Analysis of arterial pressure regulating systems in renal post-transplantation hypertension

Molecular remodeling of the renin-angiotensin system after kidney transplantation

Objective:

We aimed at assessing the molecular adaptation of the renin-angiotensin system (RAS) after successful kidney transplantation (KTX).

Materials and methods:

In this prospective, exploratory study we analyzed 12 hemodialysis (HD) patients, who received a KTX and had excellent graft function six to 12 months thereafter. The concentrations of plasma Angiotensin (Ang) peptides (Ang I, Ang II, Ang-(1–7), Ang-(1–5), Ang-(2–8), Ang-(3–8)) were simultaneously quantified with a novel mass spectrometry-based method. Further, renin and aldosterone concentrations were determined by standard immunoassays.

Results:

Ang values showed a strong inter-individual variability among HD patients. Yet, despite a continued broad dispersion of Ang values after KTX, a substantial improvement of the renin/Ang II correlation was observed in patients without RAS blockade or on angiotensin receptor blocker (HD: renin/Ang II R² = 0.660, KTX: renin/Ang II R² = 0.918). Ang-(1–7) representing the alternative RAS axis was only marginally detectable both on HD and after KTX.

Conclusions:

Following KTX, renin-dependent Ang II formation adapts in non-ACE inhibitor-treated patients. Thus, a largely normal RAS regulation is reconstituted after successful KTX. However, individual Ang concentration variations and a lack of potentially beneficial alternative peptides after KTX call for individualized treatment. The long-term post-transplant RAS regulation remains to be determined.

Pressure natriuresis and the renal control of arterial blood pressure

The regulation of extracellular fluid volume by renal sodium excretion lies at the centre of blood pressure homeostasis. Renal perfusion pressure can directly regulate sodium reabsorption in the proximal tubule. This acute pressure natriuresis response is a uniquely powerful means of stabilizing long-term blood pressure around a set point. By logical extension, deviation from the set point can only be sustained if the pressure natriuresis mechanism is impaired, suggesting that hypertension is caused or sustained by a defect in the relationship between renal perfusion pressure and sodium excretion. Here we describe the role of pressure natriuresis in blood pressure control and outline the cascade of biophysical and paracrine events in the renal medulla that integrate the vascular and tubular response to altered perfusion pressure. Pressure natriuresis is impaired in hypertension and mechanistic insight into dysfunction comes from genetic analysis of blood pressure disorders. Transplantation studies in rats show that blood pressure is determined by the genotype of the kidney and Mendelian hypertension indicates that the distal nephron influences the overall natriuretic efficiency. These approaches and the outcomes of genome-wide-association studies broaden our view of blood pressure control, suggesting that renal sympathetic nerve activity and local inflammation can impair pressure natriuresis to cause hypertension. Understanding how these systems interact is necessary to tackle the global burden of hypertension.

A linear relationship between the ex-vivo sodium mediated expression of two sodium regulatory pathways as a surrogate marker of salt sensitivity of blood pressure in exfoliated human renal proximal tubule cells: the virtual renal biopsy

BACKGROUND

Salt sensitivity (SS) of blood pressure (BP) affects 25% of adults, shares comorbidity with hypertension, and has no convenient diagnostic test. We tested the hypothesis that urine-derived exfoliated renal proximal tubule cells (RPTCs) could diagnose the degree of an individual's SS of BP.

METHODS

Subjects were selected who had their SS of BP determined 5 y prior to this study (salt-sensitive: ≥7 mm Hg increase in mean arterial pressure (MAP) following transition from a random weekly diet of low (10 mmol/day) to high (300 mmol/day) sodium (Na(+)) intake, N=4; inverse salt-sensitive (ISS): ≥7 mm Hg increase in MAP transitioning from a high to low Na(+) diet, N=3, and salt-resistant (SR): <7 mm Hg change in MAP transitioned on either diet, N=5). RPTC responses to 2 independent Na(+) transport pathways were measured.

RESULTS

There was a negative correlation between the degree of SS and dopamine-1 receptor (D1R) plasma membrane recruitment (y=-0.0107x+0.68 relative fluorescent units (RFU), R(2)=0.88, N=12, P<0.0001) and angiotensin II-stimulated intracellular Ca(++) (y=-0.0016x+0.0336, R(2)=0.7112, P<0.001, N=10) concentration over baseline.

CONCLUSIONS

Isolating RPTCs from urine provides a personalized cell-based diagnostic test of SS index that offers advantages over a 2-week controlled diet with respect to cost and patient compliance. Furthermore, the linear relationship between the change in MAP and response to 2 Na(+) regulatory pathways suggests that an individual's RPTC response to intracellular Na(+) is personalized and predictive.

