To investigate coupled, charge-translocating transport, it is imperative that the specific transporter current-voltage (IV) relationship of the transporter is separated from the overall membrane IV relationship. We report here a case study in which the currents mediated by the K+-H+ symporter, responsible for high-affinity K+ uptake in Arabidopsis thaliana (L.) Heynh. cv. Columbia roots, are analyzed with an enzyme kinetic reaction scheme. The model explicitly incorporates changes in membrane voltage and external substrate, and enables the derivation of the underlying symport IV relationships from the experimentally obtained difference IV data. Data obtained for high-affinity K+ transport in A. thaliana root protoplasts were best described by a 1:1 coupled K+-H+ symport-mediated current with a parallel, outward non-linear K+ pathway. Furthermore, the large predictive value of the model was used to describe symport behaviour as a function of the external K+ concentration and the cytoplasmic K+ concentration. Symport activity is a complex function of the external K+ concentration, with first-order saturating kinetics in the micromolar range and a strong activity reduction when external K+ is in the millimolar range and the membrane depolarises. High cytoplasmic K+ levels inhibit symport activity. These responses are suggested to be part of the feedback mechanisms to maintain cellular K+ homeostasis. The general suitability of the model for analysis of carrier-mediated transport is discussed.
