Ultrastructural localization and translocation of nitric oxide synthase in the endothelium of the human cerebral artery

Re-evaluation of the immunohistochemical distribution of isoforms of nitric oxide synthase in the human prostate: A light and electron microscopical study

Up until today, there are still uncertainties regarding the occurrence of isoforms of the nitric oxide synthase (eNOS, nNOS) in the human prostate. While nNOS was exclusively seen in slender nerve fibres branching within the transition zone, eNOS was reported in glandular structures and also in small vessels interspersing the tissue. This study aimed to re-evaluate by means of light and electron microscopy (LM, EM), the distribution of eNOS and nNOS in the transition zone of the human prostate. Tissue specimens were obtained from 16 patients who underwent surgery for pelvic malignancies. Using specific antibodies in conjunction with advanced fixation and staining procedures, the occurrence of eNOS and nNOS was investigated. nNOS was detected in nerve fibres interspersing the tissue and was also seen in glandular structures. EM revealed that in glandular epithelial cells immunoreaction for nNOS was limited to the cytoplasmic compartment. Vascular endothelial cells of small vessels transversing glandular structures significantly stained for eNOS, while epithelial layers of prostatic glandules appeared free of eNOS. The results implicate that, in the prostate, nNOS is a mediator of stromal and glandular tissue function, and counteract the assumption of eNOS activity in glandular epithelial cells as a source of NO synthesis.

Difference in vulnerability of cerebral arterial bifurcation and straight portion for development of endothelial damage

Cerebral atherosclerotic changes develop most commonly on the lateral side of the bifurcation, one of the sites where the wall shear stress is lowest in the cerebral arteries. Endothelial cells exposed to the lower shear stress appear to be more vulnerable to stimuli such as free radicals. We hypothesized that the difference in endothelial vulnerability on cerebral arteries may be involved in the local preference of atherogenesis. To clarify this hypothesis, the present study was carried out by using a laser-dye technique that causes cell damage by heat and free radicals. A helium-neon laser in the presence of circulating Evans blue was used to illuminate three sites of the rat middle cerebral artery; the straight portion, the apex of the bifurcation, and the lateral side of the bifurcation. The magnitude of endothelial damage was morphologically estimated with the electron microscope. After the laser irradiation, the straight portion and the lateral side of the bifurcation developed severe endothelial damage. However, the apex of the bifurcation developed no appreciable damage, showing significantly milder changes compared with other sites. The results suggest that endothelial cells are more vulnerable to stimuli by free radicals at the straight portion and the lateral side of the bifurcation than at the apex. We conclude that the imbalance between the strength of stress stimuli, such as free radicals, and the vulnerability of endothelium is likely to be one of the key requirements for the development of cerebral atherosclerotic changes.

Immunocytochemical distribution of nitric oxide synthase in the human seminal vesicle: a light and electron microscopical study

Although nitric oxide (NO) has been proven to be one of the most important non-adrenergic, non-cholinergic mediators in the control of human reproductive tract organs, to date information on the significance of NO-mediated signal transduction in the control of human seminal vesicle (SV) function is still sparse.()Recent investigations have underlined the significance of NO in the maintenance of sperm capacitation and viscosity of the seminal plasma as well as in the control of mammalian seminal vesicle smooth muscle tone. In order to further investigate the functional impact of NO on the regulation of normal SV function, we examined the distribution of NADPH-diaphorase (NADPH-d), endothelial nitric oxide synthase (eNOS) and neuronal nitric oxide synthase (nNOS) in the cellular anatomy of human SV by means of light and electron microscopical immunocytochemistry (LM, EM) in combination with the tyramide signal amplification technique. Human SV were obtained from 15 patients who had undergone surgery for pelvic malignancies (carcinoma of the prostate or urinary bladder). SV specimens were fixed, sectioned and examined by LM and EM for the presence of NAPDH-d, eNOS and nNOS using specific antibodies and advanced staining procedures. LM revealed a dense NADPH-d reaction in glandular epithelial structures, whereas no substantial labeling was detected in the fibromuscular stroma. EM showed that the NADPH-d reaction product was abundantly detectable attached to membranes of the endoplasmic reticulum, mitochondria and the nuclei of glandular epithelial cells. nNOS staining was found in nerve fibers branching within the SV tissue. eNOS staining was present in small vessels but was only observed to a minor degree in glandular and subglandular structures and the smooth muscle stroma. Our results support the hypothesis that human SV is a site of NO production. The distribution of NADPH-d may give rise to the speculation that NO is mainly involved in the regulation of SV secretory activity. The sparse correlation between NADPH-d-, eNOS- and nNOS-staining might hint at the existence of a previously unidentified NOS isoform in human SV.

