KINETICS OF HIGH-AFFINITY K-INDUCED CHANGES IN CURRENT-VOLTAGE RELATIONSHIPS - A MODELING APPROACH TO THE ANALYSIS OF CARRIER-MEDIATED TRANSPORT( UPTAKE IN PLANTS, DERIVED FROM K+)

To investigate coupled, charge-translocating transport, it is imperati ve 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 k inetic reaction scheme. The model explicitly incorporates changes in m embrane voltage and external substrate, and enables the derivation of the underlying symport IV relationships from the experimentally obtain ed 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+ pat hway. Furthermore, the large predictive value of the model was used to describe symport behaviour as a function of the external K+ concentra tion and the cytoplasmic K+ concentration. Symport activity is a compl ex function of the external K+ concentration, with first-order saturat ing kinetics in the micromolar range and a strong activity reduction w hen external K+ is in the millimolar range and the membrane depolarise s. High cytoplasmic K+ levels inhibit symport activity. These response s are suggested to be part of the feedback mechanisms to maintain cell ular K+ homeostasis. The general suitability of the model for analysis of carrier-mediated transport is discussed.