Role of nitric oxide in the control of the renal medullary circulation

Endothelium-Derived Dopamine and 6-Nitrodopamine in the Cardiovascular System

The review deals with the release of endothelium-derived dopamine and 6-nitrodopamine (6-ND) and its effects on isolated vascular tissues and isolated hearts. Basal release of both dopamine and 6-ND is present in human isolated umbilical cord vessels, human popliteal vessels, nonhuman primate vessels, and reptilia aortas. The 6-ND basal release was significantly reduced when the tissues were treated with Nω-nitro-l-arginine methyl ester and virtually abolished when the endothelium was mechanically removed. 6-Nitrodopamine is a potent vasodilator, and the mechanism of action responsible for this effect is the antagonism of dopamine D2-like receptors. As a vasodilator, 6-ND constitutes a novel mechanism by which nitric oxide modulates vascular tone. The basal release of 6-ND was substantially decreased in endothelial nitric oxide synthase knockout (eNOS-/-) mice and not altered in neuronal nitric oxide synthase knockout (nNOS-/-) mice, indicating a nonneurogenic source for 6-ND in the heart. Indeed, in rat isolated right atrium, the release of 6-ND was not affected when the atria were treated with tetrodotoxin. In the rat isolated right atrium, 6-ND is the most potent endogenous positive chronotropic agent, and in Langendorff's heart preparation, it is the most potent endogenous positive inotropic agent. The positive chronotropic and inotropic effects of 6-ND are antagonized by β1-adrenoceptor antagonists at concentrations that do not affect the effects induced by noradrenaline, adrenaline, and dopamine, indicating that blockade of the 6-ND receptor is the major modulator of heart chronotropism and inotropism. The review proposes that endothelium-derived catecholamines may constitute a major mechanism for control of vascular tone and heart functions, in contrast to the overrated role attributed to the autonomic nervous system.

Vascular endothelial growth factor signaling is necessary for expansion of medullary microvessels during postnatal kidney development

Postnatal inhibition or deletion of angiotensin II (ANG II) AT1 receptors impairs renal medullary mircrovascular development through a mechanism that may include vascular endothelial growth factor (VEGF). The present study was designed to test if VEGF/VEGF receptor signaling is necessary for the development of the renal medullary microcirculation. Endothelial cell-specific immunolabeling of kidney sections from rats showed immature vascular bundles at postnatal day (P) 10 with subsequent expansion of bundles until P21. Medullary VEGF protein abundance coincided with vasa recta bundle formation. In human fetal kidney tissue, immature vascular bundles appeared early in the third trimester (GA27-28) and expanded in size until term. Rat pups treated with the VEGF receptor-2 (VEGFR2) inhibitor vandetanib (100 mg·kg−1·day−1) from P7 to P12 or P10 to P16 displayed growth retardation and proteinuria. Stereological quantification showed a significant reduction in total length (386 ± 13 vs. 219 ± 16 m), surface area, and volume of medullary microvessels. Vascular bundle architecture was unaffected. ANG II-AT1A/1B−/− mice kidneys displayed poorly defined vasa recta bundles whereas mice with collecting duct principal cell-specific AT1A deletion displayed no medullary microvascular phenotype. In conclusion, VEGFR2 signaling during postnatal development is necessary for expansion of the renal medullary microcirculation but not structural patterning of the vasa recta bundles, which occurs through an AT1-mediated mechanism.

The multifaceted role of the renal microvasculature during acute kidney injury

Pediatric acute kidney injury (AKI) represents a complex disease process for clinicians as it is multifactorial in cause and only limited treatment or preventatives are available. The renal microvasculature has recently been implicated in AKI as a strong therapeutic candidate involved in both injury and recovery. Significant progress has been made in the ability to study the renal microvasculature following ischemic AKI and its role in repair. Advances have also been made in elucidating cell-cell interactions and the molecular mechanisms involved in these interactions. The ability of the kidney to repair post AKI is closely linked to alterations in hypoxia, and these studies are elucidated in this review. Injury to the microvasculature following AKI plays an integral role in mediating the inflammatory response, thereby complicating potential therapeutics. However, recent work with experimental animal models suggests that the endothelium and its cellular and molecular interactions are attractive targets to prevent injury or hasten repair following AKI. Here, we review the cellular and molecular mechanisms of the renal endothelium in AKI, as well as repair and recovery, and potential therapeutics to prevent or ameliorate injury and hasten repair.

