NFAT regulation of cystathionine γ-lyase expression in endothelial cells is impaired in rats exposed to intermittent hypoxia.

Sleep apnea is a risk factor for cardiovascular disease, and intermittent hypoxia (IH, 20 episodes/h of 5% O2-5% CO2 for 7 h/day) to mimic sleep apnea increases blood pressure and impairs hydrogen sulfide (H2S)-induced vasodilation in rats. The enzyme that produces H2S, cystathionine γ-lyase (CSE), is decreased in rat mesenteric artery endothelial cells (EC) following in vivo IH exposure. In silico analysis identified putative nuclear factor of activated T cell (NFAT) binding sites in the CSE promoter. Therefore, we hypothesized that IH exposure reduces Ca2+ concentration ([Ca2+]) activation of calcineurin/NFAT to lower CSE expression and impair vasodilation. In cultured rat aortic EC, inhibiting calcineurin with cyclosporine A reduced CSE mRNA, CSE protein, and luciferase activity driven by a full-length but not a truncated CSE promoter. In male rats exposed to sham or IH conditions for 2 wk, [Ca2+] in EC in small mesenteric arteries from IH rats was lower than in EC from sham rat arteries (Δfura 2 ratio of fluorescence at 340 to 380 nm from Ca2+ free: IH = 0.05 ± 0.02, sham = 0.17 ± 0.03, P < 0.05), and fewer EC were NFATc3 nuclear positive in IH rat arteries than in sham rat arteries (IH = 13 ± 3, sham = 59 ± 11%, P < 0.05). H2S production was also lower in mesenteric tissue from IH rats vs. sham rats. Endothelium-dependent vasodilation to acetylcholine (ACh) was lower in mesenteric arteries from IH rats than in arteries from sham rats, and inhibiting CSE with ß-cyanoalanine diminished ACh-induced vasodilation in arteries from sham but not IH rats but did not affect dilation to the H2S donor NaHS. Thus, IH lowers EC [Ca2+], NFAT activity, CSE expression and activity, and H2S production while inhibiting NFAT activation lowers CSE expression. The observations that IH exposure decreases NFATc3 activation and CSE-dependent vasodilation support a role for NFAT in regulating endothelial H2S production.NEW & NOTEWORTHY This study identifies the calcium-regulated transcription factor nuclear factor of activated T cells as a novel regulator of cystathionine γ-lyase (CSE). This pathway is basally active in mesenteric artery endothelial cells, but, after exposure to intermittent hypoxia to mimic sleep apnea, nuclear factor of activated T cells c3 nuclear translocation and CSE expression are decreased, concomitant with decreased CSE-dependent vasodilation.

Hydrogen sulfide-induced vasodilation mediated by endothelial TRPV4 channels.

Hydrogen sulfide (H2S) is a recently described gaseous vasodilator produced within the vasculature by the enzymes cystathionine γ-lyase and 3-mercaptopyruvate sulfurtransferase. Previous data demonstrate that endothelial cells (EC) are the source of endogenous H2S production and are required for H2S-induced dilation. However, the signal transduction pathway activated by H2S within EC has not been elucidated. TRPV4 and large-conductance Ca2+-activated K channels (BK channels) are expressed in EC. H2S-induced dilation is inhibited by luminal administration of iberiotoxin and disruption of the endothelium. Calcium influx through TRPV4 may activate these endothelial BK channels (eBK). We hypothesized that H2S-mediated vasodilation involves activation of TRPV4 within the endothelium. In pressurized, phenylephrine-constricted mesenteric arteries, H2S elicited a dose-dependent vasodilation blocked by inhibition of TRPV4 channels (GSK2193874A, 300 nM). H2S (1 µM) increased TRPV4-dependent (1.8-fold) localized calcium events in EC of pressurized arteries loaded with fluo-4 and Oregon Green. In pressurized EC tubes, H2S (1 µM) and the TRPV4 activator, GSK101679A (30 nM), increased calcium events 1.8- and 1.5-fold, respectively. H2S-induced an iberiotoxin-sensitive outward current measured using whole cell patch-clamp techniques in freshly dispersed EC. H2S increased K+ currents from 10 to 30 pA/pF at +150 mV. Treatment with Na2S increased the level of sulfhydration of TRPV4 channels in aortic ECs. These results demonstrate that H2S-mediated vasodilation involves activation of TRPV4-dependent Ca2+ influx and BK channel activation within EC. Activation of TRPV4 channels appears to cause calcium events that result in the opening of eBK channels, endothelial hyperpolarization, and subsequent vasodilation.

