Evidence for the role of nitric oxide in macula densa control of glomerular hemodynamics

The Role of Macula Densa Nitric Oxide Synthase 1 Beta Splice Variant in Modulating Tubuloglomerular Feedback

Abnormalities in renal electrolyte and water excretion may result in inappropriate salt and water retention, which facilitates the development and maintenance of hypertension, as well as acid-base and electrolyte disorders. A key mechanism by which the kidney regulates renal hemodynamics and electrolyte excretion is via tubuloglomerular feedback (TGF), an intrarenal negative feedback between tubules and arterioles. TGF is initiated by an increase of NaCl delivery at the macula densa cells. The increased NaCl activates luminal Na-K-2Cl cotransporter (NKCC2) of the macula densa cells, which leads to activation of several intracellular processes followed by the production of paracrine signals that ultimately result in a constriction of the afferent arteriole and a tonic inhibition of single nephron glomerular filtration rate. Neuronal nitric oxide (NOS1) is highly expressed in the macula densa. NOS1β is the major splice variant and accounts for most of NO generation by the macula densa, which inhibits TGF response. Macula densa NOS1β-mediated modulation of TGF responses plays an essential role in control of sodium excretion, volume and electrolyte hemostasis, and blood pressure. In this article, we describe the mechanisms that regulate macula densa-derived NO and their effect on TGF response in physiologic and pathologic conditions. © 2023 American Physiological Society. Compr Physiol 13:4215-4229, 2023.

SARS-CoV-2 infection and diabetes: Pathophysiological mechanism of multi-system organ failure

Since the discovery of the coronavirus disease 2019 outbreak, a vast majority of studies have been carried out that confirmed the worst outcome of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection in people with preexisting health conditions, including diabetes, obesity, hypertension, cancer, and cardiovascular diseases. Likewise, diabetes itself is one of the leading causes of global public health concerns that impose a heavy global burden on public health as well as socio-economic development. Both diabetes and SARS-CoV-2 infection have their independent ability to induce the pathogenesis and severity of multi-system organ failure, while the co-existence of these two culprits can accelerate the rate of disease progression and magnify the severity of the disease. However, the exact pathophysiology of multi-system organ failure in diabetic patients after SARS-CoV-2 infection is still obscure. This review summarized the organ-specific possible molecular mechanisms of SARS-CoV-2 and diabetes-induced pathophysiology of several diseases of multiple organs, including the lungs, heart, kidneys, brain, eyes, gastrointestinal system, and bones, and sub-sequent manifestation of multi-system organ failure.

Fumarase Overexpression Abolishes Hypertension Attributable to endothelial NO synthase Haploinsufficiency in Dahl Salt-Sensitive Rats

Human blood pressure salt sensitivity is associated with changes in urinary metabolites related to fumarase (Fh) and nitric oxide (NO) metabolism, and fumarase promotes NO production through an arginine regeneration pathway. We examined the role of the fumarase-NO pathway in the development of hypertension using genetically engineered rat models. Dahl salt-sensitive (SS) rats with heterozygous mutation of eNOS (endothelial NO synthase or Nos3; SS-Nos3^+/-) were bred with SS rats with a hemizygous Fh transgene. SS-Nos3^+/- rats without the Fh transgene (SS-Nos3^+/-/Fh^0/0) developed substantial hypertension with a mean arterial pressure of 134.2±3.7 mm Hg on a 0.4% NaCl diet and 178.0±3.5 mm Hg after 14 days on a 4% NaCl diet. Mean arterial pressure decreased remarkably to 123.1±1.4 mm Hg on 0.4% NaCl, and 143.3±1.5 mm Hg on 4% NaCl in SS-Nos3^+/- rats with a Fh transgene (SS-Nos3^+/-/Fh^0/1), and proteinuria, renal fibrosis, and tubular casts were attenuated in SS-Nos3^+/-/Fh^0/1 rats compared with SS-Nos3^+/-/Fh^0/0 rats. eNOS protein abundance decreased in rats with the Nos3 heterozygous mutation, which was not influenced by Fh overexpression in rats on the 0.4% NaCl diet. However, the decrease in NO metabolite in the renal outer medulla of SS-Nos3^+/-/Fh^0/0 rats on the 0.4% NaCl diet was reversed in SS-Nos3^+/-/Fh^0/1 rats, and levels of L-arginine, but not the other 12 amino acids analyzed, were significantly higher in SS-Nos3^+/-/Fh^0/1 rats than in SS-Nos3^+/+/Fh^0/0 rats. In conclusion, fumarase has potent effects in restoring NO production and blunting the development of hypertension attributable to eNOS haploinsufficiency.

Lack of contribution of nitric oxide synthase to cholinergic vasodilation in murine renal afferent arterioles

We have previously reported significant increases in neuronal nitric oxide synthase (NOS) immunostaining in renal arterioles of angiotensin type 1A receptor (AT1A) knockout mice, and in arterioles and macula densa cells of AT1A/AT1B knockout mice. The contribution of nitric oxide derived from endothelial and macula densa cells in the maintenance of afferent arteriolar tone and acetylcholine-induced vasodilation was functionally determined in kidneys of wild-type, AT1A, and AT1A/AT1B knockout mice. Acetylcholine-induced changes in arteriolar diameters of in vitro blood-perfused juxtamedullary nephrons were measured during control conditions, in the presence of the nonspecific NOS inhibitor, N^ω-nitro-l-arginine methyl ester (NLA), or the highly selective neuronal NOS inhibitor, N⁵-(1-imino-3-butenyl)-l-ornithine (VNIO). Acetylcholine (0.1 mM) produced a significant vasoconstriction in afferent arterioles of AT1A/AT1B mice (-10.9 ± 5.1%) and no changes in afferent arteriolar diameters of AT1A knockout mice. NLA (0.01-1 mM) or VNIO (0.01-1 μM) induced significant dose-dependent vasoconstrictions (-19.8 ± 4.0% 1 mM NLA; -7.8 ± 3.5% 1 μM VNIO) in afferent arterioles of kidneys of wild-type mice. VNIO had no effect on afferent arteriole diameters of AT1A knockout or AT1A/AT1B knockout mice, suggesting nonfunctional neuronal nitric oxide synthase. These data indicate that acetylcholine produces a significant renal afferent arteriole vasodilation independently of nitric oxide synthases in wild-type mice. AT1A receptors are essential for the manifestation of renal afferent arteriole responses to neuronal nitric oxide synthase-mediated nitric oxide release.

Effects of Nitric Oxide on Renal Proximal Tubular Na+ Transport

Nitric oxide (NO) has a wide variety of physiological functions in the kidney. Besides the regulatory effects in intrarenal haemodynamics and glomerular microcirculation, in vivo studies reported the diuretic and natriuretic effects of NO. However, opposite results showing the stimulatory effect of NO on Na⁺ reabsorption in the proximal tubule led to an intense debate on its physiological roles. Animal studies have showed the biphasic effect of angiotensin II (Ang II) and the overall inhibitory effect of NO on the activity of proximal tubular Na⁺ transporters, the apical Na⁺/H⁺ exchanger isoform 3, basolateral Na⁺/K⁺ ATPase, and the Na⁺/HCO₃⁻ cotransporter. However, whether these effects could be reproduced in humans remained unclear. Notably, our recent functional analysis of isolated proximal tubules demonstrated that Ang II dose-dependently stimulated human proximal tubular Na⁺ transport through the NO/guanosine 3′,5′-cyclic monophosphate (cGMP) pathway, confirming the human-specific regulation of proximal tubular transport via NO and Ang II. Of particular importance for this newly identified pathway is its possibility of being a human-specific therapeutic target for hypertension. In this review, we focus on NO-mediated regulation of proximal tubular Na⁺ transport, with emphasis on the interaction with individual Na⁺ transporters and the crosstalk with Ang II signalling.

