alpha(2)-adrenergic receptor-mediated increase in NO production buffers renal medullary vasoconstriction

Nitric Oxide Synthase Inhibition Induces Renal Medullary Hypoxia in Conscious Rats

Background Renal hypoxia, implicated as crucial factor in onset and progression of chronic kidney disease, may be attributed to reduced nitric oxide because nitric oxide dilates vasculature and inhibits mitochondrial oxygen consumption. We hypothesized that chronic nitric oxide synthase inhibition would induce renal hypoxia. Methods and Results Oxygen-sensitive electrodes, attached to telemeters, were implanted in either renal cortex (n=6) or medulla (n=7) in rats. After recovery and stabilization, baseline oxygenation ( pO 2) was recorded for 1 week. To inhibit nitric oxide synthase, N-ω-nitro-l-arginine (L-NNA; 40 mg/kg/day) was administered via drinking water for 2 weeks. A separate group (n=8), instrumented with blood pressure telemeters, followed the same protocol. L-NNA rapidly induced hypertension (165±6 versus 108±3 mm Hg; P<0.001) and proteinuria (79±12 versus 17±2 mg/day; P<0.001). Cortical pO 2, after initially dipping, returned to baseline and then increased. Medullary pO 2 decreased progressively (up to -19±6% versus baseline; P<0.05). After 14 days of L-NNA, amplitude of diurnal medullary pO 2 was decreased (3.7 [2.2-5.3] versus 7.9 [7.5-8.4]; P<0.01), whereas amplitudes of blood pressure and cortical pO 2 were unaltered. Terminal glomerular filtration rate (1374±74 versus 2098±122 μL/min), renal blood flow (5014±336 versus 9966±905 μL/min), and sodium reabsorption efficiency (13.0±0.8 versus 22.8±1.7 μmol/μmol) decreased (all P<0.001). Conclusions For the first time, we show temporal development of renal cortical and medullary oxygenation during chronic nitric oxide synthase inhibition in unrestrained conscious rats. Whereas cortical pO 2 shows transient changes, medullary pO 2 decreased progressively. Chronic L-NNA leads to decreased renal perfusion and sodium reabsorption efficiency, resulting in progressive medullary hypoxia, suggesting that juxtamedullary nephrons are potentially vulnerable to prolonged nitric oxide depletion.

Increased Blood Pressure Variability Prior to Chronic Kidney Disease Exacerbates Renal Dysfunction in Rats

Increased blood pressure variability (BPV), which can be experimentally induced by sinoaortic denervation (SAD), has emerged as a new marker of the prognosis of cardiovascular and renal outcomes. Considering that increased BPV can lead to organ-damage, the goal of the present study was to evaluate the effects of SAD on renal function in an experimental model of chronic kidney disease (CKD). SAD was performed in male Wistar rats 2 weeks before 5/6 nephrectomy and the animals were evaluated 4 weeks after the induction of CKD. Our data demonstrated that BPV was increased in SAD and CKD animals and that the combination of both conditions (SAD+CKD) exacerbated BPV. The baroreflex sensitivity index was diminished in the SAD and CKD groups; this reduction was more pronounced when SAD and CKD were performed together. 5/6 nephrectomy led to hypertension, which was higher in SAD+CKD animals. Regarding renal function, the combination of SAD and CKD resulted in reduced renal plasma and blood flow, increased renal vascular resistance and augmented uraemia when compared to CKD animals. Glomerular filtration rate and BPV were negatively correlated in SAD, CKD, and SAD+CKD animals. Moreover, SAD+CKD animals presented a higher level of glomerulosclerosis when compared to all other groups. Cardiac and renal hypertrophy, as well as oxidative stress, was also further increased when SAD and CKD were combined. These results show that SAD prior to 5/6 nephrectomy exacerbates renal dysfunction, suggesting that previous augmented BPV should be considered as an important factor to the progression of renal diseases.

Role of the paraventricular nucleus in the reflex diuresis to pulmonary lymphatic obstruction in rabbits

The changes in urine flow and renal sympathetic nerve activity (RSNA) due to pulmonary lymphatic obstruction (PLO) were examined in anesthetized, artificially ventilated New Zealand white rabbits. PLO was produced by pressurizing an isolated pouch created in the right external jugular vein at the points of entry of the right lymphatic ducts. During this maneuver, urine flow increased from 8.5 ± 0.3 mL/10 min to 12 ± 0.5 mL/10 min (P < 0.0001) and RSNA increased from 24.0 ± 4 to 40.0 ± 5 μV·s (P < 0.0001). Bilateral lesioning of the paraventricular nucleus (PVN) of the hypothalamus or cervical vagotomy abolished these responses. PLO increased c-fos gene expression in the PVN. The increase in urine flow due to PLO was attenuated by muscimol and abolished by kynurenic acid microinjections into the PVN. The results show that (i) neurons in the PVN are an important relay site in the reflex arc, which is activated by PLO; and (ii) this activation is regulated by glutamatergic and partly by GABAergic input to the PVN.

Impacts of nitric oxide and superoxide on renal medullary oxygen transport and urine concentration

The goal of this study was to investigate the reciprocal interactions among oxygen (O2), nitric oxide (NO), and superoxide (O2 (-)) and their effects on medullary oxygenation and urinary output. To accomplish that goal, we developed a detailed mathematical model of solute transport in the renal medulla of the rat kidney. The model represents the radial organization of the renal tubules and vessels, which centers around the vascular bundles in the outer medulla and around clusters of collecting ducts in the inner medulla. Model simulations yield significant radial gradients in interstitial fluid oxygen tension (Po2) and NO and O2 (-) concentration in the OM and upper IM. In the deep inner medulla, interstitial fluid concentrations become much more homogeneous, as the radial organization of tubules and vessels is not distinguishable. The model further predicts that due to the nonlinear interactions among O2, NO, and O2 (-), the effects of NO and O2 (-) on sodium transport, osmolality, and medullary oxygenation cannot be gleaned by considering each solute's effect in isolation. An additional simulation suggests that a sufficiently large reduction in tubular transport efficiency may be the key contributing factor, more so than oxidative stress alone, to hypertension-induced medullary hypoxia. Moreover, model predictions suggest that urine Po2 could serve as a biomarker for medullary hypoxia and a predictor of the risk for hospital-acquired acute kidney injury.

