Cardioprotective effects of atrasentan, an endothelin-A receptor antagonist, but not of nitric oxide in diabetic mice with myocyte-specific overexpression of endothelial nitric oxide synthase

Enhanced Cardiorenal Protective Effects of Combining SGLT2 Inhibition, Endothelin Receptor Antagonism and RAS Blockade in Type 2 Diabetic Mice

Treatments with sodium-glucose 2 cotransporter inhibitors (SGLT2i) or endothelin receptor antagonists (ERA) have shown cardiorenal protective effects. The present study aimed to evaluate the cardiorenal beneficial effects of the combination of SGLT2i and ERA on top of renin-angiotensin system (RAS) blockade. Type 2 diabetic mice (db/db) were treated with different combinations of an SGLT2i (empagliflozin), an ERA (atrasentan), and an angiotensin-converting enzyme inhibitor (ramipril) for 8 weeks. Vehicle-treated diabetic mice and non-diabetic mice were included as controls. Weight, blood glucose, blood pressure, and kidney and heart function were monitored during the study. Kidneys and heart were collected for histological examination and to study the intrarenal RAS. Treatment with empagliflozin alone or combined significantly decreased blood glucose compared to vehicle-treated db/db. The dual and triple therapies achieved significantly greater reductions in diastolic blood pressure than ramipril alone. Compared to vehicle-treated db/db, empagliflozin combined with ramipril or in triple therapy significantly prevented GFR increase, but only the triple combination exerted greater protection against podocyte loss. In the heart, empagliflozin alone or combined reduced cardiac isovolumetric relaxation time (IVRT) and left atrium (LA) diameter as compared to vehicle-treated db/db. However, only the triple therapy was able to reduce cardiomyocyte area. Importantly, the add-on triple therapy further enhanced the intrarenal ACE2/Ang(1-7)/Mas protective arm of the RAS. These data suggest that triple therapy with empagliflozin, atrasentan and ramipril show synergistic cardiorenal protective effects in a type 2 diabetic mouse model.

Cardiac and renal effects of atrasentan in combination with enalapril and paricalcitol in uremic rats

BACKGROUND/AIMS

The search for new therapies providing cardiorenal protection in chronic kidney disease (CKD) has led to treatments that combine conventional renin-angiotensin-aldosterone-system inhibitors with other drugs that exhibit potential in disease management.

METHODS

In rats made uremic by renal ablation, we examined the effects of addition of the endothelin-A receptor antagonist atrasentan to a previously examined combination of enalapril (angiotensin converting enzyme inhibitor) and paricalcitol (vitamin D receptor activator) on cardiac and renal parameters. The effects of the individual and combined drugs were examined after a 3-month treatment.

RESULTS

A decrease in systolic blood pressure, serum creatinine and proteinuria, and improvement of renal histology in uremic rats were attributed to enalapril and/or paricalcitol treatment; atrasentan alone had no effect. In heart tissue, individual treatment with the drugs blunted the increase in cardiomyocyte size, and combined treatment additively decreased cardiomyocyte size to normal levels. Perivascular fibrosis was blunted in uremic control rats with atrasentan or enalapril treatment.

CONCLUSIONS

We found distinct cardiac and renal effects of atrasentan. Combination treatment with atrasentan, enalapril and paricalcitol provided positive effects on cardiac remodeling in uremic rats, whereas combination treatment did not offer further protective effects on blood pressure, proteinuria or renal histology.

Elevated expression of liver X receptor alpha (LXRα) in myocardium of streptozotocin-induced diabetic rats

The present study was designed to investigate the myocardial expression of liver X receptor alpha (LXRα) in a streptozotocin (STZ)-induced diabetic rat model. Immunohistochemical staining, quantitative real-time RT-PCR, and Western blot analysis were used to determine the expression of LXRα in the myocardium of STZ-induced diabetic rats. The myocardial expression of LXRα target genes, long-chain acyl-CoA synthetase 3 (ACSL3), fatty acid transporter protein (FAT/CD36), ATP-binding cassette transporter A1 (ABCA1), and ABCG1 were also detected. Bisulfite sequencing analysis was employed to examine the methylation status of the CpG island at the LXRα promoter region in the myocardium of STZ-induced diabetic rats. We found that LXRα mRNA and protein expression in the left ventricles, right ventricles, and atria of diabetic rats were gradually increased during the progression of diabetic cardiomyopathy (DCM). The mRNA expression levels of ACSL3 and FAT/CD36 and the protein expression levels of ABCA1 and ABCG1 were also markedly increased in different heart chambers of diabetic rats. Moreover, there was a significant difference in the methylation status of LXRα gene between the ventricles of control and diabetic rats (P < 0.05). Our findings suggest that elevated expression of LXRα may be involved in the progression of DCM, and demethylation of LXRα is likely to be responsible for its increased expression in myocardial tissues.

