Effects of NG-monomethyl-L-arginine and L-arginine on acetylcholine renal response

Nitric oxide inhibits basolateral 10-pS Cl- channels through the cGMP/PKG signaling pathway in the thick ascending limb of C57BL/6 mice

We used patch-clamp techniques to examine whether nitric oxide (NO) decreases NaCl reabsorption by suppressing basolateral 10-pS Cl^- channels in the thick ascending limb (TAL). Both the NO synthase substrate l-arginine (l-Arg) and the NO donor S-nitroso-N-acetylpenicillamine significantly inhibited 10-pS Cl^- channel activity in the TAL. The inhibitory effect of l-Arg on Cl^- channels was completely abolished in the presence of the NO synthase inhibitor or NO scavenger. Moreover, inhibition of soluble guanylyl cyclase abrogated the effect of l-Arg on Cl^- channels, whereas the cGMP analog 8-bromo-cGMP (8-BrcGMP) mimicked the effect of l-Arg and significantly decreased 10-pS Cl^- channel activity, indicating that NO inhibits basolateral Cl^- channels by increasing cGMP production. Furthermore, treatment of the TAL with a PKG inhibitor blocked the effect of l-Arg and 8-BrcGMP on Cl^- channels, respectively. In contrast, a phosphodiesterase 2 inhibitor had no significant effect on l-Arg or 8-BrcGMP-induced inhibition of Cl^- channels. Therefore, we conclude that NO decreases basolateral 10-pS Cl^- channel activity through a cGMP-dependent PKG pathway, which may contribute to the natriuretic and diuretic effects of NO in vivo.

TRPV4 activation mediates flow-induced nitric oxide production in the rat thick ascending limb

Nitric oxide (NO) regulates renal function. Luminal flow stimulates NO production in the thick ascending limb (TAL). Transient receptor potential vanilloid 4 (TRPV4) is a mechano-sensitive channel activated by luminal flow in different types of cells. We hypothesized that TRPV4 mediates flow-induced NO production in the rat TAL. We measured NO production in isolated, perfused rat TALs using the fluorescent dye DAF FM. Increasing luminal flow from 0 to 20 nl/min stimulated NO from 8 ± 3 to 45 ± 12 arbitrary units (AU)/min (n = 5; P < 0.05). The TRPV4 antagonists, ruthenium red (15 μmol/l) and RN 1734 (10 μmol/l), blocked flow-induced NO production. Also, luminal flow did not increase NO production in the absence of extracellular calcium. We also studied the effect of luminal flow on NO production in TALs transduced with a TRPV4shRNA. In nontransduced TALs luminal flow increased NO production by 47 ± 17 AU/min (P < 0.05; n = 5). Similar to nontransduced TALs, luminal flow increased NO production by 39 ± 11 AU/min (P < 0.03; n = 5) in TALs transduced with a control negative sequence-shRNA while in TRPV4shRNA-transduced TALs, luminal flow did not increase NO production (Δ10 ± 15 AU/min; n = 5). We then tested the effect of two different TRPV4 agonists on NO production in the absence of luminal flow. 4α-Phorbol 12,13-didecanoate (1 μmol/l) enhanced NO production by 60 ± 11 AU/min (P < 0.002; n = 7) and GSK1016790A (10 ηmol/l) increased NO production by 52 ± 15 AU/min (P < 0.03; n = 5). GSK1016790A (10 ηmol/l) did not stimulate NO production in TRPV4shRNA-transduced TALs. We conclude that activation of TRPV4 channels mediates flow-induced NO production in the rat TAL.

ATP mediates flow-induced NO production in thick ascending limbs

Mechanical stimulation caused by increasing flow induces nucleotide release from many cells. Luminal flow and extracellular ATP stimulate production of nitric oxide (NO) in thick ascending limbs. However, the factors that mediate flow-induced NO production are unknown. We hypothesized that luminal flow stimulates thick ascending limb NO production via ATP. We measured NO in isolated, perfused rat thick ascending limbs using the fluorescent dye DAF FM. The rate of increase in dye fluorescence reflects NO accumulation. Increasing luminal flow from 0 to 20 nl/min stimulated NO production from 17 ± 16 to 130 ± 37 arbitrary units (AU)/min (P < 0.02). Increasing flow from 0 to 20 nl/min raised ATP release from 4 ± 1 to 21 ± 6 AU/min (P < 0.04). Hexokinase (10 U/ml) plus glucose, which consumes ATP, completely prevented the measured increase in ATP. Luminal flow did not increase NO production in the presence of luminal and basolateral hexokinase (10 U/ml). When flow was increased with the ATPase apyrase in both luminal and basolateral solutions (5 U/ml), NO levels did not change significantly. The P2 receptor antagonist suramin (300 μmol/l) reduced flow-induced NO production by 83 ± 25% (P < 0.03) when added to both and basolateral sides. Luminal hexokinase decreased flow-induced NO production from 205.6 ± 85.6 to 36.6 ± 118.6 AU/min (P < 0.02). Basolateral hexokinase also reduced flow-induced NO production. The P2X receptor-selective antagonist NF023 (200 μmol/l) prevented flow-induced NO production when added to the basolateral side but not the luminal side. We conclude that ATP mediates flow-induced NO production in the thick ascending limb likely via activation of P2Y receptors in the luminal and P2X receptors in the basolateral membrane.

Regulation of renal NaCl transport by nitric oxide, endothelin, and ATP: clinical implications

NaCl absorption along the nephron is regulated not just by humoral factors but also by factors that do not circulate or act on the cells where they are produced. Generally, nitric oxide (NO) inhibits NaCl absorption along the nephron. However, the effects of NO in the proximal tubule are controversial and may be biphasic. Similarly, the effects of endothelin on proximal tubule transport are biphasic. In more distal segments, endothelin inhibits NaCl absorption and may be mediated by NO. Adenosine triphosphate (ATP) inhibits sodium bicarbonate absorption in the proximal tubule, NaCl absorption in thick ascending limbs via NO, and water reabsorption in collecting ducts. Defects in the effects of NO, endothelin, and ATP increase blood pressure, especially in a NaCl-sensitive manner. In diabetes, disruption of NO-induced inhibition of transport may contribute to increased blood pressure and renal damage. However, our understanding of how NO, endothelin, and ATP work, and of their role in pathology, is rudimentary at best.

Shear stress increases nitric oxide production in thick ascending limbs

We showed that luminal flow stimulates nitric oxide (NO) production in thick ascending limbs. Ion delivery, stretch, pressure, and shear stress all increase when flow is enhanced. We hypothesized that shear stress stimulates NO in thick ascending limbs, whereas stretch, pressure, and ion delivery do not. We measured NO in isolated, perfused rat thick ascending limbs using the NO-sensitive dye DAF FM-DA. NO production rose from 21 ± 7 to 58 ± 12 AU/min (P < 0.02; n = 7) when we increased luminal flow from 0 to 20 nl/min, but dropped to 16 ± 8 AU/min (P < 0.02; n = 7) 10 min after flow was stopped. Flow did not increase NO in tubules from mice lacking NO synthase 3 (NOS 3). Flow stimulated NO production by the same extent in tubules perfused with ion-free solution and physiological saline (20 ± 7 vs. 24 ± 6 AU/min; n = 7). Increasing stretch while reducing shear stress and pressure lowered NO generation from 42 ± 9 to 17 ± 6 AU/min (P < 0.03; n = 6). In the absence of shear stress, increasing pressure and stretch had no effect on NO production (2 ± 8 vs. 8 ± 8 AU/min; n = 6). Similar results were obtained in the presence of tempol (100 μmol/l), a O(2)(-) scavenger. Primary cultures of thick ascending limb cells subjected to shear stresses of 0.02 and 0.55 dyne/cm(2) produced NO at rates of 55 ± 10 and 315 ± 93 AU/s, respectively (P < 0.002; n = 7). Pretreatment with the NOS inhibitor l-NAME (5 mmol/l) blocked the shear stress-induced increase in NO production. We concluded that shear stress rather than pressure, stretch, or ion delivery mediates flow-induced stimulation of NO by NOS 3 in thick ascending limbs.

