Niwanthi W. Rajapakse

High-Fiber Diet and Acetate Supplementation Change the Gut Microbiota and Prevent the Development of Hypertension and Heart Failure in Hypertensive Mice

Background: Dietary intake of fruit and vegetables is associated with lower incidence of hypertension, but the mechanisms involved have not been elucidated. Here, we evaluated the effect of a high-fiber diet and supplementation with the short-chain fatty acid acetate on the gut microbiota and the prevention of cardiovascular disease. Methods: Gut microbiome, cardiorenal structure/function, and blood pressure were examined in sham and mineralocorticoid excess–treated mice with a control diet, high-fiber diet, or acetate supplementation. We also determined the renal and cardiac transcriptome of mice treated with the different diets. Results: We found that high consumption of fiber modified the gut microbiota populations and increased the abundance of acetate-producing bacteria independently of mineralocorticoid excess. Both fiber and acetate decreased gut dysbiosis, measured by the ratio of Firmicutes to Bacteroidetes, and increased the prevalence of Bacteroides acidifaciens. Compared with mineralocorticoid-excess mice fed a control diet, both high-fiber diet and acetate supplementation significantly reduced systolic and diastolic blood pressures, cardiac fibrosis, and left ventricular hypertrophy. Acetate had similar effects and markedly reduced renal fibrosis. Transcriptome analyses showed that the protective effects of high fiber and acetate were accompanied by the downregulation of cardiac and renal Egr1, a master cardiovascular regulator involved in cardiac hypertrophy, cardiorenal fibrosis, and inflammation. We also observed the upregulation of a network of genes involved in circadian rhythm in both tissues and downregulation of the renin-angiotensin system in the kidney and mitogen-activated protein kinase signaling in the heart. Conclusions: A diet high in fiber led to changes in the gut microbiota that played a protective role in the development of cardiovascular disease. The favorable effects of fiber may be explained by the generation and distribution of one of the main metabolites of the gut microbiota, the short-chain fatty acid acetate. Acetate effected several molecular changes associated with improved cardiovascular health and function.

Role of L-arginine in nitric oxide production in health and hypertension.

1 l‐Arginine is the substrate for vascular nitric oxide (NO) formation. Under normal physiological conditions, intracellular l‐arginine levels far exceed the Km of NO synthase for l‐arginine. However, endogenous NO formation is dependent on extracellular l‐arginine concentrations, giving rise to the concept of the ‘l‐arginine paradox’. 2 Nitric oxide production in epithelial and endothelial cells is closely coupled to cellular l‐arginine uptake, indicating that l‐arginine transport mechanisms play a major role in the regulation of NO‐dependent function. 3 Consistent with the data in endothelial and epithelial cells are functional data indicating that exogenous l‐arginine can increase renal vascular and tubular NO bioavailability and thereby influence kidney perfusion, function and arterial pressure. The integrated effect of increased cellular l‐arginine transport is to lower arterial pressure. Therefore, the use of l‐arginine in the treatment of hypertension warrants investigation. 4 Low NO bioavailability is central to the development and maintenance of hypertension and to related endothelial dysfunction and target organ damage. We propose that l‐arginine can interrupt the vicious cycle that initiates and maintains low NO in hypertension by increasing the formation of NO.

Nitric oxide in responses of regional kidney blood flow to vasoactive agents in anesthetized rabbits.