Inhibition of Nitric Oxide Synthase 1 Induces Salt-Sensitive Hypertension in Nitric Oxide Synthase 1α Knockout and Wild-Type Mice

We recently showed that α, β, and γ splice variants of neuronal nitric oxide synthase (NOS1) expressed in the macula densa and NOS1β accounts for most of the NO generation. We have also demonstrated that the mice with deletion of NOS1 specifically from the macula densa developed salt-sensitive hypertension. However, the global NOS1 knockout (NOS1KO) strain is neither hypertensive nor salt sensitive. This global NOS1KO strain is actually an NOS1αKO model. Consequently, we hypothesized that inhibition of NOS1β in NOS1αKO mice induces salt-sensitive hypertension. NOS1αKO and C57BL/6 wild-type (WT) mice were implanted with telemetry transmitters and divided into 7-nitroindazole (10 mg/kg/d)-treated and nontreated groups. All of the mice were fed a normal salt (0.4% NaCl) diet for 5 days, followed by a high-salt diet (4% NaCl). NO generation by the macula densa was inhibited by >90% in WT and NOS1αKO mice treated with 7-nitroindazole. Glomerular filtration rate in conscious mice was increased by ≈ 40% after a high-salt diet in both NOS1αKO and WT mice. In response to acute volume expansion, glomerular filtration rate, diuretic and natriuretic response were significantly blunted in the WT and knockout mice treated with 7-nitroindazole. Mean arterial pressure had no significant changes in mice fed a high-salt diet, but increased ≈ 15 mm Hg similarly in NOS1αKO and WT mice treated with 7-nitroindazole. We conclude that NOS1β, but not NOS1α, plays an important role in control of sodium excretion and hemodynamics in response to either an acute or a chronic salt loading.

Macula Densa Nitric Oxide Synthase 1β Protects against Salt-Sensitive Hypertension

Nitric oxide (NO) is an important negative modulator of tubuloglomerular feedback responsiveness. We recently found that macula densa expresses α-, β-, and γ-splice variants of neuronal nitric oxide synthase 1 (NOS1), and NOS1β expression in the macula densa increases on a high-salt diet. This study tested whether upregulation of NOS1β expression in the macula densa affects sodium excretion and salt-sensitive hypertension by decreasing tubuloglomerular feedback responsiveness. Expression levels of NOS1β mRNA and protein were 30- and five-fold higher, respectively, than those of NOS1α in the renal cortex of C57BL/6 mice. Furthermore, macula densa NO production was similar in the isolated perfused juxtaglomerular apparatus of wild-type (WT) and nitric oxide synthase 1α-knockout (NOS1αKO) mice. Compared with control mice, mice with macula densa-specific knockout of all nitric oxide synthase 1 isoforms (MD-NOS1KO) had a significantly enhanced tubuloglomerular feedback response and after acute volume expansion, significantly reduced GFR, urine flow, and sodium excretion. Mean arterial pressure increased significantly in MD-NOS1KO mice (P<0.01) but not NOS1flox/flox mice fed a high-salt diet. After infusion of angiotensin II, mean arterial pressure increased by 61.6 mmHg in MD-NOS1KO mice versus 32.0 mmHg in WT mice (P<0.01) fed a high-salt diet. These results indicate that NOS1β is a primary NOS1 isoform expressed in the macula densa and regulates the tubuloglomerular feedback response, the natriuretic response to acute volume expansion, and the development of salt-sensitive hypertension. These findings show a novel mechanism for salt sensitivity of BP and the significance of tubuloglomerular feedback response in long-term control of sodium excretion and BP.

Concurrent activation of multiple vasoactive signaling pathways in vasoconstriction caused by tubuloglomerular feedback: a quantitative assessment

Tubuloglomerular feedback (TGF) describes the negative relationship between (a) NaCl concentration at the macula densa and (b) glomerular filtration rate or glomerular capillary pressure. TGF-induced vasoconstriction of the afferent arteriole results from the enhanced effect of several vasoconstrictors with an effect size sequence of adenosine = 20-HETE > angiotensin II > thromboxane = superoxide > renal nerves > ATP. TGF-mediated vasoconstriction is limited by the simultaneous release of several vasodilators with an effect size sequence of nitric oxide > carbon monoxide = kinins > adenosine. The sum of the constrictor effects exceeds that of the dilator effects by the magnitude of the TGF response. The validity of the additive model used in this analysis can be tested by determining the effect of combined inhibition of some or all agents contributing to TGF. Multiple independent contributors to TGF are consistent with the variability of TGF and of the factors contributing to TGF resetting.

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.

Role of soluble guanylate cyclase in renal hemodynamics and autoregulation in the rat

We studied the influence of soluble guanylate (sGC) on renal blood flow (RBF), glomerular filtration rate (GFR), and RBF autoregulation and its role in mediating the hemodynamic effects of endogenous nitric oxide (NO). Arterial pressure (AP), heart rate (HR), RBF, GFR, urine flow (UV), and the efficiency and mechanisms of RBF autoregulation were studied in anesthetized rats during intravenous infusion of sGC activator cinaciguat before and (except GFR) also after inhibition of NO synthase (NOS) by Nω-nitro-l-arginine methyl ester. Cinaciguat (0.1, 0.3, 1, 3, 10 μg·kg−1·min−1, n = 7) reduced AP and increased HR, but did not significantly alter RBF. In clearance experiments (FITC-sinistrin, n = 7) GFR was not significantly altered by cinaciguat (0.1 and 1 μg·kg−1·min−1), but RBF slightly rose (+12%) and filtration fraction (FF) fell (−23%). RBF autoregulatory efficiency (67 vs. 104%) and myogenic response (33 vs. 44 units) were slightly depressed ( n = 9). NOS inhibition ( n = 7) increased AP (+38 mmHg), reduced RBF (−53%), and greatly augmented the myogenic response in RBF autoregulation (97 vs. 35 units), attenuating the other regulatory mechanisms. These changes were reversed by 77, 78, and 90% by 1 μg·kg−1·min−1 cinaciguat. In vehicle controls ( n = 3), in which cinaciguat-induced hypotension was mimicked by aortic compression, the NOS inhibition-induced changes were not affected. We conclude that sGC activation leaves RBF and GFR well maintained despite hypotension and only slightly impairs autoregulation. The ability to largely normalize AP, RBF, RBF autoregulation, and renovascular myogenic response after NOS inhibition indicates that these hemodynamic effects of NO are predominantly mediated via sGC.

Telmisartan protects 5/6 Nx rats against renal injury by enhancing nNOS-derived NO generation via regulation of PPARγ signaling

A 5/6 nephrectomized (Nx) rat model was employed to address the impact of telmisartan on CKD related renal injury and the underlying molecular mechanisms. It was noted that telmisartan provided protection for rats against 5/6 Nx induced lethality. Telmisartan treated 5/6 Nx rats manifested improved renal function as characterized by the higher GFR but lower urinary albumin, BUN and Scr as compared with that of control rats. Telmisartan treatment also significantly decreased systolic blood pressure and alleviated glomerulosclerosis and interstitial fibrosis. Mechanistic studies revealed that telmisartan possesses the capability to increase NO generation in the kidney. Further studies demonstrated that telmisartan promotes PPARγ expression, by which it specifically enhances nNOS expression in the kidneys after 5/6 Nx insult. Particularly, blockade of PPARγ signaling by GW9662 abolished the protective effect conferred by telmisartan, indicating that telmisartan induction of renal nNOS expression along with NO generation is dependent on PPARγ signaling. Together, our data support that telmisartan could be a promising drug for treatment of chronic kidney diseases in diverse clinical settings.

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.

