Direct Measurement of the λ of the Lumped Constant of the Deoxyglucose Method in Rat Brain: Determination of λ and Lumped Constant from Tissue Glucose Concentration or Equilibrium Brain/Plasma Distribution Ratio for Methylglucose

Steady-state distribution spaces of 2-[¹⁴C]deoxyglucose ([¹⁴C]DG), glucose, and 3-O-[¹⁴C]methylglucose at various concentrations of glucose in brain and plasma ranging from hypoglycemic to hyperglycemic levels have been determined by direct chemical analyses in the brains of conscious rats. The hexose concentrations were measured chemically in freeze-blown brain extracted with ethanol to avoid the degradation of acid-labile products of [¹⁴C]DG back to free [¹⁴C]DG that has been found to occur with the more commonly used perchloric acid extraction of brain. Corrections were also made for nonphosphorylatable, labeled products of [¹⁴C]DG found in the nonacidic fractions of the brain extracts, which were previously included with the assayed [¹⁴C]DG, and for the contribution of the hexose contents in the blood in the brain, which was found to be particularly critical for the determination of the glucose distribution space, especially in hypoglycemic states. From the measured contents of the hexoses in brain and plasma, the relationships of the tissue concentrations and distribution spaces of each of the hexoses and of the Λ (i.e., ratio of tissue distribution space of DG to that of glucose) of the DG method to the tissue glucose concentration were derived. The Λ was then quantitatively related to the measured equilibrium ratio for [¹⁴C]methylglucose over the full range of brain and plasma glucose levels. By combining these new data with the values for the lumped constant, the factor that converts rate of DG phosphorylation to glucose phosphorylation, previously determined in rats over the same range of plasma glucose levels, the phosphorylation coefficient was calculated and the lumped constant graphed as a function of the measured distribution space in brain for [¹⁴C]methylglucose.