Nitric oxide decreases expression of osmoprotective genes via direct inhibition of TonEBP transcriptional activity

During antidiuresis, renal medullary cells adapt to the hyperosmotic interstitial environment by increased expression of osmoprotective genes, which is driven by a common transcriptional activator, tonicity-responsive enhancer binding protein (TonEBP). Because nitric oxide (NO) is abundantly produced in the renal medulla, the present studies addressed the effect of NO on expression of osmoprotective genes and TonEBP activation in MDCK cells. Several structurally unrelated NO donors blunted tonicity-induced up-regulation of TonEBP target genes involved in intracellular accumulation of organic osmolytes. These effects were mediated by reduced transcriptional activity of TonEBP, as assessed by tonicity-responsive elements- and aldose reductase promoter-driven reporter constructs. Neither total TonEBP abundance nor nuclear translocation of TonEBP was affected by NO. Furthermore, 8-bromo-cGMP and peroxynitrite failed to reproduce the inhibitory effect of NO, indicating that NO acts directly on TonEBP rather than through classical NO signaling pathways. In support of this notion, electrophoretic mobility shift assays showed reduced binding of TonEBP to its target sequence in nuclear extracts prepared from MDCK cells treated with NO in vivo and in nuclear extracts exposed to NO in vitro. Furthermore, immunoprecipitation of S-nitrosylated proteins and the biotin-switch method identified TonEBP as a target for S-nitrosylation, which correlates with reduced DNA binding and transcriptional activity. These observations disclose a novel direct inhibitory effect of NO on TonEBP, a phenomenon that may be relevant for regulation of osmoprotective genes in the renal medulla.

Role of vasoactive substances on endometrial and ovarian function

In this review, it is proposed that the vasoactive agents endothelin (ET), nitric oxide (NO)/NO synthase (NOS) and carbon monoxide(CO)/heme oxygenase(HO) act directly on human endometrial functions and on ovarian functions in the normal menstrual cycle and in implantation periods. These vasoactive substances are likely to be important autocrine/paracrine factors that regulate a variety of physiological and pathological processes. The main actions of these agents are differentiation and implantation in the endometrial functions, and follicular growth, luteinization and atresia in the ovarian functions, in the tight connection between endometrial and ovarian systems during normal menstrual periods and during implantation (Reprod Med Biol 2007; 6: 157-164).

Regulation of thick ascending limb transport: role of nitric oxide

Nitric oxide (NO) plays a role in many physiological and pathophysiological processes. In the kidney, NO reduces renal vascular resistance, increases glomerular filtration rate, alters renin release, and inhibits transport along the nephron. The thick ascending limb is responsible for absorbing 20-30% of the filtered load of NaCl, much of the bicarbonate that escapes the proximal nephron, and a significant fraction of the divalent cations reclaimed from the forming urine. Additionally, this nephron segment plays a role in K+ homeostasis. This article will review recent advances in our understanding of the role NO plays in regulating the transport processes of the thick ascending limb. NO has been shown to inhibit NaCl absorption primarily by reducing Na+-K+-2Cl- cotransport activity. NO also inhibits bicarbonate absorption by reducing Na+/H+ exchange activity. It has also been reported to enhance luminal K+ channel activity and thus is likely to alter K+ secretion. The source of NO may be vascular structures such as the afferent arteriole or vasa recta, or the thick ascending limb itself. NO is produced by NO synthase 3 in this segment, and several factors that regulate its activity both acutely and chronically have recently been identified. Although the effects of NO on thick ascending limb transport have received a great deal of attention recently, its effects on divalent ion absorption and many other issues remain unexplored.

Increased expression of sodium transporters in rats chronically inhibited of nitric oxide synthesis

The present study was done to determine whether endogenous nitric oxide (NO) plays a role in the regulation of sodium transporters in the kidney. Male Sprague-Dawley rats were treated with NG-nitro-L-arginine methyl ester (L-NAME, 100 mg/L drinking water) for 4 weeks. Control rats were supplied with tap water without drugs. Expression of Na, K-ATPase, type 3 Na/H exchanger (NHE3), Na/K/2Cl cotransporter (BSC1), and thiazide-sensitive Na/Cl cotransporter (TSC) proteins was determined in the kidney by Western blot analysis. Catalytic activity of Na,K-ATPase was also determined. The treatment with L-NAME significantly and steadily increased the systemic blood pressure. Total and fractional excretion of urinary sodium decreased significantly, while creatinine clearance remained unaltered. Neither plasma renin activity nor aldosterone concentration was significantly altered. The alpha1 subunit expression and the catalytic activity of Na, K-ATPase were increased in the kidney. The expression of NHE3, BSC1 and TSC was also increased significantly. These results suggest that endogenously-derived NO exerts a tonic inhibitory effect on the expression of sodium transporters, including Na, K-ATPase, NHE3, BSC1, and TSC, in the kidney.

Vascular and renal effects of dopamine during extracellular volume expansion: Role of nitric oxide pathway

OBJECTIVE

The aim of the study was to determine the possible role of NO-system activation in vascular and renal effects of the dopaminergic system and the probable interaction between both systems during acute volume expansion in rats.

DESIGN AND METHODS

Expanded (10% bw) and non-expanded anaesthetized male Wistar rats were treated with haloperidol, a DA receptor antagonist (3 mg/kg bw, ip). Mean arterial pressure, diuresis, natriuresis, renal plasma flow, glomerular filtration rate, nitrites and nitrates excretion (NOx) were determined. NADPH diaphorase activity was measured using a histochemistry technique in kidney, aorta and renal arteries. NOS activity in kidney and aorta from expanded and non-expanded animals was determined with L-[U14C]-arginine substrate, in basal conditions and after DA (1 microM) administration.

RESULTS

The hypotensive effect of L-arg and hypertension induced by L-NAME were not modified by haloperidol. This blocker reverted the increase in diuresis, natriuresis and RPF induced by L-arg in both groups. Dopaminergic blockade induced a decrease in NOx excretion and in NADPH-diaphorase activity in glomeruli, proximal tubule and medullar collecting duct and in endothelium and vascular smooth muscle of renal arteries. DA induced an increase in NOS activity in renal medulla and cortex in both groups, but no changes in the aorta were observed.

CONCLUSIONS

Our results suggest that renal DA would be associated with the renal response induced by NO during extracellular volume expansion. NO-system activation would be one of the mechanisms involved in renal DA activity during saline load, but NO appears not to be involved in DA vascular effects.

Evaluation of the nitric oxide production in rat renal artery smooth muscle cells culture exposed to radiocontrast agents

BACKGROUND

Radiocontrast agents (RC), substances largely used in diagnostic procedures, present the nephrotoxicity as one of its major side effects, which could be due to an altered synthesis of vasodilators. The aim of the present study was to evaluate the nitric oxide (NO) production in rat renal artery smooth muscle cells primary culture (rVSMC) exposed to RC.

METHODS

The cells were treated for 72 hours with mannitol at 10% (MT10; 600 mOsm/kg H2O) or 35% (MT35; 2100 mOsm/kg H2O), with the nonionic iobitridol (IBT), the low-osmolality ioxaglate (IXG), the high-osmolality ioxitalamate (IXT), the nonionic, iso-osmolar iodixanol (IDX), and with lipopolysaccharide (LPS). We determined the NO and osmolality in the cell culture media and the cellular viability.

RESULTS

By the Griess and chemiluminescence methods, the NO was not different in MT10 and IDX, but decreased in MT35, IBT, IXG, and IXT when compared with the control; it was increased in LPS and also decreased in all RC+LPS when compared with LPS. MT35, IXT, and IXT+LPS decreased the cellular viability, and the media osmolality was increased in MT35 and IXT compared with the control.

CONCLUSION

The RC (except IDX) significantly reduced NO in rVSMC, which was more pronounced after IXT treatment (57.3%). This was not related to the reduced cell viability (15.8%) or to its high osmolality, because in MT35, with similar osmolality as IXT, NO decreased only 11.0% relatively to the control. Neither the media osmolality nor the cell viability was altered by IXG or IBT. The decreased NO could explain the vasoconstriction and, therefore, the acute renal failure by RC.

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.