Glycoxidative stress and cardiovascular complications in experimentally-induced diabetes: effects of antioxidant treatment

Diabetes mellitus (DM) is a common metabolic disease, representing a serious risk factor for the development of cardiovascular complications, such as coronary heart disease, peripheral arterial disease and hypertension. Oxidative stress (OS), a feature of DM, is defined as an increase in the steady-state levels of reactive oxygen species (ROS) and may occur as a result of increased free radical generation and/or decreased anti-oxidant defense mechanisms. Increasing evidence indicates that hyperglycemia is the initiating cause of the tissue damage in DM, either through repeated acute changes in cellular glucose metabolism, or through long-term accumulation of glycated biomolecules and advanced glycation end products (AGEs). AGEs are formed by the Maillard process, a non-enzymatic reaction between ketone group of the glucose molecule or aldehydes and the amino groups of proteins that contributes to the aging of proteins and to the pathological complications of DM. In the presence of uncontrolled hyperglycemia, the increased formation of AGEs and lipid peroxidation products exacerbate intracellular OS and results in a loss of molecular integrity, disruption in cellular signaling and homeostasis, followed by inflammation and tissue injury such as endothelium dysfunction, arterial stiffening and microvascular complications. In addition to increased AGE production, there is also evidence of multiple pathways elevating ROS generation in DM, including; enhanced glucose auto-oxidation, increased mitochondrial superoxide production, protein kinase C-dependent activation of NADPH oxidase, uncoupled endothelial nitric oxide synthase (eNOS) activity, increased substrate flux through the polyol pathway and stimulation of eicosanoid metabolism. It is, therefore, not surprising that the correction of these variables can result in amelioration of diabetic cardiovascular abnormalities. A linking element between these phenomena is cellular redox imbalance due to glycoxidative stress (GOS). Thus, recent interest has focused on strategies to prevent, reverse or retard GOS in order to modify the natural history of diabetic cardiovascular abnormalities. This review will discuss the links between GOS and diabetes-induced cardiovascular disorders and the effect of antioxidant therapy on altering the development of cardiovascular complications in diabetic animal models.

Endothelin receptor selectivity in chronic kidney disease: rationale and review of recent evidence

Endothelin (ET) is a potent vasoconstrictory peptide with proinflammatory and profibrotic properties that exerts its biological effects through two pharmacologically distinct receptor subtypes, namely ET(A) and ET(B). In addition to its substantial contribution to normal renal function, a large body of evidence suggests that derangement of the renal ET system is involved in the initiation and progression of chronic kidney disease (CKD) in diabetes, hypertension and glomerulonephritis. Thus, the use of ET receptor antagonists (ERAs) may offer potential novel treatment strategies in CKD. Recent literature on the role of the renal ET system in the healthy kidney was reviewed. In addition, an unbiased PubMed search was performed for studies published during the last 5 years that addressed the effects of ERAs in CKD. A particular objective was to extract information regarding whether selective or nonselective ERAs may have therapeutic potential in humans. ET-1 acts primarily as an autocrine or paracrine factor in the kidney. In normal physiology, ET-1 promotes diuresis and natriuresis by local production and action through ET(B) receptors in the renal medulla. In pathology, ET-1 mediates vasoconstriction, mesangial-cell proliferation, extracellular matrix production and inflammation, effects that are primarily conveyed by ET(A) receptors. Results obtained in animal models and in humans with the use of ERAs in CKD are encouraging; nevertheless, it is still under debate which receptor subtype should be targeted. According to most studies, selective inhibition of ET(A) receptors appears superior compared with nonselective ERAs because this approach does not interfere with the natriuretic, antihypertensive and ET clearance effects of ET(B) receptors. Although preliminary data in humans are promising, the potential role of ERAs in patients with CKD and the question of which receptor subtype should be targeted can only be clarified in randomized clinical trials.

Fibrosis in diabetes complications: pathogenic mechanisms and circulating and urinary markers

Diabetes mellitus is characterized by a lack of insulin causing elevated blood glucose, often with associated insulin resistance. Over time, especially in genetically susceptible individuals, such chronic hyperglycemia can cause tissue injury. One pathological response to tissue injury is the development of fibrosis, which involves predominant extracellular matrix (ECM) accumulation. The main factors that regulate ECM in diabetes are thought to be pro-sclerotic cytokines and protease/anti-protease systems. This review will examine the key markers and regulators of tissue fibrosis in diabetes and whether their levels in biological fluids may have clinical utility.

Elevated expression of urotensin II and its receptor in diabetic cardiomyopathy

OBJECTIVE

Urotensin II (UII) is a somatostatin-like peptide recently identified to have several cardiovascular effects, including potent vasoactive, cardiac inotropic and chronotropic properties. Our aim was to determine the degree of expression of UII and UII receptor (UT) in the myocardium of rats with streptozotocin (STZ)-induced diabetes.

METHODS

Real-time polymerase chain reaction, Western blot, and immunohistochemistry were used to determine the degree of expression and location of UII and UT in the myocardium of STZ-induced diabetic rats.

RESULTS

UII and UT expression were significantly enhanced in the myocardium of rats with diabetes compared with healthy controls on both messenger RNA and protein levels. Both UII and UT protein expression were mainly concentrated in the cardiomyocytes, endothelial cells, cardiac fibroblasts, and smooth muscle cells of diabetic cardiomyopathy (DCM).

