Ultrastructure of hypoxic hypertensive pulmonary vascular disease

Deletion of STAT5a/b in vascular smooth muscle abrogates the male bias in hypoxic pulmonary hypertension in mice: implications in the human disease

Chronic hypoxia typically elicits pulmonary hypertension (PH) in mice with a male-dominant phenotype. There is an opposite-sex bias in human PH, with a higher prevalence in women, but greater survival (the "estrogen paradox"). We investigated the involvement of the STAT5a/b species, previously established to mediate sexual dimorphism in other contexts, in the sex bias in PH. Mice with heterozygous or homozygous deletions of the STAT5a/b locus in vascular smooth muscle cells (SMCs) were generated in crosses between STAT5a/b(fl/fl) and transgelin (SM22α)-Cre(+/+) parents. Wild-type (wt) males subjected to chronic hypoxia showed significant PH and pulmonary arterial remodeling, with wt females showing minimal changes (a male-dominant phenotype). However, in conditional STAT5(+/-) or STAT5(-/-) mice, hypoxic females showed the severest manifestations of PH (a female-dominant phenotype). Immunofluorescence studies on human lung sections showed that obliterative pulmonary arterial lesions in patients with idiopathic pulmonary arterial hypertension (IPAH) or hereditary pulmonary arterial hypertension (HPAH), both male and female, overall had reduced STAT5a/b, reduced PY-STAT5 and reduced endoplasmic reticulum (ER) GTPase atlastin-3 (ATL3). Studies of SMCs and endothelial cell (EC) lines derived from vessels isolated from lungs of male and female IPAH patients and controls revealed instances of coordinate reductions in STAT5a, STAT5b and ATL3 in IPAH-derived cells, including SMCs and ECs from the same patient. Taken together, these data provide the first definitive evidence for a contribution of STAT5a/b to the sex bias in PH in the hypoxic mouse and implicate reduced STAT5 in the pathogenesis of the human disease.

Subcellular mechanisms in pulmonary arterial hypertension: combinatorial modalities that inhibit anterograde trafficking and cause bone morphogenetic protein receptor type 2 mislocalization

Abstract The natural history of familial pulmonary arterial hypertension (PAH) typically involves mutations in and/or haploinsuffciency of BMPR2 (gene for bone morphogenetic protein receptor type 2) but with low penetrance (10%-15%), delayed onset (in the third or fourth decade), and a gender bias (two- to fourfold more prevalent in postpubertal women). Thus, investigators have sought an understanding of "second-hit" modalities that might affect BMPR2 anterograde trafficking and/or function. Indeed, vascular lung lesions in PAH have been reported to contain enlarged "vacuolated" endothelial and smooth muscle cells with dilated endoplasmic reticulum (ER) cisternae, increased ER structural protein reticulon 4 (also called Nogo-B), and enlarged and fragmented Golgi apparatus. We recently replicated this cellular phenotype in primary human pulmonary arterial endothelial cells and human pulmonary arterial smooth muscle cells in culture by acute knockdown of the estradiol 17β (E2)-responsive proteins signal transducer and activator of transcription 5a (STAT5a) and STAT5b using small interfering RNAs (siRNAs). We have now investigated whether functional haploinsufficiences of these molecules, alone or in combination with other modalities, might interfere with anterograde membrane trafficking using (a) the quantitative tsO45VSV-G-GFP trafficking assay and (b) assays for cell-surface localization of Flag-tagged BMPR2 molecules. The G glycoprotein of the vesicular stomatitis virus (VSV-G) trafficking assay was validated in EA.hy926 endothelial cells by showing that cells exposed to monocrotaline pyrrole displayed reduced anterograde trafficking. Thereafter, the combinatorial knockdowns of STAT5a, STAT5b, BMPR2, and/or endothelial nitric oxide synthase as well as exposure to E2 or 2-methoxyestradiol were observed to significantly inhibit VSV-G trafficking. These combinations also led to intracellular trapping of wild-type Flag-tagged BMPR2. Overexpression of the PAH disease-derived F14 and KDF mutants of BMPR2, which were trapped in the ER/Golgi, also inhibited VSV-G trafficking in trans. Moreover, probenecid, a chemical chaperone in clinical use today, partially restored cell-surface localization of the KDF but not the F14 mutant. These data identify several combinatorial modalities that inhibit VSV-G anterograde trafficking and cause mislocalization of BMPR2. These modalities merit consideration in defining aspects of the late-developing and gender-biased natural history of human PAH.