Interactions between vasoconstrictors and vasodilators in regulating hemodynamics of distinct vascular beds

We examined whether interactions between angiotensin II (Ang II), endothelin (ET), nitric oxide (NO), and prostaglandins (PGs) differentially regulate perfusion to distinct vascular beds. For this, we blocked either angiotensin AT1 or ET receptors or both and then sequentially inhibited NO and PG synthesis in anesthetized dogs. Blocking Ang II or ET had similar effects on systemic hemodynamics: Mean arterial pressure fell slightly without altering cardiac output. Blocking both caused a synergistic fall in mean arterial pressure and increased cardiac output. Pulmonary vascular resistance was not altered by blocking Ang II, ET, or both but progressively increased during NO and PG blockade in group 2 (which had unblocked ET receptors), suggesting that endogenous ET exerts pulmonary vasoconstriction that is tempered by NO and PGs. In the kidney, blocking Ang II increased regional blood flow (RBF), glomerular filtration rate (GFR), and fractional excretion of sodium (FENa). In contrast, blocking ET did not alter RBF, and it decreased GFR and FENa. Combined Ang II and ET blockade markedly increased RBF without altering GFR, and FENa was maintained at the levels as when only ET was blocked. Sequentially inhibiting NO and PGs decreased RBF when Ang II or ET were blocked but had little effect when both were blocked. Finally, Ang II or ET blockade did not alter iliac blood flow. Inhibiting NO and PGs decreased iliac blood flow when Ang II or ET but not both were blocked. These results suggest that regional differences in the interactions between endogenous Ang II, ET, NO, and PGs are important determinants in systemic, pulmonary, and regional hemodynamics.

Role of oxidative stress in angiotensin-induced hypertension

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

Differential renal response to Nomega-nitro-L-arginine methyl ester and L-arginine in rats with hypertensive or diabetic nephropathy

The present experiments were designed to assess the renal functional response to alterations in nitric oxide formation in animals with different forms of nephropathy. To address this issue, the effects of Nomega-nitro-L-arginine methyl ester (L-NAME) or L-arginine were assessed in animal models exhibiting arterial hypertension due to chronic nitric oxide inhibition (L-NAME, 50 mg/l in drinking water for 12 weeks) or diabetes mellitus (streptozotocin, 60 mg/kg IP). Vehicle-treated, age-matched animals served as controls. Following 12 weeks of pretreatment, mean arterial pressure (MAP), renal hemodynamics, urinary albumin, and electrolyte excretion were determined in standard clearance experiments prior to and following infusion of L-NAME (50 microg/kg/min), l-arginine (5 mg/kg/min), or saline vehicle. In control animals, L-NAME resulted in an increase in MAP and renal vascular resistance and a decline in glomerular filtration rate and renal plasma flow, as expected. L-arginine had no effect on renal hemodynamics. In nitric oxide-depleted hypertensive animals, L-NAME had no additional effect on MAP or renal hemodynamics. Infusion of L-arginine reduced elevated MAP but did not reverse changes in renal hemodynamics. Diabetic rats demonstrated glomerular hyperfiltration and proteinuria. No significant changes in MAP or renal hemodynamics were observed following infusion of L-NAME or L-arginine, respectively. However, L-NAME increased urinary albumin excretion in the absence of hemodynamic changes. The effects of nitric oxide on vascular tone were shown to be dependent on the vascular bed and the underlying disease. Variations in local nitric oxide formation and susceptibility may account for the differential response of the systemic and renal vasculature and contribute to the degree of renal functional impairment observed in different systemic diseases.

Effects of indomethacin on responses of regional kidney perfusion to vasoactive agents in rabbits

1. To determine whether differential release of products of arachidonic acid metabolism, via the cyclo-oxygenase pathway, underlies the diversity of responses of regional kidney perfusion to vasoactive agents, we tested the effects of intravenous indomethacin on responses to renal arterial bolus doses of vasoactive agents in pentobarbitone-anaesthetized rabbits. 2. Total renal blood flow (RBF) and regional kidney perfusion were determined by transit time ultrasound flowmetry and laser-Doppler flowmetry, respectively. 3. Responses of regional kidney blood flow to vasoactive agents were diverse: noradrenaline reduced cortical but not medullary perfusion, [Phe 2,Ile 3,Orn 8]-vasopressin reduced medullary perfusion more than cortical perfusion, endothelin-1 and angiotensin II increased medullary perfusion in the face of reduced cortical perfusion, while acetylcholine, bradykinin and the nitric oxide donor methylamine hexamethylene methylamine (MAHMA) NONOate all increased both cortical and medullary perfusion. 4. Indomethacin administration was followed by reductions in total RBF (17 +/- 6%), cortical perfusion (13 +/- 5%) and medullary perfusion (40 +/- 8%). Angiotensin II- and endothelin-1-induced increases in medullary perfusion were abolished by indomethacin, but indomethacin had no significant effects on responses of regional kidney perfusion to acetylcholine, bradykinin, MAHMA NONOate, noradrenaline and [Phe 2,Ile 3,Orn 8]-vasopressin. 5. Our results suggest that vasodilator cyclo-oxygenase products contribute to the maintenance of resting renal vascular tone, particularly in vascular elements controlling medullary perfusion. Cyclo-oxygenase products also appear to mediate endothelin-1- and angiotensin II-induced increases in medullary perfusion. However, regionally specific engagement of cyclo-oxygenase-dependent arachidonic acid metabolism does not appear to contribute to the differential effects of noradrenaline and [Phe 2,Ile 3,Orn 8]-vasopressin on cortical and medullary perfusion.

Effects of dopamine and nitric oxide on arterial pressure and renal function in volume expansion

1. The aim of the present study was to investigate the role of dopamine (DA) in the hypotensive and renal effects of L-arginine during extracellular fluid volume expansion (10% bodyweight). 2. Animals were randomized to non-expanded and expanded groups. Both groups received different treatments: L-arginine (250 mg/kg, i.v.), N(G)-nitro-L-arginine methyl ester (L-NAME; 1 mg/kg, i.v.), haloperidol (3 mg/kg, i.p.) and L-arginine + haloperidol (n = 8). Mean arterial pressure (MAP), diuresis, natriuresis, kaliuresis, glomerular filtration rate, renal plasma flow (RPF) and nitrite and nitrate (NO(x)) excretion were determined. 3. The increase in MAP induced by L-NAME was greater in expanded than in non-expanded rats (42 +/- 3 vs 32 +/- 3 mmHg, respectively; P < 0.01). Administration of haloperidol did not modify the L-arginine hypotensive effect. 4. Blockade of nitric oxide synthase diminished urine flow in non-expanded (4.15 +/- 0.56 vs 0.55 +/- 0.11 microL/min per 100 g; P < 0.01) and expanded animals (24.42 +/- 3.67 vs 17.85 +/- 2.16 microL/min per 100 g; P < 0.01). Diuresis induced by L-arginine was reduced by DA blockade in both non-expanded (17.15 +/- 2.11 vs 6.82 +/- 0.61 microL/min per 100 g; P < 0.01) and expanded animals (44.26 +/- 8.45 vs 25.43 +/- 5.12 microL/min per 100 g; P < 0.01). 5. Sodium excretion decreased with L-NAME treatment in non-expanded (0.22 +/- 0.03 vs 0.06 +/- 0.01 microEq/min per 100 g; P < 0.01) and expanded animals (3.72 +/- 0.70 vs 1.89 +/- 0.23 microEq/min per 100 g; P < 0.01). Natriuresis induced by L-arginine was diminished by haloperidol both in non-expanded (0.94 +/- 0.13 vs 0.43 +/- 0.04 microEq/min per 100 g; P < 0.01) and expanded rats (12.77 +/- 0.05 vs 3.53 +/- 0.75 microEq/min per 100 g; P < 0.01). Changes in kaliuresis changes seen following treatment with L-arginine, L-NAME and L-arginine + haloperidol followed a pattern similar to that observed for sodium excretion in both groups of rats. 6. L-arginine enhanced RPF in non-expanded animals (11.96 +/- 0.81 vs 14.52 +/- 1.05 mL/min per 100 g; P < 0.01). Glomerular filtration rate was increased by extracellular volume expansion (3.08 +/- 0.28 vs 5.42 +/- 0.46 mL/min per 100 g; P < 0.01). 7. The increase in NOx induced by acute volume expansion (0.18 +/- 0.03 vs 0.52 +/- 0.08 nmol/min per 100 g; P < 0.01) was diminished following the administration of haloperidol (0.52 +/- 0.08 vs 0.26 +/- 0.06 nmol/min per 100 g; P < 0.01). 8. Although DA does not participate in the actions of nitric oxide on vascular tone, both systems would play an important role in renal function adaptation during extracellular fluid volume expansion.

