Turgor-controlled K+ fluxes and their pathways in Escherichia coli.

Escherichia coli like most gram-negative bacteria with walls maintains a cytoplasmic osmolarity exceeding that of the medium; the resulting hydrostatic pressure (turgor pressure) pushes the cytoplasmic membrane against the peptidoglycan and creates a tension in the two envelopes. Potassium is the only cation which takes part in the regulation of cellular osmolarity. The adaptation of intracellular K+ concentration to external osmolarity involves K+ turgor-controlled fluxes. When the medium osmolarity is raised an osmodependent influx of K+ can be observed; this is carried out by the K+ transport system TrkA which can also taken up rubidium. A specific and unidirectional pathway allows K+ ions to flow out of the cell when the medium osmolarity is decreased; this pathway reveals two characteristics: it has no affinity for rubidium and it can be blocked by the blockers of eukaryotic K+ channels. Osmodependent fluxes are turned on immediately after the medium osmolarity is disturbed; in contrast, they are turned off gradually as the rate of K+ fluxes approach zero. The rate of K+ influx seems to depend on the level of internal osmolarity and not on the extent of the increase in medium osmolarity. The rate of the efflux is directly proportional to the decrease in medium osmolarity and is independent on the level of internal osmolarity.