Radiotracer studies of cystic fibrosis transmembrane conductance regulator expressed in Xenopus oocytes

We measured fluxes of radiotracers in Xenopus oocytes expressing the cystic fibrosis transmembrane conductance regulator (CFTR). Addition of adenosine 3',5'-cyclic monophosphate (cAMP)-elevating agents [forskolin and 3-isobutyl-1-methylxanthine (I/F)] led to large increases in uptake of ³⁶Cl, ¹²⁵I, and ⁸²Br into oocytes expressing wild-type CFTR or ΔF508 CFTR but not sham-injected oocytes. I/F also stimulated halide efflux from CFTR and ΔF508 oocytes in the sequence Cl > Br > I. cAMP-induced increases in ³⁶Cl efflux from ΔF508 oocytes were ~20% of those in CFTR oocytes. Increases in halide efflux were blocked by diphenylamine-2-carboxylic acid but not by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid. The phorbol ester, phorbol 12-myristate 13-acetate, also stimulated ³⁶Cl efflux from CFTR oocytes. ATP uptakes into CFTR and sham oocytes were similar, and both were reduced by I/F. However, ATP uptake into I/F-treated CFTR oocytes was slightly greater (~40%) than into I/F-treated sham oocytes. Urea uptake into CFTR and sham oocytes was similar and in both cases was increased by I/F. However, the I/F-induced increase in urea uptake into CFTR oocytes was significantly greater than for sham oocytes. I/F stimulated formate uptake into CFTR oocytes but not into sham oocytes. Fluxes of ²²Na, ⁸⁶Rb, ³⁵SO₄, ³²PO₄, and mannitol were unaltered by expression and activation of CFTR.