Poor glycemic control is related to increased nitric oxide activity within the renal circulation of patients with type 2 diabetes

OBJECTIVE

Experimental studies have shown that glucose releases endothelial nitric oxide (NO) and that NO contributes to renal hyperperfusion in models of diabetes. To examine whether this translates into the human condition, we studied the relationship between glycemic control and renal NO activity in patients with type 2 diabetes.

RESEARCH DESIGN AND METHODS

A total of 113 patients with type 2 diabetes and a wide range of HbA1c concentrations were included. Renal plasma flow (RPF) and glomerular filtration rate (GFR) were determined by constant infusion input clearance. Functional NO activity in the renal circulation was determined as change of RPF to infusion of the NO synthase (NOS) inhibitor N(G)-monomethyl-L-arginine (L-NMMA) (4.25 mg/kg). As additional markers, we measured urinary excretion of NO (UNOx) and L-arginine-to-asymmetrical dimethylarginine (ADMA) ratio in plasma.

RESULTS

Subjects within the highest tertile of HbA1c concentration had increased RPF (low, medium, and high tertiles 576 ± 17 vs. 585 ± 22 vs. 627 ± 33 mL/min/m(2), P = 0.05 by one-way ANOVA), while GFR was similar across tertiles. The response of RPF to NOS blockade was augmented in subjects with higher HbA1c levels (-55 ± 7 vs. -64 ± 8 vs. -86 ± 8 mL/min, P = 0.04 by one-way ANOVA). Further, L-arginine-to-ADMA ratio and UNOx were increased in subjects with higher HbA1c levels.

CONCLUSIONS

In line with experimental evidence, we could demonstrate in humans that poor glycemic control is related to higher NO activity and hyperperfusion of the kidney. The renal NO system may thus be a novel therapeutic target for improving renal hemodynamics in patients with diabetes.

Salt sensitivity of tubuloglomerular feedback in the early remnant kidney

We previously reported internephron heterogeneity in the tubuloglomerular feedback (TGF) response 1 wk after subtotal nephrectomy (STN), with 50% of STN nephrons exhibiting anomalous TGF (Singh P, Deng A, Blantz RC, Thomson SC. Am J Physiol Renal Physiol 296: F1158-F1165, 2009). Presently, we tested the theory that anomalous TGF is an adaptation of the STN kidney to facilitate increased distal delivery when NaCl balance forces the per-nephron NaCl excretion to high levels. To this end, the effect of dietary NaCl on the TGF response was tested by micropuncture in STN and sham-operated Wistar rats. An NaCl-deficient (LS) or high-salt NaCl diet (HS; 1% NaCl in drinking water) was started on day 0 after STN or sham surgery. Micropuncture followed 8 days later with measurements of single-nephron GFR (SNGFR), proximal reabsorption, and tubular stop-flow pressure (PSF) obtained at both extremes of TGF activation, while TGF was manipulated by microperfusing Henle's loop (LOH) from the late proximal tubule. Activating TGF caused SNGFR to decline by similar amounts in Sham-LS, Sham-HS and STN-LS [ΔSNGFR (nl/min) = -16 ± 2, -11 ± 3, -11 ± 2; P = not significant by Tukey]. Activating TGF in STN-HS actually increased SNGFR by 5 ± 2 nl/min (P < 0.0005 vs. each other group by Tukey). HS had no effect on the PSF response to LOH perfusion in sham [ΔPSF (mmHg) = -9.6 ± 1.1 vs. -9.8 ± 1.0] but eliminated the PSF response in STN (+0.3 ± 0.9 vs. -5.7 ± 1.0, P = 0.0002). An HS diet leads to anomalous TGF in the early remnant kidney, which facilitates NaCl and fluid delivery to the distal nephron.

A re-appraisal of volume status and renal function impairment in chronic heart failure: combined effects of pre-renal failure and venous congestion on renal function

The association between cardiac failure and renal function impairment has gained wide recognition over the last decade. Both structural damage in the form of systemic atherosclerosis and (patho) physiological hemodynamic changes may explain this association. As regards hemodynamic factors, renal impairment in chronic heart failure is traditionally assumed to be mainly due to a decrease in cardiac output and a subsequent decrease in renal perfusion. This will lead to a decrease in glomerular filtration rate and a compensatory increase in tubular sodium retention. The latter is a physiological renal response aimed at retaining fluids in order to increase cardiac filling pressure and thus renal perfusion. In heart failure, however, larger increases in cardiac filling pressure are needed to restore renal perfusion and thus more volume retention. In this concept, in chronic heart failure, an equilibrium exists where a certain degree of congestion is the price to be paid to maintain adequate renal perfusion and function. Recently, this hypothesis was challenged by new studies, wherein it was found that the association between right-sided cardiac filling pressures and renal function is bimodal, with worse renal function at the highest filling pressures, reflecting a severely congested state. Renal hemodynamic studies suggest that congestion negatively affects renal function in particular in patients in whom renal perfusion is also compromised. Thus, an interplay between cardiac forward failure and backward failure is involved in the renal function impairment in the congestive state, presumably along with other factors. Only few data are available on the impact of intervention in volume status on the cardio-renal interaction. Sparse data in cardiac patients as well as evidence from cohorts with primary renal disease suggest that specific targeting of volume overload may be beneficial for long-term outcome, in spite of a certain further decrease in renal function, at least in the context of current treatment where possible reflex neurohumoral activation is ameliorated by the background treatment by blockers of the renin-angiotensin-aldosterone system.

Endogenously produced 20-HETE modulates myogenic and TGF response in microperfused afferent arterioles

Previous studies have indicated that 20-hydroxyeicosatetraeonic acid (20-HETE) modulates vascular tone in large cerebral and renal arteries through inhibition of the large conductance, calcium sensitive potassium (BK) channel activity. However, the role of 20-HETE in modulating tubuloglomerular feedback (TGF) and the myogenic response in the afferent arteriole (Af-Art) is unknown. The present study examined the effects of inhibitors of the synthesis and action of 20-HETE on the myogenic and TGF responses of isolated rabbit and mouse Af-Arts. Luminal diameter decreased by 9.2±0.5% in mice and 8.9±1.3% in rabbit Af-Art when the perfusion pressure was increased from 60 to 120 mmHg. Administration of a 20-HETE synthesis inhibitor, HET0016 (1 μM), or a selective 20-HETE antagonist, 6, 15-20-hydroxyeicosadienoic acid (6, 15-20-HEDE, 10 μM) completely blocked the myogenic response of both rabbit and mouse Af-Art, while addition of 5, 14-20-HEDE (10 μM), a 20-HETE agonist, restored the myogenic response in vessels treated with HET0016. Increases in NaCl concentration from 10 to 80 mM of the solution perfusing the macula densa constricted the Af-Art of rabbits by 6.0±1.4 μm (n=5). Addition of a 20-HETE agonist to the tubular perfusate potentiated the TGF-mediated vasoconstrictor response. This response was blocked by addition of a 20-HETE antagonist (6, 15-20-HEDE, 10 μM) to the vascular perfusate. These studies indicate that locally produced 20-HETE plays an important role in modulating the myogenic and TGF responsiveness of the Af-Art and may help explain how deficiencies in the renal formation of 20-HETE could promote the development of hypertension induced glomerular injury.