Adenovirus-mediated gene transfer to human cerebral arteries

Gene therapy is being investigated as a putative treatment option for cardiovascular diseases, including cerebral vasospasm. Because there is presently no information regarding gene transfer to human cerebral arteries, the principal objective of this study was to characterize adenovirus-mediated expression and function of recombinant endothelial nitric oxide synthase (eNOS) gene in human pial arteries. Pial arteries (outer diameter 500 to 1,000 microm) were isolated from 30 patients undergoing temporal lobectomy for intractable seizures and were studied using histologic staining, histochemistry, electron microscopy, and isometric force recording. Gene transfer experiments were performed ex vivo using adenoviral vectors encoding genes for bovine eNOS (AdCMVeNOS) and Escherichia coli beta-galactosidase (AdCMVLacZ). In transduced arteries, studied 24 hours after exposure to vectors, expression of recombinant beta-galactosidase and eNOS was detected by histochemistry, localizing mainly to the adventitia (n = 4). Immunoelectron microscopy localized recombinant eNOS in adventitial fibroblasts. During contractions to U46619, bradykinin-induced relaxations were significantly augmented in AdCMVeNOS-transduced rings compared with control and AdCMVLacZ-transduced rings (P < 0.01; n = 6). The NOS inhibitor L-nitroarginine methylester (L-NAME) caused significantly greater contraction in AdCMVeNOS-transduced rings (P < 0.001; n = 4) and inhibited bradykinin-induced relaxations in control and transduced rings (P < 0.001; n = 6). The current findings suggest that in AdCMVeNOS-transduced human pial arteries, expression of recombinant eNOS occurs mainly in adventitial fibroblasts where it augments relaxations to NO-dependent agonists such as bradykinin. Findings from the current study might be beneficial in future clinical applications of gene therapy for the treatment or prevention of cerebral vasospasm.

Receptor-regulated translocation of endothelial nitric-oxide synthase

The endothelial nitric-oxide synthase (eNOS) is activated by transient increases in intracellular Ca2+ elicited by stimulation of diverse receptors, including bradykinin B2 receptors on endothelial cells. eNOS and B2 receptors are targeted to specialized signal-transducing domains in the plasma membrane termed plasmalemmal caveolae. Targeting to caveolae facilitates eNOS activation following receptor stimulation, but in resting cells, eNOS is tonically inhibited by its interactions with caveolin, the scaffolding protein in caveolae. We used a quantitative approach exploiting immunofluorescence microscopy to investigate regulation of the subcellular distribution of eNOS in endothelial cells by bradykinin and Ca2+. In resting cells, most of the eNOS is localized at the cell membrane. However, within 5 min following addition of bradykinin, nearly all the eNOS translocates to structures in the cell cytosol; following more protracted incubations with bradykinin, most of the cytosolic enzyme subsequently translocates back to the cell membrane. The bradykinin-induced internalization of eNOS is completely abrogated by the intracellular Ca2+ chelator BAPTA; conversely, Ca2+-mobilizing drugs and agonists promote eNOS translocation. These results establish that eNOS targeting to the membrane is labile and is subject to receptor-regulated Ca2+-dependent reversible translocation, providing another point for regulation of NO-dependent signaling in the vascular endothelium.

The endothelial nitric-oxide synthase-caveolin regulatory cycle

Nitric oxide production in the vascular endothelium is promoted by diverse agonists that transiently increase intracellular Ca2+ concentration and activate the endothelial nitric-oxide synthase (eNOS), a Ca2+/calmodulin-dependent enzyme. eNOS is acylated by the fatty acids myristate and palmitate and is targeted thereby to plasmalemmal signal-transducing domains termed caveolae. eNOS enzyme activity is markedly attenuated by its interactions with caveolin, the structural scaffolding protein of caveolae. We have discovered that in living cells, the eNOS-caveolin heteromeric complex undergoes cycles of dissociation and re-association modulated by Ca2+-mobilizing agonists. Calcium ionophore A23187 and the muscarinic cholinergic agonist carbachol both promote the dissociation of eNOS from caveolin in cultured cells, associated with translocation of eNOS from caveolae. As [Ca2+]i returns to basal levels, eNOS re-associates with caveolin, and the inhibited enzyme complex is then restored to caveolae, a process accelerated by palmitoylation of the enzyme. These data establish an eNOS-caveolin regulatory cycle, wherein enzyme activation is modulated by reversible protein-protein interactions controlled by Ca2+/calmodulin and by enzyme palmitoylation. Alterations in this cycle are likely to have an important influence on nitric oxide-dependent signaling in the vascular wall.