Regulation of endothelin synthesis by extracellular matrix in human endothelial cells

BACKGROUND

Vascular diseases are characterized by the presence of structural changes and the progressive loss of endothelial function. Although the biochemical basis of these structural changes have started to be outlined, it seems that accumulation of normal extracellular matrix proteins as well as the appearance of interstitial collagens, mainly collagen type I, characterize this process. On the other hand, a role for endothelial vasoactive factors has been proposed in the genesis of endothelial dysfunction, and it is generally accepted that changes in extracellular matrix composition may modify cell behavior.

METHODS

Experiments were designed to test the influence of the supporting matrix on endothelin-1 (ET-1) synthesis by endothelial cells. Northern blot experiments were performed to analyze the prepro-endothelin-1 (prepro-ET-1) mRNA expression. ET-1 production was measured by ELISA.

RESULTS

Cells grown on collagen type I (Col I) showed an increase of prepro-ET-1 mRNA level when compared with cells cultured on collagen type IV (Col IV). According to these results, the release of ET-1 to culture medium was also higher in Col I-grown cells than in those cultured on Col IV. Treatment of cells with a peptide that interferes with Col I integrins (D6Y), or with protein tyrosine kinase inhibitors such as genistein and herbimycin, completely abolished the effect of Col I. Moreover, experiments with antibodies against integrins suggest that these cell surface receptors could be involved in the modulation of ET-1 system by extracellular matrix.

CONCLUSIONS

These results suggest that the presence of an abnormal extracellular matrix could stimulate endothelin synthesis by human endothelial cells, through integrin activation.

Cyclooxygenase-2 products compensate for inhibition of nitric oxide regulation of renal perfusion

Cyclooxygenase (COX)-2 is in the macula densa, cosegregating with neuronal nitric oxide synthase (nNOS). It is hypothesized that in response to acute inhibition of NOS, the influence of COX-2-derived prostanoids is exaggerated, compensating for renal vasoconstriction. Blood pressure (BP) and renal blood flow (RBF) were measured after selective COX-2 inhibition with NS-398 followed by NOS inhibition with L-nitro arginine methyl ester (L-NAME) or after L-NAME followed by NS-398. BP was 106 +/- 4 mmHg and was unaffected by NS-398. L-NAME after NS-398 increased BP by 27 +/- 2 mmHg, decreased RBF by one-half, and doubled renal vascular resistance (RVR; P < 0.001). Initial L-NAME increased BP by 26 +/- 3 mmHg (P < 0.001) and decreased RBF by 44% (P < 0.001), doubling RVR. After L-NAME, NS-398 induced a further 7 +/- 3-mmHg rise in BP (P < 0.05), decreased RBF by 20% (P < 0.025), and increased RVR by 23% (P < 0.01). The constrictor response to COX-2 inhibition after L-NAME could not be duplicated by either selective nNOS inhibition or NOS-independent renal vasoconstriction. Acute NOS inhibition unmasked renal vasoconstriction with COX-2 inhibition, suggesting that the influence of COX-2-derived vasodilator eicosanoids is exaggerated to maintain renal perfusion, compensating for the acute loss of NO.

Role of nitric oxide in the regulation of nephron transport

Nitric oxide (NO) plays an important role in various physiological processes in the kidney. In vivo experiments first suggested that the natriuretic and diuretic effects caused by NO may be due to decreased NaCl and fluid absorption by the nephron. In the last 10 years, several reports have directly demonstrated a role for NO in modulating transport in different tubule segments. The effects of NO on proximal tubule transport are still controversial. Both stimulation and inhibition of net fluid and bicarbonate have been reported in this segment, whereas only inhibitory effects of NO have been found in Na/H exchanger and Na/K-ATPase activity. The effects of NO in the thick ascending limb are more homogeneous than in the proximal tubule. In this segment, NO decreases net Cl and bicarbonate absorption. A direct inhibitory effect of NO on the Na-K-2Cl cotransporter and the Na/H exchanger has been reported, while NO was found to stimulate apical K channels in this segment. In the collecting duct, NO inhibits Na absorption and vasopressin-stimulated osmotic water permeability. An inhibitory effect of NO on H-ATPase has also been reported in intercalated cells of the collecting duct. Overall, the reported effects of NO in the different nephron segments mostly agree with the natriuretic and diuretic effects observed in vivo. However, the net effect of NO on transport is still controversial in some segments, and in cases like the distal tubule, it has not been studied.

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

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

Interaction of O(2)(-) and NO in the thick ascending limb

Nitric oxide (NO) is an important regulator of NaCl absorption by the thick ascending limb of the loop of Henle (THAL). The free radical superoxide (O(2)(-)) reacts with NO, decreasing its bioavailability. O(2)(-) is produced by mitochondria and various oxidases, some of which are present in the THAL. However, the ability of the THAL to produce O(2)(-) and its interaction with NO have not been studied. We hypothesized that NO bioavailability is decreased by O(2)(-). THALs were isolated and perfused and NO production was measured with an NO-selective microelectrode. Addition of L-Arg (250 micromol/L), but not D-arginine, to the bath increased NO release by 34.8 +/- 11.8 pA (n=7). The response to L-Arg was completely abolished by the NO synthase inhibitor L-NAME (n=7). Scavenging THAL O(2)(-) with the superoxide dismutase (SOD) mimetic Tempol (50 micromol/L) increased L-Arg-induced NO release. At all concentrations of L-Arg tested (50, 100, 250, 500, and 750 micromol/L), further addition of Tempol to the bath significantly increased NO release by THALs. Addition of SOD (300 U/mL) to the bath increased L-Arg-induced NO levels by 82% (n=5; P<0.02). Pretreatment of THALs with the SOD inhibitor diethyl-dithiocarbamate (250 micromol/L) blunted L-Arg-induced NO release by 63% compared with untreated tubules (n=5; P<0.05). Finally, we tested the effect of Tempol on NO-induced inhibition of THAL chloride transport. Addition of L-Arg decreased THAL Cl(-) absorption by 35%. Subsequent addition of Tempol (50 micromol/L) to the bath further decreased Cl(-) absorption by 35% (n=6; P<0.05). We conclude that NO bioavailability in the THAL is decreased by O(2)(-). In addition, we believe our studies are the first to show that endogenous O(2)(-) may act as a physiological regulator of nephron NaCl transport.

[The arginine paradox]

L-Arginine has attracted major interest because it has been identified as the natural substrate of nitric oxide synthase and is now recognized as a major player in the regulation of biological function. The arginine paradox refers to the phenomenon that exogenous L-arginine causes NO-mediated biological effects despite the fact that nitric oxide synthases (NOS) are theoretically saturated with the substrate L-arginine. There have been several explanations for this phenomenon, although none of them can explain the arginine paradox fully: (1) L-arginine-induced insulin, which has vasodilatory actions. (2) Neither extracellular nor intracellular concentration determines the NOS activity but rather the L-arginine amount transported across the plasma membrane may do so. (3) Endogenous NOS inhibitors reduce the enzyme sensitivity to L-arginine. These inhibitors include, NG, NG-dimethyl-L-arginine, L-citrulline, argininosuccinic acid and agmatine. (4) Intracellular L-citrulline, an NOS product, is a potent inhibitor of NOS so that the cells may need extra L-arginine to compete with L-citrulline inhibition.

