Identification of an endothelial-like type III NO synthase in LLC-PK1 kidney epithelial cells

Decreased concentration of exhaled nitric oxide (NO) in patients with cystic fibrosis

Nitric oxide (NO) is produced by various cell types in the human respiratory tract. Endogenously produced nitric oxide is detectable in the exhaled air of healthy individuals. Exhaled NO has been shown to be increased in airway inflammation, most probably due to cytokine-mediated activation of NO synthases. To assess whether NO can serve as a marker of inflammation in cystic fibrosis (CF) lung disease, we measured exhaled NO in CF patients with a chemiluminescence analyser. Single breath measurements were performed in 27 stable CF patients (age range, 6-40 years) and 30 non-smoking controls (age range, 6-37 years). Exhaled NO concentrations were 9.1 +/- 3.6 ppb in the controls and 5.9 +/- 2.6 ppb (P < 0.001) in CF patients. To account for room air NO concentrations on the measurement of exhaled NO, we also calculated the difference between exhaled NO and ambient NO concentrations. Difference values were also significantly lower in CF compared with controls (P < 0.0001). In CF patients there was a positive correlation between exhaled NO and forced vital capacity (r = 0.43, P = 0.033), suggesting that exhaled NO is lower in patients with severe lung disease than in those with mild disease. We conclude that measurements of exhaled NO in CF does not reflect activity of CF airway inflammation. The decreased concentrations of exhaled NO may be due to inhibitory effects of inflammatory cytokines on NO syntheses in the airways and alveolar epithelial cells or to increased retention in airway secretions.

Intrarenal localization of nitric oxide synthase isoforms and soluble guanylyl cyclase

1. Nitric oxide (NO) plays an important role in the regulation of renal function. To date, five isoforms of NO synthase (NOS) and four subunits of soluble guanylyl cyclase have been cloned. The kidney contains four isoforms of NOS and all subunits of soluble guanylyl cyclase. 2. This review focuses on the intrarenal location of the isoforms of NOS and the subunits of soluble guanylyl cyclase.

Nitric oxide, the kidney and hypertension

1. According to the renal body fluid feedback mechanism for long-term control, persistent hypertension can only occur as a result of a reduction in renal sodium excretory function or a hypertensive shift in the pressure natriuresis relationship. Although an abnormal relationship between renal perfusion pressure and renal sodium excretion has been identified in every type of hypertension where it has been sought, factors responsible for this effect are still unclear. 2. Nitric oxide (NO) is produced within the kidney and plays an important role in the control of many intrarenal processes that regulate the renal response to changes in perfusion pressure and, thus, help determine systemic vascular volume and blood pressure. Numerous studies have shown that long-term inhibition of NO synthesis results in a chronic hypertensive shift in renal pressure natriuresis. 3. Recent studies have shown that certain animal models of genetic hypertension and forms of human hypertension areas are associated with a decrease in NO synthesis. Reductions in NO synthesis reduce renal sodium excretory function, not only through direct action on the renal vasculature, but through modulation of other vasoconstrictor processes and through direct and indirect alterations in tubular sodium transport. 4. The causes and consequences of the disregulation of NO in hypertension and other renal disease processes remain an important area of investigation.

Role of nitric oxide in acute renal failure

Nitric oxide synthase has been identified in several epithelial cells in the kidney, including proximal tubular cells, thick ascending limb, inner medullary collecting duct, and interstitial cells. Nitric oxide (NO) plays an important role in renal hemodynamics and sodium tubular transport. We have demonstrated that NO participates in hypoxia/reoxygenation (H/R) injury in isolated rat proximal tubules (PT) suspensions. In this in vitro model L-arginine addition enhanced H/R injury while L-NAME almost completely prevented injury. These effects were less intense in chronic supplemented rats with L-arginine and L-NAME, suggesting that NO synthase manipulation had interfered with PT susceptibility to H/R injury. In contrast, L-arginine protected IMCD cells in culture from hypercholesterolemic rats against hypoxia. Moreover, acute infusion of L-arginine before bilateral renal artery clamping was protective while L-arginine chronic administration and L-NAME were deleterious in this ARF model. The L-arginine protection was not observed in unilateral renal clamping plus contralateral nephrectomy in normal rats, but L-arginine was protective in hypercholesterolemic rats. Taken together, these results suggest that the net effect of NO stimulation is variable, and that it is the result of a balance between beneficial hemodynamic effects and cytotoxicity.

