Blockade of pressure natriuresis induced by inhibition of renal synthesis of nitric oxide in dogs

Thick Ascending Limb Sodium Transport in the Pathogenesis of Hypertension

The thick ascending limb plays a key role in maintaining water and electrolyte balance. The importance of this segment in regulating blood pressure is evidenced by the effect of loop diuretics or local genetic defects on this parameter. Hormones and factors produced by thick ascending limbs have both autocrine and paracrine effects, which can extend prohypertensive signaling to other structures of the nephron. In this review, we discuss the role of the thick ascending limb in the development of hypertension, not as a sole participant, but one that works within the rich biological context of the renal medulla. We first provide an overview of the basic physiology of the segment and the anatomical considerations necessary to understand its relationship with other renal structures. We explore the physiopathological changes in thick ascending limbs occurring in both genetic and induced animal models of hypertension. We then discuss the racial differences and genetic defects that affect blood pressure in humans through changes in thick ascending limb transport rates. Throughout the text, we scrutinize methodologies and discuss the limitations of research techniques that, when overlooked, can lead investigators to make erroneous conclusions. Thus, in addition to advancing an understanding of the basic mechanisms of physiology, the ultimate goal of this work is to understand our research tools, to make better use of them, and to contextualize research data. Future advances in renal hypertension research will require not only collection of new experimental data, but also integration of our current knowledge.

Renal Nitric Oxide Deficiency and Chronic Kidney Disease in Young Sheep Born with a Solitary Functioning Kidney

Previously, we demonstrated that renal hemodynamic responses to nitric oxide (NO) inhibition were attenuated in aged, hypertensive sheep born with a solitary functioning kidney (SFK). NO is an important regulator of renal function, particularly, in the postnatal period. We hypothesized that the onset of renal dysfunction and hypertension in individuals with a SFK is associated with NO deficiency early in life. In this study, renal and cardiovascular responses to L-NAME infusion (N^w-nitro-L-arginine methyl ester) were examined in 6-month old lambs born with a SFK, induced by fetal unilateral nephrectomy (uni-x). Renal responses to L-NAME were attenuated in uni-x sheep with the fall in glomerular filtration rate (GFR) and urinary sodium excretion (U_NaV) being less in the uni-x compared to sham lambs (%ΔGFR; −41 ± 3 vs −54 ± 4: P = 0.03, %ΔU_NaV; −48 ± 5 vs −76 ± 3, P = 0.0008). 24 hour-basal urinary nitrate and nitrite (NOx) excretion was less in the uni-x animals compared to the sham (NOx excretion μM/min/kg; sham: 57 ± 7; uni-x: 38 ± 4, P = 0.02). L-NAME treatment reduced urinary NOx to undetectable levels in both groups. A reduction in NO bioavailability in early life may contribute to the initiation of glomerular and tubular dysfunction that promotes development and progression of hypertension in offspring with a congenital nephron deficit, including those with a SFK.

Proximal tubule NHE3 activity is inhibited by beta-arrestin-biased angiotensin II type 1 receptor signaling

Physiological concentrations of angiotensin II (ANG II) upregulate the activity of Na+/H+ exchanger isoform 3 (NHE3) in the renal proximal tubule through activation of the ANG II type I (AT1) receptor/G protein-coupled signaling. This effect is key for maintenance of extracellular fluid volume homeostasis and blood pressure. Recent findings have shown that selective activation of the beta-arrestin-biased AT1 receptor signaling pathway induces diuresis and natriuresis independent of G protein-mediated signaling. This study tested the hypothesis that activation of this AT1 receptor/beta-arrestin signaling inhibits NHE3 activity in proximal tubule. To this end, we determined the effects of the compound TRV120023, which binds to the AT1R, blocks G-protein coupling, and stimulates beta-arrestin signaling on NHE3 function in vivo and in vitro. NHE3 activity was measured in both native proximal tubules, by stationary microperfusion, and in opossum proximal tubule (OKP) cells, by Na+-dependent intracellular pH recovery. We found that 10−7 M TRV120023 remarkably inhibited proximal tubule NHE3 activity both in vivo and in vitro. Additionally, stimulation of NHE3 by ANG II was completely suppressed by TRV120023 both in vivo as well as in vitro. Inhibition of NHE3 activity by TRV120023 was associated with a decrease in NHE3 surface expression in OKP cells and with a redistribution from the body to the base of the microvilli in the rat proximal tubule. These findings indicate that biased signaling of the beta-arrestin pathway through the AT1 receptor inhibits NHE3 activity in the proximal tubule at least in part due to changes in NHE3 subcellular localization.

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.

The effect of cadmium toxicity on renal nitric oxide synthase isoenzymes

The aim of this study was to assess the cadmium (Cd) toxicity on renal nitric oxide synthase (NOS) isoenzymes. The study was carried out on 18 inbred male (Cd group: 10 and control group: 8) Wistar rats. Cd group received drinking water containing 15 mg/L Cd for 30 days; and at the end of the 30 days, plasma Cd was analysed. One kidney was snap frozen to assess the endothelial NOS (eNOS), inducible NOS (iNOS) and neuronal NOS (nNOS) expressions by Western blot analyses, and the other kidney was preserved for histopathological examination. Plasma Cd levels were significantly elevated in the Cd group. The Western blot analyses found higher levels of eNOS, iNOS and nNOS in the Cd group but only eNOS and nNOS levels were statistically significant. There was no difference in pathological assessment of the renal tissues. Cd toxicity increases NOS isoenzyme levels and may affect renal physiology.

Lack of hemodynamic effects after extended heme synthesis inhibition by succinylacetone in rats

Hypertyrosinemia (HT) is a life-threatening condition caused in large part by the buildup of tyrosine metabolites and their derivatives. One such metabolite is succinylacetone (SA), a potent irreversible inhibitor of heme biosynthesis. Heme is a key component of numerous enzymes involved in arterial blood pressure (BP) regulation, including nitric-oxide synthase (NOS) and its downstream mediator soluble guanylyl cyclase (sGC). Because NOS and sGC are important regulators of cardiovascular function, we hypothesized that inhibition of heme supply to these enzymes by SA would result in the induction of a measurable hypertensive response. Male Sprague-Dawley rats were treated with SA (80 mg x kg(-1) x day(-1) i.p.) for 14 days, resulting in a marked increase in urinary SA and delta-aminolevulinic acid (P < 0.001 for both parameters) and decreased heme concentrations in kidney, liver, spleen, and vascular tissues (P < 0.05 for all parameters). After SA treatment, systemic nitrite/nitrate excretion was reduced by 72% (P < 0.001), and renal NOS and sGC activities were decreased by 32 (P < 0.05) and 38% (P < 0.01), respectively. SA administration also compromised the ex vivo sensitivity of aorta to endothelium-dependent and -independent vasodilation. Despite these effects, SA treatment failed to induce any changes in BP, as assessed by radiotelemetry. Moreover, BP profiles in the SA-treated animals were less responsive to altered sodium intake. The present results demonstrate that extended inhibition of heme synthesis with SA affects hemoenzyme function, albeit without consequent effects on BP regulation and sodium excretion.

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.

