Online

O'Brien, Olivia Mae

Electrical and Biomedical Engineering

2023

M.S.

Stroke in pregnancy is a global leading cause of maternal morbidity and mortality. Pregnancy is a hypercoagulable state characterized by changes in the levels of hemostatic factors, including prothrombin and thrombin, starting at the beginning of gestation. While this hypercoagulability evolved to help the body face the hemostatic challenge of delivery, it can increase the risk of prothrombotic complications and cardiovascular conditions, such as stroke. Pregnant women have a 3-fold greater risk of stroke compared to nonpregnant age-matched people. In the presence of preeclampsia (PE), stroke risk increases to 6-fold. PE is a serious hypertensive disorder of pregnancy associated with new-onset hypertension and extensive multi-organ system dysfunction. PE complicates 6 - 8% of pregnancies worldwide and is the second leading cause of maternal morbidity and mortality in the United States. This disorder further heightens the already hypercoagulable state of pregnancy. Thrombin is a key coagulatory protein that works to arrest bleeding by cleaving fibrinogen into fibrin. However, thrombin is also known to be vasoactive via protease-activated receptors (PARs). PAR types 1 and 2 are found in vasculature throughout the body and can have impacts on vascular tone. In physiological conditions, endothelial cells primarily mediate the vascular effects of PARs. Yet PARs in vascular smooth muscle cells can be induced under pathological conditions and elicit vasoconstriction. In stroke, additional vasoconstriction could increase cerebrovascular resistance and ultimately increase hypoxia and ischemia, worsening neuronal injury in critical brain regions. To investigate thrombin in the brain and cerebrovasculature in an experimental model of preeclampsia (ePE), this study sought to determine the reactivity of brain parenchymal arterioles (PAs), the expression of PARs in those blood vessels, and the levels of prothrombin and thrombin in circulating plasma and cerebrospinal fluid (CSF). PAs from nonpregnant (NP), healthy late-pregnant (LP), and ePE rats were isolated and pressurized within an arteriograph system to study their reactivity to thrombin. PAR expression in the smooth muscle cells of PAs were examined via Western blot and immunohistochemistry. Prothrombin and thrombin levels in plasma and CSF were assessed via enzyme-linked immunoassays. This study found that thrombin was elevated 12- and 14-fold in plasma from rats with ePE compared to NP and LP rats, respectively. Prothrombin was found in ePE plasma, whereas NP and LP prothrombin levels were undetectable. In CSF, thrombin was found in all groups and was elevated significantly in the LP CSF (NP: 0.137 ± 0.014 ng/mL; LP: 0.241 ± 0.015 ng/mL, p < 0.05; ePE: 0.192 ± 0.028 ng/mL). In isolated PAs, thrombin produced modest vasoconstriction that was similar between groups and appeared endothelium-independent. PARs 1 and 2 were found expressed on PAs, which could mediate thrombin's vasoactivity. This study demonstrates the presence of thrombin in brains from NP, LP, and ePE animals, as well as thrombin receptors on cerebral intraparenchymal vessels that were modestly reactive to thrombin. The presence of thrombin in CSF suggests its expression and secretion by cells within the brain, the role of which requires further investigation. Together, the results of this study highlight that thrombin plays not only a coagulatory but a vasoactive role in cerebrovasculature during pregnancy and PE, and may contribute to maternal stroke outcome.