The endothelial L-arginine/nitric oxide pathway and the renal circulation

Sodium nitrite therapy rescues ischemia-induced neovascularization and blood flow recovery in hypertension

Arterial hypertension is a major risk factor that can lead to complication of peripheral vascular disease due, in part, to endothelial dysfunction. Because sodium nitrite (SN) can be converted to nitric oxide (NO), which counteracts endothelial dysfunction, we explored the effect of nitrite on neovascularization following hind limb ischemia in different models of hypertension (HT). Chronic delivery of angiotensin II (Ang II, 400 ng/kg/min) or N(omega)-nitro-L-arginine-methyl-ester (L-NAME, 0.1 g/L) was used for a 2-week period to induce hypertension. Mice were subjected to femoral artery ligation-induced ischemia in the hind limb followed by treatment with SN (50 mg/L) for 2 weeks. SN significantly reduced systolic arterial blood pressure in mice receiving Ang II and L-NAME but had no effect in sham animals. After 2 weeks, blood flow and microangiography showed 60 % ± 1.0 recovery in sham compared with 40 % ± 1.3 in HT mice. Importantly, sham and HT mice treated with SN showed a 100 % blood flow recovery associated with normalization in capillary density. The inhibition of xanthine-oxido-reductase (allopurinol) or VEGFR (SU-5416) prevented the neovascularization in HT mice treated with SN. Cyclic GMP (cGMP) content in the hind limb was significantly increased in mice treated with SN compared with non-treated mice. Nitrite/nitrate content was only increased in the sham group treated with SN. Immunoprecipitation and Western blot analysis revealed an increase in eNOS/Akt/VEGFR phosphorylation in skeletal muscle from mice treated with SN compared with non-treated mice. Our findings indicate that SN therapy rescues the neovascularization and blood flow recovery in the ischemic hind limb of sham and HT mice likely through the Akt/NO/cGMP and VEGFR pathways.

Renoprotection, renin inhibition, and blood pressure control: the impact of aliskiren on integrated blood pressure control

Hypertension (HTN) is an important factor in progressive loss of renal function. The kidney can be both a contributor to and a target of HTN. The functional integrity of the kidney is vital for the maintenance of cardiovascular homeostasis. Chronic activation of the renin system causes HTN and, ultimately, end-organ damage. Direct renin inhibitors (DRIs) inhibit plasma renin activity (PRA), thereby preventing the conversion of angiotensinogen to angiotensin I; consequently, the levels of both Ang I and Ang II are reduced. There is no compensatory increase in PRA activity with DRIs as seen with angiotensin-converting enzyme inhibitors (ACEIs) or angiotensin receptor blockers (ARBs). There are reasons to speculate that renin inhibition might prove to be a superior strategy for blocking the renin-angiotensin-aldosterone system compared with ACEIs or ARBs. Evidence for the efficacy of aliskiren (a DRI) is considered to be relatively strong, based on published, short-term, double-blind, randomized, controlled trials showing that aliskiren is as effective as other antihypertensive agents in reducing blood pressure (BP), with no rebound effects on BP after treatment withdrawal. When combined with diuretics, fully additive BP reduction is seen. When given with an ACEI or ARB, aliskiren produces significant additional BP reduction indicative of complimentary pharmacology and more complete renin-angiotensin system blockade.

Suppressed apoptosis in the inflamed gastric mucosa of Helicobacter pylori-colonized iNOS-knockout mice

Deregulated cell turnover in Helicobacter pylori (H. pylori)-colonized gastric mucosa has been suggested to be linked to the gastric carcinogenesis pathway. We previously reported attenuation of apoptosis and enhancement of cellular proliferation in the H. pylori-colonized gastric mucosa of Mongolian gerbils as compared to that in mice, which might reflect a specific link between H. pylori colonization and carcinogenesis in the Mongolian gerbils; the difference between the two strains could be attributable to differences in the host genetic background. Inducible-type nitric oxide synthase (iNOS) is thought to participate in not only the inflammatory response, but also in the regulation of gastric mucosal cell turnover in H. pylori-colonized gastric mucosa. Thus, the present study was designed to examine gastric leukocyte activation and epithelial cell apoptosis in the gastric mucosa following H. pylori inoculation in iNOS-knockout mice.