Regulation of the cerebral circulation: role of endothelium and potassium channels

Several new concepts have emerged in relation to mechanisms that contribute to regulation of the cerebral circulation. This review focuses on some physiological mechanisms of cerebral vasodilatation and alteration of these mechanisms by disease states. One mechanism involves release of vasoactive factors by the endothelium that affect underlying vascular muscle. These factors include endothelium-derived relaxing factor (nitric oxide), prostacyclin, and endothelium-derived hyperpolarizing factor(s). The normal vasodilator influence of endothelium is impaired by some disease states. Under pathophysiological conditions, endothelium may produce potent contracting factors such as endothelin. Another major mechanism of regulation of cerebral vascular tone relates to potassium channels. Activation of potassium channels appears to mediate relaxation of cerebral vessels to diverse stimuli including receptor-mediated agonists, intracellular second messenger, and hypoxia. Endothelial- and potassium channel-based mechanisms are related because several endothelium-derived factors produce relaxation by activation of potassium channels. The influence of potassium channels may be altered by disease states including chronic hypertension, subarachnoid hemorrhage, and diabetes.

Immunocytochemistry of endothelial nitric oxide synthase in the rat brain: a light and electron microscopical study using the tyramide signal amplification technique

There are many inconsistencies in the literature about the cellular and subcellular distribution of the endothelial isoform of nitric oxide synthase (eNOS) in the brain. We have re-investigated its localization by light and electron microscopical (LM, EM) immunocytochemistry and the NADPH-diaphorase reaction. Using bovine aortic tissue as a positive control the protocols for the fixation and staining procedure were optimized. Only cryosections immersion-fixed with aceton and a mixture of aldehydes exhibited a clear-cut immunostaining. In rat brain tissue the endothelium of the entire vasculature showed immunoreactivity and, in addition to that, the epithelial cells of the choroid plexuses, whereas neurons never displayed any signs of immunostaining. EM immunoprecipitates were seen irregularly distributed in the cytosol or attached to endocellular membranes. EM NADPH-diaphorase histochemistry using the tetrazolium salt BSPT provided incoherent pictures in so far as the reaction product was exclusively bound to membranes. The restriction of eNOS within brain tissue to the vasculature may have implications for the differential significance of NOS isoforms in brain function.

Dynamic changes in NADPH-diaphorase staining reflect activity of nitric oxide synthase: evidence for a dopaminergic regulation of striatal nitric oxide release

In fixed tissue, neuronal NADPH-diaphorase staining results from nitric oxide synthase (NOS) activity. Neuronal NOS only synthesizes nitric oxide once activated by the binding of Ca2+/calmodulin. We show here that neuronal NADPH-diaphorase staining is also dependent on Ca2+/calmodulin, implying that only activated NOS is detected. In addition, in bovine pulmonary endothelial cells, carbachol and bradykinin dramatically and rapidly increase the intensity of NADPH-diaphorase staining. Furthermore, administration of MK801, an NMDA antagonist, decreases neuronal NADPH-diaphorase staining. This suggests that the intensity of the NADPH-diaphorase staining is related to the level of enzyme activation at the moment of tissue fixation. The potential of exploiting this observation to detect cellular activation of NOS is illustrated by the observations that the intensity of NADPH-diaphorase staining in rat striatal neurones is decreased following systemic treatment with the D1-like dopamine receptor antagonist SCH23390, and increased by the D2-like antagonist eticlopride. These results therefore provide strong evidence that the NADPH-diaphorase reaction can be used to monitor NOS activity at a cellular level of resolution, and reveal a dopaminergic regulation of NOS activity in the striatum mediated by D1-like and D2-like dopamine receptors.

Weibel-Palade bodies as a storage site of calcitonin gene-related peptide and endothelin-1 in blood vessels of the rat carotid body

BACKGROUND

The vasculature of the carotid body has been considered to play a role in the regulation of blood flow into this organ. This light and electron microscope immunocytochemistry deals with endothelium-dependent vasomotion by vasodilatory calcitonin gene-related peptide (CGRP) and vasoconstrictive endothelin-1 (ET-1).

