Angiotensin II type 2 receptors in the kidney: evidence for endothelial-cell-mediated renal vasodilatation

Serum symmetric dimethylarginine concentrations in enalapril- or telmisartan-treated dogs with proteinuric chronic kidney disease

Introduction

Renin-angiotensin-aldosterone system inhibition (RAASi) reduces intraglomerular pressure and is a standard therapy for dogs with proteinuric chronic kidney disease (CKD). RAASi can acutely decrease glomerular filtration rate (GFR); however, its effects on the marker of GFR serum symmetric dimethylarginine (SDMA) concentration in dogs have not been specifically evaluated. The objective of this study was to evaluate changes, relative to pretreatment values, in serum SDMA concentrations in dogs with proteinuric CKD receiving RAASi therapy.

Methods

This retrospective study used banked samples from 29 dogs with proteinuric CKD treated with enalapril (0.5 mg/kg PO q12h; n = 16) or telmisartan (1 mg/kg PO q24h; n = 13) alone (n = 22) or in combination with amlodipine if severely hypertensive (n = 7). Serum SDMA, creatinine, and urea nitrogen (SUN) concentrations were measured before and 7 and 30 days after starting RAASi. Percentage and absolute changes in these biomarkers were calculated for each dog and time point. A linear mixed model was used to test whether changes significantly differed from zero (α < 0.05).

Results

Overall, mean ± SEM Day 7 and 30 percentage change in SDMA were  - 4.8 ± 3.6% and  - 3.2 ± 3.4%, respectively; in creatinine were 7.4 ± 3.3% and 3.0 ± 3.1%, respectively; and in SUN were 22.1 ± 6.8% and 16.7 ± 6.2%, respectively. Mean changes varied according to whether all dogs, those on RAASi alone, or those co-treated with amlodipine were evaluated. In dogs receiving RAASi alone, at day 7, there were significant mean percentual increases in creatinine (9%; p = 0.023) and SUN (23%; p = 0.005), but SDMA was unchanged. In dogs co-treated with amlodipine, a significant absolute decrease in mean SDMA (-2.29 μg/dL; p = 0.026) occurred at days 7 and 30, while mean creatinine was unchanged and mean SUN increased.

Discussion

Proteinuric dogs receiving RAASi had low-magnitude changes in serum SDMA and creatinine, and moderate-magnitude changes in SUN concentrations. The direction of change in SDMA did not consistently match that of creatinine and SUN.

Renal Denervation Influences Angiotensin II Types 1 and 2 Receptors

The sympathetic and renin-angiotensin systems (RAS) are two critical regulatory systems in the kidney which affect renal hemodynamics and function. These two systems interact with each other so that angiotensin II (Ang II) has the presynaptic effect on the norepinephrine secretion. Another aspect of this interaction is that the sympathetic nervous system affects the function and expression of local RAS receptors, mainly Ang II receptors. Therefore, in many pathological conditions associated with an increased renal sympathetic tone, these receptors' expression changes and renal denervation can normalize these changes and improve the diseases. It seems that the renal sympathectomy can alter Ang II receptors expression and the distribution of RAS receptors in the kidneys, which influence renal functions.

Treatment of Diabetic Kidney Disease With Hypertension Control and Renin Angiotensin System Inhibition

The global incidence and prevalence of diabetes continues to expand due primarily to the influences of obesity and the contribution of obesity to the progression of type 2 diabetes mellitus. The rising prevalence of type 2 diabetes has driven an increase in rates of CKD in the past 3 decades in the United States. In turn, so have the rates for complications related to type 2 diabetes including CKD, eg, diabetic kidney disease (DKD). Although incident rates for DKD have stabilized in the recent years, diabetes continues to be the leading cause of ESRD in the United States. The United Kingdom Prospective Diabetes Study data and other population-level studies support that lowering blood pressure reduces kidney disease and cardiovascular disease in patients with type 2 diabetes. Furthermore, strategies targeting renin-angiotensin-aldosterone system interruption have shown to improve DKD outcomes to a greater extent than other classes of antihypertensive regimens.