METHODS

iNOS-knockout mice (iNOS(-/-)) and their iNOS(+/+) littermates were orally inoculated with the Sydney strain of H. pylori (SS1, 10(8) colony-forming units [CFU]). H. pylori infection was confirmed by microaerobic bacterial culture. The stomach of each mouse was evaluated 14 weeks and 30 weeks after the inoculation. Gastric mucosal accumulation of polymorphonuclear leukocytes (PMN) was assessed by determining the myeloperoxidase (MPO) activity and histological score based on the updated Sydney system. The level of apoptosis was determined by estimation of the cytoplasmic levels of mono- and oligonucleosomes and by the terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling method.

RESULTS

The SS1-inoculated mice showed persistent H. pylori colonization for 12 weeks. While gastric mucosal PMN infiltration increased following SS1 inoculation in both iNOS(+/+) and iNOS(-/-)strains, enhanced DNA fragmentation was observed in only SS1-colonized iNOS(+/+) mice, and not in the iNOS(-/-) mice. In conclusion, although the recruitment of PMN in response to H. pylori was evoked even in the gastric mucosa of iNOS(-/-) mice, epithelial cell apoptosis induced by H. pylori was attenuated in this strain. These data suggest that iNOS may play an important role in promoting apoptosis in the H. pylori-infected inflamed gastric mucosa, and that persistent inflammation without apoptosis in iNOS(-/-) mice with H. pylori infection may be linked to preneoplastic transformation.

Renal blood flow in hypercholesterolemic pigs is increased by chronic antioxidant treatment

Oxygen radical species can influence vascular tone, and antioxidants may have hemodynamic and vascular effects. To date, the vascular effects of chronic intervention with a combination of antioxidant vitamins E and C on renal blood flow (RBF) in hypercholesterolemia (which increases oxidative stress) have not been fully defined. The aim of this intervention study was to explore the involvement of increased oxidative stress in pig RBF disturbance by using chronic dietary antioxidant vitamin intervention. Responses of RBF to the acetylcholine (Ach) were measured in vivo using electron beam computed tomography (EBCT). Acetylcholine significantly increased RBF in normal and hypercholesterolemic + vitamins (P < 0.05 for both), but not in hypercholesterolemic pigs (P=0.1). In normocholesterolemic + vitamins pigs, Ach infusion did not induce any further increase in RBF, but RBF was similar to that observed in normal and hypercholesterolemic + vitamins under the same conditions, and tended to be higher than in hypercholesterolemic pigs (P=0.06). Thus, antioxidants improve RBF in hypercholesterolemic pigs and this effect may help to prevent renal diseases and hypertension in animals.

Renal vascular function in hypercholesterolemia is preserved by chronic antioxidant supplementation

Hypercholesterolemia impairs systemic vascular reactivity in response to endothelium-dependent vasodilators, which may be mediated partly through increased formation of lipid peroxides. However, it is unclear whether these pathophysiological mechanisms play a role in renal vascular impairment in experimental hypercholesterolemia. Hence, pigs were studied after a 3-mo normal (n = 7) or high cholesterol (HC) (n = 7) diet, HC diet supplemented daily with antioxidant vitamins E (100 IU/kg) and C (1000 mg; HC+vitamins, n = 5), or normal diet supplemented with vitamins (N+vitamins, n = 5). Renal blood flow was measured with electron-beam computed tomography before and during infusion of acetylcholine (Ach). Endothelial function, endothelial and inducible nitric oxide synthase (NOS), and nitrotyrosine immunoreactivity were studied in renal arteries ex vivo. Despite similar cholesterol levels, LDL oxidizability (lag time, malondialdehyde, and relative electrophoretic mobility) was increased in pigs that were fed the HC diet but was significantly decreased in pigs that were fed the HC+vitamins diet. Renal blood flow response to Ach was blunted in pigs that were fed the HC diet but was preserved in pigs that were fed the HC+vitamins diet. Maximal relaxation to Ach was attenuated in pigs that were fed the HC diet compared with those that were fed the normal diet (51.5 +/- 6.4% versus 97.0 +/- 2.9%; P < 0.01) but was preserved in pigs that were fed the HC+vitamins diet (103.1 +/- 3.0%; P = 0.39) and N+vitamins diet (87.7 +/- 3.0%; P = 0.1), as were relaxation responses to calcium ionophore A23187. Vascular smooth-muscle relaxation to diethylamine was enhanced in endothelium-denuded HC vessel but was restored in pigs that were on the HC+vitamins regimen. In HC, immuno-reactivity of endothelial NOS was decreased, that of inducible NOS was increased, and both were preserved in pigs that were fed the HC+vitamins and N+vitamins diets, whereas nitrotyrosine was not detected. The present study demonstrates that antioxidant intervention in experimental HC reduces LDL oxidizability and preserves renal vascular responses to endothelium-dependent vasodilators. Therefore, this beneficial effect potentially can protect the kidney from hypercholesterolemia-induced damage.