Aldosterone blunts tubuloglomerular feedback by activating macula densa mineralocorticoid receptors

Chronic aldosterone administration increases glomerular filtration rate, whereas inhibition of mineralocorticoid receptors (MRs) markedly attenuates glomerular hyperfiltration and hypertension associated with primary aldosteronism or obesity. However, the mechanisms by which aldosterone alters glomerular filtration rate regulation are poorly understood. In the present study, we hypothesized that aldosterone suppresses tubuloglomerular feedback (TGF) via activation of macula densa MR. First, we observed the expression of MR in macula densa cells isolated by laser capture microdissection and by immunofluorescence in rat kidneys. Second, to investigate the effects of aldosterone on TGF in vitro, we microdissected the juxtaglomerular apparatus from rabbit kidneys and perfused the afferent arteriole and distal tubule simultaneously. Under control conditions, TGF was 2.8±0.2 μm. In the presence of aldosterone (10(-8) mol/L), TGF was reduced by 50%. The effect of aldosterone to attenuate TGF was blocked by the MR antagonist eplerenone (10(-5) mol/L). Third, to investigate the effect of aldosterone on TGF in vivo, we performed micropuncture, and TGF was determined by maximal changes in stop-flow pressure P(sf) when tubular perfusion rate was increased from 0 to 40 nL/min. Aldosterone (10(-7) mol/L) decreased ΔP(sf) from 10.1±1.4 to 7.7±1.2 mm Hg. In the presence of l-NG-monomethyl arginine citrate (10(-3) mol/L), this effect was blocked. We conclude that MRs are expressed in macula densa cells and can be activated by aldosterone, which increases nitric oxide production in the macula densa and blunts the TGF response.

Modulation of the myogenic response in renal blood flow autoregulation by NO depends on endothelial nitric oxide synthase (eNOS), but not neuronal or inducible NOS

Nitric oxide (NO) blunts the myogenic response (MR) in renal blood flow (RBF) autoregulation. We sought to clarify the roles of NO synthase (NOS) isoforms, i.e. neuronal NOS (nNOS) from macula densa, endothelial NOS (eNOS) from the endothelium, and inducible NOS (iNOS) from smooth muscle or mesangium. RBF autoregulation was studied in rats and knockout (ko) mice in response to a rapid rise in renal artery pressure (RAP). The autoregulatory rise in renal vascular resistance within the first 6 s was interpreted as MR, from ∼6 to ∼30 s as tubuloglomerular feedback (TGF), and ∼30 to ∼100 s as the third regulatory mechanism. In rats, the nNOS inhibitor SMTC did not significantly affect MR (67 ± 4 vs. 57 ± 4 units). Inhibition of all NOS isoforms by l-NAME in the same animals markedly augmented MR to 78 ± 4 units. The same was found when SMTC was combined with angiotensin II to reproduce the hypertension and vasoconstriction seen with l-NAME (58 ± 3 vs. 54 ± 7 units, l-NAME 81 ± 2 units), or when SMTC was replaced by the nNOS inhibitor NPA (57 ± 5 vs. 56 ± 7 units, l-NAME 79 ± 4 units) or by the iNOS inhibitor 1400W (50 ± 1 vs. 55 ± 4 units, l-NAME 81 ± 3 units). nNOS-ko mice showed the same autoregulation as wild-types (MR 36 ± 4 vs. 38 ± 3 units) and the same response to l-NAME (111 ± 9 vs. 114 ± 10 units). eNOS-ko had similar autoregulation as wild-types (44 ± 8 vs. 33 ± 4 units), but failed to respond to l-NAME (37 ± 7 vs. 78 ± 16 units). We conclude that the attenuating effect of NO on MR depends on eNOS, but not on nNOS or iNOS. In eNOS-ko mice MR is depressed by NO-independent means.

Luminal flow regulates NO and O2(-) along the nephron

Urinary flow is not constant but in fact highly variable, altering the mechanical forces (shear stress, stretch, and pressure) exerted on the epithelial cells of the nephron as well as solute delivery. Nitric oxide (NO) and superoxide (O(2)(-)) play important roles in various processes within the kidney. Reductions in NO and increases in O(2)(-) lead to abnormal NaCl and water absorption and hypertension. In the last few years, luminal flow has been shown to be a regulator of NO and O(2)(-) production along the nephron. Increases in luminal flow enhance fluid, Na, and bicarbonate transport in the proximal tubule. However, we know of no reports directly addressing flow regulation of NO and O(2)(-) in this segment. In the thick ascending limb, flow-stimulated NO and O(2)(-) formation has been extensively studied. Luminal flow stimulates NO production by nitric oxide synthase type 3 and its translocation to the apical membrane in medullary thick ascending limbs. These effects are mediated by flow-induced shear stress. In contrast, flow-induced stretch and NaCl delivery stimulate O(2)(-) production by NADPH oxidase in this segment. The interaction between flow-induced NO and O(2)(-) is complex and involves more than one simply scavenging the other. Flow-induced NO prevents flow from increasing O(2)(-) production via cGMP-dependent protein kinase in thick ascending limbs. In macula densa cells, shear stress increases NO production and this requires that the primary cilia be intact. The role of luminal flow in NO and O(2)(-) production in the distal tubule is not known. In cultured inner medullary collecting duct cells, shear stress enhances nitrite accumulation, a measure of NO production. Although much progress has been made on this subject in the last few years, there are still many unanswered questions.

T-type Ca channel blockade as a determinant of kidney protection

Voltage-dependent Ca channels are classified into several subtypes based on the isoform of their α1 subunits. Traditional Ca channels blockers (CCBs), including nifedipine and amlodipine, act predominantly on L-type Ca channels, whereas novel CCBs such as efonidipine, benidipine and azelnidipine inhibit both L-type and T-type Ca channels. Furthermore, cilnidipine blocks L-type and N-type Ca channels. These CCBs exert divergent actions on renal microvessels. L-type CCBs preferentially dilate afferent arterioles, whereas both L-/T-type and L-/N-type CCBs potently dilate afferent and efferent arterioles. The distinct actions of CCBs on the renal microcirculation are reflected by changes in glomerular capillary pressure and subsequent renal injury: L-type CCBs favor an increase in glomerular capillary pressure, whereas L-/T-type and L-/N-type CCBs alleviate glomerular hypertension. The renal protective action of L-/T-type CCBs is also mediated by non-hemodynamic mechanisms, i.e., inhibition of the inflammatory process and inhibition of Rho kinase and aldosterone secretion. Finally, a growing body of evidence indicates that T-type CCBs offer more beneficial action on proteinuria and renal survival rate than L-type CCBs in patients with chronic kidney disease (CKD). Similarly, in CKD patients treated with renin-angiotensin blockers, add-on therapy with N-type CCBs is more potent in reducing proteinuria than that with L-type CCBs, although no difference is found in the subgroup with diabetic nephropathy. Thus, the strategy for hypertension treatment with CCBs has entered a new era: treatment selection depends not only on blood pressure control but also on the subtypes of CCBs.

Adenosine A2A receptor activation attenuates tubuloglomerular feedback responses by stimulation of endothelial nitric oxide synthase

Adenosine A(2) receptors have been suggested to modulate tubuloglomerular feedback (TGF) responses by counteracting adenosine A(1) receptor-mediated vasoconstriction, but the mechanisms are unclear. We tested the hypothesis that A(2A) receptor activation blunts TGF by release of nitric oxide in the juxtaglomerular apparatus (JGA). Maximal TGF responses were measured in male Sprague-Dawley rats as changes in proximal stop-flow pressure (ΔP(SF)) in response to increased perfusion of the loop of Henle (0 to 40 nl/min) with artificial tubular fluid (ATF). The maximal TGF response was studied after 5 min intratubular perfusion (10 nl/min) with ATF or ATF + A(2A) receptor agonist (CGS-21680; 10(-7) mol/l). The interaction with nitric oxide synthase (NOS) isoforms was tested by perfusion with a nonselective NOS inhibitor [N(ω)-nitro-L-arginine methyl ester hydrochloride (L-NAME); 10(-3) mol/l] or a selective neuronal NOS (nNOS) inhibitor [N(ω)-propyl-L-arginine (L-NPA); 10(-6) mol/l] alone, and with the A(2A) agonist. Blood pressure, urine flow, and P(SF) at 0 nl/min were similar among the groups. The maximal TGF response (ΔP(SF)) with ATF alone (12.3 ± 0.6 mmHg) was attenuated by selective A(2A) stimulation (9.5 ± 0.4 mmHg). L-NAME enhanced maximal TGF responses (18.9 ± 0.4 mmHg) significantly more than L-NPA (15.2 ± 0.7 mmHg). Stimulation of A(2A) receptors did not influence maximal TGF response during nonselective NOS inhibition (19.0 ± 0.4) but attenuated responses during nNOS inhibition (10.3 ± 0.4 mmHg). In conclusion, adenosine A(2A) receptor activation attenuated TGF responses by stimulation of endothelial NOS (eNOS), presumably in the afferent arteriole. Moreover, NO derived from both eNOS and nNOS in the JGA may blunt TGF responses.

