Functional asymmetry of phosphate transport and its regulation in opossum kidney cells: phosphate "adaptation"

The polarity (apical vs basolateral cell surface) of the up-regulatory response ("adaptation") to low medium phosphate (Pi) concentration on apical and basolateral Pi transport systems was investigated in opossum kidney (OK) cell monolayers grown on permeant supports. Incubation of cultures in low-Pi medium, given either only to the apical or simultanously to the apical and basolateral compartments, increased the rate of transport of both the apical and the basolateral Na/Pi cotransport systems. The basolateral Na-independent, 4,4-diisothiocyanatostilbene-2,2'-disulphonic-acid-sensitive Pi transport system was unaffected by Pi deprivation. Incubation with low-Pi medium from only the basolateral side failed to elicit any "adaptive" response in Pi transport. When cells were Pi-limited either apically or on both sides for short periods of time, adaptation was apparent within 2 h and close to maximal by 6 h, and the alteration in Pi transport was consistant with an increase in Jmax for both the apical and basolateral Na/Pi cotransport systems. These data suggest that apical Na-dependent Pi influx is important in signalling the adaptive response to low extracellular Pi.