Ultrastructural study of perivascular nerve fibres and endothelial cells of the rat basilar artery immunolabelled with monoclonal antibodies to neuronal and endothelial nitric oxide synthase

Hypoxia releases S-nitrosocysteine from carotid body glomus cells-relevance to expression of the hypoxic ventilatory response

We have provided indirect pharmacological evidence that hypoxia may trigger release of the S-nitrosothiol, S-nitroso-L-cysteine (L-CSNO), from primary carotid body glomus cells (PGCs) of rats that then activates chemosensory afferents of the carotid sinus nerve to elicit the hypoxic ventilatory response (HVR). The objective of this study was to provide direct evidence, using our capacitive S-nitrosothiol sensor, that L-CSNO is stored and released from PGCs extracted from male Sprague Dawley rat carotid bodies, and thus further pharmacological evidence for the role of S-nitrosothiols in mediating the HVR. Key findings of this study were that 1) lysates of PGCs contained an S-nitrosothiol with physico-chemical properties similar to L-CSNO rather than S-nitroso-L-glutathione (L-GSNO), 2) exposure of PGCs to a hypoxic challenge caused a significant increase in S-nitrosothiol concentrations in the perfusate to levels approaching 100 fM via mechanisms that required extracellular Ca2+, 3) the dose-dependent increases in minute ventilation elicited by arterial injections of L-CSNO and L-GSNO were likely due to activation of small diameter unmyelinated C-fiber carotid body chemoafferents, 4) L-CSNO, but not L-GSNO, responses were markedly reduced in rats receiving continuous infusion (10 μmol/kg/min, IV) of both S-methyl-L-cysteine (L-SMC) and S-ethyl-L-cysteine (L-SEC), 5) ventilatory responses to hypoxic gas challenge (10% O2, 90% N2) were also due to the activation of small diameter unmyelinated C-fiber carotid body chemoafferents, and 6) the HVR was markedly diminished in rats receiving L-SMC plus L-SEC. This data provides evidence that rat PGCs synthesize an S-nitrosothiol with similar properties to L-CSNO that is released in an extracellular Ca2+-dependent manner by hypoxia.

On P2X receptors in the brain: microvessels. Dedicated to the memory of the late Professor Geoffrey Burnstock (1929-2020)

This tribute article presents selected immunocytochemical and transmission electron microscope data on the location of ATP-gated P2X receptor in the rat brain, as studied in the 1990s in Prof G. Burnstock's laboratory at University College London. There are examples of immuno-ultrastructural findings and introductory information about pre- and post-synaptic location of P2X receptors in the rat cerebellum and endocrine hypothalamus to support the concept of purinergic transmission in the central nervous system. Then findings of diverse immunoreactivity for P2X1, P2X2, P2X4, and P2X6 receptors associated with brain microvessels are shown, including vascular endothelium and pericytes as well as perivascular astrocytes and neuronal components. These findings imply the involvement of P2X receptors and hence purinergic signalling in the neurovascular unit, at least in microvessels in the rat cerebellum and hypothalamic paraventricular and supraoptic nuclei examined here. Various aspects of P2X receptors in brain microvessels are discussed.

The endothelium of basilar artery of diabetic rat treated with epoetin delta

Erythropoiesis-stimulating agents (ESAs) are used to treat anemia associated with renal failure. It is now known that these agents also show a broad range of cell- and tissue-protective effects. In the current study, we explored whether an ESA, epoetin delta, affects vascular pathology linked to diabetes mellitus (DM). In a rat model of streptozotocin-induced DM, we investigated, by pre-embedding electron-immunocytochemistry, whether epoetin delta affects DM-induced structural changes in cerebrovascular endothelium of the rat basilar artery and influences the subcellular distribution of endothelial nitric oxide synthase (eNOS). Epoetin delta treatment influenced DM-induced changes to the distribution of eNOS in, and the structure of, the endothelial cell. This may indicate potential beneficial effects of epoetin delta on cerebrovascular endothelium and suggests eNOS as a possible target molecule of epoetin delta in DM.