Intrarenal artery superoxide is mainly NADPH oxidase-derived and modulates endothelium-dependent dilation in elderly patients

AIMS

The present study was performed to investigate the contribution of NADPH oxidases (Nox) to superoxide formation in human renal proximal resistance arteries and to test whether superoxide formation contributes to acute vasoconstrictor responses and endothelium-dependent vasodilation in these vessels.

METHODS AND RESULTS

Arcuate and proximal interlobular artery segments were from patients who underwent nephrectomy because of a renal tumour. Vessels were dissected from tumour-free parts of the kidneys. Additional intrarenal arteries were obtained from rats. Superoxide formation was measured by lucigenin-enhanced chemiluminescence, expression of Nox isoforms was analysed by RT-PCR, and functional studies were performed by small vessel wire myography. Sixty per cent of superoxide formation in human arcuate and proximal interlobular arteries was due to Nox activity. mRNA expression analyses revealed the presence of Nox2 and Nox4 but not Nox1. Phenylephrine and endothelin-1 induced powerful concentration-dependent vasoconstrictions that were unaffected by superoxide scavengers. Vasopressin elicited small and variable vasoconstrictions with signs of tachyphylaxis. Endothelium-dependent vasodilation was blunted by tiron and Nomega-nitro-L-arginine methyl ester but not by superoxide dismutase or catalase. Exogenous hydrogen peroxide elicited vasoconstriction.

CONCLUSION

Nox activity is the major source of superoxide formation in renal proximal resistance arteries from elderly patients. Acute vasoconstrictor responses to alpha1-adrenoreceptor activation and to endothelin-1 do not depend on superoxide formation, while endothelium-dependent vasodilation in intrarenal arteries is reactive oxygen species-dependent.

Multidrug resistance-related protein 2 genotype of the donor affects kidney graft function

OBJECTIVES

We tested the effect of kidney-specific multidrug resistance-related protein (MRP2, ABCC2) deficiency on renal organic solute disposition as well as on renal protein and gene expression. Furthermore, we investigated whether a particular kidney donor ABCC2 genotype is associated with delayed graft function in patients.

METHODS

A new MRP2-deficient rat strain was established. Renal cross-transplantations were performed between congenic MRP2-deficient and wild-type rats. Renal disposition of MRP2 substrates was investigated in native and transplanted rats. Proteomic analyses and transcriptional profiling were performed in rat kidney graft cortices. Ninety-eight human kidney donor-recipient pairs were genotyped for five ABCC2 polymorphisms. The relationship between delayed graft function and ABCC2 genetic variants in donors and recipients was analyzed by backward stepwise logistic regression.

RESULTS

In rats, the absence of renal MRP2 reduced renal bilirubin glucuronide excretion at pathologic plasma concentrations, modified renal p-aminohippurate excretion and did not affect renal morphine-6-glucuronide excretion. Renal MRP2 deficiency led to renal cortical protein or mRNA upregulation of glutathione transferase isoenzymes, glutaredoxin 2, and heme oxygenase-1. In patients, a particular donor ABCC2 genotype was associated with an increased incidence of delayed graft function.

CONCLUSION

Kidney graft-specific MRP2 deficiency has mild effects on the renal excretion of some organic solutes under experimental conditions and induces a protein and gene expression pattern indicative of activated antioxidant defense mechanisms. This suggests that MRP2 is a determinant of the redox status in tubular epithelial cells and thus of the susceptibility to renal damage under conditions of treatment with multiple drugs and increased oxygen radical formation.

COLON ASCENDENS STENT PERITONITIS-A MODEL OF SEPSIS ADOPTED TO THE RAT

The colon ascendens stent peritonitis (CASP) procedure creates an intestinal leakage of feces, resulting in diffuse peritonitis and polymicrobial sepsis. Mouse models of CASP have been used to study sepsis experimentally. The aim of the present study was to establish CASP sepsis in rats and to provide basic functional characteristics of this model. In analogy to the mouse model, 3 degrees of severity of CASP sepsis, 2 sublethal and 1 lethal, were established depending on the stent diameter. Radio-telemetric recordings in a sublethal model showed that the nonsurvivors remained hemodynamically stable until approximately 1 h before death, when heart rate and blood pressure fell rapidly. Intestinal microcirculatory changes were analyzed 3, 6, 12, and 18 h after CASP surgery using intravital microscopy in a sublethal model. After 18 h, the numbers of the leukocytes firmly adhering to the endothelium and of the ones temporarily interacting were significantly increased. The levels of IL-6 and IL-1beta increased continuously during the CASP experiments while remaining unchanged in the sham group. TNF-alpha and IL-10 levels of CASP animals reached a maximum after 12 h. In conclusion, a rat model of CASP sepsis has been established and characterized with regard to alterations in cardiovascular and microcirculatory function as well as plasma cytokine levels. In experimental settings where genetically engineered animals are not required, it will facilitate detailed examination of dynamic changes in integrated organ function during the course of sepsis and the investigation of treatment strategies.

