The role of arginine, homoarginine and nitric oxide in pregnancy

Normal pregnancy leads to profound maternal hemodynamic changes, including increased blood volume and vasodilatation. Several vasodilator mediators are implicated, including prostaglandins, carbon monoxide and nitric oxide (NO). Pre-eclampsia (PE) affects 3-10 % of pregnancies and is associated with increased maternal and perinatal morbidity and mortality. Around 8 % of pregnancies are complicated by intra-uterine growth restriction (IUGR), also associated with increased perinatal mortality and morbidity. PE and IUGR often co-exist. NO is essential for the formation of healthy endothelium, and in pregnancy promotes endovascular invasion by the cytotrophoblast. As interstitial trophoblasts invade the maternal spiral arteries in the uterine wall, they produce NO which acts on artery walls to create a low-resistance, high-caliber uteroplacental unit. If this process fails, the result is a high-resistance uteroplacental circulation. The hypoperfused and ischemic placenta releases antiangiogenic factors which mediate generalized endothelial dysfunction, oxidative stress and inflammatory mediators. It is these mediators that are implicated in both the fetal and maternal syndromes of PE and IUGR. Studies of NO and its modulator amino acids, including the precursors arginine and homoarginine and the NO synthesis inhibitor asymmetric dimethylarginine (ADMA), have investigated their role in both normal and pathological pregnancies. Many studies of PE (and, to a lesser extent, IUGR) have investigated maternal circulating ADMA, arginine and homoarginine levels. This article reviews and discusses the role of these amino acids in pregnancy. The results have shed some light on their role in these pathologies, but some of the findings have been conflicting and more research is needed. Nevertheless, therapeutic interventions that manipulate these guanidine-amino acids and their interactions hold real promise for the management of pregnancies complicated by PE and/or IUGR, and the results of ongoing studies are eagerly awaited.