Pharmacological interventions into the renin-angiotensin system with ACE inhibitors and angiotensin II receptor antagonists: effects beyond blood pressure lowering

Hypertension is recognized as an important risk factor for cardiovascular morbidity and mortality. Lowering of blood pressure has been shown to minimize the risk of cardiovascular events, with the majority of antihypertensives demonstrating a similar ability to reduce coronary events and stroke for a given reduction in blood pressure. Agents that modify the activity of the renin-angiotensin system (RAS) have been proposed to exhibit additional effects that might go beyond simple blood pressure lowering. The RAS is a crucial system that regulates extracellular fluid volume and blood pressure. Proposed potential benefits of RAS blockade that go beyond blood pressure lowering include a reduction in platelet aggregation and thrombosis, blunting of cardiac and vascular remodeling, favorable metabolic effects and reno- and cerebro-protection. However, factors such as treatment adherence, duration of action of antihypertensive agents and differences in effects on central versus brachial blood pressure may also result in apparent differences in efficacy of different antihypertensives. The aim of this review article is to examine the available data from clinical studies of antihypertensive drugs for evidence of effects that might legitimately be claimed to go beyond simple blood pressure lowering.

Cellular and Molecular Mechanisms of Chronic Kidney Disease with Diabetes Mellitus and Cardiovascular Diseases as Its Comorbidities

Chronic kidney disease (CKD), diabetes mellitus (DM), and cardiovascular diseases (CVD) are complex disorders of partly unknown genesis and mostly known progression factors. CVD and DM are the risk factors of CKD and are strongly intertwined since DM can lead to both CKD and/or CVD, and CVD can lead to kidney disease. In recent years, our knowledge of CKD, DM, and CVD has been expanded and several important experimental, clinical, and epidemiological associations have been reported. The tight cellular and molecular interactions between the renal, diabetic, and cardiovascular systems in acute or chronic disease settings are becoming increasingly evident. However, the (patho-) physiological basis of the interactions of CKD, DM, and CVD with involvement of multiple endogenous and environmental factors is highly complex and our knowledge is still at its infancy. Not only single pathways and mediators of progression of these diseases have to be considered in these processes but also the mutual interactions of these factors are essential. The recent advances in proteomics and integrative analysis technologies have allowed rapid progress in analyzing complex disorders and clearly show the opportunity for new efficient and specific therapies. More than a dozen pathways have been identified so far, including hyperactivity of the renin–angiotensin (RAS)–aldosterone system, osmotic sodium retention, endothelial dysfunction, dyslipidemia, RAS/RAF/extracellular-signal-regulated kinase pathway, modification of the purinergic system, phosphatidylinositol 3-kinase (PI 3-kinase)-dependent signaling pathways, and inflammation, all leading to histomorphological alterations of the kidney and vessels of diabetic and non-diabetic patients. Since a better understanding of the common cellular and molecular mechanisms of these diseases may be a key to successful identification of new therapeutic targets, we review in this paper the current literature about cellular and molecular mechanisms of CKD.

Inhibition of RAS in diabetic nephropathy

Diabetic kidney disease (DKD) is a progressive proteinuric renal disorder in patients with type 1 or type 2 diabetes mellitus. It is a common cause of end-stage kidney disease worldwide, particularly in developed countries. Therapeutic targeting of the renin-angiotensin system (RAS) is the most validated clinical strategy for slowing disease progression. DKD is paradoxically a low systematic renin state with an increased intrarenal RAS activity implicated in its pathogenesis. Angiotensin II (AngII), the main peptide of RAS, is not only a vasoactive peptide but functions as a growth factor, activating interstitial fibroblasts and mesangial and tubular cells, while promoting the synthesis of extracellular matrix proteins. AngII also promotes podocyte injury through increased calcium influx and the generation of reactive oxygen species. Blockade of the RAS using either angiotensin converting enzyme inhibitors, or angiotensin receptor blockers can attenuate progressive glomerulosclerosis in animal models, and slows disease progression in humans with DKD. In this review, we summarize the role of intrarenal RAS activation in the pathogenesis and progression of DKD and the rationale for RAS inhibition in this population.