Sensory nerves in lung and airways

Sensory nerves innervating the lung and airways play an important role in regulating various cardiopulmonary functions and maintaining homeostasis under both healthy and disease conditions. Their activities conducted by both vagal and sympathetic afferents are also responsible for eliciting important defense reflexes that protect the lung and body from potential health-hazardous effects of airborne particulates and chemical irritants. This article reviews the morphology, transduction properties, reflex functions, and respiratory sensations of these receptors, focusing primarily on recent findings derived from using new technologies such as neural immunochemistry, isolated airway-nerve preparation, cultured airway neurons, patch-clamp electrophysiology, transgenic mice, and other cellular and molecular approaches. Studies of the signal transduction of mechanosensitive afferents have revealed a new concept of sensory unit and cellular mechanism of activation, and identified additional types of sensory receptors in the lung. Chemosensitive properties of these lung afferents are further characterized by the expression of specific ligand-gated ion channels on nerve terminals, ganglion origin, and responses to the action of various inflammatory cells, mediators, and cytokines during acute and chronic airway inflammation and injuries. Increasing interest and extensive investigations have been focused on uncovering the mechanisms underlying hypersensitivity of these airway afferents, and their role in the manifestation of various symptoms under pathophysiological conditions. Several important and challenging questions regarding these sensory nerves are discussed. Searching for these answers will be a critical step in developing the translational research and effective treatments of airway diseases.

β-Adrenergic receptor stimulation increases surface NKCC2 expression in rat thick ascending limbs in a process inhibited by phosphodiesterase 4

The thick ascending limb of the loop of Henle (THAL) reabsorbs ∼30% of the filtered NaCl in a process mediated by the apical Na-K-2Cl cotransporter NKCC2. Stimulation of β-adrenergic receptors in the THAL enhances NaCl reabsorption and increases intracellular cAMP. We found that intracellular cAMP stimulates NKCC2 trafficking to the apical membrane via protein kinase A (PKA). Several cAMP-specific phosphodiesterases (PDE) have been identified in rat THALs, and PDE4 decreases cAMP generated by β-adrenergic stimulation in other cells. However, it is not known whether β-adrenergic receptors activation stimulates NKCC2 trafficking. Thus we hypothesized that β-adrenergic receptor stimulation enhances THAL apical membrane NKCC2 expression via the PKA pathway and PDE4 blunts this effect. THAL suspensions were obtained from Sprague-Dawley rats, and surface NKCC2 expression was measured by surface biotinylation and Western blot. Incubation of THALs with the β-adrenergic receptor agonist isoproterenol at 0.5 and 1.0 μM increased surface NKCC2 by 17 ± 1 and 29 ± 5% respectively (P < 0.05). Preventing cAMP degradation with 3-isobutyl-methylxanthine (IBMX; a nonselective phosphodiesterase inhibitor) enhanced isoproterenol-stimulated surface NKCC2 expression to 51 ± 7% (P < 0.05 vs. isoproterenol). The β-adrenergic receptor antagonist propranolol or the PKA inhibitor H-89 completely blocked isoproterenol + IBMX-induced increase on surface NKCC2, while propranolol or H-89 alone had no effect. Selective inhibition of PDE4 with rolipram (20 μM) potentiated the effect of isoproterenol on surface NKCC2 and increased cAMP levels. We concluded that β-adrenergic receptor stimulation enhances surface NKCC2 expression in the THALs via PKA and PDE4 blunts this effect.

Medullary thick ascending limb buffer vasoconstriction of renal outer-medullary vasa recta in salt-resistant but not salt-sensitive rats

We have demonstrated previously that paracrine signaling occurs between medullary thick ascending limb (mTAL) and the contractile pericytes of outer-medullary vasa recta (VR), termed "tubular-vascular cross-talk." The aim of the current study was to determine whether tubular-vascular cross-talk has a functional effect on vasoconstrictor responses to angiotensin II and to determine whether this is altered in the Dahl salt-sensitive (SS) rat. Studies were performed on salt-resistant consomic SS.13 Brown Norway (BN) and SS rats using a novel outer medullary tissue strip preparation in which freshly isolated VRs within VR bundles were perfused either alone or in combination with nearby mTAL. In VRs from SS.13BN rats, angiotensin II (1 µmol/L) increased VR bundle intracellular Ca2+ concentration 19±9 nmol/L (n=8) and reduced focal diameter in perfused VRs by -20±7% (n=5). In the presence of nearby mTAL, however, VR bundle intracellular Ca2+ concentration (-9±8 nmol/L; n=8) and VR diameter (-1±4%, n=7) in SS.13BN rats were unchanged by angiotensin II. In contrast, in Dahl SS rats, angiotensin II resulted in rapid and sustained increase in VR bundle intracellular Ca2+ concentration (89±48 nmol/L, n=7; 50±24%, n=8) and a reduction in VR diameter of (-17±7%, n=7; -11±4%, n=5) in both isolated VRs and VRs with nearby mTAL, respectively. In VRs with mTAL from SS13BN rats, inhibiton of purinergic receptors resulted in an increase in VR bundle intracellular Ca2+ concentration, indicating that purinergic signaling buffers vasoconstriction. Importantly, our in vitro data were able to predict medullary blood flow responses to angiotensin II in SS and SS.13BN rats in vivo. We conclude that paracrine signaling from mTAL buffers angiotensin II vasoconstriction in Dahl salt-resistant SS.13BN rats but not SS rats.

Differential involvement of D1 and D2 dopamine receptors in L-DOPA-induced angiogenic activity in a rat model of Parkinson's disease

Angiogenesis occurs in the brains of Parkinson's disease patients, but the effects of dopamine replacement therapy on this process have not been examined. Using rats with 6-hydroxydopamine lesions, we have compared angiogenic responses induced in the basal ganglia by chronic treatment with either L-DOPA, or bromocriptine, or a selective D1 receptor agonist (SKF38393). Moreover, we have asked whether L-DOPA-induced angiogenesis can be blocked by co-treatment with either a D1- or a D2 receptor antagonist (SCH23390 and eticlopride, respectively), or by an inhibitor of extracellular signal-regulated kinases 1 and 2 (ERK1/2) (SL327). L-DOPA, but not bromocriptine, induced dyskinesia, which was associated with endothelial proliferation, upregulation of immature endothelial markers (nestin) and downregulation of endothelial barrier antigen in the striatum and its output structures. At a dose inducing dyskinesia (1.5 mg/kg/day), SKF38393 elicited angiogenic changes similar to L-DOPA. Antagonism of D1- but not D2 class receptors completely suppressed both the development of dyskinesia and the upregulation of angiogenesis markers. In fact, L-DOPA-induced endothelial proliferation was markedly exacerbated by low-dose D2 antagonism (0.01 mg/kg eticlopride). Inhibition of ERK1/2 by SL327 attenuated L-DOPA-induced dyskinesia and completely inhibited all markers of angiogenesis. These results highlight the specific link between treatment-induced dyskinesias and microvascular remodeling in the dopamine-denervated brain. L-DOPA-induced angiogenesis requires stimulation of D1 receptors and activation of ERK1/2, whereas the stimulation of D2 receptors seems to oppose this response.

