Effects of P‐glycoprotein expression on cyclic AMP and volume‐activated ion fluxes and conductances in HT‐29 colon adenocarcinoma cells

The tissue distribution of P‐glycoprotein (Pgp) and the structurally related cystic fibrosis transmembrane conductance regulator (CFTR) is apparently mutually exclusive, particularly in epithelia; where one protein is expressed the other is not. To study the possible function(s) of Pgp and its potential effects on CFTR expression in epithelia, HT‐29 colon adenocarcinoma cells, which constitutively express CFTR, were pharmacologically adapted to express the classical multidrug resistance (MDR) phenotype (Pgp⁺). Concomitant with the appearance of Pgp and MDR phenotype (drug resistance, reduced drug accumulation and increased drug efflux), CFTR levels and cAMP‐stimulated Cl conductances were markedly decreased compared to wild‐type HT‐29 (Pgp⁻) cells (as shown using the whole cell patch clamp technique). Removal of drug pressure led to the gradual decrease in Pgp levels and MDR phenotype, as evidenced by increased rhodamine 123 accumulation (Pgp‐Rev). Concomitantly, CFTR levels and cAMP‐stimulated Cl⁻ conductances incresed. The cell responses of Pgp/Rev cells were heterogeneous with respect to both Pgp and CFTR functions. We also studied the possible contribution of Pgp to hypotonically activated (HCS) ion conductances. K⁺ and Cl⁻ effluxes from Pgp⁻ cells were markedly increased by HCS. This increase was twice as high as that induced by the cation ionophore gramicidin; it was blocked by the Cl⁻ channel blocker DIDS (4,4′‐disothiocyano‐2,2′‐disulfonic stilbene) and required extracellular Ca²⁺. In Pgp⁺ cells, the HCS‐induced fluxes were not significantly different from those of Pgp⁻ cells. Verapamil (10 μM), which caused 80% reversal of Pgp‐associated drug extrusion, failed to inhibit the HCS‐evoked Cl⁻ efflux of Pgp⁺ cells. Similarly, HCS increased Cl⁻ conductance to the same extent in Pgp⁻, Pgp⁺ and Pgp‐Rev cells. Verapamil (100 μM), but not 1,9‐dideoxyforskolin (50 and 100 μM), partially inhibited the HCS‐evoked whole cell current (WCC) in all three lines. Since the inhibition by verapamil was not detected in the presence of the K⁺ channel blocker Ba²⁺ (3 mM), it is suggested that verapamil affects K⁺ and not Cl⁻ conductance. We conclude that hypotonically activated Cl⁻ and K⁺ conductances are similar in HT‐29 cells irrespective of Pgp expression. Expression of high levels of Pgp in HT‐29 cells confers no physiologically significant capacity for cell volume regulation. © 1994 Wiley‐Liss, Inc.