Tubular reabsorption and diabetes-induced glomerular hyperfiltration

Elevated glomerular filtration rate (GFR) is a common observation in early diabetes mellitus and closely correlates with the progression of diabetic nephropathy. Hyperfiltration has been explained to be the result of a reduced load of sodium and chloride passing macula densa, secondarily to an increased proximal reabsorption of glucose and sodium by the sodium-glucose co-transporters. This results in an inactivation of the tubuloglomerular feedback (TGF), leading to a reduced afferent arteriolar vasoconstriction and subsequently an increase in GFR. This hypothesis has recently been questioned due to the observation that adenosine A(1)-receptor knockout mice, previously shown to lack a functional TGF mechanism, still display a pronounced hyperfiltration when diabetes is induced. Leyssac demonstrated in the 1960s (Acta Physiol Scand58, 1963:236) that GFR and proximal reabsorption can work independently of each other. Furthermore, by the use of micropuncture technique a reduced hydrostatic pressure in Bowman's space or in the proximal tubule of diabetic rats has been observed. A reduced pressure in Bowman's space will increase the pressure gradient over the filtration barrier and can contribute to the development of diabetic hyperfiltration. When inhibiting proximal reabsorption with a carbonic anhydrase inhibitor, GFR decreases and proximal tubular pressure increases. Measuring intratubular pressure allows a sufficient time resolution to reveal that net filtration pressure decreases before TGF is activated which highlights the importance of intratubular pressure as a regulator of GFR. Taken together, these results imply that the reduced intratubular pressure observed in diabetes might be crucial for the development of glomerular hyperfiltration.

Kidney is in trouble with mediators

The kidneys receive 20-25 % of cardiac output and play a main role in the control of cardiovascular homeostasis. It is an endocrine organ that regulates and produces many substances, scavenger particles and immune complexes. Cytokines, growth factors, reactive oxygen metabolites, bioactive lipids, proteases, vasoactive substances such as nitric oxide (NO), adrenomedullin (AM), urotensin-II (U-II), have been released in several diseases, and kidney is one of mostly affected organs in body. Some of these mediators act in a paracrine fashion while some act in autocrine. They play important roles in modulating the cardiovascular responses, renal hemodynamics, and probably in mediating the clinical and laboratory manifestations of several renal diseases. These mediators are like "a double edged sword". While small amounts of them mediate many physiological events, little excess may cause the damage to the healthy cells. Many investigators have searched the role(s) of mediators in several diseases. However, the findings are mostly like the model of "chicken and egg", and indistinguishable as to whether they are the causes of, or results of the diseases. We will discuss mainly the possible roles of NO, AM and U-II in children with several renal diseases and summarize what is known, and what must be known about these mediators.

Salt-sensitive splice variant of nNOS expressed in the macula densa cells

Neuronal nitric oxide synthase (nNOS), which is abundantly expressed in the macula densa cells, attenuates tubuloglomerular feedback (TGF). We hypothesize that splice variants of nNOS are expressed in the macula densa, and nNOS-beta is a salt-sensitive isoform that modulates TGF. Sprague-Dawley rats received a low-, normal-, or high-salt diet for 10 days and levels of the nNOS-alpha, nNOS-beta, and nNOS-gamma were measured in the macula densa cells isolated with laser capture microdissection. Three splice variants of nNOS, alpha-, beta-, and gamma-mRNAs, were detected in the macula densa cells. After 10 days of high-salt intake, nNOS-alpha decreased markedly, whereas nNOS-beta increased two- to threefold in the macula densa measured with real-time PCR and in the renal cortex measured with Western blot. NO production in the macula densa was measured in the perfused thick ascending limb with an intact macula densa plaque with a fluorescent dye DAF-FM. When the tubular perfusate was switched from 10 to 80 mM NaCl, a maneuver to induce TGF, NO production by the macula densa was increased by 38 +/- 3% in normal-salt rats and 52 +/- 6% (P < 0.05) in the high-salt group. We found 1) macula densa cells express nNOS-alpha, nNOS-beta, and nNOS-gamma, 2) a high-salt diet enhances nNOS-beta, and 3) TGF-induced NO generation from macula densa is enhanced in high-salt diet possibly from nNOS-beta. In conclusion, we found that the splice variants of nNOS expressed in macula densa cells were alpha-, beta-, and gamma-isoforms and propose that enhanced level of nNOS-beta during high-salt intake may contribute to macula densa NO production and help attenuate TGF.

Nitric oxide synthase gene polymorphisms in children with minimal change nephrotic syndrome

AIMS

Nitric oxide (NO) attenuates many functions within the kidney, and all NO synthase (NOS) isoforms are constitutively expressed in the kidney. But the exact role of NO in renal diseases is still debatable. The aim of the present study was to investigate endothelial (eNOS), and neuronal (nNOS) NOS gene polymorphisms in children with minimal change nephrotic syndrome (MCNS).

MATERIALS AND METHODS

Eighty-six Turkish children with clinical MCNS, ranging in age from 2 to 10 years, were compared with 114 healthy age- and sex-matched controls. The glu 298 Asp (G/T) polymorphism of the eNOS, and C276T (C/T) polymorphism of nNOS genes were genotyped using polymerase chain reaction.

RESULTS

The distribution of GG, TG, and TT genotypes for eNOS was 52%, 33% and 15% in MCNS compared with 61%, 26% and 13% in the controls (P > 0.05). The distribution of CC, TC, and TT genotypes for nNOS was 16%, 66% and 18% in MCNS compared with 10%, 43% and 47% in the controls. TT genotype distribution of nNOS was found to be lower in patients (P = 0.003). The eNOS and nNOS gene polymorphisms were not associated with gender, positive family history, frequency of relapses, or response to steroid.

CONCLUSIONS

The present study is the first to investigate eNOS and nNOS gene polymorphisms in children with MCNS. The nNOS gene polymorphism may be associated with MCNS in children, but further studies in a larger population with different glomerular diseases are needed to confirm the results.

The protective effects of green tea extract against L-arginine toxicity to cultured human mesangial cells

The aim of this study was to investigate whether green tea extract (GTE) has the protective effects on excess L-arginine induced toxicity in human mesangial cell. Human mesangial cells treated with L-arginine were cultured on Dulbecco's modified eagle medium in the presence and absence of inducible nitric oxide synthase (iNOS) inhibitor and GTE. The cell proliferation was determined by 3 (4,5-dimethylthiazol-2-yl)-2, 5-diphengltetrqzolium bromide, a tetrazole assay. The iNOS mRNA and its protein expression were detected by reverse transcription polymerase chain reaction and Western blot, respectively. The concentration of nitric oxide (NO) was measured by NO enzyme-linced immuno sorbent assay kit. L-arginine significantly inhibited the proliferation of human mesangial cells, and induced the secretion of NO to the media. NO production by L-arginine was significantly suppressed by GTE and iNOS inhibitor (p<0.01). The expression level of iNOS mRNA and its protein that was significantly increased by L-arginine was decreased by iNOS inhibitor but not by GTE. GTE protected the mesangial cells from the NO-mediated cytotoxicity by scavenging the NO rather than by iNOS gene expression. Therefore, we conclude that GTE has some protective effect for renal cells against oxidative injury possibly by polyphenols contained in GTE.