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

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

Role of guanylyl cyclase and cytochrome P-450 on renal response to nitric oxide

The present study evaluated whether inhibition of guanylyl cyclase (GC) with 1H-(1,2,4)oxadiazolo[4,3-a]quinoxaline-1-one (ODQ) and methylene blue (MB) or inhibition of the renal metabolism of arachidonic acid by cytochrome P-450 (CYP450) enzymes with 1-aminobenzotriazole (ABT) and N-hydroxy-N'-(4 butyl-2-methyl phenyl)formamidine (HET0016) alters the renal tubular and vascular effects of a nitric oxide (NO) donor in vivo. Intrarenal infusion of ODQ or MB at a dose of 170 nmol. kg(-1). min(-1) lowered renal blood flow (RBF) by 30 and 15%, respectively; glomerular filtration rate (GFR) by 26 and 18%, respectively; and sodium and water excretion by approximately 35%. In rats pretreated with nitro-L-arginine methyl ester (37 nmol. kg(-1). min(-1)) to block the endogenous production of NO, intrarenal infusion of the NO donor S-nitroso-N-acetylcysteine (S-NO-NAC; 50 nmol. kg(-1). min(-1)) increased RBF (18%), sodium (73%), and water excretion (61%). ODQ or MB administration blocked the effect of S-NO-NAC on RBF but not the diuretic and natriuretic response. Pretreatment of rats with ABT or HET0016 also abolished the renal vasodilatory response to the NO donor and reduced its diuretic and natriuretic effect. These results indicate that both activation of GC and inhibition of CYP450 enzymes contribute to the renal vascular actions of NO, whereas the natriuretic and diuretic actions of NO appear to be largely CYP450 dependent.

Endothelin inhibits thick ascending limb chloride flux via ET(B) receptor-mediated NO release

Endothelin-1 (ET-1) inhibits transport in various nephron segments, and the thick ascending limb of the loop of Henle (TALH) expresses ET-1 receptors. In many tissues, activation of ET(B) receptors stimulates release of NO, and we recently reported that endogenous NO inhibits TALH chloride flux (J(Cl)). However, the relationship between ET-1 and NO in the control of nephron transport has not been extensively studied. We hypothesized that ET-1 decreases NaCl transport by cortical TALHs through activation of ET(B) receptors and release of NO. Exogenous ET-1 (1 nM) decreased J(Cl) from 118.3 +/- 15.0 to 62.7 +/- 13.6 pmol. mm(-1). min(-1) (48.3 +/- 8.2% reduction), whereas removal of ET-1 increased J(Cl) in a separate group of tubules from 87.6 +/- 10.7 to 115.2 +/- 10.3 pmol. mm(-1). min(-1) (34.5 +/- 6.2% increase). To determine whether NO mediates the inhibitory effects of ET-1 on J(Cl), we examined the effect of inhibiting of NO synthase (NOS) with N(G)-nitro-L-arginine methyl ester (L-NAME) on ET-1-induced changes in J(Cl). L-NAME (5 mM) completely prevented the ET-1-induced reduction in J(Cl), whereas D-NAME did not. L-NAME alone had no effect on J(Cl). These data suggest that the effects of ET-1 are mediated by NO. Blockade of ET(B) receptors with BQ-788 prevented the inhibitory effects of 1 nM ET-1. Activation of ET(B) receptors with sarafotoxin S6c mimicked the inhibitory effect of ET-1 on J(Cl) (from 120.7 +/- 12.6 to 75.4 +/- 13.3 pmol. mm(-1). min(-1)). In contrast, ET(A) receptor antagonism with BQ-610 did not prevent ET-1-mediated inhibition of TALH J(Cl) (from 96.5 +/- 10.4 to 69.5 +/- 8.6 pmol. mm(-1). min(-1)). Endothelin increased intracellular calcium from 96.9 +/- 14.0 to 191.4 +/- 11.9 nM, an increase of 110.8 +/- 26.1%. We conclude that exogenous endothelin indirectly decreases TALH J(Cl) by activating ET(B) receptors, increasing intracellular calcium concentration, and stimulating NO release. These data suggest that endothelin acts as a physiological regulator of TALH NO synthesis, thus inhibiting chloride transport and contributing to the natriuretic effects of ET-1 observed in vivo.

The role of intrarenal nitric oxide in the natriuretic response to dopamine-receptor activation

Dopamine and dopamine-1 receptor agonists produce diuresis and natriuresis by causing changes in renal hemodynamics and by the activation of dopamine-1 receptors located within the various regions of the nephron. Nitric oxide plays an important role in the maintenance of systemic and regional hemodynamics. The present study was undertaken to investigate the effect of locally generated nitric oxide on renal function and its potential influence on the renal responses to dopamine-1 receptor agonists. The intrarenal infusion of a nitric oxide synthase inhibitor, L-NAME, (50 microg/kg min for 90 min) in anesthetized rats produced significant decreases in urine volume, urinary sodium excretion, glomerular filtration rate and fractional sodium excretion. These changes in renal function were associated with a concomitant decrease in urinary nitrate excretion, an indicator of nitric oxide release. However, L-NAME at this dose did not produce any significant changes in mean arterial pressure or heart rate. Intravenous infusion of fenoldopam (1 microg/kg min for 30 min), a selective dopamine-1 receptor agonist, produced diuresis and natriuresis without causing any changes in mean arterial pressure and heart rate. These renal effects of fenoldopam were significantly attenuated in animals that received the simultaneous infusion of L-NAME (intrarenal). Similar results were obtained with dopamine in that the natriuretic and diuretic response to dopamine was also attenuated during simultaneous infusion of dopamine with L-NAME. In addition, the diuresis and natriuresis produced by fenoldopam and dopamine was associated with increases in urinary nitrate excretion. Interestingly, these increases in the nitrate levels seen with fenoldopam and dopamine were also significantly reduced in the presence of L-NAME. These results indicate that intrarenal nitric oxide plays an important role in regulating renal sodium excretion and that an intact renal nitric oxide system is required for the full expression of diuretic and natriuretic response seen during dopamine-1 receptor activation.

eNOS mediates L-arginine-induced inhibition of thick ascending limb chloride flux

We recently reported that the rat thick ascending limb (THAL) possesses an active isoform of nitric oxide synthase (NOS) that is substrate-limited in vitro. NO produced by THAL NOS inhibits chloride flux. Protein and transcript for each of the primary NOS isoforms-endothelial (eNOS), inducible (iNOS), and neuronal (nNOS)-have been demonstrated in THALs. However, the NOS isoform that mediates NO-induced inhibition of chloride flux is unknown. We hypothesized that NO produced from eNOS in the THAL inhibits NaCl transport. THALs from male eNOS, iNOS, and nNOS knockout mice and C57BL/6J wild-type controls were perfused in vitro and the response of transepithelial chloride flux (J(Cl)) to L-arginine (L-Arg), the substrate for NOS, and spermine NONOate (SPM), an NO donor was measured. We first tested whether isolated mouse THALs could synthesize NO and whether this NO inhibits transport. Addition of 0. 5 mmol/L L-Arg to the bath decreased J(Cl) from 105.8+/-17.5 to 79. 2+/-15.8 pmol/mm per minute (P<0.01) in C57BL/6J wild-type mice, whereas addition of D-Arginine had no effects on J(Cl.) In contrast, addition of 0.5 mmol/L L-Arg to the bath did not alter J(Cl) of THALs from eNOS knockout mice. When 10 micromol/L SPM was added to the bath of eNOS knockout THALs, J(Cl) decreased from 89.1+/-8.6 to 74.8+/-7.5 pmol/mm/min (P<0.05). Thus the lack of responsiveness of eNOS knockout THALs to L-Arg was not due to an inability to respond to NO. We next evaluated the role of iNOS and nNOS in the response to L-Arg. Addition of 0.5 mmol/L L-Arg to the bath decreased J(Cl) in THALs from iNOS and nNOS knockout mice by 37.7+/-6.4% (P<0.05) and 31.8+/-8.3% (P<0.01), respectively. We conclude that eNOS is the active isoform of NOS in the THAL under basal conditions. Mouse THAL eNOS responds to exogenous L-Arg by increasing NO production, which, in turn, inhibits J(Cl).

Free radical scavengers to prevent reperfusion injury following experimental warm liver ischaemia. Is there a real physiological benefit?