Acute angiotensin II infusions elicit pressure natriuresis in mice and reduce distal fractional sodium reabsorption

Acute angiotensin II (Ang II) infusions into mice increase arterial pressure (AP) and elicit pressure natriuresis. We used this model of pressure natriuresis to delineate the distal nephron responses to AP-mediated increases in distal sodium delivery. In the first group, we measured changes in urinary sodium excretion (U(Na)V) in male C57/BL6 anesthetized mice (n=9) before and during acute Ang II infusions (5 ng/g of body weight per minute). Acute Ang II infusions increased AP (98+/-3 to 126+/-5 mm Hg; P<0.001), urine flow (2.7+/-0.5 to 6.0+/-0.8 microL/min; P<0.01), and U(Na)V (0.6+/-0.2 to 1.3+/-0.2 microEq/min; P<0.05). There were significant relationships between U(Na)V and urine flow (y=0.207x+0.030; P<0.0001) and between U(Na)V and AP (y=0.027x-2.100). In a separate series, distal sodium delivery and fractional reabsorption of distal sodium delivery were determined in control (n=12) and Ang II-infused mice (n=8) by comparing U(Na)V before and after blockade of the 2 major distal nephron sodium transporters with amiloride (5 mg/kg of body weight) plus bendroflumethiazide (12 mg/kg of body weight). A positive relationship was found between U(Na)V (y=0.015x-1.100; P<0.0001) or distal sodium delivery (y=0.027x-0.900; P<0.0001) and AP. An inverse relationship was found between fractional reabsorption of distal sodium delivery and AP (y=-0.511x+128.300; P<0.01). These data indicate that Ang II-mediated pressure natriuresis involves an increase in distal sodium delivery combined with a reduced distal nephron fractional sodium reabsorption, suggesting that increased AP prevents the distal nephron transport mechanisms from accommodating the increased distal delivery.

Collecting duct-derived endothelin regulates arterial pressure and Na excretion via nitric oxide

Mice with a collecting duct-specific deletion of endothelin-1 are hypertensive and have impaired Na excretion. Because endothelin-1 activates NO synthase (NOS) in the collecting duct, we hypothesized that impaired renal NO production in knockout mice exacerbates the hypertensive state. Control and knockout mice were treated chronically with N(G)-nitro-l-arginine methyl ester, and blood pressure (BP) and urinary nitrate/nitrite excretion were assessed. On a normal Na diet, knockout systolic BP was 18 mm Hg greater than in controls. N(G)-nitro-l-arginine methyl ester increased BP in control mice by 30 mm Hg and 10 mm Hg in collecting duct-specific deletion of endothelin-1 knockout mice, thereby abolishing the difference in systolic BP between the groups. A high-Na diet increased BP similarly in both groups. Urinary nitrate/nitrite excretion was lower in knockout mice than in controls on normal or high Na intake. In separate experiments, renal perfusion pressure was adjusted in anesthetized mice, and urinary nitrate/nitrite and Na excretion were determined. Similar elevations of BP increased urinary Na and nitrate/nitrite excretion in control mice but to a significantly lesser extent in knockout mice. Isoform-specific NOS activity and expression were determined in renal inner medulla homogenates from control and knockout mice. NOS1 and NOS3 activities were lower in knockout than in control mice given normal or high-Na diets. However, NOS1 or NOS3 protein expressions were similar in both groups on normal or high-Na intake. These data demonstrate that collecting duct-derived endothelin-1 is important in the following: (1) chronic N(G)-nitro-l-arginine methyl ester-induced hypertension; (2) full expression of pressure-dependent changes in sodium excretion; and (3) control of inner medullary NOS1 and NOS3 activity.

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.

Inhibition of Na-K-ATPase in thick ascending limbs by NO depends on O2- and is diminished by a high-salt diet

A high-salt diet enhances nitric oxide (NO)-induced inhibition of transport in the thick ascending limb (THAL). Long exposures to NO inhibit Na-K-ATPase in cultured cells. We hypothesized that NO inhibits THAL Na-K-ATPase after long exposures and a high-salt diet would augment this effect. Rats drank either tap water or 1% NaCl for 7-10 days. Na-K-ATPase activity was assessed by measuring ouabain-sensitive ATP hydrolysis by THAL suspensions. After 2 h, spermine NONOate (SPM; 5 microM) reduced Na-K-ATPase activity from 0.44 +/- 0.03 to 0.30 +/- 0.04 nmol P(i).microg protein(-1).min(-1) in THALs from rats on a normal diet (P < 0.03). Nitroglycerin also reduced Na-K-ATPase activity (P < 0.04). After 20 min, SPM had no effect (change -0.07 +/- 0.05 nmol P(i).microg protein(-1).min(-1)). When rats were fed high salt, SPM did not inhibit Na-K-ATPase after 120 min. To investigate whether ONOO(-) formed by NO reacting with O(2)(-) was involved, we measured O(2)(-) production. THALs from rats on normal and high salt produced 35.8 +/- 0.3 and 23.7 +/- 0.8 nmol O(2)(-).min(-1).mg protein(-1), respectively (P < 0.01). Because O(2)(-) production differed, we studied the effects of the O(2)(-) scavenger tempol. In the presence of 50 microM tempol, SPM did not inhibit Na-K-ATPase after 120 min (0.50 +/- 0.05 vs. 0.52 +/- 0.07 nmol P(i).microg protein(-1).min(-1)). Propyl gallate, another O(2)(-) scavenger, also prevented SPM-induced inhibition of Na-K-ATPase activity. SPM inhibited pump activity in tubules from rats on high salt when O(2)(-) levels were increased with xanthine oxidase and hypoxanthine. We concluded that NO inhibits Na-K-ATPase after long exposures when rats are on a normal diet and this inhibition depends on O(2)(-). NO donors do not inhibit Na-K-ATPase in THALs from rats on high salt due to decreased O(2)(-) production.

Renal interstitial guanosine cyclic 3', 5'-monophosphate mediates pressure-natriuresis via protein kinase G

Pressure-natriuresis is the physiological protective mechanism whereby elevation of blood pressure induces a rapid increase in renal sodium (Na+) excretion. Pressure-natriuresis abnormalities are common to all forms of hypertension. We tested the hypothesis that pressure-natriuresis is mediated by renal interstitial (RI) cGMP and protein kinase G (PKG). We used anesthetized, uninephrectomized Sprague-Dawley rats and a standard pressure-natriuresis model in which bilateral adrenalectomy and renal denervation was done on rats. Renal perfusion pressure (RPP) was adjusted by manipulating clamps above and below the renal artery, and RI cGMP was quantified by microdialysis. RI cGMP increased from 3.1+/-0.5 to 5.5+/-0.4 fmol/min (P<0.05) when RPP was raised from 100 to 140 mm Hg. This increase in RI cGMP was eliminated by RI infusion of soluble guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,2-alpha]quinoxalin-1-one (ODQ). Raising RPP from 100 to 140 mm Hg increased urinary sodium excretion from 0.2+/-0.1 to 0.8+/-0.1 micromol/min, fractional sodium excretion from 0.2+/-0.1% to 0.8+/-0.1%, and fractional lithium excretion from 20.1+/-3.0% to 62.7+/-3.7% (all P<0.05). These responses were eliminated by RI infusion of nitric oxide synthase inhibitor N-nitro-l-arginine methyl ester, ODQ, and PKG inhibitors Rp-8-pCPT-cGMP and Rp-8-Br-cGMP. Increasing RPP from 100 to 140 mm Hg decreased fractional proximal sodium reabsorption without influencing fractional distal Na+ reabsorption or glomerular filtration rate. In conclusion, pressure-natriuresis is mediated by RI cGMP and a PKG signaling pathway in target renal proximal tubule cells.