EFFECTS OF MITRAL REGURGITATION ON THE REFLEX DIURESIS TO PULMONARY LYMPHATIC OBSTRUCTION IN RABBITS

Increasing the extravascular fluid of the airways acutely by obstructing pulmonary lymph drainage causes a reflex diuresis mediated by neuronal nitric oxide synthase in the renal medulla. The authors examined this reflex in rabbits with a chronic increase in extravascular fluid of the airways resulting from surgically induced mitral regurgitation. Intact rabbits served as controls. Renal neuronal (nNOS) and endothelial (eNOS) nitric oxide synthase expressions were also examined. The reflex was absent in rabbits with mitral regurgitation. There were significant increases in medullary and cortical nNOS mRNA compared to controls. The observed changes in mRNA levels correlated with nNOS protein levels. eNOS mRNA was unaffected.

Nitric oxide reduces flow-induced superoxide production via cGMP-dependent protein kinase in thick ascending limbs

We have shown that increased luminal flow induces O(2)(-) and nitric oxide (NO) production in thick ascending limbs (TALs). However, the interaction of flow-stimulated NO and O(2)(-) in TALs is unclear. We hypothesized that NO inhibits flow-induced O(2)(-) production in TALs via cGMP-dependent protein kinase (PKG). We measured flow-stimulated O(2)(-) production in rat TALs using dihydroethidium in the absence and presence of L-arginine (0.3 mM), the substrate for NO synthase. The addition of L-arginine reduced flow-induced net O(2)(-) production from 68 +/- 9 to 17 +/- 4 AU/s (P < 0.002). The addition of the NO synthase inhibitor N(G)-nitro-l-arginine methyl ester (L-NAME; 5 mM) in the presence of L-arginine stimulated production (L-arginine: 15 +/- 4 AU/s vs. L-arginine + L-NAME: 63 +/- 7 AU/s; P < 0.002). The guanylate cyclase inhibitor LY-83583 (10 microM) also enhanced flow-induced net O(2)(-) production in the presence of L-arginine (L-arginine: 7 +/- 4 AU/s vs. L-arginine + LY-83583: 53 +/- 7 AU/s; P < 0.01). In the presence of LY-83583, L-arginine only reduced flow-induced net O(2)(-) by 36% (LY-83583: 80 +/- 7 AU/s vs. LY-83583 + L-arginine: 51 +/- 3 AU/s; P < 0.006). The cGMP analog dibutyryl (db)-cGMP reduced flow-induced net O(2)(-) from 39 +/- 9 to 7 +/- 3 AU/s (P < 0.03). The PKG inhibitor KT-5823 (5 microM) partially restored flow-induced net O(2)(-) in the presence of L-arginine (L-arginine: 4 +/- 4 AU/s vs. L-arginine + KT-5823: 32 +/- 9 AU/s; P < 0.03) and db-cGMP (db-cGMP: 9 +/- 7 AU/s vs. db-cGMP + KT-5823: 54 +/- 5 AU/s; P < 0.01). Phosphodiesterase II inhibition had no effect on arginine-inhibited O(2)(-) production. We conclude that 1) NO reduces flow-stimulated O(2)(-) production, 2) this occurs primarily via the cGMP/PKG pathway, and 3) O(2)(-) scavenging by NO plays a minor role.

Dissociation of metabolic and neurovascular responses to levodopa in the treatment of Parkinson's disease

We compared the metabolic and neurovascular effects of levodopa (LD) therapy for Parkinson's disease (PD). Eleven PD patients were scanned with both [15O]-H2O and [18F]-fluorodeoxyglucose positron emission tomography in the unmedicated state and during intravenous LD infusion. Images were used to quantify LD-mediated changes in the expression of motor- and cognition-related PD covariance patterns in scans of cerebral blood flow (CBF) and cerebral metabolic rate for glucose (CMR). These changes in network activity were compared with those occurring during subthalamic nucleus (STN) deep brain stimulation (DBS), and those observed in a test-retest PD control group. Separate voxel-based searches were conducted to identify individual regions with dissociated treatment-mediated changes in local cerebral blood flow and metabolism. We found a significant dissociation between CBF and CMR in the modulation of the PD motor-related network by LD treatment (p < 0.001). This dissociation was characterized by reductions in network activity in the CMR scans (p < 0.003) occurring concurrently with increases in the CBF scans (p < 0.01). Flow-metabolism dissociation was also evident at the regional level, with LD-mediated reductions in CMR and increases in CBF in the putamen/globus pallidus, dorsal midbrain/pons, STN, and ventral thalamus. CBF responses to LD in the putamen and pons were relatively greater in patients exhibiting drug-induced dyskinesia. In contrast, flow-metabolism dissociation was not present in the STN DBS treatment group or in the PD control group. These findings suggest that flow-metabolism dissociation is a distinctive feature of LD treatment. This phenomenon may be especially pronounced in patients with LD-induced dyskinesia.

A model of nitric oxide tubulovascular cross talk in a renal outer medullary cross section

We developed a two-dimensional model of NO transport in a cross section of the inner stripe (IS) of the rat outer medulla to determine whether tubular and vascular generation of NO result in significant NO concentration (C(NO)) differences between the periphery and the center of vascular bundles and thereby affect medullary blood flow distribution. Following the approach of Layton and Layton (Layton AT, Layton HE. Am J Physiol Renal Physiol 289: F1346-F1366, 2006), the structural heterogeneity of the IS was incorporated in a representative unit consisting of four concentric regions centered on a vascular bundle. Our model suggests that the diffusion distance of NO in the interstitium is limited to a few micrometers. We predict that, under basal conditions, epithelial NO generation raises the average C(NO) in pericytes surrounding peripheral descending vasa recta (DVR) by a few nanomoles relative to that in pericytes surrounding central DVR. The short descending limbs and long ascending limbs are found to exert the greatest effect on C(NO) in pericytes; long descending limbs and short ascending limbs only have a moderate effect, whereas outer medullary collecting ducts, which are situated far from the vascular bundle center, do not affect pericyte C(NO). Our results suggest that selective stimulation of epithelial NO production should significantly raise the periphery-to-center DVR diameter ratio, thereby increasing the outer medulla-to-inner medulla blood flow ratio. However, concomitant increases in epithelial superoxide (O(2)(-)) production would counteract this effect. This model confirms the importance of NO and O(2)(-) interactions in mediating tubulovascular cross talk.