Subunit interactions of endothelial nitric-oxide synthase. Comparisons to the neuronal and inducible nitric-oxide synthase isoforms

Endothelial nitric-oxide synthase (eNOS) is comprised of two identical subunits. Each subunit has a bidomain structure consisting of an N-terminal oxygenase domain containing heme and tetrahydrobiopterin (BH4) and a C-terminal reductase domain containing binding sites for FAD, FMN, and NADPH. Each subunit is also myristoylated and contains a calmodulin (CaM)-binding site located between the oxygenase and reductase domains. In this study, wild-type and mutant forms of eNOS have been expressed in a baculovirus system, and the quaternary structure of the purified enzymes has been analyzed by low temperature SDS-PAGE. eNOS dimer formation requires incorporation of the heme prosthetic group but does not require myristoylation or CaM or BH4 binding. In order to identify domains of eNOS involved in subunit interactions, we have also expressed eNOS oxygenase and reductase domain fusion proteins in a yeast two-hybrid system. Corresponding human neuronal NOS (nNOS) and murine inducible NOS (iNOS) fusion proteins have also been expressed. Comparative analysis of NOS domain interactions shows that subunit association of eNOS and nNOS involves not only head to head interactions of oxygenase domains but also tail to tail interactions of reductase domains and head to tail interactions between oxygenase and reductase domains. In contrast, iNOS subunit association involves only oxygenase domain interactions.

Local secretion of nitric oxide and the control of mammary blood flow

Our objective was to test the hypothesis that local production of the vasorelaxant nitric oxide could regulate mammary blood flow. In four lactating Saanen goats, the response of mammary blood flow to intraarterial infusion of the nitric oxide donor diethylamine NONOate and the inhibitor of nitric oxide synthesis N omega-nitro-arginine was measured. Diethylamine NONOate induced a rapid and sustained increase of mammary blood flow in the infused gland only, suggesting a direct effect on vasculature of the mammary gland. In contrast, infusion of N omega-nitro-arginine decreased mammary blood flow by up to 35%, and the coinfusion of arginine, the nitric oxide precursor, with N omega-nitro-arginine markedly reduced its ability to decrease mammary blood flow. The distribution of nitric oxide synthase was investigated in cryosections of caprine and bovine mammary tissue by histochemical staining for NADPH-diaphorase activity and by immunocytochemistry using specific antibodies against two nitric oxide synthase isoforms. Both techniques revealed nitric oxide synthase in the vascular endothelium and secretory epithelium of the two species. Only antibodies against nitric oxide synthase-III showed specific staining. These results suggest that the mammary gland produces and responds to nitric oxide and, further, raise the possibility that the epithelium may control its own blood supply by secreting nitric oxide.

Angiotensin II-induced renal responses in anesthetized rabbits: effects of N omega-nitro-L-arginine methyl ester and losartan

Intrarenal arterial infusion of angiotensin II (4 ng/kg/min) reduced renal blood flow, glomerular filtration rate and urinary Na+ excretion (UNaV) without affecting fractional Na+ excretion (FENa) in anesthetized rabbits. Losartan (10 micrograms/kg/min) abolished these angiotensin II-induced renal responses. The renal blood flow, glomerular filtration rate and UNaV responses were potentiated during intrarenal arterial infusion of N omega-nitro-L-arginine methyl ester (L-NAME, 10 micrograms/kg/min). A high dose of L-NAME (50 micrograms/kg/min) also potentiated the renal blood flow and UNaV responses but not the glomerular filtration rate response. Angiotensin II reduced FENa during L-NAME infusion at either dose. In L-NAME-pretreated rabbits, losartan abolished the angiotensin II-induced renal blood flow and glomerular filtration rate responses, but the reduction in FENa still remained. The present study suggests that in the rabbit kidney (1) nitric oxide attenuates the angiotensin II-induced (angiotensin AT1 receptor-mediated) vasoconstriction and (2) angiotensin II can evoke losartan-resistant tubular Na+ reabsorption, but the tubular action is concealed by nitric oxide.

