Single cell regulatory architecture of human pancreatic islets suggests sex differences in β cell function and the pathogenesis of type 2 diabetes

Biological sex affects the pathogenesis of type 2 and type 1 diabetes (T2D, T1D) including the development of β cell failure observed more often in males. The mechanisms that drive sex differences in β cell failure is unknown. Studying sex differences in islet regulation and function represent a unique avenue to understand the sex-specific heterogeneity in β cell failure in diabetes. Here, we examined sex and race differences in human pancreatic islets from up to 52 donors with and without T2D (including 37 donors from the Human Pancreas Analysis Program [HPAP] dataset) using an orthogonal series of experiments including single cell RNA-seq (scRNA-seq), single nucleus assay for transposase-accessible chromatin sequencing (snATAC-seq), dynamic hormone secretion, and bioenergetics. In cultured islets from nondiabetic (ND) donors, in the absence of the in vivo hormonal environment, sex differences in islet cell type gene accessibility and expression predominantly involved sex chromosomes. Of particular interest were sex differences in the X-linked KDM6A and Y-linked KDM5D chromatin remodelers in female and male islet cells respectively. Islets from T2D donors exhibited similar sex differences in differentially expressed genes (DEGs) from sex chromosomes. However, in contrast to islets from ND donors, islets from T2D donors exhibited major sex differences in DEGs from autosomes. Comparing β cells from T2D and ND donors revealed that females had more DEGs from autosomes compared to male β cells. Gene set enrichment analysis of female β cell DEGs showed a suppression of oxidative phosphorylation and electron transport chain pathways, while male β cell had suppressed insulin secretion pathways. Thus, although sex-specific differences in gene accessibility and expression of cultured ND human islets predominantly affect sex chromosome genes, major differences in autosomal gene expression between sexes appear during the transition to T2D and which highlight mitochondrial failure in female β cells.

Urinary Angiotensinogen Displays Sexual Dimorphism in Non-Diabetic Humans and Mice with Overweight

Increased body weight (BW) induces inappropriate renin-angiotensin system (RAS) activation. The activation of the intrarenal RAS is associated with increased urinary angiotensinogen (uAGT), blood pressure (BP), and kidney damage. Here, we examined uAGT excretion levels in young non-diabetic human subjects with overweight (OW) and non-diabetic mice with high-fat diet (HFD)-induced OW. Human subjects (women and men; 20-28 years old) included two groups: (a) overweight (OW, n = 17, BMI ≥ 25); and (b) controls (normal weight (NW; n = 26, BMI ≤ 25). In these subjects, we measured BP, albuminuria, and protein levels of uAGT by ELISA adjusted by urinary creatinine (expressed by uAGT/uCrea). Mice (female and male C57BL/6J mice, 8 ± 2 weeks of age) also included two groups: HFD or normal fat diet (NFD) fed for 8 weeks. We measured BW, fasting blood glucose (FBG), BP by telemetry, albuminuria, and uAGT by ELISA. In humans: (i) no significant changes were observed in BP, albuminuria, and FBG when comparing NW and OW subjects; (ii) multivariate logistic regression analysis of independent predictors related to uAGT/uCrea levels demonstrated a strong association between uAGT and overweight; (iii) urinary reactive oxygen species (ROS) were augmented in men and women with OW; (iv) the uAGT/uCrea ratio was higher in men with OW. However, the uAGT/uCrea values were lower in women even with OW. In mice: (i) males fed an HFD for 8 weeks became OW while females did not; (ii) no changes were observed either in FBG, BP, or albuminuria; (iii) kidney ROS were augmented in OW male mice after 28 weeks but not in females; (iv) OW male mice showed augmented excretion of uAGT but this was undetectable in females fed either NFD or HFD. In humans and mice who are OW, the urinary excretion of AGT differs between males and females and overcomes overt albuminuria.

Succinate metabolism and membrane reorganization drives the endotheliopathy and coagulopathy of traumatic hemorrhage

Acute hemorrhage commonly leads to coagulopathy and organ dysfunction or failure. Recent evidence suggests that damage to the endothelial glycocalyx contributes to these adverse outcomes. The physiological events mediating acute glycocalyx shedding are undefined, however. Here, we show that succinate accumulation within endothelial cells drives glycocalyx degradation through a membrane reorganization-mediated mechanism. We investigated this mechanism in a cultured endothelial cell hypoxia-reoxygenation model, in a rat model of hemorrhage, and in trauma patient plasma samples. We found that succinate metabolism by succinate dehydrogenase mediates glycocalyx damage through lipid oxidation and phospholipase A2-mediated membrane reorganization, promoting the interaction of matrix metalloproteinase 24 (MMP24) and MMP25 with glycocalyx constituents. In a rat hemorrhage model, inhibiting succinate metabolism or membrane reorganization prevented glycocalyx damage and coagulopathy. In patients with trauma, succinate levels were associated with glycocalyx damage and the development of coagulopathy, and the interaction of MMP24 and syndecan-1 was elevated compared to healthy controls.

Architecture of androgen receptor pathways amplifying glucagon-like peptide-1 insulinotropic action in male pancreatic β cells

Male mice lacking the androgen receptor (AR) in pancreatic β cells exhibit blunted glucose-stimulated insulin secretion (GSIS), leading to hyperglycemia. Testosterone activates an extranuclear AR in β cells to amplify glucagon-like peptide-1 (GLP-1) insulinotropic action. Here, we examined the architecture of AR targets that regulate GLP-1 insulinotropic action in male β cells. Testosterone cooperates with GLP-1 to enhance cAMP production at the plasma membrane and endosomes via: (1) increased mitochondrial production of CO2, activating the HCO3--sensitive soluble adenylate cyclase; and (2) increased Gαs recruitment to GLP-1 receptor and AR complexes, activating transmembrane adenylate cyclase. Additionally, testosterone enhances GSIS in human islets via a focal adhesion kinase/SRC/phosphatidylinositol 3-kinase/mammalian target of rapamycin complex 2 actin remodeling cascade. We describe the testosterone-stimulated AR interactome, transcriptome, proteome, and metabolome that contribute to these effects. This study identifies AR genomic and non-genomic actions that enhance GLP-1-stimulated insulin exocytosis in male β cells.

Detrimental effects of transient cerebral ischemia on middle cerebral artery mitochondria in female rats

Mitochondrial numbers and dynamics in brain blood vessels differ between young male and female rats under physiological conditions, but how these differences are affected by stroke is unclear. In males, we found that mitochondrial numbers, possibly due to mitochondrial fission, in large middle cerebral arteries (MCAs) increased following transient middle cerebral artery occlusion (tMCAO). However, mitochondrial effects of stroke on MCAs of female rats have not been studied. To address this disparity, we conducted morphological, biochemical, and functional studies using electron microscopy, Western blot, mitochondrial respiration, and Ca2+ sparks activity measurements in MCAs of female, naïve or sham Sprague-Dawley rats before and 48 h after 90 min of tMCAO. Adverse changes in mitochondrial characteristics and the relationship between mitochondria and sarcoplasmic reticulum (SR) in MCAs were present on both sides. However, mitochondria and mitochondrial/SR associations were often within the range of normal appearance. Mitochondrial protein levels were similar between ipsilateral (ipsi) and contralateral (contra) sides. Nonrespiratory oxygen consumption, maximal respiration, and spare respiratory capacity were similar between ipsi and contra but were reduced compared with sham. Basal respiration, proton leak, and ATP production were similar among MCAs. Ca2+ sparks activity increased in sham and ipsi MCAs exposed to a mitochondrial ATP-sensitive potassium channel opener: diazoxide. Our results show that tMCAO has effects on mitochondria in MCAs on both the ipsi and contra sides. Mitochondrial responses of cerebral arteries to tMCAO in females are substantially different from responses seen previously in male rats suggesting the need for specific sex-based therapies.NEW & NOTEWORTHY We propose that differences in mitochondrial characteristics of males and females, including mitochondrial morphology, respiration, and calcium sparks activity contribute to sex differences in protective and repair mechanisms in response to transient ischemia-reperfusion.