Impact of nitric-oxide-mediated vasodilation and oxidative stress on renal medullary oxygenation: a modeling study

The goal of this study was to investigate the effects of nitric oxide (NO)-mediated vasodilation in preventing medullary hypoxia, as well as the likely pathways by which superoxide (O2(-)) conversely enhances medullary hypoxia. To do so, we expanded a previously developed mathematical model of solute transport in the renal medulla that accounts for the reciprocal interactions among oxygen (O2), NO, and O2(-) to include the vasoactive effects of NO on medullary descending vasa recta. The model represents the radial organization of the vessels and tubules, centered around vascular bundles in the outer medulla and collecting ducts in the inner medulla. Model simulations suggest that NO helps to prevent medullary hypoxia both by inducing vasodilation of the descending vasa recta (thus increasing O2 supply) and by reducing the active sodium transport rate (thus reducing O2 consumption). That is, the vasodilative properties of NO significantly contribute to maintaining sufficient medullary oxygenation. The model further predicts that a reduction in tubular transport efficiency (i.e., the ratio of active sodium transport per O2 consumption) is the main factor by which increased O2(-) levels lead to hypoxia, whereas hyperfiltration is not a likely pathway to medullary hypoxia due to oxidative stress. Finally, our results suggest that further increasing the radial separation between vessels and tubules would reduce the diffusion of NO towards descending vasa recta in the inner medulla, thereby diminishing its vasoactive effects therein and reducing O2 delivery to the papillary tip.

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.

Renal endothelial dysfunction in acute kidney ischemia reperfusion injury

Acute kidney injury is associated with alterations in vascular tone that contribute to an overall reduction in GFR. Studies in animal models indicate that ischemia triggers alterations in endothelial function that contribute significantly to the overall degree and severity of a kidney injury. Putative mediators of vasoconstriction that may contribute to the initial loss of renal blood flow and GFR are highlighted. In addition, there is discussion of how intrinsic damage to the endothelium impairs homeostatic responses in vascular tone as well as promotes leukocyte adhesion and exacerbating the reduction in renal blood flow. The timing of potential therapies in animal models as they relate to the evolution of AKI, as well as the limitations of such approaches in the clinical setting are discussed. Finally, we discuss how acute kidney injury induces permanent alterations in renal vascular structure. We posit that the cause of the sustained impairment in kidney capillary density results from impaired endothelial growth responses and suggest that this limitation is a primary contributing feature underlying progression of chronic kidney disease.

Nitric oxide synthesis in the adult and developing kidney

Nitric oxide (NO) is synthesized within the adult and developing kidney and plays a critical role in the regulation of renal hemodynamics and tubule function. In the adult kidney, the regulation of NO synthesis is very cell type specific and subject to distinct control mechanisms of NO synthase (NOS) isoforms. Endothelial NOS (eNOS) is expressed in the endothelial cells of glomeruli, peritubular capillaries, and vascular bundles. Neuronal NOS (nNOS) is expressed in the tubular epithelial cells of the macula densa and inner medullary collecting duct. Furthermore, in the immature kidney, the expression of eNOS and nNOS shows unique patterns distinct from that is observed in the adult. This review will summarize the localization and presumable function of NOS isoforms in the adult and developing kidney.

Distinct roles of central and peripheral prostaglandin E2 and EP subtypes in blood pressure regulation

Prostaglandin E(2) (PGE(2)) is a major prostanoid with a wide variety of biological activities. PGE(2) can influence blood pressure (BP) both positively and negatively. In particular, centrally administered PGE(2) induces hypertension whereas systemic administration of PGE(2) produces a hypotensive effect. These physiologically opposing effects are generated by the existence of multiple EP receptors, namely EP(1-4), which are G protein-coupled receptors with distinct signaling properties. This review highlights the distinct roles of PGE(2) in BP regulation and the involvement of specific EP receptor subtypes.

Pathophysiology of acute kidney injury

Acute kidney injury (AKI) is the leading cause of nephrology consultation and is associated with high mortality rates. The primary causes of AKI include ischemia, hypoxia, or nephrotoxicity. An underlying feature is a rapid decline in glomerular filtration rate (GFR) usually associated with decreases in renal blood flow. Inflammation represents an important additional component of AKI leading to the extension phase of injury, which may be associated with insensitivity to vasodilator therapy. It is suggested that targeting the extension phase represents an area potential of treatment with the greatest possible impact. The underlying basis of renal injury appears to be impaired energetics of the highly metabolically active nephron segments (i.e., proximal tubules and thick ascending limb) in the renal outer medulla, which can trigger conversion from transient hypoxia to intrinsic renal failure. Injury to kidney cells can be lethal or sublethal. Sublethal injury represents an important component in AKI, as it may profoundly influence GFR and renal blood flow. The nature of the recovery response is mediated by the degree to which sublethal cells can restore normal function and promote regeneration. The successful recovery from AKI depends on the degree to which these repair processes ensue and these may be compromised in elderly or chronic kidney disease (CKD) patients. Recent data suggest that AKI represents a potential link to CKD in surviving patients. Finally, earlier diagnosis of AKI represents an important area in treating patients with AKI that has spawned increased awareness of the potential that biomarkers of AKI may play in the future.

