The properties of a protein-kinase-A(PKA)-activated Cl--conductive pathway( s) in alkaline phosphatase-enriched microsomes from the rat inner medulla (IMV) were investigated. Transcripts of cystic fibrosis transmembrane regulator (CFTR) were detected by reverse transcription/polymerase analysis of total RNA from the inner medulla, while immunoblot analysis using anti-CFTR antibodies detected a 170-kDa protein in the IMV. The PKA C1--conductive pathway(s) was studied by measuring the rate of valinomycin-induced microsomal swelling by light scattering. PKA increased the rate of valinomycin-induced swelling of vesicles consistent with the presence of C1--conductive pathway(s). The pharmacological properties and anion selectivity of the PKA-activated Cl--conductive pathway(s) were similar to those of the CFTR CI-channel. Our results show that a CFTR C1 channel and possibly another CAMP-activated pathway(s) may participate in Cl-secretion in the rat inner medulla.Keywords: kidney; inner medulla; cystic fibrosis transmembrane regulator; chloride conductance; chloride channel.The main function of the tubules in the kidney inner medulla, which are mostly collecting ducts (IMCD), is to modify the composition of urine before it leaves the kidney [l], and thus control the concentration of renal excretion. Most functional investigations of IMCD have focused on sodium absorption [2, 31, but recent studies suggest that chloride secretion may also contribute to the regulation of final urine composition and volume [4]. Studies on isolated perfused IMCD from the rat have also shown that CAMP stimulates C1-se.cretion [4-61. It has been suggested that the cystic fibrosis transmembrane regulator (CFTR) chloride channel mediates the electrogenic chloride secretion in mouse IMCD-K2 cells 171 and rat primary cultures of IMCD cells [8]. This is supported by the presence of transcripts for CFTR protein along the nephron [9], and a protein immunologically related to CFTR in the proximal and distal tubules of the human kidney [lo]. To add to the evidence for the presence of a functional CFTR in the IMCD, we performed the present study on microsomes from rat inner medulla (IMV). The goal was to determine whether the CAMP-activated CI-. fluxes in IMV have pharmacological features and anion selectivity similar to those of the CFTR Cl-channel.Reverse transcription, (RT)-PCR, and western blot analysis indicated that CFTR protein is expressed in the inner medulla of the rat kidney. The functional assay based on valinomycininduced vesicle swelling provided evidence that protein-kinase-A (PKA) regulated CI--conductive pathways are present in the inner medulla vesicles (IMV) and that a protein with the pharniacological properties of the CFTR-C1-channel is involved in the regulation of one of these pathways. MATERIALS AND METHODSMaterials. ATP, PKA, valinomycin, phenylmethylsulfonyl fluoride (PhMeSO,F), leupeptin, pepstatin A, Hepes, Tris, 4 acetamido-4'-isothiocyanato-stilbene-2,2'-disulfonic acid, BSA fraction V, and Ponceau S were obtained froin...