Sodium Transport and the Origin of the Membrane Potential in the Cyanobacterium Synechococcus R-2 (Anacystis Nidulans) PCC 7942

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Summary Synechococcus R-2 (PCC 7942) actively extruded Na + in the light and dark, and over a very wide range of pH o (5 to 10), [Na + ] o and [K + ] o (0.1 to 300 mol m -3 ). The Na + motive force or Na + electrochemical potential (ΔμNa + i,o ) was -93 ± 3 mV in 0.1 mol m -3 Na + o , rose to a maximum of -150 ± 6mV in 50 mol m -3 Na + o then fell off to -112 ± 3 mV in 300 mol m -3 Na + o . [Na + ] i was virtually independent of pH o (5 to 10), E Na + i,o ranged from 24 ± llmV (pH o 7) to 8 ± 3mV (PH o 10). ΔμNa + i,o decreased in parallel to ΔΨ i.o in increasing [K + ] o . Under control conditions ([Na + ] o = 18mol m -3 ), [Na + ] i was 10.87 ± 0.98molm -3 , ΔμNa + i,o = -139 ± 3mV (light); [Na + ] i = 28.9 ± 2.3mol m -3 , ΔμNa + i,o = -115 ± 3mV (dark). Na + fluxes and permeabilities were much higher in the light: ϕNa + = 275 ± 38 nmol m -2 s -1 , P Na + in = 3 ± 0.4 nm s -1 (light); ϕNa + = 5.97 ± 0.60 nmol m -2 s -1 , P Na + in = 0.073 ± 0.007 nm S -1 (dark). ϕNa + in the light was insensitive to dichlorophenyldimethylurea (DCMU). Dicyclohexylcarbodiimide (DCCD) inhibited ϕNa + in the light by about 75% and [Na + ] i doubled to 21.2 ± 2.4 (4) molm -3 but in the dark ϕNa + increased to 16.5 ± 3.8nmolm -2 s -1 (20) and [Na + ] i increased to 136 ± 34 mol m -3 (20). ϕNa + out in the light was directly proportional to [Na + ] o . In alkaline pH o the proton motive force (pmf i,o ) was near zero but ΔμNa + i,o exceeded -1OOmV. The active extrusion of Na + could not be driven by pmf i,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 + out /2 Na + in antiport at pH o less than or equal to 9. The Na + conductance, under control conditions, was G Na + in = 2.44 (± 0.49) Sm -2 , which is comparable to the conductance of the proton circuits found in giant algal cells, but G Na + in fell off rapidly in lower [Na + ] o . Δϕ i,o is apparently generated by electrogenic pumping of Na + in the light and dark and is not a K + / Na + Goldman diffusion potential.