Nitric oxide synthases in mammals

Nitric oxide synthase in the human pituitary gland

Nitric oxide (NO) is generated by the NO synthase family of isozymes, which is present in many mammalian cells. The constitutive NO synthase isozymes generate NO, which acts via signal transduction mechanisms in the regulation of many functions including vascular tone and blood pressure, and the inducible isozymes mediate immunological mechanisms by cytotoxic and cytostatic effects. To determine whether NO has a role in anterior pituitary cell function, immunohistochemistry and in situ hybridization analyses were used to study NO synthase expression in normal and neoplastic human pituitary tissues. Brain NO synthase was localized in the anterior pituitary in secretory and in folliculo-stellate cells and in the posterior pituitary. Pituitary adenomas had higher levels of brain NO synthase protein and mRNA compared with normal pituitaries. Endothelial NO synthase was also present in anterior and posterior pituitary cells and in endothelial cells of the pituitary. Immunoblotting studies with brain NO synthase antibodies detected a slowly migrating approximately 155-kd band and more rapidly migrating approximately 90-kd and approximately 60-kd bands. Endothelial NO synthase, but not macrophage NO synthase, was also detected in the pituitary by immunoblotting studies, confirming the immunohistochemical observations. These findings indicate that NO synthase is expressed in normal and neoplastic human pituitary tissues with increased levels of brain NO synthase protein and mRNA in adenomas compared with non-neoplastic pituitary cells and suggest that NO may play a regulatory role in hormone secretion in anterior pituitary cells.

Differential regulation of cyclic GMP levels in the frontal cortex and the cerebellum of anesthetized rats by nitric oxide: an in vivo microdialysis study

A microdialysis method combined with a sensitive radioimmunoassay was used to monitor extracellular cyclic GMP (cGMP) levels in the frontal cortex and the cerebellum of anesthetized rats in vivo. Basal cGMP release remained constant throughout the perfusion period and was approximately 2 fmol/30 min in the frontal cortex and approximately 4 fmol/30 min in the cerebellum. The nitric oxide (NO) donor sodium nitroprusside (SNP) stimulated cGMP release transiently in both regions. However, the maximal response was 3-fold in the frontal cortex (obtained with 5 microM SNP) but 90-fold in the cerebellum (obtained with 1 mM SNP). Perfusion with the NO synthase (NOS) inhibitor NG-nitro-L-arginine methyl ester (L-NAME) suppressed cerebellar cGMP release by 74% indicating that NO is the major regulator of basal cGMP levels in the cerebellum. Quite opposite, L-NAME exhibited no potency in the frontal cortex suggesting that other activators of guanylyl cyclase may regulate basal cortical cGMP levels in vivo.

Expression of rat brain nitric oxide synthase in baculovirus-infected insect cells and characterization of the purified enzyme

Rat brain nitric oxide synthase was expressed to a high level in baculovirus-infected insect cells and purified to apparent homogeneity by affinity chromatography. The enzyme had a specific activity of approximately 1 mumol of citrulline.min-1.mg of protein-1 and contained 0.93, 0.45, 0.18 and 0.23 mol of haem, (6R)-5,6,7,8-tetrahydro-L-biopterin (H4biopterin), FAD and FMN per mol of subunit respectively.

Cross-talk between cyclooxygenase and nitric oxide pathways: prostaglandin E2 negatively modulates induction of nitric oxide synthase by interleukin 1

The inflammatory cytokine interleukin 1 beta (IL-1 beta) induces both cyclooxygenase (COX) and nitric oxide synthase (NOS) with increases in the release of prostaglandin (PG) and nitric oxide (NO) by mesangial cells. Recently, activation of the COX enzyme by NO has been described. However, the effects of COX products (PGs) on the NO pathway have not been fully clarified. Thus we determined the effect of COX inhibition and exogenous PGs on NO production and NOS induction in rat mesangial cells. A COX inhibitor, indomethacin, enhanced IL-1 beta-induced steady-state level of the inducible NOS (iNOS) mRNA and nitrite production. The effect of indomethacin was dose dependently reversed by the replacement of endogenous PGE2 with exogenous PGE2, which is the predominant product of the COX pathway in rat mesangial cells. In contrast to PGE2, a stable analog of PGI2, carba prostacyclin, enhanced IL-1 beta-induced iNOS mRNA levels and nitrite production. Forskolin, an activator of the adenylate cyclase, mimicked the effect of carba prostacyclin but not PGE2. These data suggest that (i) endogenous PGE2 downregulates iNOS induction, (ii) this inhibitory effect of PGE2 on iNOS induction is not mediated by activation of adenylate cyclase, and (iii) exogenous PGI2 stimulates COX induction possibly by activation of adenylate cyclase.

