Urinary Nitrate Excretion Rates Research Articles

Role of nitric oxide in the vasodilator effect of recombinant human growth hormone in patients with dilated cardiomyopathy

Dilated cardiomyopathy is characterized by elevated arterial vascular resistance and impaired nitric oxide (NO)-dependent vasodilation. Insulin-like growth factor-I (IGF-I) has been shown to stimulate endothelial NO-synthase resulting in endothelium-dependent vasodilation. Growth hormone (GH) substitution therapy leads in GH-deficient patients to significant increases of IGF-I which may alter systemic vascular resistance by stimulating NO production. This study was designed to evaluate the effects of treatment with recombinant human growth hormone (GH) on NO production and NO-dependent vascular effects in patients with dilated cardiomyopathy. 50 patients with dilated cardiomyopathy were randomly assigned to double-blind treatment with 2 I.U. of GH or placebo for 3 months. Central hemodynamics were determined by Swan-Ganz catheter and cardiac output was obtained by the thermodilution method. Serum GH and IGF-I levels were measured and systemic NO production was determined from urinary nitrate and cyclic GMP excretion rates in 42 patients. GH treatment caused in comparison to the placebo group a significant increase of IGF-I by 91 ng/ml (P = 0.0001). Urinary excretion rates of nitrate and cyclic GMP increased also significantly by 38 mumol/mmol creatinine (P = 0.027) and 65 nmol/mmol creatinine (P = 0.003), respectively. The parallel increase of both marker molecules indicates increased systemic NO production during GH treatment. GH treatment induces a significant, but moderate increase of systemic NO production in patients with dilated cardiomyopathy. This effect may be mediated by IGF-I stimulating endothelial NO synthase.

Renal handling of circulating nitrates in anesthetized dogs.

Nitric oxide (NO) is rapidly oxidized to nitrite (NO-2) and then to nitrate (NO-3) in biological tissues. Although urinary excretion rates of NO-3 are often used as an index of NO production in the body, very little is known regarding the kidney's ability to excrete circulating NO-3. We have evaluated the renal responses to systemic administration of sodium nitrate (NaNO3) in eight anesthetized dogs treated with the NO synthase inhibitor, nitro-L-arginine (NLA; 50 microg . kg-1 . min-1), intrarenally to minimize renal production of NO. Urinary and plasma concentrations of NO-3/NO-2 (NOX) were determined by the Greiss reaction after enzymatic reduction of NO-3 to NO-2. NLA treatment alone resulted in reductions in urinary NOX excretion rates (UNOXV, 1.13 +/- 0.2 to 0.53 +/- 0.1 nmol . min-1 . g-1) and an increase in fractional reabsorption of NOX (FRNOX, 93.8 +/- 0.6 to 97 +/- 0.6%) without changes in arterial plasma concentrations (ANOX, 18.7 +/- 1.4 to 21.2 +/- 3.7 microM). Administration of NaNO3 (10, 20, 30, and 40 microg . kg-1 . min-1) resulted in dose-dependent increases in ANOX (34.5 +/- 8.0, 46.4 +/- 7.3, 60.7 +/- 6.3, and 78.1 +/- 6.3 microM), UNOXV (1.8 +/- 0.7, 4.2 +/- 1.8, 7.0 +/- 2.0, and 11.4 +/- 3.3 nmol . min-1 . g-1), and decreases in FRNOX (93.8 +/- 2.3, 90.3 +/- 3.5, 88.6 +/- 3.2, and 84.6 +/- 3.5%). Absolute net tubular reabsorption of NO-3 showed a linear relationship with filtered loads, with no evidence of a transport maximum. These data show that, in the absence of additions from intrarenal sources, urinary excretion rates of nitrate increases progressively in response to increases in its circulating levels without exhibiting a transport maximum but with progressive decreases in fractional reabsorption.

