Mechanisms underlying pressure-related natriuresis: the role of the renin-angiotensin and prostaglandin systems. State of the art lecture

Nocturia and Blood Pressure Elevation in Adolescents

Nocturia has been increasingly recognized as a manifestation of various non-urological conditions including hypertension. In adults, blood pressure (BP) elevation has been identified as a robust correlate of nocturia, but such a relationship has not been studied in pediatric populations where nocturia is often attributed to hormonal, sleep, physiological or psychological disorders. Accordingly, this study aimed to determine the relationship between nocturia and BP elevation in adolescents. We prospectively studied 100 patients, aged 10-18 years, recruited from pediatric clinics at our institution. Nocturia (defined as ≥ 1 voids on voiding diary analysis) was present in 45% of the study sample (range: 1-4 voids/night). 37% of subjects self-reported awakening to urinate, and 34% of subjects had BP elevation according to age-dependent thresholds from current Pediatrics guidelines. On multivariate analyses, BP elevation was strongly associated with nocturia determined by both voiding diary (OR 26.2, 95% CI: 6.5, 106.0) and self-report. Conversely, nocturia was associated with increased odds of elevated BP by diary (26.3, 95% CI: 6.5, 106.4) and self-report (OR 8.1, 95% CI: 3.2, 20.5). In conclusion, nocturia appears to be common and is strongly associated with BP elevation in adolescents. These findings suggest that eliciting a history of nocturia holds promise as a simple method of identifying adolescents at risk for hypertension.

Evening home blood pressure and pulse rate: age-specific associations with nocturia severity

Previous studies have reported a significant relationship between hypertension and nocturia. However, the underlying pathophysiology associated with pulse rate (PR) remains unclear. In the Japan Morning Surge-Home Blood Pressure Study, a self-administered nocturia questionnaire and evening home blood pressure (BP) and PR measurements (taken on a mean of 11.2 days) were performed on 4310 patients with one or more cardiovascular risk factors (mean: 64.9 years old; 47% male). According to the number of nighttime voids, the study population was divided into three groups (no voids: n = 2382; 1 void: n = 847; ≥2 voids per night: n = 1082). In the multinomial logistic regression analysis adjusted for confounders, diuretic use (OR, 1.23; 95%CI, 1.01-1.50; p < 0.05) was significantly associated with one nocturnal void, whereas evening home systolic BP (SBP) (OR per 1 SD, 1.14; 95%CI, 1.05-1.24; p < 0.01) and evening home PR (OR per 1 SD, 1.12; 95%CI: 1.02-1.24; p < 0.05) were significantly associated with multiple nocturnal voids. In the younger group (<65 years), only evening home PR was significantly related to multiple nighttime voids (p < 0.01), whereas in the older group (≥65 years), only evening home SBP was significantly related to multiple nighttime voids (p = 0.02). In this study, both higher evening home PR and higher evening home SBP were associated with multiple nighttime voids, with the former playing a greater role in the younger participants, and the latter more often associating the older group. An age-stratified approach to reduce the burden of BP or PR might be important to improve sleep quality.

Unlocking the Full Potential of SGLT2 Inhibitors: Expanding Applications beyond Glycemic Control

The number of diabetic patients has risen dramatically in recent decades, owing mostly to the rising incidence of type 2 diabetes mellitus (T2DM). Several oral antidiabetic medications are used for the treatment of T2DM including, α-glucosidases inhibitors, biguanides, sulfonylureas, meglitinides, GLP-1 receptor agonists, PPAR-γ agonists, DDP4 inhibitors, and SGLT2 inhibitors. In this review we focus on the possible effects of SGLT2 inhibitors on different body systems. Beyond the diabetic state, SGLT2 inhibitors have revealed a demonstrable ability to ameliorate cardiac remodeling, enhance myocardial function, and lower heart failure mortality. Additionally, SGLT2 inhibitors can modify adipocytes and their production of cytokines, such as adipokines and adiponectin, which enhances insulin sensitivity and delays diabetes onset. On the other hand, SGLT2 inhibitors have been linked to decreased total hip bone mineral deposition and increased hip bone resorption in T2DM patients. More data are needed to evaluate the role of SGLT2 inhibitors on cancer. Finally, the effects of SGLT2 inhibitors on neuroprotection appear to be both direct and indirect, according to scientific investigations utilizing various experimental models. SGLT2 inhibitors improve vascular tone, elasticity, and contractility by reducing oxidative stress, inflammation, insulin signaling pathways, and endothelial cell proliferation. They also improve brain function, synaptic plasticity, acetylcholinesterase activity, and reduce amyloid plaque formation, as well as regulation of the mTOR pathway in the brain, which reduces brain damage and cognitive decline.

Current Understanding of Pressure Natriuresis

Pressure natriuresis refers to the concept that increased renal perfusion pressure leads to a decrease in tubular reabsorption of sodium and an increased sodium excretion. The set point of blood pressure is the point at which pressure natriuresis and extracellular fluid volume are in equilibrium. The term "abnormal pressure natriuresis" usually refers to the expected abnormal effect of a certain level of blood pressure on sodium excretion. Factors that cause abnormal pressure natriuresis are known. Sympathetic nerve system, genetic factors, and dietary factors may affect an increase in renal perfusion pressure. An increase in renal perfusion pressure increases renal interstitial hydrostatic pressure (RIHP). Increased RIHP affects tubular reabsorption through alterations in tight junctional permeability to sodium in proximal tubules, redistribution of apical sodium transporters, and/or release of renal autacoids. Renal autocoids such as nitric oxide, prostaglandin E2, kinins, and angiotensin II may also regulate pressure natriuresis by acting directly on renal tubule sodium transport. In addition, inflammation and reactive oxygen species may mediate pressure natriuresis. Recently, the use of new drugs associated with pressure natriuretic mechanisms, such as angiotensin receptor neprilysin inhibitor and sodium glucose co-transporter 2 inhibitors, has been consistently demonstrated to reduce mortality and hypertension-related complications. Therefore, the understanding of pressure natriuresis is gaining attention as an antihypertensive strategy. In this review, we provide a basic overview of pressure natriuresis to the target audience of nephrologists.

Establishment and evaluation of a reversible two-kidney, one-clip renovascular hypertensive rat model

The aim of the present study was to establish and evaluate a novel and reversible two-kidney, one-clip renovascular hypertensive rat model with a titanium vascular clip. A total of 40 male Sprague-Dawley rats were evenly and randomly divided into a sham-operated group, and 3, 7, 12 and 28D groups (namely removing the vascular clip in the renovascular hypertensive model after 3, 7, 12 and 28 days, respectively). The systolic blood pressure (SBP) and plasma renin activity (PRA) were measured, and color duplex imaging was conducted before placing the clips, as well as before and after removing them. After placing the vascular clips, SBP and PRA in the 3, 7, 12 and 28D groups were significantly increased (SBP: Sham-operated vs. 3D groups, P=0.020; 3 vs. 7D groups, P=0.008; 7 vs. 28D groups, P=0.019; 12 vs. 28D groups, P=0.039, and between other groups P<0.001. PRA: 3 vs. 7D groups, P=0.001; 7 vs. 12D groups, P=0.004; 12 vs. 28D groups, P=0.040, and between other groups, P<0.001). After removing the clips, SBP were significantly reduced in the 3 and 7D groups (P=0.023, 0.040, 0.066 and 0.314 in the 3, 7, 12 and 28D groups, respectively), but were still significantly higher than that before placing clips in the 7, 12 and 28D groups (P=0.067, P=0.005, P<0.001 and P<0.001 in the 3, 7, 12 and 28D groups, respectively). After removing the clips, PRA was significantly reduced in each group (P<0.001, P<0.001, P=0.012 and P=0.049 in 3, 7, 12 and 28D groups, respectively), but still higher than that before placing the clips (P<0.001, P=0.001, P=0.001 and P=0.003 in 3, 7, 12 and 28D groups, respectively). Vascular imaging also indicates this model has a reversible property. In conclusion, a reversible renovascular hypertension rat model is feasible, and provides a basis for research on clinical ischemic nephropathy and renal artery revascularization.

