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1 Laboratory of Cerebral Metabolism, National Institute of Mental Health, Bethesda, Maryland 20892; and 2 Department of Medical Physics, University of Wisconsin Medical School, Madison, Wisconsin 53706
Methylglucose can be used to assay brain glucose levels because the equilibrium brain-to-plasma distribution ratio for methylglucose (C*e/C*p) is quantitatively related to brain (Ce) and plasma (Cp) glucose contents. The relationship between Ce and C*e/C*p predicted by Michaelis-Menten kinetics has been experimentally confirmed when glucose utilization rate (CMRGlc) is maintained at normal, resting levels and Cp is varied in conscious rats. Theoretically, however, Ce and C*e/C*p should change when CMRGlc is altered and Cp is held constant; their relationship in such conditions was, therefore, examined experimentally. Drugs were applied topically to brains of conscious rats with fixed levels of Cp to produce focal alterations in CMRGlc, and Ce and C*e/C*p were measured. Plots of Ce as a function of C*e/C*p for each Cp produced straight lines; their slopes decreased as Cp increased. The results confirm that a single theoretical framework describes the relationship between Ce and C*e/C*p as either glucose supply or demand is altered over a wide range; they also validate the use of methylglucose to estimate local Ce under abnormal conditions.
cerebral glucose utilization; brain hexose transport
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