Jaime Herrera-Acosta

Is There a Pathogenetic Role for Uric Acid in Hypertension and Cardiovascular and Renal Disease

Hyperuricemia is associated with hypertension, vascular disease, renal disease, and cardiovascular events. In this report, we review the epidemiologic evidence and potential mechanisms for this association. We also summarize experimental studies that demonstrate that uric acid is not inert but may have both beneficial functions (acting as an antioxidant) as well as detrimental actions (to stimulate vascular smooth muscle cell proliferation and induce endothelial dysfunction). A recently developed experimental model of mild hyperuricemia also provides the first provocative evidence that uric acid may have a pathogenic role in the development of hypertension, vascular disease, and renal disease. Thus, it is time to reevaluate the role of uric acid as a risk factor for cardiovascular disease and hypertension and to design human studies to address this controversy.

Hyperuricemia Causes Glomerular Hypertrophy in the Rat

Background/Aims: Rats with mild hyperuricemia develop systemic hypertension, interstitial renal disease, afferent arteriolopathy, and increased renin expression [Mazzali et al.: Am J Physiol 2002;6:F991–F997]. We hypothesized that hyperuricemia might also induce glomerular changes. Methods: We reviewed renal biopsies of rats previously made hyperuricemic for 7 weeks with the uricase inhibitor, oxonic acid. Controls included normal rats and oxonic acid-treated rats administered allopurinol, benziodarone, hydrochlorothiazide, or enalapril. Glomeruli were examined for size (computer image analysis) and structure (histology). An additional group of rats were administered oxonic acid or control diet for 6 months. Results: Renal biopsies showed that hyperuricemic rats had a 30% increase in glomerular tuft area (p < 0.01); these changes were prevented by allopurinol and benziodarone. Control of blood pressure with hydrochlorothiazide did not prevent the development of glomerular hypertrophy, whereas enalapril partially reduced the glomerular hypertrophy. Prolonged hyperuricemia was associated with the development of microalbuminuria (p < 0.05) and glomerulosclerosis (22 vs. 10%, p < 0.05) compared to control rats. Conclusions: Hyperuricemic rats develop glomerular hypertrophy that can be prevented in part by ACE inhibitor therapy. Prolonged hyperuricemia is associated with the development of glomerulosclerosis in the rat.

Role of NO in cyclosporin nephrotoxicity: effects of chronic NO inhibition and NO synthases gene expression

The role of nitric oxide (NO) during cyclosporin renal vasoconstriction was evaluated by glomerular hemodynamic and histological changes produced by chronic NO synthesis inhibition and neuronal (nNOS), inducible (iNOS), and endothelial (eNOS) NO synthases mRNA expression in renal cortex and medulla. Uninephrectomized rats treated during 7 days with vehicle (Veh), cyclosporin A (CsA) 30 mg/kg, CsA + nitro-l-arginine methyl ester (l-NAME), and Veh +l-NAME (10 mg/dl) in the drinking water were studied. Increase in arterial pressure and afferent and efferent resistances, as well as decrease in glomerular plasma flow, ultrafiltration coefficient, and single-nephron glomerular filtration rate were significantly greater with CsA +l-NAME than with CsA alone. The increase in afferent resistance was higher with CsA +l-NAME than with Veh +l-NAME. In addition, glomerular thrombosis, proximal tubular vacuolization, and arteriolar thickening were more prominent. In renal cortex, eNOS mRNA expression exhibited a 2.7-fold increase in CsA, whereas, in medulla, nNOS and iNOS expression were lower in CsA than in Veh, while eNOS tended to increase. Our results support the hypothesis that NO synthesis is enhanced at cortical level during CsA nephrotoxicity, counterbalancing predominantly preglomerular vasoconstriction. Higher NO production could be the result of increased eNOS mRNA expression.The role of nitric oxide (NO) during cyclosporin renal vasoconstriction was evaluated by glomerular hemodynamic and histological changes produced by chronic NO synthesis inhibition and neuronal (nNOS), inducible (iNOS), and endothelial (eNOS) NO syntheses mRNA expression in renal cortex and medulla. Uninephrectomized rats treated during 7 days with vehicle (Veh), cyclosporin A (CsA) 30 mg/kg, CsA + nitro-L-arginine methyl ester (L-NAME), and Veh + L-NAME (10 mg/dl) in the drinking water were studied. Increase in arterial pressure and afferent and efferent resistances, as well as decrease in glomerular plasma flow, ultrafiltration coefficient, and single-nephron glomerular filtration rate were significantly greater with CsA + L-NAME than with CsA alone. The increase in afferent resistance was higher with CsA + L-NAME than with Veh + L-NAME. In addition, glomerular thrombosis, proximal tubular vacuolization, and arteriolar thickening were more prominent. In renal cortex, eNOS mRNA expression exhibited a 2.7-fold increase in CsA, whereas, in medulla, nNOS and iNOs expression were lower in CsA than in Veh, while eNOS tended to increase. Our results support the hypothesis that NO synthesis is enhanced at cortical level during CsA nephrotoxicity, counterbalancing predominantly preglomerular vasoconstriction. Higher NO production could be the result of increased eNOS mRNA expression.