Abstract WP245: Sex-dependent Regulation Of Mitochondrial Respiratory Function In Mouse Brain Microvessels By Peroxynitrite Decomposition Catalyst

Background: Peroxynitrite (PN) is a strong oxidizing and nitrating molecule. PN is a potent inhibitor of mitochondrial respiration and mediates the ischemia-reperfusion injury following stroke. In isolated mouse brain mitochondria, we observed that PN donors inhibit mitochondrial respiratiory function whereas PN decomposition catalyst, Fe (III) tetrakis (1-methyl-4-pyridyl) porphyrin pentachlorideporphyrin pentachloride (FeTMPyP), enhanced mitochondrial state III and state IVo mitochondrial respiration. In addition, we demonstrated that mitochondrial respiration is the primary contributor of cellular energy in brain microvessels (BMVs). The present study tested the hypothesis that FeTMPyP negatively regulates the mitochondrial respiration in the mouse BMVs.

Methods: BMVs were isolated from male and female mice (C57Bl/6, 2-4 months) using a combination of filters with pore sizes of 300μm and 40μm followed by gradient centrifugation. BMVs were treated with FeTMPyP at 37 o C for 60 minutes. Oxygen consumption rates (OCR) were measured using the Agilent Seahorse XFe24 analyzer and various respiratory parameters were determined following Mitostress test.

Results: In the male BMVs, basal respiration, ATP production, and non-mitochondrial respiration were not altered by FeTMPyP treatment. Contrary to our hypothesis, in the male BMVs, PN scavenger decreased the mitochondrial maximal respiration by 24.6% (2.1 ± 0.4 vs 2.8 ± 0.3 picomoles of O 2 /min/μg protein;) whereas the spare respiratory capacity was reduced by 33.3% (1.2 ± 0.3 vs 1.8 ± 0.3 picomoles of O 2 /min/μg protein) (n=17 each, p<0.05). Proton leak was elevated by 70% (0.7 ± 0.1 vs 0.4 ± 0.1 picomoles of O 2 /min/μg protein ) by PN scavenger in male BMVs. In contrast, in the female BMVs, the PN scavenger failed to alter the mitochondrial respiratory parameters (n=15 each, p=NS). Interestingly, FeTMPyP increased non-mitochondrial respiration by 63.8% (0.95 ± 0.2 vs 0.58 ± 0.1 picomoles of O 2 /min/μg protein) in the female BMVs (n=17 each, p<0.05).

Conclusion: BMVs display sex-dependent respenses to endogenous PN. Notably, in female mouse BMVs, PN appears to act as an antioxidant as PN inhibited the non-mitochondrial respiration which contributes to extramitochondrial superoxide generation.

Abstract TP248: Nos Inhibitor Is An Effective Adjuvant To Reperfusion Therapy In Ischemic Stroke In Mice

Background: Inhibition nitric oxide synthase (NOS) has been shown to paradoxically protect against ischemic brain injury by an unknown mechanism. Recently, we found that NOS inhibitor, N (gamma)-nitro-L-arginine methyl ester (L-NAME), protects primary cultures of endothelial cells and neurons against oxygen-glucose deprivation-reoxygenation injury by preventing NOS uncoupling. Thus, the objective of the present study was to demonstrate the effectiveness of locally administered L-NAME against ischemia-reperfusion (I-R) injury.

Methods: Transient focal cerebral ischemia in male mice (C57Bl/6, 2-4 months) was achieved by filament method of MCAO with a 60 min occlusion. We employed a novel approach to inject L-NAME (1 mg/Kg dosages at a rate of 60 ug/minute) locally into the ischemic zone by threading a catheter up the middle cerebral artery. This was timed to the onset of reperfusion by rapidly followng the removal of the occlusion with silicon suture. Following 24-hour reperfusion, infarct size was determined by 2,3,5-Triphenyltetrazolium chloride (TTC) staining and neurological functioning was recorded employing standard neurological score (0-4 range) and Rotarod performance test.

Results: Admnistration of FITC Dextran 40K and fluroscence microscopy of the brain slices 1 hour post-ischemia confirmed the effective delivery of injected L-NAME into ischemic tissue. Localized injection of L-NAME was protective against I-R injury and promoted significant recovery of neurological and motor function (Figure).

Conclusion: NOS inhibition could serve as an adjuvant for reperfusion therapies of ischemic stroke.

Androgen excess in pancreatic β cells and neurons predisposes female mice to type 2 diabetes

Androgen excess predisposes women to type 2 diabetes (T2D), but the mechanism of this is poorly understood. We report that female mice fed a Western diet and exposed to chronic androgen excess using dihydrotestosterone (DHT) exhibit hyperinsulinemia and insulin resistance associated with secondary pancreatic β cell failure, leading to hyperglycemia. These abnormalities are not observed in mice lacking the androgen receptor (AR) in β cells and partially in neurons of the mediobasal hypothalamus (MBH) as well as in mice lacking AR selectively in neurons. Accordingly, i.c.v. infusion of DHT produces hyperinsulinemia and insulin resistance in female WT mice. We observe that acute DHT produces insulin hypersecretion in response to glucose in cultured female mouse and human pancreatic islets in an AR-dependent manner via a cAMP- and mTOR-dependent pathway. Acute DHT exposure increases mitochondrial respiration and oxygen consumption in female cultured islets. As a result, chronic DHT exposure in vivo promotes islet oxidative damage and susceptibility to additional stress induced by streptozotocin via AR in β cells. This study suggests that excess androgen predisposes female mice to T2D following AR activation in neurons, producing peripheral insulin resistance, and in pancreatic β cells, promoting insulin hypersecretion, oxidative injury, and secondary β cell failure.

Abstract 058: Transforming Growth Factor β1 Antagonizes Npr1 Expression and Vascular Signaling: Role of Transcription Factor δEF1 Transforming Growth Factor β1 Antagonizes Npr1 Expression and Vascular Signaling: Role of Transcription Factor δEF1

The objective of the present study was to examine the repressive effect of transforming growth factor beta 1 (TGF-β1) in the regulation of Npr1 (coding for guanylyl cyclase/natriuretic peptide receptor-A; GC-A/NPRA) gene expression and vascular signaling. The rat thoracic aortic vascular smooth muscle cells (RTASMC) and denuded aortic rings were cultured in Dulbecco’s modified Eagle’s medium containing 10% fetal bovine serum and treated with TGF-β1 in a time-and dose-dependent manner. Treatment with TGF-β1 decreased NPRA mRNA and protein levels by 62% (0.42 ± 0.05 vs. control, 0.9 ± 0.02, p < 0.01) and 55% (9603 ± 860 vs. control, 22211 ± 1449, p < 0.01), respectively. TGF-β1 treatment significantly increased delta EF1 (δEF1) protein expression by 2.4-fold (907.9 ± 36.5. vs. control, 378.5 ± 10.3; p < 0.001) and enhanced its recruitment to Npr1 promoter. TGF-β1-treated RTASMCs and denuded aortic rings showed significant increases in α-smooth muscle actin (α-SMA) and collagen type 1 alpha 2 (COL1A2) protein expression, which were markedly attenuated by ANP treatments. The TGF-β1-pretreated cells showed 2.6-fold increase in α-SMA (control, 1523 ± 143, TGF-β1, 3997 ± 182 and TGF-β1 + ANP, 2172 ± 135) and 3.4-fold increase in COL1A2 (control, 1250 ± 77, TGF-β1, 4234 ± 110 and TGF-β1 + ANP, 1546 ± 57), respectively. In ex vivo experiments of denuded-aortic rings, TGF-β1 decreased Npr1 mRNA and protein levels by 62% (0.39 ± 0.06 vs. control 1.10 ± 0.01) and 70% (2609 ± 69 vs. control 5775 ± 123), respectively, and significantly (p < 0.0) increased the expression of TGF-β1-responsive proteins, namely α-SMA (2.6-fold) and COL1A2 (3.1-fold). Treatment with increasing concentrations of ANP (IC50=6x10 -9 M), relaxed denuded aortic rings contracted with prostaglandin F2α (PGF2α); however, pretreatment with TGF-β1 significantly attenuated ANP-mediated vascular relaxation after PFG2α contraction (ANP-treated, 68.68 ± 9.4 vs. TGF-β1 + ANP-treated 88.85 ± 4.7). The endothelium-intact vessels were not affected by TGF-β1 incubation. Together, the present results suggest that an antagonistic cascade exists between TGF-β1 pathways and ANP/NPRA signaling, which might be critical in the vascular remodeling and regulation of hypertension and cardiovascular homeostasis.