Diuretic usage for protection against end-organ damage in liver cirrhosis and heart failure

Volume overload is common in liver cirrhosis, heart failure, and chronic kidney disease, being an independent risk factor for mortality. Loop diuretics have been widely used for treating volume overload in these patients. However, there is a tendency to increase the dose of loop diuretics partly because of diuresis resistance. Neurohormonal factors are also enhanced in these patients, which play a role in volume overload and organ ischemia. Loop diuretics cannot improve neurohormonal factors and could result in end-organ damage. The water diuretic tolvaptan has been approved for use for volume overload in heart failure and liver cirrhosis. Despite causing similar increases in urine volume, its characteristics differ from those of loop diuretics. Renal blood flow is maintained with tolvaptan but decreased with furosemide in heart failure patients. Neurohormonal factors and blood pressure are not markedly altered by tolvaptan administration. It is expected that these mechanisms of tolvaptan can protect against worsening renal function by volume overload diseases compared with loop diuretics. It has also been reported that some patients do not respond well to tolvaptan. Loop diuretics and tolvaptan share the same mechanism with regard to decreasing renal interstitial osmolality, which plays a fundamental role in water diuresis. Thus, a high dose of loop diuretics could result in resistance to tolvaptan, so tolvaptan should be administered before increasing the loop diuretic dose. Therefore, volume control without enhancing end-organ damage can be achieved by adding tolvaptan to a tolerable dose of Na-sparing diuretics.

Angiotensin II type 1 receptor antagonists in animal models of vascular, cardiac, metabolic and renal disease

We have reviewed the effects of angiotensin II type 1 receptor antagonists (ARBs) in various animal models of hypertension, atherosclerosis, cardiac function, hypertrophy and fibrosis, glucose and lipid metabolism, and renal function and morphology. Those of azilsartan and telmisartan have been included comprehensively whereas those of other ARBs have been included systematically but without intention of completeness. ARBs as a class lower blood pressure in established hypertension and prevent hypertension development in all applicable animal models except those with a markedly suppressed renin-angiotensin system; blood pressure lowering even persists for a considerable time after discontinuation of treatment. This translates into a reduced mortality, particularly in models exhibiting marked hypertension. The retrieved data on vascular, cardiac and renal function and morphology as well as on glucose and lipid metabolism are discussed to address three main questions: 1. Can ARB effects on blood vessels, heart, kidney and metabolic function be explained by blood pressure lowering alone or are they additionally directly related to blockade of the renin-angiotensin system? 2. Are they shared by other inhibitors of the renin-angiotensin system, e.g. angiotensin converting enzyme inhibitors? 3. Are some effects specific for one or more compounds within the ARB class? Taken together these data profile ARBs as a drug class with unique properties that have beneficial effects far beyond those on blood pressure reduction and, in some cases distinct from those of angiotensin converting enzyme inhibitors. The clinical relevance of angiotensin receptor-independent effects of some ARBs remains to be determined.

Recent advances in renal hemodynamics: insights from bench experiments and computer simulations

It has been long known that the kidney plays an essential role in the control of body fluids and blood pressure and that impairment of renal function may lead to the development of diseases such as hypertension (Guyton AC, Coleman TG, Granger Annu Rev Physiol 34: 13-46, 1972). In this review, we highlight recent advances in our understanding of renal hemodynamics, obtained from experimental and theoretical studies. Some of these studies were published in response to a recent Call for Papers of this journal: Renal Hemodynamics: Integrating with the Nephron and Beyond.

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

Dominant factors that govern pressure natriuresis in diuresis and antidiuresis: a mathematical model

We have developed a whole kidney model of the urine concentrating mechanism and renal autoregulation. The model represents the tubuloglomerular feedback (TGF) and myogenic mechanisms, which together affect the resistance of the afferent arteriole and thus glomerular filtration rate. TGF is activated by fluctuations in macula densa [Cl(-)] and the myogefnic mechanism by changes in hydrostatic pressure. The model was used to investigate the relative contributions of medullary blood flow autoregulation and inhibition of transport in the proximal convoluted tubule to pressure natriuresis in both diuresis and antidiuresis. The model predicts that medullary blood flow autoregulation, which only affects the interstitial solute composition in the model, has negligible influence on the rate of NaCl excretion. However, it exerts a significant effect on urine flow, particularly in the antidiuretic kidney. This suggests that interstitial washout has significant implications for the maintenance of hydration status but little direct bearing on salt excretion, and that medullary blood flow may only play a signaling role for stimulating a pressure-natriuresis response. Inhibited reabsorption in the model proximal convoluted tubule is capable of driving pressure natriuresis when the known actions of vasopressin on the collecting duct epithelium are taken into account.

Nocturia in arterial hypertension: a prevalent, underreported, and sometimes underestimated association

Nocturia is a risk factor for morbidity and mortality but is frequently overlooked and underreported by patients and unrecognized by physicians. Epidemiologic studies reported that nocturnal voiding is associated not only with aging and benign prostatic hyperplasia, but also with many other clinical conditions. The majority of epidemiologic studies reported a significant relationship between nocturia and hypertension. However, the cause-and-effect relationship between them has not been established. Some physiopathological changes in hypertension are conducive to result in nocturia. These include the effects of hypertension on glomerular filtration and tubular transport, resetting of the kidney pressure-natriuresis relationship, atrial stretch and release of atrial natriuretic peptide when congestive heart failure complicates hypertension, and peripheral edema. Another link between hypertension and nocturia is obstructive sleep apnea. Furthermore, some evidence supports the relationship between nondipping behavior of blood pressure and an increased prevalence of nocturia. The use of some classes of antihypertensive agents may result in nocturia. The present review aims to provide a comprehensive evaluation of the epidemiologic evidence and physiopathological links that correlate hypertension and nocturia. Emphasis is placed on the need to take a pro-active attitude to detect and treat this hazardous condition.

Intakes of garlic and dried fruits are associated with lower risk of spontaneous preterm delivery

Several studies have found associations between microbial infections during pregnancy and preterm delivery (PTD). We investigated the influence of food with antimicrobial and prebiotic components on the risk of spontaneous PTD. A literature search identified microbes associated with spontaneous PTD. Subsequently, 2 main food types (alliums and dried fruits) were identified to contain antimicrobial components that affect the microbes associated with spontaneous PTD; they also contained dietary fibers recognized as prebiotics. We investigated intake in 18,888 women in the Norwegian Mother and Child Cohort (MoBa), of whom 950 (5%) underwent spontaneous PTD (<37 gestational weeks). Alliums (garlic, onion, leek, and spring onion) [OR: 0.82 (95% CI: 0.72, 0.94), P = 0.005] and dried fruits (raisins, apricots, prunes, figs, and dates) [OR: 0.82 (95% CI: 0.72, 0.94); P = 0.005] were associated with a decreased risk of spontaneous PTD. Intake of alliums was related to a more pronounced risk reduction in early spontaneous PTD (gestational weeks 28-31) [OR: 0.39 (95% CI: 0.19, 0.80)]. The strongest association in this group was with garlic [OR: 0.47 (95% CI: 0.25-0.89)], followed by cooked onions. Intake of dried fruits showed an association with preterm prelabor rupture of membranes (PPROM) [OR: 0.74 (95% CI: 0.65, 0.95)]; the strongest association in this group was with raisins [OR: 0.71 (95% CI: 0.56, 0.92)]. The strongest association with PPROM in the allium group was with garlic [OR: 0.74 (95% CI: 0.56, 0.97)]. In conclusion, intake of food with antimicrobial and prebiotic compounds may be of importance to reduce the risk of spontaneous PTD. In particular, garlic was associated with overall lower risk of spontaneous PTD. Dried fruits, especially raisins, were associated with reduced risk of PPROM.