Intermittent hypoxia in rats reduces activation of Ca2+ sparks in mesenteric arteries.

Ca(+) sparks are vascular smooth muscle cell (VSMC) Ca(2+)-release events that are mediated by ryanodine receptors (RyR) and promote vasodilation by activating large-conductance Ca(2+)-activated potassium channels and inhibiting myogenic tone. We have previously reported that exposing rats to intermittent hypoxia (IH) to simulate sleep apnea augments myogenic tone in mesenteric arteries through loss of hydrogen sulfide (H2S)-induced dilation. Because we also observed that H2S can increase Ca(2+) spark activity, we hypothesized that loss of H2S after IH exposure reduces Ca(2+) spark activity and that blocking Ca(2+) spark generation reduces H2S-induced dilation. Ca(2+) spark activity was lower in VSMC of arteries from IH compared with sham-exposed rats. Furthermore, depolarizing VSMC by increasing luminal pressure (from 20 to 100 mmHg) or by elevating extracellular [K(+)] increased spark activity in VSMC of arteries from sham rats but had no effect in arteries from IH rats. Inhibiting endogenous H2S production in sham arteries prevented these increases. NaHS or phosphodiesterase inhibition increased spark activity to the same extent in sham and IH arteries. Depolarization-induced increases in Ca(2+) spark activity were due to increased sparks per site, whereas H2S increases in spark activity were due to increased spark sites per cell. Finally, inhibiting Ca(2+) spark activity with ryanodine (10 µM) enhanced myogenic tone in arteries from sham but not IH rats and blocked dilation to exogenous H2S in arteries from both sham and IH rats. Our results suggest that H2S regulates RyR activation and that H2S-induced dilation requires Ca(2+) spark activation. IH exposure decreases endogenous H2S-dependent Ca(2+) spark activation to cause membrane depolarization and enhance myogenic tone in mesenteric arteries.

Modulation of hydrogen sulfide by vascular hypoxia.

Hydrogen sulfide (H2S) has emerged as a key regulator of cardiovascular function. This gasotransmitter is produced in the vasculature and is involved in numerous processes that promote vascular homeostasis, including vasodilation and endothelial cell proliferation. Although H2S plays a role under physiological conditions, it has become clear in recent years that hypoxia modulates the production and action of H2S. Furthermore, there is growing evidence that H2S is cytoprotective in the face of hypoxic insults. This review focuses on the synthesis and signaling of H2S in hypoxic conditions in the vasculature, and highlights recent studies providing evidence that H2S is a potential therapy for preventing tissue damage in hypoxic conditions.

Endothelin-1-induced vasoconstriction does not require intracellular Ca²âº waves in arteries from rats exposed to intermittent hypoxia.