Hormonal and cytokine effects of uric acid

Purpose of reviewCurrent evidence supports the role of soluble uric acid as a true mediator of injury, exerting its effects through the induction of growth factors, cytokines, hormones and autacoids. In the present review, we summarize recent studies on the mechanisms involved in the uric acid deleterious effects. Recent findingsAlthough uric acid is considered an antioxidant in plasma, recent clinical and epidemiological studies have found that hyperuricemia is associated with mortality and development of hypertension, cardiovascular and chronic renal diseases. Experimental studies suggest that uric acid induce its detrimental effects at the cellular level entering to vascular smooth muscle cells (VSMC) via an organic anion transport system, and followed by the activation of specific MAP kinases, nuclear transcription factors, with stimulation of COX-2, PDGF A and C chain, PDGF alpha receptor, and various inflammatory mediators, including C-reactive protein and monocyte chemoattractant protein-1. Physiologically, these effects translate into a rise of arterial pressure, VSMC hypertrophy, tubulointerstitial infiltration and glomerular hypertension in the setting of renal vasoconstriction. Uric acid also promotes endothelial dysfunction through inactivation of NO and arresting the proliferation of endothelial cells. Thus, arteriosclerosis induced by hyperuricemia may be a novel mechanism for the development of essential hypertension. SummarySoluble uric acid has important biologic roles. While it acts as an antioxidant, there is also evidence that uric acid has pro-inflammatory and proliferative effects on VSMC, and causes dysfunction of endothelial cells. These cellular mechanisms may translate into why uric acid is associated with renal and cardiovascular disease.

Uric Acid – A Uremic Toxin?

Uric acid might often be regarded as a simple marker of renal disease. Although it is well known that hyperuricemia causes gout which is associated with renal insufficiency and cardiovascular disease, one might think that it could attribute to the intrarenal urate crystal, but not to uric acid per se. In order to clarify the role of uric acid in the kidney, we hypothesized that uric acid causes renal disease. To generate mild hyperuricemia without intrarenal crystal in rats, we used low doses of an uricase inhibitor (2% oxonic acid). Hyperuricemia induced systemic hypertension, glomerular hypertrophy/hypertension, afferent arteriolar sclerosis, and macrophage infiltration in normal rat kidney. In progressive renal disease, such as cyclosporine nephropathy and remnant kidney in rat, uric acid accelerated the progression of renal disease. Thus, we concluded that uric acid is not a simple marker, but a cause of renal disease.

A single pathway for the development of essential hypertension.

The majority of subjects (85–90%) with elevated blood pressure are labeled as having essential hypertension that by definition has no identifiable underlying cause. This review aims to challenge this concept through an appraisal of both past and recent studies, providing an argument that some forms of essential hypertension may have underlying mild renal disease as an etiologic factor. While it is well established that nearly all patients with essential hypertension have mild structural changes in their kidneys, many view these changes as being secondary to the hypertension. Recent animal studies provide evidence that multiple initiating factors may lead to minor structural alterations in the renal microvasculature and tubulointerstitium, which in turn are important in the etiology of salt-sensitive hypertension. Despite these minor structural abnormalities, the usual screening tests for renal causes of hypertension (eg serum creatinine and urinary protein excretion) may not be significantly altered, at least initially. Data from several animal models suggest that sodium retention may result from the combined effects of preglomerular arteriolosclerosis and renal interstitial inflammation leading to the activation of various signaling pathways (including the COX-2, renin-angiotensin, and nitric oxide systems) and increased oxidative stress. Therapies aimed at remodeling the renal microvasculature, reducing the immune cell infiltrate, and ameliorating intrarenal hypoxia during the early phase of hypertensive disease may have an important role in future treatment strategies for certain forms of hypertension and require further study.

