Substrate induced currents through the monoamine transporters and single cell uptake of the fluorescent substrate APP+

<p>The concentrative power of the transporters for dopamine (DAT), norepinephrine (NET) and serotonin (SERT) is thought to be fueled by the transmembrane Na⁺ gradient, but it is conceivable that they can also tap other energy sources, e.g. membrane voltage and/or the transmembrane K⁺ gradient. We address this by recording uptake of endogenous substrates or the fluorescent substrate APP⁺ ((4-(4-dimethylamino)phenyl-1-methylpyridinium) under voltage control in cells expressing DAT, NET or SERT. We show that DAT and NET differ from SERT in intracellular handling of K⁺. In DAT and NET, substrate uptake was voltage-dependent due to the transient nature of intracellular K⁺ binding, which precluded K⁺ antiport. SERT, however, antiports K⁺ and achieves voltage-independent transport. Thus, there is a trade-off between maintaining constant uptake and harvesting membrane potential for concentrative power, which we conclude to occur due to subtle differences in the kinetics of co-substrate ion binding in closely related transporters.</p>