Hemodynamic responses elicited by systemic injections of flavin adenine dinucleotide in anesthetized rats

Flavin adenine dinucleotide (FAD) elicits an endothelium-dependent vasodilation in isolated rat mesenteric beds via activation of P2Y-purinoceptors. The aims of this study were to characterize the hemodynamic responses elicited by systemic injections of FAD and flavin mononucleotide (FMN) in anesthetized rats and to determine the role of nitric oxide synthase (NOS), cyclooxygenase, P2Y/P2X-purinoceptors, and muscarinic receptor in these responses. FAD (0.05-1.0 micromol/kg, iv) elicited dose-dependent decreases in heart rate (HR), mean arterial blood pressure (MAP), and hindquarter vascular resistance (HQR), whereas it elicited an initial increase and then a decrease in mesenteric (MR) vascular resistance. The FAD-induced responses were not affected by the P2Y/P2X-purinoceptor antagonist suramin, the muscarinic receptor antagonist methyl-atropine, or the cyclooxygenase inhibitor indomethacin. The vasodilator actions of FAD were unaffected by the NOS inhibitor N-nitro-L-arginine methyl ester (L-NAME), whereas the bradycardia elicited by higher doses of FAD were diminished by L-NAME. FMN did not elicit hemodynamic responses in the absence or presence of L-NAME. In summary, FAD-induced bradycardia depends, in part, on the activation of NOS, whereas the vasodilator actions of FAD are not obviously due to newly synthesized nitrosyl factors. These findings and those in our companion manuscript support the concepts that the adenine moiety confers biological activity to FAD, which releases preformed pools of nitrosyl factors.

Flavin adenine dinucleotide may release preformed stores of nitrosyl factors from the vascular endothelium of conscious rats

This study determined whether flavin adenine dinucleotide (FAD) may elicit vasodilation in conscious rats via release of preformed endothelium-derived nitrosyl factors. Injections 1-6 (inj(1-6)) of FAD (2.5 micromol/kg, IV) elicited pronounced and equivalent vasodilator responses in saline-treated rats. Inj(1) of FAD elicited pronounced vasodilation in L-NAME-treated rats pretreated with the nitric oxide (NO) synthesis inhibitor, NG-nitro-L-arginine (L-NAME; 50 micromol/kg, IV), whereas Inj(2-6) elicited progressively smaller responses such that inj(6) elicited minor responses. The vasodilator responses elicited by the endothelium-dependent agonist, acetylcholine, were markedly attenuated in L-NAME-treated rats that had received inj(1-6) of FAD but not in saline-treated rats that had received inj(1-6) of FAD. The vasodilator actions of L-S-nitrosocysteine and the NO donor, sodium nitroprusside, were not diminished after the injections of FAD in saline- or in L-NAME-treated rats. Binding studies demonstrated that the densities of muscarinic M3 receptors were increased in thoracic aorta endothelium of rats treated with L-NAME + inj(1-6) of saline or L-NAME + inj(1-6) of FAD as compared to rats treated with saline + inj(1-6) of saline or saline + inj(1-6) of FAD. The progressive loss of response to injections of FAD in L-NAME-treated rats coupled with the loss of response to acetylcholine suggests that FAD elicits the use-dependent depletion of vesicular pools of nitrosyl factors in endothelial cells that cannot be replenished in the absence of NO synthesis.

The developmental stage and cell type dependent phosphorylation of eNOS in murine enteric mucosa and myenteric plexus