Diabetes and endothelial dysfunction: a clinical perspective

The main etiology for mortality and a great percent of morbidity in patients with diabetes mellitus is atherosclerosis. A hypothesis for the initial lesion of atherosclerosis is endothelial dysfunction, defined pragmatically as changes in the concentration of the chemical messengers produced by the endothelial cell and/or by blunting of the nitric oxide-dependent vasodilatory response to acetylcholine or hyperemia. Endothelial dysfunction has been documented in patients with diabetes and in individuals with insulin resistance or at high risk for developing type 2 diabetes. Factors associated with endothelial dysfunction in diabetes include activation of protein kinase C, overexpression of growth factors and/or cytokines, and oxidative stress. Several therapeutic interventions have been tested in clinical trials aimed at improving endothelial function in patients with diabetes. Insulin sensitizers may have a beneficial effect in the short term, but the virtual absence of trials with cardiovascular end-points preclude any definitive conclusion. Two trials offer optimism that treatment with ACE inhibitors may have a positive impact on the progression of atherosclerosis. Although widely used, the effect of hypolipidemic agents on endothelial function in diabetes is not clear. The role of antioxidant therapy is controversial. No data have been published regarding the effects of hormonal replacement therapy on endothelial dysfunction in postmenopausal women with type 2 diabetes.

Impaired renal vascular endothelial function in vitro in experimental hypercholesterolemia

Hypercholesterolemia (HC) induces alterations in systemic vascular reactivity, which can manifest as an attenuated endothelium-dependent relaxation, partly consequent to an impairment in nitric oxide (NO) activity. To determine whether experimental HC has a similar effect on renal vascular function, renal artery segments obtained from pigs fed a HC (n=5) or normal (n=5) diet were studied in vitro. Endothelium-dependent relaxation was examined using increasing concentrations of acetylcholine (Ach), calcium ionophore A23187, and Ach following pre-incubation with N(G)-monomethyl-L-arginine or L-arginine (L-ARG). The NO-donor diethylamine (DEA) was used to examine smooth muscle relaxation response and cyclic GMP generation in endothelium-denuded vessels. The expression of endothelial NO synthase (eNOS) in the renal arteries was examined using Western blotting. Endothelium-dependent relaxation to Ach was significantly attenuated in the HC group compared to normal (53.3+/-9.1 vs. 98.8+/-3.7%, P<0.005), but normalized after pre-incubation with L-ARG (82.3+/-13.8%, P=0.21). Receptor-independent endothelium-dependent relaxation to A23187 was also significantly blunted in HC (75.2+/-10.5 vs. 115.5+/-4.2%, P<0. 017). Smooth muscle relaxation and cyclic GMP generation in response to DEA were greater in denuded HC vessels, while relaxation of intact vessels to nitroprusside was unaltered. In the HC vessels eNOS was almost undetectable. In conclusion, experimental HC attenuates in vitro endothelium-dependent relaxation of the porcine renal artery, possibly due to low bioavailability of NO. These vascular alterations in HC could play a role in the pathogenesis of renal disease or hypertension, supporting a role for HC as a risk factor for renovascular disease.

Attenuation of contrast material-induced renal artery vasoconstriction by nitric oxide donors

RATIONALE AND OBJECTIVES

The authors studied the role of the endothelium and associated endothelial pathways in contrast material-induced renal vasoconstriction.