AT2R agonist, compound 21, is reno-protective against type 1 diabetic nephropathy

The hemodynamic and nonhemodynamic effects of angiotensin II on diabetic complications are considered to be primarily mediated by the angiotensin II type 1 receptor subtype. However, its biological and functional effect mediated through the angiotensin II type 2 receptor subtype is still unclear. Activation of the angiotensin II type 2 receptors has been postulated to oppose angiotensin II type 1 receptor-mediated actions and thus attenuate fibrosis. This study aimed to elucidate the reno-protective role of the novel selective angiotensin II type 2 receptor agonist, Compound 21, in an experimental model of type 1 diabetic nephropathy. Compound 21 treatment significantly attenuated diabetes mellitus-induced elevated levels of cystatin C, albuminuria, mesangial expansion, and glomerulosclerosis in diabetic mice. Moreover, Compound 21 markedly inhibited the expression of various proteins implicated in oxidative stress, inflammation, and fibrosis, in association with decreased extracellular matrix production. These findings demonstrate that monotherapy of Compound 21 is protective against the progression of experimental diabetic nephropathy by inhibiting renal oxidative stress, inflammation, and fibrosis.

Maternal low-protein diet in rat pregnancy programs blood pressure through sex-specific mechanisms

Animal models support human epidemiological studies in demonstrating a relationship between impaired fetal growth and risk of adult hypertension. Undernutrition during pregnancy exerts programming effects on the developing kidney, and modulation of angiotensin receptor (ATR) expression has been observed persisting into adult life. Fetal overexposure to glucocorticoids is thought to be central to the nutritional programming of blood pressure and may act through an interaction with ATR expression. Pregnant female Wistar rats were fed a control ( n = 6) or a maternal low-protein diet (MLP; n = 17) throughout pregnancy. The glucocorticoid dependency of MLP effects was tested using metyrapone, an inhibitor of corticosterone synthesis. MLP-fed rats were injected twice daily with metyrapone, metyrapone plus corticosterone, or vehicle over days 1–14 of pregnancy. At delivery, all animals were fed standard laboratory chow. MLP-exposed offspring 4 wk of age exhibited increased systolic blood pressure compared with controls ( P < 0.05), which proved to be glucocorticoid dependent in males only. AT1R mRNA expression was independent of in utero dietary treatment. AT2R mRNA expression was downregulated in MLP-exposed females only ( P < 0.05) and in a glucocorticoid-independent manner. Male offspring exhibited glucocorticoid-dependent hypertension with no modulation of renal ATR mRNA expression. In contrast, female offspring exhibited glucocorticoid-independent hypertension associated with reduced expression of renal AT2R mRNA. These data do not support the hypothesis that an interaction between glucocorticoid and ATR mRNA expression underlies the nutritional programming of blood pressure but instead suggest two independent mechanisms acting in a sex-specific manner.

AT1 and AT2 receptor in the kidney: role in health and disease

The renin angiotensin system plays an important role in the control of body fluid and electrolyte homeostasis and blood pressure regulation. Angiotensin II is the most effector hormone of this system and functions mainly through stimulation of its subtype receptors, namely, the AT1 and AT2 receptors. Most of the known physiological and pathologic effects of angiotensin II are mediated through stimulation of the AT1 receptor. The knowledge about the involvement of the AT2 receptor in physiological and pathologic processes is still evolving. In the kidney, both the AT1 and AT2 receptors contribute to the regulation of renal hemodynamic and tubular functions. Also, these receptors regulate renal cellular growth and matrix formation. However, AT2 receptor possesses functions that counteract the effects of the AT1 receptor. The balance between the AT1 and AT2 receptors can determine the renal status in health and disease.

The renin-angiotensin-aldosterone system and the kidney: effects on kidney disease