Adverse effects of pneumoperitoneum on renal function: involvement of the endothelin and nitric oxide systems

Increased intra-abdominal pressure (IAP) during laparoscopy adversely affects kidney function. The mechanism underlying this phenomenon is largely unknown. This study was designed to investigate the involvement of endothelin (ET)-1 and nitric oxide (NO) systems in IAP-induced renal dysfunction. Rats were subjected to IAP of 14 mmHg for 1 h, followed by a deflation for 60 min (recovery). Four additional groups were pretreated with 1) ABT-627, an ET(A) antagonist; 2) A-192621, an ET(B) antagonist; 3) nitroglycerine; and 4) N(G)-nitro-L-arginine methyl ester, a NO synthase inhibitor, before IAP. Urine flow rate (V), absolute Na+ excretion (U(Na)V), glomerular filtration rate (GFR), and renal plasma flow (RPF) were determined. Significant reductions in kidney function and hemodynamics were observed when IAP was applied. V decreased from 8.1 +/- 1.0 to 5.8 +/- 0.5 microl/min, U(Na)V from 1.08 +/- 0.31 to 0.43 +/- 0.10 microeq/min, GFR from 1.84 +/- 0.12 to 1.05 +/- 0.06 ml/min (-46.9 +/- 2.7% from baseline), and RPF from 8.62 +/- 0.87 to 3.82 +/- 0.16 ml/min (-54 +/- 3.5% from baseline). When the animals were pretreated with either ABT-627 or A-192621, given alone or combined, the adverse effects of IAP on GFR, RPF, V, and U(Na)V were significantly augmented. When the animals were pretreated with nitroglycerine, the adverse effects of pneumoperitoneum on GFR and RPF were substantially improved. In contrast, pretreatment with N(G)-nitro-L-arginine methyl ester remarkably aggravated pneumoperitoneum-induced renal dysfunction. In conclusion, decreased renal excretory function and hypofiltration are induced by increased IAP. These effects are related to impairment of renal hemodynamics and could be partially ameliorated by pretreatment with nitroglycerine and aggravated by NO and ET blockade.

Evaluation of Intra-Renal Oxygenation by BOLD MRI

This is a review of blood oxygenation level-dependent (BOLD) MRI as applied to the kidney. It has been shown that BOLD MRI measurements reflect changes in renal oxygenation, especially in the medulla. Renal medulla functions in a hypoxic milieu and is extremely sensitive to further decrease in blood flow or increase in oxygen consumption. Availability of a non-invasive technique such as BOLD MRI should allow for better understanding of the factors involved in the maintenance of renal oxygenation status, not only in animal models, but also in humans.

Nitric oxide and the vascular endothelium

The vascular endothelium synthesises the vasodilator and anti-aggregatory mediator nitric oxide (NO) from L-arginine. This action is catalysed by the action of NO synthases, of which two forms are present in the endothelium. Endothelial (e)NOS is highly regulated, constitutively active and generates NO in response to shear stress and other physiological stimuli. Inducible (i)NOS is expressed in response to immunological stimuli, is transcriptionally regulated and, once activated, generates large amounts of NO that contribute to pathological conditions. The physiological actions of NO include the regulation of vascular tone and blood pressure, prevention of platelet aggregation and inhibition of vascular smooth muscle proliferation. Many of these actions are a result of the activation by NO of the soluble guanylate cyclase and consequent generation of cyclic guanosine monophosphate (cGMP). An additional target of NO is the cytochrome c oxidase, the terminal enzyme in the electron transport chain, which is inhibited by NO in a manner that is reversible and competitive with oxygen. The consequent reduction of cytochrome c oxidase leads to the release of superoxide anion. This may be an NO-regulated cell signalling system which, under certain circumstances, may lead to the formation of the powerful oxidant species, peroxynitrite, that is associated with a variety of vascular diseases.

Effect of renal medullary H2O2 on salt-induced hypertension and renal injury

Dahl salt-sensitive (SS) and consomic, salt-resistant SS-13(BN) rats possess substantial differences in blood pressure salt-sensitivity even with highly similar genetic backgrounds. The present study examined whether increased oxidative stress, particularly H2O2, in the renal medulla of SS rats contributes to these differences. Blood pressure was measured using femoral arterial catheters in three groups of rats: 1) 12-wk-old SS and consomic SS-13(BN) rats fed a 0.4% NaCl diet, 2) SS rats fed a 4% NaCl diet and chronically infused with saline or catalase (6.9 microg x kg(-1) x min(-1)) directly into the renal medulla, and 3) SS-13(BN) fed high salt (4%) and infused with saline or H2O2 (347 nmol x kg(-1) x min(-1)) into the renal medullary interstitium. After chronic blood pressure measurements, renal medullary interstitial H2O2 concentration ([H2O2]) was collected by microdialysis and analyzed with Amplex red. Blood pressure and [H2O2] were both significantly higher in SS (126 +/- 3 mmHg and 145 +/- 17 nM, respectively) vs. SS-13(BN) rats (116 +/- 2 mmHg and 56 +/- 14 nM) fed a 0.4% diet. Renal interstitial catalase infusion significantly decreased [H2O2] (96 +/- 41 vs. 297 +/- 52 nM) and attenuated the hypertension (146 +/- 2 mmHg catalase vs. 163 +/- 4 mmHg saline) in SS rats after 5 days of high salt (4%). H2O2 infused into the renal medulla of consomic SS-13(BN) fed high salt (4%) for 7 days accentuated the salt sensitivity (145 +/- 2 mmHg H2O2 vs. 134 +/- 1 mmHg saline). [H2O2] was also increased in the treated group (83 +/- 1 nM H2O2 vs. 44 +/- 9 nM saline). These data show that medullary production of H2O2 may contribute to salt-induced hypertension in SS rats and that chromosome 13 of the Brown Norway contains gene(s) that protect against renal medullary oxidant stress.

