Effects of cyclosporin A on the synthesis, excretion, and metabolism of endothelin in the rat

Increasing evidence suggests that endothelin, a potent vasoconstrictor, is implicated in cyclosporin A (CsA)-induced nephrotoxicity. Increased levels of urinary and circulating endothelin have been described in CsA-treated humans and animals. The exact mechanisms by which CsA induces these increases are still unknown, and no data indicate whether these elevated levels reflect increased synthesis or decreased clearance of endothelin. In the present study, we investigated the effects of CsA administration (50 mg/kg per day i.p. for 6 days) to rats on plasma and urinary levels of endothelin; expression of endothelin-1 (ET-1), ET-3, and endothelin-converting enzyme in renal tissue; clearance of infused 125I-ET-1; and degradation of 125I-ET-1 by recombinant neutral endopeptidase. Rats given CsA for 6 days developed severe renal insufficiency, as shown by a 74% decrease in creatinine clearance rate (Ccr) (P < .006). Ccr was remarkably improved in CsA-treated rats that received bosentan, the combined antagonist of both endothelin A and endothelin B receptors. Urinary excretion of endothelin increased from an undetectable level to 31.7 +/- 6.0 pg/24 h (P < .001), and plasma levels of endothelin were unchanged (2.8 +/- 0.2 to 3.1 +/- 0.2 pg/mL). Reverse transcription followed by quantitative polymerase chain reaction revealed that ET-1 mRNA in the renal medulla increased by 59% (P < .006), whereas the expression of both ET-3 and endothelin-converting enzyme was unchanged. In other rats, neither acute nor chronic treatment with CsA affected either the clearance of 125I-ET-1 from the blood or the renal and pulmonary uptake of the peptide. Moreover, CsA did not affect the degradation of 125I-ET-1 by highly purified recombinant neutral endopeptidase, a well-known endothelinase. Taken together, these data suggest that the elevated urinary endothelin levels obtained after CsA treatment originate from the kidney and reflect increased renal synthesis of ET-1. Moreover, the production of endothelin appears to be regulated at the mRNA transcription level, and expressions of ET-1 and ET-3 are regulated independently.