Inhibition of sympathetic vasoconstriction is a major principle of vasodilation by nitric oxide in vivo

The objective of this study was to determine whether vasodilator effects of nitric oxide (NO) can be explained by the inhibition of vasoconstriction caused by peripheral sympathetic nerve activity (SNA) in vivo. For this purpose, we studied the effects of systemic inhibition of NO synthesis during experimental variation of SNA in anesthetized cats. Intravenous infusion of NG-nitro-L-arginine methyl ester (L-NAME, 10 mg/kg) in baroreceptor-intact animals (n = 6) caused increases in mean arterial blood pressure (MAP) from 105.8 +/- 3.4 to 192.0 +/- 4.3 mm Hg that were associated with slight decreases in preganglionic SNA recorded from the white ramus of the third thoracic segment. Higher SNA appeared in completely baroreceptor-denervated cats (n = 10) than in the intact cats, but no changes in nerve activity occurred after the subsequent administration of L-NAME. In contrast, MAP increased from 123.3 +/- 4.0 to 245.8 +/- 5.1 mm Hg. In baroreceptor-denervated cats, reversible suppression of peripheral SNA produced by cooling of the ventral surface of the rostral ventrolateral medulla oblongata (RVLM) caused significant hypotension (61.1 +/- 2.6 mm Hg) and almost completely reversed the hypertension caused by L-NAME (76.0 +/- 3.7 mm Hg). Intravenous administration of the alpha 1-adrenergic receptor antagonist prazosin after L-NAME reduced MAP to a similar extent. In contrast, hypertension induced by angiotensin II could not be reversed by RVLM cooling. The pressor effects of intravenously administered noradrenaline during RVLM cooling were markedly potentiated by L-NAME and attenuated by the NO-donor compound S-nitroso-N-acetylpenicillamine (SNAP).(ABSTRACT TRUNCATED AT 250 WORDS)

Tumor cells suppress cytokine-induced nitric-oxide (NO) production in cerebral endothelial cells

Nitric oxide (NO) produced by endothelial cells (EC) has been shown to exert cytotoxic activity on tumor cells. In order to analyze events involved in brain metastasis, the modulation of NO production in rat-brain-derived EC was investigated. NO release was increased by tumor necrosis factor-alpha (TNF-alpha), interferon-gamma (IFN-gamma), interleukin-1 beta, lipopolysaccharide or forskolin in EC219 cells, a rat-brain-microvessel-derived EC line. Dexamethasone decreased NO release by cytokine-activated EC219 cells. Tumor cells (DHD/K12/PROb, a rat colon-carcinoma cell line) were highly adherent to EC219 cells, and adhesion was not modified by TNF-alpha plus IFN-gamma, or by dexamethasone. Addition of tumor cells or tumor-cell-conditioned medium significantly inhibited NO release induced by any of the stimuli examined, but only if added during the initial phase of endothelial-cell activation. Tumor-derived suppression of NO release was also observed in primary cultures of cerebral EC. NO synthase (NOS) activity in cytosol extracts of the cerebral EC line was Ca(2+)-independent and required both NADPH and tetrahydrobiopterin. NOS activity was increased by TNF-alpha and IFN-gamma, and significantly reduced by tumor-cell-conditioned medium. These results suggest that rat colon-carcinoma cells may have developed a protective mechanism involving the release of (a) soluble factor(s) which inhibit(s) NO production by cerebral EC.

Localization of nitric oxide synthase activity in the human lower urinary tract and its correlation with neuroeffector responses

OBJECTIVES

The present study was designed to correlate the localization of nitric oxide synthase (NOS) activity to nerve-induced smooth muscle responses in the human lower urinary tract.

METHODS

Nerve-induced smooth muscle activity was studied in the human lower urogenital tract. NOS activity was studied by measurement of citrulline formation and guanylate cyclase activity.

RESULTS

Nerve-induced contractions in the human detrusor muscle, bladder neck, and prostatic urethra were not significantly enhanced by the NOS inhibitor N omega-nitro-L-arginine methyl ester (L-NAME). In the prostatic urethra, relaxations to transmural nerve stimulation were obtained after increase in tension. The relaxations were abolished by L-NAME and restored by L-arginine. Nerve-induced relaxations were occasionally obtained in the bladder neck, whereas nerve-induced relaxations were never obtained in the detrusor muscle. Citrulline formation was highest in the prostatic urethra, it was intermediate in the bladder neck, and it was less pronounced in the detrusor muscle. Guanylate cyclase activity was also highest in the prostatic urethra, whereas there was no significant difference in guanylate cyclase activity in the bladder neck and detrusor muscle.