Regulation of blood flow in the mammary microvasculature

Although milk yield of cows and goats is known to be closely related to the total flow of blood through the udder, a number of studies suggest that milk yield can vary independently. No studies have attempted to measure the proportion of total flow that is nutritive. Within the mammary gland, capillary networks form a basket-like architecture surrounding each alveolus. Notably, flow in individual capillaries is not constant and varies among capillaries. Capillary flow (measured by intravital microscopy) was decreased by oxytocin, which generally increased total flow in the mammary artery, suggesting that the proportion of total flow that is nutritive can vary. In addition to classic metabolic regulators (e.g., carbon dioxide and oxygen) of tissue blood flow, the mammary gland produces a number of vasodilatory compounds, including parathyroid hormone-related protein, insulin-like growth factor-I, prostacyclin, nitric oxide, and endothelin. All of these compounds have been shown to alter mammary blood flow. Mammary tissue also contains kallikrein and angiotensin-converting enzyme, which convert circulating kinins and angiotensin, respectively, into potent vasoactive compounds. A number of these compounds are produced by epithelial cells themselves, providing a mechanism for the functioning epithelium to control its own blood supply and, hence, nutrient flow for milk synthesis. In this review, we examine the nature of the mammary microcirculation, its behavior under different conditions, and some of the regulatory features of the mammary microvasculature.

Renal nitric oxide synthases in transgenic sickle cell mice

The alpha H beta S [beta MDD] mouse is a useful model for studying renal functional abnormalities in sickle cell disease. We previously reported that these mice develop a urine concentrating defect when chronically exposed to a low oxygen environment. In the present study, we measured glomerular filtration rate (GFR), urinary excretion of NO2 s+ NO3, the stable products of nitric oxide (NO), and the abundance of endothelial constitutive nitric oxide synthase (NOS III) and inducible nitric oxide synthase (NOS II) in the kidneys by Western blot. Immunohistochemistry was also carried out. We found that GFR is significantly higher in the transgenic mice than in controls. The urinary NO2 + NO3/creatinine ratio was also higher. The Western blots revealed that both NOS III and NOS II are markedly increased in the kidneys of transgenic mice as compared to normal control mice. Immunohistochemistry localized NOS III reactivity in proximal convoluted cells in the cortex of control and alpha H beta S [beta MDD] mice. NOS II immunostaining was not seen in control mice but was clearly evident in glomeruli and distal nephron segments of the alpha H beta S [beta MDD] mice. These observations suggest that NOS II is induced in glomeruli and distal nephrons of the alpha H beta S [beta MDD] mice. An increase in synthesis of NO may occur in the glomeruli as a result of NOS II induction, and this may contribute to the hyperfiltration in these mice.

Expression of nitric oxide synthase in mucosal cells of the canine colon

The expression of nitric oxide synthase (NOS) in the mucosa of the canine colon was investigated with in situ hybridzation, immunohistochemistry (using isoform specific antibodies), western analysis, and NADPH diaphorase (NADPH-d) histochemistry. In situ hybridization using a common probe for known isoforms of NOS showed that NOS mRNA was strongly expressed in mucosal cells. A gradient in the degree of hybridization was noted from the base of the crypts to the luminal surface. This gradient was also apparent using an endothelial NOS (eNOS)-specific probe. Neural NOS-like immunoreactivity (nNOS-LI) was observed in columnar epithelial cells, and the same population of cells was stained with NADPH-d. Endothelial NOS-like immunoreactivity (eNOS-LI) was also found in mucosal cells; however, this eNOS-LI was confined to mucous cells. These cells were not stained with NADPH-d. The existence of eNOS in mucosal cells was confirmed by in situ hybridization using the probe which specifically hybridized with mRNA of eNOS and by western blots which demonstrated the expression of a 135-kDa protein in mucosal homogenates. The differential expression of NOS isoforms and the gradient in expression along the length of the crypts suggest complex roles for NO in the development of colonic epithelial cells and in secretion and transport functions of the colonic mucosa.

Expression of constitutive endothelial nitric oxide synthase in human blood platelets

Nitric oxide (NO) activates the soluble isoform of guanylate cyclase in platelets and inhibits platelet function. Several studies suggest the existence of a pathway for NO synthesis in platelets as a form of feedback inhibition, but the identity of the NO synthase (NOS) isoform present within platelets is unknown. We isolated human platelets, and synthesized cDNA from platelet RNA for analysis by PCR. Primers for human neuronal or inducible NOS failed to yield a PCR signal. However, primers specific for endothelial NOS (ecNOS) amplified a DNA band of the expected size. Analysis of nucleotide sequence revealed that the amplified DNA is ecNOS. NOS enzyme activity was detected in the platelet particulate subcellular fraction, as previously demonstrated for ecNOS in other cells. Thus, ecNOS is present in human platelets, and may play a role in the regulation of platelet function by an endogenous NO pathway.