Sleep apnea is associated with cardiovascular disease, and patients with sleep apnea have elevated plasma endothelin (ET)-1 concentrations. Rats exposed to intermittent hypoxia (IH), a model of sleep apnea, also have increased plasma ET-1 concentrations and heightened constriction to ET-1 in mesenteric arteries without an increase in global vascular smooth muscle cell Ca(2+) concentration ([Ca(2+)]). Because ET-1 has been shown to increase the occurrence of propagating Ca(2+) waves, we hypothesized that ET-1 increases Ca(2+) wave activity in mesenteric arteries, rather than global [Ca(2+)], to mediate enhanced vasoconstriction after IH exposure. Male Sprague-Dawley rats were exposed to sham or IH conditions for 7 h/day for 2 wk. Mesenteric arteries from sham- and IH-exposed rats were isolated, cannulated, and pressurized to 75 mmHg to measure ET-1-induced constriction as well as changes in global [Ca(2+)] and Ca(2+) wave activity. A low concentration of ET-1 (1 nM) elicited similar vasoconstriction and global Ca(2+) responses in the two groups. Conversely, ET-1 had no effect on Ca(2+) wave activity in arteries from sham rats but significantly increased wave frequency in arteries from IH-exposed rats. The ET-1-induced increase in Ca(2+) wave frequency in arteries from IH rats was dependent on phospholipase C and inositol 1,4,5-trisphosphate receptor activation, yet inhibition of phospholipase C and the inositol 1,4,5-trisphosphate receptor did not prevent ET-1-mediated vasoconstriction. These results suggest that although ET-1 elevates Ca(2+) wave activity after IH exposure, increases in wave activity are not associated with increased vasoconstriction.

Hydrogen sulfide dilates rat mesenteric arteries by activating endothelial large-conductance Ca²âº-activated K⁺ channels and smooth muscle Ca²âº sparks.

We have previously shown that hydrogen sulfide (H2S) reduces myogenic tone and causes relaxation of phenylephrine (PE)-constricted mesenteric arteries. This effect of H2S to cause vasodilation and vascular smooth muscle cell (VSMC) hyperpolarization was mediated by large-conductance Ca(2+)-activated potassium channels (BKCa). Ca(2+) sparks are ryanodine receptor (RyR)-mediated Ca(2+)-release events that activate BKCa channels in VSMCs to cause membrane hyperpolarization and vasodilation. We hypothesized that H2S activates Ca(2+) sparks in small mesenteric arteries. Ca(2+) sparks were measured using confocal microscopy in rat mesenteric arteries loaded with the Ca(2+) indicator fluo-4. VSMC membrane potential (Em) was measured in isolated arteries using sharp microelectrodes. In PE-constricted arteries, the H2S donor NaHS caused vasodilation that was inhibited by ryanodine (RyR blocker), abluminal or luminal iberiotoxin (IbTx, BKCa blocker), endothelial cell (EC) disruption, and sulfaphenazole [cytochrome P-450 2C (Cyp2C) inhibitor]. The H2S donor NaHS (10 µmol/l) increased Ca(2+) sparks but only in the presence of intact EC and this was blocked by sulfaphenazole or luminal IbTx. Inhibiting cystathionine γ-lyase (CSE)-derived H2S with ß-cyano-l-alanine (BCA) also reduced VSMC Ca(2+) spark frequency in mesenteric arteries, as did EC disruption. However, excess CSE substrate homocysteine did not affect spark activity. NaHS hyperpolarized VSMC Em in PE-depolarized mesenteric arteries with intact EC and also hyperpolarized EC Em in arteries cut open to expose the lumen. This hyperpolarization was prevented by ryanodine, sulfaphenazole, and abluminal or luminal IbTx. BCA reduced IbTx-sensitive K(+) currents in freshly dispersed mesenteric ECs. These results suggest that H2S increases Ca(2+) spark activity in mesenteric artery VSMC through activation of endothelial BKCa channels and Cyp2C, a novel vasodilatory pathway for this emerging signaling molecule.

Phosphoinositide-dependent kinase-1 and protein kinase Cδ contribute to endothelin-1 constriction and elevated blood pressure in intermittent hypoxia.

