**2. Nitric oxide and uteroplacental circulation**

The endothelial cells in the uteroplacental circulation play an important physiological role in the maintenance of vasodilation of placental vessels, since these are not innervated. These endothelial cells produce prostacyclin and nitric oxide, causing vasodilation and also preventing platelet aggregation and platelet adhesion to endothelial cells [19].

 Nitric oxide (NO) regulates implantation and trophoblastic invasion as well as embryonic development [20]. In addition, vascular tone in the placenta is controlled by several vasoactive mediators, with NO being the most important [21].

Nitric oxide participates at the onset of placental vasculogenesis. The onset of vasculogenesis requires the expression of vascular endothelial growth factor (VEGF), the mitogenic effects of which are mediated by nitric oxide. There is no well-established level of NO required for adequate placental angiogenesis. Elevated levels of NO may prevent angiogenesis, and its effect on cell survival and proliferation depends on its concentration [22].

NO has an important role in facilitating pregnancy-induced expansive remodeling in the uterine circulation, especially in the larger arteries [7].

The NO signaling has an important role in the expansive circumferential gestational remodeling of the uterine circulation. It provides an interesting link to the theory that preeclampsia results from elevated levels of sFlt-1, a soluble receptor for vascular endothelial growth factor and placenta growth factor, in preeclamptic women [23].

An excess of soluble receptor would reduce the availability of these ligands to the maternal vascular wall and fetal growth retardation. The sFlt-1, when infused in pregnant rats, promotes glomerular proteinuria and endotheliosis, characteristics of the preeclampsia picture [24].

 Since both placenta growth factor and vascular endothelial growth factor stimulate endothelial NO release, a reduction in their signaling would create a vasoconstrictor imbalance and increase peripheral resistance and blood pressure. Thus, a reduction in NO signaling also impacts vessel remodeling in a way that would further increase uterine vascular resistance. This effect on structure, combined with loss of function (vasodilation), would further mitigate the increases in uterine [7].

In addition to the decrease in the synthesis of nitric oxide in the uteroplacental circulation in preeclampsia, the endothelium-dependent relaxation in response to acetylcholine is impaired in preeclamptic arteries. Additionally, the increased plasma fibronectin levels in preeclampsia may reflect fibronectin which has been shed by injured endothelial cells. Furthermore, soluble circulating endothelial cell adhesion molecules such as soluble intercellular adhesion molecule-1 (sICAM-1), soluble vascular adhesion molecule-1 (sVCAM-1), and sE-selectin are significantly increased in preeclampsia compared to normal pregnancies. This suggests that there is an altered and pathological endothelial phenotype in preeclampsia [25].

## **3. Animal models for the study of nitric oxide during pregnancy**

Animal models using rats or mice are very useful for the study of the pathogenesis, diagnosis, and treatment of preeclampsia. N-Nitro-L-arginine methyl

ester (L-NAME) is an inhibitor of NO synthase, and it has been shown to promote arterial hypertension in pregnant rats [26, 27].

 The administration of L-NAME in adult rats, in addition to causing hypertension [28], promotes cardiac and aortic tissue damage [29], proteinuria, and glomerular endotheliosis [30].

Several animals have already been used as models of experimental hypertension, such as rhesus monkeys, dogs, and sheep. Most of the experimental studies use rats and mice. In these animals, four categories of preeclampsia are produced: (i) animals with surgically induced reduced uteroplacental blood flow, (ii) animals with preclinical symptoms induced by drugs, (iii) genetic animal models, and (iv) animals with preexisting hypertension developing preeclampsia [31].

 Few studies address the effects of inhibition of nitric oxide on fetal development. Most of the work on this subject is from the 1990s. In pregnant rats, this nitric oxide synthesis inhibitor causes fetal growth restriction by a reduction in cellular proliferation due to induction of apoptosis [32], reducing the body weight and causing hemorrhagic necrosis of neonate's hind limbs [33, 34].

This suggests that L-NAME crosses the placental barrier and affects the fetal NO synthesis, leading to cell death in the limbs because the NO has a role in limb and digit developments [35]. Reactive oxygen species (ROS) formation by L-NAME induces hemorrhages, oxidative stress, and limb reduction defects [30].

The administration of L-arginine, the precursor of nitric oxide, in mice during gestation promoted an increase in fetal weight presumably due to the contribution of NO in improving fetal-maternal circulation by vasodilation and subsequently increased blood volume and viscosity in the fetal-maternal circulation [36].

## **4. Effects of inhibition of nitric oxide on fetal heart development in animal models**

 The first studies on the importance of nitric oxide in cardiac development go back to the beginning of the year 2000. The role of nitric oxide on fetal cardiovascular development is only partially known. In addition, in women who had preeclampsia, their children are at greater risk of developing cardiovascular disease later in life [37].

The nitric oxide probably contributes to the transformation of the epitheliummesenchyme in the areas of the endocardial cushion, myocardial survival and angiogenesis, and myocardial remodeling. Impaired production of NO in the heart leads to structural congenital abnormalities, resulting in heart failure and increased mortality [38].

Inhibition of nitric oxide during cardiac development is known to promote bicuspid aortic valve defects [39], congenital septal defects, and increase in cardiomyocyte apoptosis [40].

Apoptosis occurs in situations of cardiac remodeling during or after pathological processes and was observed in the myocardium of newborns from rats with hypertension induced by L-NAME. Thus, apoptosis can also occur in postnatal maturation of the heart and other tissues of the cardiovascular system, which need to adapt to the new hemodynamic role [41].

In newborns from L-NAME mothers, the most significant change in the myocardium involved the microvasculature. The wall/lumen ratio of arterioles was significantly higher in neonates of L-NAME and spontaneously hypertensive rats (SHR) than of normotensive mothers at 2 and 15 days postnatal [42].

It is possible that the myocardial vascular changes induced by the blockade of nitric oxide synthesis in rats are due to the activation of the local angiotensin

### *Maternal and Fetal Complications Due to Decreased Nitric Oxide Synthesis during Gestation DOI: http://dx.doi.org/10.5772/intechopen.85383*

I-converting enzyme (ACE). Takemoto et al. [43] found an increase in ACE in the coronary arteries and increase of the wall/lumen ratio in the myocardial vasculature of adult rats treated with L-NAME.

A decreased NO generation induces the synthesis of growth-promoting factors from the endothelium. The ACE activation would increase the formation of angiotensin II, which in turn directly induces vascular smooth muscle proliferation [44].

 Possibly, the factors involved in hyperplasia/hypertrophy of the smooth muscle cells of the microvasculature of newborns born to hypertensive mothers who received L-NAME during pregnancy are activation of the renin-angiotensin system and activation of the sympathetic nervous system that contributes to the remodeling of intramyocardial vessels [42].

In rats treated with L-NAME, the blood pressure increases via the reninangiotensin system, and, therefore, angiotensin II can promote the narrowing of the lumen of the microvasculature [45].

In neonates of hypertensive rats induced by L-NAME, in addition to cardiac microvasculature being affected, the pyloric musculature is also compromised, observing hypertrophy and hyperplasia of smooth muscle cells [46].

The rat offspring from L-NAME parents, with sustained NO-induced hypertension, had a remarkably higher blood pressure [47]. In addition to impairment in cardiovascular development, there are other damages in the offspring of rats treated with L-NAME as in the hippocampus, affecting cognitive and learning abilities [48].
