Selective inhibition of nitric oxide production during cardiac allograft rejection causes a small increase in graft survival.

Nitric oxide production is increased in allograft rejection and may have both beneficial and deleterious effects on graft function and survival. In animal models, conventional immunosuppressive agents have been shown to decrease nitric oxide production. The aim of our study was to determine what effect augmentation and selective inhibition of nitric oxide production may have on graft survival by using the model of heterotopic cardiac transplantation in the rat. L-Arginine, the naturally occurring substrate for nitric oxide production, was administered subcutaneously at 200 mg/kg/day. L-NG-monomethyl-L-arginine (L-NMMA) is a selective inhibitor of nitric oxide synthase and was administered at 500 mg/kg/day to allograft recipients from the day of operation. Endogenous nitric oxide production was quantified by analysis of urinary nitrate excretion, and time to rejection was determined by graft palpation. L-Arginine did not significantly alter urinary nitrate excretion by iso- or allografts, suggesting that nitric oxide production is not a substrate-limited process in this model. Graft survival in this group was unchanged. L-NMMA produced a small increase in graft survival from 5.1 +/- 0.1 to 6.3 +/- 0.3 days compared with control allografts (P = 0.001) and abolished the rise in urinary nitrate excretion seen with control allografts. Lower doses of L-NMMA produced dose-related decrements in urinary nitrate excretion, but did not alter graft survival. We found that allograft rejection can proceed to graft loss despite complete inhibition of the increase in nitric oxide production that occurs during untreated rejection. The small increase in graft survival suggests that nitric oxide plays a minor role as a cytotoxic effector molecule in this model of acute rejection.