mPGES-1 deletion impairs aldosterone escape and enhances sodium appetite

Aldosterone (Aldo) is a major sodium-retaining hormone that reduces renal sodium excretion and also stimulates sodium appetite. In the face of excess Aldo, the sodium-retaining action of this steroid is overridden by an adaptive regulatory mechanism, a phenomenon termed Aldo escape. The underlying mechanism of this phenomenon is not well defined but appeared to involve a number of natriuretic factors such prostaglandins (PGs). Here, we investigated the role of microsomal prostaglandin E synthase-1 (mPGES-1) in the response to excess Aldo. A 14-day Aldo infusion at 0.35 mg x kg(-1) x day(-1) via an osmotic minipump in conjunction with normal salt intake did not produce obvious disturbances in fluid metabolism in WT mice as suggested by normal sodium and water balance, plasma sodium concentration, hematocrit, and body weight, despite the evidence of a transient sodium accumulation on days 1 or 2. In a sharp contrast, the 14-day Aldo treatment in mPGES-1 knockoute (KO) mice led to increased sodium and water balance, persistent reduction of hematocrit, hypernatremia, and body weight gain, all evidence of fluid retention. The escaped wild-type (WT) mice displayed a remarkable increase in urinary PGE(2) excretion in parallel with coinduction of mPGES-1 in the proximal tubules, accompanied by a remarkable, widespread downregulation of renal sodium and water transporters. The increase in urinary PGE(2) excretion together with the downregulation of renal sodium and water transporters were all significantly blocked in the KO mice. Interestingly, compared with WT controls, the KO mice exhibited consistent increases in sodium and water intake during Aldo infusion. Together, these results suggest an important role of mPGES-1 in antagonizing the sodium-retaining action of Aldo at the levels of both the central nervous system and the kidney.

Nitric oxide produced by endothelial nitric oxide synthase promotes diuresis

Extracellular fluid volume is highly regulated, at least in part, by peripheral resistance and renal function. Nitric oxide (NO) produced by NO synthase type 3 (NOS 3) in the nonrenal vasculature may promote fluid retention by reducing systemic vascular resistance and arterial pressure. In contrast, NO produced by renal NOS 3 promotes water excretion by reducing renal vascular resistance, increasing glomerular filtration, and inhibiting reabsorption along the nephron. Thus, the net effect of NO from NOS 3 on urinary volume (UV) is unclear. We hypothesized that NO produced by NOS 3 promotes water excretion primarily due to renal tubular effects. We gave conscious wild-type and NOS 3 -/- mice an acute volume load and measured UV, blood pressure, plasma renin concentration (PRC), Na(+), vasopressin, and urinary Na(+) and creatinine concentrations. To give the acute volume load, we trained mice to drink a large volume of water while in metabolic cages. On the day of the experiment, water was replaced with 1% sucrose, and mice had access to it for 1 h. Volume intake was similar in both groups. Over 3 h, wild-type mice excreted 62 +/- 10% of the volume load, but NOS 3 -/- excreted only 42 +/- 5% (P < 0.05). Blood pressure in NOS 3 -/- was 118 +/- 3 compared with 110 +/- 2 mmHg in wild-type mice (P < 0.05), but it did not change following volume load in either strain. PRC, vasopressin, and glomerular filtration rate were similar between groups. Urinary Na(+) excretion was 49.3 +/- 7.0 in wild-type vs. 37.8 +/- 6.4 mumol/3 h in NOS 3 -/- mice (P < 0.05). Bumetanide administration eliminated the difference in volume excretion between wild-type and NOS 3 -/- mice. We conclude that 1) NO produced by NOS 3 promotes water and Na(+) excretion and 2) the renal epithelial actions of NO produced by NOS 3 supersede the systemic and renal vascular actions.

Salt Loading Increases Urinary Excretion of Linoleic Acid Diols and Triols in Healthy Human Subjects

Increased dietary linoleic acid has been associated with reduced blood pressure in clinical and animal studies possibly mediated by prostaglandins. Urinary linoleate and prostaglandin metabolite excretion were investigated in subjects exposed to a salt-loading/salt-depletion regimen. Twelve healthy subjects were recruited from the New Orleans population (before Hurricaine Katrina) and admitted to the Tulane-Louisiana State University-Charity Hospital General Clinical Research Center after a 5-day outpatient lead-in phase on a 160-mmol sodium diet. On inpatient day 1, the subjects were maintained on the 160-mmol sodium diet, and a 24-hour urine specimen was collected. On day 2, the subjects received 2 L of IV normal saline over 4 hours and continued on a 160-mmol Na + diet (total: 460 mmol of sodium). Two 12-hour urine collections were obtained. On day 3, the subjects received three 40-mg oral doses of furosemide, two 12-hour urine collections were obtained, and the subjects were given a 10-mmol sodium diet. Urinary oxidized lipids were measured by high-performance liquid chromatography-tandem quadrupole mass spectroscopy. The excretion of the urinary linoleate metabolites, dihydroxyoctadecamonoenoic acids, and trihydroxyoctadecamonoenoic acids increased significantly during intravenous salt loading as compared with day 1 and the salt-depleted periods. The urinary excretion of 6-keto- prostaglandin F1α was unaffected by salt loading but was dramatically increased 7- to 10-fold by salt depletion. Prostaglandin E2 excretion was positively correlated with sodium excretion. The salt-stimulated production of linoleic acid diols and triols may inhibit tubular sodium reabsorption, thereby assisting in the excretion of the sodium load.

Renal disease: the anesthesiologist's perspective

The kidney is a remarkable organ whose functions include maintaining fluid and electrolyte balance, excreting metabolic waste products, and controlling vascular tone. Blood flow within the kidney is very heterogeneous, which places the metabolically active medulla at high risk for ischemic injury. A number of mediators play a role in the modulation of renal blood flow, including angiotensin II, dopamine, vasopressin, prostaglandins, atrial natriuretic peptide, endothelin, and nitric oxide. Early markers of renal injury elicit strong interest, although currently there is no reliable marker available. Surgery causes the release of catecholamines, renin, angiotensin, and AVP that lead to a redistribution of renal blood flow and a decrease in GFR. Additionally, general anesthesia often results in some degree of hypotension and depressed cardiac output, which further reduces renal perfusion and potentially jeopardizes renal function. A careful anesthetic plan is imperative in the patient with renal insufficiency or failure because acute renal failure in the perioperative period is associated with a high morbidity and mortality. Factors including advanced age, diabetes, underlying renal insufficiency, and heart failure place a patient at high risk for developing acute renal failure. It is imperative to maintain euvolemia, normotension, and cardiac output, and to avoid nephrotoxic agents to optimize renal blood flow and renal perfusion as the best prevention of renal dysfunction. Further studies are needed to establish if any therapies exist to prevent or treat renal dysfunction effectively.

High prevalence of ACE DD genotype among north Indian end stage renal disease patients

BACKGROUND

The Renin-Angiotensin system (RAS) is a key regulator of both blood pressure and kidney functions and their interaction. In such a situation, genetic variability in the genes of different components of RAS is likely to contribute for its heterogeneous association in the renal disease patients. Angiotensin converting enzyme-1 (ACE-1) is an important component of RAS which determines the vasoactive peptide Angiotensin-II.