Obstructive sleep apnea (OSA) is associated with cardiovascular complications including hypertension. Previous findings from our laboratory indicate that exposure to intermittent hypoxia (IH), to mimic sleep apnea, increases blood pressure in rats. IH also increases endothelin-1 (ET-1) constrictor sensitivity in a protein kinase C (PKC) δ-dependent manner in mesenteric arteries. Because phosphoinositide-dependent kinase-1 (PDK-1) regulates PKCδ activity, we hypothesized that PDK-1 contributes to the augmented ET-1 constrictor sensitivity and elevated blood pressure following IH. Male Sprague-Dawley rats were exposed to either sham or IH (cycles between 21% O(2)/0% CO(2) and 5% O(2)/5% CO(2)) conditions for 7 h/day for 14 or 21 days. The contribution of PKCδ and PDK-1 to ET-1-mediated vasoconstriction was assessed in mesenteric arteries using pharmacological inhibitors. Constrictor sensitivity to ET-1 was enhanced in arteries from IH-exposed rats. Inhibition of PKCδ or PDK-1 blunted ET-1 constriction in arteries from IH but not sham group rats. Western analysis revealed similar levels of total and phosphorylated PDK-1 in arteries from sham and IH group rats but decreased protein-protein interaction between PKCδ and PDK-1 in arteries from IH- compared with sham-exposed rats. Blood pressure was increased in rats exposed to IH, and treatment with the PDK-1 inhibitor OSU-03012 [2-amino-N-{4-[5-(2-phenanthrenyl)-3-(trifluoromethyl)-1H-pyrazol-1-yl]-phenyl}-acetamide] (33 mg/day) lowered blood pressure in IH but not sham group rats. Our results suggest that exposure to IH unmasks a role for PDK-1 in regulating ET-1 constrictor sensitivity and blood pressure that is not present under normal conditions. These novel findings suggest that PDK-1 may be a uniquely effective antihypertensive therapy for OSA patients.

Matrix metalloproteinase 2 and 9 dysfunction underlie vascular stiffness in circadian clock mutant mice.

OBJECTIVE: To determine if elasticity in blood vessels is compromised in circadian clock-mutant mice (Bmal1-knockout [KO] and Per-triple KO) and if matrix metalloproteinases (MMPs) might confer these changes in compliance. METHODS AND RESULTS: High-resolution ultrasonography in vivo revealed impaired remodeling and increased pulse-wave velocity in the arteries of Bmal1-KO and Per-triple KO mice. In addition, compliance of remodeled arteries and naïve pressurized arterioles ex vivo from Bmal1-KO and Per-triple KO mice was reduced, consistent with stiffening of the vascular bed. The observed vascular stiffness was coincident with dysregulation of MMP-2 and MMP-9 in Bmal1-KO mice. Furthermore, inhibition of MMPs improved indexes of pathological remodeling in wild-type mice, but the effect was abolished in Bmal1-KO mice. CONCLUSIONS: Circadian clock dysfunction contributes to hardening of arteries, which may involve impaired control of the extracellular matrix composition.

Preventing increased blood pressure in the obese Zucker rat improves severity of stroke.

Obesity is a risk factor for stroke, but the determinants of increased stroke risk in obesity are unknown. We have previously reported that obese Zucker rats (OZRs) have a worse stroke outcome and display evidence of remodeling of the middle cerebral artery (MCA), in parallel with hypertension, compared with lean controls. This study tested the hypothesis that hypertension is an essential determinant of cerebral vascular remodeling and increased stroke damage in OZRs. Blood pressure was measured by telemetry in lean and obese rats with and without hydrochlorthiazide (HCT; 2 mg.kg(-1).day(-1)) from 8 to 15 wk of age. A separate group of rats was also chronically fed a low-sodium (LS) diet. Vessel structure was assessed in isolated, pressurized MCAs. Cerebral ischemia was induced for 60 min using an intralumenal suture technique, followed by 24 h of reperfusion. HCT treatment effectively prevented the increase in blood pressure in obese rats; however, the LS diet did not lower pressure. Importantly, infarct size was normalized by HCT after ischemia-reperfusion injury. Additionally, HCT improved the changes in MCA structure observed in untreated OZRs. There were no benefits of the LS diet on stroke injury or vessel structure. These results indicate that increased pressure is essential for driving the changes in infarct size in OZRs.

Obesity increases blood pressure, cerebral vascular remodeling, and severity of stroke in the Zucker rat.