CONCLUSIONS

The nerve-induced smooth muscle responses and the localization of NOS activity were in good agreement. Thus, in areas where marked relaxations to nerve stimulation were obtained, there was also a high NOS activity. The data suggest that nitric oxide is a mediator for the neurogenic dilation of the bladder neck and urethra during the micturition reflex.

Tetrahydrobiopterin is a limiting factor of nitric oxide generation in interleukin 1 beta-stimulated rat glomerular mesangial cells

Treatment of mesangial cells with recombinant human interleukin 1 beta (IL-1 beta) triggers the expression of a macrophage-type of nitric oxide (NO) synthase and the subsequent increase of cellular concentration of cGMP and nitrite production. Tetrahydrobiopterin (BH4) is an essential cofactor for NO synthase, and in the present study we investigated its impact on inducible NO synthesis in mesangial cells. Inhibition of GTP-cyclohydrolase I, the rate-limiting enzyme for BH4 synthesis, with 2,4-diamino-6-hydroxy-pyrimidine (DAHP) potently suppresses IL-1 beta-induced nitrite production and elevation of cellular cGMP levels. This inhibitory effect of DAHP is reversed by sepiapterin, which provides BH4 via the pterin salvage pathway. Most importantly, sepiapterin dose-dependently augments IL-1 beta-stimulated NO synthesis, indicating that the availability of BH4 limits the production of NO in cytokine-induced mesangial cells. N-acetylserotonin, an inhibitor of the BH4 synthetic enzyme sepiapterin reductase, completely abolishes IL-1 beta-stimulated nitrite production, whereas methotrexate, which inhibits the pterin salvage pathway, displays only a moderate inhibitory effect, thus suggesting that mesangial cells predominantly synthesize BH4 by de novo synthesis from GTP. In conclusion, these data demonstrate that BH4 synthesis is an absolute requirement for, and limits IL-1 beta induction of NO synthesis in mesangial cells. Inhibition of BH4 synthesis may provide new therapeutic approaches to the treatment of pathological conditions involving increased NO formation.

Nitric oxide synthase activity in the human urogenital tract

Nitric oxide (NO) has been suggested as a nonadrenergic non-cholinergic neurotransmitter in the urogenital tract and has previously been shown to have a smooth muscle relaxing effect in the urogenital organs both in various animals and in humans. It has been shown that NO is a mediator of the erection and the dilatation of the bladder neck and urethra. The aim of the study was to analyse nitric oxide synthase (NOS) activity in the human urogenital tract. NOS activity was measured by the conversion of L-[U-14C] arginine to L-[U-14C] citrulline. In the upper urinary tract there was Ca(2+)-dependent NOS activity in the renal pelvis, but no significant NOS activity could be found in the ureter. In the lower urinary tract we found high Ca(2+)-dependent NOS activity in the urethra, intermediate activity in the bladder neck and comparatively low activity in the detrusor muscle. In the male genital tract the testis and epididymis had no significant NOS activity. The vas deferens, prostate, seminal vesicle and corpus cavernosum were found to have high levels of Ca(2+)-dependent NOS activity. Ca(2+)-independent NOS activity was not obtained in the urogenital tract. Our results correspond well with previous functional studies indicating NO to be an important nerve-induced mediator of erection and in the micturition reflex, but also suggest that NO may be involved in several other functions in the human urogenital tract.

