-13]D-beta OHB inhibited metabolism of [1-C-13]d-glucose, and reduced total label incorporation and total metabolite pools. When glucose levels were reduced, beta OHB was able to partially restore the loss of glutamate and GABA caused by hypoglycaemia, but was not able to supplement levels of lactate, glutamine or alanine or to prevent the increase in aspartate. Under depolarizing conditions glucose was the preferred substrate over beta OHB, even in hypoglycaemic conditions where comparatively less beta OHB was incorporated except into aspartate isotopomers. Inhibition of glutamine synthesis with MSO had no significant effect on incorporation of label from [U-C-13]D-beta OHB into GABA C2,1 indicating that the majority of this GABA was synthesized in GABAergic neurons from [U-C-13]D-beta OHB rather than from Gln C4,5 imported from astrocytes.