Mechanisms underlying the differential control of blood flow in the renal medulla and cortex

There is much evidence that the medullary circulation plays a key role in regulating renal salt and water handling and, accordingly, the long-term level of arterial pressure. It has also recently become clear that various regulatory factors can affect medullary blood flow (MBF) differently from cortical blood flow (CBF). It appears likely that the influence of hormonal and neural factors on the control of arterial pressure is mediated partly through their impact on MBF. In this review, we focus on the mechanisms underlying the differential control of MBF and CBF, particularly the relative insensitivity of MBF to vasoconstrictors such as angiotensin II, endothelin-1 and the sympathetic nerves. The vascular architecture of the kidney appears to be arranged in a way that protects the renal medulla from ischaemic insults, with juxtamedullary arterioles, the source of MBF, having larger calibre than their counterparts in other kidney regions. Indeed, recent studies using vascular casting methodology suggest that juxtamedullary glomerular arterioles are not the chief regulators of MBF, which is consistent with the idea that outer medullary descending vasa recta play a key role in MBF control. Release of vasoactive paracrine factors such as nitric oxide and various eicosanoids from the vascular endothelium, and probably also from the tubular epithelium, appear to differentially modulate responses of MBF and CBF to hormonal and neural factors. The prevailing intrarenal hormonal milieu and existing haemodynamic conditions also appear to strongly modulate these responses, indicating that multiple control systems interact to regulate regional kidney blood flow at an integrative level.

Effect of Dietary Sodium Intake on the Expression of Endothelin-Converting Enzyme in the Renal Medulla

BACKGROUND/AIM

Endothelin-converting enzyme (ECE) catalyzes the generation of endothelin-1 (ET-1) from its inactive precursor big-ET-1. Previous studies suggested that the ET-1 system is involved in the regulation of sodium excretion by the kidney. In particular, ET-1 via the ET(B) receptor-mediated signaling has been shown to increase renal medullary blood flow and decrease sodium transport in the collecting duct, both acting to promote renal sodium excretion. The present study was designed to evaluate the possibility that alterations in dietary salt intake may regulate the ECE-1.

METHODS

Wistar rats were fed for 3 days either with a diet containing low salt (0.01% NaCl), normal salt (0.5% NaCl), or high salt intake, either by high salt diet (4% NaCl) or normal salt diet plus 0.9% saline drinking. The expression of and immunoreactive protein levels of ECE-1 in the renal medulla was studied by RT-PCR, Northern blotting and Western blotting techniques.

RESULTS

The expression of ECE-1 mRNA (by RT-PCR and Northern blotting), as well as the immunoreactive levels of ECE-1, were significantly higher in the renal medulla of rats exposed to high salt intake than in rats on normal salt diet.

CONCLUSION

The findings suggest that upregulation of ECE-1, leading to increased generation of ET-1 in the renal medulla, may be a compensatory mechanism promoting enhanced sodium excretion by the kidney in response to high salt intake.

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.

Cardiovascular and renal control in NOS-deficient mouse models

Nitric oxide (NO) plays an essential role in the maintenance of cardiovascular and renal homeostasis. Endogenous NO is produced by three different NO synthase (NOS) isoforms: endothelial NOS (eNOS), inducible NOS (iNOS), and neuronal NOS (nNOS). To investigate which NOS is responsible for NO production in different tissues, NOS knockout (-/-) mice have been generated for the three isoforms. This review focuses on the regulation of cardiovascular and renal function in relation to blood pressure homeostasis in the different NOS-/- mice. Although regulation of vascular tone and cardiac function in eNOS-/- has been extensively studied, far less is known about renal function in these mice. eNOS-/- mice are hypertensive, but the mechanism responsible for their high blood pressure is still not clear. Less is known about cardiovascular and renal control in nNOS-/- mice, probably because their blood pressure is normal. Recent data suggest that nNOS plays important roles in cardiac function, renal homeostasis, and regulation of vascular tone under certain conditions, but these are only now beginning to be studied. Inasmuch as iNOS is absent from the cardiovascular system under physiological conditions, it may become important to blood pressure regulation only during pathological conditions related to inflammatory processes. However, iNOS is constitutively expressed in the kidney, where its function is largely unknown. Overall, the study of NOS knockout mice has been very useful and produced many answers, but it has also raised new questions. The appearance of compensatory mechanisms suggests the importance of the different isoforms to specific processes, but it also complicates interpretation of the data. In addition, deletion of a single gene may have physiologically significant effects in addition to those being studied. Thus the presence or absence of a specific phenotype may not reflect the most important physiological function of the absent gene.