Insulin secretion in rats with chronic nitric oxide synthase blockade

Nitric oxide, which is produced from L-ar-ginine by a nitric oxide-synthase enzyme, has been shown to be a ubiquitous messenger molecule. Recently, it has been suggested that nitric oxide might influence insulin secretion by activating the soluble guanylate cyclase and generating cyclic guanosine monophosphate (cGMP). We have investigated the role of the nitric oxide pathway in insulin secretion by evaluating the insulin response to several secretagogues in rats in which nitric oxide-synthase was chronically inhibited by oral administration of the L-arginine analogue, NG-nitro-L-arginine methyl ester (L-NAME). Blood pressure and aortic wall cGMP content were used as indices of nitric oxide-synthase blockade. Insulin secretion was evaluated after an intravenous bolus of D-glucose, L-arginine or D-arginine. Chronic L-NAME administration induced a 30% increase in blood pressure and a seven-fold drop in arterial cGMP content. Body weight, fasting plasma glucose and insulin were not influenced by L-NAME administration. First-phase insulin secretion (1 + 3 min) in response to glucose was not significantly different in L-NAME and control rats. The areas under the insulin curve were similar in both groups. Insulin secretion in response to D-arginine or L-arginine in L-NAME-treated and control rats were also similar. In conclusion, chronic nitric oxide-synthase blockade increases blood pressure and decreases aortic cGMP content, but does not alter insulin secretion in response to several secretagogues. Chronic oral administration of L-NAME in the rat provides an adequate animal model for studying the L-arginine nitric oxide-pathway.

Identification of imidazole as L-arginine-competitive inhibitor of porcine brain nitric oxide synthase

Imidazole acts as a heme-site inhibitor of nitric oxide synthase (NOS). We used this compound to investigate whether the substrate L-arginine binds directly to the heme or to a separate domain of brain NOS. Enzyme kinetic experiments showed that imidazole enhanced the apparent Km for L-arginine without affecting maximal enzyme activity, and binding studies revealed that the inhibitor displaced the radioligand NG-nitro-L-[3H]arginine in a concentration-dependent fashion. These results demonstrate that imidazole exerts its effects on NOS in an L-arginine-competitive manner and that the substrate site of the enzyme may be identical with the prosthetic heme group.

Endothelial NOS and the blockade of LTP by NOS inhibitors in mice lacking neuronal NOS

Long-term potentiation (LTP) is a persistent increase in synaptic strength implicated in certain forms of learning and memory. In the CA1 region of the hippocampus, LTP is thought to involve the release of one or more retrograde messengers from the postsynaptic cell that act on the presynaptic terminal to enhance transmitter release. One candidate retrograde messenger is the membrane-permeant gas nitric oxide (NO), which in the brain is released after activation of the neuronal-specific NO synthase isoform (nNOS). To assess the importance of NO in hippocampal synaptic plasticity, LTP was examined in mice where the gene encoding nNOS was disrupted by gene targeting. In nNOS- mice, LTP induced by weak intensity tetanic stimulation was normal except for a slight reduction in comparison to that in wild-type mice and was blocked by NOS inhibitors, just as it was in wild-type mice. Immunocytochemical studies indicate that in the nNOS- mice as in wild-type mice, the endothelial form of NOS (eNOS) is expressed in CA1 neurons. These findings suggest that eNOS, rather than nNOS, generates NO within the postsynaptic cell during LTP.

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.

Induction of calcium-dependent nitric oxide synthases by sex hormones

We have examined the effects of pregnancy and sex hormones on calcium-dependent and calcium-independent nitric oxide synthases (NOSs) in the guinea pig. Pregnancy (near term) caused a > 4-fold increase in the activity of calcium-dependent NOS in the uterine artery and at least a doubling in the heart, kidney, skeletal muscle, esophagus, and cerebellum. The increase in NOS activity in the cerebellum during pregnancy was inhibited by the estrogen-receptor antagonist tamoxifen. Treatment with estradiol (but not progesterone) also increased calcium-dependent NOS activity in the tissues examined from both females and males. Testosterone increased calcium-dependent NOS only in the cerebellum. No significant change in calcium-independent NOS activity was observed either during pregnancy or after the administration of any sex hormone. Both pregnancy and estradiol treatment increased the amount of mRNAs for NOS isozymes eNOS and nNOS in skeletal muscle, suggesting that the increases in NOS activity result from enzyme induction. Thus both eNOS and nNOS are subject to regulation by estrogen, an action that could explain some of the changes that occur during pregnancy and some gender differences in physiology and pathophysiology.

Physiological concentrations of melatonin inhibit nitric oxide synthase in rat cerebellum

In the present paper we show the inhibitory effect of melatonin on rat cerebellar nitric oxide synthase (NOS) activity. NO production was monitored by the stoichiometric conversion of L-arginine to L-citrulline. The inhibitory effect of melatonin was dose-dependent, with an IC50 value of about 0.1 mM. However, a significant inhibition of enzyme activity (> 22%) was observed at 1 nM melatonin which is in the range of the physiological serum concentration of the hormone at night. The inhibitory effect of melatonin was observed exclusively in the presence of Ca++. Results suggest a new and important role of the pineal hormone melatonin on central nervous system processes, i.e., by modulating NO production.