Role of Mitochondria in Cerebral Vascular Function: Energy Production, Cellular Protection, and Regulation of Vascular Tone

Mitochondria not only produce energy in the form of ATP to support the activities of cells comprising the neurovascular unit, but mitochondrial events, such as depolarization and/or ROS release, also initiate signaling events which protect the endothelium and neurons against lethal stresses via pre-/postconditioning as well as promote changes in cerebral vascular tone. Mitochondrial depolarization in vascular smooth muscle (VSM), via pharmacological activation of the ATP-dependent potassium channels on the inner mitochondrial membrane (mitoKATP channels), leads to vasorelaxation through generation of calcium sparks by the sarcoplasmic reticulum and subsequent downstream signaling mechanisms. Increased release of ROS by mitochondria has similar effects. Relaxation of VSM can also be indirectly achieved via actions of nitric oxide (NO) and other vasoactive agents produced by endothelium, perivascular and parenchymal nerves, and astroglia following mitochondrial activation. Additionally, NO production following mitochondrial activation is involved in neuronal preconditioning. Cerebral arteries from female rats have greater mitochondrial mass and respiration and enhanced cerebral arterial dilation to mitochondrial activators. Preexisting chronic conditions such as insulin resistance and/or diabetes impair mitoKATP channel relaxation of cerebral arteries and preconditioning. Surprisingly, mitoKATP channel function after transient ischemia appears to be retained in the endothelium of large cerebral arteries despite generalized cerebral vascular dysfunction. Thus, mitochondrial mechanisms may represent the elusive signaling link between metabolic rate and blood flow as well as mediators of vascular change according to physiological status. Mitochondrial mechanisms are an important, but underutilized target for improving vascular function and decreasing brain injury in stroke patients. © 2016 American Physiological Society. Compr Physiol 6:1529-1548, 2016.

Dynamics of enhanced mitochondrial respiration in female compared with male rat cerebral arteries

Mitochondrial respiration has never been directly examined in intact cerebral arteries. We tested the hypothesis that mitochondrial energetics of large cerebral arteries ex vivo are sex dependent. The Seahorse XFe24 analyzer was used to examine mitochondrial respiration in isolated cerebral arteries from adult male and female Sprague-Dawley rats. We examined the role of nitric oxide (NO) on mitochondrial respiration under basal conditions, using N(ω)-nitro-l-arginine methyl ester, and following pharmacological challenge using diazoxide (DZ), and also determined levels of mitochondrial and nonmitochondrial proteins using Western blot, and vascular diameter responses to DZ. The components of mitochondrial respiration including basal respiration, ATP production, proton leak, maximal respiration, and spare respiratory capacity were elevated in females compared with males, but increased in both male and female arteries in the presence of the NOS inhibitor. Although acute DZ treatment had little effect on mitochondrial respiration of male arteries, it decreased the respiration in female arteries. Levels of mitochondrial proteins in Complexes I-V and the voltage-dependent anion channel protein were elevated in female compared with male cerebral arteries. The DZ-induced vasodilation was greater in females than in males. Our findings show that substantial sex differences in mitochondrial respiratory dynamics exist in large cerebral arteries and may provide the mechanistic basis for observations that the female cerebral vasculature is more adaptable after injury.

Abstract 383: 17 beta-Estradiol Enhances Mitochondrial Function of Cerebral Arteries From Ovariectomized Female Rats

Previous studies have provided indirect evidence that circulating sex hormones alter the function of the cerebral circulation perhaps via effects on mitochondrial dynamics. However, effects of estrogen on mitochondrial respiration have never been directly examined. We have previously observed a difference in the mitochondrial function of cerebral arteries from male and female rats but the exact mechanisms are not clear.

We tested the hypothesis that mitochondrial respiration in isolated cerebral arteries from ovariectomized (OVX) Sprague Dawley rats treated with a 21 d release, 0.5 mg of 17 β-estradiol pellet (OVX+E) was enhanced compared with arteries from placebo treated OVX rats. The Seahorse Bioscience XFe24 system was used to measure mitochondrial oxygen consumption rate (pM/min/μg protein) in cerebral arteries. Western blot was used to investigate the arterial expression of proteins. Radioimmunoassay was used to measure serum estradiol level.

Treatment with 17 β-estradiol resulted in a higher serum estradiol level (146.9±18.16 pg/ml) and uterus weight (0.15±0.0058 g) in the OVX+E compared with the OVX (14.7±1.2 pg/ml, 0.07±0.003, respectively; p<0.05). The components of mitochondrial respiration in pM/min/μg protein including basal respiration (147±9), ATP production (44±4), proton leak (102±7), and maximal respiration (212±13) were elevated in OVX+E compared with OVX (105±13, 21±4, 50±6, 138±10, respectively; p<0.05). Expression of the mitochondrial DNA encoded Complexes I and III, the nuclear DNA encoded Complexes II, IV, V, and the voltage-dependent anion channel protein were similar in all groups. However, the ratios of phosphorylated endothelial and neuronal nitric oxide (NO) synthase normalized to total protein were significantly (p<0.05) elevated in the OVX+E (1.43±0.06, 1.15±0.2, respectively) compared with OVX group (0.92±0.06, 0.53±0.19, respectively).

Our findings provide direct evidence for sex-specific differences in mitochondrial function on freshly isolated cerebral arteries. Thus, estradiol replacement enhances the efficacy of the oxidative phosphorylation resulting in an increased mitochondrial respiration which is not due to increased mitochondrial protein expression but may be due to enhanced NO.

An ex vivo model for anti-angiogenic drug testing on intact microvascular networks

New models of angiogenesis that mimic the complexity of real microvascular networks are needed. Recently, our laboratory demonstrated that cultured rat mesentery tissues contain viable microvascular networks and could be used to probe pericyte-endothelial cell interactions. The objective of this study was to demonstrate the efficacy of the rat mesentery culture model for anti-angiogenic drug testing by time-lapse quantification of network growth. Mesenteric windows were harvested from adult rats, secured in place with an insert, and cultured for 3 days according to 3 experimental groups: 1) 10% serum (angiogenesis control), 2) 10% serum + sunitinib (SU11248), and 3) 10% serum + bevacizumab. Labeling with FITC conjugated BSI-lectin on Day 0 and 3 identified endothelial cells along blood and lymphatic microvascular networks. Comparison between day 0 (before) and 3 (after) in networks stimulated by 10% serum demonstrated a dramatic increase in vascular density and capillary sprouting. Growing networks contained proliferating endothelial cells and NG2+ vascular pericytes. Media supplementation with sunitinib (SU11248) or bevacizumab both inhibited the network angiogenic responses. The comparison of the same networks before and after treatment enabled the identification of tissue specific responses. Our results establish, for the first time, the ability to evaluate an anti-angiogenic drug based on time-lapse imaging on an intact microvascular network in an ex vivo scenario.

