Urinary Nitric Oxide Excretion Research Articles

DOMINANT EFFECT OF SUPPLEMENTED-SUCROSE ON THE LOW PROTEIN DIET-INDUCED INCREASE IN BLOOD PRESSURE OF SPRAGUE-DAWLEY RATS

A low-protein diet (LPD) is known to affect the regulation of hemodynamics, and could contribute to the genesis of hypertension. We investigated the mechanism for the LPD-induced elevation of blood pressure in 52 Sprague-Dawley rats. Rats fed the LPD for 8 weeks showed a significantly higher blood pressure than those fed on a normal-protein diet (NPD) when the LPD included sucrose as a predominant component of carbohydrate (LPD with a high sucrose content, 135 ±2 mmHg; NPD, 124 ±2 mmHg; p<0.05). However, LPD with a low sucrose content, in which corn starch was the main component of carbohydrate, did not have a hypertensive effect (125 ±2 mmHg). Urinary epinephrine and norepinephrine excretion was significantly higher in the LPD high-sucrose group than in the NPD and LPD low-sucrose groups, and there was a significant positive correlation between urinary norepinephrine excretion and systolic blood pressure. Urinary nitric oxide excretion was no different between these groups, and 2 % L-arginine administration exerted no antihypertensive effect on the LPD-induced elevation of blood pressure. Sodium restriction also did not attenuate the LPD-induced elevation of blood pressure. These results suggest that the effect of LPD on blood pressure could be interpreted as the effect of the high sucrose content supplemented to the LPD rather than the direct effect of protein restriction, and that the stimulation of sympathetic nervous activity was associated with this elevation of blood pressure.

Localization of protein inhibitor of neuronal nitric oxide synthase in rat kidney.

We have recently demonstrated that in rats with 5/6 nephrectomy (5/6 Nx), renal cortical and inner medullary neuronal NOS (nNOS) expression is downregulated, associated with decreased urinary excretion of nitric oxide (NO) products. Recently, a novel 89-amino acid protein [protein inhibitor of nNOS (PIN)] was isolated from rat brain and shown to inhibit nNOS activity. The present studies localized PIN in the rat kidney and determined the effect of 5/6 Nx on PIN expression. By RT-PCR, PIN mRNA was detected in the kidney cortex and inner medulla. Immunohistochemistry revealed staining for PIN in glomerular and vasa rectae endothelial cells. PIN was also localized to the apical membranes of inner medullary collecting duct (IMCD) cells. Two weeks after 5/6 Nx, inner medullary PIN expression was significantly upregulated (sham, 0.18+/-0.07 vs. 5/6 Nx, 0.58+/-0.13 arbitrary units; n = 6, P<0.02), as determined by Western blotting. In summary, our data show that PIN, a specific inhibitor of nNOS activity, is expressed in the IMCD, a site of high nNOS expression in the kidney. PIN expression is upregulated in the inner medulla of 5/6 Nx rats. Inhibition of nNOS activity by PIN may have important implications for the regulation of NO synthesis in the IMCD of normal and remnant kidneys.

Plasma and 24-h NO(x) and cGMP during normal pregnancy and preeclampsia in women on a reduced NO(x) diet.

We tested the hypothesis that nitric oxide (NO) biosynthesis increases during normal human pregnancy and decreases in preeclampsia. The major metabolites of NO, nitrate and nitrite (NO(x)), were measured in both the plasma and 24-h urine of women subjected to a reduced NO(x) diet. In this way, the plasma and urinary levels mainly reflected endogenous production rather than dietary intake. Moreover, we assessed cGMP, a second messenger of NO, in the same samples. Both NO(x) and cGMP assays were validated in our laboratory. We first conducted a cross-sectional study of nonpregnant women (n = 15), normal pregnant women in the first (n = 9), second (n = 17) and third (n = 22) trimesters, as well as women with preeclampsia (n = 15) and transient hypertension of pregnancy (n = 7). We also performed a serial study in the same women (n = 9) before, during, and after pregnancy. Taken together, the results of the two investigations suggested marked increases in cGMP production especially during the first trimester when the maternal circulation is rapidly vasodilating. In contrast, whole body NO production as estimated by the plasma level and urinary excretion of NO(x) was not elevated during the first trimester. Finally, unequivocal demonstration of reduced NO biosynthesis in preeclampsia was not forthcoming.