Dexamethasone increases eNOS gene expression and prevents renal vasoconstriction induced by cyclosporin.

Cyclosporin A (CsA)-induced renal vasoconstriction (RV) is attributed to an imbalance in vasoactive factors release. Dexamethasone (Dex) exerts a renal vasodilatory effect by a mechanism not yet characterized. This study evaluates whether the effect of Dex is mediated by NO and whether it prevents CsA-induced RV. Micropuncture studies were performed in six groups of uninephrectomized rats treated for 7 days with the following: vehicle (Veh); Veh + 4 mg/kg dexamethasone (Veh+Dex); 30 mg/kg CsA; CsA+Dex; vehicle + 10 mg/kg nitro-L-arginine methyl ester (Veh+L-NAME); and Veh+Dex+L-NAME. NO synthase (NOS) isoform mRNA levels were evaluated in renal cortex and medulla by semiquantitative RT-PCR analysis in the first four groups. Dex produced renal vasodilation, which was blocked by concomitant L-NAME administration, and the effect of Dex was associated with higher cortical and medullary endothelial NOS (eNOS) and cortical inducible NOS (iNOS) mRNA levels. In the CsA group, Dex prevented RV, restoring glomerular hemodynamics to control values. These changes were associated with further enhancement of eNOS and restoration of medullary iNOS and neuronal NOS (nNOS) expression. We conclude that Dex prevents CsA-induced RV, and its vasodilator effect could be mediated by increased intrarenal generation of NO, secondary to enhanced expression of eNOS and iNOS.Cyclosporin A (CsA)-induced renal vasoconstriction (RV) is attributed to an imbalance in vasoactive factors release. Dexamethasone (Dex) exerts a renal vasodilatory effect by a mechanism not yet characterized. This study evaluates whether the effect of Dex is mediated by NO and whether it prevents CsA-induced RV. Micropuncture studies were performed in six groups of uninephrectomized rats treated for 7 days with the following: vehicle (Veh); Veh + 4 mg/kg dexamethasone (Veh+Dex); 30 mg/kg CsA; CsA+Dex; vehicle + 10 mg/kg nitro-l-arginine methyl ester (Veh+l-NAME); and Veh+Dex+l-NAME. NO synthase (NOS) isoform mRNA levels were evaluated in renal cortex and medulla by semiquantitative RT-PCR analysis in the first four groups. Dex produced renal vasodilation, which was blocked by concomitant l-NAME administration, and the effect of Dex was associated with higher cortical and medullary endothelial NOS (eNOS) and cortical inducible NOS (iNOS) mRNA levels. In the CsA group, Dex prevented RV, restoring glomerular hemodynamics to control values. These changes were associated with further enhancement of eNOS and restoration of medullary iNOS and neuronal NOS (nNOS) expression. We conclude that Dex prevents CsA-induced RV, and its vasodilator effect could be mediated by increased intrarenal generation of NO, secondary to enhanced expression of eNOS and iNOS.

Nifedipine Prevents Changes in Nitric Oxide Synthase mRNA Levels Induced by Cyclosporine

Cyclosporine toxicity mainly affects kidney and liver function. We have previously shown that cyclosporine nephrotoxicity alters kidney nitric oxide synthase mRNA pattern of expression. To determine if nitric oxide synthase expression changes are mediated directly by cyclosporine or by secondary hemodynamic alterations induced by cyclosporine, we evaluated if these effects are tissue specific and if nifedipine-induced vasodilation prevents these alterations. Uninephrectomized Wistar rats treated for 7 days with olive oil, cyclosporine (30 mg/kg), nifedipine (3 mg/kg), and nifedipine+cyclosporine were studied. In vehicle and cyclosporine groups, the gene expression of the neuronal, inducible, and endothelial nitric oxide synthases in cerebellum, heart, intestine, liver, renal cortex, and medulla was evaluated. The administration of cyclosporine was associated with nephrotoxicity and hepatotoxicity, increased endothelial nitric oxide synthase mRNA levels in renal cortex and liver, and a decrease in inducible nitric oxide synthase and neuronal nitric oxide synthase in renal medulla. The mRNA levels of the 3 nitric oxide synthase isoforms were not affected in any other tissue. Nifedipine did not alter nitric oxide synthase expression in the control group but prevented changes associated with cyclosporine. These results suggest that cyclosporine-induced changes in the pattern of expression of the nitric oxide synthases may be secondary to its hemodynamic effects.

