Introduction
Hypertensive disorders of pregnancy such as preeclampsia and gestational hypertension are associated with a higher risk of cardiovascular disease for both mother and offspring in later life. Women with these complications during pregnancy have a 4-fold increased risk of developing hypertension (Bellamy, Casas, Hingorani & Williams, 2007; McDonald, Malinowski, Zhou, Yusuf & Devereaux, 2008) and a 2-fold increased risk of developing stroke after pregnancy (Bellamy et al., 2007). Offspring from mothers with hypertensive disorders of pregnancy are also likely to have a higher risk of hypertension and other cardiovascular disease in later in life (Ferreira, Peeters & Stehouwer, 2009; Kajantie, Eriksson, Osmond, Thornburg & Barker, 2009).
The maternal cardiovascular system must undergo significant remodelling to support the growing fetus. By term, the uteroplacental bed receives more than 12% of cardiac output. In order to accommodate this increase in flow, vascular resistance is reduced by outward remodeling and vasodilatation of more proximal uterine arteries, together with trophoblast invasion and remodelling of spiral arteries (Kelly, Stone & Poston, 2000). Failure of this vascular adaptation is associated with adverse outcomes including pregnancy induced hypertension and pre-eclampsia. Therefore, understanding the mechanisms underlying vascular adaptations and how they impact blood pressure regulations during pregnancy are of great importance.
Nitric oxide (NO), generated by endothelial nitric oxide synthase (eNOS), is a key mediator of vascular adaptation in pregnancy. During pregnancy, enhanced uteroplacental vasodilation is driven by increased NOS-derived NO in humans, mice, and rats (Anumba, Robson, Boys & Ford, 1999; Miller, Jenkin & Walker, 1999; Sladek, Magness & Conrad, 1997; Williams, Vallance, Neild, Spencer & Imms, 1997). Increased uterine artery calibre is associated with both enhanced eNOS expression, activity, and NO bioavailability (Cooke & Davidge, 2003; Nelson, Steinsland, Wang, Yallampalli, Dong & Sanchez, 2000; Sullivan, Grummer, Yi & Bird, 2006) (Dimmeler, Fleming, Fisslthaler, Hermann, Busse & Zeiher, 1999; Kublickiene, Cockell, Nisell & Poston, 1997). Loss of eNOS leads to impaired uterine artery remodelling during pregnancy mediated in part by a reduced activation of matrix metalloproteinase in eNOS- knockout mice (van der Heijden, Essers, Fazzi, Peeters, De Mey & van Eys, 2005). NO bioavailability is also central to physiological adaptation in the more distal uteroplacental circulation with increased expression of eNOS observed in remodelling spiral arteries and in cytotrophoblasts and syncytiotrophoblasts (Lyall, Bulmer, Kelly, Duffie & Robson, 1999). However, simple strategies attempting to restore or augment NO with NO donors in pre-eclampsia have been disappointing (Meher & Duley, 2007). One possible explanation for these results is the failure to specifically target eNOS uncoupling and consequently altered NO/ROS signalling. To date, there is less certainty regarding the role of eNOS coupling and NO bioavailability in the uterine circulation in pregnancy induced hypertension and pre-eclampsia.
The generation of NO by eNOS requires the small molecule tetrahydrobiopterin (BH4). Loss of BH4 results in eNOS uncoupling, loss of NO generation and increased production of reactive oxygen species (ROS). Biosynthesis of BH4 is catalyzed by GTPCH (GTP cyclohydrolase 1, encoded by Gch1 , the rate-limiting enzyme for de novo BH4 biosynthesis. We have previously shown that Gch1 expression is a key determinant of BH4 bioavailability, NOS regulation and NO and superoxide generation (Crabtree et al., 2009; Vasquez-Vivar, Martasek, Whitsett, Joseph & Kalyanaraman, 2002). Recently, we have demonstrated that maternal endothelial cell BH4 deficiency, due to loss of Gch1 , leads to progressive hypertension and fetal growth restriction during pregnancy (Chuaiphichai et al., 2021). However, the mechanism mediating these changes and how loss of endothelial cell-specific Gch1 alters uterine artery function both before and during pregnancy and if these changes are reversable are yet to be answered. To address this question, we assessed vascular function and blood pressure in pregnant and non-pregnant endothelial cell specific Gch1 knock out mice.