Nitric Oxide–Dependent Renal Vasodilatation Is Not Altered in Rat with rHuEpo–Induced Hypertension

Background: Recombinant human erythropoietin (rHuEpo) is the treatment of choice in anemia associated with end–stage renal disease. Its major side effect is hypertension, which occurs in 8–30% of uremic patients. The exact mechanism of rHuEpo–induced hypertension has not been fully elucidated, and several possibilities have been proposed, such as a direct vascular effect of the drug with a shift in the balance of constrictor and relaxing endothelial factors (endothelins and nitric oxide (NO)). Recent papers suggested an enhanced rather than reduced activity of endogenous NO system in rats with normal renal function and rHuEpo–induced hypertension. Our study was designed to verify whether, in spite of enhanced activity of the renal NO system, rHuEpo may affect endothelium–dependent (acetylcholine–induced) and/or endothelium–independent (sodium nitroprusside–induced) vasorelaxation and to evaluate basal NO release by the infusion of N^G–nitro–L–arginine methyl ester (L–NAME) in an isolated and perfused rat kidney model. Methods: To investigate this hypothesis, we have determined systemic and renal NO activity in Wistar rats treated with a hypertensive dose of rHuEpo (150 IU/kg b.w. every other day for 2 weeks) by measuring stable NO metabolites (NO₂+NO₃) in the urine and have also evaluated variations in renal vascular resistance after the injection of Ach, SNP and the infusion of L–NAME. Results: Hematocrit, hemoglobin concentration and arterial blood pressure were significantly increased in the treated group as compared with the controls. Urinary excretion of NO₂+NO₃ was significantly higher in treated than in the controls (438±66 vs. 294±36 nM/ml/min, p<0.01, respectively). There were no significant differences in the dose–response curves to Ach and SNP between the two groups. The renal vasoconstriction following the infusion of L–NAME was also similar in the two groups. Conclusions: The analysis of our results seems to indicate that the endogenous NO system activity was enhanced in rHuEpo–induced hypertension in rats with normal renal function and a resistance to NO was not developed in renal circulation. Further studies seem to be necessary to better clarify the exact mechanisms underlying the development of rHuEpo–induced hypertension.

Urinary Excretion of Nitric Oxide, Cyclic Gmp, and Catecholamines During Rest and Activity Period in Transgenic Hypertensive Rats

Dysregulation of the system of nitric oxide (NO)-cyclic 3',5'-guanosine monophosphate (cGMP) might be involved in the development of hypertension in transgenic hypertensive TGR(mREN2)27 (TGR) rats. The present study was performed to determine possible differences in the day-night pattern and the urinary excretion rates of NO and cGMP in TGR rats in comparison to normotensive Sprague-Dawley (SPRD) controls. In addition, the urinary excretion of creatinine and catecholamines was measured in both rat strains. The day-night excretion patterns of NO, cGMP, catecholamines, and creatinine were preserved in TGR rats. Urinary excretion of NO was significantly decreased in TGR rats, whereas cGMP, the second messenger of NO, was elevated in the transgenic animals. Catecholamines and creatinine excretion rates did not differ between the strains. In conclusion, data suggest that a reduced NO synthesis could contribute to the increased blood pressure in the severely hypertensive rats. However, these data make it unlikely that the disturbances in the nitric oxide-cGMP system and the sympathetic nervous system are mainly responsible for the inverse circadian blood pressure rhythm in TGR rats.

The contribution of nitric oxide to renal vascular wall thickening in rats with L-NAME-induced hypertension.

We investigated the mechanisms of renal vascular wall thickening in a rat model of N-nitro L-arginine methyl ester (L-NAME)-induced hypertension. To separate the effects of L-NAME-induced hypertension from other effects of nitric oxide (NO) inhibition, we created two models of L-NAME-induced hypertension: both had the same blood pressure level but NO inhibition was moderate in one group (group M) and severe in the other (group S). Urinary excretion of nitrates and nitrites was lower in group S than in group M. Wall thickening and lipid deposition in renal vessels were significantly greater in group S than in groups M. Simple and multiple regression analyses indicated that renal vascular wall thickening was more strongly correlated with lipid deposition than with blood pressure. The number of vessels positive for staining with Sudan black B was negatively correlated with urinary NO excretion. Expression of fibronectin and transforming growth factor-beta was greater in the Sudan black B-positive than in the Sudan black B-negative vessels, suggesting that extracellular matrix production was increased in vessels with lipid deposition. Lipid deposition and increased production of extracellular matrix may contribute to renal vascular wall thickening in L-NAME-induced hypertension. Some mechanisms independent of hypertension play important roles in vascular wall thickening induced by NO inhibition.