Mathematical model of nitric oxide convection and diffusion in a renal medullary vas rectum

In this study, the generation, convection, diffusion, and consumption of nitric oxide (NO) in and around a single renal medullary descending or ascending vas rectum in rat were modeled using CFD. The vascular lumen (with a core RBC-rich layer and a parietal layer), the endothelium, the pericytes and the interstitium were represented as concentric cylinders. We accounted for the generation of NO by vascular endothelial cells, and that by the epithelial cells of medullary thick ascending limbs (mTALs) and inner medullary collecting ducts (IMCDs), the latter via interstitial boundary conditions. Luminal velocity profiles were obtained by modeling blood flow dynamics. Our results suggest that convection (i.e., blood flow per se) does not significantly affect NO concentrations along the cortico-medullary axis, because the latter are mostly determined by the rate of NO production and that of NO consumption by hemoglobin. However, the shear stress-mediated effects of blood flow on NO generation rates, and therefore NO concentrations, were predicted to be important. Finally, we found that unless epithelial NO generation rates (per unit tubular surface area) are at least 10 times lower than endothelium NO generation rates, NO production by mTALs and IMCDs affects vascular NO concentrations, with possible consequences for medullary blood flow distribution.

Nitric oxide in the kidney: functions and regulation of synthesis

In the kidney nitric oxide (NO) has numerous important functions including the regulation of renal haemodynamics, maintenance of medullary perfusion, mediation of pressure-natriuresis, blunting of tubuloglomerular feedback, inhibition of tubular sodium reabsorption and modulation of renal sympathetic neural activity. The net effect of NO in the kidney is to promote natriuresis and diuresis. Significantly, deficient renal NO synthesis has been implicated in the pathogenesis of hypertension. All three isoforms of nitric oxide synthase (NOS), namely neuronal NOS (nNOS or NOS1), inducible NOS (iNOS or NOS2) and endothelial NOS (eNOS or NOS3) are reported to contribute to NO synthesis in the kidney. The regulation of NO synthesis in the kidney by NOSs is complex and incompletely understood. Historically, many studies of NOS regulation in the kidney have emphasized the role of variations in gene transcription and translation. It is increasingly appreciated, however, that the constitutive NOS isoforms (nNOS and eNOS) are also subject to rapid regulation by post-translational mechanisms such as Ca(2+) flux, serine/threonine phosphorylation and protein-protein interactions. Recent studies have emphasized the role of post-translational regulation of nNOS and eNOS in the regulation of NO synthesis in the kidney. In particular, a role for phosphorylation of nNOS and eNOS at both activating and inhibitory sites is emerging in the regulation of NO synthesis in the kidney. This review summarizes the roles of NO in renal physiology and discusses recent advances in the regulation of eNOS and nNOS in the kidney by post-translational mechanisms such as serine/threonine phosphorylation.

Angiotensin II and nitric oxide in neural control of intrarenal blood flow

We investigated the roles of the renin-angiotensin system and the significance of interactions between angiotensin II and nitric oxide, in responses of regional kidney perfusion to electrical renal nerve stimulation (RNS) in pentobarbital sodium-anesthetized rabbits. Under control conditions, RNS (0.5–8 Hz) reduced total renal blood flow (RBF; −89 ± 3% at 8 Hz) and cortical perfusion (CBF; −90 ± 2% at 8 Hz) more than medullary perfusion (MBF; −55 ± 5% at 8 Hz). Angiotensin II type 1 (AT1)-receptor antagonism (candesartan) blunted RNS-induced reductions in RBF ( P = 0.03), CBF ( P = 0.007), and MBF ( P = 0.04), particularly at 4 and 8 Hz. Nitric oxide synthase inhibition with NG-nitro-l-arginine (l-NNA) enhanced RBF ( P = 0.003), CBF ( P = 0.001), and MBF ( P = 0.03) responses to RNS, particularly at frequencies of 2 Hz and less. After candesartan pretreatment, l-NNA significantly enhanced RNS-induced reductions in RBF ( P = 0.04) and CBF ( P = 0.007) but not MBF ( P = 0.66). Renal arterial infusion of angiotensin II (5 ng·kg−1·min−1) selectively enhanced responses of MBF to RNS in l-NNA-pretreated but not in vehicle-pretreated rabbits. In contrast, greater doses of angiotensin II (5–15 ng·kg−1·min−1) blunted responses of MBF to RNS in rabbits with intact nitric oxide synthase. These results suggest that endogenous angiotensin II enhances, whereas nitric oxide blunts, neurally mediated vasoconstriction in the renal cortical and medullary circulations. In the renal medulla, but not the cortex, angiotensin II also appears to be able to blunt neurally mediated vasoconstriction.

Urinary responses to acute moxonidine are inhibited by natriuretic peptide receptor antagonist

We have previously shown that acute intravenous injections of moxonidine and clonidine increase plasma atrial natriuretic peptide (ANP), a vasodilator, diuretic and natriuretic hormone. We hypothesized that moxonidine stimulates the release of ANP, which would act on its renal receptors to cause diuresis and natriuresis, and these effects may be altered in hypertension. Moxonidine (0, 10, 50, 100 or 150 microg in 300 microl saline) and clonidine (0, 1, 5 or 10 microg in 300 microl saline) injected intravenously in conscious normally hydrated normotensive Sprague-Dawley rats (SD, approximately 200 g) and 12-14-week-old Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) dose-dependently stimulated diuresis, natriuresis, kaliuresis and cGMP excretion, with these effects being more pronounced during the first hour post-injection. The actions of 5 microg clonidine and 50 microg moxonidine were inhibited by yohimbine, an alpha2-adrenoceptor antagonist, and efaroxan, an imidazoline I1-receptor antagonist. Moxonidine (100 microg) stimulated (P<0.01) diuresis in SHR (0.21+/-0.04 vs 1.16+/-0.06 ml h(-1) 100 g(-1)), SD (0.42+/-0.06 vs 1.56+/-0.19 ml h(-1) 100 g(-1)) and WKY (0.12+/-0.04 vs 1.44+/-0.21 ml h(-1) 100 g(-1)). Moxonidine-stimulated urine output was lower in SHR than in SD and WKY. Moxonidine-stimulated sodium and potassium excretions were lower in SHR than in SD, but not WKY, demonstrating an influence of strain but not of pressure. Pretreatment with the natriuretic peptide antagonist anantin (5 or 10 microg) resulted in dose-dependent inhibition of moxonidine-stimulated urinary actions. Anantin (10 microg) inhibited (P<0.01) urine output to 0.38+/-0.06, 0.12+/-0.01, and 0.16+/-0.04 ml h(-1) 100 g(-1) in SD, WKY, and SHR, respectively. Moxonidine increased (P<0.01) plasma ANP in SD (417+/-58 vs 1021+/-112 pg ml(-1)) and WKY (309+/-59 vs 1433+/-187 pg ml(-1)), and in SHR (853+/-96 vs 1879+/-229 pg ml(-1)). These results demonstrate that natriuretic peptides mediate the urinary actions of moxonidine through natriuretic peptide receptors.