Endogenous and exogenous nitric oxide inhibits norepinephrine release from rat heart sympathetic nerves

This study was designed to elucidate whether nitric oxide (NO) controls norepinephrine (NE) release from sympathetic nerves of the rat heart. Hearts were perfused in the Langendorff mode with Tyrode's solution. The right sympathetic nerve was stimulated with trains of 1 or 3 Hz and NE release was measured. The NO synthase (NOS) inhibitor NG-nitro-L-arginine (L-NNA) enhanced the evoked NE release in a concentration-dependent manner. This facilitation was independent of the increase in perfusion pressure and was stereospecifically reversed by L-arginine but not D-arginine. Another NOS inhibitor, NG-methyl-L-arginine, produced a similar increase in NE release. The NO-donor compound S-nitroso-N-acetyl-D,L-penicillamine, added in the presence of L-NNA, restored the suppression of NE release in a concentration-dependent fashion. A similar suppression was achieved with 3-morpholinosydnonimine. These results demonstrated that NE release is under the inhibitory control of endogenous NO. Western blots demonstrated the presence of neuronal NOS I and endothelial NOS III in the hearts. Perfusion of the hearts with a low concentration of the detergent CHAPS produced functional damage of the endothelium, as evidenced by an increase in perfusion pressure and a conversion of the acetylcholine-induced coronary vasodilation to a constriction. However, CHAPS treatment did not produce a facilitation of NE release (as did the NOS inhibitors), and L-NNA still increased NE release in CHAPS-treated hearts. Double-labeling immunofluorescence histochemistry showed NOS I immunoreactivity in stellate ganglion cells and in neurons of the heart, some of which also stained positive for tyrosine hydroxylase.(ABSTRACT TRUNCATED AT 250 WORDS)

Nitric oxide synthase: expression and expressional control of the three isoforms

Three isozymes of nitric oxide synthase (NOS) have been identified. Their cDNA- and protein structures as well as their genomic DNA structures have been described. NOS I (ncNOS, originally discovered in neurons) and NOS III (ecNOS, originally discovered in endothelial cells) are low output, Ca(2+)-activated enzymes whose physiological function is signal transduction. NOS II (iNOS, originally discovered in cytokine-induced macrophages) is a high output enzyme which produces toxic amounts of NO that represent an important component of the antimicrobial, antiparasitic and antineoplastic activity of these cells. Depending on the species, NOS II activity is largely (human) or completely (mouse and rat) Ca(2+)-independent. In the human species, the NOS isoforms I, II and III are encoded by three different genes located on chromosomes 12, 17 and 7, respectively. The amino acid sequences of the three human isozymes (deduced from the cloned cDNAs) show less than 59% identity. Across species, amino acid sequences are more than 90% conserved for NOS I and III, and greater 80% identical for NOS II. All NOS produce NO by oxidizing a guanidino nitrogen of L-arginine utilizing molecular oxygen and NADPH as co-substrates. All isoforms contain FAD, FMN and heme iron as prosthetic groups and require the cofactor BH4. NOS I and III are constitutively expressed in various cells. Nevertheless, expression of these isoforms is subject to regulation. Expression is enhanced by e.g. estrogens (for NOS I and III), shear stress, TGF-beta 1, and (in certain endothelial cells) high glucose (for NOS III). TNF-alpha reduces the expression of NOS III by a post-transcriptional mechanism destabilizing the mRNA. The regulation of the NOS I expression seems to be very complex as reflected by at least 8 different promoters transcribing 8 different exon 1 sequences which are expressed differently in different cell types. Expression of NOS II is mainly regulated at the transcriptional level and can be induced in many cell types with suitable agents such as LPS, cytokines, and other compounds. Whether some cells can express NOS II constitutively is still under debate. Pathways resulting in the induction of the NOS II promoter may vary in different cells. Activation of transcription factor NF-kappa B seems to be an essential step for NOS II induction in most cells. The induction of NOS II can be inhibited by a wide variety of immunomodulatory compounds acting at the transcriptional levels and/or post-transcriptionally.

