Transmembrane potential-mediated coupling between H+ pump operation and K+ fluxes in Elodea densa leaves hyperpolarized by fusicoccin, light or acid load

In isolated Elodea densa leaves, the relationships between H+ extrusion K+ fluxes and membrane potential (Em) were investigated for two different conditions of activation of the ATP-dependent H+ pump. The 'basal condition' (darkness, no pump activator present) was characterized by low values of and K+ uptake, wide variability of the K+ uptake ratio, relatively low membrane polarization and Em values more positive than EK for external K+ concentrations ([K+]o) of up to 2 mol m-3. A net K+ uptake was seen already at [K+]o below 1mol m-3, suggesting that K+ influx in this condition was a thermodynamically uphill process involving an active mechanism. When the H+ pump was stimulated by fusicoccin (FC), by cytosol acidification, or by light (the 'high polarization condition'), K+ influx largely dominated K+ and Cl- efflux, and the H+ extrusion/k+ uptake ratio approached unity. In the range 50 mmol m-3-5 mol m-3 [K+]o, Em was consistently more negative than EK. The curve of K+ influx at [K+]o ranging from 50 to 5000mmol m-3 fitted a monophasic, hyperbolic curve, with an apparent half saturation value = 0.2 mol m-3. Increasing [K+]o progressively depolarized Ems counteracting the strong hyperpolarizing effect of FC. The effects of K+ in depolarizing Em were well correlated with the effects on both K+ influx and H+ extrusion, suggesting a cause-effect chain: K+o influx leads to depolarization leads to activation of H+ extrusion. Cs+ competitively inhibited K+ influx much more strongly in the 'high polarization' than in the 'basal' condition (50% inhibition at [Cs+]/[K+]o ratios of 1:14 and 1:2, respectively) thus confirming the involvement of different K+ uptake systems in the two conditions. These results suggest that in E. densa leaves two distinct modes of interactions rule the relationships between H+ pump, membrane polarization and K+ transport. At low membrane polarization, corresponding to a low state of activation of the PM H+-ATPase and to Em values more positive than EK, K+ influx would mainly depend on an active transport mechanism, whereas in the condition of high activation of the H+-ATPase and of Em more negative than EK the hyperpolarization-activated voltage-gated K+ influx, even at very low, micromolar extracellular K+ concentrations. The linkage between H+-ATPase and K+ fluxes would depend mainly (although not exclusively) on the interplay between hyperpolarization by the H+ pump, controlling voltage-gated K+ channels, and depolarization by K+ net uptake, facilitating H+ extrusion by the pump.