CONCLUSIONS

Our results suggest a possible role for the UII/UT system in the pathophysiology of DCM.

Mechanisms of ET-1-induced endothelial dysfunction

There is now increasing evidence that endothelial dysfunction is an early event in the pathophysiology of cardiovascular diseases and can be corrected with certain therapies such as angiotensin converting enzyme inhibitors angiotensin type I receptor antagonists and stains independently of blood pressure lowering effects. Restoring endothelial function appears to be a crucial target since endothelial dysfunction predicts cardiovascular events in various situations such as coronary artery disease peripheral artery disease, or hypertension and in patients undergoing vascular surgery. Preclinical and clinical data strongly support that endothelin receptor antagonists belong to this restricted class of pharmacological agents able to act on the endothelium, and offer a potential therapeutic approach for numerous diseases associated with endothelial dysfunction. The purpose of this review will be therefore, 1) to propose mechanisms by which ET-1 can cause endothelial dysfunction; 2) to provide an overview of pathological situations associated with endothelial dysfunction related to ET-1; and 3) to assemble evidence on efficacy of endothelin receptor antagonists for improvement of endothelial function.

Chronic endothelin-A receptor antagonism is as protective as angiotensin converting enzyme inhibition against cardiac dysfunction in diabetic rats

BACKGROUND AND PURPOSE

Diabetes mellitus is associated with a specific cardiomyopathy. We compared the cardioprotective effects of an endothelin-A receptor blocker (ET(A)-RB) with those of an angiotensin-converting enzyme inhibitor (ACE-I) in rats with streptozotocin (STZ)-induced diabetes.

EXPERIMENTAL APPROACH

Diabetic rats were left untreated or received either the ET(A)-RB atrasentan or the ACE-I ramipril (each 3 mg kg(-1) per day) orally for 8 weeks. Isolated isovolumic heart function was studied during normoxia and in response to ischaemia-reperfusion. Cardiac fibrosis, tissue oxidative stress and tissue nitric oxide synthase (NOS) activity were determined.

KEY RESULTS

Basal left ventricular systolic contractility was lower in diabetic compared to nondiabetic hearts and ET(A)-RB or ACE-I treatment significantly antagonised the decline. Following 15 min of no-flow ischaemia, reperfusion systolic function was depressed and left-ventricular end-diastolic pressure (LVEDP) was elevated in diabetic hearts. ET(A)-RB or ACE-I treatment significantly improved recovery of reperfusion systolic and diastolic function, without differences between groups. Hydroxyproline (an index of tissue fibrosis) and malondialdehyde (a measure of tissue oxidative stress) were elevated at the end of reperfusion in diabetic, compared to nondiabetic hearts. Either treatment reduced hydroxyproline and malondialdehyde to control level. Constitutive NOS activity was similar in nondiabetic and diabetic hearts and unaffected by ET(A)-RB or ACE-I treatment.

CONCLUSIONS AND IMPLICATIONS

These results suggest that in experimental type 1 diabetes ET(A)-RB is as effective as an ACE-I in ameliorating myocardial functions during normoxia and ischaemia-reperfusion. Combining the two treatments neither afforded additive effects, nor diminished any protection effect seen with either drug.

Role of myocardial nitric oxide in diabetic ischemia-reperfusion dysfunction: studies in mice with myocyte-specific overexpression of endothelial nitric-oxide synthase

We investigated the role of nitric oxide (NO) in myocardial ischemia-reperfusion injury of diabetic mice with myocyte-specific overexpression of endothelial NO synthase (NOS). Four weeks after diabetes induction with streptozotocin (blood glucose approximately 29 mM), isolated isovolumic heart function and cellular NO metabolites in response to brief normothermic ischemia-reperfusion were determined. Under normoxic conditions transgenic (TG) hearts from nondiabetic and diabetic animals generated less left-ventricular developed pressure compared with wild-type (WT) control hearts, and this abnormality was unaffected by NOS inhibition. During ischemia, the rise in end-diastolic pressure was less in the TG than WT group of nondiabetic hearts, whereas the transgene had no effect in the diabetic group. Similarly, the transgene also improved reperfusion systolic and diastolic function in nondiabetic but not in diabetic hearts. NOS inhibition worsened reperfusion function in diabetic hearts. Postischemic nitrite and cGMP formation were higher in nondiabetic TG than WT hearts, but in diabetic hearts cGMP was no longer elevated. The formation of reactive oxygen species (superoxide and peroxynitrite) during early reperfusion, measured by electron spin resonance spectroscopy, was similar in nondiabetic WT and TG hearts, but it was significantly higher in diabetic TG hearts. Stimulating endogenous NO production with 10 microM bradykinin more strongly reduced myocardial O(2) consumption in diabetic TG than diabetic WT hearts perfused in normoxia, whereas there was no difference after ischemia-reperfusion. Thus, providing additional endogenous NO is sufficient to protect nondiabetic hearts against ischemia-induced injury, but for a similar protection in diabetic hearts, effective scavenging of reactive oxygen species is also important.