Betz et al. [327] reported Tmax = 1.6 µmol g−1 min−1 and Kt= 8.6 mM (based on blood rather than plasma) in the dog based on tracer uptake rate after 1 min of hypoxia, while Pardridge and Oldendorf [538] investigated transport of five different hexoses into rat brains and found Tmax = 1.6 µmol g−1 min−1 for all, but differing apparent dissociation constants with 9 mM for glucose, which they took to imply that the conformation changes of the carrier rather than binding of the substrates were rate limiting. This is plausible because relatively low affinity binding of small substrates to sites is often diffusion controlled and rapid while conformation changes of large molecules may well be much slower. At a plasma concentration of 6 mM these values correspond to a glucose clearance of 100 µL g−1 min−1. Mason et al. [334] lists many values of Tmax and Kt determined from flux studies prior to 1992. These range from 0.5 to 6.7 µmol g−1 min−1 for Tmax and 4.9–11 mM for Kt.