Shawn B. Bender

Direct regulation of blood pressure by smooth muscle cell mineralocorticoid receptors

Hypertension is a cardiovascular risk factor present in over two-thirds of people over age 60 in North America; elevated blood pressure correlates with increased risk of heart attack, stroke and progression to heart and kidney failure. Current therapies are insufficient to control blood pressure in almost half of these patients. The mineralocorticoid receptor (MR), acting in the kidney, is known to regulate blood pressure through aldosterone binding and stimulation of sodium retention. However, recent studies support the concept that the MR also has extrarenal actions and that defects in sodium handling alone do not fully explain the development of hypertension and associated cardiovascular mortality. We and others have identified functional MR in human vascular smooth muscle cells (SMCs), suggesting that vascular MR might directly regulate blood pressure. Here we show that mice with SMC-specific deficiency of the MR have decreased blood pressure as they age without defects in renal sodium handling or vascular structure. Aged mice lacking MR in SMCs (SMC-MR) have reduced vascular myogenic tone, agonist-dependent contraction and expression and activity of L-type calcium channels. Moreover, SMC-MR contributes to angiotensin II–induced vascular oxidative stress, vascular contraction and hypertension. This study identifies a new role for vascular MR in blood pressure control and in vascular aging and supports the emerging hypothesis that vascular tone contributes directly to systemic blood pressure.

Vascular Effects of Exercise: Endothelial Adaptations Beyond Active Muscle Beds

Endothelial adaptations to exercise training are not exclusively conferred within the active muscle beds. Herein, we summarize key studies that have evaluated the impact of chronic exercise on the endothelium of vasculatures perfusing nonworking skeletal muscle, brain, viscera, and skin, concluding with discussion of potential mechanisms driving these endothelial adaptations.

Mineralocorticoid Receptor–Mediated Vascular Insulin Resistance: An Early Contributor to Diabetes-Related Vascular Disease?

Two-thirds of adults in the U.S. are overweight or obese, and another 26 million have type 2 diabetes (T2D). Patients with diabetes and/or the metabolic syndrome have a significantly increased risk of heart attack and stroke compared with people with normal insulin sensitivity. Decreased insulin sensitivity in cardiovascular tissues as well as in traditional targets of insulin metabolic signaling, such as skeletal muscle, is an underlying abnormality in obesity, hypertension, and T2D. In the vasculature, insulin signaling plays a critical role in normal vascular function via endothelial cell nitric oxide production and modulation of Ca2+ handling and sensitivity in vascular smooth muscle cells. Available evidence suggests that impaired vascular insulin sensitivity may be an early, perhaps principal, defect of vascular function and contributor to the pathogenesis of vascular disease in persons with obesity, hypertension, and T2D. In the overweight and obese individual, as well as in persons with hypertension, systemic and vascular insulin resistance often occur in concert with elevations in plasma aldosterone. Indeed, basic and clinical studies have demonstrated that elevated plasma aldosterone levels predict the development of insulin resistance and that aldosterone directly interferes with insulin signaling in vascular tissues. Furthermore, elevated plasma aldosterone levels are associated with increased heart attack and stroke risk. Conversely, renin–angiotensin–aldosterone system and mineralocorticoid receptor (MR) antagonism reduces cardiovascular risk in these patient populations. Recent and accumulating evidence in this area has implicated excessive Ser phosphorylation and proteosomal degradation of the docking protein, insulin receptor substrate, and enhanced signaling through hybrid insulin/IGF-1 receptor as important mechanisms underlying aldosterone-mediated interruption of downstream vascular insulin signaling. Prevention or restoration of these changes via blockade of aldosterone action in the vascular wall with MR antagonists (i.e., spironolactone, eplerenone) may therefore account for the clinical benefit of these compounds in obese and diabetic patients with cardiovascular disease. This review will highlight recent evidence supporting the hypothesis that aldosterone and MR signaling represent an ideal candidate pathway linking early promoters of diabetes, especially overnutrition and obesity, to vascular insulin resistance, dysfunction, and disease.

