TRANSMEMBRANE POTENTIAL-MEDIATED COUPLING BETWEEN H+ PUMP OPERATION AND K+ FLUXES IN ELODEA DENSE LEAVES HYPERPOLARIZED BY FUSICOCCIN, LIGHT OR ACID LOAD

In isolated Elodea densa leaves, the relationships between H+ extrusio n (-Delta H+), K+ fluxes and membrane potential (E(m)) were investigat ed for two different conditions of activation of the ATP-dependent Hpump. The 'basal condition' (darkness, no pump activator present) was characterized by low values of -Delta H+ and K+ uptake (Delta K+), wid e variability of the -Delta H+/Delta K+ ratio, relatively low membrane polarization and E(m) values more positive than E(K) for external Kconcentrations ([K+](0)) of up to 2 mol m(-3). A net K+ uptake was see n already at [K+](0) below 1 mol m(-3), suggesting that K+ influx in t his condition was a therm modynamically uphill process involving an ac tive 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 -Delta H+/De lta K+ ratio approached unity, In the range 50 mmol m(-3) -5 mol m(-3) [K+](0), E(m) was consistently more negative than E(K). The curve of K+ influx at [K+](0) ranging from 50 to 5000 mmol m(-3) fitted a monop hasic, hyperbolic curve, with an apparent half saturation value = 0.2m ol m(-3). Increasing [K+](0) progressively depolarized E(m), counterac ting the strong hyperpolarizing effect of FC. The effects of K+ in dep olarizing E(m) were well correlated with the effects on both K+ influx and -Delta H+, suggesting a cause-effect chain: K-0(+) influx --> dep olarization --> activation of H+ extrusion, Cs+ competitively inhibite d K+ influx much more strongly in the 'high polarization' than in the 'basal' condition (50% inhibition at [Cs+]/[K+](0) ratios of 1:14 and 1:2, respectively) thus confirming the involvement of different K+ upt ake systems in the two conditions, These results suggest that in E. de nsa leaves two distinct modes of interactions rule the relationships b etween H+ pump, membrane polarization and K+ transport, At low membran e polarization, corresponding to a low state of activation of the PM H +-ATPase and to E(m) values more positive than E(K), K+ influx would m ainly depend on an active transport mechanism, whereas in the conditio n of high activation of the H+-ATPase and of E(m) more negative than E (K) the hyperpolarization-activated voltage-gated K+ channels would be come the dominant system for passive K+ influx, even at very low, micr omolar extracellular K+ concentrations. The linkage between H+-ATPase and K+ fluxes would depend mainly (although not exclusively) on the in terplay between hyperpolarization by the H+ pump, controlling voltage- gated K+ channels, and depolarization by K+ net uptake, facilitating H + extrusion by the pump.