Nitric oxide and the control of renin secretion

Research during recent years has established nitric oxide as a unique signaling molecule that plays important roles in the regulation of the cardiovascular, nervous, renal, immune and other systems. Nitric oxide has also been implicated in the control of the secretion of hormones by the pancreas, hypothalamus, pituitary and other endocrine glands, and evidence is accumulating that it contributes to the regulation of the secretion of renin by the kidneys. The enzyme nitric oxide synthetase is present in vascular and tubular elements of the kidney, particularly in cells of the macula densa, a structure that plays an important role in the control of renin secretion. Guanylyl cyclase, a major target for nitric oxide, is also present in the kidney and is responsive to changes in nitric oxide levels. Drugs that inhibit nitric oxide synthesis generally suppress renin release in vivo and in vitro, suggesting a stimulatory role for the L-arginine-nitric oxide pathway in the control of renin secretion. Under some conditions, however, blockade of nitric oxide synthesis increases renin secretion. Recent studies indicate that nitric oxide not only contributes to the regulation of basal renin secretion, but also participates in the renin secretory responses to activation of the renal baroreceptor, macula densa and beta adrenoceptor mechanisms that regulate renin secretion. Future research should clarify the mechanisms by which nitric oxide regulates the secretion of renin and establish the physiological significance of this regulation.

Functional analysis of the human endothelial nitric oxide synthase promoter. Sp1 and GATA factors are necessary for basal transcription in endothelial cells

To gain insights into the mechanisms of endothelial nitric oxide synthase (eNOS) gene expression, we have cloned the eNOS promoter and fused it to a luciferase reporter gene to map regions of the promoter important for basal transcription in bovine aortic endothelial cells (BAEC). Transfection of BAEC with F1 luciferase (LUC) (-1600 to +22 nucleotides) yielded a 35-fold increase in promoter. Progressive deletion from -1600 to -1033 (F2 and F3 LUC) did not significantly influence eNOS promoter activity. Further deletion from -1033 to -779 (F4 LUC) resulted in an approximate 40% reduction in basal promoter activity, and still further deletion from -779 to -494 (F5 LUC) did not markedly influence activity. Deletion from -494 to -166 (F6 LUC) reduced eNOS promoter activity by 40-50%. Specific mutation of the consensus GATA site (-230) in the F3 LUC construct reduced luciferase activity (by 25-30%). Gel shift analysis and antibody depletion using BAEC nuclear extracts demonstrated in vitro binding of GATA-2 to the oligonucleotide sequence containing the -230 GATA site. Next, we mutated the Sp1 site (-103) in the F3 and F6 LUC constructs and in the F3 GATA mutant construct. Expression of these Sp1 mutants in BAEC resulted in a 85-90% reduction in normalized luciferase activity. Gel shift and antibody supershift analysis using a BAEC nuclear extracts demonstrated four specific, DNA-protein complexes binding to the eNOS Sp-1 site, with the slowest migrating form composed of Sp1 and another nuclear protein. These data demonstrate that the Sp1 site is an important cis-element in the core eNOS promoter.

Nitric oxide and airway disease

Nitric oxide (NO) may play an important role in regulating airway function and in the pathophysiology of inflammatory airway diseases. Endothelium-derived NO may be important in regulating airway blood flow and, indirectly, plasma exudation. NO is the neurotransmitter of bronchodilator nerves in human airways and counteracts the bronchoconstriction due to cholinergic neural mechanisms. Inducible NO synthase (iNOS) is expressed in human epithelial cells in response to pro-inflammatory cytokines and oxidants, probably via activation of the transcription factor nuclear factor kappa B (NF-kappa B). There is increased expression of iNOS in the epithelium of asthmatic patients and in lung macrophages in bronchiectasis. This may account for the increased concentration of NO in the exhaled air of patients with inflammatory airways disease. Increased NO production in the airways may result in hyperaemia, plasma exudation, mucus secretion and indirectly increased proliferation of Th2 lymphocytes responsible for eosinophilic inflammation. Glucocorticoids inhibit the induction of iNOS in epithelial cells and reduce the elevated exhaled NO to normal values. Selective inhibitors of iNOS may be useful in the treatment of inflammatory airway diseases, such as asthma, in the future.