Role of adrenergic receptors in the reflex diuresis in rabbits during pulmonary lymphatic obstruction

The role of adrenergic receptors in the reflex diuresis in response to pulmonary lymphatic drainage was examined in anaesthetized, artificially ventilated New Zealand White rabbits. Pulmonary lymphatic drainage was obstructed by raising the pressure in a pouch created from the right external jugular vein. This pulmonary lymphatic obstruction results in a reflex increase in urine flow and sodium excretion. This reflex is abolished by renal denervation and by administration of L-NAME, a non-selective inhibitor of nitric oxide synthase. Also, infusion of the relatively selective neuronal nitric oxide synthase blocker, 7-nitroindazole sodium salt, into the renal medulla abolished the reflex diuresis. In this study the effects of adrenergic receptor antagonists on the reflex increase in urine were observed. Both ureters were cannulated in order to determine urine flow from both kidneys separately. Prazosin, an alpha1 adrenergic receptor antagonist, was infused into the renal medulla of the right kidney, while the left kidney acted as control. Administration of prazosin in this manner did not block the reflex diuresis in response to pulmonary lymphatic obstruction in either kidney. However, rauwolscine, an alpha2 adrenergic receptor antagonist, abolished the reflex increase in urine and sodium excretion in the ipsilateral kidney while preserving it in the contralateral kidney. These findings suggest that the increase in urine flow in rabbits caused by pulmonary lymphatic obstruction is dependent upon activation of alpha2 adrenergic receptors within the renal medulla.

Evidence-based renal protection in cardiac surgery

Acute renal dysfunction is a common serious complication of cardiac surgery. Although a diversity of mechanisms exist by which the kidney can be damaged during cardiac surgery, atheroembolism, ischemia-reperfusion, and inflammation are believed to be primary contributors to perioperative renal insult. In addition, the high metabolic demands of active tubular reabsorption and the oxygen diffusion shunt characteristic of renal circulation make the kidney particularly vulnerable to ischemic injury. Remote effects of acute renal injury likely contribute to the strong association of this condition with other major postoperative morbidities and mortality and justify the search for renoprotective agents, even when dialysis is never required. Nonpharmacologic preventive strategies include procedure planning that is based on risk stratification, avoidance of nephrotoxins, and meticulous perioperative clinical care, including optimizing intravascular volume and attention to modifiable risk factors such as minimizing hemodilution. Although numerous pharmacologic interventions to prevent or treat acute renal injury have shown promise in animal models, randomized placebo-controlled clinical trials that have looked at measures of significant adverse outcomes such as death and dialysis have not confirmed a benefit.

Determinants of basal nitric oxide concentration in the renal medullary microcirculation

In this study, we modeled the production, transport, and consumption of nitric oxide (NO) in the renal medullary microcirculation under basal conditions. To yield agreement with reported NO concentrations of ∼60–140 nM in medullary tissues (Zou AP and Cowley AW Jr. Hypertension 29: 194–198, 1997; Am J Physiol Regul Integr Comp Physiol 279: R769–R777, 2000) and 3 nM in plasma (Stamler JS, Jaraki O, Osborne J, Simon DI, Keaney J, Vita J, Singel D, Valeri CR, and Loscalzo J. Proc Natl Acad Sci USA 89: 7674–7677, 1992), the permeabilities of red blood cells (RBCs), vascular walls, and pericytes to NO are all predicted to lie between 0.01 and 0.1 cm/s, and the NO production rate by vasa recta endothelium is estimated to be on the order of 10−14μmol·μm−2·s−1. Our results suggest that the concentration of NO in RBCs, which is essentially controlled by the kinetics of NO scavenging by hemoglobin, is ∼0.01 nM, that is, 103times lower than that in plasma, pericytes, and interstitium. Because the basal concentration of NO in pericytes is on the order of 10 nM, it may be too low to active guanylate cyclase, i.e., to induce vasorelaxation. Our simulations also indicate that basal superoxide concentrations may be too low to affect medullary NO levels but that, under pathological conditions, superoxide may be a very significant scavenger of NO. We also found that although oxygen is a negligible NO scavenger, medullary hypoxia may significantly enhance NO concentration gradients along the corticomedullary axis.

α-Adrenoceptor subtypes mediating regional kidney blood flow responses to renal nerve stimulation

The mechanisms underlying the relative insensitivity of the renal medullary circulation to renal sympathetic nerve stimulation (RNS) remain unknown. Therefore, we tested the effects of systemic alpha(1)- and alpha(2)-adrenoceptor blockade on responses to electrical RNS in pentobarbitone anaesthetized rabbits. Renal blood flow (RBF), cortical laser Doppler flux (CLDF), and to a lesser extent medullary LDF (MLDF) were reduced by RNS in a frequency-dependent manner. Prazosin decreased responses of RBF and CLDF, but not MLDF, to RNS. For example, during the control period 4 Hz stimulation reduced RBF, CLDF and MLDF by 85+/-3%, 89+/-2%, and 20+/-12%, respectively, but after prazosin, corresponding responses were 39+/-3%, 42+/-5% and 28+/-7%, respectively. Prazosin markedly blunted pressor and renal vasoconstrictor responses to intravenous phenylephrine, without altering pressor responses to intravenous xylazine. Rauwolscine enhanced renal vasoconstrictor responses to RNS, although this was statistically significant for RBF and CLDF but not MLDF. For example, during the control period 2 Hz stimulation reduced RBF, CLDF and MLDF by 63+/-7%, 58+/-7%, and 29+/-17%, respectively, and after rauwolscine, corresponding responses were 83+/-4%, 87+/-1%, and 53+/-12%, respectively. Rauwolscine markedly blunted renal vasoconstrictor responses to renal arterial guanabenz, but not phenylephrine. These data suggest that alpha(1)-adrenoceptors contribute to RNS-induced vasoconstriction in the renal cortex, but contribute less in vascular elements controlling medullary perfusion. Activation of alpha(2)-adrenoceptors appears to blunt RNS-induced renal vasoconstriction, but this mechanism does not underlie the relative insensitivity of medullary perfusion to RNS.

Role of renal NO production in the regulation of medullary blood flow

The unique role of nitric oxide (NO) in the regulation of renal medullary function is supported by the evidence summarized in this review. The impact of reduced production of NO within the renal medulla on the delivery of blood to the medulla and on the long-term regulation of sodium excretion and blood pressure is described. It is evident that medullary NO production serves as an important counterregulatory factor to buffer vasoconstrictor hormone-induced reduction of medullary blood flow and tissue oxygen levels. When NO synthase (NOS) activity is reduced within the renal medulla, either pharmacologically or genetically [Dahl salt-sensitive (S) rats], a super sensitivity to vasoconstrictors develops with ensuing hypertension. Reduced NO production may also result from reduced cellular uptake of l-arginine in the medullary tissue, resulting in hypertension. It is concluded that NO production in the renal medulla plays a very important role in sodium and water homeostasis and the long-term control of arterial pressure.

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.

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.

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.