MATERIALS AND METHODS

Isometric contractions in human and rabbit renal artery rings with intact and denuded endothelium were stimulated with phenylephrine and increasing concentrations of the ionic contrast material diatrizoate, the nonionic contrast materials iopamidol and iomeprol, and the dimeric contrast material iodixanol in a tissue perfusion bath. Rings with intact endothelium were incubated with endothelium-stimulating compounds such as the NO synthetase inhibitor Ng nitro-L-arginine methyl ester (L-NAME) to study the endothelium-mediated vasomotor regulation and the NO-liberating substances molsidomine (SIN-1) and nitroprusside (NPR) to study the endothelial-mediated vasorelaxation before being stimulated with contrast material.

RESULTS

Contrast material-induced, dose-dependent, reversible renal artery contractions are dependent on the type of contrast material. No differences in the contractions were found between intact and denuded rings. L-NAME had no effect on contrast material-induced contractions. Contractions were inhibited by the NO donors SIN-1 and NPR. SIN-1 was the most potent inhibitor.

CONCLUSION

Contrast material-induced renal vasoconstriction is endothelium-independent. Selective pharmacologic stimulation of the endothelium by NO donors, however, may still be useful in the prophylaxis of contrast material-induced renal vasoconstriction and, thus, potentially nephrotoxicity.

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.

Computed tomography-derived intrarenal blood flow in renovascular and essential hypertension

The effect of renal artery stenosis on intrarenal perfusion and volume in renovascular hypertensive patients is unclear. Alterations in these attributes may ultimately be involved in deterioration of renal function. We measured whole kidney, cortical, and medullary perfusion and volume with electron beam computed tomography (EBCT) in 33 hypertensive patients, with well-preserved renal function, scheduled for renal angiography. EBCT-derived whole kidney perfusion was lower in patients with atherosclerotic renal artery stenosis (RAS; N = 20) than in fibromuscular dysplasia (FMD; N = 10) or essential hypertension (N = 28; P < 0.05), as was cortical perfusion (2.44 +/- 0.16 vs. 3.26 +/- 0.17 and 3.07 +/- 0.09 ml/min/cc tissue, respectively, P < 0.005), but medullary perfusion was similar. Whole kidney, cortical, and medullary perfusion correlated inversely with degree of stenosis in FMD, but not in atherosclerotic RAS. Renal volumes were similar. These results demonstrate that, in contrast to patients with FMD, in patients with atherosclerotic RAS the decrease in cortical perfusion is not directly related to the degree of stenosis in the main renal artery. Factors other than the stenosis itself may play a role in the pathophysiology of atherosclerotic RAS and associated renal failure.

Gas-generating systems in acute renal allograft rejection in the rat. Co-induction of heme oxygenase and nitric oxide synthase

Gases are now viewed as biologic messengers, and in this regard, carbon monoxide and nitric oxide are incriminated in signaling processes in neural tissue. Carbon monoxide is generated by heme oxygenase (HO), an enzyme inducible by heme, cytokines, and oxidative stress and considered an antioxidant response; nitric oxide is generated by nitric oxide synthase, an enzyme also inducible by cytokines. Since mononuclear cells infiltrate the acutely rejecting kidney, and foster within the kidney oxidative stress and a cytokine-enriched milieu, we examined the expression of these enzymes in acute renal allograft rejection (AR) (Brown Norway kidney to a Lewis rat; n = 17) and in control isografts (Lewis kidney to a Lewis rat; n = 17). No immunosuppressives were used. We found marked induction of HO mRNA and protein in renal allografts at day 5 after transplantation. Prominent expression of HO protein, as detected by immunofluorescence, was observed in the mononuclear cells infiltrating the renal allograft. More than 80% of these cells were macrophages, as identified by positive staining with ED1 antibody. ED1+ cells were rare in isografts and did not stain for HO. We also found co-expression of mRNA and protein for the inducible isoform of nitric oxide synthase (iNOS) in AR at day 5 after transplantation. Induction of HO and iNOS may reflect the cellular effect of diverse cytokines elaborated in the rejecting kidney. HO may enable the macrophage to degrade heme-containing proteins released from erythrocytes and other damaged cells; alternatively, induction of HO may defend the macrophage against oxidant injury. Increased nitric oxide, as a result of iNOS activity, may antagonize the vasoconstrictive effects of a number of mediators (i.e., thromboxane and endothelin) present in acute rejection; conversely, nitric oxide may prove cytotoxic through a number of recognized effects. Our studies provide the first demonstration of the induction of HO in the rejecting renal allograft as well as the first demonstration in vivo for the induction of HO in macrophages at the site of an inflammatory response. Such expression, linked as it is to the expression of iNOS, indicates that the macrophage mimics the behavior of neural cells by generating these gaseous messengers; thus, neural cells are not alone in deploying these mediators. Through a number of effects, these products of HO and iNOS may influence the nature and severity of tissue injury in AR.