Possible regulation of capacitative Ca2+ entry into colonic epithelial cells by NO and cGMP

A possible role of the nitric oxide (NO)/cGMP pathway in the regulation of Ca2+ entry into HT29/B6 human colonic epithelial cells was investigated using digital image processing of Fura-2 fluorescence and immunoblotting for nitric oxide synthase (NOS). We tested the hypothesis that Ca2+ store depletion causes increased NOS activity and [NO], which is stimulatory to Ca2+ entry by increasing guanylate cyclase (GC) and [cGMP]. Cells were incubated in 95 mM K(+)-containing solutions to depolarize the cell membrane potential and thereby exclude effects of NO and CGMP on K+ or Cl- channels, which might affect Ca2+ entry. Sodium nitroprusside (SNP, 0.5 microM and 30 microM), a NO donor, only slightly raised intracellular [Ca2+] ([Ca2+]i) in resting cells, but in 100 microM carbachol-stimulated cells the sustained, elevated Ca2+ plateau (reflecting Ca2+ entry) as well as Ba2+ entry were increased by 0.5 microM SNP, while 5, 10 or 30 microM SNP either had no effect or were inhibitory. Pretreatment of cells with the NOS inhibitor N-nitro-L-arginine (1 mM) reduced carbachol-stimulated Ca2+ entry, and simultaneous treatment with 0.5 microM (but not 30 microM) SNP restored Ca2+ influx. 8-Br-cGMP (1 mM) had little effect on [Ca2+]i or on rates of Ca2+ or Ba2+ influx into resting cells, but there were large effects on cells in which capacitative Ca2+ entry was activated by carbachol or cyclopiazonic acid (10 microM). The GC inhibitor LY83583 (10 microM) reduced carbachol-stimulated Ca2+ entry, and this entry was restored with 8-Br-cGMP. Western blotting revealed that endothelial-type NOS was present in the particulate fraction of cells. The data are consistent with the notion that Ca2+ entry into HT29/B6 cells is regulated by endothelial NOS/NO and GC/cGMP, but effects are most pronounced in store-depleted cells. Thus, NO and cGMP appear to potentiate the action of messengers released from the store during the emptying process, but NO and cGMP have only small effects of their own to open the Ca2+ channel in the plasma membrane. High [SNP] appeared to be inhibitory while low [SNP] was stimulatory, indicating that a precise range of [NO] may be required for effective stimulation of Ca2+ entry.

Effect of lipopolysaccharide on nitric oxide synthase activity in rat proximal tubules

Renal proximal tubules isolated from the rat possess nitric oxide synthase (NOS) activity that is calcium/calmodulin dependent and stereoselectively inhibited by NG-monomethyl-arginine (NMMA). In the absence of added Ca2+ and calmodulin, activity was reduced 84 +/- 13% compared with the activity in the presence of 2 mM Ca2+ and 25 micrograms/mL calmodulin. Inhibition by EGTA (10 mM) was 95 +/- 5% and by calmidazolium (R24571, 250 microM) was 99 +/- 1%. Inhibition by L-NMMA (100 microM) was 78 +/- 13% and by D-NMMA (100 microM) was 7 +/- 7%. The majority of NOS activity was found in the soluble fraction. NOS activity in isolated proximal tubules was also examined 6 hr after a single i.v. injection of lipopolysaccharide. Activity was increased significantly (P < 0.05) in the soluble fraction by 2-fold [from 0.320 +/- 0.052 to 0.648 +/- 0.046 (nmol/mg protein/30 min)] and in the particulate fraction by 3-fold [from 0.081 +/- 0.030 to 0.256 +/- 0.034 (nmol/mg protein/30 min)]. All activities were inhibited by EGTA. These data demonstrate that proximal tubules express a calcium/calmodulin-dependent NOS activity that is increased in vivo by lipopolysaccharide.

Nitric oxide synthases: gene structure and regulation

The NOSs are a family of complex enzymes that catalyze the five-electron oxidation of L-arginine to form NO and L-citrulline. They are best characterized as cytochrome P-450-like hemeproteins that depend on molecular oxygen, NADPH, flavins, and tetrahydrobiopterin. The three human NOS isoforms identified to date, ecNOS, nNOS, and iNOS, are found on human chromosomes 7, 12, and 17, respectively. Regulation of NO synthesis and release occurs at the levels of enzyme activity and mRNA synthesis. The nNOS mRNA is structurally diverse as a consequence of alternative promoters and alternate splicing. The iNOS gene is predominantly regulated at the level of transcription by synergistic combinations of proinflammatory cytokines and bacterial wall products. Changes in mRNA levels of the ecNOS following endothelium activation are mediated by altered rates of transcription as well as by the intriguing process of changes in mRNA stability. Given the essential role of the NO pathway in a wide variety of physiological and pathophysiological process, it is possible that the three isoforms of NOS contribute to polygenic genetic diversity in neurological, immune, and cardiovascular biology. Further studies are needed to determine the mechanisms of gene regulation of NOS in health and disease.

