2‐Deoxyglucose Incorporation into Rat Brain Glycogen During Measurement of Local Cerebral Glucose Utilization by the 2‐Deoxyglucose Method

The incorporation of 14 C into glycogen in rat brain has been measured under the same conditions that exist during the measurement of local cerebral glucose utilization by the autoradiographic 2‐[ 14 C]deoxyglucose method. The results demonstrate that approximately 2% of the total 14 C in brain 45 min after the pulse of 2‐[ 14 C]deoxyglucose is contained in the glycogen portion, and, in fact, incorporated into α‐1‐4 and α‐1‐6 deoxyglucosyl linkages. When the brain is removed by dissection, as is routinely done in the course of the procedure of the 2‐[ 14 C]deoxyglucose method to preserve the structure of the brain for autoradiography, the portion of total brain 14 C contained in glycogen falls to less than 1%, presumably because of postmortem glycogenolysis which restores much of the label to deoxyglucose‐phosphates. In any case, the incorporation of the 14 C into glycogen is of no consequence to the validity of the autoradiographic deoxyglucose method, not because of its small magnitude, but because 2‐[ 14 C]deoxyglucose is incorporated into glycogen via [ 14 C]deoxyglucose‐6‐phosphate, and the label in glycogen represents, therefore, an additional “trapped” product of deoxyglucose phosphorylation by hexokinase. With the autoradiographic 2‐[ 14 C]deoxyglucose method, in which only total 14 C concentration in the brain tissue is measured by quantitative autoradiography, it is essential that all the labeled products derived directly or indirectly from [ 14 C]deoxyglucose phosphorylation by hexokinase be retained in the tissue; their chemical identity is of no significance.