Cyclosporine stimulates Na⁺-K⁺-Cl^- cotransport activity in cultured mouse medullary thick ascending limb cells

Background. Cyclosporine (CsA) has been shown to alter the activity of plasma membrane transporters in kidney epithelial cells. In this study, we have investigated the effects of CsA on Na⁺,K⁺-ATPase and Na⁺-K⁺-Cl^- cotransport activities in cultured cells derived from microdissected mouse medullary thick ascending limb (mTAL) cells. Methods. Experiments were carried out on subcultured confluent mouse TAL cells. Reverse transcription-polymerase chain reaction experiments showed that they expressed the mNKCC2 electroneutral Na⁺-K⁺-Cl^- cotransporter and ROM-K1 and ROMK2 potassium channel mRNA. Western blotting also revealed the presence of the 40 kD ROMK protein using an anti-ROMK antibody. The effect of CsA (100 ng/mL) on ion transport was assessed by measuring the influx and efflux of rubidium (⁸⁶Rb⁺) and ³⁶Cl^-, used as tracers of K⁺ and Cl^- movements, on cells grown on Petri dishes or permeable filters. Results. CsA inhibited by 38% the ouabain-sensitive component of ⁸⁶Rb⁺ influx mediated by the Na⁺,K⁺-ATPase pumps. CsA also increased by 38% the ouabain-resistant furosemide-sensitive component (Or-Fs) of ⁸⁶Rb⁺ influx, reflecting the Na⁺-K⁺-Cl^- cotransport activity and stimulated the basolateral efflux of ³⁶Cl^- from mTAL cells grown on filters. The CsA-stimulated basal efflux of Cl^- was prevented by the basal addition of the Cl^- channel blocker 5-nitro-2-(3-phenylpropylamino) benzoate (NPPB, 10^-4 mol/L). Apical addition of the K⁺ channel blocking agent Ba²⁺ (10^-4 mol/L) partially prevented the CsA-stimulated basal efflux of Cl^-. Adding Ba²⁺ to the luminal side of cells grown on Petri dishes also prevented the rise in apical ⁸⁶Rb⁺ efflux and the increased Or-Fs component of ⁸⁶Rb⁺ influx caused by CsA. Conclusion. These results indicated that CsA may stimulate the Na⁺-K⁺-Cl- cotransport activity and also suggested that this immunosuppressive agent may interfere in the recycling of apical K⁺ in this model of cultured mouse TAL cells.