METHODS

In the present study, we have investigated 127 ESRD patients and 150 normal healthy controls from north India to deduce the association between ACE gene polymorphism and ESRD. The inclusion criteria for patients included a constantly elevated serum creatinine level above normal range (ranging from 3.4 to 15.8) and further the patients were recommended for renal transplantation. A total of 150 normal healthy controls were also genotyped for ACE I/D polymorphism. The criterion of defining control sample as normal was totally based on the absence of any kidney disease determined from the serum creatinin level. Genotyping of ACE I/D were assayed by polymerase chain reaction (PCR) based DNA amplification using specific flanking primers Based on the method described elsewhere.

RESULTS

The difference of DD and II genotypes was found highly significant among the two groups (p = 0.025; OR = 3.524; 95% CI = 1.54-8.07). The combined genotype DD v/s ID+II comparison validated that DD genotype is a high risk genotype for ESRD (p = 0.001; OR = 5.74; 95% CI limit = 3.4-8.5). However, no correlation was obtained for different biochemical parameters of lipid profile and renal function among DD and non DD genotype. Interestingly, approximately 87% of the DD ESRD patients were found hypertensive in comparison to the 65% patients of non DD genotype

CONCLUSION

Based on these observations we conclude that ACE DD genotype implicate a strong possible role in the hypertensive state and in renal damage among north Indians. The study will help in predetermining the timing, type and doses of anti-hypertensive therapy for ESRD patients.

Mechanisms mediating pressure natriuresis: what we know and what we need to find out

1. It is well established that pressure natriuresis plays a key role in long-term blood pressure regulation, but our understanding of the mechanisms underlying this process is incomplete. 2. Pressure natriuresis is chiefly mediated by inhibition of tubular sodium reabsorption, because both total renal blood flow and glomerular filtration rate are efficiently autoregulated. Inhibition of active sodium transport within both the proximal and distal tubules likely makes a contribution. Increased renal interstitial hydrostatic pressure (RIHP) likely inhibits sodium reabsorption by altering passive diffusion through paracellular pathways in 'leaky' tubular elements. 3. Nitric oxide and products of cytochrome P450-dependent arachidonic acid metabolism are key signalling mechanisms in pressure natriuresis, although their precise roles remain to be determined. 4. The key unresolved question is, how is increased renal artery pressure 'sensed' by the kidney? One proposal rests on the notion that blood flow in the renal medulla is poorly autoregulated, so that increased renal artery pressure leads to increased renal medullary blood flow (MBF), which, in turn, leads to increased RIHP. An alternative proposal is that the process of autoregulation of renal blood flow leads to increased shear stress in the preglomerular vasculature and, so, release of nitric oxide and perhaps products of cytochrome P450-dependent arachidonic acid metabolism, which, in turn, drive the cascade of events that inhibit sodium reabsorption. 5. Central to the arguments underlying these opposing hypotheses is the extent to which MBF is autoregulated. This remains highly controversial, largely because of the limitations of presently available methods for measurement of MBF.

Animal models of hypertension: an overview

Hypertension is a multifactorial disease involving complex interactions between genetic and environmental factors. Development of experimental models of hypertension allowed dissection and isolation of various factors associated with regulation of blood pressure, inheritance of hypertensive traits, and cellular responses to injury. The phenotype-driven approach is taking advantage of selective breeding of animals (primarily rats) that exhibit a desired phenotype, like the useful SHR. Genotype-driven models include transgenic techniques, in which mice are the most successful for selective deletion or overexpression of target genes. Notably, a combination of comparative genomics strategies and phenotypic correlates enhances the utility of hypertension models and their clinical relevance. Indeed, experimental models enabled development of targeted interventions aimed at decreasing not only blood pressure but also target organ injury. Continued utilization of experimental models simulating human hypertension, particularly those that combine other clinically relevant comorbidities like obesity or hypercholesterolemia, may afford development of effective strategies to address this common disease. Nevertheless, a cautious approach is mandatory when experimental findings in these models are extrapolated to human hypertension.

The endothelin receptor antagonist ameliorates the hypertensive phenotypes of transgenic hypertensive mice with renin-angiotensin genes and discloses roles of organ specific activation of endothelin system in transgenic mice

Endothelin (ET)-1 and ET-2 are potent vasoconstrictor peptides with mitogenic activity. In this study, we investigated roles of ET system in renin-angiotensin system (RAS)-mediated hypertension, using transgenic hypertensive mice (THM) with over-expression of both human renin and angiotensinogen genes. In the first step, it was revealed that expression of ET system was locally enhanced, i.e. increases in cardiac preproET-1 mRNA and renal preproET-2 mRNA in THM, compared with the control (wild type) mice. In the next step, we studied the chronic effects of an ET antagonist (SB209670) on THM. Blood pressure (BP) in THM was significantly higher than that in the normal mice during the investigation. However, in the later phase of the study, from 12 to 20 weeks of treatment, THM receiving SB 209670 showed significantly lower BP than that in THM receiving saline. SB 209670 treatment for 20 weeks significantly attenuated phenotypes of cardiac hypertrophy, vascular wall thickening and hypertensive nephropathy observed in THM, suggesting that the ETA/B receptor antagonist is also effective even in the extraordinarily activated RAS condition. These findings suggest that organ specifically activated ET system in THM develops the phenotypes, hypertension, cardiac hypertrophy, and hypertensive nephropathy.

Increased renal vascular sensitivity to angiotensin II in hypertension is due to decreased response to prostaglandins

OBJECTIVES

An enhanced sensitivity to angiotensin II in the renal circulation has been demonstrated in the pre-hypertensive phase both in the spontaneously hypertensive rat and in man. To further characterize this abnormality and the role of prostanoids, renal haemodynamics in normotensive young men with a positive (PFH) or a negative (NFH) family history of hypertension were studied.

METHODS

Renal vascular reactivity was assessed during infusion of angiotensin II with and without inhibition of prostaglandin synthesis. Normotensive men with PFH (n = 13) and with NFH (n = 10) with a mean age of 38 years were given on two different occasions: (i). angiotensin II infusion i.v. (0.1, 0.5 and 1.0 ng/kg per min) and (ii). angiotensin II infusion after inhibition of prostaglandin synthesis with indomethacin (150 mg daily three consecutive days). Glomerular filtration rate (GFR) and renal plasma flow were measured with renal clearances of chromium edetic acid and para-aminohippuric acid.

RESULTS

Before angiotensin II challenge, the groups did not differ with respect to blood pressure, body mass index, plasma renin activity, GFR, renal blood flow (RBF) or urinary sodium excretion. There was no significant difference in systolic or diastolic blood pressure response to angiotensin II between the two groups. In PFH, the lowest angiotensin II dose caused a significant decrease in RBF and increase in renal vascular resistance (RVR) from baseline (P < 0.01 for both). In NFH, only the highest angiotensin II dose produced a significant decrease in RBF and increase in RVR (P < 0.01 for both). During inhibition of prostaglandin synthesis, all three angiotensin II doses caused a significant decrease in RBF (P < 0.02) and increase in RVR (P < 0.02) also in NFH. The renal haemodynamic difference between PFH and NFH was thus eliminated.

CONCLUSIONS

These findings indicate that young human subjects with a positive family history of hypertension have a defective vasodilator prostaglandin system, which is responsible for increased renal vascular sensitivity to angiotensin II. Enhanced renal vasoconstriction may be an early event leading to the generation of primary hypertension.