Physiology of the renal medullary microcirculation

Perfusion of the renal medulla plays an important role in salt and water balance. Pericytes are smooth muscle-like cells that impart contractile function to descending vasa recta (DVR), the arteriolar segments that supply the medulla with blood flow. DVR contraction by ANG II is mediated by depolarization resulting from an increase in plasma membrane Cl(-) conductance that secondarily gates voltage-activated Ca(2+) entry. In this respect, DVR may differ from other parts of the efferent microcirculation of the kidney. Elevation of extracellular K(+) constricts DVR to a lesser degree than ANG II or endothelin-1, implying that other events, in addition to membrane depolarization, are needed to maximize vasoconstriction. DVR endothelial cytoplasmic Ca(2+) is increased by bradykinin, a response that is inhibited by ANG II. ANG II inhibition of endothelial Ca(2+) signaling might serve to regulate the site of origin of vasodilatory paracrine agents generated in the vicinity of outer medullary vascular bundles. In the hydropenic kidney, DVR plasma equilibrates with the interstitium both by diffusion and through water efflux across aquaporin-1. That process is predicted to optimize urinary concentration by lowering blood flow to the inner medulla. To optimize urea trapping, DVR endothelia express the UT-B facilitated urea transporter. These and other features show that vasa recta have physiological mechanisms specific to their role in the renal medulla.

Platelet-activating factor and solute transport processes in the kidney

We examined the hemodynamic and tubular transport mechanisms by which platelet-activating factor (PAF) regulates salt and water excretion. In anesthetized, renally denervated male Wistar rats, with raised systemic blood pressure and renal arterial blood pressure maintained at normal levels, intrarenal PAF infusion at 2.5 ng. min(-1) x kg(-1) resulted in a small fall in systemic blood pressure (no change in renal arterial blood pressure) and an increase in renal blood flow and urinary water, sodium, and potassium excretion rates. The PAF-induced changes in cardiovascular and renal hemodynamic function were abolished and renal excretory function greatly attenuated by treating rats with a nitric oxide synthase inhibitor. To determine whether a tubular site of action was involved in the natriuretic effect of PAF, cortical proximal tubules were enzymatically dissociated from male Wistar rat kidneys, and oxygen consumption rates (Qo(2)) were used as an integrated index of transcellular sodium transport. PAF at 1 nM maximally inhibited Qo(2) in both untreated and nystatin-stimulated (sodium entry into renal cell is not rate limiting) proximal tubules by approximately 20%. Blockade of PAF receptors or Na(+)-K(+)-ATPase pump activity with BN-52021 or ouabain, respectively, abolished the effect of PAF on nystatin-stimulated proximal tubule Qo(2). Inhibition of nitric oxide synthase or guanylate cyclase systems did not alter PAF-mediated inhibition of nystatin-stimulated proximal tubule Qo(2), whereas phospholipase A(2) or cytochrome-P-450 monooxygenase inhibition resulted in a 40-60% reduction. These findings suggest that stimulation of PAF receptors on the proximal tubule decreases transcellular sodium transport by activating phospholipase A(2) and the cytochrome-P-450 monooxygenase pathways that lead to the inhibition of an ouabain-sensitive component of the basolateral Na(+)-K(+)-ATPase pump. Thus PAF can activate both an arachidonate pathway-mediated suppression of proximal tubule sodium transport and a nitric oxide pathway-mediated dilatory action on renal hemodynamics that likely contributes to the natriuresis and diuresis observed in vivo.

Chronic renal hypoxia after acute ischemic injury: effects of L-arginine on hypoxia and secondary damage

Ischemic acute renal failure (ARF) results in the permanent loss of peritubular capillaries and predisposes the progression of chronic renal failure. The present study was undertaken to determine whether renal hypoxia, which may represent an important mediator in disease progression, is persistently exacerbated after recovery from ARF. Rats were subjected to ischemia-reperfusion injury and allowed to recover for 5 or 20 wk. Immunohistochemistry of the hypoxia-sensitive marker 2-pimonidizole at 5 wk revealed an overall increase in incorporation in the outer medullary region after recovery from ARF compared with sham-operated controls. Unilateral nephrectomy, in combination with ischemia-reperfusion injury resulted in greater 2-pimonidizole staining than that observed in the bilateral injury model. In addition, in the unilateral ischemia-nephrectomy model, proteinuria, interstitial fibrosis, and renal functional loss developed significantly faster than in the bilateral model of ARF when animals were allowed to recover for 20 wk. l-Arginine in the drinking water ( approximately 0.5 g/day) increased total renal blood flow approximately 30%, decreased pimonidizole staining, and attenuated manifestations of chronic renal disease. These data suggest that a reduction in the peritubular capillary density after ARF results in a persistent reduction in renal Po(2) and that hypoxia may play an important role in progression of chronic renal disease after ARF.