In order to clarify the developmental regulation of the eNOS activity in intestine by phosphorylation, we examined the immunohistochemical localizations of the eNOS phosphorylation sites at Ser(1177), Ser(116) and at Thr(495) in cells of the mouse enteric mucosa and myenteric plexus at E13.5, E14.5, E16.5, E18.5, E20.5 and P3. In addition, in cells of the E16.5 stage the protein levels of eNOS and the phosphorylation sites of eNOS at Ser(1177), Ser(116) and at Thr(495) were investigated by immunoblot. From E14.5 to P3, phosphorylation residues of eNOS at Ser(1177) and at Ser(116) were detected with different staining intensities in the enteric mucosa epithelium. In ganglion cells of the myenteric plexus Ser(116) was identified at E18.5 to P3. The absence of phosphorylated Thr(495) in cells of intestine during all developmental stages, was confirmed by immunoblot at E16.5. The immunoblot levels of eNOS and eNOS phosphorylated at Ser(1177) and at Ser(116) were comparable with the immunohistochemical results of E16.5 mouse intestine. It was concluded that development of epithelial cells of the enteric mucosa may be modulated by phosphorylation of eNOS at Ser(1177) and at Ser(116). The phosphorylation of eNOS in cells of the myenteric plexus is modulated at Ser(116). These data suggest that there is a developmental stage and cell type dependent phosphorylation of eNOS.

Vasorelaxing effect of BAY 41-2272 in rat basilar artery: involvement of cGMP-dependent and independent mechanisms

Decreases in intrinsic NO cause cerebral vasospasms because of the dysregulation of cGMP formation by NO-mediated pathways. Because 5-cyclopropyl-2-{1-(2-fluorobenzyl)-1H-pyrazolo[3,4-b]pyridin-3-yl}pyrimidin-4-ylamine (BAY 41-2272) is a potent soluble guanylyl cyclase (sGC) stimulator in an NO-independent manner, this study aimed to investigate the mechanisms underlying the relaxant effects of BAY 41-2272 in the rat basilar artery. BAY 41-2272 (0.0001 to 1 micromol/L) induced relaxations in a concentration-dependent manner, with pEC50 values of 8.13+/-0.03 and 7.63+/-0.05 in intact and denuded rings, respectively. The sGC inhibitor 1H-[1,2,4] oxadiazolo [4,3,-a]quinoxalin-1-one (ODQ) markedly displaced the curve for BAY 41-2272 to the right in intact or denuded rings (&10-fold). The NO synthesis inhibitor NG-nitro-L-arginine methyl ester caused a rightward shift in the curve for BAY 41-2272 (4-fold), whereas the phosphodiesterase type 5 inhibitor sildenafil enhanced BAY 41-2272-induced relaxations (3- to 4-fold). The Na+-K+-ATPase inhibitor ouabain caused 3-fold rightward shifts in the curves for BAY 41-2272. Ca2+-induced contractions in K+ depolarized rings were significantly attenuated by BAY 41-2272 in an ODQ-insensitive manner. The NO donor glyceryl trinitrate and BAY 41-2272 caused rightward shifts in the contractile responses to serotonin. Their coincubation caused a synergistic inhibition of serotonin-induced contractions. BAY 41-2272 and glyceryl trinitrate increased cGMP levels (but not cAMP) by 10-fold and 4-fold above baseline, respectively, in an ODQ-sensitive manner. cGMP levels increased by 50-fold after coincubation. BAY 41-2272 potently relaxes the rat basilar artery in a synergistic fashion with NO. Targeting the sGC with selective activators, such as BAY 41-2272, may represent a new therapy to treat cerebrovascular disease.

Basilar artery of the capybara (Hydrochaeris hydrochaeris): an ultrastructural study

The present study investigated the ultrastructural features of the basilar artery of the largest rodent species, the capybara. The study suggests that the general ultrastructural morphological organization of the basilar artery of the capybara is similar to that of small rodents. However, there are some exceptions. The basilar artery of the capybara contains a subpopulation of 'granular' vascular smooth muscle cells resembling monocytes and/or macrophages. The possibility cannot be excluded that the presence of these cells reflects the remodelling processes of the artery due to animal maturation and the regression of the internal carotid artery. To clarify this issue, more systemic studies are required involving capybaras of various ages.