Endothelial nitric oxide synthase polymorphisms and hypertension

The human endothelial nitric oxide synthase (eNOS) gene is highly polymorphic. Evidence for the involvement of eNOS single nucleotide polymorphisms in the development of essential hypertension is limited, though the eNOS Glu298Asp polymorphism appears to influence the blood pressure response to exercise. This variant also influences endothelial function, with its effects becoming manifest during the adaptive vascular changes of pregnancy. Carriers of eNOS Asp298 may be at risk of developing pre-eclampsia. Molecular studies have indicated that intact eNOS Asp298 has equivalent enzymatic activity to eNOS Glu298, but undergoes selective proteolysis in native cells and tissues such that the steady state level of active eNOS may be reduced in carriers of this allele. Carriers of eNOS Asp298, particularly if exposed to adverse environmental infuences on endothelial function, may be at increased risk of developing atherosclerosis and cerebrovascular disease.

Renal effects of nitric oxide synthase inhibition in conscious water-loaded dogs

The renal effects of the nitric oxide (NO) synthase inhibitor nitro-L-arginine methyl ester (L-NAME) were investigated in conscious dogs undergoing sustained water diuresis and replacement of urinary sodium losses. Experiments were performed with and without additional extracellular volume expansion (isotonic saline, 2% body wt). L-NAME (10 microg. kg(-1). min(-1)) infused during water diuresis decreased urine flow (2.5 +/- 0.2 to 1.5 +/- 0.3 ml/min), free water clearance (1.9 +/- 0.2 to 1.0 +/- 0.2 ml/min), and sodium excretion (4.0 +/- 1.7 to 2.1 +/- 0.6 micromol/min). Arterial blood pressure increased from 112 +/- 2 to 126 +/- 3 mmHg, but creatinine clearance did not measurably change. Plasma endothelin and vasopressin concentrations and plasma renin activity (PRA) were unchanged. Urinary endothelin concentration increased (3.4 +/- 0.8 to 6.2 +/- 1.7 pg/ml), but the excretion rate remained constant. L-Arginine infusion (0.6 mg. kg(-1). min(-1)) along with L-NAME abolished the renal effects but not the blood pressure increase. Volume expansion increased urine flow (2.5 +/- 0.4 to 5.7 +/- 0.5 ml/min) and sodium excretion (3.8 +/- 1.6 to 76.5 +/- 14.5 micromol/min). L-NAME attenuated the renal effects of volume expansion: urine flow increased to 2.8 +/- 0.7 ml/min and sodium excretion to 34 +/- 17 micromol/min. PRA decreased with control volume expansion but not during L-NAME. Urinary endothelin levels were elevated by L-NAME, decreased with volume expansion in all series, but excretion rate remained constant. Infusion of L-arginine partially reversed these effects of L-NAME. The results demonstrate that NO synthase inhibition increases blood pressure and blunts the renal responses to water and saline loading.

Cellular regulation of endothelial nitric oxide synthase

Renal function is highly dependent on endothelium-derived nitric oxide (NO). Several renal disorders have been linked to impaired NO bioavailability. The enzyme that is responsible for the synthesis of NO within the renal endothelium is endothelial NO synthase (eNOS). eNOS-mediated NO generation is a highly regulated cellular event, which is induced by calcium-mobilizing agonists and fluid shear stress. eNOS activity is regulated at the transcriptional level but also by a variety of modifications, such as acylation and phosphorylation, by its cellular localization, and by protein-protein interactions. The present review focuses on the complex regulation of eNOS within the endothelial cell.

Hemodynamic and renal effects of acute and progressive nitric oxide synthesis inhibition in anesthetized dogs

This study evaluated the effects of progressive nitric oxide (NO) inhibition in the regulation of systemic and regional hemodynamics and renal function in anesthetized dogs. The N(G)-nitro-L-arginine methyl ester group (n = 9) received progressive doses of 0.1, 1, 10, and 50 microg. kg(-1). min(-1). Renal (RBF), mesenteric (MBF), iliac (IBF) blood flows, mean arterial pressure (MAP), pulmonary pressures, cardiac output (CO), and systemic and pulmonary vascular resistances were measured. During N(G)-nitro-L-arginine methyl ester infusion, MAP and systemic vascular resistances increased in a dose-dependent manner. Mean pulmonary pressure and pulmonary vascular resistances increased in both the N(G)-nitro-L-arginine methyl ester and the control group, but the increase was more marked in the N(G)-nitro-L-arginine methyl ester group during the last two infusion periods. CO decreased progressively, before any significant change in blood pressure was noticeable in the N(G)-nitro-L-arginine methyl ester group. IBF decreased significantly from the first N(G)-nitro-L-arginine methyl ester dose, whereas RBF and MBF only decreased significantly during the highest N(G)-nitro-L-arginine methyl ester dose. Urinary volume and sodium excretion only increased significantly in the time control group during the two last time periods. The pulmonary vasculature was more sensitive than the systemic vasculature, whereas skeletal muscle and renal vasculatures showed a greater sensitivity to the inhibition of NO production than the mesenteric vasculature. NO synthesis inhibition induces a progressive antidiuretic and antinatriuretic effect, which is partially offset by the increase in blood pressure.

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

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

Endothelial function in hypertension: the role of superoxide anion

Much attention has been focused on the role of nitric oxide in hypertension and cardiovascular disease. More recently, the role of superoxide anion and its interaction with nitric oxide has been investigated in this context. This review will concentrate on the role of superoxide in human and experimental hypertension, paying particular attention to the potential sources of superoxide within the vasculature and discussing some of the molecular mechanisms surrounding its production and dismutation. We discuss what is known about the human superoxide dismutase enzymes. We conclude that the balance between nitric oxide and superoxide is more important than the absolute levels of either alone.

Responses to acute changes in arterial pressure on renal medullary nitric oxide activity in dogs

A direct relationship between renal arterial pressure (RAP) and cortical tissue nitric oxide (NO) activity in the canine kidney was reported earlier. We have conducted further experiments to evaluate medullary NO responses to alterations in RAP with the use of a NO-selective microelectrode inserted into the renal medulla of 6 anesthetized, sodium-replete dogs. Graded reductions in RAP (from 140 to 80 mm Hg) elicited decreases in medullary tissue NO concentration, with a slope of 10.2+/-4.5 nmol x L(-1) x mm Hg(-1). These changes in NO levels were associated with decreases in urinary excretion rate of nitrate and nitrite (U(NOx)V; control value, 1.7+/-0.03 nmol x min(-1) x g(-1); slope, 0.02+/-0.004 nmol x min(-1) x g(-1) x mm Hg(-1)) and sodium excretion (U(Na)V; control, 3.2+/-0.7 micromol x min(-1) x g(-1); slope, 0.06+/-0.02 micromol x min(-1) x g(-1) x mm Hg(-1)) without changes in glomerular filtration rate control (0.84+/-0.06 mL x min(-1) x g(-1)). Intra-arterial administration of the NO synthase inhibitor N(omega)-nitro-L-arginine (NLA; 50 microg x kg(-1) x min(-1)) decreased medullary NO concentration by 218+/-55 nmol x L(-1) (n=5) and attenuated the relationship between RAP and NO concentration (slope, 2.7+/-2.2 nmol x L(-1) x mm Hg(-1)). NLA infusion decreased U(NOx)V (0.8+/-0.06 nmol x min(-1) x g(-1)) and U(Na)V (1.1+/-0.2 micromol x min(-1) x g(-1)) without changes in glomerular filtration rate and attenuated RAP versus U(Nox)V and U(Na)V relationships. Total and regional blood flows, as measured by electromagnetic and laser Doppler needle flow probes, respectively, remained autoregulated both before and during NLA infusion. These data support the hypothesis that acute changes in RAP elicit changes in intrarenal NO production, which may participate in the mediation of pressure natriuresis.