Role of oxidative stress in angiotensin-induced hypertension

Infusion of ANG II at a rate not sufficient to evoke an immediate vasoconstrictor response, produces a slow increase in blood pressure. Circulating levels of ANG II may be within ranges found in normotensive individuals, although inappropriately high with respect to sodium intake. When ANG II levels are dissociated from sodium levels, oxidative stress (OXST) occurs, which can increase blood pressure by several mechanisms. These include inadequate production or reduction of bioavailability of nitric oxide, alterations in metabolism of arachidonic acid, resulting in an increase in vasoconstrictors and decrease in vasodilators, and upregulation of endothelin. This cascade of events appears to be linked, because ANG II hypertension can be blocked by inhibition of any factor located distally, blockade of ANG II, OXST, or endothelin. Such characteristics are shared by other models of hypertension, such as essential hypertension, hypertension induced by reduction in renal mass, and renovascular hypertension. Thus these findings are clinically important because they reveal 1) uncoupling between ANG II and sodium, which can trigger pathological conditions; 2) the various OXST mechanisms that may be involved in hypertension; and 3) therapeutic interventions for hypertension developed with the knowledge of the cascade involving OXST.

Role of nitric oxide and prostaglandin E2 in acute renal hypoperfusion

Although acute renal ischaemia alters the production of various paracrines, there has been little investigation examining the role of intrarenal vasoactive substances. In the present study, we investigated the role of intrarenal nitric oxide and prostaglandins in modulating the acute renal hypoperfusion-induced alterations in renal function. After a 90% clipping of the left renal artery for 60 min, the clip was released, and the renal haemodynamics and sodium excretion were evaluated in both clipped and non-clipped kidneys of anaesthetized dogs. Furthermore, the changes in renal contents of nitrate/nitrite (NOx) and prostaglandin E2 (PGE2) were assessed by using the renal microdialysis technique. The release of the clipping elicited a gradual recovery of renal plasma flow and glomerular filtration rate, and a sustained increase in fractional sodium excretion (FENa) in the clipped kidney. Renal interstitial NOx was reduced in both the cortex (from 8.2 +/- 1.1 to 2.5 +/- 0.3 micromol/L, P < 0.01) and medulla (from 10.1 +/- 0.9 to 3.1 +/- 0.2 micromol/L, P < 0.01), but the levels gradually elevated after declamping. The treatment with nitro-l-arginine methylester only modestly impaired the recovery of renal plasma flow (RPF; at hour 4) and glomerular filtration rate (GFR; at hours 3 and 4 after declamping), without affecting FENa. Conversely, the renal PGE2 levels increased prominently upon the onset of ischaemia (medulla, from 149 +/- 19 to 378 +/- 39 pg/mL, P < 0.01; cortex, from 107 +/- 13 to 302 +/- 34 pg/mL, P < 0.01). Furthermore, the pretreatment with a non-specific cyclo-oxygenase (COX) inhibitor, sulpyrine, and a COX-2-specific inhibitor, NS398, prominently inhibited the increases in FENa induced by the acute renal arterial clipping in a similar manner. In conclusion, in acute renal hypoperfusion, nitric oxide (NO) plays a permissive role in the recovery of the renal haemodynamics. In contrast, sustained increases in renal PGE2 in both clipped and non-clipped kidneys indicate that the COX-2-mediated PGE2 contributes importantly to the failure of the sodium reabsorption in response to acute renal hypoperfusion.

Importance of the renal medullary circulation in the control of sodium excretion and blood pressure

The control of renal medullary perfusion and the impact of alterations in medullary blood flow on renal function have been topics of research interest for almost four decades. Many studies have examined the vascular architecture of the renal medulla, the factors that regulate renal medullary blood flow, and the influence of medullary perfusion on sodium and water excretion and arterial pressure. Despite these studies, there are still a number of important unanswered questions in regard to the control of medullary perfusion and the influence of medullary blood flow on renal excretory function and blood pressure. This review will first address the vascular architecture of the renal medulla and the potential mechanisms whereby medullary perfusion may be regulated. The known extrarenal and local systems that influence the medullary vasculature will then be summarized. Finally, this review will present an overview of the evidence supporting the concept that selective changes in medullary perfusion can have a potent influence on sodium and water excretion with a long-term influence on arterial blood pressure regulation.

Stenosis-dependent role of nitric oxide and prostaglandins in chronic renal ischemia

The role of nitric oxide (NO) and prostaglandins (PG) in modifying renal hemodynamics was examined in clipped and nonclipped kidneys of unilateral renal artery stenosis. Chronic unilateral renal ischemia was established by 4-wk-clipping the left renal artery of canine kidneys, and renal interstitial nitrate+nitrite and PGE(2) contents were evaluated by the microdialysis technique. Unilateral renal artery stenosis caused 45 +/- 1 and 73 +/- 1% decrements in renal plasma flow (RPF) in moderately and severely clipped kidneys and 21 +/- 3% decrements in nonclipped kidneys with severe stenosis. Renal nitrate+nitrite decreased in moderately (-31 +/- 1%) and severely clipped kidneys (-63 +/- 4%). N(omega)-nitro-L-arginine methyl ester reduced RPF (-56 +/- 3%) and glomerular filtration rate (GFR; -54 +/- 3%) in moderately clipped kidneys, whereas this inhibitory effect was abolished in severely clipped kidneys. In contrast, renal PGE(2) contents increased modestly in moderate clipping and were markedly elevated in severely clipped kidneys (from 111 +/- 7 to 377 +/- 22 pg/ml); sulpyrine impaired renal hemodynamics only in severely clipped kidneys. In contralateral nonclipped kidneys, although renal PGE(2) was not increased, sulpyrine reduced RPF (-32 +/- 1%) and GFR (-33 +/- 3%) in severe stenosis. Collectively, NO plays a substantial role in maintaining renal hemodynamics both under basal condition and in moderate renal artery stenosis, whereas the contributory role shifts from NO to PG as renal artery stenosis progresses. Furthermore, because intrarenal angiotensin II is reported to increase in nonclipped kidneys, unilateral severe ischemia may render the nonclipped kidney susceptible to PG inhibition.

Mechanisms of pressure natriuresis

A central component of the feedback system for long-term control of arterial pressure is the pressure-natriuresis mechanism, whereby increases in renal perfusion pressure lead to decreases in sodium reabsorption and increases in sodium excretion. The specific intrarenal mechanism for the decrease in tubular reabsorption in response to increases in renal perfusion pressure appears to be related to increases in hemodynamic factors such as medullary blood flow and renal interstitial hydrostatic pressure (RIHP), and renal autocoids such as nitric oxide, prostaglandins, kinins, and angiotensin II. Increases in renal perfusion pressure are associated with significant increases in RIHP, nitric oxide, prostaglandin E2, and kinins, and decreases in angiotensin II. The mechanism whereby RIHP increases in the absence of discernible changes in whole kidney renal blood flow and peritubular capillary hydrostatic and/or oncotic pressures may be related to increases in renal medullary flow as a result of nitric oxide-induced reductions in renal medullary vascular resistance. Several lines of investigation support an important quantitative role for RIHP in mediating pressure natriuresis. Preventing RIHP from increasing in response to increases in renal perfusion pressure markedly attenuates pressure natriuresis. Furthermore, direct increases in RIHP, comparable to increases measured in response to increases in renal perfusion pressure, have been shown to significantly decrease tubular reabsorption of sodium in the proximal tubule and increase sodium excretion. The exact mechanism whereby RIHP influences tubular reabsorption is unknown, but may be related to alterations in tight junctional permeability to sodium in proximal tubules, redistribution of apical sodium transporters, and/or release of renal autacoids such as prostaglandin E2.

Effect of angiotensin-converting enzyme inhibitor on renal function in ovarian hyperstimulation syndrome in the rabbit

OBJECTIVE

To investigate renal function and whether captopril prevents alterations in the handling of sodium and water in the ovarian hyperstimulation syndrome (OHSS) in the rabbit.

DESIGN

Experimental study

SETTING

Physiology laboratory.