Endothelial nitric oxide synthase phosphorylated at Ser116 is localized in nerve fibers of the rat glandulae palatinae

It is known that total endothelial nitric oxide synthase (eNOS) is localized in peripheral nerve fibers, but the existence of the phosphorylation site/s of eNOS in peripheral nerve fibers is unknown. In the perfusion-fixed and decalcified sections of rat palates, eNOS and eNOS phosphorylated at Ser(116) were identified in the nerve fibers of the glandulae palatinae. The localization of eNOS phosphorylated at Ser(1177) and Thr(495) in nerve fibers was undetectable. It is concluded that eNOS is phosphorylated at Ser(116) in nerve fibers of the glandulae palatinae under basal conditions. The basal Ser(1177) and Thr(495) phosphorylation of eNOS are missing in nerve fibers of the glandulae palatinae.

Further characterisation of particulate neuronal nitric oxide synthase in rat small intestine

Neuronal NO-synthase (nNOS) was investigated in rat longitudinal muscle/myenteric plexus (LM/MP) tissue at the cellular and subcellular level. Using preparations and double immune staining and light and electron microscopy, we concluded that, in these preparations, nNOS is only present in neuronal cells. However, in spite of numerous attempts to morphologically identify the NOS-containing subcellular structure, no firm conclusions were possible. Consequently, the problem was approached by biochemical methods including gradient centrifugation followed by analysis of the fractions. Using a protocol involving gentle homogenisation of the tissue, we found that about 10% of the nNOS immune reactivity was particle-bound confirming previous results (Biochem. Pharmacol. 60 (2000) 145). However, applying a different protocol including strong homogenisation, we now demonstrated that about 50% of the immune reactive nNOS was sedimentable. The results suggested that particulate nNOS is associated with one single subcellular structure, which is different from the plasma membrane, rough and smooth endoplasmic reticulum, mitochondria and lysosomes. The equilibrium sedimentation characteristics of the nNOS containing particles corresponded partly to those containing vasoactive intestinal polypeptide (VIP) or synaptobrevin. Application of non-equilibrium centrifugation conditions, however, demonstrated that almost no co-localisation occurred. We conclude that, in the LM/MP tissue, nNOS is about 50% particle-bound in a subcellular structure, which is different from the VIP-containing particle and from synaptobrevin-containing exocytotic particles.

Impaired Nitric Oxide Function in the Basilar Artery of the Obese Zucker Rat

The effect of insulin-resistance syndrome on vascular function has been examined in isolated basilar arteries using the obese Zucker rat (OZR) and age-matched lean littermate controls (lean Zucker rat; LZR) at 36 weeks of age. The OZR showed significantly reduced oral glucose tolerance and increased body weight, blood pressure, proteinuria, plasma levels of triglycerides, cholesterol, and insulin compared with the LZR. The contractile response to serotonin was significantly increased in the OZR. Furthermore, contractions to serotonin in LZR but not OZR were enhanced in the presence of the nitric oxide synthase inhibitor Nomega-nitro-L-arginine methyl ester (NAME). Relaxations to acetylcholine (ACh), histamine, and A23187 were significantly reduced in precontracted arteries from the OZR. In the presence of NAME, histamine responses were significantly reduced whereas ACh and A23187 responses were almost abolished. Relaxations to free-radical nitric oxide (NO) and papaverine were not different in arteries from the OZR, even though responses to sodium nitroprusside were reduced in the OZR. Western blot and immunofluorescent quantitative analyses of eNOS content in cerebral microvessel fractions and basilar artery preparations, respectively, were not significantly different between OZR and LZR. The results suggest impairment in endothelial function resulting in reduced NO function in the basilar artery from the OZR.

Synaptic localization of nitric oxide synthase and soluble guanylyl cyclase in the hippocampus

