Electrogenic transporters

When mitochondria are loaded with glutamate, cysteine sulfinic acid exchanges with glutamate, and efflux of protons can be observed. No proton movements are observed in conjunction with the cysteine sulfinic acid/aspartate exchange. Therefore, cysteine sulfinic acid must be transported electrogenically as the anion. 

Neurotransmitter transport can be electrogenic if it results in the net translocation of electrical charge (e.g. if more cations than anions are transferred into the cell interior). Moreover, some transporters may direction-ally conduct ions in a manner akin to ligand-gated ion channels this ion flux is not coupled to substrate transport and requires a separate permeation pathway associated with the transporter molecule. In the case of the monoamine transporters (DAT, NET, SERT) the sodium current triggered by amphetamine, a monoamine and psychostimulant (see Fig. 4) is considered responsible for a high internal sodium concentration... 

From the point of view of the stoichiometry of the transported ions during active transport, the electroneutral pump, where there is no net charge transfer or change in the membrane potential, must be distinguished from the electrogenic pump connected with charge transfer. 

RJ Alpern. (1985). Mechanism of basolateralmembrane II -OH /IICOL transport in the rat proximal convoluted tubule. A sodium-coupled electrogenic process. J Gen Physiol 86 613-636. 

Kanai, Y., et al. Electrogenic properties of the epithelial and neuronal high affinity glutamate transporter. J. Biol. Chem. 1995, 270, 16561-16568. 

Busch, A. E., et al. Electrogenic properties and substrate specificity of the polyspecific rat cation transporter rOCTl. /. Biol. Chem. 1996, 271, 32599-32604. 

Meyer-Wentrup F, Karbach U, Gor-boulev V, Arndt P, Koepsell H. Membrane localization of the electrogenic cation transporter rOCTl in rat liver. Bio-chem Biophys Res Commun 1998 248(3) 673-678. 

Figure 2. Sodium and chloride uptake across an idealised freshwater-adapted gill epithelium (chloride cell), which has the typical characteristics of ion-transporting epithelia in eukaryotes. In the example, the abundance of fixed negative charges (muco-proteins) in the unstirred layer may generate a Donnan potential (mucus positive with respect to the water) which is a major part of the net transepithelial potential (serosal positive with respect to water). Mucus also contains carbonic anhydrase (CA) which facilitates dissipation of the [H+] and [HCO(] to CO2, thus maintaining the concentration gradients for these counter ions which partly contribute to Na+ import (secondary transport), whilst the main driving force is derived from the electrogenic sodium pump (see the text for details). Large arrow indicates water flow...

Figure 15.2 Ion transporters in human vaginal-ectocervical epithelial cells, sorted into the apical and basolateral domains of the plasma membrane. Black solid symbols depict electrogenic transporters hatched symbols depict electroneutral transporters. Squares are tight junctions. MCT—monocarboxylate transporter NBC— Na+/HCO co-transporter AE—acid (Cl /HCOj ) exchanger NHE (types 1 or 3)—Na+/H+ exchangers.

The transport is, therefore, electronentral. If the transport does, in fact, resnlt in the transfer of a net charge, it is termed electrogenic (e.g. the proton pnmps in the inner membrane). 

Figure 9.19 Adenine nucleotide translocase and phosphate transfer into the matrix. Phosphate is transported into the mitochondria with protons in a symport transport system. The adenine nucleotide translocase transports ADP into and ATP out of the mitochondria, i.e. it is electrogenic. The charge is neutralised by H movement into the matrix from the proton motive force which utilises about 25% of the energy in the proton motive force.

Transport by ASBT is electrogenic with a 2 1 ratio of Na rbile acids and membrane potential may regulate transport function. ASBT is essential for the enterohepatic circulation as shown by ASBT gene knockout mice that developed bile-acid malabsorption with no enterohepatic circulation. This is summarised in Figure 2.4. 

OTEIER EPITEIEUA Electrogenic and electroneutral ion transporters and their regulation in tracheal epithelium, 192, 549 transformation of airway epithelial cells with persistence of cystic fibrosis or normal ion transport phenotypes, 192, 565 cell culture of bovine corneal endothelial cells and its application to transport studies, 192, 571 methods for studying eccrine sweet gland function in vivo and in vitro, 192, 583. 

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