FIG. 3.

Na⁺ efflux from cells elicited by an inwardly directed ethidium gradient. Energy-starved cells of E. coli HIT-1/pMVP36 were incubated in a buffer consisting of 100 mM MOPS–tetramethylammonium hydroxide (pH 7.0), 2 mM MgSO₄, and 100 μM NaCl. Flux of Na⁺ into and out of cells was measured with an Na⁺ electrode at 25°C. The first arrow indicates when an anaerobic solution of serine (final concentration, 100 μM) was added to the cell suspension under anaerobic conditions to elicit Na⁺ uptake into cells. The second arrow indicates when an anaerobic solution of ethidium bromide (final concentration, 200 μM) was added to the assay mixture. Upward deflection represents the uptake of Na⁺, and downward deflection represents the efflux of Na⁺.