SPecific role of p-glycoprotein (pgp) in the swelling-activated cl^- current (i_cl^s) of cells not exposed to CHEMOTHERAPEUTIC DRUGS

We previously reported that the magnitude and voltage-dependence of 'cis 'n Pgp-expressing and non-expressing cells is similar, and that the anti-Pgp monoclonal antibody C219 inhibits lc,s only in cells expressing Pgp (Han et al., 1 995). However, drug selection per se might affect tcis. In order to test whether Pgp has an effect on lc,s in Pgp-expressing cells not exposed to chemotherapeutic drugs, we transfected mouse fibroblasts (BALB/C 3T3I with hamster Pgp cDNA. The transfected cells expressed functional Pgp in the plasma membrane as assessed by rhodamine 123 efflux and Western-blot analysis with C219. The parental cells and cells transfected with the vector alone lacked Pgp expression. Both the Pgp-transfected and the parental cells displayed similar ICIS characteristics. After 8-10 min of reducing the bath osmolarity by 22%, IC(S in both cell lines was similar in magnitude (ICIS at +80 mV, 2.9±0.5 nA, control, and 2.4±0.3, transfected). ICIS was outwardly rectifying, with a rectification ratio (ICIS at +80 mV divided by ICI5 at -80 mV) of 1.2 in both cell lines. Removal of ATP from the pipette solution abolished lc,s independently of Pgp expression. The sensitivities to CI channel blockers was similar in both cell lines: NPPB (200 jjM) reduced IC(S by at least 90%, while DIDS (500 /jM] had no effect. These results show that several of the properties of ICIS in the BALB/C 3T3 cells are not altered by Pgp expression. However, if C219 is present in the pipette solution, lc,s is blocked only in the Pgp transfected cells. These results, obtained in Pgp-expressing cells not exposed to chemotherapeutic drugs, confirm a specific role for Pgp in ICI5. [Supported by Searle Research and Development and NIH grant 5F31 DK08865].