Impaired coronary function in Wistar Ottawa Karlsburg W rats—a new model of the metabolic syndrome

The metabolic syndrome is associated with an increased risk for coronary heart disease. The underlying mechanisms are not well understood. The present study was designed to investigate coronary function in Wistar Ottawa Karlsburg W (WOKW) rats, a new animal model of the metabolic syndrome. The responses of coronary artery segments from WOKW and Dark Agouti (DA) control rats of different ages to several physiological vasoconstrictors and vasodilators were measured in a small vessel wire myograph, and potential mechanisms involved in the differential responses between the two strains were investigated. WOKW showed increased alpha(1)-adrenoceptor-mediated coronary constriction at 3 and 10 months of age, as well as seriously blunted beta-adrenoceptor-mediated coronary relaxation at 16 months of age. Responses to non-adrenergic agonists were not altered in WOKW compared to DA. The alpha(1)-adrenoceptor-mediated coronary constriction in WOKW was completely blocked by rho-kinase inhibition. Induced hyperinsulinemia did not cause increased alpha(1)-adrenoceptor-mediated coronary constriction or impaired beta-adrenoceptor-mediated coronary relaxation in DA. The association between blunted coronary beta-adrenoceptor responsiveness and the metabolic syndrome was confirmed in Zucker diabetic fatty rats. We conclude that the metabolic syndrome in WOKW rats is associated with impaired coronary function due to altered adrenoceptor sensitivity. The latter may contribute to inappropriately elevated coronary tone in insulin-resistant subjects, especially when sympathetic activity to the heart is increased.

Angiotensin in the kidney: a key to understanding hypertension?

A crosstransplantation study between genetically matched angiotensin AT1 receptor knockout and wild-type mice revealed that renal AT1 receptors are required for the development of angiotensin II-induced hypertension (). However, in this experimental setting, hypertension-related left ventricular hypertrophy seemed to depend on blood pressure elevation rather than on the expression of AT1 receptors in the heart.

Mechanisms of disease: the role of GRK4 in the etiology of essential hypertension and salt sensitivity

Hypertension and salt sensitivity of blood pressure are two conditions the etiologies of which are still elusive because of the complex influences of genes, environment, and behavior. Recent understanding of the molecular mechanisms that govern sodium homeostasis is shedding new light on how genes, their protein products, and interacting metabolic pathways contribute to disease. Sodium transport is increased in the proximal tubule and thick ascending limb of Henle of the kidney in human essential hypertension. This Review focuses on the counter-regulation between the dopaminergic and renin-angiotensin systems in the renal proximal tubule, which is the site of about 70% of total renal sodium reabsorption. The inhibitory effect of dopamine is most evident under conditions of moderate sodium excess, whereas the stimulatory effect of angiotensin II is most evident under conditions of sodium deficit. Dopamine and angiotensin II exert their actions via G protein-coupled receptors, which are in turn regulated by G protein-coupled receptor kinases (GRKs). Polymorphisms that lead to aberrant action of GRKs cause a number of conditions, including hypertension and salt sensitivity. Polymorphisms in one particular member of this family-GRK4-have been shown to cause hyperphosphorylation, desensitization and internalization of a member of the dopamine receptor family, the dopamine 1 receptor, while increasing the expression of a key receptor of the renin-angiotensin system, the angiotensin II type 1 receptor. Novel diagnostic and therapeutic approaches for identifying at-risk subjects, followed by selective treatment of hypertension and salt sensitivity, might center on restoring normal receptor function through blocking the effects of GRK4 polymorphisms.

The Impact of Daily Sodium Intake on Posttransplant Hypertension in Kidney Allograft Recipients

INTRODUCTION

Posttransplant hypertension is a well-known risk factor for long-term allograft failure and mortality in kidney recipients. Although dietary sodium restriction is a widely recommended nonpharmacological measure for control of blood pressure (BP), no detailed investigation has been conducted regarding the impact of dietary sodium restriction on this condition.

METHODS

Thirty-two patients on antihypertensive treatment completed the study. They were randomly divided into two groups: controls (group 1) versus strict sodium diet (group 2; 80 to 100 mmol sodium daily). After randomization, 24-hour urine for sodium measurement, BP, and allograft functions were recorded at baseline and after 3 months. BP treatment was reevaluated at each visit throughout the study.