Nitric oxide bioavailability and its potential relevance to the variation in susceptibility to the renal and vascular complications in patients with type 2 diabetes

OBJECTIVE

We compared the renal and systemic vascular (renovascular) response to a reduction of bioavailable nitric oxide (NO) in type 2 diabetic patients without nephropathy and of African and Caucasian heritage.

RESEARCH DESIGN AND METHODS

Under euglycemic conditions, renal blood flow was determined by a constant infusion of paraminohippurate and changes in blood pressure and renal vascular resistance estimated before and after an infusion of L-Ng-monomethyl-L-arginine.

RESULTS

In the African-heritage group, there was a significant fall in renal blood flow (Delta-46.0 ml/min per 1.73 m(2); P < 0.05) and rise in systolic blood pressure (Delta 10.0 mmHg [95% CI 2.3-17.9]; P = 0.017), which correlated with an increase in renal vascular resistance (r(2) = 0.77; P = 0.004).

CONCLUSIONS

The renal vasoconstrictive response associated with NO synthase inhibition in this study may be of relevance to the observed vulnerability to renal injury in patients of African heritage.

Redox control of renal function and hypertension

Loss of redox homeostasis and formation of excessive free radicals play an important role in the pathogenesis of kidney disease and hypertension. Free radicals such as reactive oxygen species (ROS) are necessary in physiologic processes. However, loss of redox homeostasis contributes to proinflammatory and profibrotic pathways in the kidney, which in turn lead to reduced vascular compliance and proteinuria. The kidney is susceptible to the influence of various extracellular and intracellular cues, including the renin-angiotensin-aldosterone system (RAAS), hyperglycemia, lipid peroxidation, inflammatory cytokines, and growth factors. Redox control of kidney function is a dynamic process with reversible pro- and anti-free radical processes. The imbalance of redox homeostasis within the kidney is integral in hypertension and the progression of kidney disease. An emerging paradigm exists for renal redox contribution to hypertension.

Possible mediators of connecting tubule glomerular feedback

In the renal cortex, the connecting tubule (CNT) returns to the glomerular hilum and contacts the afferent arteriole (Af-Art). Increasing Na delivery to the CNT dilates the Af-Art by activating epithelial Na channels, a process that we call connecting tubule glomerular feedback (CTGF). However, the mediator(s) of CTGF are unknown. We tested the hypothesis that Na reabsorption by the CNT induces release of arachidonic acid metabolites that diffuse to and dilate the Af-Art. Microdissected rabbit Af-Arts and adherent CNTs were simultaneously microperfused. CTGF was measured as the increase in diameter of norepinephrine-preconstricted Af-Arts in response to switching NaCl concentration in the lumen of the CNT from 10 to 80 mmol/L. Under control conditions, CTGF was repeatable and completely reversed norepinephrine-induced vasoconstriction. In the presence of 5,8,11,14-eicosatetraynoic acid, an inhibitor of arachidonic acid metabolism, CTGF was completely blocked (-0.7+/-0.3 versus 7.3+/-0.5 microm), suggesting that arachidonic acid metabolites mediate CTGF. Because both cyclooxygenase-derived prostaglandins and epoxygenase-derived epoxyeicosatrienoic acids are known vasodilatory arachidonic acid metabolites, we tested whether indomethacin or MS-PPOH (a cyclooxygenase and an epoxygenase inhibitor) could block CTGF. Both indomethacin and MS-PPOH partially blocked CTGF (2.3+/-0.8 versus 6.5+/-0.5 microm, and 2.9+/-0.8 versus 6.6+/-1.1 microm, respectively). When combined, they completely blocked CTGF (-0.4+/-0.3 versus 6.6+/-1.1 microm). We confirmed these findings by using the epoxyeicosatrienoic acid antagonist 14,15-EEZE. The combination of indomethacin plus 14,15-EEZE completely abolished CTGF (-0.3+/-0.2 versus 8.0+/-1.0 microm). We conclude that increasing Na concentrations in the CNT stimulate release of prostaglandins and epoxyeicosatrienoic acids, which mediate CTGF.

Heme oxygenase metabolites inhibit tubuloglomerular feedback (TGF)

Tubuloglomerular feedback (TGF) is the mechanism by which the macula densa (MD) senses increases in luminal NaCl concentration and sends a signal to constrict the afferent arteriole (Af-Art). The kidney expresses constitutively heme oxygenase-2 (HO-2) and low levels of HO-1. HOs release carbon monoxide (CO), biliverdin, and free iron. We hypothesized that renal HOs inhibit TGF via release of CO and biliverdin. Rabbit Af-Arts and attached MD were simultaneously microperfused in vitro. The TGF response was determined by measuring Af-Art diameter before and after increasing NaCl in the MD perfusate. When HO activity was inhibited by adding stannous mesoporphyrin (SnMP) to the MD perfusate, the TGF response increased from 2.1+/-0.2 to 4.1+/-0.4 microm (P=0.003, control vs. SnMP, n=7). When a CO-releasing molecule, (CORM-3; 50 microM), was added to the MD perfusate, the TGF response decreased by 41%, from 3.6+/-0.3 to 2.1+/-0.2 microm (P<0.001, control vs. CORM-3, n=12). When CORM-3 at 100 microM was added to the perfusate, it completely blocked the TGF response, from 4.2+/-0.4 to -0.2+/-0.3 microm (P<0.001, control vs. CORM-3, n=6). When biliverdin was added to the perfusate, the TGF response decreased by 79%, from 3.4+/-0.3 to 0.7+/-0.4 microm (P=0.001, control vs. biliverdin, n=6). The effects of SnMP and CORM-3 were not blocked by inhibition of nitric oxide synthase. We concluded that renal HO inhibits TGF probably via release of CO and biliverdin. HO regulation of TGF is a novel mechanism that could lead to a better understanding of the control of renal microcirculation and function.

Adenosine and kidney function: potential implications in patients with heart failure

Therapy of heart failure is more difficult when renal function is impaired. Here, we outline the effects on kidney function of the autacoid, adenosine, which forms the basis for adenosine A(1) receptor (A(1)R) antagonists as treatment for decompensated heart failure. A(1)R antagonists induce a eukaliuretic natriuresis and diuresis by blocking A(1)R-mediated NaCl reabsorption in the proximal tubule and the collecting duct. Normally, suppressing proximal reabsorption will lower glomerular filtration rate (GFR) through the tubuloglomerular feedback mechanism (TGF). But the TGF response, itself, is mediated by A(1)R in the preglomerular arteriole, so blocking A(1)R allows natriuresis to proceed while GFR remains constant or increases. The influence of A(1)R over vascular resistance in the kidney is augmented by angiotensin II while A(1)R activation directly suppresses renin secretion. These interactions could modulate the overall impact of A(1)R blockade on kidney function in patients taking angiotensin II blockers. A(1)R blockers may increase the energy utilized for transport in the semi-hypoxic medullary thick ascending limb, an effect that could be prevented with loop diuretics. Finally, while the vasodilatory effect of A(1)R blockade could protect against renal ischaemia, A(1)R blockade may act on non-resident cells to exacerbate reperfusion injury, where ischaemia to occur. Despite these uncertainties, the available data on A(1)R antagonist therapy in patients with decompensated heart failure are promising and warrant confirmation in further studies.