Endothelin mediates renal vascular memory of a transient rise in perfusion pressure due to NOS inhibition

We investigated the renal responses to NO synthase (NOS) inhibition with N-monomethyl-L-arginine (L-NMA; 30 mg/kg) in anesthetized rats in which renal perfusion pressure (RPP) to the left kidney was mechanically adjusted. Acute L-NMA increased blood pressure (BP, approximately 20%) and renal vascular resistance (RVR) rose ( approximately 50%) in the right kidneys that were always exposed to high RPP. In group 1, the left kidney was exposed to a transient increase (5 min) in RPP which was then normalized, and the rise in RVR was similar to the right kidney. In group 2 the left kidney was never exposed to high RPP, and the rise in RVR was attenuated relative to the right kidney. In group 3, rats were pretreated with the endothelin (ET) receptor antagonist Bosentan, immediately before exposure of the left kidney to a transient increase in RPP, and the rise in RVR was also attenuated relative to the right kidney. NOS inhibition resulted in a natriuresis and diuresis in the right kidneys, and approximately 50% of the natriuresis persisted in the left kidney of group 2, in the absence of any rise in RPP. ET antagonism completely prevented the natriuresis and diuresis in response to acute L-NMA in both left and right kidneys. These data suggest that transient exposure to high RPP by NOS inhibition prevents an appropriate vasodilatory response when RPP is lowered, due to the intrarenal action of ET.

Intrarenal nitric oxide activity and pressure natriuresis in anesthetized dogs

Recent studies have indicated that changes in intrarenal nitric oxide (NO) production participate in mediating arterial pressure-induced changes in urinary sodium excretion. Until recently, however, the means to measure changes in intrarenal NO activity in vivo have not been available. For the present study, changes in renal tissue NO activities were assessed directly using an NO-selective microelectrode inserted into the cortical tissue of anesthetized dogs. Control studies demonstrated that the electrode was responsive to intrarenal bolus injections of acetylcholine and to the NO donor S-nitroso-acetylpenicillamine (SNAP). Intrarenal nitro-L-arginine (50 microg x kg(-1) x min(-1)) decreased renal tissue NO concentration by 593+/-127 nmol/L (P<0.05; n=7). Infusions of SNAP (1, 2, and 3 microg x kg(-1) x min(-1) for 25 minutes) in nitro-L-arginine-treated dogs (n=5) resulted in dose-dependent increases in renal tissue NO activity, which showed a positive correlation with changes in urinary excretion rates of NO metabolites, nitrates and nitrites, (r=0.62, P<0.05) and sodium (r=0.78, P<0.01). During graded reductions of renal arterial pressure within the autoregulatory range (144+/-3 to 73+/-2 mm Hg; n=10), there were decreases in tissue NO activity that were positively correlated with changes in renal arterial pressure (r=0.45; P<0.05), urinary nitrate/nitrite excretion (r=0.64, P<0.005), and urinary sodium excretion (r=0.46; P<0.05). These data support the hypothesis that acute changes in renal arterial pressure result in alterations in intrarenal NO activity, which may be responsible for the associated changes in sodium excretion.

Candidate genes in the regulation of Na+ transport by inner medullary collecting duct cells from Dahl rats

Recently, we reported that primary cultures of inner medullary collecting duct cells from Dahl salt-sensitive (S) rats absorb more Na+ than do cells cultured from Dahl salt-resistant (R) rats. To begin to evaluate the molecular basis for this difference, we selected four candidate gene products that on the basis of their physiology and genetics could participate in regulation of Na+ transport by these cells. During 24-hour exposure, inhibitors of the cytochrome P450 enzymes had no effect on Na+ transport by either S or R monolayers. Twenty-four-hour exposure to NG-monomethyl-L-arginine (0.5 mmol/L), a nonspecific inhibitor of NO synthase, also had no effect on Na+ transport by either S or R monolayers. Neither atrial natriuretic peptide 1-28 (100 nmol/L) nor 8-Br-cyclic GMP (100 micromol/L) had any short-term effect on Na+ transport by either S or R monolayers. 18-Hydroxy-11-deoxycorticosterone (100 nmol/L), an adrenocorticoid hormone that is produced in greater amounts in S rats, stimulated Na+ transport by both S and R monolayers via the mineralocorticoid receptor; however, its effect was less potent than aldosterone. Congenic rats in which the R isoform of the 11beta-hydroxylase gene was bred onto the S background had monolayers that transported Na+ at a rate similar to the S rats. These results demonstrate that neither cytochrome P450 genes, NO synthase genes, the atrial natriuretic peptide receptor gene, nor the 11beta-hydroxylase gene is a likely candidate to explain the difference in Na+ transport between S and R inner medullary collecting duct monolayers in primary culture.

Inducible nitric oxide synthase and blood pressure

In the present studies, the influence of inducible nitric oxide synthase (NOS) inhibition with aminoguanidine on renal function and blood pressure was examined in rats. Intravenous aminoguanidine infusion (60 mg x kg-1 x hr-1) for 40 minutes to anesthetized Sprague-Dawley rats (n=7) resulted in no significant changes in mean arterial pressure or renal cortical blood flow, while medullary blood flow was slightly increased. Despite minimal effects on renal blood flow, urine flow was significantly decreased from 14.2+/-2.7 to 10.4+/-2.3 microL x min-1 x g kidney wt-1 during aminoguanidine infusion. To examine the possible effects of inducible NOS on blood pressure, aminoguanidine (10 mg x kg-1 x h-1 IV) was infused chronically into uninephrectomized rats maintained on a high salt (4.0% NaCl) diet. Mean arterial pressure significantly increased from 104+/-2 to 118+/-3 mm Hg after 6 days of aminoguanidine infusion (n=7) and returned to levels not different from those in the control group after 2 days of postcontrol infusion. Calcium-independent NOS activity in the renal medulla, a tissue that expresses inducible NOS in normal rats, was significantly decreased by 49% in the aminoguanidine-infused group (n=6) compared with that activity in the vehicle-infused control animals (n=6). In contrast, calcium-dependent NOS activity in the renal medulla was not significantly altered by aminoguanidine infusion, indicating specificity of aminoguanidine for inducible NOS in these experiments. In a final group of rats (n=5), oral L-arginine administration in drinking water (2% wt/vol) increased plasma arginine levels from 118+/-5 to 232+/-16 micromol/L and blocked the increase in arterial pressure after 6 days of aminoguanidine infusion. The present experiments provide evidence supporting a role for inducible NOS in the control of arterial pressure, possibly by renal tubular effects.