The renin-angiotensin-aldosterone system regulates renal vasomotor activity, maintains optimal salt and water homeostasis, and controls tissue growth in the kidney. However, pathologic consequences can result from overactivity of this cascade, involving it in the pathophysiology of kidney disease. An activated renin-angiotensin-aldosterone system promotes both systemic and glomerular capillary hypertension, which can induce hemodynamic injury to the vascular endothelium and glomerulus. In addition, direct profibrotic and proinflammatory actions of angiotensin II and aldosterone may also promote kidney damage. The majority of the untoward effects associated with angiotensin II appear to be mediated through its binding to the angiotensin II type 1 receptor. Aldosterone can also induce renal injury by binding to its receptor in the kidney. An understanding of this system is important to appreciate that inhibitors of this cascade can reduce the progression of chronic kidney disease in proteinuric disease states. Pharmacologic agents that can interfere with this cascade include angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, and aldosterone receptor antagonists. This paper will provide an overview of the renin-angiotensin system, review its role in kidney disease, examine the renal effects of inhibition of this cascade in experimental animal models, and review clinical studies utilizing renin-angiotensin-aldosterone inhibitors in patients with diabetic and nondiabetic nephropathies.

The renin-angiotensin-aldosterone system: cardiorenal effects and implications for renal and cardiovascular disease states

The renin-angiotensin-aldosterone system (RAAS) plays an integral role in maintaining vascular tone, optimal salt and water homeostasis, and cardiac function in humans. However, it has been recognized in recent years that pathologic consequences may also result from overactivity of the RAAS. Clinical disease states such as renal artery stenosis, hypertension, diabetic and nondiabetic nephropathies, left ventricular hypertrophy, coronary atherosclerosis, myocardial infarction, and congestive heart failure (CHF) are examples. Part of the adverse cardiorenal effects of the RAAS may be related to the prominent role that this system plays in the activation of the sympathetic nervous system, the dysregulation of endothelial function and progression of atherosclerosis, as well as inhibition of the fibrinolytic system. Also, direct profibrotic actions of angiotensin II and aldosterone in the kidney and heart promote end organ injury. Current basic science and clinical research supports the use of inhibitors of the RAAS, including angiotensin converting enzyme inhibitors, angiotensin receptor blockers, and aldosterone antagonists in treating hypertension, improving diabetic nephropathy and other forms of chronic kidney disease, preventing or ameliorating CHF, and optimizing prognosis after myocardial infarction.

The effects of losartan on renal function in the newborn rabbit

The low GFR of newborns is maintained by various factors including the renin-angiotensin system. We previously established the importance of angiotensin II in the newborn kidney, using the angiotensin-converting enzyme inhibitor perindoprilat. The present study was designed to complement these observations by evaluating the role of angiotensin-type 1 (AT(1)) receptors, using losartan, a specific AT(1)-receptor blocker. Increasing doses of losartan were infused into anesthetized, ventilated, newborn rabbits. Renal function and hemodynamic variables were assessed using inulin and para-aminohippuric acid clearances as markers of GFR and renal plasma flow, respectively. Losartan 0.1 mg/kg slightly decreased mean blood pressure (-11%) and increased diuresis (+22%). These changes can be explained by inhibition of the AT(1)-mediated vasoconstrictive and antidiuretic effects of angiotensin, and activation of vasodilating and diuretic AT(2) receptors widely expressed in the neonatal period. GFR and renal blood flow were not modified. Losartan 0.3 mg/kg decreased mean blood pressure significantly (-15%), probably by inhibiting systemic AT(1) receptors. GFR significantly decreased (-25%), whereas renal blood flow remained stable. The decrease in filtration fraction (-21%) indicates predominant efferent vasodilation. At 3 mg/kg, the systemic hypotensive effect of losartan was marked (mean blood pressure, -28%), with decreased GFR and renal blood flow (-57% and -51%, respectively), a stable filtration fraction, and an increase in renal vascular resistance by 124%. The renal response to this dose can be considered as reflex vasoconstriction of afferent and efferent arterioles, rather than specific receptor antagonism. We conclude that under physiologic conditions, the renin-angiotensin is critically involved in the maintenance of GFR in the immature kidney.

Angiotensin II type 2 receptors: signalling and pathophysiological role

The signalling mechanisms and biological significance of the angiotensin II type 2 receptor have long been unknown. In recent years, studies, first in cell culture models but now increasingly also in vivo, have shed some light on the molecular events occurring after a stimulation of the receptor with its ligand as well as on its physiological effects and its significance for pathophysiological processes. There is increasing evidence that the angiotensin II type 2 receptor is involved in different pathophysiological processes, such as myocardial infarction, heart and kidney failure, and stroke.