Measurement of Cerebral Glucose Utilization Using Washout After Carotid Injection in the Rat

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Abstract
The carotid injection technique, used previously to quantitate the kinetics of blood-brain barrier transport of metabolic substrates, may be modified to analyze the rate of cerebral glucose utilization. A 0.2-ml solution of [14C]glucose (GF) and [3H]methylglucose (M), an internal reference, is rapidly injected into the carotid artery, followed by microwave fixation of brain at various times up to 4 min after injection. The brain radioactivity is separated into a fraction containing neutral hexoses (GF and M) and a fraction containing metabolites of glucose. The GF/M ratio is related to the rate constant (k3) of brain glucose utilization by the simple, linear equation: In(GF/M) = In(GF°/M°) –k3t, where GF°/M°= the brain uptake index of glucose, relative to methylglucose, at 5-15 s after injection, and t= the time after carotid injection, e.g., 1–4 min. It is assumed that (a) the rate of influx due to recirculation of label is minimal during the 4-min circulation period; and (b) the rate constants of glucose efflux (k2) and methylglucose efflux (k2*) are identical. Independent estimates of k2 and k2* showed these parameters to be identical: k2= 0.14 + 0.08 min-I; k2*= 0.14 ± 0.02 min-I. A logarithmic plot of GF/M ratios versus time was linear (r = 0.99), and was described by the slope k2= 0.21 ± 0.02 min−1. Assuming glucose is uniformly distributed in brain, then the glycolytic rate = k3× brain glucose = (0.21 min−1) (2.6 μmol g−1) = 0.55 μmol min−1 g−1 for the cortex of the barbiturate-anesthetized rat. These studies provide the basis for a simple method of measurement of regional brain glycolysis that does not require either the use of correction factors, e.g., the lumped constant, or the use of differentially labeled glucose.