Low Nitric Oxide Production in Patients with Chronic Renal Failure

Background: Rats with chronic renal failure have a low nitric oxide (NO) production and a diminished NO excretion. The supplementation of L-arginine has an inhibitory effect on the progression of renal insufficiency. Methods: The present study was designed to determine whether chronic renal failure patients have a low NO production. Plasma and urine nitrate (NO₃) and nitrite (NO₂), stable metabolites of NO, were measured in 83 consecutive patients with chronic renal failure. The 83 chronic renal failure patients were divided into three groups: group 1, mild renal failure (creatinine clearance >60 ml/min/1.73 m²); group 2, moderate renal failure (creatinine clearance >30 <60 ml/min/1.73 m²), and group 3, severe renal failure (creatinine clearance <30 ml/min/1.73 m²). Thirty-three healthy volunteers served as controls. Results: The daily urinary NO excretion was significantly lower in patients with moderate and severe renal failure as compared with those with mild renal failure and normal controls. The lowest values were found in the severe renal failure group. When the 24-hour urinary NO excretion or NO per milligram creatinine and the NO clearance were correlated with the renal function in all patients as a group, these parameters were directly correlated with the creatinine clearance and inversely correlated with the serum creatinine level. The plasma NO concentration was not different between the three chronic renal failure groups, but higher than in the controls. Plasma NO in renal failure patients was not correlated with the creatinine clearance or serum creatinine levels. Conclusions: Chronic renal failure is a state of NO deficiency. Treatment strategies to increase NO production (L-arginine supplementation or other NO compounds) may prove to be useful in maintaining the renal function and slow the progression of renal disease.

Garlic prevents hypertension induced by chronic inhibition of nitric oxide synthesis

It has been reported that garlic activates nitric oxide synthase in vitro and that chronic inhibition of nitric oxide (NO) synthesis by N ω-nitro-L-arginine-methyl-ester (L-NAME) induces arterial hypertension in rats. In this work, we studied the effect of oral administration of L-NAME for 4 weeks on control and garlic-fed rats. Basal systolic blood pressure was recorded 4 weeks after garlic supplementation, and on weeks 1, 2, 3, and 4 after L-NAME treatment. At the end of the study, the in vivo NO production was evaluated indirectly by measuring the urinary excretion of the stable end products of NO metabolism, nitrite (NO 2 .) and nitrate (NO 3 .). It was found that L-NAME induced arterial hypertension on weeks 1–4 in control rats but not in garlic-fed rats, whose blood pressure remained essentially as the basal values. Also, during this time period, blood pressure remained unchanged in garlic-fed rats without L-NAME treatment. Urinary excretion of NO 2 − NO 3 − decreased in L-NAME-treated rats, increased in garlic-fed rats, and remained unchanged in garlic-fed rats treated with L-NAME. It was concluded that garlic blocks the L-NAME-induced hypertension by antagonizing in vivo the inhibitory effect of L-NAME on NO production.

Dietary calcium inhibits the intestinal colonization and translocation of Salmonella in rats

BACKGROUND & AIMS: Dietary calcium decreases the cytotoxicity of intestinal contents and intestinal epitheliolysis by precipitating cytotoxic surfactants such as bile acids. A decreased luminal cytotoxicity might not only strengthen the barrier function of the gut mucosa but also reinforce the protective, endogenous microflora. We hypothesized, therefore, that dietary calcium increases the resistance to intestinal infections.METHODS: Rats on a low-, medium-, or high- calcium purified diet were orally infected with a single dose of Salmonella enteritidis. The kinetics of fecal Salmonella excretion was studied to determine the colonization resistance. Intestinal bacterial translocation was quantitated by measuring urinary oxidation products of nitric oxide (NOx) excretion and culturing bacteria from tissues.RESULTS: Compared with the low-calcium group, the medium- and high- calcium diet-fed rats had a substantially improved colonization resistance. Calcium supplementation also reduced translocation of Salmonella, considering the diminished urinary NOx excretion and viable Salmonella counts in the Ileal Peyer's patches and spleen. Dietary calcium decreased the bile acid concentration and cytotoxicity of fecal water. Several indicators of fecal bacterial mass were significantly increased by supplemental calcium.CONCLUSIONS: Dietary calcium improves the colonization resistance and reduces the severity of gut- derived systemic infections, which is probably attributable to its luminal cytoprotective effects.(Gastroenterology 1997 Aug;113(2):550-7)