Segmentation features and structural organization of the intrapulmonary artery of the yak

This study aims to systematically investigate intrapulmonary artery segmentation, blood vessel wall characteristics and structure organization, and the interrelation between intrapulmonary artery structure and plateau hypoxia adaptation in yak. The normal intrapulmonary artery structure of the yak had been studied using histological methods and transmission electron microscopy. The intrapulmonary artery of the yak was also examined using morphometric analysis and angiography. Results showed that the elastic intrapulmonary artery is divided into two types, namely, classical and transitional elastic segments. The muscular intrapulmonary artery is divided into three types, namely, transitional, classical muscular, and muscular arteriole segments. In the transitional elastic artery, elastic fibers and smooth muscles are linked through three models of ends, lateral branches, and branch tops. Two phenomena are possible for the transition from the elastic intrapulmonary artery to the muscular artery. One phenomenon postulates that a less elastic membrane is first increased and then suddenly decreased, and another supposes that the elastic membrane is gradually reduced and assembled in one to two layers before entering the transitional muscular artery. The smooth muscle of the intrapulmonary artery tunica media had more apophysis; it was physically connected with elastic membrane or fiber and composed of functionally resilient unit of the intrapulmonary arterial wall. Glycogenosomes increased in the muscular intrapulmonary artery smooth muscle cells. It exist one to two layers intact smooth muscle in intrapulmonary arteriole, the presence of intact smooth muscle in the intrapulmonary arteriole of the yak is a kind of structure adaptation to low-oxygen environment.

Non-genomic STAT5-dependent effects at the endoplasmic reticulum and Golgi apparatus and STAT6-GFP in mitochondria

STAT protein species are well-known as transcription factors that regulate nuclear gene expression. Recent novel lines of research suggest new non-genomic functions of STAT5A/B and STAT6. It was discovered in human pulmonary arterial endothelial cells that STAT5A, including STAT5A-GFP, constitutively associated with the Golgi apparatus, and both STAT5A and B with the endoplasmic reticulum. Acute siRNA-mediated knockdown of STAT5A/B led to the rapid development of a dramatic cystic change in the endoplasmic reticulum (ER) characterized by deposition of the ER structural protein reticulon-4 (RTN4; also called Nogo-B) and the ER-resident GTPase atlastin-3 (ATL3) along cyst membranes and cyst-zone boundaries, accompanied by Golgi fragmentation. Functional consequences included reduced anterograde trafficking, an ER stress response (increased GRP78/BiP) and eventual mitochondrial fragmentation. This phenotype was "non-genomic" in that it was elicited in enucleated cytoplasts. In cross-immunopanning assays STAT5A and B species associated with ATL3, and the ER-lumen spacer CLIMP63 (also called cytoskeleton-associated protein 4, CKAP4) but not RTN4. From a disease significance perspective we posit that STAT5, which is known to be affected by estradiol-17β and prolactin, represents the gender-sensitive determinant in the pathogenesis of idiopathic pulmonary hypertension (IPAH), a disease which includes ER/Golgi dysfunctions but with a 2- to 4-fold higher prevalence in postpubertal women. A separate line of recent research produced evidence for the association of STAT6-GFP, but not STAT3-GFP, STAT3-DsRed, or STAT3-Flag, with mitochondria in live-cell, immunofluorescence, and immunoelectron microscopy. An N-terminal truncation of STAT6-GFP (1-459), which lacked the SH2 domain and Tyr-phosphorylation site, constitutively associated with mitochondria. Thus, the emergent new of biology STAT proteins includes non-genomic roles-structurally and functionally-in the three closely related membrane organelles consisting of the endoplasmic reticulum, Golgi apparatus, and mitochondria.

Definitive evidence using enucleated cytoplasts for a nongenomic basis for the cystic change in endoplasmic reticulum structure caused by STAT5a/b siRNAs

