Letters to the Editor

To the Editor: Recent articles by Redies and associates in this journal and elsewhere provide interesting data on the kinetics of deoxyglucose (DG) and fluorodeox yglucose (FDG) metabolism in rat and ferret brain (Redies et aI., 1987a,b; Redies and Diksic, 1989; Redies et aI., 1989). Their findings with regard to loss of phosphorylated DG and FDG from brain may be summarized as follows: (a) the rate constant for this loss is, on average, somewhat greater than 1 %/min; (b) loss occurs from the beginning of the experiment (there is no delay); and (c) there is sig nificant regional variation in the rate constants of loss. One might reasonably conclude from these ob servations that experimenters using the deoxyglu cose method should use a mathematical model that takes into account loss of product, on a regional basis. Given the range of brain regional rate con stants for FDG loss of 0.8-1.7% (Redies et aI., 1989), failure to account for loss will result in un derestimates of regional rates of glucose utilization of approximately 25-50% at 45 min after intrave nous FDG. The range for DG, 0.9-1.4% (Redies et aI., 1989), will cause underestimates of approxi mately 28-42%. The authors accept the need to account for loss of product in long experiments (120--180 min) but ar gue that this is unnecessary in 45 min experiments. Instead, they state that at 45 min one can ignore the rate constant for loss and use different values for the rate constants for transport out, for phosphory lation, and for the lumped constant (the ratio of the unidirectional rate of tracer phosphorylation to the net rate of glucose phosphorylation). This approach is unsatisfactory for three reasons.