EFFECTS OF SARAFOTOXIN S6c ON RENAL HAEMODYNAMICS AND URINE FORMATION IN ANAESTHETIZED DOGS

1. The effects of sarafotoxin S6c (S6c), a selective endothelin ETB receptor agonist, on renal haemodynamics and urine formation were examined in anaesthetized dogs. 2. Intrarenal arterial infusion of S6c at a rate of 1 or 5 ng/kg per min produced a transient increase in renal blood flow (RBF), with no change in systemic blood pressure and heart rate; RBF then decreased gradually to below the basal value. There were significant and dose-dependent increases in urine flow and free water clearance and decreases in urine osmolality during S6c infusion, whereas urinary excretion of sodium and glomerular filtration rate (GFR) remained unchanged. Simultaneously, S6c administration elicited a marked increase in urinary excretion of nitric oxide (NO) metabolites, NO2- and NO3- (UNOxV). 3. In dogs simultaneously administered S6c (5 ng/kg per min) and NG-nitro-L-arginine (NOARG; 40 micrograms/kg per min), a NO synthase inhibitor, the renal vasodilator effect of S6c was abolished and marked reductions in RBF and GFR were observed. The S6c-induced diuretic action was not affected by NOARG. In the presence of NOARG, there was a small amount of UNOxV at the basal level and the administration of S6c did not increase UNOxV. 4. These results suggest that an intrarenal arterial infusion of S6c enhances the production of NO in the kidney and that this enhancement contributes to the peptide-induced renal vasodilation. In contrast, it is unlikely that S6c-induced water diuresis is related to NO production stimulated by this peptide.

Experimentally induced acute hyperinsulinemia stimulates endogenous nitric oxide production in humans: Detection using urinary [formula omitted] excretion

Insulin-mediated glucose metabolism in skeletal muscle is associated with a proportional increase in muscle perfusion. The vasodilatory effect of insulin is thought to be mediated in part by endothelium-derived nitric oxide (NO). The present study was performed to determine whether acute hyperinsulinemia has any stimulatory effect on endogenous NO production in humans. Bolus intravenous injection of insulin (0.1 IU/kg body weight) caused a significant increase in urinary excretion of NO 2 − NO 3 − together with a significant decrease in blood pressure, whereas saline infusion alone had no effect on these parameters. The increased NO response to insulin was almost comparable to that obtained with infusion of 30 g l-arginine. The acute effect of hyperinsulinemia on endogenous NO formation supports the concept that NO may mediate the vasodilatory action of insulin in humans.

Renal Nitric Oxide Synthase Activity is Altered in BioBreeding Diabetic Rats.• 1700

The BioBreeding diabetic (BB) rat, a model of insulin dependent diabetes mellitus, is resistant to diabetic nephropathy despite prolonged hyperglycemia. We have previously shown that this resistance is associated with increased urinary excretion of nitric oxide (NO) metabolites and maintenance of high glomerular filtration rates. To assess whether chronic NO synthesis inhibition contributes to the progression of nephropathy, we treated BB rats with nitro-L-arginine-methylester (L-NAME). We studied 4 groups: BioBreeding diabetic rats (BB)(n=12), D + 10 mg/kg L-NAME daily in drinking water (BB+NAME)(n=8), BioBreeding diabetes-resistant controls (C)(n=12), and C+ NAME (n=6). After 6 months of treatment, we measured systolic blood pressure(SBP: mmHg), plasma glucose (Pgluc: mg/dl), creatinine clearance(Ccr: ml/min × 100 g bw), urine protein excretion (UprotV: mg/day × 100 g bw), percent albumin in urine protein (%alb), urine NO metabolite excretion (UNOV: mmol/day × 100 g bw × 103): Table Kidneys were removed and renal NO synthase activity was assayed by conversion of [3H]L-arginine to[3H]L-citrulline (fmol[3H]/min × mg protein). NO synthase activity was similar in BB, BB+NAME and C+NAME (64±12, 66±9, 72.9±9, p=ns) but was significantly higher in C (94±23, p<0.05). Basal renal NO synthase activity in BB rats is low and does not change with chronic NO synthesis inhibition. The maintenance of normal renal function and high urinary NO metabolite excretion may be due to increased systemic rather than renal NO production.

Decreased urinary excretion of nitric oxide in acute rejection episodes in pediatric renal allograft recipients.