Functional evidence suggests that nitric oxide released from CA1 pyramidal cells can act as a retrograde messenger to mediate hippocampal long-term potentiation, but the failure to find neuronal nitric oxide synthase (NOS-I) in the dendritic spines of these cells has cast doubt on this suggestion. We hypothesized that NOS-I may be in spines but in a form inaccessible to antibody when using standard histological fixation procedures. Supporting this hypothesis, we found that after a weak fixation protocol shown previously to enhance staining of synaptic proteins, CA1 pyramidal cells exhibit clear immunoreactivity for NOS-I. Confocal microscopy revealed that numerous dendritic spines in the stratum radiatum contained the NR2 subunit of the NMDA receptor and the adaptor protein postsynaptic density-95, and a subset of these spines also contained NOS-I. Quantitative studies showed that only approximately 8% of synaptic puncta (identified by synaptophysin staining) were associated with NOS-I, and approximately 9% contained the beta subunit of soluble guanylyl cyclase (sGC), a major target of NO. However, the majority of NOS-I-positive synaptic puncta was associated with sGC and vice versa. Postembedding immunogold electron microscopy showed that NOS-I concentrates just inside the postsynaptic plasma membrane of asymmetric axospinous synapses in the stratum radiatum of CA1, whereas sGCbeta concentrates just inside the presynaptic membrane. Together, these findings support the possibility that NO may act as a retrograde messenger to help mediate homosynaptic plasticity in a subpopulation of synapses in the stratum radiatum of CA1.

Endothelin in the middle cerebral artery: a case of multiple system atrophy

In this study, we show the changes in the wall of the middle cerebral artery of a subject who suffered multiple system atrophy with autonomic failure. An electron-immunocytochemical approach was employed to reveal the presence of endothelin-1. Our results demonstrate the presence of immunoreactive endothelin-1 in the endothelial cells of the intima, vascular smooth muscle cells and macrophages of the media and neointima, and perivascular nerves/axons varicosities at the adventitial-medial border of the artery. It is concluded that endothelin-1 may, therefore, play a number of roles within diseased cerebral artery. The finding of endothelin-1-positive varicosities of autonomic innervation to this artery suggests an influence of neural endothelin on vascular smooth muscle in multiple system atrophy with autonomic failure. However, the presence of features such as neointima formation, wall irregularities and foam cells suggest the coexistence of atherosclerosis.

Functional importance of neuronal nitric oxide synthase in the endothelium of rat basilar arteries

The function of the neuronal isoform of nitric oxide synthase (nNOS) was studied by comparing the effects of the specific nNOS blocker 7-nitro indazole monosodium salt (7-NINA) with that of the general NOS inhibitor N(G)-nitro-L-arginine (L-NA) in isolated rat basilar arteries (BAs). 7-NINA had no significant effect on the resting tone of the vessels, while both L-NA and 1H-[1,2,4]oxadiazolo[4, 3-a]quinoxalin-1-one (ODQ), a selective inhibitor of the soluble guanylyl cyclase, induced contraction. The relaxant effect of bradykinin was attenuated in the presence of L-NA but was not changed by 7-NINA. In contrast, 7-NINA markedly reduced the acetylcholine-induced, endothelium-dependent relaxation. These results demonstrate that nNOS contributes significantly to the relaxant effect of acetylcholine, indicating the functional importance of this isoenzyme in the cerebrovascular endothelium.

Ultrastructural localization of neuronal nitric oxide synthase in the laterodorsal tegmental nucleus of wild-type and knockout mice

The cellular and subcellular distribution of neuronal nitric oxide synthase and its related reduced beta-nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase activity was compared in wild-type and homozygous knockout mice, in which the gene for neuronal nitric oxide synthase has been disrupted, resulting in a lack of the predominant splice isoform alpha. In the laterodorsal tegmental nucleus, used as a model structure, the cholinergic principal neurons also exhibited an intensive neuronal nitric oxide synthase immunoreactivity. Using the tetrazolium salt 2-(2-benzothiazolyl)-5-styryl-3-(4'-phthalhydrazidyl)-tetrazo++ +-lium chloride (BSPT), these neurons were filled with NADPH-diaphorase reaction product, whereas the equivalent neurons of knockout mice showed, if at all, only traces of neuronal nitric oxide synthase immunoreactivity in parallel to a diminished NADPH-diaphorase labelling. Subcellularly, the neuronal nitric oxide synthase-related diaminobenzidine product was, apparently owing to diffusion artifact, more or less evenly distributed in the cytosol of the neuronal perikarya and dendrites of wild-type mice. In contrast, the BSPT reaction product formazan was closely and discretely attached to endocellular membranes. In the intensely NADPH-diaphorase stained neurons of wild-type mice, 85% of the mitochondria were, at least partly, labelled for BSPT-formazan, whilst in the equivalent neurons of mutant mice, only 13% of mitochondria were NADPH-diaphorase positive. Related to the NADPH-diaphorase activity in the principal neurons of wild-type mice, only 10% of membranes of the endoplasmic reticulum, 27% of mitochondrial membranes and 26% of the nuclear envelope exhibited NADPH-diaphorase activity in the mutant mice. Our findings with the BSPT histochemistry suggest that residues of NADPH-diaphorase positivity in mutant mice are attributed to the alternative splice isoforms beta and/or gamma of neuronal nitric oxide synthase. The splice isoform a is located predominantly at the membranes of the endoplasmic reticulum.