STAT5a/b species are well known as transcription factors that regulate nuclear gene expression. In a novel line of research in human pulmonary arterial endothelial cells (HPAECs), we previously observed that STAT5a associated with the Golgi apparatus and that siRNA-mediated knockdown of STAT5a/b led to the rapid development of a dramatic cystic change in the endoplasmic reticulum (ER) characterized by deposition along cyst membranes and tubule-to-cyst boundaries of the proteins reticulon-4 (RTN4; also called Nogo-B) and the ER-resident GTPase atlastin-3 (ATL3) and Golgi fragmentation. We now report that STAT5a can be observed in ER sheets in digitonin-permeabilized HPAECs and that anti-STAT5a cross- immunopanned ATL3 but not RTN4. Moreover, there was marked accumulation of the 63-kDa cytoskeleton-linking membrane protein and ER-spacer CLIMP63 (also called cytoskeleton-associated protein 4, CKAP4) and KDEL-mCherry within the cysts. That the STAT5a/b-siRNA-induced cystic ER phenotype developed in the presence of the transcription inhibitor 5,6-dichloro-1-β-d-ribofuranosylbenzimidazole (DRB) had suggested that the mechanism was independent of the transcription factor functions of STAT5a/b, i.e., was "nongenomic." We have now definitively tested the requirement for the nucleus in eliciting the STAT5a/b-siRNA-induced cystic ER phenotype. Enucleated HPAEC cytoplasts were prepared using adherent 35-mm cultures using the cytochalasin B-centrifugation method (typically yielding 65-75% enucleation). STAT5a/b siRNAs readily elicited the cystic ER phenotype including the marked luminal accumulation of CLIMP63 and Golgi fragmentation in the recovered HPAEC cytoplasts demonstrably lacking a nucleus. These studies provide unequivocal evidence using enucleated cytoplasts for a nongenomic mechanism(s) underlying the cystic change in ER structure elicited by STAT5a/b knockdown.

Nongenomic STAT5-dependent effects on Golgi apparatus and endoplasmic reticulum structure and function

We report unexpected nongenomic functions of signal transducer and activator of transcription (STAT) 5 species in the cytoplasm aimed at preserving the structure and function of the Golgi apparatus and rough endoplasmic reticulum (ER) in vascular cells. Immunoimaging and green fluorescent protein-tagged-STAT5a protein localization studies showed the constitutive association of nonphosphorylated STAT5a, and to a lesser extent STAT5b, with the Golgi apparatus and of STAT5a with centrosomes in human pulmonary arterial endothelial and smooth muscle cells. Acute knockdown of STAT5a/b species using small interfering RNAs (siRNAs), including in the presence of an mRNA synthesis inhibitor (5,6-dichloro-1-β-d-ribofuranosylbenzimidazole), produced a dramatic phenotype within 1 day, consisting of dilatation and fragmentation of Golgi cisternae, a marked tubule-to-cyst change in the ER, increased accumulation of reticulon-4 (RTN4)/Nogo-B and atlastin-3 (ATL3) at cyst-zone boundaries, cystic separation of the outer and inner nuclear membranes, accompanied by scalloped/lunate distortion of the nucleus, with accumulation of RTN4 on convex sides of distorted nuclei. These cells showed inhibition of vesicular stomatitis virus G protein glycoprotein trafficking, mitochondrial fragmentation, and reduced mitochondrial function. STAT5a/b(-/-) mouse embryo fibroblasts also showed altered ER/Golgi dynamics. RTN4 knockdown using siRNA did not affect development of the cystic phenotype; ATL3 siRNA led to effacement of cyst-zone boundaries. In magnetic-bead cross-immunopanning assays, ATL3 bound both STAT5a and STAT5b. Remarkably, this novel cystic ER/lunate nucleus phenotype was characteristic of vascular cells in arterial lesions of idiopathic pulmonary hypertension, an unrelentingly fatal human disease. These data provide evidence of a STAT-family protein regulating the structure of a cytoplasmic organelle and implicate this mechanism in the pathogenesis of a human disease.

Carbon monoxide promotes hypoxic pulmonary vascular remodeling

CO is a biologically active gas that produces cellular effects by multiple mechanisms. Because cellular binding of CO by heme proteins is increased in hypoxia, we tested the hypothesis that CO interferes with hypoxic pulmonary vascular remodeling in vivo. Rats were exposed to inspired CO (50 parts/million) at sea level or 18,000 ft of altitude [hypobaric hypoxia (HH)], and changes in vessel morphometry and pulmonary pressure-flow relationships were compared with controls. Vascular cell single strand DNA (ssDNA) and proliferating cell nuclear antigen (PCNA) were assessed, and changes in gene and protein expression of smooth muscle alpha-actin (sm-alpha-actin), beta-actin, and heme oxygenase-1 (HO-1) were evaluated by Western analysis, RT-PCR, and immunohistochemistry. After 21 days of HH, vascular pressure at constant flow and vessel wall thickness increased and lumen diameter of small arteries decreased significantly. The presence of CO, however, further increased both pulmonary vascular resistance (PVR) and the number of small muscular vessels compared with HH alone. CO + HH also increased vascular PCNA and nuclear ssDNA expression compared with hypoxia, suggesting accelerated cell turnover. CO in hypoxia downregulated sm-alpha-actin and strongly upregulated beta-actin. CO also increased lung HO activity and HO-1 mRNA and protein expression in small pulmonary arteries during hypoxia. These data indicate an overall propensity of CO in HH to promote vascular remodeling and increase PVR in vivo.

