Voltage dependence of the carrier-mediated ion transport

With regards to the common features of carrier-mediated transport, voltage dependence was studied, using an asymmetric, six-state model. Our study shows that for an ion exchanger, transporting one kind of ion via exchange with another kind, the ion flux as a function of the membrane potential shows a sigmoidal curve with a shallow slope, saturation behavior, and possibly a negative slope. These features are mainly due to the transport of ions with charges of the same sign in the opposite direction. Membrane potential depolarization can facilitate only one transport and hinder another. As a result, the ion flux cannot increase dramatically and has an upper limitation because the exchanging rate depends on competition of the two inversely voltage-dependent transport processes. In contrast, for unidirectional ion transporters, the ion flux will monotonically increase as a function of the membrane potential. Both the maximum ion flux and the voltage sensitivity are much higher than those of the ion exchanger.