Impaired function of coronary BKCa channels in metabolic syndrome

The role of large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels in regulation of coronary microvascular function is widely appreciated, but molecular and functional changes underlying the deleterious influence of metabolic syndrome (MetS) have not been determined. Male Ossabaw miniature swine consumed for 3-6 mo a normal diet (11% kcal from fat) or an excess-calorie atherogenic diet that induces MetS (45% kcal from fat, 2% cholesterol, 20% kcal from fructose). MetS significantly impaired coronary vasodilation to the BK(Ca) opener NS-1619 in vivo (30-100 microg) and reduced the contribution of these channels to adenosine-induced microvascular vasodilation in vitro (1-100 microM). MetS reduced whole cell penitrem A (1 microM)-sensitive K(+) current and NS-1619-activated (10 microM) current in isolated coronary vascular smooth muscle cells. MetS increased the concentration of free intracellular Ca(2+) and augmented coronary vasoconstriction to the L-type Ca(2+) channel agonist BAY K 8644 (10 pM-10 nM). BK(Ca) channel alpha and beta(1) protein expression was increased in coronary arteries from MetS swine. Coronary vascular dysfunction in MetS is related to impaired BK(Ca) channel function and is accompanied by significant increases in L-type Ca(2+) channel-mediated coronary vasoconstriction.

Dipeptidylpeptidase Inhibition Is Associated with Improvement in Blood Pressure and Diastolic Function in Insulin-Resistant Male Zucker Obese Rats

Diastolic dysfunction is a prognosticator for future cardiovascular events that demonstrates a strong correlation with obesity. Pharmacological inhibition of dipeptidylpeptidase-4 (DPP-4) to increase the bioavailability of glucagon-like peptide-1 is an emerging therapy for control of glycemia in type 2 diabetes patients. Accumulating evidence suggests that glucagon-like peptide-1 has insulin-independent actions in cardiovascular tissue. However, it is not known whether DPP-4 inhibition improves obesity-related diastolic dysfunction. Eight-week-old Zucker obese (ZO) and Zucker lean rats were fed normal chow diet or diet containing the DPP-4 inhibitor, linagliptin (LGT), for 8 weeks. Plasma DPP-4 activity was 3.3-fold higher in ZO compared with Zucker lean rats and was reduced by 95% with LGT treatment. LGT improved echocardiographic and pressure volume-derived indices of diastolic function that were impaired in ZO control rats, without altering food intake or body weight gain during the study period. LGT also blunted elevated blood pressure progression in ZO rats involving improved skeletal muscle arteriolar function, without reducing left ventricular hypertrophy, fibrosis, or oxidative stress in ZO hearts. Expression of phosphorylated- endothelial nitric oxide synthase (eNOS)(Ser1177), total eNOS, and sarcoplasmic reticulum calcium ATPase 2a protein was elevated in the LGT-treated ZO heart, suggesting improved Ca(2+) handling. The ZO myocardium had an abnormal mitochondrial sarcomeric arrangement and cristae structure that were normalized by LGT. These studies suggest that LGT reduces blood pressure and improves intracellular Cai(2+) mishandling and cardiomyocyte ultrastructure, which collectively result in improvements in diastolic function in the absence of reductions in left ventricular hypertrophy, fibrosis, or oxidative stress in insulin-resistant ZO rats.

Low-Dose Mineralocorticoid Receptor Blockade Prevents Western Diet–Induced Arterial Stiffening in Female Mice

Women are especially predisposed to development of arterial stiffening secondary to obesity because of consumption of excessive calories. Enhanced activation of vascular mineralocorticoid receptors impairs insulin signaling, induces oxidative stress, inflammation, and maladaptive immune responses. We tested whether a subpressor dose of mineralocorticoid receptor antagonist, spironolactone (1 mg/kg per day) prevents aortic and femoral artery stiffening in female C57BL/6J mice fed a high-fat/high-sugar western diet (WD) for 4 months (ie, from 4–20 weeks of age). Aortic and femoral artery stiffness were assessed using ultrasound, pressurized vessel preparations, and atomic force microscopy. WD induced weight gain and insulin resistance compared with control diet–fed mice and these abnormalities were unaffected by spironolactone. Blood pressures and heart rates were normal and unaffected by diet or spironolactone. Spironolactone prevented WD-induced stiffening of aorta and femoral artery, as well as endothelial and vascular smooth muscle cells, within aortic explants. Spironolactone prevented WD-induced impaired aortic protein kinase B/endothelial nitric oxide synthase signaling, as well as impaired endothelium-dependent and endothelium-independent vasodilation. Spironolactone ameliorated WD-induced aortic medial thickening and fibrosis and the associated activation of the progrowth extracellular receptor kinase 1/2 pathway. Finally, preservation of normal arterial stiffness with spironolactone in WD-fed mice was associated with attenuated systemic and vascular inflammation and an anti-inflammatory shift in vascular immune cell marker genes. Low-dose spironolactone may represent a novel prevention strategy to attenuate vascular inflammation, oxidative stress, and growth pathway signaling and remodeling to prevent development of arterial stiffening secondary to consumption of a WD.