Free radical scavengers have been utilized to prevent the consequences of ischemia, however, results do not seem conclusive. In our study we analyzed the blood flow, function, and histology of rat liver tissue after warm liver ischemia, in order to assess the effect of free radicals in liver reperfusion injury. N-acetyl cysteine (NAC), tocopherol, allopurinol, and superoxide dismutase (SOD), pharmacological agents expected to protect from injury mediated by free radicals, were investigated. Laser Doppler flowmetry and photometry were utilized to measure post-ischemic microcirculatory changes as an expression of ischemia-reperfusion injury in a model of segmental liver ischemia in the rat, with an ischemic time of 45 min. Galactose elimination capacity, ALT and histology were used to assess the functional and morphological consequences of ischemia after 24 h of reperfusion. The overall mean blood flow over 1 hour after reperfusion was of 33.9% (SD 11.2) of the normal, non-ischemic control. NAC (31.2% SD 10.9) did not show any protective effect and in some cases the effect seemed to be negative. Tocopherol (41.7% SD 5.1) marginally improved post ischemic liver tissue blood flow. Treatment with allopurinol did not show any beneficial effects (37.5% SD 14.2). Only animals treated with SOD showed an improvement of the post ischemic liver microcirculation (57.9% SD 14.4)(P < 0.001) and function. Only SOD produced statistically significant differences in galactose elimination capacity, compared with those of the ischemic control group. This moderately protective effect of SOD is encouraging, however, the relevance of all these compounds in a broader pathophysiological setting remains unproven.

Relaxing effect of insulin in renal arteries from diabetic rats

Abnormal renal vasomotor tone exists in the early stages of diabetes mellitus. Insulin has been proposed to modulate renal function and to possess vasodilatory effects. The present study was initiated in order to evaluate the direct effect of insulin on isolated renal arteries. Twelve insulin-treated streptozotocine diabetic rats with diabetes for 50 days were compared with 15 weight-matched control rats. The contractile responses to 60 mM K+ and 10(-4) M noradrenaline, and the insulin- (0.8-6.4 I.U./ml) induced relaxation of vessels precontracted with noradrenaline, were similar in diabetic and control rats. There was a tendency towards greater relaxation in diabetic (71%) than in control rats (54%). Nw-nitro-L-arginine methyl ester (L-NAME) (10(-4) M) given before noradrenaline tended to attenuate the insulin-induced relaxation, while addition of L-arginine (10(-6) M) to L-NAME attenuated the relaxation in diabetic but increased it in control rats (P < 0.05). The effect of insulin was tested further in control rats and was not influenced by administration of a single dose (10(-6) M) of indomethacin or propranolol given instead of L-NAME. The effect of a single dose of methylene-blue, given before noradrenaline, was tested in control rats in varying doses between 2 x 10(-6) and 2 x 10(-4) M. In the highest concentration it made no difference whether insulin was given or not and there was a similar relaxing effect in diabetic and control arteries. In conclusion, the present study showed that insulin per se has a relaxing effect on renal arteries. There was a tendency to greater relaxation in diabetic than in control rats, an effect which was attenuated by in-vitro-pretreatment with L-NAME as well as with L-NAME and L-arginine in diabetic vessels, while relaxation was increased in control vessels. This may indicate that the effect of insulin may be mediated through nitric oxide in diabetic but not in control rats. The effects of insulin in control vessels were not modified in vitro by indomethacin, propranolol or methylene-blue.

Sex differences and role of nitric oxide in blood flow of canine urinary bladder

Continuous measurements were made of bladder blood flow by laser Doppler flowmetry in anesthetized dogs during bladder filling and emptying. In both mucosa and muscle, perfusion was inversely proportional to intravesical pressure. There was significantly greater perfusion in the bladder mucosa of males than females at baseline and up to 10 cm water filling pressure but not in the muscle. Intra-arterial infusion of the nitric oxide synthase inhibitor NG-nitro-L-arginine produced a significant decrease in resting bladder perfusion in the mucosa only, with no differences seen in the response to intravesical pressure. Intra-arterial infusion of L-arginine produced a significant increase in the level of perfusion in the mucosa seen immediately after the bladder was drained. No changes were observed in muscle perfusion after L-arginine. These results suggest that the perfusion of the bladder mucosa differs by gender and is regulated differently than the bladder muscle, possibly related to the different function of the two layers.

Endogenous nitric oxide inhibits chloride transport in the thick ascending limb

Nitric oxide (NO) inhibits transport in various nephron segments, and the thick ascending limb of the loop of Henle (TALH) expresses NO synthase (NOS). However, the effects of NO on TALH transport have not been extensively studied. We hypothesized that endogenously produced NO directly decreases NaCl transport by the TALH. We first determined the effect of exogenously added NO on net chloride flux (JCl). The NO donor spermine NONOate (SPM; 10 microM) decreased JCl from 101.2 +/- 9.6 to 65.0 +/- 7.7 pmol. mm-1. min-1, a reduction of 35.5 +/- 6.4%, whereas controls did not decrease over time. To determine whether endogenous NO affects cortical TALH transport, we measured the effect of adding the NOS inhibitor NG-nitro-L-arginine methyl ester (L-NAME), the substrate L-arginine (L-Arg), or its enantiomer D-arginine (D-Arg) on JCl. L-NAME and D-Arg did not alter JCl; in contrast, addition of 0.5 mM L-Arg decreased JCl by 40.2 +/- 10.4% from control. The inhibition of chloride flux by 0.5 mM L-Arg was abolished by pretreatment with L-NAME, indicating that cortical TALH NOS is active, but production of NO is substrate-limited in our preparation. Furthermore, cortical TALH chloride flux increased following removal of 0.5 mM L-Arg from the bath, indicating that the reductions in chloride flux observed in response to L-Arg are not the result of NO-mediated cytotoxicity. We conclude that 1) exogenous NO decreases cortical TALH JCl; 2) cortical TALHs produce NO in the presence of L-Arg, which decreases JCl; and 3) the response of cortical TALHs to L-Arg is reversible in vitro. These data suggest an important role for locally produced NO, which may act via an autocrine mechanism to directly affect TALH sodium chloride transport. Thus TALH NO synthesis and inhibition of chloride transport may contribute to the diuretic and natriuretic effects of NO observed in vivo.

Activity of nitric oxide synthase in the ventilatory muscle vasculature

We evaluated in the in situ vascularly isolated canine diaphragm the role of nitric oxide (NO) in the regulation of basal vascular resistance and vascular responses to increased muscle activity (active hyperemia), brief occlusions of the phrenic artery (reactive hyperemia), and changes in arterial pressure. The vasculature of the left hemidiaphragm was either pump-perfused at a fixed flow rate or autoperfused with arterial blood from the femoral artery. Endothelial nitric oxide synthase (NOS) activity was inhibited by intraphrenic infusion of L-arginine analogues such as N(G)-nitro-L-arginine, N(G)-nitro-L-arginine methyl ester and argininosuccinic acid. Active hyperemia was produced by low (2 Hz) frequency stimulation of the left phrenic nerve. Reactive hyperemia was measured in response to 10, 20, 30, 60, and 120 sec duration occlusions of the left phrenic artery and was quantified in terms of postocclusive blood flow, vascular resistance, hyperemic duration, and hyperemic volume. Infusion of NOS inhibitors into the vasculature of the resting diaphragm increased phrenic vascular resistance significantly and to a similar extent. Reactive hyperemic volume and reactive hyperemic duration were also significantly attenuated after NOS inhibition, however, peak reactive hyperemic dilation was not influenced by NOS inhibition. It was also found that enhanced NO release contribute by about 41% to active dilation elicited by continuous 2 Hz stimulation. In addition, NOS inhibition had no effect on O2 consumption of the resting diaphragm, but significantly attenuated the rise in diaphragmatic O2 consumption during during 2 Hz stimulation. The decline in diaphragmatic O2 consumption was due to reduction in blood flow. These results indicate that NO release plays a significant role in the regulation of diaphragmatic vascular tone and O2 consumption.