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.

Membrane potential controls calcium entry into descending vasa recta pericytes

We tested the hypothesis that constriction of descending vasa recta (DVR) is mediated by voltage-gated calcium entry. K(+) channel blockade with BaCl(2) (1 mM) or TEACl (30 mM) depolarized DVR smooth muscle/pericytes and constricted in vitro-perfused vessels. Pericyte depolarization by 100 mM extracellular KCl constricted DVR and increased pericyte intracellular Ca(2+) ([Ca(2+)](i)). The K(ATP) channel opener pinacidil (10(-7)-10(-4) M) hyperpolarized resting pericytes, repolarized pericytes previously depolarized by ANG II (10(-8) M), and vasodilated DVR. The DVR vasodilator bradykinin (10(-7) M) also reversed ANG II depolarization. The L-type Ca(2+) channel blocker diltiazem vasodilated ANG II (10(-8) M)- or KCl (100 mM)-preconstricted DVR, and the L-type agonist BayK 8644 constricted DVR. The plateau phase of the pericyte [Ca(2+)](i) response to ANG II was inhibited by diltiazem. These data support the conclusion that DVR vasoreactivity is controlled through variation of membrane potential and voltage-gated Ca(2+) entry into the pericyte cytoplasm.

Antihypertensive effects of chronic anti-TGF-beta antibody therapy in Dahl S rats

This study examined the role of transforming growth factor-beta (TGF-beta) in the development of hypertension and renal disease in 9-wk-old male Dahl salt-sensitive (Dahl S) rats fed an 8% NaCl diet for 3 wk. The rats received an intraperitoneal injection of a control or an anti-TGF-beta antibody (anti-TGF-beta Ab) every other day for 2 wk. Mean arterial pressure was significantly lower in Dahl S rats treated with anti-TGF-beta Ab (177 +/- 3 mmHg, n = 12) than in control rats (190 +/- 4 mmHg, n = 17). Anti-TGF-beta Ab therapy also reduced proteinuria from 226 +/- 20 to 154 +/- 16 mg/day. Renal blood flow, cortical blood flow, and creatinine clearance were not significantly different in control and treated rats; however, medullary blood flow was threefold higher in the treated rats than in the controls. Despite the reduction in proteinuria, the degree of glomerulosclerosis and renal hypertrophy was similar in control and anti-TGF-beta Ab-treated rats. Renal levels of TGF-beta1 and -beta2, alpha-actin, type III collagen, and fibronectin mRNA decreased in rats treated with anti-TGF-beta Ab. To examine whether an earlier intervention with anti-TGF-beta Ab would confer additional renoprotection, these studies were repeated in a group of 6-wk-old Dahl S rats. Anti-TGF-beta Ab therapy significantly reduced blood pressure, proteinuria, and the degree of glomerulosclerosis and renal medullary fibrosis in this group of rats. The results indicate that anti-TGF-beta Ab therapy reduces blood pressure, proteinuria, and the renal injury associated with hypertension.

Renal medullary interstitial infusion is a flawed technique for examining vasodilator mechanisms in anesthetized rabbits

INTRODUCTION

In rats, medullary interstitial (IMI) infusion is a useful technique for selective delivery of pharmacological agents to the renal medulla, in both acute and chronic experimental settings. We examined the feasibility of using this technique for delivery of vasodilators in rabbits, since this larger species would provide a number of advantages, particularly in long-term studies of circulatory control.

METHODS

Rabbits were anesthetized with pentobarbitone and artificially ventilated. Catheters were placed in a side branch of the renal artery and/or the renal medullary interstitium. Renal blood flow (RBF) was determined by transit-time ultrasound flowmetry, and blood flow in the cortex and medulla was estimated by laser Doppler flowmetry.

RESULTS

Pilot studies showed that renal arterial (IRA) infusions of bradykinin (10-300 ng/kg/min) and adenosine (1-10 ng/kg/min) produced only transient renal vasodilatation. IRA infusions of methylamine hexamethylene methylamine (MAHMA) NONOate (100-1000 ng/kg/min) and acetylcholine (10-250 ng/kg/min) produced dose-dependent and sustained increases in RBF and reductions in arterial pressure at the highest doses. However, IMI infusion of the same doses did not consistently increase medullary laser Doppler flux (MLDF). After IRA MAHMA NONOate and IMI acetylcholine, RBF fell to below its resting level. IRA boluses of acetylcholine (10-1250 ng/kg), bradykinin (2-250 ng/kg), and MAHMA NONOate (100-3000 ng/kg) dose-dependently increased RBF and CLDF and MLDF.