Pentosan polysulfate treatment reduces cyclosporine-induced nephropathy in salt-depleted rats.

BACKGROUND Long-term cyclosporine (CsA) treatment leads to a decreased glomerular filtration rate, hyalinosis of afferent arterioles, and striped cortical tubulo-interstitial fibrosis. We showed previously that pentosan polysulfate (SP54) prevented the development of microvascular and interstitial lesions in mouse models of progressive glomerulosclerosis. In this study, we examined the effect of pentosan polysulfate on the development of CsA nephropathy. METHODS Pair-fed Sprague-Dawley rats were fed a low-sodium (0.03%) diet and received CsA (15 mg/kg, subcutaneously, in olive oil)/5% glucose, pentosan polysulfate (10 mg/kg, subcutaneously in 5% glucose) plus CsA, olive oil/pentosan polysulfate, or olive oil/5% glucose for 30 days. Creatinine clearance (CrCl) was determined at three time points. Afferent arteriolar lesions, glomerular volume, and tubulo-interstitial lesions were quantitated. RNA was extracted from cortex. RESULTS Severe lesions were found in the CsA group. A reduction in the number of affected arterioles (32%) and the degree of chronic tubulo-interstitial lesions (44%) was found in pentosan polysulfate/CsA-treated rats. A 20% decrease in glomerular volume was found in CsA rats, but not in pentosan polysulfate/CsA-treated rats. Pentosan polysulfate treatment did not prevent the CsA-induced decrease in CrCl (approximately 30%) at 4 weeks. CsA did not affect cortical endothelial or neuronal nitric-oxide synthase or mRNA levels, but there was small increase in neuronal nitric-oxide synthase mRNA levels in the pentosan polysulfate/CsA-treated group. CONCLUSIONS Pentosan polysulfate reduced structural renal lesions in CsA-treated, salt-depleted Sprague-Dawley rats.

Effect of Mycophenolate Mofetil on Kallikrein Expression in the Kidney of 5/6 Nephrectomized Rats

It has recently been proposed that tubulointerstitial damage plays a key role in the pathogenesis of sodium-dependent hypertension. Since components, enzymes and substrates, of the kallikrein-kinin system (KKS) are synthesized by connecting and collecting tubules, respectively, it is expected that damage of any origin, involving the tubulointerstitial compartment, may affect the functionality of these nephron segments and impair blood pressure control. Therefore, we analyzed renal kallikrein expression in the 5/6 renal ablation model, which is characterized by a progressive tubulointerstitial damage and systemic hypertension. In addition, we studied the renal expression of this enzyme after treatment of healthy and 5/6 nephrectomized rats with mycophenolate mofetil (MMF), an immunosuppressive drug known to reduce tubulointerstitial damage in this model. Twenty-six male Sprague-Dawley rats were included in this study. Seven 5/6 nephrectomized rats (Nx), 4 sham-operated (Sham) rats and 5 Nx rats treated with MMF (Nx + MMF) were studied 4 weeks after surgery. For comparison, 6 healthy rats treated with MMF at the same dose were compared with 4 vehicle-treated controls. Tubulointerstitial damage was significantly high in Nx compared with Nx-MMF and sham-operated rats. Blood pressure was significantly higher in Nx (178 ± 7.8 mm Hg) than in Sham (120 ± 2.0 mm Hg, p < 0.05) and Nx + MMF animals (154 ± 5.6 mm Hg, p < 0.05). Renal kallikrein expression, quantified by a computer image system was significantly lower in the Nx group (1,696 ± 437 density/mm2) than in Sham (9,779 ± 4,068 density/mm2, p < 0.05), and in Nx + MMF groups (4,640 ± 1,578 density/mm2, p < 0.05). Healthy animals treated with MMF did not show tubulointerstitial damage, changes in blood pressure nor changes in the expression of immunoreactive renal kallikrein suggesting that improvement in kallikrein expression after MMF treatment of 5/6 nephrectomy was not due to a direct effect of the drug on kallikrein-producing cells. Our results suggest that protection of the KKS after 5/6 nephrectomy may have additional renoprotective effects and may reduce the progression of renal disease.