Interactions between nitric oxide and angiotensin II on renal cortical and papillary blood flow

This study examined the role of angiotensin II (Ang II) on the effects of nitric oxide (NO) synthesis blockade on renal cortical and papillary blood flow in innervated and denervated kidneys of volume-expanded Munich-Wistar rats with hormonal influences on the kidney that were held constant by intravenous infusion. Cortical (CBF) and papillary (PBF) blood flow were measured by laser-Doppler flowmetry. A low dose of N omega-nitro-L-arginine methyl ester (L-NAME, 3.7 nmol x kg[-1] x min[-1]) reduced CBF only in innervated kidneys, and this effect was abolished by subsequent administration of valsartan (an AT1 antagonist). L-NAME 3.7 nmol x kg(-1) x min(-1) improved PBF autoregulation by lowering PBF to the range of 100 to 140 mm Hg of perfusion pressure, and this effect was attenuated or abolished by valsartan in innervated and denervated kidneys, respectively. These results indicate that the cortical and medullary vasoconstriction induced by a low dose of L-NAME are caused by potentiation of the vasoconstrictor influence of renal sympathetic nerves and Ang II. A higher dose of L-NAME (37 nmol x kg[-1] x min[-1]) lowered CBF and PBF in both innervated and denervated kidneys. This effect of L-NAME on the cortical circulation was abolished by valsartan, but this AT1 antagonist had no effect on the medullary vasoconstriction produced by NO synthesis blockade. Therefore, a higher dose of L-NAME induces a renal cortical vasoconstriction through potentiation of the renin-angiotensin system, whereas the fall of PBF seen after L-NAME 37 nmol x kg(-1) x min(-1) seems to be caused primarily by NO suppression. This Ang II potentiation produced by L-NAME in the renal cortex seems to be mediated by AT1 receptors, because it was unaffected by PD123319 (an AT2 antagonist). The results of the present study indicate that NO is an important modulator of the vasoconstrictor influence of Ang II in the renal cortical circulation of the rat. However, although there are some interactions between NO and renal nerves and Ang II on the medullary circulation, the renal medullary vasoconstriction produced by L-NAME appears to be caused primarily by NO suppression, with little influence of the renal vasoconstrictor systems.

Pressure natriuresis and renal medullary blood flow in dogs

In the present study, we evaluated the effects of changes in arterial pressure on regional renal blood flows and sodium excretion in anesthetized dogs during control conditions and after 5% volume expansion with isotonic saline. Medullary and cortical blood flow responses were determined with laser-Doppler needle flow probes inserted into the midmedullary and midcortical regions, and whole-kidney blood flow was assessed with an electromagnetic flow probe. Volume expansion in six dogs caused marked increases in urine flow (20.2 +/- 5.5 to 82.5 +/- 22.7 microL.min-1.g-1) and sodium excretion (3.2 +/- 0.5 to 11.1 +/- 2.7 mumol.min-1.g-1), with slight increases in glomerular filtration rate (0.92 +/- 0.03 to 1.01 +/- 0.02 mL.min-1.g-1) but no significant changes in total renal blood flow (4.7 +/- 0.3 to 5.2 +/- 0.6 mL.min-1.g-1), medullary blood flow (+6 +/- 9%), or cortical blood flow (+12 +/- 10%). During stepwise reductions in renal arterial pressure (150 to 75 mm Hg) elicited with a renal arterial occluder, both before and after volume expansion, medullary, cortical, and total renal blood flows as well as glomerular filtration rate exhibited efficient autoregulation, with slopes not significantly different from zero over this range of arterial pressure. Ther were marked increases in the slopes of the relationships between arterial pressure and urine flow (0.18 +/- 0.05 to 0.78 +/- 0.27 microL.min-1.g-1.mm Hg-1) as well as sodium excretion (0.03 +/- 0.004 to 0.10 +/- 0.03 mumol.min-1.g-1.mm Hg-1) during volume expansion. These data demonstrate that medullary blood flow is efficiently autoregulated in dogs during control and volume-expanded states and indicate that the mechanism responsible for the arterial pressure-induced changes in sodium excretion does not depend on coincident alterations in medullary blood flow.

Renal hemodynamic and excretory responses in anesthetized rats to FK409, a novel nitric oxide donor

Renal hemodynamic and excretory responses to (+/-)-(E)-4-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexenamide (FK409), a novel nitric oxide (NO) donor, were examined using anesthetized rats. When FK409 was infused into the renal artery of normal rats at 10 micrograms/kg per min, a moderate renal vasodilating effect was observed with a decrease in mean arterial blood pressure. Urine flow, urinary excretion of sodium and fractional excretion of sodium significantly increased by about 85%, 110% and 75%, respectively, compared with each control value. Simultaneously, urinary excretion of NO metabolites (UNOxV) was markedly increased with the administration of FK409. In hypertensive rats treated with NG-nitro-L-arginine (NOARG), the NO synthase inhibitor, FK409 produced a potent renal vasodilation, although the hypotensive effect of the agent was comparable to that seen in normal rats. In addition, glomerular filtration rate was significantly elevated by the agent. There were marked increases in the excretory responses, i.e., levels of urine flow, urinary excretion of sodium and fractional excretion of sodium were increased to about 3-, 6- and 5-fold of each control value, respectively. The extent of increment of UNOxV was similar to that seen in normal rats. These results clearly indicate that FK409 causes renal vasodilation and diuresis, via NO formation. Renal hemodynamic and excretory responses to the agent are sensitive in NO-depleted conditions. FK409 and related compounds may be useful for the treatment of renal diseases, in cases where the basal NO formation is impaired.

Temporal influence of the renal nerves on renal excretory function during chronic inhibition of nitric oxide synthesis

To determine whether the sympathetic nervous system contributes to the hypertension induced by long-term suppression of nitric oxide synthesis, we determined the neurally induced changes in renal excretory function during chronic administration of NG-nitro-L-arginine methyl ester (L-NAME). Studies were carried out in six conscious chronically instrumented dogs subjected to unilateral renal denervation and surgical division of the urinary bladder into two hemibladders to allow separate 24-hour urine collection from denervated and innervated kidneys. Animals were studied during acute (100 minutes) and chronic (5 days) intravenous infusion of L-NAME at 37.1 nmol/kg per minute (10 micrograms/kg per minute). During the first 100 minutes of L-NAME, there were no significant changes in mean arterial pressure (control: 96 +/- 3 mm Hg), but heart rate fell from 66 +/- 6 to 55 +/- 7 beats per minute. Changes in glomerular filtration rate were not significant, but renal plasma flow and urinary sodium excretion decreased to approximately 75% and 50% of control values, respectively; however, these changes were comparable in both kidneys. In association with these responses, plasma concentrations of norepinephrine (control: 887 +/- 130 pmol/L or 150 +/- 22 pg/mL) and epinephrine (control: 691 +/- 192 pmol/L or 108 +/- 30 pg/mL) tended to decrease. In contrast to the acute responses, mean arterial pressure increased from 92 +/- 3 to 106 +/- 3 mm Hg and heart rate decreased from 72 +/- 4 to 57 +/- 5 beats per minute by day 5 of L-NAME infusion, while renal plasma flow and glomerular filtration rate were not significantly different from control values. Most importantly, there were no significant differences in urinary sodium excretion between innervated (control: 31 +/- 2 mmol/d) and denervated (control 33 +/- 2 mmol/d) kidneys during chronic L-NAME infusion or during the recovery period. These results indicate that the renal sympathetic nerves do not play an important role in promoting sodium retention during either acute or chronic inhibition of nitric oxide synthesis in conscious dogs. Thus, increased renal sympathetic nerve activity does not contribute significantly to L-NAME-induced hypertension.

