Membrane Transport

The rapid removal of amino acid transmitters is accomplished by transport proteins located in the plasma membranes of neurons and glial cells. The active transport process is typically Na+-dependent, and driven by the transmembrane Na+ gradient that is maintained by Na+/K+ ATPase activity. Thus, agents such as cyanide that block ATP synthesis, or substances such as ouabain that interfere with establishing an ionic gradient, inhibit transmitter uptake (cf. Kanner, 1983). In addition to Na+, the coupling of other ions, e.g., K;+ or CI-, is often required for transport. GABA uptake, for example, is catalyzed by a Na+ and CI-dependent transporter, whereas the glutamate carrier requires the countertransport of K+ as well as Na+ uptake. The stoichiometry suggests that for both neurotransmitters, the transport process is electrogenic—i.e., it is capable of generating a transmembrane current when activated (see Section 4.2).

4.1.1. Uptake of Radioactive Tracers

Tracer flux measurement is frequently used to determine the rate at which cells accumulate exogenously applied amino acids (cf. Keynan and Kanner, 1988). This technique enables visualization of the translocation of label to intracellular sites, and is particularly useful when the transport process does not involve a net transfer of electrical charge. Neurotransmit-ter uptake by Müller cells and retinal neurons has been frequently demonstrated using this approach. Although both cell types are able to accumulate radiolabeled amino acids, there are significant interspecies differences in the identity of the cells that take up the transmitter (cf. Marshall and Voaden, 1975; Ehinger, 1977). Autoradiographic studies of the cellular uptake of tritium-labeled GABA provide a good example of this variability (Table 4.1).

Table 4.1. Species Comparison of Retinal Cell Types Accumulating 3H-GABA

Species

Cell type(s)

Reference

Frog

Horizontal, amacrine

Voaden et al., 1974

Goldfish

Horizontal, amacrine

Lam and Steinman, 1971; Marshall

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