Mitochondrial mechanisms in cerebral vascular control: shared signaling pathways with preconditioning

Mitochondrial-initiated events protect the neurovascular unit against lethal stress via a process called preconditioning, which independently promotes changes in cerebrovascular tone through shared signaling pathways. Activation of adenosine triphosphate (ATP)-dependent potassium channels on the inner mitochondrial membrane (mitoKATP channels) is a specific and dependable way to induce protection of neurons, astroglia, and cerebral vascular endothelium. Through the opening of mitoKATP channels, mitochondrial depolarization leads to activation of protein kinases and transient increases in cytosolic calcium (Ca(2+)) levels that activate terminal mechanisms that protect the neurovascular unit against lethal stress. The release of reactive oxygen species from mitochondria has similar protective effects. Signaling elements of the preconditioning pathways also are involved in the regulation of vascular tone. Activation of mitoKATP channels in cerebral arteries causes vasodilation, with cell-specific contributions from the endothelium, vascular smooth muscles, and nerves. Preexisting chronic conditions, such as insulin resistance and/or diabetes, prevent preconditioning and impair relaxation to mitochondrial-centered responses in cerebral arteries. Surprisingly, mitochondrial activation after anoxic or ischemic stress appears to protect cerebral vascular endothelium and promotes the restoration of blood flow; therefore, mitochondria may represent an important, but underutilized target in attenuating vascular dysfunction and brain injury in stroke patients.

Abstract 172: Impact of Pharmacological Inhibition of Neuronal Nitric Oxide Synthase on Brain Microvascular Endothelial Cells

Experimental stroke in animals with deletions of endothelial (eNOS) and neuronal (nNOS) isoforms of nitric oxide synthase have observed greater infarct injury whereas pharmacological inhibition of NOS reported varying impact. nNOS has been identified recently in endothelial cells, however, the functional role of nNOS in brain microvascular endothelial cells (MECs) has never been examined. Our objective was to identify the nNOS in MECs and study its role in the regulation of mitochondrial function and response to anoxic injury.

Methods and Results: Primary brain MECs from humans and rats were used in the studies. Immunohistochemistry identified von Willebrand factor, eNOS, and nNOS in MECs. The nNOS immunoreactivity to three antibodies raised against different sequences of nNOS was observed in the cytoplasm and also in the nucleus when cells were permeabilized. Oxygen consumption rate (OCR) measurements were made from hMECs treated with selective inhibitors of nNOS (N-ω-Propyl-L-arginine; NPA or 7-nitroindazole) and eNOS (L-N 5 -(1-Iminoethyl)ornithine; NIO) or non-specific inhibitor (N G -Nitro-L-arginine methyl ester; L-NAME) revealed that mitochondrial reserve respiratory capacity was enhanced by nNOS inhibition but diminished by eNOS inhibition or L-NAME (100 mol/L). Drug treatment of hMECs for 3 h followed 24 h later by cell viability measurements showed that nNOS inhibition increased cell proliferation whereas eNOS inhibition diminished cell viability. Exposure to oxygen-glucose deprivation in the presence of nNOS inhibition increased cell viability.

Conclusions: We identified immunoreactive nNOS in brain MECs for the first time. Pharmacological inhibition of nNOS in MECs enhanced mitochondrial capacity, promoted cell proliferation, and afforded protection against anoxic injury. Thus, in MECs, nNOS appears to function distinct from eNOS and may even counteract eNOS actions.

Abstract 368: Insulin Promotes Hypoxic Vascular Injury in Insulin Resistance

Clinical studies targeting aggressive lowering of blood glucose in type 2 diabetes (T2DM) by exogenous insulin failed to show cardiovascular protection, but paradoxically increased cardiovascular mortality. Previously, we demonstrated cerebrovascular insulin resistance (IR) and reduced mitochondria-mediated vasodilation in Zucker obese (ZO) rats with IR compared to lean (ZL) controls. Communication between endoplasmic reticula (ER) and mitochondria play a critical role in metabolic functioning and responses to hypoxia. Tethering of mitochondria to ER by mitofusin 2 (MFN2) creates Ca 2+ microdomains to aid inter-organelle communication. Overexpression of glucose regulated protein 78 kDa (GRP78), an ER chaperone and a marker of ER stress, is protective by preempting ER Ca 2+ leak. Our aim is to demonstrate the adverse impact of IR on ER-mitochondrial interactions. Levels of mitochondrial proteins (MFN2 and voltage dependent anionic channel or VDAC), ER stress marker (GRP78), and endothelial nitric oxide synthase (eNOS) were determined in cerebral arteries from ZO and ZL treated with vehicle or insulin under normoxic or hypoxia-reoxygenation (H-R) conditions. ZO arteries displayed decreased MFN2 and VDAC, elevated GRP78, and increased eNOS monomer/dimer ratio (eNOS uncoupling) at baseline. In ZL arteries, MFN2 and GRP78 proteins were increased under normoxic (1.2±0.1 and 0.97±0.27 respectively; p<0.05 versus vehicle) and decreased under H-R conditions (1.43±0.4 and 0.43±0.02 respectively; p<0.05 versus vehicle) by insulin. In contrast, in ZO arteries, MFN2 and GRP78 proteins were decreased under normoxic (0.69±0.1 and 0.71±0.01 respectively; p<0.05 versus vehicle) and increased under H-R conditions (1.76±0.12 and 1.4±0.07 respectively; p<0.05 versus vehicle) by insulin. Insulin decreased eNOS uncoupling under H-R versus normoxia in ZL, but increased it in ZO arteries. Thus, ZO arteries with IR display impaired mitochondria-ER communication at baseline which was further exacerbated by insulin under H-R leading to increased oxidative vascular injury. Our study provides a mechanistic basis for increased cardiovascular mortality following intensive glucose lowering therapy by high exogenous insulin in patients with IR and T2DM.

Abstract 208: Augmented Cerebrovascular Responses to Diazoxide, an Opener of MitokAtp Channels, Following Middle Cerebral Artery (MCA) Occlusion in Rats

Previous research has not examined the effects of ischemia on cerebral vascular responses to mitochondrial activation in experimental strokes caused by occlusion of the MCA (MCAO). We investigated the role and mechanisms of mitochondrial derived vasoreactivity in the MCA of male SD rats following 90 min ischemia/48 h reperfusion injury. Ischemia was induced ipsilaterally (I) and the contralateral (C) side was non-ischemic. Electron microscopy showed disrupted mitochondrial morphology on the I side. Western blots for expression of mitochondrial proteins (Mean±SEM of immunoband intensity normalized to β-actin and as C vs. I): DRP-1 (1±0.1 vs. 3.1±0.2); VDAC (0.4±0.1 vs. 0.8±0.2); and complex-V (1.3±0.3 vs.2.0±0.3) as well as the non-mitochondrial proteins: phosphorylated (ph) ph-nNOS (0.4±0.1 vs. 0.7±0.1); ph-eNOS (0.2±0.06 vs. 0.7±0.2); COX-1 (0.5±0.1 vs. 0.8±0.1); and COX-2 (0.16±0.04 vs. 0.3±0.03) were elevated on I compared with C. The I mitochondrial membrane potential was greater (165±7 %) compared with C using TMRE fluorescence. Mitochondrial membrane depolarization decreased the TMRE intensity in both groups (100 to 80±3 vs. 165±7 to 70±4). Vascular responses of the MCA were characterized using the isolated, pressurized artery technique. Vasodilation in response to Ach (12±2 vs. 3±1), BK (42±9 vs. 32±6), SNP (75±7 vs. 38±4), and vasoconstriction to serotonin (65±8 vs. 33±7), were significantly decreased in I compared to C MCAs. On the other hand, 50 μM DZ induced vasodilation was enhanced in I arteries compared with C (5±1 vs. 17±2). Diazoxide induced dilation was decreased in the presence of the NOS inhibitor L-NAME (5±1 to 1±2 vs. 17±2 to 3±2) and the non-selective COX inhibitor indomethacin (5±1 to 1±2 vs.17±2 to 6±3). Our results indicate that experimental stoke has an major effect on mitochondria via inducing mitochondrial biogenesis leading to altered morphology, protein expression and function. Furthermore, the nitric-oxide and prostanoid pathways appear to be involved in enhanced diazoxide mediated vasodilation after MCAO. We speculate that targeting mitochondria may be useful therapy for improving outcome in stroke patients.

