Mechanism of electrogenic cation transport by the cloned organic cation transporter 2 from rat

The organic cation transporter 2 (OCT2) is expressed in plasma membranes of kidney and brain. Its transport mechanism and substrates are debated. We s tudied substrate-induced changes of electrical current with the patch clamp technique after expression of rat OCT2 in oocytes. Activation of current, corresponding to efflux, was observed for small organic cations, e.g. choli ne. In contrast, the bigger cations quinine and tetrabutylammonium elicited no change in current. However, transport of choline could be inhibited by applying quinine or tetrabutylammonium to the cytoplasmic side. Inhibition of organic cation efflux by quinine was competitive with substrates. Quinin e at the inside also inhibited substrate influx from the outside. Current-v oltage analysis showed that both maximal turnover and apparent affinity to substrates are voltage-dependent. Substrate-induced currents with organic c ations on both membrane sides reversed as predicted from the Nernst potenti al. Our results clearly identify the electrochemical potential as driving f orce for transport at neutral pH and exclude an electroneutral H+/organic c ation(+) exchange. We suggest the existence of an electroneutral organic ca tion(+) exchange and propose a model for a carrier-type transport mechanism .