Induction and role of NO synthase in hypotensive hepatic failure

Nitric oxide (NO) plays an important role in the physiological and pathophysiological control of the vascular system. NO is synthesized by isoforms of the enzyme NO synthase (NOS). Hepatic failure is complicated by hypotension, low systemic vascular resistance, and resistance to vasoconstrictor drugs. The potential role of NO in these abnormalities was investigated by using in vitro pharmacological interventions on hepatic arteries obtained from both donor and recipient patients at the time of liver transplantation. The presence of NOS mRNA was investigated by reverse transcription polymerase chain reaction (RT-PCR) with primers designed from human endothelial NOS (eNOS) and inducible NOS (iNOS) cDNA sequences. Arteries from patients with hepatic failure had an impaired constrictor response to phenylephrine compared with those of donor arteries. The constrictor effect of phenylephrine was potentiated by NG-monomethyl-L-arginine, an inhibitor of NOS, which had no effect in donor control arteries. RT-PCR identified human eNOS mRNA in donor and recipient arteries and human iNOS mRNA in recipient arteries only. Induction of NOS in the vasculature with subsequent NO-induced vasodilatation may therefore contribute to the hemodynamic abnormalities observed in hepatic failure and potentially in other pathologies associated with endotoxemia. Whether selective inhibitors of iNOS will improve hemodynamic control or clinical outcome in these conditions requires further study.