Cerebral microcirculatory responses of insulin-resistant rats are preserved to physiological and pharmacological stimuli

OBJECTIVE

Previously, we have shown that IR impairs the vascular reactivity of the major cerebral arteries of ZO rats prior to the occurrence of Type-II diabetes mellitus. However, the functional state of the microcirculation in the cerebral cortex is still being explored.

METHODS

We tested the local CoBF responses of 11-13-week-old ZO (n = 31) and control ZL (n = 32) rats to several stimuli measured by LDF using a closed cranial window setup.

RESULTS

The topical application of 1-100 μm bradykinin elicited the same degree of CoBF elevation in both ZL and ZO groups. There was no significant difference in the incidence, latency, and amplitude of the NMDA-induced CSD-related hyperemia between the ZO and ZL groups. Hypercapnic CoBF response to 5% carbon-dioxide ventilation did not significantly change in the ZO compared with the ZL. Topical bicuculline-induced cortical seizure was accompanied by the same increase of CoBF in both the ZO and ZL at all bicuculline doses.

CONCLUSIONS

CoBF responses of the microcirculation are preserved in the early period of the metabolic syndrome, which creates an opportunity for intervention to prevent and restore the function of the major cerebral vascular beds.

Abstract 622: Increased Mitochondrial Oxidative Stress and Activated AMP- Kinase In the Brain Dorsal Medulla of Hypertensive (mRen2)27 Transgenic Rats

The deleterious actions of angiotensin (Ang) II are generally thought to be mediated by NADPH oxidase (NOX)-derived reactive oxygen species (ROS) which may stimulate mitochondrial oxidant release leading to impaired energy metabolism. Hypertensive transgenic (mRen2)27 rats (mRen2) exhibit high NOX activity in brain dorsal medullary tissue compared to normotensive Hannover Sprague-Dawley (SD) control rats; however, the extent of mitochondrial involvement is unknown. Therefore, the present study evaluated mitochondrial ROS levels, ATP and mitochondrial content in the brain dorsal medulla of age-matched mRen2 and SD rats. Additional studies assessed AMP-activated kinase (AMPK) activation since altered mitochondrial oxidant and/or energy levels are associated with a stimulated AMPK pathway that is known to be activated in response to depleted cellular energy levels.

Freshly isolated mitochondria from the dorsal medulla of 20 week old male heterozygous mRen2 [systolic blood pressure (SBP): 211 ± 4 mmHg] and SD [SBP: 120 ± 3 mmHg] rats were loaded with the sensitive ROS indicator dihydroethidium (HEt). Basal HEt fluorescence intensity was ∼16% higher in mRen2 as compared to SD rats suggesting higher ROS levels in mitochondria of the hypertensive strain [mRen2: 83 ± 4 vs. SD: 70 ± 4 mean fluorescence intensity; p=0.02, n=3]. Although ATP levels and mitochondrial content were similar between strains, AMPK was significantly activated (phosphorylated AMPK-α and β 1 subunits) in the mRen2 compared to normotensive SD rats [AMPKα- mRen2: 3.5 ± 0.4 arbitary units vs. SD: 1.0 ± 0.2; p = 0.001, n = 5; and AMPKβ 1 - mRen2: 1.6 ± 0.04 arbitary units vs. SD: 1.0 ± 0.2; p = 0.05, n = 3]. The activation of AMPK may contribute to enhanced mitochondrial biogenesis since ATP and mitochondrial content were unchanged, thus representing a compensatory response to increased energy requirements in the hypertensive strain. We conclude that targeting of mitochondrial oxidants and increased AMPK activation may serve as a potential therapy to improve mitochondrial energy metabolism in hypertension.

Abstract 3903: Dynamin-related Protein 1 Independent Apoptosis In Neurons Following Oxygen-glucose Deprivation

Background and purpose: Previous studies showed that mitochondrial (mito) fission occurs prior to apoptosis and that a key initiator is the mito fission protein dynamin-related protein 1 (Drp1). However, little information is available concerning mito dynamics in neurons due to OGD. Therefore, we investigated mito fission and mito fusion responses in cultured neurons in an in vitro model of brain ischemia.

Methods: Primary rat cortical neurons were isolated from E18 Sprague Dawley fetuses. Neurons were exposed to OGD 9 days after culturing for 3 hr followed by up to 24 hr of reoxygenation. Mito dynamics were investigated by measuring the expression of: 1) mito fission [Drp1; mito fission 1 (Fis1)] and mito fusion [mitofusin-2 (Mfn2); optic atrophy-1 (OPA1)] proteins; 2) electron transport chain proteins (complex II 70 kDa subunit, complex IV subunit I, complex V alpha subunit); 3) mito DNA; and 4) apoptosis protein Bax. We also examined neuronal and mito morphology using transmission electron microscopy. The effects of the Drp1 inhibitors 15d-Prostaglandin J 2 (2.5-20 µM) (15d-PGJ 2 ) and Mdivi-1 (2.5-250 µM) were also investigated.

Results: 3hr OGD followed by 3-24 hr recovery resulted in an increase in the level of mito DNA and enhanced expression of complex IV and V proteins. During this time, abnormal morphology, including enlarged and/or swollen mitochondria, was often observed. However, many mitochondria still retained normal morphology. Drp1 levels decreased before virtually disappearing by 6 hr of reoxygenation following OGD while Fis1 expression did not change. Bax expression increased with an increasing number of apoptotic cells during this time. Mfn2 decreased after OGD while OPA1 did not change significantly. 15d-PGJ 2 treatment dose-dependently increased cell death both in control and OGD-exposed neurons. Furthermore, 15d-PGJ 2 treatment surprisingly increased Drp1 both in controls and in OGD-exposed cells, and also increased the mito DNA. Additionally, 15d-PGJ 2 treatment itself increased the number of larger mitochondria in the neurons. However, Mdivi-1 did not have any effects.

Conclusions: Despite a lower fission/fusion protein ratio, the increase in mitochondrial size indicates a shift to enhanced fusion in neurons following OGD. In addition, apoptosis induction seems to be Drp1 independent in this model. Additionally, 15d-PGJ 2 treatment appears to decrease survival in neurons following OGD. Thus, mito dynamics are affected by OGD and 15d-PGJ 2 treatment and play a role in cell survival/death.

Abstract 107: Mitochondrial Depolarization without Reactive Oxygen Species Production leads to Augmented Cerebral Vascular Relaxation via Diverse Calcium-related events in Smooth Muscle and Endothelium

Introduction: Mitochondrial depolarization and subsequent generation of reactive oxygen species (ROS) in vascular smooth muscle cells (VSMC) trigger vasodilation by diazoxide, a putative mitochondrial K ATP channel (mitoK ATP ) opener, via activation of ‘Ca 2+ -sparks’ (CS) and Ca 2+ -activated K + channels. It is unclear if mitochondrial depolarization, independent of ROS, promotes CS. In addition, the contribution to dilation of calcium events in endothelium following mitochondrial activation is unclear. Previously, we reported reduced vasodilation of cerebral arteries of insulin resistant (IR) Zucker obese (ZO) compared to lean (ZL) rats. Therefore, we examined the effect of BMS-191095 (BMS), a mitoK ATP opener that does not affect ROS generation, on cerebral arteries of Sprague-Dawley (SD) and ZL rats. In addition, we evaluated the impact of mitochondrial dysfunction due to IR in ZO rats.