Endothelial nitric oxide synthase is expressed in cultured human bronchiolar epithelium

Nitric oxide (NO) is an important mediator of physiologic and inflammatory processes in the lung. To better understand the role of NO in the airway, we examined constitutive NO synthase (NOS) gene expression and function in NCI-H441 human bronchiolar epithelial cells, which are believed to be of Clara cell lineage. NOS activity was detected by [3H]arginine to [3H]citrulline conversion (1,070 +/- 260 fmol/mg protein per minute); enzyme activity was inhibited 91% by EGTA, consistent with the expression of a calcium-dependent NOS isoform. Immunoblot analyses with antisera directed against neuronal, inducible, or endothelial NOS revealed expression solely of endothelial NOS protein. Immunocytochemistry for endothelial NOS revealed staining predominantly in the cell periphery, consistent with the association of this isoform with the cellular membrane. To definitively identify the NOS isoform expressed in H441 cells, NOS cDNA was obtained by degenerate PCR. Sequencing of the H441 NOS cDNA revealed 100% identity with human endothelial NOS at the amino acid level. Furthermore, the H441 NOS cDNA hybridized to a single 4.7-kb mRNA species in poly(A)+ RNA isolated from H441 cells, from rat, sheep, and pig lung, and from ovine endothelial cells, coinciding with the predicted size of 4.7 kb for endothelial NOS mRNA. Guanylyl cyclase activity in H441 cells, assessed by measuring cGMP accumulation, rose 6.6- and 5.4-fold with calcium-mediated activation of NOS by thapsigargin and A23187, respectively. These findings indicate that endothelial NOS is expressed in select bronchiolar epithelial cells, where it may have autocrine effects through activation of guanylyl cyclase. Based on these observations and the previous identification of endothelial NOS in a kidney epithelial cell line, it is postulated that endothelial NOS may be expressed in unique subsets of epithelial cells in a variety of organs, serving to modulate ion flux and/or secretory function.

Nitric oxide synthase isozymes. Characterization, purification, molecular cloning, and functions

Three isozymes of nitric oxide (NO) synthase (EC 1.14.13.39) have been identified and the cDNAs for these enzymes isolated. In humans, isozymes I (in neuronal and epithelial cells), II (in cytokine-induced cells), and III (in endothelial cells) are encoded for by three different genes located on chromosomes 12, 17, and 7, respectively. The deduced amino acid sequences of the human isozymes show less than 59% identity. Across species, amino acid sequences for each isoform are well conserved (> 90% for isoforms I and III, > 80% for isoform II). All isoforms use L-arginine and molecular oxygen as substrates and require the cofactors NADPH, 6(R)-5,6,7,8-tetrahydrobiopterin, flavin adenine dinucleotide, and flavin mononucleotide. They all bind calmodulin and contain heme. Isoform I is constitutively present in central and peripheral neuronal cells and certain epithelial cells. Its activity is regulated by Ca2+ and calmodulin. Its functions include long-term regulation of synaptic transmission in the central nervous system, central regulation of blood pressure, smooth muscle relaxation, and vasodilation via peripheral nitrergic nerves. It has also been implicated in neuronal death in cerebrovascular stroke. Expression of isoform II of NO synthase can be induced with lipopolysaccharide and cytokines in a multitude of different cells. Based on sequencing data there is no evidence for more than one inducible isozyme at this time. NO synthase II is not regulated by Ca2+; it produces large amounts of NO that has cytostatic effects on parasitic target cells by inhibiting iron-containing enzymes and causing DNA fragmentation. Induced NO synthase II is involved in the pathophysiology of autoimmune diseases and septic shock. Isoform III of NO synthase has been found mostly in endothelial cells. It is constitutively expressed, but expression can be enhanced, eg, by shear stress. Its activity is regulated by Ca2+ and calmodulin. NO from endothelial cells keeps blood vessels dilated, prevents the adhesion of platelets and white cells, and probably inhibits vascular smooth muscle proliferation.