ANIMAL(S)

Six female New Zealand white rabbits were used as controls, and 13 were hyperstimulated with gonadotropins.

INTERVENTION(S)

Saline or captopril.

MAIN OUTCOME MEASURE(S)

Renal excretory and hemodynamic variables.

RESULT(S)

The 3% extracellular volume expansion in OHSS animals induced a significant elevation in mean arterial pressure by 27%, although increments in natriuresis and diuresis were similar to those observed in controls. The OHSS group had impaired pressure-natriuresis sensitivity compared with controls (0.36 +/- 0.07 microEq/min/g of Na excreted per mm Hg vs. 1.74 +/- 0.45 microEq/min/g of Na excreted per mm Hg; P<.05. Captopril significantly reduced mean arterial pressure (P<.05) and shifted the pressure-natriuresis response to the left by 0.85 +/- 0.17 microEq/min/g of Na excreted per mm Hg (P<.05).

CONCLUSION(S)

In OHSS in the rabbit model, pressure-natriuresis sensitivity is impaired. Angiotensin II may play a significant role in this phenomenon, since angiotensin-converting enzyme inhibition normalized the pressure-natriuresis relationship.

Hypokalemia induces renal injury and alterations in vasoactive mediators that favor salt sensitivity

We investigated the hypothesis that hypokalemia might induce renal injury via a mechanism that involves subtle renal injury and alterations in local vasoactive mediators that would favor sodium retention. To test this hypothesis, we conducted studies in rats with diet-induced K+ deficiency. We also determined whether rats with hypokalemic nephropathy show salt sensitivity. Twelve weeks of hypokalemia resulted in a decrease in creatinine clearance, tubulointerstitial injury with macrophage infiltration, interstitial collagen type III deposition, and an increase in osteopontin expression (a tubular marker of injury). The renal injury was greatest in the outer medulla with radiation into the cortex, suggestive of an ischemic etiology. Consistent with this hypothesis, we found an increased uptake of a hypoxia marker, pimonidazole, in the cortex. The intrarenal injury was associated with increased cortical angiontensin-converting enzyme (ACE) expression and continued cortical angiotensin II generation despite systemic suppression of the renin-angiotensin system, an increase in renal endothelin-1, a decrease in renal kallikrein, and a decrease in urinary nitrite/nitrates and prostaglandin E(2) excretion. At 12 wk, hypokalemic rats were placed on a normal-K+ diet with either high (4%)- or low (0.01%)-NaCl content. Despite correction of hypokalemia and normalization of renal function, previously hypokalemic rats showed an elevated blood pressure in response to a high-salt diet compared with normokalemic controls. Hypokalemia is associated with alterations in vasoactive mediators that favor intrarenal vasoconstriction and an ischemic pattern of renal injury. These alterations may predispose the animals to salt-sensitive hypertension that manifests despite normalization of the serum K+.

Angiotensin II and prostaglandin interactions on systemic and renal effects of L-NAME in humans

For investigation of whether interactions between prostaglandins and angiotensin II modulate renal response to acute nitric oxide synthesis inhibition in humans, seven young volunteers who were kept on a 240-mM Na diet underwent four experiments with 90 min of infusion of 3.0 microg/kg.min(-1) NG-nitro-L-arginine methyl ester (L-NAME), each preceded by a 3-d treatment with placebo (PL), 50 mg of losartan (LOS), 75 to 125 mg of indomethacin (IND), or both drugs. Mean arterial pressure (MAP), GFR, effective renal plasma flow (ERPF), and Na excretion rate (UNaV) were measured at baseline and from 0 to 45 min and 45 to 90 min of L-NAME infusion. After PL, L-NAME reduced GFR by 5% at 45 min (P < 0.05) and by 9% at 90 min (P < 0.001), ERPF by 11 to 17% (P < 0.001), and UNaV by 28 to 45% (P < 0.001). MAP, unchanged at 45 min, rose by 5% (P < 0.001) at 90 min. LOS prevented pressor but not renal effects of L-NAME. With L-NAME+IND, MAP rose even at 45 min (+5%; P < 0.001 versus baseline) with a 10% rise at 90 min (P < 0.001). Changes in GFR (-13 to -20%), ERPF (-19 to -26%), and UNaV (-51 to -70%) were greater than those with L-NAME+PL or L-NAME+LOS (P < 0.05 to 0.001). With L-NAME+IND+LOS, MAP did not increase, and GFR, ERPF, and UNaV fell much less than with L-NAME+IND alone (P < 0.02 to 0.001) with no differences versus PL or LOS alone. Angiotensin II blockade does not affect renal changes caused by L-NAME but prevents their potentiation by prostaglandin inhibition. Thus, endogenous prostaglandins counteract renal actions of endogenous angiotensin II in Na-repleted humans even when nitric oxide synthesis is inhibited.

Prostaglandin I(2)/E(2) ratios in unilateral renovascular hypertension of different severities

Differences between prostaglandins I(2) and E(2) in their renal synthesis and pathophysiological roles were investigated in unilateral renovascular hypertension of different severities in 18 patients: 6 with mild stenosis (<75% of the diameter) of the renal artery, 7 with moderate stenosis (75% to 90%), and 5 with severe stenosis (>90%). Before and after aspirin administration (10 mg/kg), renal venous and aortic plasma was assayed for 6-ketoprostaglandin F(1alpha) (instead of prostaglandin I(2)), prostaglandin E(2), and renin activity. In mild or moderate stenosis, the mean 6-ketoprostaglandin F(1alpha) level in renal venous plasma from the stenotic side was not different from that from the normal side or from aortic plasma. Prostaglandin E(2) levels and renin activity in such patients were higher on the stenotic side than on the normal side and higher in venous than in aortic plasma. Aspirin inhibited prostaglandin E(2) synthesis and suppressed renin release from stenotic kidneys and lowered blood pressure as the renin activity decreased in patients with mild or moderate stenosis. In severe stenosis, levels of 6-ketoprostaglandin F(1alpha) and prostaglandin E(2) were higher on the stenotic side than on the normal side and higher in venous than in aortic plasma. Aspirin inhibited the synthesis of both prostaglandins and suppressed renin release from the stenotic kidney. In patients with unilateral renovascular hypertension with mild or moderate stenosis of the renal artery, prostaglandin E(2), rather than I(2), seems to contribute to further acceleration of renin release. Prostaglandin I(2) may increase and participate in further renin release when the stenosis is severe.

Combination of hypercholesterolemia and hypertension augments renal function abnormalities

Hypercholesterolemia and hypertension are both risk factors for end-stage renal disease. This study was designed to examine whether their coexistence augmented impairment in renal function and redox status. Regional renal hemodynamics and function in response to vasoactive challenges with acetylcholine or sodium nitroprusside were quantified by using electron-beam computed tomography in pigs after 12 weeks of either a normal (n=10) or hypercholesterolemic (n=10) diet, renovascular hypertension (n=7), or combined hypercholesterolemia+hypertension (n=6). The hypercholesterolemic and hypercholesterolemic+hypertensive groups had significantly increased serum cholesterol levels, whereas in the hypertensive and hypercholesterolemic+hypertensive groups, mean arterial pressure was significantly elevated compared with the group fed a normal diet. Basal regional renal perfusion and glomerular filtration rates were similar among the groups. In response to acetylcholine, cortical perfusion increased in normal animals (15.6+/-4.7%, P=0.002) but not in hypercholesterolemic or hypertensive animals (8.0+/-7.4% and 8.2+/-5.9%, respectively; P>0.05). Moreover, in the hypercholesterolemic+hypertensive group, cortical perfusion response was further attenuated (2.5+/-4.8%, P=0.02) and significantly different from the group fed a normal diet (P<0.05). The response to sodium nitroprusside followed a similar pattern, and the impairment was augmented in the hypercholesterolemic+hypertensive group. The functional abnormalities in hypercholesterolemia or hypertension were associated with a decrease in systemic and/or renal tissue levels of oxygen radical scavengers that was again accentuated in hypercholesterolemia+hypertension. These results demonstrate that concurrent hypercholesterolemia and hypertension have a greater detrimental effect on renal perfusion responses compared with hypercholesterolemia or hypertension alone, associated with a marked pro-oxidant shift in redox status. These effects may potentially augment renal functional impairment and play a role in the initiation and progression of renal injury in hypertension and atherosclerosis.