METHODS

After adult male rats were perfused with a solution of periodate-lysine-paraformaldehyde through the left ventricle, the carotid artery bifurcations were isolated and utilized for light and electron microscope immunolabelings with CGRP and ET-1 primary antisera.

RESULTS

By light microscope immunocytochemistry, immunoreactions to CGRP were seen along the endothelium of the carotid body artery (CBA) and its branches, and those of ET-1 were observed along the endothelium of the intralobular capillaries in addition to the above vessels. By immunoelectron microscopy, immunoreactive gold particles of CGRP and ET-1 were identified in the rough endoplasmic reticulum (rER) and in the Weibel-Palade (WP) bodies of endothelial cells of the CBA and its branches. Colocalization of both immunoreactive gold particles was observed in the same WP body. Immunoreactive gold particles of CGRP were also identified in the rER, Golgi apparatus, and specific granules of the dark glomus cells.

CONCLUSIONS

Conceivably, CGRP and ET-1 are synthesized in the rER of these endothelial cells and are stored in the WP bodies for the autoregulation of blood flow.

Ontogenesis of NADPH-diaphorase activity in the olfactory bulb of the rat

The enzyme NADPH-diaphorase, which has been shown to correspond to nitric oxide synthase, is present in discrete neuron populations in the olfactory bulb of the adult rat. The ontogenesis of NADPH-diaphorase activity was studied and compared with the ontogenesis of tyrosine hydroxylase containing cells from embryonic day E15 to postnatal day P30. In the main olfactory bulb, scanty NADPH-diaphorase reactive neurons were first present at E21 in an immature phenotype. The periglomerular positive cells increased in number and acquired their adult morphology in the postnatal period. No colocalization of tyrosine hydroxylase with NADPH-diaphorase was observed at any developmental stage studied. In the granule cell layer, a population of rather bipolar neurons transiently expressed NADPH-diaphorase from P3 to P15; a population of large multipolar cells permanently expressed NADPH-diaphorase from P3 to P30. In the accessory olfactory bulb, NADPH-diaphorase staining appeared in the granule cell layer at P3, and then in the granule cell projections towards the mitral cells. From E21 to P7, neural processes often seemed to contact blood vessels. Endothelial cells showed a diffuse and faint staining at all stages; moreover patches of high NADPH-diaphorase staining were transiently present on blood vessels from E15 to P7. The presence of both permanent and transient expression of NADPH-diaphorase during olfactory bulb genesis is discussed according to the hypotheses of the function of NO during development.

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

The ultrastructural distribution of neuronal (type I) and endothelial (type III) isoforms of nitric oxide synthase in perivascular axons and endothelial cells was examined in the Wistar rat cerebral basilar artery, employing monoclonal antibodies specific either to type I or type III of nitric oxide synthase in pre-embedding peroxidase-antiperoxidase immunocytochemistry. Both neuronal and endothelial nitric oxide synthase were localized in similar proportions of perivascular axons (25.2% +/- 2.8 and 28.7% +/- 6.5, respectively) but in different proportions in vascular endothelial cells (6.2% +/- 0.9 and 27.1% +/- 2.4, respectively). The intracellular distribution of immunoreactivity to neuronal and endothelial nitric oxide synthase was similar both in axons and endothelial cells; e.g. the labelling of the membranes of mitochondria and synaptic/cytoplasmic vesicles. However, the intensity of immunoreactivity was most prominent in profiles positive for endothelial nitric oxide synthase. The neuronal and endothelial nitric oxide synthase-positive axon varicosities were characterized by the presence of small spherical agranular vesicles; agranular vesicles in nerve varicosities positive for neuronal nitric oxide synthase were significantly larger than those in nerve varicosities positive for endothelial nitric oxide synthase (43.2 nm +/- 0.5 versus 37.7 nm +/- 0.8; p < 0.001). Varicosities positive for endothelial nitric oxide synthase also contained large granular vesicles (116.6 nm +/- 5.9) with labelled cores. In conclusion, the present data demonstrate that monoclonal antibodies to neuronal and endothelial nitric oxide synthase immunoreact with subpopulations of both perivascular axons and endothelial cells of rat basilar artery. The significance of neuronal and endothelial isoforms of nitric oxide synthase for the basilar artery is discussed.