Mineralocorticoid Receptor Antagonism Treats Obesity-Associated Cardiac Diastolic Dysfunction

Patients with obesity and diabetes mellitus exhibit a high prevalence of cardiac diastolic dysfunction (DD), an independent predictor of cardiovascular events for which no evidence-based treatment exists. In light of renin-angiotensin-aldosterone system activation in obesity and the cardioprotective action of mineralocorticoid receptor (MR) antagonists in systolic heart failure, we examined the hypothesis that MR blockade with a blood pressure–independent low-dose spironolactone (LSp) would treat obesity-associated DD in the Zucker obese (ZO) rat. Treatment of ZO rats exhibiting established DD with LSp normalized cardiac diastolic function, assessed by echocardiography. This was associated with reduced cardiac fibrosis, but not reduced hypertrophy, and restoration of endothelium-dependent vasodilation of isolated coronary arterioles via a nitric oxide–independent mechanism. Further mechanistic studies revealed that LSp reduced cardiac oxidative stress and improved endothelial insulin signaling, with no change in arteriolar stiffness. Infusion of Sprague-Dawley rats with the MR agonist aldosterone reproduced the DD noted in ZO rats. In addition, improved cardiac function in ZO-LSp rats was associated with attenuated systemic and adipose inflammation and an anti-inflammatory shift in cardiac immune cell mRNAs. Specifically, LSp increased cardiac markers of alternatively activated macrophages and regulatory T cells. ZO-LSp rats had unchanged blood pressure, serum potassium, systemic insulin sensitivity, or obesity-associated kidney injury, assessed by proteinuria. Taken together, these data demonstrate that MR antagonism effectively treats established obesity-related DD via blood pressure–independent mechanisms. These findings help identify a particular population with DD that might benefit from MR antagonist therapy, specifically patients with obesity and insulin resistance.

Altered Mechanism of Adenosine-Induced Coronary Arteriolar Dilation in Early-Stage Metabolic Syndrome

Onset of the combined metabolic syndrome (MetS) is a complex progressive process involving numerous cardiovascular risk factors. Although patients with established MetS exhibit reduced coronary flow reserve and individual components of the MetS reduce microvascular vasodilation, little is known concerning the impact of early-stage MetS on the mechanisms of coronary flow control. Therefore, we tested the hypothesis that coronary arteriolar dilation to adenosine is attenuated in early-stage MetS by reduced A2 receptor function and diminished K+ channel involvement. Pigs were fed control or high-fat/cholesterol diet for 9 weeks to induce early-stage MetS. Coronary atheroma was determined in vivo with intravascular ultrasound. In vivo coronary dilation was determined by intracoronary adenosine infusion. Further, apical coronary arterioles were isolated, cannulated and pressurized to 60 cmH2O for in vitro pharmacologic assessment of adenosine dilation. Coronary atheroma was not different between groups, indicating early-stage MetS. Coronary arteriolar dilation to adenosine (in vivo) and 2-chloroadenosine (2-CAD; in vitro) was similar between groups. In control arterioles, 2-CAD-mediated dilation was reduced only by selective A2A receptor inhibition, whereas only dual A2A/2B inhibition reduced this response in MetS arterioles. Arteriolar A2B, but not A2A, receptor protein expression was reduced by MetS. Blockade of voltage-dependent K+ (Kv) channels reduced arteriolar sensitivity to 2-CAD in both groups, whereas ATP-sensitive K+ (KATP) channel inhibition reduced sensitivity only in control arterioles. Our data indicate that the mechanisms mediating coronary arteriolar dilation to adenosine are altered in early-stage MetS prior to overt decrements in coronary vasodilator reserve.