Thromboxane-B2, prostaglandin-E2 and hypertension in the rat 2-kidney 1-clip model: a possible mechanism of the garlic induced hypotension

Serum collected from unilaterally clipped and unclipped rats before and after treatment with water, garlic or cilazapril and subsequent to measuring blood pressure was assayed for thromboxane-B2 and prostaglandin-E2. The unclipped rats' thromboxane-B2 and prostaglandin-E2 levels were about 23 ng/ml and 2 ng/ml, respectively, and blood pressure was 126+/-3 mmHg. These values were not affected by either water or garlic administration. The clipped rats' thromboxane-B2 and prostaglandin-E2 concentrations were close to 34 ng/ml and 4 ng/ml, respectively, and declined only in response to garlic (by 15 ng/ml and 3 ng/ml) and cilazapril (by 12 ng/ml and 1.5 ng/ml). The blood pressure of these rats was 196+/-7 mmHg and again was reduced only by garlic to 169+/-14 mmHg and cilazapril to 137+/-5 mmHg. The no-treatment and water-treatment readings were significantly higher in the clipped rats. The data suggest that prostanoid system activity in the 2-kidney 1-clip rat is enhanced and mostly toward maintaining the hypertension. Furthermore, the blood pressure lowering effects of garlic and cilazapril might have been induced partially by a greater reduction in the synthesis of vasoconstrictor prostanoids.

Eicosanoid regulation of the renal vasculature

Even though it has been recognized that arachidonic acid metabolites, eicosanoids, play an important role in the control of renal blood flow and glomerular filtration, several key observations have been made in the past decade. One major finding was that two distinct cyclooxygenase (COX-1 and COX-2) enzymes exist in the kidney. A renewed interest in the contribution of cyclooxygenase metabolites in tubuloglomerular feedback responses has been sparked by the observation that COX-2 is constitutively expressed in the macula densa area. Arachidonic acid metabolites of the lipoxygenase pathway appear to be significant factors in renal hemodynamic changes that occur during disease states. In particular, 12(S)- hydroxyeicosatetraenoic acid may be important for the full expression of the renal hemodynamic actions in response to angiotensin II. Cytochrome P-450 metabolites have been demonstrated to possess vasoactive properties, act as paracrine modulators, and be a critical component in renal blood flow autoregulatory responses. Last, peroxidation of arachidonic acid metabolites to isoprostanes appears to be involved in renal oxidative stress responses. The recent developments of specific enzymatic inhibitors, stable analogs, and gene-disrupted mice and in antisense technology are enabling investigators to understand the complex interplay by which eicosanoids control renal blood flow.

Renal effects of prolonged cyclooxygenase inhibition when angiotensin II levels are elevated

We examined the renal functional and hemodynamic changes induced by prolonged cyclooxygenase (COX) inhibition when angiotensin II levels are elevated during several consecutive days. The effects induced by the infusion of either initially subpressor or pressor angiotensin II doses (1 and 5 ng/kg/min) were examined in dogs with or without the simultaneous infusion of meclofenamate (5 microg/kg/min). Experiments were performed in conscious permanently instrumented dogs. Infusion of the lower angiotensin II dose alone (n = 6) caused a late 12+/-2% increase in arterial pressure, a 25+/-6% decrease in renal blood flow (RBF), and a transitory decrease in urinary sodium excretion. COX inhibition reduced the hypertension and renal vasoconstriction, but enhanced the sodium retention, induced by the lower dose angiotensin II infusion (n = 6). The higher angiotensin II dose (n = 6) caused a 25+/-4% increase in arterial pressure, a 24+/-5% decrease in RBF, and a transitory decrease in urinary sodium excretion. Finally, COX inhibition did not modify the renal effects elicited by the higher angiotensin II dose (n = 6). The results of this study suggest that endogenous prostaglandins play an important role in the regulation of the renal and systemic changes induced by prolonged administration of initially subpressor angiotensin II doses. It has also been demonstrated that prolonged COX inhibition does not modify the renal functional and hemodynamic changes elicited by the long-term infusion of a pressor angiotensin II dose.

Mechanisms of big endothelin-1-induced diuresis and natriuresis : role of ET(B) receptors

Endothelin-1 (ET-1) at high concentrations has marked antidiuretic and antinatriuretic activities, whereas its precursor, big endothelin-1 (big ET-1), has surprisingly potent diuretic and natriuretic actions. The mechanisms underlying the excretory effects of big ET-1 have not been fully elucidated. To explore these mechanisms, we examined the effects of a highly selective ET(B) antagonist (A-192621.1), a calcium channel blocker (verapamil), a nitric oxide synthase inhibitor (N-nitro-L-arginine methyl ester [L-NAME]), and a cyclooxygenase inhibitor (indomethacin) on the systemic and renal actions of big ET-1 in anesthetized rats. An intravenous bolus injection of incremental doses of big ET-1 (0.3, 1. 0, and 3.0 nmol/kg) produced a significant hypertensive effect that was dose dependent and prolonged (from 113+/-7 mm Hg to a maximum of 148+/-6 mm Hg). The administration of big ET-1 induced marked diuretic and natriuretic responses (urinary flow rate increased from 8.5+/-1 to 110+/-14 microL/min, and fractional excretion of sodium increased from 0.38+/-0.13% to 7.51+/-1.24%). Glomerular filtration rate and renal plasma flow significantly decreased only at the highest dose of big ET-1. Pretreatment with A-192621.1 (3 mg/kg plus 3 mg. kg(-1). h(-1)) significantly abolished the diuretic (17+/-5 microL/min to a maximum of 19+/-3 microL/min) and natriuretic (0. 29+/-0.1% to a maximum of 1.93+/-0.37%) responses induced by big ET-1. Moreover, A-192621.1 potentiated the decline in glomerular filtration rate and renal plasma flow and the increase in mean arterial blood pressure produced by the low doses of big ET-1. Similar to A-192621.1, pretreatment with a nitric oxide synthase inhibitor (L-NAME, 10 mg/kg plus 5 mg. kg(-1). h(-1)) significantly and comparably reduced the diuretic and natriuretic actions of big ET-1 and augmented the hypoperfusion/hypofiltration and systemic vasoconstriction induced by high doses of the peptide. Pretreatment with verapamil (2 mg. kg(-1). h(-1)) slightly inhibited the diuretic/natriuretic effects of the high-dose big ET-1 and completely prevented the increase in mean arterial blood pressure provoked by the peptide. Unlike verapamil and L-NAME, only indomethacin administration was associated with significant natriuretic/diuretic responses and did not influence the pressor effect and renal actions of big ET-1. Taken together, these results suggest that big ET-1-induced diuretic and natriuretic responses are mediated mainly by stimulation of nitric oxide production coupled to ET(B) receptor subtype activation.

Pressor and renal effects of cross-linked hemoglobin in anesthetized cirrhotic rats

BACKGROUND/AIMS

Cross-linked hemoglobin (XL-Hb), a hemoglobin-based oxygen carrier, is currently under investigation as a blood substitute. In the present study we have evaluated its pressor and renal effects in a rat model of liver cirrhosis by bile duct ligation.

