Kinetics of unidirectional glucose transport into the isolated dog brain

An indicator dilution technique with ²²Na as the intravascular marker was used to measure unidirectional transport of D glucose 6^-3H from blood into the isolated dog brain. The rate of unidirectional glucose transport, v, was calculated from the equation v = (E - 0.036) A Fp/W, where E is the fractional extraction of glucose from the blood, A is the arterial plasma glucose concentration, Fp/W is the plasma flow rate per unit weight of brain, and 0.036 is a correction for glucose diffusion. The diffusion correction was based on the fractional extraction of D fructose 6^-3H, a hexose that is not transported into brain. The results of experiments in which Fp/W was varied while A was held constant indicate that an increased flow rate results in an increased rate of glucose uptake. Because of the flow effect, the kinetics of unidirectional glucose transport were studied in six brains in which Fp/W was held constant while A was varied between 4.3 and 60 mM. An apparent K(m) for glucose transport of 8.26 ± 1.67 mM and a V(max) of 1.75 + .11 μmoles/g of brain per minute were calculated. The kinetics were not significantly altered in the presence of pentobarbital (25 mg/l) or insulin (4 μg/100 ml). This technique is generally applicable to any isolated, perfused tissue.