Pressure natriuresis and autoregulation of inner medullary blood flow in canine kidney

We have evaluated the responses to changes in arterial pressure on regional blood flows in the renal medulla and sodium excretion simultaneously in denervated kidneys of six anesthetized sodium-replete dogs. Renal regional blood flow responses were determined using laser-Doppler needle flow probes and whole-kidney blood flow was assessed using an electromagnetic flow probe. The responses to stepwise reductions in renal arterial pressure (140 to 70 mm Hg) were examined first with a laser-Doppler needle probe inserted in the outer medulla and then repeated after advancing the same probe in the inner medulla. There were no differences in the control values of total renal blood flow (4.4 +/- 0.7 to 4.5 +/- 0.5 mL.min-1.g-1), glomerular filtration rate (0.89 +/- 0.7 to 0.94 +/- 0.9 mL.min-1.g-1), sodium excretion (3.6 +/- 0.6 to 3.4 +/- 0.5 mumol.min-1.g-1), and urinary excretion rate of nitric oxide metabolites (nitrate/nitrite, 1.6 +/- 0.2 to 1.5 +/- 0.2 nmol.min-1.g-1) at the start of both experimental periods. During changes in renal arterial pressure, inner medullary blood flow exhibited efficient autoregulation similar to that in outer medullary blood flow. Usual excretory responses to reductions in renal arterial pressure as well as autoregulation of cortical and whole-kidney blood flows and glomerular filtration rate were observed in these dogs. The slopes of the relationship between arterial pressure and sodium excretion (0.046 +/- 0.007 to 0.044 +/- 0.009 mumol.min-1.g-1.mm Hg-1) or nitrate/nitrite excretion (0.014 +/- 0.003 to 0.013 +/- 0.003 nmol.min-1.g-1.mm Hg-1) were similar in both experimental periods. These data indicate that blood flow to the inner medulla is efficiently autoregulated as in outer medulla and cortex of the kidney in anesthetized dogs and demonstrate further that the arterial pressure-induced natriuretic responses do not require associated changes in medullary blood flow.

Neural nitric oxide synthase in the renal medulla and blood pressure regulation

We studied the effect of selective inhibition of the neural isoform of nitric oxide synthase in the rat renal medulla in conscious Sprague-Dawley rats. Continuous renal medullar interstitial infusion of an antisense oligonucleotide complementary to the initiation region of the mRNA for neural nitric oxide synthase increased blood pressure 14 +/- 1 mm Hg in rats maintained on a high sodium intake. Medullary interstitial infusion of saline vehicle or a scrambled oligonucleotide probe failed to alter blood pressure in separate groups of high salt control rats. Renal medullary interstitial infusion of the antisense oligonucleotide significantly decreased the level of neural nitric oxide synthase in the renal medulla by 53 +/- 8% and decreased total renal medullary nitric oxide synthase activity by 28 +/- 8%. No alterations were detected in the levels of inducible nitric oxide synthase or beta-actin in the antisense oligonucleotide-infused rats. To confirm the antisense oligonucleotide data, we administered a mechanistically different inhibitor of neural nitric oxide synthase, 7-nitroindazole, to an additional group of rats maintained on a high salt diet. Direct renal medullary interstitial infusion of this selective enzyme inhibitor significantly increased mean arterial pressure (15 +/- 6 mm Hg) and decreased total renal medullary nitric oxide synthase activity by 37 +/- 12% in rats on a high sodium diet. The present experiments demonstrate a role for the neural isoform of nitric oxide synthase in the long-term control of blood pressure in the presence of a high salt diet.

Effect of salt intake and inhibitor dose on arterial hypertension and renal injury induced by chronic nitric oxide blockade

Long-term nitric oxide blockade by N omega -nitro-L-arginine methyl ester (L-NAME) leads to severe and progressive hypertension. The role of salt intake in this model is unclear. To verify whether salt dependence in this model is related to the extent of nitric oxide inhibition, we gave adult male Munich-Wistar rats a low salt, standard salt, or high salt diet and oral L-NAME treatment at either 3 or 25 mg/kg per day. At 10 to 15 days of treatment, the slope of the pressure-natriuresis line was decreased in rats receiving low-dose L-NAME compared with untreated controls. In rats treated with the higher dose, the line was shifted to the right but remained parallel to that obtained in untreated controls. Renal vascular resistance was moderately increased in rats receiving low-dose L-NAME, whereas high-dose L-NAME induced a marked vasoconstriction that was aggravated by salt overload. Low-dose L-NAME treatment induced hypertension only when associated with sodium overload. In rats receiving high-dose L-NAME, hypertension was aggravated by sodium excess but was not ameliorated by sodium restriction. Long-term (6 weeks) L-NAME treatment was associated with progressive hypertension, which was aggravated by salt overload, and with the development of albuminuria, focal glomerular collapse, glomerulosclerosis, and renal interstitial expansion. These abnormalities were worsened by salt overload and largely prevented by salt restriction. In the model of chronic nitric oxide blockade, salt dependence is a function of the inhibitor dose, and renal injury varies directly with the level of salt intake.

Influence of dietary sodium intake on renal medullary nitric oxide synthase

We previously reported that chronic systemic treatment of rats with a nitric oxide synthase inhibitor leads to a selective decrease in renal medullary blood flow, retention of sodium, and the development of hypertension. In the present studies, we used protein blotting techniques to determine the whole tissue distribution and relative quantitation of the different nitric oxide synthase isoforms in the renal cortex and medulla of Sprague-Dawley rats maintained on a low (0.4% NaCl) or high (4.0% NaCl) dietary salt intake. Neural, endothelial, and inducible nitric oxide synthase were readily detectable in homogenized renal inner and outer medullas. Only endothelial nitric oxide synthase was detectable in the renal cortex. Densitometric comparison of Western blots from equal amounts of total inner medullary tissue protein indicated that endothelial, inducible, and neural nitric oxide synthase were increased by 145%, 49%, and 119%, respectively, in rats maintained on a high NaCl diet compared with rats on a low NaCl diet. No significant differences in nitric oxide synthase levels were detected in the outer medulla, renal cortex, or aorta of rats maintained on low and high NaCl diets. In separate studies, continuous intravenous infusion of N(G)-nitro-L-arginine methyl ester (8.6 mg/kg per day) for 11 days in chronically instrumented rats increased mean arterial pressure 32 +/- 3 mm Hg in rats on a high NaCl diet (n=5) but only increased pressure 17 +/- 3 mm Hg in rats on a low NaCl diet (n=6). These data indicate that increased levels of renal medullary nitric oxide synthase may be important in the chronic adaptation to increased sodium intake.