Automated enzymatic determination of urinary nitrates in humans

Urinary nitrate determinations are used to evaluate the in vivo synthesis of nitric oxide, the major endothelial-derived relaxing factor. In the present study, the enzymatic method using the Aspergillus nitrate reductase was applied to an autoanalyzer to determine urinary nitrate in 17 healthy men. This method was fast and required a small amount of biologic sample (7 μL). Linearity of the assay was between 0.062 μmol/mL and 1.00 μmol/mL; accuracy, measured as percentage of nitrate recovered, ranged from 96% to 118%; between-day imprecision, assessed as coefficient of variation, ranged from 3.5% to 4.5%. The mean urinary nitrate concentration was 0.73 ± 0.29 μmol/mL (range, 0.26 to 1.26 μmol/mL). To carefully quantify nitrate production, urinary nitrate excretion rates were also measured. Rates averaged 0.71 ± 0.18 μmol/min (range, 0.44 to 0.98 μmol/min). Nitrate excretion rates were much less variable than nitrate concentrations. Nitrate excretion rates, but not concentrations, differed significantly in smokers (n = 7, 0.54 ± 0.08 μmol/min) and nonsmokers (n = 10, 0.85,± 0.11 μmol/min). In conclusion, (1) the nitrate reductase assay applied to an autoanalyzer is an accurate, fast, and convenient method for the determination of urinary nitrates in humans; (2) urinary nitrate excretion rate is less variable than nitrate concentration; and (3) cigarette smoking is associated with a diminished nitrate excretion rate. Further studies are needed to assess the nitric oxide generation rate in these subjects.

Elevatedl-Arginine/Dimethylarginine Ratio Contributes to Enhanced Systemic NO Production by Dietaryl-Arginine in Hypercholesterolemic Rabbits

Dietary supplementation of L-arginine, the precursor of endogenous NO, has been shown to enhance endothelial function in the cholesterol-fed rabbits. However, the mechanism by which dietary L-arginine accomplishes these effects has been unclear. In the present study we have assessed the plasma concentrations of L-arginine and of asymmetrical dimethylarginine (ADMA), a known endogenous inhibitor of NO synthase, in cholesterol-fed rabbits with or without dietary supplementation of L-arginine. Urinary nitrate excretion rates were assessed as an index of endogenous NO formation. Plasma L-arginine levels were not different between control and cholesterol-fed rabbits, but they were elevated nearly threefold in rabbits fed cholesterol + L-arginine. Plasma ADMA concentration increased about two-fold in hyper-cholesterolemia, but was unaffected by dietary L-arginine. Thus, dietary L-arginine elevated the plasma L-arginine/ADMA ratio above the normal level, and partly restored urinary nitrate excretion, which was decreased by hypercholesterolemia. We conclude that elevation of the L-arginine/ADMA ratio may at least partly explain the restored NO formation by exogenous L-arginine in hypercholesterolemia.

Changes in urinary nitrate and nitrite during treatment of ulcerative colitis.

The urinary excretion rates of nitrate (NO3) and nitrite (NO2) were monitored in 14 patients with active ulcerative colitis during treatment using hydrocortisone and sulfasalazine. During the active phase of the disease, the NO3 excretion was significantly higher in the patients than in healthy controls (n = 6, p < 0.05), although it varied considerably among the patients. During the healing phase, the NO3 excretion decreased concurrently with improvement of symptoms and colorectal ulceration, but the NO2 excretion increased. During the inactive phase of the disease, the NO3 and NO2 excretions were significantly lower than during the active phase, and the NO2/NO3 ratio resembled that in the healthy controls. In contrast, a patient who failed to respond to treatment showed continuously high NO3 and NO2 excretion rates. These results indicate that urinary NO3 and NO2 excretions vary with the disease state in ulcerative colitis.

