Summary: , a facultative alkalophile, can maintain a δpH of up to 1·7 pH units, acid inside, and can rapidly adjust the cytoplasmic pH (pH) in response to a shift in external pH (pH), demonstrating effective pH homeostasis. The presence of Na accelerated the attainment of a new steady-state pH during a shift in the alkaline direction but slowed the attainment of new steady state following a shift in pH 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 pH indicated that pH exerted some feedback control over Na influx. A model for the involvement of Na in pH regulation comprising an electrogenic Na/H antiporter and a sodium influx channel regulated by pH is proposed. Intrinsic to this model is the suggestion that the Na/H antiporter is not the sole site of feedback control by pH.


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