Induction of heme oxygenase in toxic renal injury: a protective role in cisplatin nephrotoxicity in the rat

Cellular content of heme is regulated by heme oxygenase, the rate limiting enzyme in the degradation of heme. Induction of heme oxygenase is a protective response in an in vivo model of heme protein mediated renal injury, the glycerol model of acute renal failure. In addition to heme, heme oxygenase is induced by diverse forms of oxidative stress, the functional significance of which is currently unknown. We examined whether heme oxygenase is induced, and the functional significance of such induction, in two in vivo models of oxidant-induced toxic nephropathy, namely, cisplatin and gentamicin nephropathies; nephrotoxicity in these models is not dependent on the delivery of a burden of heme proteins to the kidney as occurs in the glycerol model. We demonstrate induction of heme oxygenase mRNA and protein in the kidney as early as 6 and 12 hours after a single dose of cisplatin (6 mg/kg i.v.). Pretreatment with tin protoporphyrin, a competitive inhibitor of heme oxygenase, led to higher serum creatinine values on days 3 through 5 and lower inulin clearances on day 5; tin protoporphyrin also exacerbated renal injury in this model. Renal hemodynamics studied at day 2 after cisplatin demonstrate reduced renal blood flow rates, increased renal vascular resistance and increased fractional excretion of sodium in rats treated with tin protoporphyrin. Tin protoporphyrin alone had no significant effect on serum creatinine and renal hemodynamics in rats with intact, disease-free kidneys. We confirmed that tin protoporphyrin prevented the increase in heme oxygenase activity induced by cisplatin. Induction of heme oxygenase by cisplatin was associated with increased kidney heme content and ferritin content.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of prostaglandin production by nitric oxide; an in vivo analysis

1. Endotoxin E. Coli lipopolysaccharide (LPS)-treatment in conscious, restrained rats increased plasma and urinary prostaglandin (PG) and nitric oxide (NO) production. Inducible cyclo-oxygenase (COX-2) and nitric oxide synthase (iNOS) expression accounted for the LPS-induced PG and NO release since the glucocorticoid, dexamethasone inhibited both effects. Thus, LPS (4 mg kg-1) increased the plasma levels of nitrite/nitrate from 14 +/- 1 to 84 +/- 7 microM within 3 h and this rise was inhibited to 35 +/- 1 microM by dexamethasone. Levels of 6-keto PGF1 alpha in the plasma were below the detection limit of the assay (< 0.2 ng ml-1). However, 3 h after the injection of LPS these levels rose to 2.6 +/- 0.2 ng ml-1 and to 0.7 +/- 0.01 ng ml-1 after LPS in rats that received dexamethasone. 2. The induced enzymes were inhibited in vivo with selective COX and NOS inhibitors. Furthermore, NOS inhibitors, that did not affect COX activity in vitro markedly suppressed PG production in the LPS-treated animals. For instance, the LPS-induced increased in plasma nitrite/nitrate and 6-keto PGF1 alpha at 3 h was decreased to 18 +/- 2 microM and 0.5 +/- 0.02 ng ml-1, 23 +/- 1 microM and 0.7 +/- 0.01 ng ml-1, 29 +/- 2 microM and 1 +/- 0.01 ng ml-1 in rats treated with LPS in the presence of the NOS inhibitors NG-monomethyl-L-arginine, NG-nitro arginine methyl ester and aminoguanidine, respectively. 3. The intravenous infusion of the NO donors sodium nitroprusside (SNP) or glyceryl trinitrate (GTN)increased prostaglandin production in normal animals (for instance urinary PGE2 excretion was increased from 96 +/- 10 to 576 +/- 12 pg min-1 and 400 +/- 24 pg min-1 in the presence of GTN or SNP respectively).4. Proteinuria was measured in order to evaluate the roles of NO and PG in renal damage associated with the in vivo injection of LPS. Interestingly, dexamethasone and the NOS inhibitors attenuated proteinuria in the LPS-treated rats. The COX inhibitors had no effect. It therefore appears that NO and not PG contributes to the LPS-induced renal damage; these findings support the potential use of NOS inhibitors in the treatment of renal inflammation.5. This study demonstrates the regulatory contribution of NO on the in vivo production of prostanoids and suggests that in inflammatory diseases that are driven by both NO and the prostaglandins, NOS inhibitors may act to reduce inflammation by the dual inhibition of cytotoxic NO and pro-inflammatory PG.