Importance of nitric oxide and prostaglandins in the control of rat renal papillary blood flow

The role of nitric oxide and prostaglandins in the control of rat renal papillary blood flow has been studied in anesthetized Munich-Wistar rats by use of laser Doppler flowmeter. Acute administration of N omega-nitro-L-arginine methyl ester (L-NAME) 10 mg/kg IV (n=8) increased mean arterial pressure by 27.8 +/- 3.6%, decreased papillary blood flow by 39.4 +/- 3.8%, and decreased renal blood flow by 47.4 +/- 1.9%. The subsequent administration of indomethacin (7.5 mg/kg IV) further decreased papillary blood flow (35.2 +/- 2.5%) without significant changes in mean arterial pressure or renal blood flow. In a second group (n = 6), administration of indomethacin before L-NAME decreased papillary blood flow by 39.6 +/- 2.1% without significantly altering mean arterial ressure or renal blood flow. The subsequent injection of L-NAME further decreased papillary blood flow (32.9 +/- 1.8%) and renal blood flow (49.8 +/- 6.6%) while increasing mean arterial pressure to a level not significantly different from that found in the first group. Autoregulation studies showed that L-NAME but not indomethacin reduced the renal perfusion pressure-renal blood flow relationship without altering autoregulation. However, both nitric oxide and prostaglandins importantly affected the renal perfusion pressure-papillary blood flow relationship because L-NAME and indomethacin significantly decreased this relationship in an additive fashion. Although both drugs reduced the sensitivity of the pressure-papillary flow relationship, only L-NAME affected autoregulation so that papillary blood flow was autoregulated at higher renal perfusion pressures. Thus, the present results indicate that both nitric oxide and prostaglandins control a similar percentage of rat renal papillary blood flow, but nitric oxide seems to be more important than prostaglandins as a mediator of the pressure-blood flow relationship. In contrast, only nitric oxide modifies the renal blood flow level, although it does not disturb whole-kidney blood flow autoregulation.

Nitric oxide synthase isoform activities in kidney of Dahl salt-sensitive rats

An abnormal L-arginine-nitric oxide axis has been suggested to be relevant to the genesis of salt-sensitive hypertension. In the present study we investigated the activities of three isoforms of nitric oxide synthase (NOS) in the kidney of Dahl salt-sensitive and salt-resistant rats. Five-week-old Dahl Iwai salt-sensitive (n = 9) and salt-resistant (n = 10) rats were maintained on a high salt diet (4% sodium chloride) for 4 weeks. We measured calcium-dependent and calcium-independent NOS activities in each particulate and soluble fraction of kidney by conversion of L-[3H]arginine to L-[3H]citrulline. Systolic blood pressure was elevated significantly (P < .001) in salt-sensitive but not salt-resistant rats. Calcium-dependent NOS activity in the soluble fraction was significantly lower in salt-sensitive rats than in salt-resistant rats (25.8 +/- 9.0 versus 48.2 +/- 19.2 disintegrations per microgram protein, respectively; P < .01). There were no differences in calcium-dependent NOS activity in the particulate fraction and calcium-independent NOS activity in the soluble fraction between groups. Renal norepinephrine content was lower in salt-sensitive rats than in salt-resistant rats (P < .05) and was positively correlated with calcium-dependent NOS activity in the soluble fraction (P < .01). Although no differences in endothelial and inducible-type NOS activity were observed a significant reduction in calcium-dependent NOS activity in the soluble fraction of the kidney of salt-sensitive rats suggests that the decreased neural-type NOS activity may in part be involved in the mechanism of salt-sensitive hypertension, possibly through alterations in renal sympathetic nervous activity and sodium handling.

The renal medulla and hypertension

We review evidence supporting the conclusion that renal dysfunction underlies the development of all forms of hypertension in humans and experimental animals. Indexes of global renal function are generally normal in the early stages of most genetic forms of hypertension, but renal function is clearly impaired in long-established hypertension. Studies in our laboratory over the past decade summarized below have established that the renal medulla plays an important role in sodium and water homeostasis and in the long-term control of arterial pressure. Development of implanted optical fibers for measurement of cortical and medullary blood flows with laser-Doppler flowmetry and techniques for delivery of vasoactive compounds into the medullary interstitial space enabled us to examine determinants of medullary flow (nitric oxide, atrial natriuretic peptides, kinins, eicosanoids, vasopressin, renal sympathetic nerves, etc). We have shown in spontaneously hypertensive rats that the initial changes of renal function begin as a reduction of medullary blood flow in the absence of changes of cortical flow. Long-term medullary interstitial infusion of captopril, which preferentially increased medullary blood flow, resulted in a lowering of arterial pressure. In normal Sprague-Dawley rats, selective reduction of medullary flow with medullary interstitial or intravenous infusion of small amounts of NG-nitro-L-arginine methyl ester resulted in hypertension. These and other studies we review show that although blood flow to the inner renal medulla comprises less than 1% of the total renal blood flow, changes in flow to this region can have a major effect on sodium and water homeostasis and on the long-term control of arterial blood pressure.

Relation between pressure natriuresis and urinary excretion of nitrate/nitrite in anesthetized dogs

Alterations in intrarenal nitric oxide (NO) formation during changes in renal arterial pressure (RAP) have been suggested as a mechanism mediating pressure natriuresis. To test this hypothesis further, we examined the relation between RAP and the urinary excretion rate of nitrate/nitrite (NO3-/NO2-; NO metabolites) in anesthetized sodium-replete dogs before (n = 9) and during (n = 6) intrarenal infusion of the NO synthesis inhibitor nitro-L-arginine (NLA; 50 micrograms.kg-1.min-1). Urinary NO3-/NO2- concentrations were measured with the Griess reaction and spectrophotometry methods after enzymatic reduction of NO3- to NO2- in the samples. During control conditions, there were decreases in the urinary NO3-/NO2- excretion rate in response to reductions in RAP (150 to 75 mm Hg; slope, 0.04 +/- 0.01 nmol.min-1.g-1.mm Hg-1) in association with decreases in urinary sodium excretion (UNaV). There was a positive correlation between changes in NO3-/NO2- excretion rate and changes in RAP (r = .48; P < .005) or UNaV (r = .59; P < .001). NLA infusion resulted in decreases in NO3-/NO2- excretion rate (4.8 +/- 1.4 to 1.0 +/- 0.3 nmol.min-1.g-1) in association with reductions in UNaV (4.3 +/- 0.3 to 0.7 +/- 0.2 microL.min-1.g-1), fractional excretion of sodium (2.9 +/- 0.2% to 0.5 +/- 0.1%), and renal blood flow (4.8 +/- 0.3 to 3.3 +/- 0.2 mL.min-1.g-1), without changes in glomerular filtration rate. Furthermore, there was a marked attenuation of the NO3-/NO2- and sodium excretory responses to alterations in RAP during NO synthesis inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of nitric oxide on papillary blood flow and pressure natriuresis

