Summary: Exiguobacterium aurantiacum, a facultative alkalophile, can maintain a dpH of up to 1·7 pH units, acid inside, and can rapidly adjust the cytoplasmic pH (pHi) in response to a shift in external pH (pHo), demonstrating effective pHi homeostasis. The presence of Na+ accelerated the attainment of a new steady-state pHi during a shift in the alkaline direction but slowed the attainment of new steady state following a shift in pHo in the acid direction. Measurements of internal Na+ following the addition of 6 mm-NaCl to cells incubated under conditions whereby the cells either could (+ 0·68 mm-NaCl) or could not (0·08 mm-NaCl) regulate pHi indicated that pHi exerted some feedback control over Na+ influx. A model for the involvement of Na+ in pHi regulation comprising an electrogenic Na+/H+ antiporter and a sodium influx channel regulated by pHi is proposed. Intrinsic to this model is the suggestion that the Na+/H+ antiporter is not the sole site of feedback control by pHi.
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