Tetrahydrobiopterin improves aging-related impairment of endothelium-dependent vasodilation through increase in nitric oxide production

Deficiency of tetrahydrobiopterin (BH4), an essential cofactor for nitric oxide (NO) synthase, decreases NO production and increases reactive oxygen species. The purpose of this study was to elucidate the effects of aging on endothelial function and to determine whether the degree of BH4 deficiency is related to aging and oxidative stress. We evaluated forearm blood flow (FBF) responses to acetylcholine (ACh), an endothelium-dependent vasodilator, and isosorbide dinitrate (ISDN), an endothelium-independent vasodilator, before and after co-infusion of BH4 (500 mg/min) in 37 healthy men (mean age, 41+/-18 yr; range, 19-81 yr). FBF was measured using strain-gauge plethysmograph. Urinary 8-hydroxy-2'-deoxyguanosine (8-OHdG) and serum malondialdehyde-modified low-density lipoprotein (MDA-LDL) were measured as indices of oxidative stress. Both ACh and ISDN increased the FBF in a dose-dependent manner in all subjects. Co-infusion of BH4 resulted in a significant increase in ACh-induced vasodilation (from 22.3+/-6.7 to 30.1+/-7.5 mL/min/100 mL tissue, P<0.05). Aging was found to be significantly correlated with ACh-induced vasodilation (r=-0.47, P=0.006), urinary 8-OHdG (r=0.38, P=0.02), serum MDA-LDL (r=0.36, P=0.02), and the change in ACh-induced vasodilation after co-infusion of BH4 (r=0.45, P=0.007). The FBF response to ISDN did not correlate with any parameters. Infusion of N(G)-monomethyl-L-arginine, an NO synthase inhibitor, abolished the BH4-induced enhancement of forearm vasorelaxation evoked by ACh. The increase in FBF after ISDN was not altered by BH4. These findings suggest that a deficiency of BH4 may be involved in the pathogenesis of disturbances in endothelium-dependent vasodilation related to aging through decrease in NO production and increase in oxidative stress.

Accumulated endogenous nitric oxide synthase inhibitors in inhibiting urethral relaxation following estrogen supplementation in ovariectomized rabbits

PURPOSE

We investigated the possible role of the endogenous nitric oxide (NO) synthase (NOS) inhibitors N-monomethyl-L-arginine (L-NMMA) and asymmetrical N, N-dimethyl-L-arginine (ADMA) in inhibiting urethral relaxation following estrogen supplementation in ovariectomized rabbits.

MATERIALS AND METHODS

A total of 16 mature Japanese White female rabbits were divided into 2 groups. In the control group rabbits were sacrificed 2 weeks after bilateral ovariectomy. In the estrogen group estradiol was administered subcutaneously for 2 weeks with the aid of sustained release pellet from 2 weeks after ovariectomy until sacrifice. Isolated urethra was cut into transverse strips for functional study and processed to determine endogenous NOS inhibitors, NOS activity, dimethylarginine dimethylaminohydrolase (DDAH) activity as a metabolizing enzyme of endogenous NOS inhibitors and cyclic guanosine monophosphate production.

RESULTS

Electrical field stimulation produced NO mediated and neurogenic relaxation of the urethral strip in the presence of guanethidine and atropine under contraction with phenylephrine. Relaxation was significantly decreased in the estrogen group and accompanied by decreased cyclic guanosine monophosphate production. Sodium nitroprusside induced relaxation was not different between the 2 groups. The content of L-NMMA plus ADMA in the urethra was significantly increased in the estrogen group. Ca dependent NOS activity in the urethra remained unaffected. DDAH activity was significantly lower in the estrogen group.

CONCLUSIONS

Estrogen supplementation leads to decreased NO mediated and neurogenic urethral relaxation through the accumulation of L-NMMA and ADMA in the urethra. The accumulation of NOS inhibitors is possibly brought about by impaired DDAH activity.

Endogenous methylarginines regulate neuronal nitric-oxide synthase and prevent excitotoxic injury

Nitric oxide (NO) has a critical role in neuronal function; however, high levels lead to cellular injury. While guanidino-methylated arginines (MA) including asymmetric dimethylarginine (ADMA) and N(G)-methyl-l-arginine (NMA) are potent competitive inhibitors of nitric oxide synthase (NOS) and are released upon protein degradation, it is unknown whether their intracellular concentrations are sufficient to critically regulate neuronal NO production and secondary cellular function or injury. Therefore, we determine the intrinsic neuronal MA concentrations and their effects on neuronal NOS function and excitotoxic injury. Kinetic studies demonstrated that the K(m) for l-arginine is 2.38 microm with a V(max) of 0.229 micromol mg(-1) min(-1), while K(i) values of 0.67 microm and 0.50 microm were determined for ADMA and NMA, respectively. Normal neuronal concentrations of all NOS-inhibiting MA were determined to be approximately 15 microm, while l-arginine concentration is approximately 90 microm. These MA levels result in >50% inhibition of NO generation from neuronal NOS. Down-modulation or up-modulation of these neuronal MA levels, respectively, dramatically enhanced or suppressed NO-mediated excitotoxic injury. Thus, neuronal MA profoundly modulate NOS function and suppress NO mediated injury. Pharmacological modulation of the levels of these intrinsic NOS inhibitors offers a novel approach to modulate neuronal function and injury.