Methods: Fluorescence microscopy was used to measure the CS and global calcium (Fluo-4 AM), mitochondrial membrane potential (TMRE), ROS (mitoSOX), and nitric oxide (DAF) in segments of pressurized, cerebral arteries. Diameters were determined continuously with an automated system.

Results: In SD arteries, BMS (50 umol/L) depolarized mitochondria without producing ROS. BMS also enhanced generation of CS (54±9 versus 117±18 sparks/min in response to vehicle and BMS respectively, p<0.05). Baseline CS activity (sparks/min) was reduced in ZO compared to ZL (156±21 versus 231±26, p<0.05). BMS and DZ responses were diminished in ZO (207±48 and 257±32, p=NS) versus ZL (318±54 and 366±63). In addition, BMS increased global calcium and nitric oxide production by endothelium.

Conclusions: Mitochondrial depolarization induces Ca 2+ -sparks in VSM and increased global calcium in endothelium via ROS-independent mechanisms. Thus, mitochondria can be depolarized without an associated increase in ROS levels. In addition, mitochondrial dysfunction associated with IR impaired CS activity leading to reduced mitochondrial mediated vasodilation. Thus, even a relatively mild metabolic disease such as IR, is able to disrupt coupling between mitochondrial status and vascular tone.

Impaired vascular responses of insulin-resistant rats after mild subarachnoid hemorrhage

Insulin resistance (IR) impairs cerebrovascular responses to several stimuli in Zucker obese (ZO) rats. However, cerebral artery responses after subarachnoid hemorrhage (SAH) have not been described in IR. We hypothesized that IR worsens vascular reactions after a mild SAH. Hemolyzed blood (300 μl) or saline was infused (10 μl/min) into the cisterna magna of 11-13-wk-old ZO (n = 25) and Zucker lean (ZL) rats (n = 25). One day later, dilator responses of the basilar artery (BA) and its side branch (BA-Br) to acetylcholine (ACh, 10(-6) M), cromakalim (10(-7) M, 10(-6) M), and sodium nitroprusside (10(-7) M) were recorded with intravital videomicroscopy. The baseline diameter of the BA was increased both in the ZO and ZL rats 24 h after the hemolysate injection. Saline-injected ZO animals showed reduced dilation to ACh (BA = 9 ± 3 vs. 22 ± 4%; and BA-Br = 23 ± 5 vs. 37 ± 7%) compared with ZL rats. Hemolysate injection blunted the response to ACh in both the ZO (BA = 4 ± 2%; and BA-Br = 12 ± 3%) and ZL (BA = 7 ± 2%; and BA-Br = 11 ± 3%) rats. Cromakalim (10(-6) M)-induced dilation was significantly reduced in the hemolysate-injected ZO animals compared with the saline control (BA = 13 ± 3 vs. 26 ± 5%; and BA-Br = 28 ± 8 vs. 44 ± 9%) and in the hemolysate-injected ZL rats compared with their saline control (BA = 24 ± 4 vs. 32 ± 4%; but not BA-Br = 39 ± 6 vs. 59 ± 9%). No significant difference in sodium nitroprusside reactivity was observed. Western blot analysis of the BA showed a lower baseline level of neuronal nitric oxide synthase expression and an enhanced cyclooxygenase-2 level in the hemolysate-injected ZO animals. In summary, cerebrovascular reactivity to both endothelium-dependent and -independent stimuli is severely compromised by SAH in IR animals.

Mitochondrial-mediated suppression of ROS production upon exposure of neurons to lethal stress: mitochondrial targeted preconditioning

Preconditioning represents the condition where transient exposure of cells to an initiating event leads to protection against subsequent, potentially lethal stimuli. Recent studies have established that mitochondrial-centered mechanisms are important mediators in promoting development of the preconditioning response. However, many details concerning these mechanisms are unclear. The purpose of this review is to describe the initiating and subsequent intracellular events involving mitochondria which can lead to neuronal preconditioning. These mitochondrial specific targets include: 1) potassium channels located on the inner mitochondrial membrane; 2) respiratory chain enzymes; and 3) oxidative phosphorylation. Following activation of mitochondrial ATP-sensitive potassium (mitoK(ATP)) channels and/or increased production of reactive oxygen species (ROS) resulting from the disruption of the respiratory chain or during energy substrate deprivation, morphological changes or signaling events involving protein kinases confer immediate or delayed preconditioning on neurons that will allow them to survive otherwise lethal insults. While the mechanisms involved are not known with certainty, the results of preconditioning are the enhanced neuronal viability, the attenuated influx of intracellular calcium, the reduced availability of ROS, the suppression of apoptosis, and the maintenance of ATP levels during and following stress.

Systemic administration of diazoxide induces delayed preconditioning against transient focal cerebral ischemia in rats

Diazoxide is the prototypical opener of mitochondrial ATP-sensitive potassium channels (mitoK(ATP)) and protects neurons in vivo and in vitro against chemical and anoxic stresses. While we have previously shown that diazoxide administration induces acute preconditioning against transient cerebral ischemia in rats, the potential for delayed preconditioning of diazoxide has not been examined. The purpose of this study was to determine whether diazoxide promotes delayed preconditioning following 90 min of middle cerebral artery occlusion (MCAO) in male Wistar rats. Diazoxide (10 mg/kg) or vehicle was injected intraperitoneally 24 h before MCAO. Infarct volumes were measured 72 h after reperfusion. In animals anesthetized with halothane, treatment with diazoxide exhibited a 35% reduction (48.3+/-3.0% to 31.3+/-4.8%) and 18% reduction (35.1+/-2.2% to 28.9+/-2.1%) in cortical and subcortical infarct volumes, respectively. Administration of the mitoK(ATP) blocker 5-hydroxydecanoate attenuated this beneficial effect. In contrast, diazoxide did not induce delayed preconditioning in isoflurane-anesthetized rats. These findings support the concept that diazoxide produces delayed preconditioning via mitoK(ATP) activation but that physiological status can affect induction of preconditioning.

ET(B) receptor-deficient rats exhibit reduced contraction to ET-1 despite an increase in ET(A) receptors

Several disease states, including hypertension, are associated with elevations in plasma endothelin-1 (ET-1) and variable changes in vascular contraction to ET-1. The spotting lethal (sl) rat carries a deletion of the endothelin-B (ET(B)) receptor gene that prevents expression of functional ET(B) receptors, resulting in elevated plasma ET-1. On a normal diet, these rats are normotensive and thus provide an opportunity to study the vascular effects of chronically elevated ET-1 in the absence of hypertension. Studies were performed in rats homozygous for the ET(B) deficiency (sl/sl; n = 8) and in transgenic rats heterozygous for the ET(B) deficiency (sl/+; n = 8). Plasma ET-1 was elevated in sl/sl rats (3.85 +/- 0.55 pg/ml) compared with sl/+ rats (0.31 +/- 0.11 pg/ml). Mean arterial blood pressure in conscious unrestrained sl/sl and sl/+ rats was 101 +/- 5 and 107 +/- 6 mmHg, respectively. Concentration-dependent contractions to ET-1 (10(-11)-10(-8) M) were reduced in mesenteric small arteries (150-250 microm) from sl/sl rats, as indicated by an approximately 10-fold increase in EC(50). A selective ET(A) antagonist, A-127722 (30 nM), abolished contraction to ET-1 in both groups, whereas a selective ET(B) antagonist had no effect. Also, ET(B) agonists (IRL-1620 and sarafatoxin 6c) produced neither contraction nor relaxation in either group, indicating that contraction to ET-1 in this vascular segment was exclusively ET(A) dependent. Despite increased plasma ET-1, protein expression of ET(A) receptors in membrane protein isolated from mesenteric small arteries was increased in sl/sl compared with sl/+ rats, as shown by Western blotting. These results indicate that, in ET(B)-deficient rats, ET(A)-induced contraction is reduced in vessels normally lacking ET(B)-mediated effects. Reduced contraction may be related to elevated plasma ET-1 and occurs in the presence of increased ET(A) receptor protein expression, suggesting an uncoupling of ET(A) receptor expression from functional activity.