METHODS

Experiments were performed 3 weeks after surgery in anesthetized rats In the first protocol, the ability of XL-Hb to recover blood pressure after a hypotensive hemorrhage (0.5 ml/min, 10 min) was analyzed. In the second protocol, the pressor and renal effects produced by the administration of XL-Hb were evaluated during a period of 3 h.

RESULTS

After a hypotensive hemorrhage (0.5 ml/min, 10 min), resuscitation with XL-Hb resulted in greater and faster recovery of blood pressure than with the administration of blood. In non-hemorrhaged rats, administration of XL-Hb (5% of blood volume) reversibly increased blood pressure in bile duct ligation and in control rats, but this effect was of longer duration in the control animals. XL-Hb also induced brisk increases in water and sodium excretion in both groups of animals, but the response of the control animals was more intense and sustained than that of the bile duct ligation rats. Glomerular filtration rate and renal blood flow showed slight decreases, but they were well maintained around the baseline levels. All the parameters studied were normalized 3 h later. In additional experiments, the effect of a bolus of L-NAME (10 mg/kg), an inhibitor of nitric oxide synthase, 1 h after the administration of XL-Hb was partially reduced, suggesting that the effect of XL-Hb may be secondary to the disappearance of circulating nitric oxide.

CONCLUSIONS

XL-Hb seems to be effective as a resuscitative solution in case of hemorrhage in cirrhotic rats Moreover, this blood substitute only moderately and reversibly elevates blood pressure and does not adversely affects renal function.

Noninvasive evaluation of a novel swine model of renal artery stenosis

Intrarenal hemodynamics and excretory function distal to renal artery stenosis are difficult to quantify noninvasively. In this study, a swine model of chronic unilateral renal artery stenosis, achieved by implantation of an intravascular device that leads to a gradual and progressive luminal area narrowing, was developed and evaluated. Bilateral cortical and medullary volumes, blood flows, and segmental tubular dynamics were assessed in the intact kidneys of seven pigs using electron-beam computerized tomography before and 1 mo after implantation of the device. Within 1 mo, a 66% angiographic stenosis was significantly correlated with a 25% increase in BP. The volume and blood flow were markedly lower in the stenotic compared with the contralateral kidney and cortex, while the medulla exhibited minimal changes. In the stenotic kidney, intratubular contrast content has decreased in all nephron segments, especially in the distal tubule, where it correlated with an increase in serum creatinine and stenosis severity. In the contralateral kidney, dilution of proximal tubular fluid correlated with the increase in BP, likely due to pressure-natriuresis. In conclusion, the swine model closely resembles human renovascular hypertension. In the stenotic kidney, the hemodynamic impairment of the cortex is dissociated from the relatively preserved renal medulla, and the earliest effect on excretory function is observed in the distal nephron, where the fall in the amount of fluid reaching that segment is directly proportional to the renal arterial compromise. Electron-beam computerized tomography shows promise to noninvasively quantify, follow-up, and study changes in concurrent, in vivo intrarenal hemodynamics and segmental tubular function in renovascular hypertension.

Role of nitric oxide in modulating renal function and arterial pressure during chronic aldosterone excess

Chronic aldosterone (Aldo) excess is associated with transient sodium retention, extracellular fluid volume expansion, renal vasodilation, and hypertension. The purpose of this study was to determine the role of nitric oxide (NO) in mediating the renal vasodilation and the escape from the sodium-retaining actions of Aldo. To achieve this goal, we examined the long-term effects of Aldo (15 microgram. kg-1. min-1 for 7 days) in conscious, chronically instrumented control dogs (n = 9) and in dogs (n = 12) pretreated with the NO synthesis inhibitor NG-nitro-L-arginine methyl ester (L-NAME; 10 microgram. kg-1. min-1). In control dogs, Aldo caused a transient sodium retention (126 +/- 6 to 56 +/- 2 meq/day) followed by a return of sodium excretion to normal levels. Aldo also increased renal plasma flow by 15% (205 +/- 13 to 233 +/- 16 ml/min), glomerular filtration rate by 20% (72 +/- 3 to 87 +/- 5 ml/min), and arterial pressure from 90 +/- 3 to 102 +/- 3 mmHg. Aldo increased urinary nitrate/nitrite excretion by 60% in the control dogs. Although the sodium-retaining (144 +/- 7 to 56 +/- 7 meq/day) and arterial pressure (122 +/- 6 to 136 +/- 5 mmHg) responses to Aldo were the same in dogs pretreated with L-NAME compared with control, the renal hemodynamic response was markedly attenuated. The results of this study suggest that NO plays an important role in mediating the renal vasodilation during chronic Aldo excess.

Effect of indomethacin on the renal response to angiotensin II receptor blockade in healthy subjects

BACKGROUND

Non-steroidal anti-inflammatory drugs are known to promote sodium retention and to blunt the blood pressure lowering effects of several classes of antihypertensive agents including beta-blockers, diuretics and angiotensin converting enzyme (ACE) inhibitors. The purpose of the present study was to investigate the acute and sustained effects of indomethacin on the renal response to the angiotensin II receptor antagonist valsartan and to the ACE inhibitor enalapril.

METHODS

Twenty normotensive subjects maintained on fixed sodium intake (100 mmol sodium/day) were randomized to receive for one week: valsartan 80 mg o.d., enalapril 20 mg o.d., valsartan 80 mg o.d. + indomethacin 50 mg bid and enalapril 20 mg o.d. + indomethacin 50 mg bid. This single-blind study was designed as a parallel (valsartan vs. enalapril) and cross-over trial (valsartan or enalapril vs. valsartan + indomethacin or enalapril + indomethacin). Renal hemodynamics and urinary electrolyte excretion were measured for six hours after the first and seventh administration of each treatment regimen.

RESULTS

The results show that valsartan and enalapril have comparable renal effects characterized by no change in glomerular filtration rate and significant increases in renal plasma flow and sodium excretion. The valsartan- and enalapril-induced renal vasodilation is not significantly blunted by indomethacin. However, indomethacin similarly abolishes the natriuresis induced by the angiotensin II antagonist and the ACE inhibitor.

CONCLUSIONS

This observation suggests that although angiotensin receptor antagonists do not affect prostaglandin metabolism, the administration of a non-steroidal anti-inflammatory drug blunts the natriuretic response to angiotensin receptor blockade.

Bradykinin may be involved in neuropeptide Y-induced diuresis, natriuresis, and calciuresis

Neuropeptide Y (NPY) can cause diuresis, natriuresis, and calciuresis in rats independently of the pressure-natriuresis mechanism (A. Bischoff and M. C. Michel. Pflügers Arch. 435: 443-453, 1998). Because this is seen in systemic but not intrarenal NPY infusion, we have investigated the possible mediator of tubular NPY effects in anesthetized rats. In the present study, infusion of NPY (2 micrograms . kg-1 . min-1) enhanced renovascular resistance by approximately 8 mmHg . ml-1 . min and enhanced urine and sodium excretion by approximately 450 microliter/15 min and approximately 60-85 micromol/15 min, respectively. Acute renal denervation did not alter renovascular or tubular NPY effects, indicating that a neuronally released mediator is not involved. Treatment with the angiotensin II-receptor antagonist losartan prevented the decline of the renovascular response with time but did not modify tubular NPY effects. The bradykinin B2-receptor antagonist icatibant accelerated the decline of the renovascular NPY effects with time; concomitantly, it attenuated NPY-induced diuresis and natriuresis and abolished NPY-induced calciuresis. The converting-enzyme inhibitor ramiprilat prevented the decline of the renovascular response with time; concomitantly, it magnified the NPY-induced diuresis, natriuresis, and calciuresis. We conclude that bradykinin may be involved in NPY-induced diuresis, natriuresis, and, in particular, calciuresis.