Endogenous nitric oxide enhances prostaglandin production in a model of renal inflammation

The interaction between nitric oxide (NO) and cyclooxygenase (COX) was studied in a rabbit model of renal inflammation, the ureteral obstructed hydronephrotic kidney (HNK). Ex vivo perfusion of the HNK but not the control kidney (e.g., unobstructed contralateral kidney, CLK), led to a time-dependent release of nitrite (NO2-), a breakdown product of NO. Stimulation of the HNK with bradykinin (BK) evoked a time-dependent increase in prostaglandin E2 (PGE2) production. NG-monomethyl-L-arginine (L-NMMA), which blocks the activity of both constitutive and inducible nitric oxide synthase (cNOS and iNOS), aminoguanidine, a recently described selective iNOS inhibitor, dexamethasone, or cycloheximide abolished the release of NO2- and attenuated the exaggerated BK-induced PGE2 production. This supports the existence of iNOS and COX-2 in the HNK. In the CLK, BK elicited release of both NO2- and PGE2 but this did not augment with time. L-NMMA but not aminoguanidine, dexamethasone, or cycloheximide attenuated NO2- and PGE2 release indicative of the presence of constitutive but not inducible NOS or COX. The current study suggests that the endogenous release of NO from cNOS in the CLK activates a constitutive COX resulting in optimal PGE2 release by BK. In addition, in the HNK, NO release from iNOS activates the induced COX resulting in markedly increased release of proinflammatory prostaglandin. The broader implication of this study is that the cyclooxygenase isozymes are potential receptor targets for nitric oxide.

Changes in nitric oxide precursor, L-arginine, and metabolites, nitrate and nitrite, with aging

The present study was performed to test the hypothesis that excretion of nitric oxide metabolites, nitrate and nitrite, are decreased with progressive aging in rats and that a decrease in nitric oxide precursor, L-arginine, also decreases with aging. Urinary nitrate/nitrite excretory rates and serum L-arginine levels were measured in male Sprague Dawley rats, ranging in ages from 3 to 25 months. Proteinuria increased dramatically with aging. Conversely, urinary nitrate/nitrite excretion decreased by 50% and 80% in rats, aged 12 months and 17 months, respectively. There was no further decrease in urinary nitrate/nitrite excretion in very old rats, aged 23-24 months. Glomerular filtration rate (GFR) was also measured in some of the rats, aged 3-5 mos and 17 mos. GFR was not different between old and young rats, suggesting that a decrease in GFR could not account for the decrease in urinary nitrate/nitrite excretion in the old rats. However, serum L-arginine levels were decreased with aging, by 30% and 50% in rats, aged 13-15 months and 24-25 months, respectively, when compared with young rats. These data confirm our hypothesis and suggest that nitric oxide (NO) production may decrease with aging and that one mechanism by which nitric oxide production could be decreased with age is a lack of the endogenous substrate, L-arginine. Because NO has been implicated to be involved in many physiological processes, age-related decreases in NO production could have far-reaching adverse effects in the aging individual.