Changes In Delta Psi Research Articles

Characterization of the transport of potassium ions in the cyanobacterium Anabaena variabilis Kütz.

Interrelationships between potassium-ion transport and transplasmalemma electrical-potential difference (delta psi m) have been investigated in Anabaena variabilis (ATCC 29413) by measuring K+ translocation and membrane potential in parallel. At pH 7.0, 5 mmol . dm-3 external K+, there was a thirtyfold accumulation of K+. The K+ equilibrium potential was lower (more negative) than the measured membrane potential by up to 20 mV, (delta psi K+ = -90 mV; delta psi m = -70 mV to -75 mV, respectively). Dark pretreatment and low temperature (4 degrees C) reduced internal K+ and depolarized delta psi m. External pH affected K+ translocation and membrane potential; delta psi m was hyperpolarized at high external pH; transplasmalemma K+ fluxes and internal K+ concentration were also increased at high pH. The effects of pH upon delta psi m, coupled with the finding that the membrane potential was relatively insensitive to external K+, suggest that delta psi m is unlikely to be due primarily to a diffusion potential of K+, but that the membrane potential is maintained by an electrogenic proton-extrusion mechanism. There was no close (obligate) link between K+ transport and changes in delta psi m. Carbonylcyanide m-chlorophenylhydrazone decreased K+ fluxes, internal K+ and delta psi m when added in amounts up to 100 mumol . dm-3. However, delta psi K+ was always more negative than delta psi m. Valinomycin up to concentrations of 50 mumol . dm-3 increased transplasmalemma K+ fluxes by up to 300%, while changes in delta psi m were negligible. Internal K+ was unaffected by valinomycin. N,N'-Dicyclohexylcarbodiimide at concentrations up to 100 mumol . dm-3, reduced K+ flux rates and caused a hyperpolarization of delta psi m. These observations suggest that delta psi m is primarily due to electron transport reactions at the plasmalemma and that K+ transport is energy-dependent. In the presence of dicyclohexylcarbodiimide, internal K+ redistributed in accordance with the membrane potential, suggesting that passive uniport in response to delta psi m (i.e. secondary active transport) is not usually important but may operate when primary active mechanisms are blocked.

The electrochemical proton gradient in Mycoplasma cells.

The electrochemical proton gradient, delta mu H+ generated upon glycolysis by Mycoplasma mycoides var. Capri cells has been determined. The components, the transmembrane pH gradient, delta pH, and the membrane potential, delta psi, were measured using several methods. The determination of the delta pH was conducted by measuring the transmembrane distribution of weak acids (acetate and butyrate) and of a weak base (methylamine), using flow dialysis and filtration techniques. The transmembrane electrical potential was determined from the distribution of the lipophilic cation Ph3MeP+ and of Rb+ or K+ in the presence of valinomycin. At extra-cellular pH 7.2, glycolyzing Mycoplasma cells maintain an internal pH more alkaline (0.5 pH unit) than that of the milieu and an electrical potential of - 85 mV, interior negative. The delta mu H+ in M. mycoides var. Capri cells is thus about - 115 mV. When the external pH was altered from 7.7 to 5.7 delta psi decreased from - 90 mV to - 60 mV. On other hand although the internal pH decreased, delta pH was found to increase from 0.2 to 1.0 pH unit. Since the changes in delta psi were largely compensated by the changes in delta pH, delta mu H+ remained practically constant at about - 115 mV throughout the pH range tested. Finally, inhibition of delta pH by N,N'-dicyclohexylcarbodiimide, carbonylcyanide-p-trifluoromethoxyphenylhydrazone or nigericin confirmed that chemiosmotic phenomena contribute to energy transduction across the membranes of M. mycoides var. Capri cells.