Cardiovascular effects of L-arginine

Most of the known cardiovascular effects of L-arginine are exerted via its conversion to nitric oxide by nitric oxide synthase. Accumulating evidence indicates that supplemental administration of L-arginine is sufficient to restore endothelium-derived nitric oxide production in many disorders in which endothelium-derived nitric oxide production is altered. L-arginine may enhance nitric oxide production by competing as a substrate with an endogenous antagonist for nitric oxide synthase. In other cases, L-arginine may act by competing with molecular oxygen as a substrate so as to reduce the production of superoxide anion. It is likely that other mechanisms exist by which the nitric oxide synthase pathway can be perturbed. Regardless of the mechanism, a wide array of cardiovascular disorders characterized by endothelial dysfunction are reversible by L-arginine.

Role of endothelium-derived nitric oxide-endothelin balance in contrast medium-induced acute renal vasoconstriction in dogs

RATIONALE AND OBJECTIVES

The authors evaluated the involvement of nitric oxide and endothelin in radiographic contrast medium-induced changes in renal hemodynamics.

METHODS

Eleven anesthetized healthy dogs were each studied during three periods. Thirty minutes before the first, second, and third periods, the dogs received 1 mL per kilogram of body weight of isotonic saline, L-N-nitro-L-arginine-methyl-ester (L-Name, 10 mg/kg intravenously), and L-arginine (500 mg/ kg intravenously), respectively. Renal blood flow (RBF) and mean arterial blood pressure were continuously monitored. The glomerular filtration rate (GFR) was evaluated by means of polyfructosan clearance.

RESULTS

Contrast medium induced a significant (P < .05) decrease in RBF and GFR and a significant (P < .05) increase in urinary endothelin excretion. L-Name enhanced the effect of contrast media on RBF and GFR. L-arginine attenuated the effect of L-Name on the contrast medium-induced reduction of GFR.

CONCLUSION

These findings support the hypothesis that acute contrast medium-induced intrarenal vasoconstriction may involve an imbalance of endothelial vasoactive agents, nitric oxide, and endothelin, and they confirm the involvement of hemodynamic changes in contrast medium-induced nephropathy.

Renal vasodilatory effect of endothelial stimulation in patients with chronic congestive heart failure

OBJECTIVES

This study sought to examine the vasodilatory response of the renal circulation to endothelial stimulation in patients with chronic heart failure.

BACKGROUND

Renal blood flow is often reduced in patients with chronic congestive heart failure and may lead to deterioration of renal function. Stimulation of renal endothelium has been shown to cause renal vasodilation in animals and in isolated human renal artery. The vasoregulatory role of the renal endothelium in patients with heart failure has not been evaluated.

METHODS

Renal vasodilatory effect of endothelial stimulation with acetylcholine was assessed and compared with that of endothelial independent vasodilation with nitroglycerin. Both drugs were infused into the main renal artery. Renal artery cross-sectional area was measured with intravascular ultrasound and renal blood flow velocity with the aid of an intravascular Doppler technique.

RESULTS

Both drugs caused a significant and comparable increase in renal artery cross-sectional area (maximal increase [mean +/- SE] 14 +/- 5% with acetylcholine, 15 +/- 5% with nitroglycerin; both changes < 0.05 vs. baseline). Acetylcholine also caused a significant reduction in renal vascular resistance (maximal reduction 55+/- 6%) and increase in renal blood flow (maximal increase 136 +/- 54%). In contrast, nitroglycerin administration showed no significant effect on renal vascular resistance and blood flow.

CONCLUSIONS

Stimulation of endothelium-derived nitric oxide with acetylcholine results in a significant vasodilatory effect on both conductance and resistance renal blood vessels and leads to a marked reduction in renal vascular resistance and enhancement of renal blood blow. Nitroglycerin, an exogenous nitric oxide donor, caused a selective vasodilatory effect on renal conductance but not on resistance blood vessels and failed to increase renal blood flow. These data suggest the possibility that stimulation of endogenous nitric oxide production in the kidney could be used as a therapeutic target for enhancement of renal flow in patients with heart failure.

Ginsenosides stimulate endogenous production of nitric oxide in rat kidney

Ginsenosides (GS), saponins purified from Panax ginseng, increase renal blood flow in rats. Nitric oxide (NO) is thought to be the substance endogenously released by GS in preconstricted lungs and cultured endothelial cells. The present study aims to determine whether GS could stimulate endogenous release of NO in rat kidney and whether GS affected the activity of NO synthase in kidney tissues. The serum and urine levels of the stable NO metabolites, nitrite (NO2) and nitrate (NO3) and urinary cGMP levels were measured 8 hr after a single intraperitoneal injection of GS (200 mg/kg) into rats. The effects of the NO synthesis inhibitor, N omega-nitro-L-arginine methyl ester and the NO precursor, L-arginine, on the GS-induced changes were also determined. The activity of NO synthase, as determined by conversion of [14C]-L-arginine to [14C]-L-citrulline, in whole kidney, glomeruli and cortical tubules was also investigated. A single injection of GS resulted in endogenous production of NO as reflected by increase in serum and urine levels of NO2/NO3 and urinary cGMP levels, which were inhibited by the addition of N omega-nitro-L-arginine methyl ester and restored by L-arginine. GS also stimulated the activity of NO synthase in whole kidney as well as glomeruli and cortical tubules, and this increase was significantly prevented by N omega-nitro-L-arginine methyl ester. It was concluded that stimulation in endogenous production of NO by GS may contribute to its antinephritic action and may play a protective role in the kidney.

Mechanisms of action of atrial natriuretic factor and C-type natriuretic peptide

After secretion by the heart, atrial natriuretic factor (ANF) circulates in plasma, whereas C-type natriuretic peptide (CNP), which is found in abundance in the endothelium, may regulate vascular tone in a paracrine manner. However, there is little information on the effect of CNP on renal microvessels. We hypothesized that CNP dilates the afferent arteriole via the nitric oxide pathway, whereas ANF acts directly on vascular smooth muscle cells. When we perfused rat kidneys with minimal essential medium and bovine serum albumin at 100 mm Hg and examined the juxtamedullary afferent arterioles, neither CNP nor ANF was found to have any effect. When the peptides were added to arterioles preconstricted with norepinephrine, CNP and ANF dilated them in a similar fashion; diameters increased by 25 +/- 4% (n=7) and 29 +/- 6% (n=6) at 10(-7) mol/L, respectively (P < .008). Pretreatment with 10(-4) mol/L N-nitro-L-arginine methyl ester (L-NAME) or 5 x 10(-6) mol/L indomethacin blocked CNP-induced dilation; dilation by ANF was unaffected by indomethacin (52 +/- 25%, n=5) and potentiated by L-NAME (73 +/- 14%, n=5). Thus, CNP dilates the afferent arterioles via the prostaglandin/nitric oxide pathway, whereas ANF dilates them directly. This difference may be important in controlling glomerular hemodynamics.

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.

Glomerular actions of nitric oxide

NO, a simple molecule synthesized from L-arginine by NO synthases, has been identified to play an important role in cell communication, cell defense and cell injury. The half life of NO is very short because NO either reacts with superoxide anion (O2-), and/or binds to heme molecules or Fe-S groups present in proteins. The biological effects of NO depend on both the concentration of NO at the site of action as well as upon the specific location where NO is generated. Small quantities of NO are generated by cNOS such as that present in the vascular endothelium, while large quantities of nitric oxide are synthesized by iNOS in response to cytokines or bacterial products. Within the kidney NO generated by endothelial cNOS participates in the regulation of the glomerular microcirculation by modifying the tone of the afferent arteriole and mesangial cells (Fig. 4). In addition, NO generated by macula densa and the afferent arteriole control glomerular hemodynamics via TGF and by modulating renin release. Therefore NO is important in the physiologic regulation of glomerular capillary blood pressure, glomerular plasma flow and the glomerular ultrafiltration coefficient. Through its actions on glomerular pressures and flows, NO may also regulate the macro- and micromolecular traffic through the mesangium. Chronic NO insufficiency causes hypertension and glomerular damage and may be causally involved in the genesis of salt dependent hypertension. Increased NO production may be involved in the early pathogenic hemodynamic changes in diabetes and in the physiologic hemodynamic responses to normal pregnancy. Maintenance of the antithrombogenic properties of the endothelium is another important action of NO which inhibits platelet aggregation and adhesion. Large quantities of NO such as that synthesized by either glomerular cells or macrophages during glomerular inflammation may lead to glomerular injury. A better understanding of the physiology and pathophysiology of NO in the kidney will lead to the development of new therapeutic avenues.

