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

Abstract

The polarity (apical vs basolateral cell surface) of the up-regulatory response (“adaptation”) to low medium phosphate (P_i) concentration on apical and basolateral P_i transport systems was investigated in opossum kidney (OK) cell monolayers grown on permeant supports. Incubation of cultures in low-P_i 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/P_i cotransport systems. The basolateral Na-independent, 4,4-diisothiocyanatostilbene-2, 2′-disulphonic-acid-sensitive P_i transport system was unaffected by P_i deprivation. Incubation with low-P_i medium from only the basolateral side failed to elicit any “adaptive” response in P_i transport. When cells were P_i-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 P_i transport was consistant with an increase in J _max for both the apical and basolateral Na/P_i cotransport systems. These data suggest that apical Na-dependent P_i influx is important in signalling the adaptive response to low extracellular P_i.