Crosstalk between the connecting tubule and the afferent arteriole regulates renal microcirculation

The renal afferent arterioles (Af-Arts) account for most of the renal vascular resistance, which is controlled similar to other arterioles and by tubuloglomerular feedback (TGF). The latter signal is generated by sensing sodium chloride concentrations in the macula densa; this in turn results in a signal which acts through the extraglomerular mesangium leading to constriction of the Af-Art. In the outer renal cortex, the connecting tubule (CNT) returns to the glomerular hilus and contacts the Af-Art suggesting that crosstalk may exist here as well. To investigate this, we simultaneously perfused a microdissected Af-Art and adherent CNT. Increasing the sodium chloride concentration perfusing the CNT significantly dilated preconstricted Af-Arts. We called this crosstalk 'connecting tubule glomerular feedback' (CTGF) to differentiate it from TGF. We tested whether entry of Na(+) and/or CI(-) into the CNT is required to induce CTGF by replacing Na(+) with choline(+). Increasing choline chloride concentration did not dilate the Af-Art. To test whether epithelial Na channels (ENaCs) mediate CTGF, we blocked ENaC with amiloride and found that the dilatation induced by CTGF was completely blocked. Inhibiting sodium chloride cotransporters with hydrochlorothiazide failed to prevent Af-Art dilatation. Finally, we tested whether nitric oxide released by the CNT mediates CTGF by the addition of a non-selective nitric oxide synthase inhibitor to the CNT. This potentiated CTGF rather than blocking it. We suggest that crosstalk exists between CNTs and attached Af-Arts, which is initiated by sodium reabsorption through amiloride-sensitive channels and this can contribute to the regulation of renal blood flow and glomerular filtration.

Possible mechanism of efferent arteriole (Ef-Art) tubuloglomerular feedback

Adenosine triphosphate (ATP) is liberated from macula densa cells in response to increased tubular NaCl delivery. However, it is not known whether ATP from the macula densa is broken down to adenosine, or whether this adenosine mediates efferent arteriole (Ef-Art) tubuloglomerular feedback (TGF). We hypothesized that increased macula densa Ca(2+), release of ATP and degradation of ATP to adenosine are necessary for Ef-Art TGF. Rabbit Ef-Arts and adherent tubular segments (with the macula densa) were simultaneously microperfused in vitro while changing the NaCl concentration at the macula densa. The Ef-Art was perfused orthograde through the end of the afferent arteriole (Af-Art). In Ef-Arts preconstricted with norepinephrine (NE), increasing NaCl concentration from 10 to 80 mM at the macula densa dilated Ef-Arts from 7.5+/-0.7 to 11.1+/-0.3 microm. Buffering increases in macula densa Ca(2+) with the cell-permeant Ca(2+) chelator BAPTA-AM diminished Ef-Art TGF from 3.1+/-0.3 to 0.1+/-0.2 microm. Blocking adenosine formation by adding alpha-beta-methyleneadenosine 5'-diphosphate (MADP) blocked Ef-Art TGF from 2.9+/-0.5 to 0.1+/-0.2 microm. Increasing luminal NaCl at the macula densa from 10 to 45 mM caused a moderate Ef-Art TGF response, 1.3+/-0.1 microm. It was potentiated to 4.0+/-0.3 microm by adding hexokinase, which enhances conversion of ATP into adenosine. Our data show that in vitro changes in macula densa Ca(2+) and ATP release are necessary for Ef-Art TGF. ATP is broken down to form adenosine, which mediates signal transmission of Ef-Art TGF.

Ca2+ channel subtypes and pharmacology in the kidney

A large body of evidence has accrued indicating that voltage-gated Ca(2+) channel subtypes, including L-, T-, N-, and P/Q-type, are present within renal vascular and tubular tissues, and the blockade of these Ca(2+) channels produces diverse actions on renal microcirculation. Because nifedipine acts exclusively on L-type Ca(2+) channels, the observation that nifedipine predominantly dilates afferent arterioles implicates intrarenal heterogeneity in the distribution of L-type Ca(2+) channels and suggests that it potentially causes glomerular hypertension. In contrast, recently developed Ca(2+) channel blockers (CCBs), including mibefradil and efonidipine, exert blocking action on L-type and T-type Ca(2+) channels and elicit vasodilation of afferent and efferent arterioles, which suggests the presence of T-type Ca(2+) channels in both arterioles and the distinct impact on intraglomerular pressure. Recently, aldosterone has been established as an aggravating factor in kidney disease, and T-type Ca(2+) channels mediate aldosterone release as well as its effect on renal efferent arteriolar tone. Furthermore, T-type CCBs are reported to exert inhibitory action on inflammatory process and renin secretion. Similarly, N-type Ca(2+) channels are present in nerve terminals, and the inhibition of neurotransmitter release by N-type CCBs (eg, cilnidipine) elicits dilation of afferent and efferent arterioles and reduces glomerular pressure. Collectively, the kidney is endowed with a variety of Ca(2+) channel subtypes, and the inhibition of these channels by their specific CCBs leads to variable impact on renal microcirculation. Furthermore, multifaceted activity of CCBs on T- and N-type Ca(2+) channels may offer additive benefits through nonhemodynamic mechanisms in the progression of chronic kidney disease.

Assessment of renal autoregulation

The kidney displays highly efficient autoregulation so that under steady-state conditions renal blood flow (RBF) is independent of blood pressure over a wide range of pressure. Autoregulation occurs in the preglomerular microcirculation and is mediated by two, perhaps three, mechanisms. The faster myogenic mechanism and the slower tubuloglomerular feedback contribute both directly and interactively to autoregulation of RBF and of glomerular capillary pressure. Multiple experiments have been used to study autoregulation and can be considered as variants of two basic designs. The first measures RBF after multiple stepwise changes in renal perfusion pressure to assess how a biological condition or experimental maneuver affects the overall pressure-flow relationship. The second uses time-series analysis to better understand the operation of multiple controllers operating in parallel on the same vascular smooth muscle. There are conceptual and experimental limitations to all current experimental designs so that no one design adequately describes autoregulation. In particular, it is clear that the efficiency of autoregulation varies with time and that most current techniques do not adequately address this issue. Also, the time-varying and nonadditive interaction between the myogenic mechanism and tubuloglomerular feedback underscores the difficulty of dissecting their contributions to autoregulation. We consider the modulation of autoregulation by nitric oxide and use it to illustrate the necessity for multiple experimental designs, often applied iteratively.

Angiotensin II–nitric oxide interaction in the kidney

PURPOSE OF REVIEW

The balance of angiotensin II and nitric oxide determines the sensitivity of the tubuloglomerular feedback mechanism, renal vascular resistance and filtration rate. Angiotensin II induces nitric oxide release, but the role of angiotensin II receptors here is not fully understood. Further, the angiotensin II-nitric oxide interaction can be modulated by reactive oxygen species. This review focuses on the angiotensin II-nitric oxide interaction and their modulation by reactive oxygen species in the control of renal blood flow.

RECENT FINDINGS

Ideas about the role of angiotensin II type 1 and angiotensin II type 2 receptors are extended by the observation of angiotensin II type 1-mediated nitric oxide release with direct effects on vascular tone, tubuloglomerular feedback and sympathetic neurotransmission. Angiotensin receptors elicit disparate effects on intrarenal circulation. Angiotensin II-nitric oxide interactions are modulated by reactive oxygen species, as shown by angiotensin II type 1-mediated activation of superoxide and depression of antioxidant enzymes leading to reduced nitric oxide concentration - mechanisms that may be also important in angiotensin II-dependent hypertension.

SUMMARY

Recent studies show that angiotensin II stimulates the nitric oxide system via angiotensin II type 1 and angiotensin II type 2 receptors, whereas receptors exert different effects on renal and medullary flow. The interaction via angiotensin II type 1 is modulated by reactive oxygen species.

