• The Effect of Mineralocorticoid Receptor Antagonism on Ischemic Infarct Size

      Rigsby, Christiné Spring; Department of Physiology (2006-12)
      Stroke is the third leading cause of death and the leading cause of long-term disability in the United States, where approximately 88% of stroke occurrences are ischemic in origin. Hypertension is a primary risk factor for stroke. Elevated aldosterone levels have also been identified as a risk factor for stroke, as patients with primary aldosteronism incur increased incidences of cardiovascularrelated pathologies than do patients with essential hypertension. Previous studies from our laboratory have shown that mineralocorticoid (aldosterone) receptor (MR) activation can induce deleterious vascular remodeling and, conversely, blockade of the MR with spironolactone reduces cerebral infarct size in male spontaneously hypertensive stroke-prone rats (SHRSP). It is known from studies in SHRSP that cerebral vessel structure is directly related to infarct size. We hypothesized that chronic spironolactone treatment would alter cerebral vessel structure. Six-week-old male SHRSP were treated with spironolactone for six weeks and passive vessel structure was analyzed using a pressurized arteriograph. Spironolactone treatment prevented cerebral vessel remodeling. From a clinical standpoint, many patients present with pre-existing vascular damage; therefore, we hypothesized that chronic MR antagonism would reverse existing vascular damage. Twelve-week-old male SHRSP were treated as described above. Interestingly, spironolactone treatment partially reversed existing cerebral vessel remodeling. Recent analysis of data from the Framingham Heart Study show that females may be more sensitive to the effects of aldosterone, but few studies looking at MR blockade have been performed in females. Similar ischemic stroke and vascular analysis studies were performed in 12-week-old female SHRSP. Contrary to the male studies, MR antagonism, using spironolactone or eplerenone, did not reduce damage from ischemic stroke or improve vessel structure. MR protein expression was evaluated in cerebral arteries collected from 12-week-old male and female SHRSP using Western blot analysis. Surprisingly, female SHRSP had increased MR expression, compared to male SHRSP. These novel studies uncover an apparent sexual dimorphism in the actions of MR antagonists and expression of the MR in SHRSP. The action of the MR antagonists may be influenced by differential MR expression and this could help to explain the sex difference observed.
    • The effects of hypertension on neurovascular unit function and structure

      Iddings, Jennifer Ann; Department of Physiology (2015)
      Functional hyperemia is the regional increase in cerebral blood flow upon increases in neuronal activity which ensures that the metabolic demands of the neurons are met. Hypertension is known to impair the hyperemic response; however, the neurovascular coupling mechanisms by which this cerebrovascular dysfunction occurs have yet to be fully elucidated. The goal of this dissertation project was to test the central hypothesis that hypertension-induced impairments in functional hyperemia are mediated by a specific disruption of communication within the neurovascular unit at the parenchymal arteriole level of the cerebrovascular tree. To test our hypothesis, we measured parenchymal arteriole reactivity, vascular smooth muscle cell Ca2+ dynamics, parenchymal arteriole remodeling and cerebral vascular density in cortical brain slices from normotensive (WKY) and hypertensive (SHR) rats. We found that vasoconstriction in response to the thromboxane A2 receptor agonist U46619 and basal vascular smooth muscle cell Ca2+ oscillation frequency were increased in parenchymal arterioles from SHR. In perfused and pressurized parenchymal arterioles, myogenic tone was increased in SHR. While K+-induced parenchymal arteriole dilations were similar in WKY and SHR, metabotropic glutamate receptor activation-induced parenchymal arteriole dilations were enhanced in SHR. Further, neuronal stimulation-evoked parenchymal arteriole dilations were similar in SHR and WKY. Parenchymal arteriole wall to lumen ratio and wall thickness were increased in SHR. Vascular density was also increased in deeper cortical layers in SHR. Our data indicate that although SHR parenchymal arterioles display vascular remodeling, neurovascular coupling is not impaired in SHR, at least at the parenchymal arteriole level.
    • Increased S-nitrosylation Impairs Contraction and Relaxation in Mouse Aorta