Acute renal allograft rejection continues to have a negative effect on graft survival despite a better understanding of the molecular basis of renal allograft rejection. Nitric oxide (NO) has important biological functions in cell defense and injury and some evidence exists that it may act as an immunomodulator in allograft transplantation. To determine if NO has any role in acute renal allograft rejection in pediatric patients, acute rejection episodes in pediatric renal transplant recipients were evaluated. Four out of eleven patients who received a renal allograft in 1995 at Children's Kidney Center at The Children's Hospital of Buffalo had eight episodes of acute rejection. One patient received a living-related and three received cadaveric grafts. Stable metabolites of NO (NO-2 + NO-3 = NOx) were measured in the serum and urine samples of the patients daily. Serum levels of NO did not change significantly during acute rejection episodes. Urinary NOx levels decreased by 73+/-9% of the baseline values during episodes of acute rejection: mean +/-SE urinary nitric oxide/creatinine ratio (NOx/Cr) of 0.17+/-0.05 at baseline vs. 0.05+/-0.01 during rejection (P=0.02). Successful treatment of acute rejection by administration of high dose i.v. steroids or OKT-3 induced acute rises in the urinary NO levels to baseline values: NO/Cr = 0.17+/-0.04 (mean +/-SE). We conclude that urinary NO excretion decreases significantly during acute renal allograft rejection and that NOx concentration in the urine increases in response to successful antirejection therapy.

Renal sodium handling in experimental diabetes: role of NO

Recent studies have suggested that diabetes is a state of increased renal nitric oxide (NO) activity as assessed by urinary excretion of nitrites and nitrates (NO x ), and that NO synthase inhibitors reverse the increased glomerular filtration rate (GFR) observed in experimental diabetes. In addition to being a potent vasodilator in the renal vasculature, NO also plays a role in modulation of renal sodium excretion. To explore the role of NO in diabetes-associated alterations in renal excretory function, renal haemodynamic and sodium handling parameters were evaluated in conscious control (C) and streptozotocin diabetic rats (D) and correlated to the renal activity of NO, as assessed by urinary excretion of its metabolites NO x . To further explore this issue, the changes in renal haemodynamics and sodium handling were also assessed after NO synthase inhibition with a non-pressor dose of L-nitro-arginine-methyl-ester (L-NAME) and after administration of the NO donor, glyceryl trinitrate (GTN). Systolic blood pressure was not different between C and D rats. D rats exhibited marked hyperglycaemia ( P <0.001), and increases in GFR ( P <0.001), renal plasma flow, filtration fraction, urinary sodium excretion (U Na V, P <0.001), filtered load of sodium (FL Na , P <0.01), and a decrease in fractional reabsorption of sodium (FR Na , P <0.0001). In contrast, total reabsorption of sodium (TR Na ) was increased in D rats compared to C rats ( P <0.0001). The urinary excretion of NO was markedly increased in D rats ( P <0.01). Regression analyses performed in D rats revealed a close relationship between U Na V and GFR and a weaker correlation with urinary NO x . Although FR Na correlated only with urinary excretion of NO x , there was a strong relationship between TR Na and GFR. In contrast to D rats, control rats demonstrated only a relationship between TR Na and GFR and no other correlations were found, in D rats, NO inhibition with L-NAME (1 mg/kg body weight) resulted in a marked decrease in GFR and urinary NO x associated with decreases in FL Na and TR Na but did not influence FR Na . In contrast, in C rats the post-L-NAME decrease in NO x was not associated with significant changes in GFR and renal sodium handling. GTN-treated C rats exhibited a renal vasodilatory response and an increase in natriuresis and urinary NO x whereas no renal changes were observed in D rats during GTN administration. The present data indicate that changes in renal sodium handling before and after NO modulation in experimental diabetes are related to changes in GFR rather than to the renal activity of NO. Therefore, in contrast to the effects on renal haemodynamics, NO does not play an important role in the altered renal sodium handling observed in experimental diabetes.