DISCUSSION

We had previously validated the IMI infusion technique for intramedullary delivery of vasoconstrictors in rabbits. Our present results indicate that this technique has limited application for delivery of vasodilator agents, in part because counterregulatory vasoconstrictor mechanisms are activated.

Role of nitric oxide in regulation of the renal medulla in normal and hypertensive kidneys

Accumulating evidence favors the notion that perfusion of the medulla of the kidney is regulated through the effects of nitric oxide. Reduction of nitric oxide production in the medulla by local tissue infusion of nitric oxide synthase blockers leads to reduction of medullary blood flow, salt retention and hypertension. Conversely, infusion of L-arginine to increase nitric oxide abrogates hypertension and enhances medullary blood flow in animal models. Nitric oxide levels can also be controlled through its consumption by reactive oxygen species. Thus, medullary oxidative stress might influence blood pressure and sodium balance through changes in nitric oxide. Nitric oxide inhibits sodium chloride reabsorption by the thick ascending limb and collecting duct. The likelihood that some forms of hypertension result directly from pathological alteration of transporters, channels, regulatory elements or enzymes that affect medullary nitric oxide seems high.

Effect of nitric oxide synthase inhibition on regional blood flow during hyperthermia

The loss of compensatory splanchnic vasoconstriction during hyperthermia was assessed in rats after administration of either 0, 10, 30, or 100mg/kg N(w)-nitro-L-arginine methyl ester,L-NAME. Rectal temperature (T(re)), heart rate (HR), mean arterial blood pressure (MAP), breathing frequency (BF), and renal, mesenteric and caudal blood flows (Q(R), Q(M) and Q(C)) were measured until irreversible cardiovascular collapse occurred. HR, MAP and BF increased as T(re) rose to 42 degrees C, then fell as circulatory collapse occurred. As dose increased T(re) at collapse decreased. Q(M) decreased until a T(re) of 41.5-42 degrees C and then increased. Q(R) and Q(C) were unaffected by either hyperthermia orL-NAME. Inhibition of NO synthase did not prevent the circulatory collapse of heatstroke; the higher doses ofL-NAME may have exacerbated the onset of circulatory failure.

Renal fibrosis in mice treated with human recombinant transforming growth factor-beta2

BACKGROUND

The biologic responses to transforming growth factor-beta (TGF-beta) suggest many potential therapeutic applications; however, in the only clinical trial to examine the effect of the systemic administration of a TGF-beta isoform, patients experienced significant but reversible declines in renal function. We studied the effects of administering human recombinant TGF-beta2 to adult mice.

METHODS

The effect of daily administration of TGF-beta2 on tissue vasoconstriction, tissue levels of endothelin and angiotensin II, tissue hypoxia, and renal fibrosis were examined.

RESULTS

Daily administration of TGF-beta2 at 10 or 100 microg/kg caused apparent tissue vasoconstriction that was visualized by vascular casting, with the largest impact seen in the kidney. Tissue levels of endothelin 1 and angiotensin II were significantly elevated in kidneys of treated mice, as was urinary thromboxane beta2. Renal fibrosis was observed in the cortical tubular interstitium and vasculature, particularly at the cortical-medullary junction and medullary vasa recta; however, glomerular sclerosis was not observed. Fibrosis was correlated to focal tissue hypoxia as determined by immunohistochemical detection of tissue bound pimondazole.

CONCLUSION

We conclude that there are significant histopathologic consequences, focused in the kidney, resulting from the daily administration of high doses of human recombinant TGF-beta2, and we propose that selective vascular constriction with consequent tissue hypoxia is a contributing factor.

Inhibition of nitric oxide synthase induces renal xanthine oxidoreductase activity in spontaneously hypertensive rats

The kidney function plays a crucial role in the salt-induced hypertension of genetically salt-sensitive, hypertension-prone rats. We have previously reported that renal xanthine oxidoreductase (XOR) activity is increased in hypertension-prone rats, and even more markedly in salt-induced experimental hypertension. XOR is an enzyme involved in purine metabolism, converting ATP metabolites hypoxanthine and xanthine to uric acid. Because the possible involvement of XOR in nitric oxide metabolism has gained recent interest, we determined renal XOR activity after treating spontaneously hypertensive rats (SHRs), kept on different salt intake levels (0.2, 1.1 and 6.0% of NaCl in the chow), for three weeks with a nitric oxide synthase (NOS) inhibitor, N-omega-nitro-L-arginine methyl ester (L-NAME, 20mg/kg/d). L-NAME treatment induced renal XOR activity by 14 to 37 % (P<0.001), depending on the intake level of salt. Increased salt intake was no more able to aggravate L-NAME induced hypertension, but it did further increase the renal XOR activity (p<0.05). Treatment of SHRs with a nitric oxide donor, isosorbide-5-mononitrate (60-70 mg/kg/d for 8 weeks), markedly attenuated the salt-enhanced hypertension without a clear effect on renal XOR activity. Thus, the results indicate that the NO concentration needed to inhibit XOR is supra-physiological, and suggest that renal NO production is not impaired in the SHR model of hypertension.

