The effects of the immunosuppressant drugs cyclosporin A and tacrolimus (FK506) on nitric oxide synthesis were examined in a murine macrophage cell line (J774) and rat vascular smooth muscle cells (VSMC) in culture for 24 and 48 h, respectively.

Cyclosporin A (0.01–10 μM) inhibited by up to 90% accumulation of nitrite induced by lipopolysaccharide (LPS) in both cell lines, but FK506 (0.01–10 μM) had a weaker effect on nitrite accumulation in these cells. Cyclosporin A and FK506 (at 1 μM) also significantly inhibited nitrite production induced by recombinant murine interferon‐γ (rIFNγ) and recombinant murine interleukin‐1β (rIL‐1β) in J774 and VSMC, respectively.

In J774 cells, cyclosporin A (but not FK506) at 1 μM was inhibitory when co‐incubated with the inducing agents but not when the cells were treated with the immunosuppressant before or after the inducer. In VSMC, nitrite production was inhibited by co‐incubation of cyclosporin A or FK506 with the inducer, or when the immunosuppressants were pre‐incubated with cells. In contrast, N‐monomethyl L‐arginine (NMMA) abolished nitrite production when incubated with either cell type during or after addition of inducing agent, but not if cells were preincubated with NMMA.

RNA extracted from treated J774 and VSMC was subjected to reverse transcription–polymerase chain reaction (RT–PCR). Cyclosporin A, but not FK506, suppressed expression of mRNA for NOS2 in a concentration‐dependent manner when co‐incubated with LPS.

The fact that the potency difference between cyclosporin A and FK506 for NO suppression is the opposite to that for inhibition of interleukin‐2 generation suggests that the immunosuppressants act in J774 macrophages and VSMC through intracellular mechanisms that differ from those elucidated in T‐cells. Cyclosporin A suppresses NOS2 gene transcription, but FK506 acts post‐transcriptionally to suppress NO generation in VSMC.

Taken together the present data suggest that therapeutic concentrations of cyclosporin A, but not FK506, might well suppress NO production, but FK506 would not have this effect. Suppression of NO might contribute to the side effects of hypertension and nephrotoxicity associated with long‐term use of cyclosporin A to prevent transplant rejection.