[Biological and pathophysiological roles of endogenous methylarginines as inhibitors of nitric oxide synthase]

Protein arginine N-methyltransferases (PRMTs) catalyse the methylation of guanidinonitrogen(s) of arginine to produce NG-monomethyl-L-arginine (L-NMMA), asymmetric NG,NG-dimethyl-L-arginine (ADMA) and symmetric NG,NG-dimethyl-L-arginine (SDMA), which are subsequently released into the cytoplasm following proteolysis. Free intracellular L-NMMA and ADMA, but not SDMA, are inhibitors of all three isoforms of nitric oxide synthases (nNOS, eNOS and iNOS). L-NMMA and ADMA, but not SDMA, are actively metabolized by dimethylarginine dimethylaminohydrolase (DDAH) to L-citrulline and methylamine (and dimethylamine). Free methylarginines are detectable in cell cytosol, plasma and tissues. Elevated ADMA has been detected in the plasma of patients or experimental animals with hypercholesterolemia, renal failure, atherosclerosis, hypertension, thrombotic microangiopathy, peripheral arterial occlusive disease and in the regenerated endothelial cells after angioplasty. Moreover, in the non-cardiovascular field, ADMA was increased in the urethral tissue following ischemia and in the plasma of patients with schizophrenia and multiple sclerosis. Altered biosynthesis of NO has been implicated in the pathogenesis of these diseases, and it is possible to consider that the accumulation of endogenous L-NMMA and ADMA underlies the impaired NO generation and increased O2- production. We described herein the biosynthesis, transmembrane transport, metabolic pathway and possible pathophysiological roles of endogenous methylarginines.

Helicobacter pylori stimulates pepsinogen secretion from isolated human peptic cells

BACKGROUND

Different acid and peptic related gastroduodenal diseases are associated with both increased gastric secretion and Helicobacter pylori infection. Patients with H pylori associated gastritis or duodenal ulcer have increased serum pepsinogen levels which decrease after eradication. The mechanisms of H pylori induced gastric mucosal damage are not completely understood.

AIM

To determine the effects of H pylori on pepsinogen secretion from isolated human peptic cells.

METHODS

Dispersed human peptic cells were prepared from endoscopically obtained biopsy specimens after collagenase digestion, mechanical disruption, and density gradient centrifugation. H pylori was obtained from gastric biopsies (antrum and body), and cultured in non-selective and selective media. Isolates of H pylori were used at different concentrations (1 - 20 x 10(6) colony forming units (cfu)).

RESULTS

H pylori (10(6) - 2 x 10(7) cfu) increased basal pepsinogen secretion in a concentration dependent manner. This stimulus was not observed with Escherichia coli. The increased secretion was in addition to that observed with 0.1 mM histamine and 0.1 mM dibutyryl-cyclic adenosine monophosphate. However, H pylori did not affect either carbamylcholine (0.1-10 microM) or cholecystokinin (1 microM) stimulated pepsinogen secretion. Addition of the nitric oxide synthase inhibitor N(w)-monomethyl-L-arginine (1 mM) inhibited H pylori induced cGMP generation and pepsinogen secretion, which were also reduced in the absence of extracellular calcium. H pylori induced pepsinogen secretion was not affected by the absence/presence of the cagA gene.

CONCLUSIONS

H pylori increases pepsinogen secretion from human peptic cells through a calcium and nitric oxide mediated intracellular pathway. This effect is independent of the H pylori virulent cagA gene, and may be a mechanism of H pylori induced gastric mucosal damage.

Expression of nitric oxide synthases and effects of L-arginine and L-NMMA on nitric oxide production and fluid transport in collagenous colitis

BACKGROUND AND AIMS

Luminal nitric oxide (NO) is greatly increased in the colon of patients with collagenous and ulcerative colitis. To define the source and consequence of enhanced NO production we have studied expression of NO synthase (NOS) isoforms and nitrotyrosine in mucosal biopsies from these patients. In addition, effects on colonic fluid transfer caused by manipulating the substrate of NOS were studied in patients with collagenous colitis.

PATIENTS

Eight patients with collagenous colitis, nine with active ulcerative colitis, and 10 with uninflamed bowel were included.