Regional responsiveness of renal perfusion to activation of the renal nerves

We tested for regional differences in perfusion responses, within the renal medulla and cortex, to renal nerve stimulation in pentobarbital sodium-anesthetized rabbits. Laser-Doppler flux (LDF) was monitored at various depths below the cortical surface (1-15 mm). Basal cortical LDF (1-3 mm, approximately 200-450 U) was greater than medullary LDF (5-15 mm, approximately 70-160 U), but there were no statistically significant differences in basal LDF within these regions. The background LDF signal during aortic occlusion was similar in the cortex (2 mm, 31 U) and outer medulla (7 mm, 31 U), but slightly greater in the inner medulla (12 mm, 44 U). During electrical stimulation of the renal nerves (0.5-8 Hz), cortical LDF and total renal blood flow were similarly progressively reduced with increasing stimulus frequency. Medullary LDF (measured between 5 and 15 mm) was overall less responsive than cortical LDF. For example, 4-Hz stimulation reduced inner medullary LDF (9 mm) by 19 +/- 6% but reduced cortical LDF (1 mm) by 54 +/- 11%. However, medullary LDF responses to nerve stimulation were similar at all depths measured. Our results indicate that while the vascular elements controlling medullary perfusion are less sensitive to the effects of electrical stimulation of the renal nerves than are those controlling cortical perfusion, sensitivity within these vascular territories appears to be relatively homogeneous.

Sympathetic-renal interaction in chronic arterial pressure control

The effects of neonatal sympathectomy of donors or recipients on posttransplantation arterial pressure were investigated in spontaneously hypertensive rats (SHR) by renal transplantation experiments. Conscious mean arterial pressure (MAP) and renal vascular resistance were 136 +/- 1 mmHg and 15.5 +/- 1.2 mmHg x ml(-1) x min x g in sympathectomized SHR (n = 8) vs. 158 +/- 4 mmHg (P < 0.001) and 20.8 +/- 1.1 mmHg x ml(-1) x min x g (P < 0.05) in controls (n = 10). Seven weeks after transplantation of a kidney from neonatally sympathectomized SHR donors, MAP in SHR recipients (n = 10) was 20 mmHg lower than in controls transplanted with a kidney from hydralazine-treated SHR (n = 10) (P < 0.05) associated with reduced sodium sensitivity of MAP. Neonatal sympathectomy also lowered MAP in F1-hybrids (F1H; SHR x Wistar-Kyoto rats). Within 6 wk after transplantation, renal grafts from untreated SHR increased MAP by 20 mmHg in sympathectomized F1H (n = 10) and by 35 mmHg in sham-treated F1H (n = 8) (P < 0.05). Neonatal sympathectomy induces chronic changes in SHR kidney function leading to a MAP reduction even when extrarenal sympathetic tone is restored. Generalized reduction in sympathetic tone resets the kidney-fluid system to reduced MAP and blunts the extent of arterial pressure rise induced by an SHR kidney graft.

Physiological significance of alpha(2)-adrenergic receptor subtype diversity: one receptor is not enough

Alpha(2)-adrenergic receptors mediate part of the diverse biological effects of the endogenous catecholamines epinephrine and norepinephrine. Three distinct subtypes of alpha(2)-adrenergic receptors, alpha(2A), alpha(2B), alpha(2C), have been identified from multiple species. Because of the lack of sufficiently subtype-selective ligands, the specific biological functions of these receptor subtypes were largely unknown until recently. Gene-targeted mice carrying deletions in the genes encoding for individual alpha(2)-receptor subtypes have added important new insight into the physiological significance of adrenergic receptor diversity. Two different strategies have emerged to regulate adrenergic signal transduction. Some biological functions are controlled by two counteracting alpha(2)-receptor subtypes, e.g., alpha(2A)-receptors decrease sympathetic outflow and blood pressure, whereas the alpha(2B)-subtype increases blood pressure. Other biological functions are regulated by synergistic alpha(2)-receptor subtypes. The inhibitory presynaptic feedback loop that tightly regulates neurotransmitter release from adrenergic nerves also requires two receptor subtypes, alpha(2A) and alpha(2C). Similarly, nociception is controlled at several levels by one of the three alpha(2)-receptor subtypes. Further investigation of the specific function of alpha(2)-subtypes will greatly enhance our understanding of the relevance of closely related receptor proteins and point out novel therapeutic strategies for subtype-selective drug development.

Renal medullary nitric oxide deficit of Dahl S rats enhances hypertensive actions of angiotensin II

Studies were designed to examine the hypothesis that the renal medulla of Dahl salt-sensitive (Dahl S) rats has a reduced capacity to generate nitric oxide (NO), which diminishes the ability to buffer against the chronic hypertensive effects of small elevations of circulating ANG II. NO synthase (NOS) activity in the outer medulla of Dahl S rats (arginine-citrulline conversion assay) was significantly reduced. This decrease in NOS activity was associated with the downregulation of protein expression of NOS I, NOS II, and NOS III isoforms in this region as determined by Western blot analysis. In anesthetized Dahl S rats, we observed that a low subpressor intravenous infusion of ANG II (5 ng. kg(-1). min(-1)) did not increase the concentration of NO in the renal medulla as measured by a microdialysis with oxyhemoglobin trapping technique. In contrast, ANG II produced a 38% increase in the concentration of NO (87 +/- 8 to 117 +/- 8 nmol/l) in the outer medulla of Brown-Norway (BN) rats. The same intravenous dose of ANG II reduced renal medullary blood flow as determined by laser-Doppler flowmetry in Dahl S, but not in BN rats. A 7-day intravenous ANG II infusion at a dose of 3 ng. kg(-1). min(-1) did not change mean arterial pressure (MAP) in the BN rats but increased MAP in Dahl S rats from 120 +/- 2 to 138 +/- 2 mmHg (P < 0.05). ANG II failed to increase MAP after NO substrate was provided by infusion of L-arginine (300 microg. kg(-1). min(-1)) into the renal medulla of Dahl S rats. Intravenous infusion of L-arginine at the same dose had no effect on the ANG II-induced hypertension. These results indicate that an impaired NO counterregulatory system in the outer medulla of Dahl S rats makes them more susceptible to the hypertensive actions of small elevations of ANG II.