To determine whether differential release of nitric oxide underlies the diversity of regional kidney blood flow responses to vasoactive agents, this study examined how nitric oxide synthase blockade with IV NG-nitro-l-arginine (l-NNA), and also IV l-NNA plus co-infusion of glyceryl trinitrate, affected responses to renal arterial boluses and infusions of vasoactive agents. l-NNA, but not vehicle, or l-NNA plus glyceryl trinitrate, increased mean arterial pressure (35%) and reduced renal blood flow (20%), cortical perfusion (11%), and medullary perfusion (54%). l-NNA plus glyceryl trinitrate, but not l-NNA alone, blunted renal vasodilatation in response to boluses of bradykinin and acetylcholine, abolished increased medullary perfusion after bolus angiotensin II, and enhanced reductions in medullary perfusion, and to a lesser extent those in renal blood flow and cortical perfusion, during norepinephrine infusion. Neither l-NNA, nor l-NNA plus glyceryl trinitrate, affected responses to infusions of angiotensin II, [Phe2,Ile3,Orn8]-vasopressin, or endothelin-1. The data indicate roles for nitric oxide in angiotensin II–induced increases in medullary perfusion and in protecting medullary perfusion from norepinephrine-induced vasoconstriction. However, differential engagement of nitric oxide synthase cannot completely account for the diversity of responses of regional kidney perfusion to vasoactive agents. Effects of nitric oxide synthase blockade on renal vascular responses to vasoactive agents were revealed only when glyceryl trinitrate was co-infused to restore resting nitrergic vasodilator tone. This may reflect interactions between nitric oxide and other vasodilator mediators, in modulating renal hemodynamic responses to vasoactive agents.

Renal oxygenation in acute renal ischemia-reperfusion injury

Tissue hypoxia has been demonstrated, in both the renal cortex and medulla, during the acute phase of reperfusion after ischemia induced by occlusion of the aorta upstream from the kidney. However, there are also recent clinical observations indicating relatively well preserved oxygenation in the nonfunctional transplanted kidney. To test whether severe acute kidney injury can occur in the absence of widespread renal tissue hypoxia, we measured cortical and inner medullary tissue Po2 as well as total renal O2 delivery (Do2) and O2 consumption (Vo2) during the first 2 h of reperfusion after 60 min of occlusion of the renal artery in anesthetized rats. To perform this experiment, we used a new method for measuring kidney Do2 and Vo2 that relies on implantation of fluorescence optodes in the femoral artery and renal vein. We were unable to detect reductions in renal cortical or inner medullary tissue Po2 during reperfusion after ischemia localized to the kidney. This is likely explained by the observation that Vo2 (-57%) was reduced by at least as much as Do2 (-45%), due to a large reduction in glomerular filtration (-94%). However, localized tissue hypoxia, as evidence by pimonidazole adduct immunohistochemistry, was detected in kidneys subjected to ischemia and reperfusion, particularly in, but not exclusive to, the outer medulla. Thus, cellular hypoxia, particularly in the outer medulla, may still be present during reperfusion even when reductions in tissue Po2 are not detected in the cortex or inner medulla.

Exogenous l-Arginine Ameliorates Angiotensin II-Induced Hypertension and Renal Damage in Rats

Experiments were performed to determine whether exogenous l-arginine could ameliorate angiotensin II–induced hypertension and renal damage. Rats were instrumented with chronic indwelling femoral venous and arterial catheters for infusions of drugs and measurement of conscious arterial pressure. Arterial blood pressure significantly increased from 124±1 to 199±4 mm Hg, after 9 days of continuous infusion of angiotensin II (20 ng/kg per minute; IV; n=6 to 9). In contrast, the increase in arterial pressure after 9 days of angiotensin II infusion was significantly blunted by 45% (P=0.0003) in rats coadministered l-arginine (300 &mgr;g/kg per minute; IV; n=7 to 9). The glomerular injury index was significantly greater in rats administered angiotensin II in comparison with rats administered saline vehicle (P<0.001). Coinfusion of l-arginine significantly increased plasma nitrate/nitrite concentrations (P<0.001) and completely prevented angiotensin II–induced glomerular damage (P<0.001). Angiotensin II infusion alone and combined angiotensin II plus l-arginine infusion significantly increased urinary albumin excretion. Albuminuria in rats administered angiotensin II plus l-arginine is likely to be because of increased intraglomerular pressure. Our experiments demonstrate that l-arginine can blunt angiotensin II–induced hypertension and associated renal damage. This latter observation is most exciting because it indicates that increasing NO bioavailability, in addition to lowering arterial pressure, can greatly reduce hypertension-induced renal damage.

