Background. Cyclosporin A (CsA), a potent immunosuppressive agent, can be nephrotoxic. Because clinical studies have suggested that the intrarenal renin-angiotensin system may be involved in the mechanism responsible for CsA nephrotoxicity, we have analysed the effects of CsA on angiotensin II (Ang II) receptors in medullary thick ascending limb (mTAL) cells known to be sensitive to the action of CsA. Methods. Experiments were carried out on subcultured mouse mTAL cells. The expression of mRNA of Ang II subtype 1 and 2 (AT1 and AT2) receptors was investigated using reverse transcription-polymerase chain reaction (RT-PCR). [3H]Ang II was used for radioligand and binding studies. Fluorimetric recordings using the fluorescent dye fura-2/AM were performed to determine the effect of CsA on the intracellular calcium ([Ca2+]i) content of untreated and Ang II-treated mTAL cells. Results. Subcultured mTAL cells expressed AT1 and AT2 Ang II receptor mRNAs, and binding studies revealed that the AT1 receptors were the predominant Ang II receptor subtype (∼90%) in mTAL cells. CsA (100 ng/ml, 24 h) increased (1.7-fold) the number of Ang II receptors (untreated, 315.8; +CsA, 543.6 fmol/mg protein) without altering the KD (untreated, 7.16; +CsA, 7.06nM). CsA also significantly increased the level of [Ca2+]i measured in cultured mTAL cells both in the basal state (-CSA, 72.2; +CsA, 93.4 nM/106 cells) and in the presence of Ang II (-CSA, 97.8; +CsA, 206.3 nM/106 cells). Conclusions. These findings suggest that the increase in Ang II AT1 receptors and [Ca2+]i caused by CsA may be involved in the mechanism(s) responsible for CsA nephrotoxicity.
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