Decreased renal haemodynamic response to inhibition of nitric oxide synthase in subtotally nephrectomized rats

To assess the renal haemodynamic response to manipulations of the nitric oxide (NO) system, we examined subtotally nephrectomized (SNX) rats and control rats (CON) 28 days after their operation. Bolus infusions of the NO synthase inhibitor NG-nitro-L-arginine (L-NA) were given intravenously at doses of 2 mg/kg and 10 mg/kg. Blood pressure was measured intra-arterially, glomerular filtration rate was measured by inulin clearance and fractional changes in renal blood flow (RBF) were determined by a Doppler flow probe. Both doses of L-NA caused a similar and dose-dependent increase in mean blood pressure in both SNX and CON rats. In contrast, the decrease in RBF and the increase in the renovascular resistance index (RVRI) was less in SNX rats as compared to CON rats (RBF = -70.1 +/- 2.2% of baseline vs -52.7 +/- 5.2%, P < 0.01; RVRI = +177 +/- 9% of baseline vs +243 +/- 24%, P < 0.05). These changes were not affected by autonomic blockade (hexamethonium), or by blockade of the angiotensin II receptor (Losartan). The exogenous NO donor sodium nitroprusside (0.5 and 1.5 micrograms.kg-1.min-1) lowered mean blood pressure to a similar degree in SNX and CON rats; in contrast, RVRI decreased less in SNX rats (86.9 +/- 9.2% of baseline) than in CON rats (68.2 +/- 4.6%, P < 0.05). We conclude that the reaction of the renal vasculature to manipulations of the NO system is altered in the SNX rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunolocalization of soluble guanylyl cyclase subunits in rat kidney

Stimulation of soluble guanylyl cyclase (SGC) by nitric oxide (NO) results in the generation of cyclic guanosine monophosphate (cGMP). We recently described expression of abundant nitric oxide synthase, the enzyme by which NO is generated from L-arginine in macula densa cells of rat kidney at the protein and mRNA level. In the present study we looked for possible targets of NO in the kidney. By light and electron microscopy, we applied polyclonal antisera against four subunits (alpha 1, alpha 2, beta 1, beta 2) of SGC in immunocytochemical studies of frozen sections of rat kidney. We demonstrate the presence of alpha 1-subunit in glomerular podocytes and of beta 2-subunit in principal cells of the collecting duct. In both cell types a cytosolic localization was evident from ultrastructural analysis. Regarding the collecting duct, NO was shown by other authors to inhibit sodium reabsorption in cultured mouse cortical collecting duct principal cells. In podocytes NO may relax the contractile system of podocyte food processes, the tone of which has been suggested to counteract the elastic distension of the capillary wall.

Multiple factors contribute to acetylcholine-induced renal afferent arteriolar vasodilation during myogenic and norepinephrine- and KCl-induced vasoconstriction. Studies in the isolated perfused hydronephrotic kidney

Acetylcholine (ACh) elicits vasodilation by releasing a number of endothelium-derived relaxing factors (EDRFs). We used the isolated perfused hydronephrotic rat kidney to examine the characteristics of ACh-induced vasodilation of renal afferent arterioles during different types of underlying vasoconstriction. Basal arteriolar tone was increased by either elevating perfusion pressure to 180 mm Hg (myogenic), administering 0.3 mumol/L norepinephrine (NE), or elevating medium potassium concentration to 30 mmol/L (KCl). ACh (10 mumol/L) completely reversed myogenic and NE-induced vasoconstriction and reversed KCl-induced vasoconstriction by 80 +/- 5%. However, whereas ACh produced a sustained vasodilation during KCl- and NE-induced vasoconstriction, only a transient reversal of myogenic vasoconstriction was observed, and myogenic tone recovered within 5 to 10 minutes. ACh-induced vasodilation of arterioles preconstricted with KCl was markedly inhibited by either indomethacin (100 mumol/L) or nitro-L-arginine (100 mumol/L) and was completely abolished by pretreatment with both inhibitors. In contrast, indomethacin and nitro-L-arginine had no effect on the transient response to ACh observed during pressure-induced vasoconstriction. In vessels preconstricted with NE, nitro-L-arginine converted the normally sustained response to ACh to a transient vasodilation, which was refractory to both nitric oxide synthase and cyclooxygenase inhibition. Since this component was not observed during KCl-induced vasoconstriction, it may reflect the actions of an, as yet unidentified, endothelium-derived hyperpolarizing factor (EDHF). Our findings thus suggest that prostanoids, nitric oxide, and EDHF all contribute to ACh-induced renal afferent arteriolar vasodilation and that the relative contributions of these individual EDRFs depends on the nature of the underlying renal vascular tone.

Nitric oxide increases renal blood flow by interacting with the sympathetic nervous system

To investigate whether changes in renal blood flow induced by nondepressor doses of L-arginine, the precursor of nitric oxide, are mediated by a sympathetic neural mechanism, we examined the following in conscious rabbits: (1) the effects of intravenous infusion of L- or D-arginine (15 to 200 mumol/kg per minute) on renal blood flow and renal sympathetic nerve activity with or without intravenous infusion of a nonpressor dose of NG-monomethyl-L-arginine (L-NMMA), a nitric oxide synthase inhibitor, and (2) the effects of L-arginine on renal blood flow after renal denervation with or without L-NMMA pretreatment. In renal innervated rabbits, L-arginine (100 and 200 mumol/kg per minute) increased renal blood flow by 9 +/- 2 and 16 +/- 3 mL/min (P < .05, respectively) and decreased renal sympathetic nerve activity by 12 +/- 4% and 19 +/- 3% of control (P < .05, respectively). In contrast, no changes occurred in any variable during D-arginine infusion. L-NMMA attenuated the renal blood flow and renal sympathetic nerve activity responses to L-arginine (P < .05). In renal denervated rabbits, L-NMMA also attenuated the renal blood flow responses to L-arginine (P < .05) and abolished them (P < .05) compared with those in renal innervated rabbits. All renal blood flow responses to L-arginine were accompanied by parallel changes in plasma L-citrulline concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Nitric oxide in the kidney: synthesis, localization, and function

The characterization and cloning of constitutive and inducible nitric oxide (NO)-synthesizing enzymes and the development of specific inhibitors of the L-arginine NO pathway have provided powerful tools to define the role of NO in renal physiology and pathophysiology. There is increasing evidence that endothelium-derived NO is tonically synthesized within the kidney and that NO plays a crucial role in the regulation of renal hemodynamics and excretory function. Bradykinin and acetylcholine induce renal vasodilation by increasing NO synthesis, which in turn leads to enhancement of diuresis and natriuresis. The blockade of basal NO synthesis has been shown to result in decreases of renal blood flow and sodium excretion. These effects are partly mediated by an interaction between NO and the renin angiotensin system. Intrarenal inhibition of NO synthesis leads to reduction of sodium excretory responses to changes in renal arterial pressure without an effect on renal autoregulation, suggesting that NO exerts a permissive or a mediatory role in pressure natriuresis. Nitric oxide released from the macula densa may modulate tubuloglomerular feedback response by affecting afferent arteriolar constriction. Nitric oxide produced in the proximal tubule possibly mediates the effects of angiotensin on tubular reabsorption. In the collecting duct, an NO-dependent inhibition of solute transport is suggested. The L-arginine NO pathway is also active in the glomerulus. Under pathologic conditions such as glomerulonephritis, NO generation is markedly enhanced due to the induction of NO synthase, which is mainly derived from infiltrating macrophages. An implication of NO in the mechanism of proteinuria, thrombosis mesangial proliferation, and leukocyte infiltration is considered. In summary, the data presented on NO and renal function have an obvious clinical implication. A role for NO in glomerular pathology has been established. Nitric oxide is the only vasodilator that closely corresponds to the characteristics of essential hypertension. Using chronic NO blockade, models of systemic hypertension will provide new insights into mechanisms of the development of high blood pressure.