Effects of indomethacin on responses of regional kidney perfusion to vasoactive agents in rabbits

1. To determine whether differential release of products of arachidonic acid metabolism, via the cyclo‐oxygenase pathway, underlies the diversity of responses of regional kidney perfusion to vasoactive agents, we tested the effects of intravenous indomethacin on responses to renal arterial bolus doses of vasoactive agents in pentobarbitone‐anaesthetized rabbits.

Angiotensin II and neurohumoral control of the renal medullary circulation

1. Angiotensin (Ang) II has multiple actions in the renal medullary circulation. It can induce vasodilatation and blunt the response of medullary blood flow (MBF) to renal nerve activation through AT1 receptor‐mediated release of nitric oxide (NO) and/or vasodilator prostaglandins. These actions require high intravascular and/or intratubular AngII concentrations, so are not apparent under physiological conditions.

Role of cellular L-arginine uptake and nitric oxide production on renal blood flow and arterial pressure regulation.

Purpose of reviewL-Arginine (L-Arg) is the substrate for nitric oxide (NO) formation. Reduced NO bioavailability, particularly within the renal circulation, has been identified as a key factor in the pathogenesis of hypertension. This review focuses on the pathogenic role of abnormal L-Arg transport, particularly within the kidney, in hypertension. Recent findingsMost recent studies have attempted to restore NO bioavailability in cardiovascular diseases with the use of antioxidants to reduce NO inactivation, but this approach has failed to provide beneficial effects in the clinical setting. We argue that this may be due to reduced NO formation in hypertension, which has largely been overlooked as a means of restoring NO bioavailability in cardiovascular diseases. Recent data indicate that renal L-Arg transport plays an important role in regulating both renal perfusion and function and the long-term set point of arterial pressure in health. Perturbations in the renal L-Arg transport system can give rise to abnormal renal perfusion and function, initiating hypertension and related renal damage. SummaryAccordingly, we propose that L-Arg transporters are a new treatment target in hypertension and in disease states where renal NO bioavailability is disturbed.

Augmented endothelial-specific L-arginine transport prevents obesity-induced hypertension

Hypertension is a major clinical complication of obesity. Our previous studies show that abnormal uptake of the nitric oxide precursor L‐arginine, via the cationic amino acid transporter‐1 (CAT1), contributes to endothelial dysfunction in cardiovascular disease. In this study, we tested the hypothesis that abnormal L‐arginine transport may be a key mediator of obesity‐induced hypertension.

N‐acetylcysteine attenuates the development of cardiac fibrosis and remodeling in a mouse model of heart failure

Oxidative stress plays a central role in the pathogenesis of heart failure. We aimed to determine whether the antioxidant N‐acetylcysteine can attenuate cardiac fibrosis and remodeling in a mouse model of heart failure. Minipumps were implanted subcutaneously in wild‐type mice (n = 20) and mice with cardiomyopathy secondary to cardiac specific overexpression of mammalian sterile 20‐like kinase 1 (MST‐1; n = 18) to administer N‐acetylcysteine (40 mg/kg per day) or saline for a period of 8 weeks. At the end of this period, cardiac remodeling and function was assessed via echocardiography. Fibrosis, oxidative stress, and expression of collagen types I and III were quantified in heart tissues. Cardiac perivascular and interstitial fibrosis were greater by 114% and 209%, respectively, in MST‐1 compared to wild type (P ≤ 0.001). In MST‐1 mice administered N‐acetylcysteine, perivascular and interstitial fibrosis were 40% and 57% less, respectively, compared to those treated with saline (P ≤ 0. 03). Cardiac oxidative stress was 119% greater in MST‐1 than in wild type (P < 0.001) and N‐acetylcysteine attenuated oxidative stress in MST‐1 by 42% (P = 0.005). These data indicate that N‐acetylcysteine can blunt cardiac fibrosis and related remodeling in the setting of heart failure potentially by reducing oxidative stress. This study provides the basis to investigate the role of N‐acetylcysteine in chronic heart failure.