Chloride uptake by oat coleoptile parenchyma described by combined influx and efflux transport systems

Chloride fluxes for oat coleoptile parenchyma were measured using the MIFE system for non-invasive microelectrode ion flux measurement. The effects on chloride fluxes of membrane potential, external chloride concentration, pH, cation composition of the bathing solution, and the anion channel blocker anthracene-9-carboxylic acid were studied. From the experimental data we propose that the step-like appearance of the chloride uptake curve at concentrations higher than 1 mm is due to combined activity of influx and efflux systems. According to this hypothesis the uptake of chloride ions is made by a pH sensitive active transport system with V(max) = 4000 nmol m-2 s-, K(m) = 7 mM [Cl-]. Chloride ions are released from the cell through channels with Gaussian type kinetics, the characteristics of which are dependent upon the external bathing solution: V(max) = -2300 nmol m-2 s-1 at 25 mM [Cl-] for CaCl2, and V(max) = -2000 nmol m-2 s-1 at 15 mM [Cl-] for KCl; with both curves having the same width of 17 mM [Cl-].