Differentiation of glucose transport in human brain gray and white matter

Localized H-1 nuclear magnetic resonance spectroscopy has been applied to determine human brain gray matter and white matter glucose transport kinetics by measuring the steady-stare glucose concentration under normoglycemia and two levels of hyperglycemia. Nuclear magnetic resonance spectroscopic measurements were simultaneously performed on three 12-mL volumes. containing predominantly gray or white matter. The exact volume compositions were determined from quantitative T-1 relaxation magnetic resonance images. The absolute brain glucose concentration as a function of the plasma glucose level was fitted with two kinetic transport models, based on standard (irreversible) or reversible Michaelis-Menten kinetics. The steady-state brain glucose levels were similar for cerebral gray and white matter, although the white matter levels were consistently 15% to 20% higher. The ratio of the maximum glucose transport rate, V-max, to the cerebral metabolic utilization rate of glucose, CMRGlc, was 3.2 +/- 0.10 and 3.9 +/- 0.15 for gray matter and white matter using the standard transport model and 1.8 +/- 0.10 and 2.2 +/- 0.12 for gray matter and white matter using the reversible transport model. The Michaelis-Menten constant K-m was 6.2 +/- 0.85 and 7.3 +/- 1.1 mmol/L for gray matter acid white matter in the standard model and 1.1 +/- 0.66 and 1.7 +/- 0.88 mmol/L in the reversible model. Taking into account the threefold lower rate of CMRGlc in white matter, this finding suggests that blood-brain barrier glucose transport activity is lower by a similar amount in white matter. The regulation of glucose transport activity at the blood-brain barrier may be an important mechanism for maintaining glucose homeostasis: throughout the cerebral cortex.