The Lumped Constant in the Deoxyglucose Procedure Declines with Age in Fischer‐344 Rats

: Concentrations of [14C]2‐deoxy‐D‐glucose ([14C]DG) and of glucose were measured in plasma of arterial and sagittal sinus venous blood from awake Fischer‐344 rats at 3, 12, and 24 months of age, during continuous intravenous infusion of [14C]DG and after a steady‐state arterial plasma concentration of [14C]DG was reached. Brain extraction, i.e., the difference between arterial and venous plasma concentrations divided by the arterial plasma concentration, was calculated for both [14C]DG and glucose. Because exchange of both substances between rat plasma and erythrocytes is slow, the ratio of the brain extraction of [14C]DG to that of glucose is identical to the lumped constant in the deoxyglucose procedure of Sokoloff et al. [J. Neurochem. 28, 897–916, (1977)]. This ratio equaled 0.502 ± 0.015 (SEM) at 3 months, 0.456 ± 0.007 at 12 months, and 0.418 ± 0.006 at 24 months of age (n = 15); the means differed significantly from each other (p < 0.05). The results indicate that the lumped constant declines between 3 and 24 months of age in awake rats, and suggest that many reported age reductions in regional cerebral glucose utilization, of 15–25%, are artifactual.