Inducible nitric oxide synthase is expressed during experimental acute lung allograft rejection

Background: We recently demonstrated that inhibition of nitric oxide (NO) production ameliorated acute pulmonary allograft rejection. This study examined whether inducible NO synthase (iNOS) was expressed in the transplanted lung during acute rejection. Methods: With a rat left lung transplant model, tissue from syngeneic (Fischer 344 to Fischer 344) and allogeneic (Brown Norway to Fischer 344) transplants were harvested on postoperative day 4 and analyzed for iNOS mRNA expression (ribonuclease protection assay), iNOS enzyme activity (conversion of L-[³H]-arginine to NO and L-[³H]-citrulline), and serum nitrite/nitrate levels. Results: The iNOS mRNA was expressed in allograft lungs but was not detected in isografts or controls. The iNOS protein was present in allograft lungs, as demonstrated by high levels of L-[³H]-citrulline production compared with minimal iNOS enzyme activity in isograft and control lungs (10.1 ± 2.4 vs 0.6 ± 0.2 and 0.7 ± 0.2 pmol L-[³H]citrulline · mg^-1 · min^-1, respectively; n = 6, p < 0.001). Allografts had significantly elevated systemic serum nitrite/nitrate levels compared with isografts and controls (38 ± 6 vs 18 ± 2 and 16 ± 1 μmol/L, respectively; n = 6; p < 0.005). Conclusions: These results, together with our previous demonstration that iNOS inhibition ameliorated lung allograft rejection, suggest that (1) iNOS expression and increased NO production contributed to acute rejection of the transplanted lung, (2) iNOS inhibition may offer an alternative in management of acute lung allograft rejection, and (3) increased NO production, detected by the presence of iNOS mRNA or protein or noninvasively by measuring serum nitrite/nitrate levels, may serve as an early marker of acute allograft rejection.