Discussion
The vascular endothelium is located throughout the body and acts as not
only a mechanical barrier between blood and smooth muscle cells but also
as the largest endocrine organ of the body, which can synthesize and
release various vasoactive substances through various mechanisms to
regulate vascular tension[13-15]. It is generally
accepted that endothelial dysfunction promotes the occurrence of
cardiovascular diseases such as hypertension and that the degree of
endothelial injury is directly related to the severity of hypertension.
In patients with hypertension, endothelial cells are damaged due to
elevated blood pressure, which reduces the secretion of NO and other
vasoactive substances, thus leading to endothelial-dependent diastolic
dysfunction, which in turn becomes an initiating factor of various
complications of hypertension, forming a vicious cycle[16]. Severe preeclampsia is a specific kind of
hypertensive disease, and many studies have shown that NO levels in the
serum of patients with preeclampsia are significantly downregulated,
indicating that there are functional changes after vascular endothelial
injury in preeclampsia [17-20].
Boeldt
et al.[21] showed that maternal peripheral
endothelial dysfunction is central to the disease stage of preeclampsia.
A review of endothelial nitric oxide signaling in preeclampsia also
concluded that preeclampsia was a complex obstetric syndrome associated
with maternal vascular dysfunction in which the NO signaling pathway is
a key driver of disease progression and
severity[22]. Our study showed that the vascular
walls of placental arterioles in the SP were thickened, collagen fibers
were increased, arteriole hyaline was changed, and the endothelial cell
layer was obviously damaged. In addition, other results showed that the
relative expression of eNOS in the SP group was significantly
downregulated, indirectly indicating that NO synthesis by endothelial
cells was significantly decreased compared with that in women with
normal term pregnancy. We hypothesize that the decreased expression of
eNOS in the placental arteriole results in a decrease in NO synthesis,
leading to endothelial diastolic dysfunction and vasospasm, both of
which form a vicious cycle and participate in the changes of uterine
placental ischemia and hypoxia and the release of a variety of placental
factors, which enter maternal blood circulation, activate systemic
inflammatory reaction, damage vascular endothelial cells, and eventually
lead to hypertension and various complications, which is consistent with
the classical theory of preeclampsia due to poor
placentation[23]. Since circulating NO
reflects the total activity of all three nitric oxide synthase subtypes
(not just that in endothelial cells) and NO levels are influenced by
dietary intake, different studies have different results, and so we did
not measure blood levels of NO in the two groups, which is where this
experiment was limited.
TRPV1 was originally discovered by researchers to be specifically
activated by capsaicin, which is why it is known as the capsaicin
receptor [24]. As the most studied member of the
TRPV subfamily, TRPV1 was originally identified in the nervous system
and is present in the vagus nerve, trigeminal ganglion and dorsal
ganglion neurons[25]. In recent years, an
increasing number of studies have shown that TRPV1 also plays an
important role in the regulation of cardiovascular disease and is mainly
expressed in cardiomyocytes, smooth muscle cells, vascular endothelial
cells, inflammatory cells and peripheral vascular adipose tissue[26-29]. TRPV1 is a nonselective cationic channel
that can mediate the transmembrane flow of cations dominated by
Ca2+ when ligands bind to the receptor, which can
change intracellular Ca2+ concentrations, activate a
series of intracellular signals and participate in a variety of
intracellular physiological and pathological
processes[26, 30]. The synthesis of NO is closely
related to the increase in intracellular Ca2+.
Ca2+ binding to calmodulin in endothelial cells can
activate eNOS and promote the synthesis and release of NO, thereby
dilating blood vessels and reducing vascular
resistance[31-33]. TRPV1 may play an important
role in the physiological and pathological status of endothelial cells
to maintain normal vascular function and the pathological process of
vascular lesions. It has been confirmed in animal experiments that TRPV1
can mediate coronary artery relaxation in an endothelium-dependent
manner[4], and TRPV1 can stimulate NO synthesis
through different signaling pathways[6, 34]. It is
well known that structural dysfunction of potassium channels can disrupt
the balance of vasoconstriction and the diastole of blood vessels
themselves, increasing vascular tension and eventually leading to the
pathological state of hypertension[35, 36]. The
KATP channel is a potassium ion channel. For a long time, reports on
KATP have mainly focused on vascular smooth muscle cells, and scholars
later showed that KATP in endothelial cells also participates in the
regulation of vascular tone[37]. Studies have
shown that the KATP agonist etacarin can increase Ca2+levels in endothelial cells, which can increase the expression of eNOS,
promote the synthesis and release of NO, and indirectly relax smooth
muscle[8, 9]. Bratz et
al.[4] found that endothelium-dependent dilation
mediated by TRPV1 could be attenuated by iberiotoxin, a selective
inhibitor of Ca2+-activated K channels (BKca),
suggesting that BKca is involved in capsaicin-induced relaxation. In
addition, TEA, a nonspecific potassium channel blocker, also attenuates
TRPV1-mediated endothelium-dependent relaxation, suggesting that
multiple potassium channels are involved in TRPV1-mediated
endothelium-dependent relaxation. Based on this research, our study
showed that TRPV1 and Kir6.1/SUR2B mainly existed in the endothelial
cell layer in human placental arteriolar cells, and the relative gene
and protein expression of TRPV1 and Kir6.1/SUR2B in the SP group was
significantly downregulated compared with that in the NP group. We
hypothesized that there might be some correlation between TRPV1 and the
KATP subtype SUR2B/Kir6.1. To further verify this hypothesis, cell
experiments were carried out. We cultured human umbilical vein
endothelial cells and found that in the agonist and blocker groups, the
relative expression of SUR2B/Kir6.1 and eNOS was also significantly
upregulated and downregulated, respectively, compared with that in the
control group, indicating that activation or inhibition of TRPV1 could
upregulate or downregulate the expression of Kir6.1/SUR2B and eNOS.
Referring to the previous theory, we hypothesize that TRPV1 activation
in endothelial cells activates
Kir6.1/SUR2B by some unknown mechanism, causing hyperdifferentiation of
the endothelial membrane and thereby increasing Ca2+influx into cells. As a nonselective cation channel, TRPV1 can also
mediate the transmembrane flow of cations dominated by
Ca2+ when the ligand binds to the receptor. Through
two possible approaches, Ca2+ is further increased in
endothelial cells, which enhances the expression and activity of eNOS,
thereby increasing the synthesis and release of NO and the
endothelium-dependent vasodilation response. In patients with severe
preeclampsia, the expression of TRPV1 and Kir6.1/SUR2B is reduced due to
endothelial cell damage, and the function of TRPV1 plus KATP and NO is
also impaired, leading to endothelial dysfunction. In addition, NO
diffusion to vascular smooth muscle is correspondently reduced, and
vascular smooth muscle cannot relax properly, causing arteriole spasm in
the body and insufficient blood supply to the placenta, leading to a
series of maternal and infant complications. However, due to the
limitations of experimental conditions, we did not conduct vascular ring
tension tests and additional electrophysiological experiments, which was
a great limitation of this study.