RESULTS

At baseline, there was no significant difference in age, sex, serum creatinine, systolic and diastolic BP, antihypertensive drugs, or 24-hour urinary sodium levels between the groups. After 3 months, daily urinary sodium excretion (from 190+/-75 to 106+/-48 mEq/d, P<.0001), systolic BP (from 146+/-21 to 116+/-11 mm Hg), and diastolic BP (from 89+/-8 to 72+/-10 mm Hg) had significantly decreased in group 2, while no significant changes were observed in group 1.

CONCLUSION

Low sodium intake in combination with antihypertensive treatment appears to efficiently control BP in kidney allograft recipients with hypertension. Twenty-four-hour urinary sodium excretion should be checked regularly in these patients as a useful marker to indicate whether the patient complies with low sodium intake.

Neonatal sympathectomy reduces NADPH oxidase activity and vascular resistance in spontaneously hypertensive rat kidneys

Neonatal sympathectomy reduces arterial pressure in spontaneously hypertensive rats (SHR). In SHR transplanted with a kidney from sympathectomized SHR, arterial pressure was lower and less Na+ sensitive than in SHR transplanted with a kidney from hydralazine-treated SHR. This study was performed to identify underlying renal mechanisms. Tests for differential renal mRNA expression of nine a priori selected genes revealed robust differences for renal medullary expression of the NADPH oxidase subunit p47phox. Therefore, we investigated the effects of neonatal sympathectomy on renal mRNA expression of NADPH oxidase subunits, NADPH oxidase activity, and renal function. In 10-wk-old sympathectomized SHR fed a 0.6% NaCl diet, medullary p47phox and gp91phox expression was 40% less than in hydralazine-treated SHR. Also, after a 1.8% NaCl diet, medullary p47phox mRNA expression was lower in sympathectomized than in hydralazine-treated SHR. We found lower cortical (-30%, P<0.01) and medullary (-30%, P<0.05) NADPH oxidase activities in sympathectomized than in hydralazine-treated or untreated SHR. Glomerular filtration rate, renal blood flow, medullary blood flow, and fractional Na+ excretion in kidney grafts from sympathectomized and hydralazine-treated donors (n=8 per group) were similar at baseline and in response to a 20-mmHg rise in renal perfusion pressure. Renal vascular resistance was lower in kidneys from sympathectomized than hydralazine-treated donors (25+/-2 vs. 32+/-4 mmHg.min.ml-1, P<0.05). The results indicate that the sympathetic nervous system contributes to the level of renal NADPH oxidase activity and to perinatal programming of alterations in renal vascular function that lead to elevated renal vascular resistance in SHR.

Hypothesis: the systemic circulation as a regulated free-market economy. A new approach for understanding the long-term control of blood pressure

1. The purpose of the present paper is to discuss the long-term regulation of arterial blood pressure. Current thinking on the topic favours the theory that tight regulation is achieved through the action of a central (or primary) controller, usually assumed to be in either the brain or kidneys. 2. Here, an alternative explanation is considered; namely, that the average long-term level of arterial pressure is an emergent property of a decentralized control system. The goal of the system is to deliver nutrient-rich blood to distinct vascular regions based on their energy demand. 3. Specifically, the circulation is conceptualized as a free-market economy where tissues 'compete' for a scarce resource (the energy contained in blood) supplied by the heart-lung unit; the 'price' of the resource (analogous to the reciprocal of arterial pressure) is determined primarily by the dynamic relationship between supply and demand, not by a central mechanism. 4. Based on this concept of the circulation as an energy market, economic analogies are used to suggest novel mechanisms by which the brain and kidney may affect the long-term control of blood pressure. 5. Market-based control, a process derived from quantitative theoretical analysis of the performance of economic markets, is proposed as a new, potentially useful strategy for mathematically modelling the behaviour of the circulation.

Sympatho-renal interactions in the determination of arterial pressure: role in hypertension

Renal mechanisms and the sympathetic nervous system contribute to the development of arterial hypertension. Renal transplantation experiments in spontaneously hypertensive rats (SHRs) were performed to investigate how the sympathetic nervous system and the kidneys interact during the development and maintenance of hypertension. Our findings indicate that the rise in arterial pressure that occurs after transplantation of a kidney from a SHR into normotensive recipients is not mediated by elevations in sympathetic activity. However, chronic reductions in sympathetic tone reduce the rise in arterial pressure which can be induced by SHR renal grafts in normotensive recipients. Untreated SHRs transplanted with a kidney from sympathectomized donors have lower arterial pressure and reduced sodium sensitivity of arterial pressure compared to SHRs transplanted with a kidney from hydralazine-treated donors. It is concluded that chronic non-adapting changes in sympathetic activity modulate the degree to which renal mechanisms can cause hypertension in SHRs. Severe reduction in sympathetic tone during early ontogeny causes long-term changes in renal function that mitigate hypertension development in SHRs even when the extrarenal neuro-hormonal environment is restored.