Endothelial Mineralocorticoid Receptors Differentially Contribute to Coronary and Mesenteric Vascular Function Without Modulating Blood Pressure

Arteriolar vasoreactivity tightly regulates tissue-specific blood flow and contributes to systemic blood pressure (BP) but becomes dysfunctional in the setting of cardiovascular disease. The mineralocorticoid receptor (MR) is known to regulate BP via the kidney and by vasoconstriction in smooth muscle cells. Although endothelial cells (EC) express MR, the contribution of EC-MR to BP and resistance vessel function remains unclear. To address this, we created a mouse with MR specifically deleted from EC (EC-MR knockout [EC-MR-KO]) but with intact leukocyte MR expression and normal renal MR function. Telemetric BP studies reveal no difference between male EC-MR-KO mice and MR-intact littermates in systolic, diastolic, circadian, or salt-sensitive BP or in the hypertensive responses to aldosterone±salt or angiotensin II±L-nitroarginine methyl ester. Vessel myography demonstrated normal vasorelaxation in mesenteric and coronary arterioles from EC-MR-KO mice. After exposure to angiotensin II–induced hypertension, impaired endothelial-dependent relaxation was prevented in EC-MR-KO mice in mesenteric vessels but not in coronary vessels. Mesenteric vessels from angiotensin II–exposed EC-MR-KO mice showed increased maximum responsiveness to acetylcholine when compared with MR-intact vessels, a difference that is lost with indomethacin+L-nitroarginine methyl ester pretreatment. These data support that EC-MR plays a role in regulating endothelial function in hypertension. Although there was no effect of EC-MR deletion on mesenteric vasoconstriction, coronary arterioles from EC-MR-KO mice showed decreased constriction to endothelin-1 and thromboxane agonist at baseline and also after exposure to hypertension. These data support that EC-MR participates in regulation of vasomotor function in a vascular bed–specific manner that is also modulated by risk factors, such as hypertension.

Differential vulnerability of skeletal muscle feed arteries to dysfunction in insulin resistance: impact of fiber type and daily activity.

Functional and structural heterogeneity exists among skeletal muscle vascular beds related, in part, to muscle fiber type composition. This study was designed to delineate whether the vulnerability to vascular dysfunction in insulin resistance is uniformly distributed among skeletal muscle vasculatures and whether physical activity modifies this vulnerability. Obese, hyperphagic Otsuka Long-Evans Tokushima fatty rats (20 wk old) were sedentary (OSED) or physically active (OPA; access to running wheels) and compared with age-matched sedentary Long-Evans Tokushima Otsuka (LSED) rats. Vascular responses were determined in isolated, pressurized feed arteries from fast-twitch gastrocnemius (GFAs) and slow-twitch soleus (SFAs) muscles. OSED animals were obese, insulin resistant, and hypertriglyceridemic, traits absent in LSED and OPA rats. GFAs from OSED animals exhibited depressed dilation to ACh, but not sodium nitroprusside, and enhanced vasoconstriction to endothelin-1 (ET-1), but not phenylephrine, compared with those in LSED. Immunoblot analysis suggests reduced endothelial nitric oxide synthase phosphorylation at Ser1177 and endothelin subtype A receptor expression in OSED GFAs. Physical activity prevented reduced nitric oxide-dependent dilation to ACh, but not enhanced ET-1 vasoconstriction, in GFA from OPA animals. Conversely, vasoreactivity of SFAs to ACh and ET-1 were principally similar in all groups, whereas dilation to sodium nitroprusside was enhanced in OSED and OPA rats. These data demonstrate, for the first time, that SFAs from insulin-resistant rats exhibit reduced vulnerability to dysfunction versus GFAs and that physical activity largely prevents GFA dysfunction. We conclude that these results demonstrate that vascular dysfunction associated with insulin resistance is heterogeneously distributed across skeletal muscle vasculatures related, in part, to muscle fiber type and activity level.