This study examined whether nitric oxide synthesis blockade or potentiation (with N omega-nitro-L-arginine methyl ester [L-NAME] or N-acetylcysteine, respectively) can shift the relations between sodium excretion, papillary blood flow, and renal perfusion pressure. Papillary blood flow was measured by laser Doppler flowmetry. A low dose of L-NAME (3.7 nmol/kg per minute) reduced papillary blood flow only at high arterial pressure (140 mm Hg), but it had no effect on pressure natriuresis. Infusion of 37 nmol/kg per minute L-NAME reduced cortical blood flow by 9% at all perfusion pressures studied, lowered papillary blood flow by 8% and 19% at 120 and 140 mm Hg, respectively, and blunted the pressure-natriuresis response. The administration of 185 nmol/kg per minute L-NAME reduced cortical blood flow by 30% and decreased papillary blood flow by 25% in the range of 100 to 140 mm Hg of arterial pressure. Blockade of nitric oxide synthesis with L-NAME at all doses studied reduced papillary blood flow only at high renal perfusion pressures, but papillary blood flow remained essentially unchanged at low perfusion pressures, thus restoring papillary blood flow autoregulation. N-Acetyl-cysteine (1.8 mmol/kg) increased papillary blood flow by 9% and shifted the relations between papillary blood flow, sodium excretion, and renal perfusion pressure toward lower pressures. This effect of N-acetylcysteine on papillary blood flow was blocked by subsequent L-NAME administration.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal effects of acute amino acid infusion in hypertension induced by chronic nitric oxide blockade

L-Arginine is the physiological substrate of nitric oxide, a vasodilator that controls blood pressure and renal hemodynamics in the basal state. In the present studies, we produced chronic nitric oxide blockade by oral administration of the L-arginine analogue NG-nitro-L-arginine methyl ester, which produced sustained hypertension and increased renal vascular resistance in conscious rats. Acute excess L-arginine had little effect on blood pressure but completely normalized renal vascular resistance and increased renal plasma flow in chronically nitric oxide-blocked hypertensive rats. In contrast to L-arginine, D-arginine had no renal hemodynamic effects in either normal or chronically nitric oxide-blocked rats. Acutely administered glycine was ineffective in vasodilating the chronically nitric oxide-blocked rat kidney, in a dose that produced renal vasodilation in normal rats. These findings indicate the following: (1) Hypertension induced by chronic nitric oxide blockade due to substituted L-arginine analogue cannot be acutely reversed with excess L-arginine, suggesting that the maintenance of the hypertension is not solely caused by competitive inhibition of nitric oxide production; (2) in contrast, the kidney remains responsive to L-arginine whereas the renal vasodilator response to glycine is abolished in this model of hypertension.

Mechanism of vasoconstriction induced by chronic inhibition of nitric oxide in rats

Either acute or chronic inhibition of nitric oxide synthesis by L-arginine analogues results in increases in mean arterial pressure and reductions in renal blood flow. The role of endogenous vasoconstrictors in mediating these effects is not entirely clear. In the present study, nitric oxide was inhibited in male Sprague-Dawley rats by oral administration of nitro-L-arginine for 3 weeks. At the end of this time, mean arterial pressure was 30 to 40 mm Hg higher than in normal controls, renal blood flow and glomerular filtration rate were 25% to 30% lower, and renal vascular resistance was markedly increased. Intravenous infusion of receptor antagonists for angiotensin II, thromboxane, epinephrine, and endothelin-1 had no significant effect on the hypertension. Inhibition of prostaglandin synthesis and furosemide-induced diuresis in the presence of angiotensin blockade also had no effect on blood pressure. Renal vascular resistance was also unaffected by these interventions, except that saralasin did reduce renal resistance in both control and nitric oxide-inhibited groups. However, the absolute level of renal vascular resistance remained higher in the latter group. Calcium channel blockade partially corrected blood pressure and renal resistance, but the levels remained significantly higher than in control animals. The findings are consistent with the view that the increase in vascular smooth muscle tone caused by inhibition of nitric oxide synthesis cannot be accounted for by overexpression of common endogenous vasoconstrictors. Rather, the generalized increase in vascular smooth muscle tone appears to be due to a direct effect of reduced nitric oxide availability, which may lead to an increase in intracellular calcium concentration or sensitivity.

Hypertension induced by nitric oxide synthesis inhibition is renal nerve dependent

Recent studies have indicated that chronic administration of N omega-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide (NO) synthesis, produces marked hypertension. Although the mechanism of this form of hypertension is not well understood, several studies have demonstrated that sympathetic nerve activity is at least acutely elevated after L-NAME administration. To evaluate the potential role of the renal sympathetic nerves in L-NAME-induced hypertension, we compared the blood pressure response to L-NAME in four groups of Sprague-Dawley rats (n = 8 each): (1) sham-operated vehicle-treated, (2) sham-operated L-NAME-treated, (3) denervated vehicle-treated, and (4) denervated L-NAME-treated. After renal denervation or sham surgery, L-NAME was added to the drinking water (70 mg/100 mL) for 4 weeks, and arterial pressure was measured weekly by the tail-cuff method. L-NAME treatment caused a progressive increase in arterial pressure in sham-operated rats, rising to 154 +/- 6 mm Hg by week 4 of treatment compared with 115 +/- 2 mm Hg in the vehicle-treated sham-operated group (P < .005). In contrast, the development of hypertension was significantly delayed and attenuated in renal-denervated rats treated with L-NAME. The results of our study suggest that L-NAME-induced hypertension may be partly mediated by or is at least dependent on the integrity of the renal nerves.

Renal responses to intra-arterial administration of nitric oxide donor in dogs

Inhibition of nitric oxide synthesis by intra-arterial administration of nitro-L-arginine (NLA) leads to attenuation of the slope of the relation between renal arterial pressure (RAP) and sodium excretion without an alteration in renal autoregulatory efficiency. In the present study, we examined whether only the presence of nitric oxide or, alternatively, changes in nitric oxide production during changes in RAP are required for pressure natriuresis to occur. Anesthetized sodium-replete dogs (n = 8) were treated with NLA (50 micrograms.kg-1 x min-1) to inhibit endogenous nitric oxide formation, and S-nitroso-n-acetylpenicillamine (SNAP) was infused intra-arterially at a constant rate (2 micrograms.kg-1 x min-1) to replenish intrarenal nitric oxide levels. Renal responses to reductions in RAP within the autoregulatory range were assessed before and during NLA infusion followed by SNAP+NLA infusion. As reported previously, NLA infusion alone increased renal vascular resistance and decreased renal blood flow, urine flow, sodium excretion, and fractional excretion of sodium, with no change in glomerular filtration rate. Autoregulatory efficiency remained intact, whereas the pressure-induced natriuretic responses were attenuated. During SNAP+NLA infusion, renal blood flow increased from 2.8 +/- 0.3 to 3.5 +/- 0.3 mL.min-1 x g-1 (P < .001), without significant changes in glomerular filtration rate (0.75 +/- 0.07 to 0.81 +/- 0.05 mL.min-1 x g-1); the autoregulatory efficiency of renal blood flow and glomerular filtration rate remained intact. SNAP increased urine flow (4.8 +/- 1.8 to 10.0 +/- 2.5 microL.min-1 x g-1), sodium excretion (0.63 +/- 0.26 to 1.70 +/- 0.37 mumol.min-1 x g-1), and fractional excretion of sodium (0.55 +/- 0.20% to 1.38 +/- 0.27%).(ABSTRACT TRUNCATED AT 250 WORDS)