Lee et al. [360] made the interesting suggestion that much of the NH4+ that moves from brain microvascular endothelial cells to plasma is produced within the endothelial cells by glutaminase acting on glutamine. However, that taken alone would suggest that there should also be a substantial efflux of glutamate, which has not been observed. Alternatively the NH4+ effluxed may derive from metabolism of both glutamine and glutamate. This is considered further in Sect. 5.5.4.