Cholinergic contraction is altered in nNOS knockouts. Cooperative modulation of neural bronchoconstriction by nNOS and COX

Endogenous nitric oxide (NO) is a bronchodilator but its physiologic role in small airways is not clear. In this study, we investigated the role of endogenous NO in the regulation of bronchiolar tone in the small airways of wild type and NO synthase (NOS) isoform (eNOS and nNOS)-knockout mice. Pretreatment with the cyclooxygenase inhibitor indomethacin significantly enhanced electrical field stimulation (EFS)-induced contraction in the airways from all types of mice by approximately 60 to 170% (n = 8 in each case), whereas pretreatment with the NOS inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME) did not (n = 8). Combined pretreatment with L-NAME and indomethacin enhanced airway contraction of wild-type and eNOS-knockout mice to a significantly greater extent (i.e., by 140 to 290%) than did indomethacin alone (n = 8 for each). This potentiation by L-NAME was not seen in nNOS knockout mice (n = 8). Neither indomethacin nor L-NAME alone affected carbachol (CCh) potency or maximal efficacy in the airways of wild-type mice, whereas the combined pretreatment slightly enhanced the maximal response without altering the potency of CCh (n = 6). Our results show that both NO and prostaglandins modulate neuronal contraction of murine small airways. NO is produced by nNOS, which may be located in nerves, and its overall effects are tonically inhibited by cyclooxygenase products.

Cellular changes induced by chronic nitric oxide inhibition in intact rat basilar arteries revealed by confocal microscopy

BACKGROUND

Cellular aspects of remodelling have not been investigated fully in intact vessels due to lack of appropriate methodology.

OBJECTIVE

To determine the cellular alterations induced by chronic inhibition of nitric oxide (NO) production in intact rat basilar arteries by combined use of perfusion myography and a laser scanning confocal microscope.

METHODS

Wistar-Kyoto rats were treated with 10 mg/kg per day NG-nitro-L-arginine methyl ester (L-NAME) for 3 weeks. Basilar arteries from treated and age-matched Wistar-Kyoto rat controls were mounted on a perfusion myograph, stained with the nuclear dye Hoechst 33342 and fixed under pressure. The segments were mounted on a slide and visualized using the 364 nm line of a laser scanning confocal microscope. MetaMorph software was used to obtain optical sections from the vessel and for morphology determinations.

RESULTS

L-NAME treatment induced hypertension (systolic blood pressure control 129.2+/-2.7 mmHg and SBP L-NAME treatment 176.3+/-5.2 mmHg, P< 0.001). Compared with control rat arteries, arteries from treated rats had a reduced lumen diameter, similar wall thickness and an increased wall: lumen ratio. L-NAME treatment induced specific changes in adventitia, media and intima, namely an increase in number of adventitial cells and in adventitia thickness, a reduction in number of smooth muscle cells with no change in media thickness and reductions in number of endothelial cells, size of nuclei and luminal surface area.

CONCLUSIONS

Hypertension induced by chronic inhibition of NO production is associated with eutrophic remodelling of rat basilar artery. However, within this overall maintenance of constant volume, there are marked cellular changes in adventitia, media and intima. The separate contributions of inhibition of NO production and hypertension to the remodelling process need to be elucidated.