Dihydroxyeicosatrienoic acids are potent activators of Ca(2+)-activated K(+) channels in isolated rat coronary arterial myocytes

1. Dihydroxyeicosatrienoic acids (DHETs), which are metabolites of arachidonic acid (AA) and epoxyeicosatrienoic acids (EETs), have been identified as highly potent endogenous vasodilators, but the mechanisms by which DHETs induce relaxation of vascular smooth muscle are unknown. Using inside-out patch clamp techniques, we examined the effects of DHETs on the large conductance Ca(2+)-activated K(+) (BK) channels in smooth muscle cells from rat small coronary arteries (150-300 microM diameter). 2. 11,12-DHET potently activated BK channels with an EC(50) of 1.87 +/- 0.57 nM (n = 5). Moreover, the three other regioisomers 5,6-, 8,9- and 14,15-DHET were equipotent with 11,12-DHET in activating BK channels. The efficacy of 11,12-DHET in opening BK channels was much greater than that of its immediate precursor 11,12-EET. In contrast, AA did not significantly affect BK channel activity. 3. The voltage dependence of BK channels was dramatically modulated by 11,12-DHET. With physiological concentrations of cytoplasmic Ca(2+) (200 nM), the voltage at which the channel open probability was half-maximal (V(1/2)) was shifted from a baseline of 115.6 +/- 6.5 mV to 95.0 +/- 10.1 mV with 5 nM 11,12-DHET, and to 60.0 +/- 8.4 mV with 50 nM 11,12-DHET. 4. 11,12-DHET also enhanced the sensitivity of BK channels to Ca(2+) but did not activate the channels in the absence of Ca(2+). 11,12-DHET (50 nM) reduced the Ca(2+) EC(50) of BK channels from a baseline of 1.02 +/- 0.07 microM to 0.42 +/- 0.11 microM. 5. Single channel kinetic analysis indicated that 11,12-DHET did not alter BK channel conductance but did reduce the first latency of BK channel openings in response to a voltage step. 11,12-DHET dose-dependently increased the open dwell times, abbreviated the closed dwell times, and decreased the transition rates from open to closed states. 6. We conclude that DHETs hyperpolarize vascular smooth muscle cells through modulation of the BK channel gating behaviour, and by enhancing the channel sensitivities to Ca(2+) and voltage. Hence, like EETs, DHETs may function as endothelium-derived hyperpolarizing factors.

12-lipoxygenase in porcine coronary microcirculation: implications for coronary vasoregulation

Noncyclooxygenase metabolites of arachidonic acid (AA) have been proposed to mediate endothelium-dependent vasodilation in the coronary microcirculation. Therefore, we examined the formation and bioactivity of AA metabolites in porcine coronary (PC) microvascular endothelial cells and microvessels, respectively. The major noncyclooxygenase metabolite produced by microvascular endothelial cells was 12(S)-hydroxyeicosatetraenoic acid (HETE), a lipoxygenase product. 12(S)-HETE release was markedly increased by pretreatment with 13(S)-hydroperoxyoctadecadienoic acid but not by the reduced congener 13(S)-hydroxyoctadecadienoic acid, suggesting oxidative upregulation of 12(S)-HETE output. 12(S)-HETE produced potent relaxation and hyperpolarization of PC microvessels (EC(50), expressed as -log[M] = 13.5 +/- 0.5). Moreover, 12(S)-HETE potently activated large-conductance Ca(2+)-activated K(+) currents in PC microvascular smooth muscle cells. In contrast, 12(S)-HETE was not a major product of conduit PC endothelial AA metabolism and did not exhibit potent bioactivity in conduit PC arteries. We suggest that, in the coronary microcirculation, 12(S)-HETE can function as a potent hyperpolarizing vasodilator that may contribute to endothelium-dependent relaxation, particularly in the setting of oxidative stress.

Enhanced endothelin-1 response and receptor expression in small mesenteric arteries of insulin-resistant rats

Hyperinsulinemia, a primary feature of insulin resistance, is associated with increased endothelin-1 (ET-1) activity. This study determined the vascular response to ET-1 and receptor binding characteristics in small mesenteric arteries of insulin-resistant (IR) rats. Rats were randomized to control (C) (n = 32) or IR (n = 32) groups. The response to ET-1 was assessed (in vitro) in arteries with (Endo+) and without (Endo-) endothelium. In addition, arteries (Endo+) were pretreated with the ET(B) antagonist A-192621 or the ET(A) antagonist A-127722. Finally, binding characteristics of [(125)I]ET-1 were determined. Results showed that in Endo+ arteries the maximal relaxation (E(max)) to ET-1 was similar between C and IR groups; however, the concentration at 50% of maximum relaxation (EC(50)) was decreased in IR arteries. In Endo- arteries, the E(max) to ET-1 was enhanced in both groups. Pretreatment with A-192621 enhanced the E(max) and EC(50) to ET-1 in both groups. In contrast, A-127722 inhibited the ET-1 response in all arteries in a concentration-dependent manner; however, a greater ET-1 response was seen at each concentration in IR arteries. Maximal binding of [(125)I]ET-1 was increased in IR versus C arteries although the dissociation constant values were similar. In conclusion, we found the vasoconstrictor response to ET-1 is enhanced in IR arteries due to an enhanced expression of ET receptors and underlying endothelial dysfunction.

Cytochrome P450 activity and endothelial dysfunction in insulin resistance

Impaired endothelium-dependent relaxation attributable to nitric oxide/prostacyclin-independent factor (endothelium-dependent hyperpolarizing factor; EDHF) has been demonstrated in the small mesenteric arteries of insulin-resistant rats. The purpose of this study was to determine if modulation of the cytochrome P450 enzyme system would restore EDHF-mediated relaxation in insulin-resistant rats. Sprague-Dawley rats were randomized to control (n = 32) or insulin-resistant (n = 32) groups. Each group was further randomized to treatment (n = 48) or placebo (n = 16). Miconazole (3 days) and phenobarbital (3 and 14 days) achieved cytochrome P450 inhibition and induction, respectively. Following drug treatment, mean arterial pressure was measured and vascular function was assessed in small mesenteric arteries in vitro. Specifically, acetylcholine-induced relaxation alone and in the presence of indomethacin plus N-nitro-L-arginine (LNNA) or KCl was determined. Miconazole reduced the maximal relaxation in response to acetylcholine in control rats. Similarly, in the presence of LNNA plus indomethacin, acetylcholine-induced relaxation was impaired in the miconazole-treated control group versus the placebo group, whereas relaxation in the presence of KCl was unchanged. Miconazole did not affect relaxation in insulin-resistant arteries. In contrast, 3- and 14-day treatment with phenobarbital significantly improved acetylcholine-induced relaxation in insulin-resistant arteries. Likewise, acetylcholine-mediated relaxation in the presence of LNNA plus indomethacin was also improved after phenobarbital treatment, while relaxation in the presence of KCl was unchanged. Phenobarbital treatment did not affect the control group. Miconazole treatment increased the mean arterial pressure in control rats, while 14-day phenobarbital treatment normalized the mean arterial pressure in insulin-resistant rats. Cytochrome P450 induction results in the restoration of EDHF-mediated relaxation in small mesenteric arteries and the normalization of mean arterial pressure in insulin-resistant rats. Thus, endothelial dysfunction secondary to insulin resistance can be reversed by the induction of cytochrome P450.