Placental alpha(2)-adrenoceptors control vascular development at the interface between mother and embryo

A substantial percentage of human pregnancies are lost as spontaneous abortions after implantation. This is often caused by an inadequately developed placenta. Proper development of the placental vascular system is essential to nutrient and gas exchange between mother and developing embryo. Here we show that alpha(2)-adrenoceptors, which are activated by adrenaline and noradrenaline, are important regulators of placental structure and function. Mice with deletions in the genes encoding alpha(2A)-, alpha(2B)- and alpha(2C)-adrenoceptors died between embryonic days 9.5 and 11.5 from a severe defect in yolk-sac and placenta development. In wildtype placentae, alpha(2)-adrenoceptors are abundantly expressed in giant cells, which secrete angiogenic factors to initiate development of the placental vascular labyrinth. In placentae deficient in alpha(2A)-, alpha(2B)- and alpha(2C)-adrenoceptors, the density of fetal blood vessels in the labyrinth was markedly lower than normal, leading to death of the embryos as a result of reduced oxygen and nutrient supply. Basal phosphorylation of the extracellular signal regulated kinases ERK1 and ERK2 was also lower than normal, suggesting that activation of the mitogen-activated protein kinase (MAP kinase) pathway by alpha(2)-adrenoceptors is required for placenta and yolk-sac vascular development. Thus, alpha(2)-adrenoceptors are essential at the placental interface between mother and embryo to establish the circulatory system of the placenta and thus maintain pregnancy.

Production of superoxide through NADH oxidase in thick ascending limb of Henle's loop in rat kidney

We recently reported that NADH oxidase is one of the major enzymes responsible for superoxide (O(2)(-)*) production in the rat kidney. However, the functional significance of NADH oxidase-mediated O. production and the mechanisms regulating this enzyme activity are poorly understood. Using fluorescence microscopic imaging analysis, the present study demonstrated that thick ascending limbs of Henle's loop (TALHs) exhibited red fluorescence when incubated with dihydroethidium (DHE), suggesting that O(2)(-)* is produced in this tubular segment. Compared with other nephron segments, TALHs from both renal cortex and medulla showed the highest fluorescence intensity. By incubating cortical TALHs (cTALHs) with the substrates of NADH oxidase, xanthine oxidase, nitric oxide synthase, arachidonic acid-metabolizing enzymes, and intramitochondrial oxidases, NADH oxidase was found to be one of the most important enzymes for O(2)(-)* production in this tubular segment. The NADH oxidase inhibitor diphenyleneiodonium (DPI; 100 microM) completely blocked NADH-induced O(2)(-)* production in cTALHs. Exposure of cTALHs to low PO(2) (5-10 Torr) significantly increased O(2)(-)* production regardless of the absence or presence of NADH. Furthermore, angiotensin II (100 nM) increased NADH oxidase activity by 32%, which was completely blocked by DPI. These results suggest that NADH oxidase is a major enzyme responsible for O(2)(-)* production in the TALHs and that the production of O(2)(-)* via NADH oxidase may be regulated by renal tissue oxygenation and circulating hormones.

Blood flow-dependent changes in renal interstitial guanosine 3',5'-cyclic monophosphate in rabbits

We examined responses of renal interstitial guanosine 3',5'-cyclic monophosphate (cGMP) to changes in renal perfusion pressure (RPP) within and below the range of renal blood flow (RBF) autoregulation. A microdialysis method was used to monitor renal cortical and medullary interstitial cGMP levels in anesthetized rabbits. RPP was reduced in two steps: from ambient pressure (89 +/- 3 mmHg) to 70 +/- 2 mmHg (step 1) and then to 48 +/- 3 mmHg (step 2). RBF was maintained in step 1 but was significantly decreased in step 2 from 2.94 +/- 0.23 to 1.47 +/- 0.08 ml x min(-1) x g(-1). Basal interstitial concentrations of cGMP were significantly lower in the cortex than in the medulla (12.1 +/- 1.4 and 19.9 +/- 0.4 nmol/l, respectively). Cortical and medullary cGMP did not change in step 1 but were significantly decreased in step 2, with significantly less reduction in cGMP concentrations in the medulla than in the cortex (-25 +/- 3 and -44 +/- 3%, respectively). Over this pressure range, changes in cortical and medullary cGMP were highly correlated with changes in RBF (r = 0.94, P < 0.005 for cortex; r = 0.82, P < 0.01 for medulla). Renal interstitial nitrate/nitrite was not changed in step 1 but was significantly decreased in step 2 (-38 +/- 2% in cortex and -20 +/- 2% in medulla). Nitric oxide synthase inhibition with N(G)-nitro-L-arginine methyl ester (L-NAME, 30 mg/kg bolus, 50 mg x kg(-1) x h(-1) i.v. infusion) significantly decreased RBF (by -46 +/- 4%) and interstitial concentrations of cGMP (-27 +/- 4% in cortex and -22 +/- 4% in medulla, respectively). During L-NAME treatment, renal interstitial concentrations of cGMP in the cortex and medulla were similarly not altered in step 1. However, L-NAME significantly attenuated cGMP responses to a reduction in RPP in step 2. These results indicate that acute changes in RBF result in alterations in nitric oxide-dependent renal interstitial cGMP levels, with differential effects in the medulla compared with the cortex.

Expression and cellular localization of classic NADPH oxidase subunits in the spontaneously hypertensive rat kidney

Phagocytes generate superoxide anion (O(2)(-)) by a classic, 5-component NADPH oxidase. O(2)(-) contributes to hypertension in spontaneously hypertensive rats (SHR). Therefore, we tested the hypothesis that NADPH oxidase expression is enhanced in the SHR kidney. We also analyzed the localization of NADPH oxidase components in SHR kidney. Renal NADPH oxidase was quantified by reverse transcription-polymerase chain reaction and Western blotting and was localized in SHR and Wistar Kyoto rat (WKY) kidney by immunohistochemistry. The mRNA for 5 subunits of phagocyte NADPH oxidase, and also for MOX1 and RENOX (NOX4), was detected in adult rat kidney. Kidneys of adult (10 weeks old) SHR had a significantly (P<0.01) greater mRNA for p47phox (SHR 0.81 +/- 0.05 versus WKY 0.37 +/- 0.01, arbitrary unit), which was confirmed by Western blotting (SHR 0.58 +/- 0.04 versus WKY 0.42 +/- 0.04, arbitrary unit; P<0.05) and by immunohistochemistry. This higher p47phox protein expression was also detected in young prehypertensive SHR (SHR 0.61 +/- 0.05 versus WKY 0.39 +/- 0.04, arbitrary unit; P<0.01). The 10-week-old SHR contained more modest but significantly (P<0.05) greater protein for p67phox (SHR 0.54 +/- 0.02 versus WKY 0.46 +/- 0.02). Immunostaining localized p47phox, p67phox, and p22phox in vasculature, macula densa, distal convoluted tubule, cortical collecting duct, and outer and inner medullary collecting ducts. The kidney of SHR expresses genes for all the main components of phagocyte NADPH oxidase, RENOX, and MOX1. There is a prominent increase in the SHR kidney of the mRNA, and protein expression of p47phox in the vasculature, macula densa, and distal nephron, which precedes development of hypertension.

