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.