ELECTROGENIC ACTIVE PROTON PUMP IN HEVEA-BRASILIENSIS LATICIFEROUS CELLS - ITS ROLE IN ACTIVATING SUCROSE/H+ AND GLUCOSE/H+ SYMPORTS AT THE PLASMA-MEMBRANE

The transplasmalemmal electrical gradient recorded in laticiferous cells at steady state was -113 +/- 21 mV. Sucrose and glucose depolarize the plasmalemma of laticiferous cells by about 15 to 25 mV. Our results show that with depolarization due to sucrose (1 mM) or glucose (1 mM) a slight alkalinization (0.1 to 0.2 pH units) can be detected on the outer surface of the cell. Fructose and 3-O-methyl-glucose have no such effect. The extent of depolarization due to the addition of sugars is lower than the electrogenic component of the membrane potential produced by the functioning of the H+-excretion pump (vanadate sensitive-ATPase). Furthermore, in the presence of vanadate or DNP, with glucose or sucrose no shift in pH value was observed. The effect of phlorizin has been tested on the shift of the membrane potential due to sugar uptake across the plasmalemma: neither sucrose nor glucose demonstrate any further depolarization and alkalinization in the presence of phlorizin. Stimulation of the H+-pump by ethylene hyperpolarizes cells by approximately -40 mV and increases the extent of the depolarization induced by sugar transport. These results suggest an active transport of the sugars from the apoplasm towards the cytosol. Evidence for the existence of H+ cotransport with sucrose and/or glucose at the plasmalemma is discussed hereafter.