Sodium transport and the origin of the membrane potential in the cyanobacterium synechococcus r 2 anacystis nidulans pcc 7942
Journal of Plant Physiology 139(3): 320-330
ISSN/ISBN: 0176-1617 DOI: 10.1016/s0176-1617(11)80345-7
Synechococcus R-2 (PCC 7942) actively extruded Na+ in the light and dark, and over a very wide range of pHo (5 to 10), [Na+]o and [K+]o (0.1 to 300 mol m-3). The Na+ motive force or Na+ electrochemical potential (.DELTA.mu.Na+i,o) was -93 .+-. 3 mV in 0.1 mol m-3 Na+o, rose to a maximum of -150 .+-. 6 mV in 50 mol m-3 Na+o then fell off to - 112 .+-. 3 m V in 300 mol m-3 Na+o. [Na+]i was virtually independent of pHo (5 to 10), ENa+i,,o ranged from 24 .+-. 11 mV (pHo 7) to 8 .+-. 3 mV (pHo 10). .DELTA.mu.Na+i,o decreased in parallel to .DELTA.psi.i,o in increasing [K+]o. Under control conditions ([Na+]o = 18 mmol m-3), [Na+]i was 10.87 .+-. 0.98 mmol m-3, .DELTA.mu.Na+i,o = -139 .+-. 3 mV (light); [Na+]1i = 28.9 .+-. 2.3 mmol m-3, .DELTA.mu.Na+i,o = -115 .+-. 3 mV (dark). Na+ fluxes and permeabilities were much higher in the light: .phi.Na+ = 275 .+-. 38 nmol m-2s-1, PNa+in = 3 .+-. 0.4 nm s-1 (light); .phi.Na+ = 5.97 .+-. 0.60 nmol m-2s-1, PNa+in = 0.073 .+-. 0.007 nm s-1 (dark). .phi.Na+ in the light was insensitive to dichlorophenyldimethylurea (DCMU). Dicyclohexylcarbodiimide (DCCD) inhibited .phi.Na+ in the light by about 75% and [Na+]i doubled to 21.2 .+-. 2.4 (4) mol m-3 but in the dark .phi.Na+ increased to 16.5 .+-. 3.8 nmol m-2s-1 (20) and [Na+]i increased to 136 .+-. 34 mol m-3 (20). .phi.Na+out in the light was directly proportional to [Na+]o. In alkaline pHo the proton motive force (pmfi,o) was near zero but .DELTA.mu.Na+i,o exceeded -100 mV. The active extrusion of Na+ could not be driven by pmfi,o. In the light, NA+ was probably actively extruded by an electrogenic 3 Na+/ATP pump which in turn drives H+ efflux via a H+our/2 Na+in antiport at pHo less than or equal to 9. The Na+ conductance, under control conditions, was GNa+in = 2.44 (.+-. 0.49) S m-2, which is comparable to the conductance of the proton circuits found in giant algal cells, but GNa+in fell off rapidly in lower [Na+]o. .DELTA.psi.i,o is apparently generated by electrogenic pumping of Na+ in the light and dark and is not a K+/Na+ Goldman diffusion potential.