A POSSIBLE ROLE FOR NITRIC-OXIDE IN MODULATING THE FUNCTIONAL CYCLOSPORINE TOXICITY BY ARGININE

The renal damage consequent to cyclosporine A (CsA) administration ranges from hemodynamic alterations td irreversible chronic lesions. The initial vasoconstriction depends upon the imbalance between the various modulators of the renal vascular tone, among which the most powerful are endothelins and nitric oxide (NO). CsA could play a crucial role by inhibiting the Ca++/calmodulin-mediated activation of the constitutive NO synthase (NOS) isoform, which converts L-arginine (L-Arg) into NO and citrulline, with a 1:1 stoichiometry. To investigate the possibility of modulating CsA nephrotoxicity with L-Arg we studied six groups (G) of Lewis rats treated with daily gavage up to eight weeks: G1, CsA 40 mg/kg; G2, G1 plus L-Arg 300 mg/kg; G3, G2 plus the competitive inhibitor of NOS, N-G-nitro-L-Arg (L-NNA); G4, L-Arg alone; G5, L-NNA alone; and G6, controls receiving vehicle alone. After eight weeks L-Arg treated rats were protected against the toxic effects of CsA [creatinine (Cr) values, G2, 0.62 +/- 0.05 mg/dl vs. G1, 0.99 +/- 0.16 mg/dl, P < 0.001; proteinuria (P), G2, 7.2 +/- 1.02 mg/day vs. G1, 15.1 +/- 1.9 mg/day, P < 0.01]. The administration of L-NNA abolished the protective effect of L-Arg (G3, Cr 1.23 +/- 0.16 mg/dl; P 16.9 = 2.3; P < 0.02 and P < 0.005, respectively vs. G2). The levels of Cr in G2 rats were superimposable to control groups. The NOS activity, evaluated by measuring [H-3]citrulline formation from [H-3]L-Arg in kidney homogenates, was blocked by L-NNA in G5 (0.019 +/- 0.009 pmol/min/mg proteins vs. G6 0.047 +/- 0.002 P < 0.01). NOS activity was significantly increased versus controls in G1 (0.110 +/- 0.032, P < 0.01) and G2 (0.088 +/- 0.009, p < 0.02), while L-NNA reversed this phenomenon (G3, 0.052 +/- 0.03). The expression of mRNA encoding for cNOS and iNOS was only slightly increased in CsA-treated rats. We suggest that CsA treatment increases NOS activity in the kidney by a mechanism which does not require a de novo synthesis of the enzyme. Such an increase, that may be devoted to counterbalance the vasoconstrictive effects of the drug, is unable to reduce the toxic effect of CsA in the absence of exogenous L-Arg. Administration of L-Arg is likely to reduce CsA nephrotoxicity by accomplishing the higher request of activated NOS for its substrate, thus potentiating NO synthesis in the kidney.(1)