Mechanisms of renal blood flow autoregulation: dynamics and contributions

Autoregulation of renal blood flow (RBF) is caused by the myogenic response (MR), tubuloglomerular feedback (TGF), and a third regulatory mechanism that is independent of TGF but slower than MR. The underlying cause of the third regulatory mechanism remains unclear; possibilities include ATP, ANG II, or a slow component of MR. Other mechanisms, which, however, exert their action through modulation of MR and TGF are pressure-dependent change of proximal tubular reabsorption, resetting of RBF and TGF, as well as modulating influences of ANG II and nitric oxide (NO). MR requires < 10 s for completion in the kidney and normally follows first-order kinetics without rate-sensitive components. TGF takes 30-60 s and shows spontaneous oscillations at 0.025-0.033 Hz. The third regulatory component requires 30-60 s; changes in proximal tubular reabsorption develop over 5 min and more slowly for up to 30 min, while RBF and TGF resetting stretch out over 20-60 min. Due to these kinetic differences, the relative contribution of the autoregulatory mechanisms determines the amount and spectrum of pressure fluctuations reaching glomerular and postglomerular capillaries and thereby potentially impinge on filtration, reabsorption, medullary perfusion, and hypertensive renal damage. Under resting conditions, MR contributes approximately 50% to overall RBF autoregulation, TGF 35-50%, and the third mechanism < 15%. NO attenuates the strength, speed, and contribution of MR, whereas ANG II does not modify the balance of the autoregulatory mechanisms.

Adenosine and Kidney Function

In this review we outline the unique effects of the autacoid adenosine in the kidney. Adenosine is present in the cytosol of renal cells and in the extracellular space of normoxic kidneys. Extracellular adenosine can derive from cellular adenosine release or extracellular breakdown of ATP, AMP, or cAMP. It is generated at enhanced rates when tubular NaCl reabsorption and thus transport work increase or when hypoxia is induced. Extracellular adenosine acts on adenosine receptor subtypes in the cell membranes to affect vascular and tubular functions. Adenosine lowers glomerular filtration rate (GFR) by constricting afferent arterioles, especially in superficial nephrons, and acts as a mediator of the tubuloglomerular feedback, i.e., a mechanism that coordinates GFR and tubular transport. In contrast, it leads to vasodilation in deep cortex and medulla. Moreover, adenosine tonically inhibits the renal release of renin and stimulates NaCl transport in the cortical proximal tubule but inhibits it in medullary segments including the medullary thick ascending limb. These differential effects of adenosine are subsequently analyzed in a more integrative way in the context of intrarenal metabolic regulation of kidney function, and potential pathophysiological consequences are outlined.

Regulation of ecto-5'-nucleotidase by NaCl and nitric oxide: potential roles in tubuloglomerular feedback and adaptation

The tubuloglomerular feedback (TGF) system serves to establish an appropriate balance between tubular reabsorption and glomerular filtration rate (GFR). High salt at the macula densa activates TGF to decrease GFR. Effector molecules for the TGF signal include ATP and adenosine. Over time, the GFR will adapt by increasing even if a high salt concentration persists. A potential modulator of this TGF adaptation is nitric oxide synthase-1-derived nitric oxide (NO). In isolated glomerular preparations, we developed a system for evaluating the effects of changing dietary salt on ecto-5'-nucleotidase (ecto-5'-NT) activity, the final enzyme in the conversion of ATP to adenosine. We found observable ecto-5'-NT activity in isolated glomeruli and that this activity can be regulated by dietary salt, with high salt increasing activity. Conversely, NO decreases ecto-5'-NT activity in glomerular preparations. Moreover, NO inhibition of ecto-5'-NT activity is suppressed in the presence of dithiothreitol, suggesting nitrosylation as a reversible, oxidative stress-sensitive mechanism. The salt-induced activation of ecto-5'-NT correlates with high salt resetting of TGF. NO inhibition of enzymatic activity could be part of the adaptive phase.

Nitric oxide and superoxide in rat mesangial cells: modulation by C-reactive protein

BACKGROUND

C-reactive protein (CRP) has been linked to cardiovascular and renal disease. We evaluated the effects of CRP on the production of nitric oxide (NO) and superoxide by rat mesangial cells (RMC) and the impact on cell function.

METHODS AND RESULTS

RMC were incubated with cytokines (IFN-gamma, IL-1beta, and LPS) and CRP (10-100 microg/ml) for 24-72 h. Exposure to CRP resulted in a time- and dose-dependent reduction in NO accumulation (p<0.05). Although inducible nitric oxide synthase (iNOS) protein expression was unaltered after 48 h, CRP stimulated expression of HSP90. Steady state abundance of iNOS mRNA increased nearly threefold after a 24-h exposure to CRP. Incubation with 100 microg/ml CRP for 60-120 min resulted in a 272% increase in superoxide production that was prevented by diphenyleneiodium chloride but not L-NAME (p<0.0001).

CONCLUSION

CRP enhances superoxide release in RMC, which in turn inactivates NO and reduces net production. The functional relevance of these CRP-induced changes is supported by increased expression of HSP90 in RMC exposed to the mediator. These findings suggest that systemic inflammation, which contributes to the pathogenesis of atherosclerosis, may play a role in the progression of kidney disease.

Role of macula densa neuronal nitric oxide synthase in renal diseases

Macula densa cells have an important role in the regulation of glomerular blood flow and glomerular filtration by its regulation of afferent arteriolar vascular tone. Nitric oxide derived from neuronal nitric oxide synthase (nNOS) in macula densa can dilate afferent arterioles. Macula densa nNOS is important for renin secretion, and its expression is regulated by dietary salt, renal angiotensin II, intracellular pH, and other factors. In salt-sensitive hypertension, nNOS is suppressed, whereas in SHR or in the early phase of diabetes, nNOS is increased in macula densa along with NADPH oxidase, which limits NO bioavailability. Renal damage induced by hypertension, diabetes, and hyperlipidemia could be prevented by enhancement of nNOS in macula densa with ACEI, dipyridamole, alpha(1)-receptor blocker, a low-salt diet, or sodium bicarbonate. Sodium bicarbonate is a safe and clinically available enhancer of nNOS in macula densa that increases glomerular blood flow and prevents the reduction of GFR in radiocontrast nephropathy and chronic renal failure. In conclusion, the enhancement of nNOS in the macula densa can be a promising strategy to prevent reduction of renal function.

Oxidative stress, lipid profile and liver functions in average Egyptian long term depo medroxy progesterone acetate (DMPA) users

Depo-medroxy progesterone acetate (DMPA, Depo-Provera) is used in more than 80 countries as a long-acting contraceptive administered as a single intramuscular(i.m) injection of 150 mg/3 months. The present study was set up to investigate the effects of DMPA on 80 average Egyptian women classified into four groups comprising those using the drug for one, two, three and four years, respectively, compared to a control group (N = 20) of married non-hormonally - treated women of similar ages. The drug showed a transient significant elevation of alanine aminotransferase activity (ALT)without an apparent effect on other liver indices, namely total bilirubin (T.Bil) level,aspartate aminotransferase (AST) and alkaline phosphatase (ALP) activities. Only the low density/high density lipoproteins cholesterol ratio (LDLC/HDLC) was gradually and non-significantly (ns) increased in comparison to control group, however, neither total cholesterol (TC) nor triglycerides (TG) were affected by the drug. The lipid peroxide product malondialdehyde (MDA) was significantly elevated in an gradual manner with a corresponding decrease in reduced glutathione (GSH), without any change in blood nitric oxide (NO) levels. It can be concluded that DMPA may be considered as a safe contraceptive medication for the studied group of women, but that special care should be exercised for cardiovascular, hepatic and other patients more sensitive to the harmful effects of free radicals. Alternatively, supportive medications are advisable for each exposed case to secure against the possible irreversible adverse effects of the drug by continuous use. In addition, annual re-evaluation is much more advisable despite the proven safety of the drug.