Exercise Increases Systemic Nitric Oxide Production in Men

Our aim was to compare the effects of exercise on systemic nitric oxide formation, assessed from the urinary excretion rates of nitrate and of cyclic GMP, in endurance-trained and untrained men. Ten endurance-trained and six untrained men underwent a submaximal exercise test for 30 min at 60% of their individual maximal work capacity. Urinary excretion rates of nitrate (measured using gas chromatography) and cyclic GMP (measured using a radioimmunoassay) were assessed at hourly intervals. Urinary nitrate excretion was comparable at rest in the two groups (104 +/- 35 and 110 +/- 19 mumol/mmol creatinine; NS), more than doubled during exercise in both groups (236 +/- 88 mumol/mmol creatinine in untrained and 252 +/- 39 mumol/mmol creatinine in trained participants; P < 0.01 for both groups) and decreased rapidly after the completion of the exercise test. Excretion of urinary cyclic GMP at rest was four times higher in the endurance-trained than in the untrained men (21 +/- 5 compared with 6 +/-1 nmol/mmol creatinine; P < 0.05). During the exercise test, GMP excretion approximately doubled in both groups (52 +/- 12 and 10 +/- 1 nmol/mmol creatinine, respectively; P < or = 0.01 for both groups) and returned to baseline at the end of the test. Urinary cyclic GMP and nitrate excretion correlated well in both groups (p < 0.05). Atrial natriuretic factor is also a stimulator of cyclic GMP formation but resting plasma levels of atrial natriuretic factor did not differ between the groups (22 +/- 3 compared with 21 +/- 4 pmol/l; NS). An acute submaximal exercise test increases the formation of nitric oxide, reflected in the increase in urinary excretion of nitrate and cyclic GMP. This increase may contribute to vasodilatation during physical exercise and at least partly explain the beneficial effects of physical training in patients with vascular diseases.

Metabolic Fate of the Precursors of N-Nitroso Compounds (III)

Urinary excretion rate of nitrate in man was studied by determining the amount of nitrate ingested with foods and beverages and that excreted into urine for 24hr and 48hr.The mean nitrate concentration ±S. D. in the urine (n=33) which were collected from 11 subjects at random was 74.4±42.5ppm. The excretion amounts of nitrate in the 24hr urine obtained from 9 subjects who ingested conventional diet containing from 44.1 to 864.2mg of nitrate was from 45.6 to 236.9mg, indicating a remarkable and individual differences of urinary excretion rate of nitrate (27.4 to 133.8%). The excretion amount of urinary nitrate every 2hr in two subjects was less than 10mg when only the low-nitrate diet containing a small amount of nitrate was ingested, but increased more than 25mg every 2hr by the ingestion of 200mg of sodium nitrate or 100g of salted Chinese cabbage, containing 75.1mg of nitrate, with the low-nitrate diet. The range of the urinary excretion rates of nitrate for 48hr was from 78.2 to 86.2% (n=3). No nitrite was detected in any urine specimens, and neither nitrate nor nitrite was detected in any feces (n=6).

Metabolic Fate of the Precursors of N-Nitroso Compounds (I)

Urinary excretion rate of nitrate in man was studied by determining the amount of nitrate ingested with foods and beverages and that excreted into urine for 24hr and 48hr.The mean nitrate concentration ±S. D. in the urine (n=33) which were collected from 11 subjects at random was 74.4±42.5ppm. The excretion amounts of nitrate in the 24hr urine obtained from 9 subjects who ingested conventional diet containing from 44.1 to 864.2mg of nitrate was from 45.6 to 236.9mg, indicating a remarkable and individual differences of urinary excretion rate of nitrate (27.4 to 133.8%). The excretion amount of urinary nitrate every 2hr in two subjects was less than 10mg when only the low-nitrate diet containing a small amount of nitrate was ingested, but increased more than 25mg every 2hr by the ingestion of 200mg of sodium nitrate or 100g of salted Chinese cabbage, containing 75.1mg of nitrate, with the low-nitrate diet. The range of the urinary excretion rates of nitrate for 48hr was from 78.2 to 86.2% (n=3). No nitrite was detected in any urine specimens, and neither nitrate nor nitrite was detected in any feces (n=6).