Comparison of acute rapamycin nephrotoxicity with cyclosporine and FK506

Acute cyclosporine (CsA) nephrotoxicity is characterized by a reduction of glomerular filtration rate (GFR), hypomagnesemia and tubular injury. The mechanisms of CsA's immunosuppressive action and presumably its nephrotoxicity are mediated through inhibition of the renal phosphatase, calcineurin. FK506 (FK), which has a different chemical structure and binding immunophilin, also inhibits calcineurin. We compared the renal effects of these drugs to those of rapamycin (RAPA), which although similar in structure and intracellular binding to FK, does not work by changing calcineurin activity. Rats were given CsA (15 mg/kg/s.c.), FK (6 mg/kg/p.o.), RAPA (3 mg/kg/p.o.) or vehicle (V) for two weeks on a low salt diet. CsA and FK strikingly decreased urinary excretion of nitric oxide, renal blood flow and GFR, whereas RAPA did not. In contrast, all these three drugs caused significant hypomagnesemia associated with inappropriately high fractional excretion of magnesium, suggesting renal magnesium wasting. In addition, with all three drugs there were lesions in the rat kidneys consisting of tubular collapse, vacuolization and nephrocalcinosis. We thus showed that only the calcineurin inhibitors produced glomerular dysfunction in an acute experimental model of nephrotoxicity. The mechanism of hypomagnesemia and tubular injury induced by all three immunosuppressive drugs is unclear but may be independent of calcineurin. The mechanism of renal vasoconstriction on the other hand may be related to inhibition of calcineurin.

Supplementation of hypercholesterolaemic rabbits with L-arginine reduces the vascular release of superoxide anions and restores NO production

L-arginine, the precursor of endogenous nitric oxide (NO), has been shown to enhance endothelial function and to reduce intimal plaque area in cholesterol (Chol)-fed rabbits. We have studied endogenous NO production in such animals in vitro (endothelium-dependent relaxations) and in vivo (assessed by urinary NO 3 − excretion) before and during chronic oral adminstration of L-arginine and an inhibitor of NO synthesis, L-NAME. Vascular superoxide anion (O 2 −) production of aortic rings was measured under basal conditions and following exposure to phorbol-myristate-acetate (PMA). Cholesterol feeding reduced endothelium-dependent relaxations and decreased urinary NO 3 − excretion. These effects were potentiated by administration of L-NAME. L-arginine partly restored endothelium-dependent relaxations and increased NO 3 − excretion. PMA-stimulated O 2 − production was increased in aortic rings from rabbits given cholesterol (+159 ± 28%; mean ± S.E.M.) or cholesterol + L-NAME (+149 ± 37%) as compared with controls (−22 ± 7%). In rabbits given cholesterol + L-arginine, O 2 − production was decreased to control levels (+14 ± 17%; P < 0.05). We conclude that the systemic synthesis of NO is impaired in cholesterol-fed rabbits, as indicated by the decreased urinary excretion of NO 3 −. Enhanced O 2 − production may further contribute to the decreased biological activity of NO in hypercholesterolaemia. L-arginine restores endothelial function in hypercholesterolaemia by enhancing NO production and by protecting NO from early breakdown by O 2 −.

780-2 Exercise Increases Systemic Nitric Oxide Production in Men

The effects of submaximal exercise on systemic nitric oxide (NO) formation, as assessed by measuring the urinary excretion rates of NO - 3 and of cyclic GMP, were compared in 10 endurance-trained and 6 untrained male subjects. Urinary excretion rates of NO - 3 (by gas chromatography) and cyclic GMP (by radioimmunoassay) were assesse before and after a 30 min exercise test dlt 60% of each subject's individual maximal work capacity in hourly intervals. Urinary NO - 3 excretion in untrained and trained subjects was comparable at rest (104 ± 35 vs. 110 ± 19 μmol/mmol creatinine, p = n.s.). It more than doubled during exercise in both groups (to 236 ± 88 μmol/mmol creatinine in untrained and to 252 ± 39 μmol/mmol creatinine in trained subjects, each p &lt; 0.01) and rapidly decreased to baseline within 2 hours after the test. Resting urinary cyclic GMP excretion was fourfold higher in the athletes than in the untrained subjects (21 ± 5 vs. 6 ± 1 nmol/mmol creatinine, p &lt; 0.05). During exercise it increased about twofold in both groups (to 52 ± 12 and 10 ± 1 nmol/mmol creatinine, respectively, each p &lt; 0.01), and returned to baseline after its end. There was a correlation between urinary cyclic GMP and NO - 3 excretion (p &lt; 0.05) in both groups. We conclude that acute submaximal exercise increases the formation of NO in man, as reflected by the increased urinary excretion of NO - 3 and cyclic GMP. This may contribute to the vasodilation during physical exercise, and at least partly explain the beneficial effects of physical training in patients with cardiovascular diseases.