Metformin improves vascular function in insulin-resistant rats

This study assessed the effect of metformin treatment on insulin, mean arterial pressure (MAP), and endothelial function in insulin-resistant (IR) rats. In addition, we assessed the direct effect of metformin in vitro. Sprague-Dawley rats were randomized to control (n=28) or IR (n=28) groups. Rats were further randomized to receive metformin (300 mg/kg) or placebo for 2 weeks. MAP and insulin were measured. Subsequently, a third-order branch of the superior mesenteric artery was isolated, and endothelial function was assessed. Specifically, dose-response experiments of acetylcholine (ACh) with or without N-nitro-L-arginine (LNNA) were performed. For in vitro experiments, mesenteric arteries were removed from untreated control and IR rats and treated with metformin (100 micromol/L) before ACh+/-LNNA. MAP and insulin levels were improved in IR-metformin compared with IR-placebo rats. Maximal relaxation (E(max)) to ACh was enhanced in IR-metformin (92+/-2%) compared with IR-placebo rats (44+/-4%) (P<0.05). Relaxation in response to ACh+LNNA was greater in IR-metformin (33+/-4%) than in IR-placebo rats (12+/-4%) but remained depressed compared with control rats (E(max)=68+/-5%). The control group was not affected by metformin. In vitro treatment of arteries with metformin in response to ACh produced results similar to those in the experiments with metformin-treated rats. Although metformin improves metabolic abnormality in IR rats, this action does not appear to mediate its effect on vascular function. Both in vivo and in vitro metformin improved ACh-induced relaxation in IR rats to control levels, apparently through nitric oxide-dependent relaxation. These data suggest that metformin improves vascular function through a direct mechanism rather than by improving metabolic abnormalities.

EDHF-mediated relaxation is impaired in fructose-fed rats

Insulin resistance (IR) is associated with endothelial dysfunction. A defect in endothelium-dependent relaxation via outward potassium conductance has been observed in mesenteric arteries from IR rats. The purpose of this study was to assess whether this defect in endothelium-dependent relaxation was due to impaired endothelium-derived hyperpolarizing factor (EDHF) and to determine which specific potassium channel(s) are involved in relaxation. This was accomplished by using specific potassium channel inhibitors in the presence of nitric oxide synthase and cyclooxygenase inhibition. In addition, we sought to assess the function of smooth muscle cell adenosine triphosphate (ATP)-dependent potassium (K(ATP)) channels. Sprague-Dawley rats were randomized to control or IR. To determine EDHF-mediated relaxation, acetylcholine (ACh)-induced (10(-9)-10(-5) M) relaxation was measured (in vitro) in mesenteric arteries in the presence of indomethacin (10(-5) M) and N-nitro-L-arginine (L-NNA) (10(-4) M). Subsequently the combination of charybdotoxin (CTX) (0.1 microM) and apamin (0.5 microM) or glibenclamide (Glib) (10 microM) was added to the bath to inhibit KCa or K(ATP), respectively. In separate experiments, relaxation to pinacidil (10(-13)-10(-5) M), a K(ATP) activator, was assessed in vessels with intact endothelium, endothelium denuded, or with L-NNA. Maximal relaxation to ACh in the presence of L-NNA and indomethacin was 68+/-6% for control and 12+/-3% for IR (p<0.01). The addition of CTX + apamin almost abolished EDHF-mediated relaxation in control (Emax, 8+/-5% vs. 68+/-6%; p<0.01), whereas Glib had little affect. Neither CTX + apamin nor Glib had any affect on IR. Additionally, IR arteries were less sensitive to pinacidil than were controls (EC50, 1.5+/-0.9 microM vs. 5x10(-4)+/-3x10(-4) microM, respectively; p<0.01). Endothelial removal or L-NNA pretreatment of control arteries decreased the response to pinacidil similar to IR, whereas IR vessels were unaffected. EDHF-mediated relaxation is impaired in IR arteries. In addition, the K(Ca) channel appears to be imperative for activity of EDHF in rat small mesenteric arteries. Moreover, activation of K(ATP) channels by pinacidil is impaired in IR, and this appears to be a result of endothelial dysfunction.

Impaired endothelium-mediated relaxation in coronary arteries from insulin-resistant rats

OBJECTIVE

The insulin resistance syndrome is associated with atherosclerosis and cardiovascular events; however, the underlying mechanism of vascular dysfunction is unknown. The purpose of the current study was to assess endothelium- and smooth-muscle-mediated vasodilation in isolated coronary arteries from insulin-resistant rats and to determine whether insulin resistance alters the activity of the specific endothelium-derived relaxing factors.

METHODS

Male Sprague-Dawley rats were randomized to insulin resistance or control. Insulin resistance was induced by a fructose-rich diet. After 4 weeks of diet, coronary arteries were removed and vascular function was assessed in vitro using videomicroscopy. Acetylcholine (10(-9)-3 x 10(-5) M)- or sodium-nitroprusside (10(-9)-3 x 10(-4) M)-induced relaxations were determined. To evaluate the role of the specific endothelium-derived relaxing factors, several inhibitors were used, including N-nitro-L-arginine (LNNA), charybdotoxin/apamin (CTX/apamin), and indomethacin.

RESULTS

Studies with nitroprusside showed that smooth-muscle-dependent relaxation did not differ between insulin resistance and control groups. In contrast, maximal relaxation (E(max)) to acetylcholine was decreased in the insulin resistance group (56 +/- 7%) versus control (93 +/- 3%). LNNA pretreatment further impaired E(max) in the IR group from 56 +/- 7 to 17 +/- 2% (p < 0.01). In control, E(max) was only slightly impaired by LNNA (93 +/- 3 to 63 +/- 6%; p < 0.05). The addition of CTX/apamin also decreased relaxation in the control group (93 +/- 3 to 47 +/- 7%; p < 0.05), whereas relaxation in insulin-resistant rats was not affected (45 +/- 5% with CTX/apamin vs. 56 +/- 7% with acetylcholine alone, NS). Pretreatment with indomethacin did not affect relaxation in either group, while pretreatment with the combination of LNNA and CTX/ apamin completely abolished relaxation in both groups.

CONCLUSIONS

Endothelium-dependent relaxation is impaired in small coronary arteries from insulin-resistant rats. The mechanism of this defect is related to a decrease in an endothelium-dependent, nitric oxide/prostanoid-independent relaxing factor or endothelium-derived hyperpolarizing factor.

Endothelial dysfunction precedes hypertension in diet-induced insulin resistance

The insulin-resistant (IR) syndrome may be an impetus for the development of hypertension (HTN). Unfortunately, the mechanism by which this could occur is unclear. Our laboratory and others have described impaired endothelium-mediated relaxation in IR, mildly hypertensive rats. The purpose of the current study is to determine if HTN is most likely a cause or result of impaired endothelial function. Sprague-Dawley rats were randomized to receive a fructose-rich diet for 3, 7, 10, 14, 18, or 28 days or were placed in a control group. The control group received rat chow. After diet treatment, animals were instrumented with arterial cannulas, and while awake and unrestrained, their blood pressure (BP) was measured. Subsequently, endothelium-mediated relaxation to acetylcholine was determined (in vitro) by measuring intraluminal diameter of phenylephrine-preconstricted mesenteric arteries ( approximately 250 microM). Serum insulin levels were significantly elevated in all groups receiving fructose feeding compared with control, whereas there were no differences in serum glucose levels between groups. Impairment of endothelium-mediated relaxation starts by day 14 [mean percent maximal relaxation (Emax): 69 +/- 10% of baseline] and becomes significant by day 18 (Emax: 52 +/- 11% of baseline; P < 0.01). However, the mean BP (mmHg) does not become significantly elevated until day 28 [BP: 132 +/- 1 (day 28) vs. 116 +/- 3 (control); P < 0.05]. These findings demonstrate that both IR and endothelial dysfunction occur before HTN in this model and suggest that endothelial dysfunction may be a mechanism linking insulin resistance and essential HTN.