Measurement of human tricarboxylic acid cycle rates during visual activation by C-13 magnetic resonance spectroscopy

Measurement by C-13 magnetic resonance spectroscopy (MRS) of the incorporat ion of label from [1-C-13] glucose, initially into C4 of glutamate, allows the regional tricarboxylic acid (TCA) cycle flux (F-TCA) to be determined i n the human brain. In this study, a direct C-13 MRS approach was used at 3T , with NOE enhancement and H-1 decoupling with WALTZ16, to determine basal FTCA in six volunteers. The values found in the visual cortex are similar t o those reported in previous C-13 MRS studies, and consistent with PET meas urements of the cerebral metabolic rate for glucose, CMRglc. In two prelimi nary activation studies using light emitting diode (LED) goggles flashing a t 8 Hz, compared to darkness as control, increases in FTCA were found from 0.60 +/- 0.10 to 0.94 +/- 0.03 mu mol/min/g (56%) and from 0.34 +/- 0.14 to 0.56 +/- 0.07 mu mol/min/g (65%). These are upper estimates, but they are similar to the increases in CMRglc reported in PET studies, and strongly su ggest, in contrast to these PET studies, that cerebral glucose is metaboliz ed oxidatively, even during intense visual stimulation. This is supported b y the observation that very little C-13 label is incorporated into C3 lacta te, as would be expected if glucose were metabolized anaerobically. There i s evidence for incorporation of glucose into cerebral glycogen, but this is a relatively minor component of cerebral glucose metabolism. (C) 2001 Wile y-Liss, Inc.