Obesity is a risk factor for stroke, but the mechanisms by which obesity increases stroke risk are unknown. Because microvascular architecture contributes to the outcome of stroke, we hypothesized that middle cerebral arteries (MCAs) from obese Zucker rats (OZRs) undergo inward remodeling and develop increased myogenic tone compared with those in lean Zucker rats (LZRs). We further hypothesized that OZRs have an increased infarct after cerebral ischemia and that changes in vascular structure and function correlate with the development of hypertension in OZRs. Blood pressure was measured by telemetry in LZRs and OZRs from 6 to 17 weeks of age. Vessel structure and function were assessed in isolated MCAs. Stroke damage was assessed after ischemia was induced for 60 minutes followed by 24 hours of reperfusion. Although mean arterial pressure was similar between young rats (6 to 8 weeks old), mean arterial pressure was higher in adult (14 to 17 weeks old) OZRs than in LZRs. MCAs from OZRs had a smaller lumen diameter and increased myogenic vasoconstriction compared with those from LZRs. After ischemia, infarction was 58% larger in OZRs than in LZRs. Before the development of hypertension, MCA myogenic reactivity and lumen diameter, as well as infarct size, were similar between young LZRs and OZRs. Our results indicate that the MCAs of OZRs undergo structural remodeling and that these rats have greater cerebral injury after cerebral ischemia. These cerebrovascular changes correlate with the development of hypertension and suggest that the increased blood pressure may be the major determinant for stroke risk in obese individuals.

11beta-hydroxysteroid dehydrogenase type II inhibition causes cerebrovascular remodeling and increases infarct size after cerebral ischemia.

Direct mineralocorticoid receptor (MR) activation with deoxycorticosterone acetate has deleterious effects on the cerebral vasculature. Inhibition of 11beta-hydroxysteroid dehydrogenase type II (11betaHSD2) mimics the detrimental effects of elevated mineralocorticoids in the heart, but the effect of enzyme inactivation on the cerebral vasculature is unknown. Therefore, we hypothesized that systemic 11betaHSD2 inhibition with carbenoxolone (CBX) would cause remodeling of the middle cerebral artery (MCA) and increase the damage caused by cerebral ischemia. Six-week-old male Sprague Dawley rats were divided into control and CBX (2.5 mg/d) + 0.9% NaCl treated. After 4 wk treatment, rats were used to assess either structure and reactivity of the MCA or the response to cerebral ischemia using the MCA occlusion technique. Cerebral damage was assessed by 2,3,5-triphenyltetrazolium chloride staining and expressed as a percentage of the hemisphere infarcted. CBX treatment increased systolic blood pressure (153.2 +/- 7.3 vs. 122.1 +/- 4.4 mm Hg; P < 0.05) compared with control rats. MCAs from CBX treated rats were smaller and stiffer than control MCAs over a range of intralumenal pressures, indicating inward remodeling of the vessel. CBX treatment significantly increased ischemic cerebral infarct size compared with control rats (27.1 +/- 5.4% vs. 14.8 +/- 4.2%; P < 0.05). These data indicate that inhibition of 11betaHSD2, and, thus, disproportionate glucocorticoid activation of the MR, results in remodeling of the MCA and worsens the outcome of cerebral ischemia, further underscoring the importance of understanding the mechanism by which MR activation leads to cerebrovascular disease.

Is the mineralocorticoid receptor a potential target for stroke prevention?

In recent years, it has become increasingly clear that the extra-renal effects of aldosterone play an important role in the pathogenesis of cardiovascular disease. Stroke is one of the leading causes of death in the Western world, and MR (mineralocorticoid receptor) antagonism is a potential preventative therapy for patients at risk of both ischaemic and haemorrhagic strokes. This protective effect of MR antagonism appears to occur at the level of the cerebral vasculature and may be related to the expression and activation of the EGFR (epidermal growth factor receptor) and the degree of vessel wall collagen deposition.