METHODS

Expression of NOS isoforms was quantified by western blotting. Inducible NOS (iNOS) and nitrotyrosine were localised by immunohistochemistry. Modulation of NOS activity by topical N(G)-monomethyl-L-arginine (L-NMMA) or L-arginine was assessed during perfusion of whole colon. Plasma and perfusate nitrite/nitrate (NOx) was measured by Griess' reaction.

RESULTS

Both in collagenous and ulcerative colitis, expression of iNOS was 10(2)-10(3) higher (p<0.001) than in uninflamed bowel and localised primarily to the epithelium. Endothelial NOS was evenly expressed in all groups while neuronal NOS was undetectable. Nitrotyrosine was markedly expressed in active ulcerative colitis but rarely detected in collagenous colitis and never in uninflamed bowel. In collagenous colitis, the output of NOx was markedly increased compared with uninflamed bowel (283 (58) v <37 nmol/min; p<0.01) and fluid was net secreted. L-NMMA reduced the output of NOx by 13-66% (95% confidence intervals) and secretion of fluid by 25-109% whereas L-arginine increased the output of NOx by 3-39% and secretion of fluid by 15-93%.

CONCLUSIONS

In collagenous colitis, as opposed to ulcerative colitis, upregulation of iNOS occurs in the absence of nitrotyrosine formation and mucosal damage. Excess generation of NO may be the primary cause of diarrhoea in this condition.

A possible role of endogenous inhibitor for nitric oxide synthesis in the bovine ciliary muscle

The present experiments were designed to investigate the possible role of endogenous methylarginine derivatives such as NG-monomethyl-L-arginine, asymmetrical NG,NG-dimethyl-L-arginine and symmetrical NG,N'G-dimethyl-L-arginine for the nitric oxide synthesis in the bovine ciliary muscle. The contents of asymmetrical NG,NG-dimethyl-L-arginine and symmetrical NG,N'G-dimethyl-L-arginine in the bovine ciliary muscle were determined to be 370.2 +/- 27.6 (n = 5) and 182.4 +/- 22.9 (n = 5) pmoles g-1 wet weight, respectively by means of the automated high-performance liquid chromatography. NG-Monomethyl-L-arginine was below the assay limits. On the basis of the total tissue water content (0.792 +/- 0.006 ml g-1 wet weight, n = 14), the concentrations of asymmetrical NG,NG-dimethyl-L-arginine and symmetrical NG,N'G-dimethyl-L-arginine were tentatively estimated to be (4.7 +/- 0.3) x 10(-7) M (n = 5) and (2.3 +/- 0.3) x 10(-7) M (n = 5), respectively. A23187 (10(-7)-3 x 10(-4) M) produced a concentration-dependent relaxation of the ciliary muscle strips which had been contracted with 10(-5) M carbachol. Authentic asymmetrical NG,NG-dimethyl-L-arginine (3 x 10(-6)-3 x 10(-4) M), but not symmetrical NG,N'G-dimethyl-L-arginine (3 x 10(-4) M), inhibited the 10(-6) M A23187-induced relaxation in a concentration-dependent manner. The inhibition with asymmetrical NG,NG-dimethyl-L-arginine (10(-4) M) was reversed by an addition of 3 x 10(-3) M L-arginine, but not by 3 x 10(-3) M D-arginine. The A23187 (10(-6) M)-induced relaxation was enhanced by 3 x 10(-3) M L-arginine or superoxide dismutase (50 U ml-1), whereas it was inhibited by carboxy-PTIO (3 x 10(-4) M), a scavenger of nitric oxide, or methylene blue (10(-5) M), an inhibitor of guanylate cyclase. The carbachol-induced contraction was enhanced by asymmetrical, NG,NG-dimethyl-L-arginine (10(-5) M) and inhibited by 3 x 10(-3) M L-arginine. Any effect of prostanoid formation during the A23187-induced relaxation was ruled out by using indomethacin (10(-5) M). Sodium nitroprusside (10(-5) M), a donor of nitric oxide, also produced a relaxation, which was inhibited by methylene blue (10(-5) M) or carboxy-PTIO (3 x 10(-4) M) and was augmented by superoxide dismutase (50 U ml-1), but unaffected by asymmetrical NG,NG-dimethyl-L-arginine (3 x 10(-4) M) or L-arginine (3 x 10(-3) M). These results lead us to speculate that the nitric oxide synthesized endogenously from L-arginine may play a role for mediating relaxation of the bovine ciliary muscle and that the endogenous asymmetrical NG,NG-dimethyl-L-arginine may be involved in inhibiting the biosynthesis of nitric oxide when there are increased intracellular concentrations of the methylarginine under certain circumstances.