Sympathetic modulation of renal blood flow by rilmenidine and captopril: central vs. peripheral effects

Renal blood flow (RBF) is modulated by renal sympathetic nerve activity (RSNA). However, agents that are supposed to reduce sympathetic tone, such as rilmenidine and captopril, influence RBF also by direct arteriolar effects. The present study was designed to test to what extent the renal nerves contribute to the renal hemodynamic response to rilmenidine and captopril. We used a technique that allows simultaneous recording of RBF and RSNA to the same kidney in conscious rabbits. We compared the dose-dependent effects of rilmenidine (0.01-1 mg/kg) and captopril (0.03-3 mg/kg) on RBF and RSNA in intact and renal denervated (RNX) rabbits. Because rilmenidine and captopril lower blood pressure, studies were also performed in sinoaortically denervated (SAD) rabbits to determine the role of the baroreflex in the renal hemodynamic response. Rilmenidine reduced arterial pressure, RBF, and RSNA dose dependently. In intact rabbits (n = 10), renal conductance (RC) remained unaltered (3 +/- 5%), even after the 1-mg/kg dose, which completely abolished RSNA. In RNX rabbits (n = 6), RC fell by 18 +/- 5%, whereas in SAD rabbits (n = 7) RC increased by 30 +/- 20% after rilmenidine. In intact rabbits, captopril increased RSNA maximally by 64 +/- 8%. RSNA did not rise in SAD rabbits. Despite the differential response or absence of RSNA, captopril increased RC to a comparable degree (maximally 40-50%) in all three groups. Using spectral analysis techniques, we found that in all groups, independently of ongoing RSNA, captopril, but not rilmenidine, attenuated both myogenic (0.07-0.25 Hz) and tubuloglomerular feedback (0.01-0.07 Hz) related fluctuations in RC. We conclude that, in conscious rabbits, the renal vasodilator effect of rilmenidine depends on the level of ongoing RSNA. Its sympatholytic effect is, however, blunted by a direct arteriolar vasoconstrictor effect. In contrast, the renal vasodilator effect of captopril is not modulated by ongoing RSNA and is associated with impairment of autoregulation of RBF.

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.

Renal blood flow autoregulation in blood pressure control

Although the kidney strives to maintain its perfusion within tight boundaries, considerable blood flow fluctuations do occur. The reasons for this are the rather slow acting compensatory mechanisms of renal blood flow autoregulation, the effects of renal nerves, hormonal influences, etc. It seems that variations in renal perfusion can exert a major influence on renal excretory functions, on renin release and on blood pressure. The clinical importance of renal blood flow variability is not fully understood. In many situations, the absence of normal cardiovascular oscillations seems to be a risk factor. Large fluctuations in perfusion pressure to the kidney, however, in the long run, may induce target organ damage.

Alpha(2)-adrenergic-mediated tubular NO production inhibits thick ascending limb chloride absorption

Stimulation of alpha(2)-adrenergic receptors inhibits transport in various nephron segments, and the thick ascending limb of the loop of Henle (THAL) expresses alpha(2)-receptors. We hypothesized that selective alpha(2)-receptor activation decreases NaCl absorption by cortical THALs through activation of NOS and increased production of NO. We found that the alpha(2)-receptor agonist clonidine (10 nM) decreased chloride flux (J(Cl)) from 119.5 +/- 15.9 to 67.4 +/- 13.8 pmol. mm(-1). min(-1) (43% reduction; P < 0.02), whereas removal of clonidine from the bath increased J(Cl) by 20%. When NOS activity was inhibited by pretreatment with 5 mM N(G)-nitro-L-arginine methyl ester, the inhibitory effects of clonidine on THAL J(Cl) were prevented (81.7 +/- 10.8 vs. 71.6 +/- 6.9 pmol. mm(-1). min(-1)). Similarly, when the NOS substrate L-arginine was deleted from the bath, addition of clonidine did not decrease THAL J(Cl) from control (106.9 +/- 11.6 vs. 132.2 +/- 21.3 pmol. mm(-1). min(-1)). When we blocked the alpha(2)-receptors with rauwolscine (1 microM), we found that the inhibitory effect of 10 nM clonidine on THAL J(Cl) was abolished, verifying that alpha(2), rather than I(1), receptors mediate the effects of clonidine in the THAL. We investigated the mechanism of NOS activation and found that intracellular calcium concentration did not increase in response to clonidine, whereas pretreatment with 150 nM wortmannin abolished the clonidine-mediated inhibition of THAL J(Cl), indicating activation of phosphatidylinositol 3-kinase and the Akt pathway. We found that pretreatment of THALs with 10 microM LY-83583, an inhibitor of soluble guanylate cyclase, blocked clonidine-mediated inhibition of THAL J(Cl). In conclusion, alpha(2)-receptor stimulation decreases THAL J(Cl) by increasing NO release and stimulating guanylate cyclase. These data suggest that alpha(2)-receptors act as physiological regulators of THAL NO synthesis, thus inhibiting chloride transport and participating in the natriuretic and diuretic effects of clonidine in vivo.

Sustained influence of the renal nerves to attenuate sodium retention in angiotensin hypertension

Recent studies indicate that baroreflex suppression of renal sympathetic nerve activity is sustained for up to 5 days of ANG II infusion; however, steady-state conditions are not associated with ANG II hypertension of this short duration. Thus the major goal of this study was to determine whether neurally induced increments in renal excretory function during chronic intravenous infusion of ANG II are sustained under more chronic conditions when hypertension is stable and sodium balance is achieved. Experiments were conducted in five conscious dogs subjected to unilateral renal denervation and surgical division of the urinary bladder into hemibladders to allow separate 24-h urine collection from denervated (Den) and innervated (Inn) kidneys. ANG II was infused after control measurements for 10 days at a rate of 5 ng. kg(-1). min(-1). Twenty-four-hour control values for mean arterial pressure (MAP) and the ratio for urinary sodium excretion from Den and Inn kidneys (Den/Inn) were 92 +/- 4 mmHg and 0.99 +/- 0.05, respectively. On days 8-10 of ANG II infusion, MAP was stable (+30 +/- 3 mmHg) and sodium balance was achieved. Whereas equal amounts of sodium were excreted from the kidneys during the control period, throughout ANG II infusion there was a greater rate of sodium excretion from Inn vs. Den kidneys (day 10 Den/Inn sodium = 0.56 +/- 0.05), indicating chronic suppression of renal sympathetic nerve activity. The greater rate of sodium excretion in Inn vs. Den kidneys during renal sympathoinhibition also revealed a latent impairment in sodium excretion from Den kidneys. Although the Den/Inn for sodium and the major metabolites of nitric oxide (NO) decreased in parallel during ANG II hypertension, the Den/Inn for cGMP, a second messenger of NO, remained at control levels throughout this study. This disparity fails to support the notion that a deficiency in NO production and action in Den kidneys accounts for the impaired sodium excretion. Most importantly, these results support the contention that baroreflex suppression of renal sympathetic nerve activity is sustained during chronic ANG II hypertension, a response that may play an important role in attenuating the rise in arterial pressure.