Systemic hemodynamics and renal function in hemorrhaged dogs resuscitated with cross-linked hemoglobin

Cross-linked hemoglobin (XL-Hb) infused into dogs increases mean arterial pressure (MAP) but decreases blood flow to the renal (RBF), mesenteric (MBF), and iliac (IBF) circulations. These actions differ markedly from dextran infusion (which increases RBF, MBF, and IBF without altering MAP) and may be due to scavenging of nitric oxide by XL-Hb. However, because the hormonal milieu regulating regional circulation is altered during hemorrhage (when XL-Hb may be used), we studied whether systemic hemodynamics, RBF, MBF, IBF, and renal excretory function in hemorrhaged dogs was altered when resuscitated with XL-Hb compared with dextran (n = 6 each). Hemorrhage decreased MAP by 25% due to a 75% decline in cardiac output. RBF, MBF, and IBF all fell by 33, 64, and 72%, respectively (P<0.05 each). There was also a fall in glomerular filtration rate (GFR), urinary flow, and sodium excretion (P<0.05 each). After resuscitation, MAP, cardiac output, RBF, MBF, IBF, and GFR all recovered to basal values with either XL-Hb or dextran. Urinary flow and sodium excretion increased to above basal levels with dextran (both by 3.5-fold; P<0.05) or XL-Hb (by 7.5- and 10-fold, respectively; P<0.05). We conclude that resuscitation with XL-Hb after hemorrhage not only increases MAP, but also restores RBF, MBF, IBF, GFR, and urinary sodium and volume excretion analogously to dextran. The results contrast with those in normal dogs and suggest that nitric oxide inhibition does not impair hemodynamic and renal function recovery during hemorrhage.

Nitrite and nitrate analyses: a clinical biochemistry perspective

OBJECTIVE

To review the assays available for measurement of nitrite and nitrate ions in body fluids and their clinical applications.

DESIGN AND METHODS

Literature searches were done of Medline and Current Contents to November 1997.

RESULTS

The influence of dietary nitrite and nitrate on the concentrations of these ions in various body fluids is reviewed. An overview is presented of the metabolism of nitric oxide (which is converted to nitrite and nitrate). Methods for measurement of the ions are reviewed. Reference values are summarized and the changes reported in various clinical conditions. These include: infection, gastroenterological conditions, hypertension, renal and cardiac disease, inflammatory diseases, transplant rejection, diseases of the central nervous system, and others. Possible effects of environmental nitrite and nitrate on disease incidence are reviewed.

CONCLUSIONS

Most studies of changes in human disease have been descriptive. Diagnostic utility is limited because the concentrations in a significant proportion of affected individuals overlap with those in controls. Changes in concentration may also be caused by diet, outside the clinical investigational setting. The role of nitrite and nitrate assays (alongside direct measurements of nitric oxide in breath) may be restricted to the monitoring of disease progression, or response to therapy in individual patients or subgroups. Associations between disease incidence and drinking water nitrate content are controversial (except for methemoglobinemia in infants).

Nitric oxide in the mediation of pressure natriuresis

1. Recent studies have indicated that nitric oxide (NO) production in the kidney contributes to the regulation of renal haemodynamics and excretory function. Inhibition of nitric oxide synthase (NOS) reduces renal blood flow by approximately 25% and markedly reduces sodium excretion without reductions in filtered load. In particular, inhibition of NO synthesis markedly suppresses the slope of the arterial pressure-mediated response in sodium excretion. 2. Further studies have shown that constant intrarenal infusion of a NO donor in dogs treated with a NOS inhibitor produced diuretic and natriuretic responses but failed to restore the slope of the pressure-induced natriuretic response. These data indicate that an alteration in intrarenal NO activity, rather than the simple presence of NO during changes in arterial pressure is required for full expression of pressure natriuretic responses. 3. In support of the hypothesis that NO is involved in the mediation of pressure natriuresis, we also recently demonstrated a direct relationship between changes in arterial pressure and urinary excretion rate of sodium as well as nitrate and nitrite (a marker for endogenous NO activity) in the presence of efficient autoregulation of cortical and medullary blood flow. 4. The direct inhibitory actions of NO on tubular sodium reabsorption have also been observed in cultured tubular cells as well as isolated, perfused cortical collecting duct segments. 5. Thus, the collective data suggest that acute changes in arterial pressure induce changes in intrarenal NO production, which may directly alter tubular reabsorptive function to manifest the phenomenon of pressure natriuresis.