      Choi, Hyehun; Department of Physiology (2011-06)
      S-Nitrosylation is a ubiquitous protein modification in redox-based signaling. This modification uses nitric oxide (NO) to forms S-nitrosothiol (SNO) on cysteine residues. Thioredoxin (Trx) and Trx reductase (TrxR) play a role in limiting Snitrosylation. We hypothesized overall that S-nitrosylation of intracellular signaling molecules impairs contraction and relaxation of vascular smooth muscle cells. Aortic rings from C57BL/6 mice were used to measure vascular contraction and relaxation. The rings were treated with TrxR inhibitors, auranofin or 1-chloro-2,4-dinitrobenzene (DNCB), and/or NO donors, propylamine propylamine NONOate (PANOate) or S-nitrosocysteine (CysNO), to increase Snitrosylation. Contractile responses of aortic rings to phorbol-12,13-dibutyrate (PDBu), a PKC activator, were attenuated by auranofin, DNCB, PANOate, and CysNO. PKCa S-nitrosylation was increased by a TrxR inhibitor and CysNO; concomitantly, PKCa activity and downstream signaling were inhibited as compared to control protein. Vascular relaxation in aortic rings from normotensive (Sham) and angiotensin II (Angll)-induced hypertensive mice was measured after contraction with phenylephrine in the presence or absence of DNCB. DNCB reduced relaxation to acetylcholine (ACh) compared to vehicle, but the antioxidants, apocynin and tempol, normalized DNCB-induced impaired relaxation to ACh in sham aorta. Soluble guanylyl cyclase (sGC) S-nitrosylation was increased by DNCB, and sGC activity (cyclic GMP assay) was reduced in sham aorta. In aortic rings from Angll-treated mice, DNCB did not change relaxation to ACh compared to vehicle. DNCB decreased relaxation to sodium nitroprusside (SNP) in aortic rings from both sham and Angll mice. Total protein S-nitrosylation was enhanced in Angll aorta compared to sham, and TrxR activity was inhibited in Angll aorta compared to sham. These data suggest that PKC is inactivated by S-nitrosylation and this modification inhibits contractile responses to PDBu. TrxR inhibition reduces vascular relaxation via increasing oxidative stress and sGC S-nitrosylation. In Angll-induced hypertensive mice, augmented S-nitrosylation is associated with impaired vasodilation. Thus, TrxR and Snitrosylation may provide a critical mechanism in hypertension associated with abnormal vascular reactivity.
    • The Mechanisms Underlying VLDL-Induced Aldosterone Production

      Tsai, Ying-Ying; Department of Physiology (2014-03)
      Aldosterone is responsible for sodium retention, thus increased blood volume and pressure. Excessive production of aldosterone results in high blood pressure, as well as renal disease, stroke, and visual loss via its effects on blood pressure. Although weight gain is associated with increased blood pressure, it remains unclear how excess fat deposits increase blood pressure. Indeed, overweight and obesity issues are correlated with serious health risks. In addition to hypertension, obese patients typically have high lipoprotein levels; moreover, some studies have suggested that aldosterone levels are also elevated and represent a link between obesity and hypertension. Very low density lipoprotein (VLDL) functions to transport triglycerides from the liver to peripheral tissues. Previous studies have demonstrated that VLDL can stimulate aldosterone production. By analogy with the signaling pathways activated by Angll, including the finding that VLDL increases cytosolic calcium levels, here we show that both phospholipase C (PLC) and phospholipase D (PLD) are involved in VLDL-induced aldosterone production. The effects of VLDL on steroidogeneses are mediated via an ability of these signaling pathways to result in the induction of steroidogenic acute regulatory (StAR) protein and aldosterone synthase (CYP11B2) expression, the early and late limiting steps in aldosterone biosynthesis, presumably byincreasing the phosphorylation (activation) of their regulatory transcription factors, such as the cAMP response element binding (CREB) protein family of transcription factors.
    • Toll-like receptor 9 contributes to vascular dysfunction in hypertension

      McCarthy, Cameron; Department of Physiology (2016-03)
      Inappropriate immune system activation is common in hypertension; however, the exact mechanisms by which this occurs are not well understood. Innate immune system recognition and response to damage-associated molecular patterns (DAMPs) is becoming an increasingly accepted mechanism. Mitochondrial DNA (mtDNA) is a DAMP that is recognized by Toll-like receptor (TLR)9, and it is elevated in the circulation of spontaneously hypertensive rats (SHR). Therefore, we hypothesized that (1) inhibition of TLR9 in SHR with a TLR9 antagonist (ODN2088) or TLR9 inhibitor (chloroquine) would lower blood pressure and improve vascular function and that (2) treatment of normotensive rats with a TLR9 agonist (ODN2395) would cause vascular dysfunction and increase blood pressure. Both ODN2088 and chloroquine lowered high blood pressure in SHR and treatment with chloroquine also improved cyclooxygenase-dependent endothelial function and prevented the full recruitment of the adaptive immune system in SHR. On the other hand, treatment of normotensive rats with ODN2395 increased blood pressure and rendered their arteries less sensitive to acetylcholine-induced relaxation and more sensitive to norepinephrine-induced contraction. This dysfunctional vasoreactivity was due to cyclooxygenase activation, increased reactive oxygen species generation, and reduced nitric oxide bioavailability. In conclusion, these findings support the involvement of the innate immune system pattern recognition receptor TLR9 in the pathogenesis and maintenance of hypertension. Specifically, circulating mtDNA may activate TLR9 and contribute to high blood pressure and endothelial dysfunction in SHR.