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In recent times, there has been a tremendous expansion in knowledge of Na+-dependent glutamate (Glu) transporters. A large family has been identified using pharmacological as well as molecular biological techniques. At present, the cloned transporters do not reconstitute all of the pharmacological subtypes observed in vivo and in vitro. Furthermore, the clones provide evidence for subtypes of transport activity that were not differentiated pharmacologically. The acidic amino acids, glutamate (Glu), and aspartate (Asp) are the predominant excitatory neurotransmitters in the mammalian central nervous system (CNS). Several types of transport activities for Glu and Asp have been differentiated in mammals. Although at least two families of these transporters are localized to the plasma membrane, other subtypes of transporters appear to be selectively localized to subcellular organelles. Only one of the families of transporters that is localized to the plasma membrane appears to be coupled to an electrochemical gradient that permits transport of the acidic amino acids against their concentration gradient. This family of active Na+-dependent transporters is the focus of this chapter. The pharmacological evidence to suggest that there are several subtypes of this transport system are presented here.
Robinson et al. (Mon,) studied this question.