Biological chemistry of carbon monoxide

Carbon monoxide (CO) has many effects in biology due to its complex biochemical activities. These actions of CO depend primarily on its ability to bind heme proteins (Hp) and to inhibit or alter their biochemical functions. Whether CO is derived from exogenous or endogenous sources, its cellular activity is related to its concentration and the concentration of molecular O(2), as well as to the availability of reduced transition metals such as Fe(II). In this respect, the CO/O(2) ratio and O(2)-dependent changes in local oxidation-reduction state assume critical importance in determining the physiological effects of CO by affecting the functions of specific Hp. By interacting with Hp, CO influences electron-transport reactions in a variety of ways, which can produce either prooxidant or antioxidant effects. Similarly, Hp relationships also govern how changes in CO concentration influence the physiological and pathological effects of nitric oxide and the relationships of the two biologically active gases to metal-catalyzed oxidations. This article provides a brief update on the biochemistry of CO as it relates to Hp binding, chemical oxidative processes, and cellular function.

Expression of heme oxygenase-1 in the lung in chronic hypoxia

Heme oxygenase (HO)-1 is an oxygen-dependent enzyme that may regulate vascular tone and cell proliferation through the production of carbon monoxide (CO). We tested the hypothesis that HO-1 is upregulated in the lung in chronic hypoxia by exposing male Sprague-Dawley rats to 17,000 feet (395 Torr) for 0, 1, 3, 7, 14, or 21 days. After exposure, blood gases, carboxyhemoglobin (COHb) levels, and hematocrit were measured, and the lungs were either inflation fixed for immunohistochemistry or frozen for later measurement of HO enzyme activity, Western blot for HO-1 protein, and RT-PCR for HO-1 mRNA. The heart was excised and weighed, and the right-to-left heart weight ratio was determined. During hypoxia, the hematocrit increased progressively, reaching significantly higher values than the control value after 3 days. COHb levels increased above the control value after 1 day of hypoxia and increased progressively between 14 and 21 days, whereas arterial PO(2) and arterial PCO(2) did not vary significantly. HO-1 protein determined by Western blot increased for the first 7 days and declined thereafter; however, enzyme activity was elevated only after 1 day. Changes in HO-1 during hypoxia were localized by immunohistochemistry to inflammatory cells (early) and newly muscularized arterioles (later). Lung HO-1 mRNA normalized to glyceraldehyde-3-phosphate dehydrogenase was increased after 1 and 21 days. The data indicate that lung HO-1 protein and activity are upregulated only during early chronic hypoxia, whereas persistent COHb elevations indicate high endogenous CO production rates at nonpulmonary sites. If CO has antiproliferative properties, the lack of HO enzyme activity in the lung may be permissive for pulmonary vascular proliferation in hypoxia.

Pulmonary, vascular responses in rats exposed to chronic hypobaric hypoxia at two different altitude levels

High altitude hypoxia is known to cause pulmonary hypertension in humans. Altitudes of 5500 m and 4000 m above sea level are considered to be the upper limits for short-term human survival and long-term human residence, respectively. To study the effects of hypobaric-hypoxic environments on pulmonary vascular beds, the physiologic and morphometric differences occurring in the pulmonary arteries of rats at the equivalent of these two altitude levels were compared. One hundred and ninety male rats were housed in a double-roomed mechanical chamber and subjected for 12 weeks to hypobaric-hypoxic environments equivalent to an altitude level of 5500 m or 4000 m. After 6-8 weeks, mean pulmonary arterial pressure (PAPm) was significantly higher in rats at the 5500 m level than in those at 4000 m. The external diameter of muscular arteries (M) and of partially muscular arteries (PM) in the lungs of rats at 8 and 12 weeks, and those of nonmuscular arteries (NM) around the alveolar sacs at 2 and 8 weeks were each significantly less in rats exposed to the 5500 m level than in those at 4000 m. At 4 weeks, the ratio of medial thickness to external diameter for M and PM around the alveolar ducts and alveolar sacs were greater in rats exposed to the 5500 m level than in those at 4000 m. Erythrocyte count, hemoglobin and hematocrit tests showed polycythemic patterns during the initial hypobaric period in both 5500 m-, and 4000 m-level rats. However, from 4 to 12 weeks, the hematologic values of the 4000 m-level rats declined progressively toward the upper end of the normal range. This study suggests (i) that elevated PAPm in rats exposed to hypobaric-hypoxic environments results from structural remodeling of all M, PM and NM within the lobule, and (ii) that an environment equivalent to the 5500 m level causes more severe pulmonary vascular changes than one equivalent to the 4000 m level. The hematologic results suggest that rats exposed to the equivalent of the upper limit for long-term human residence can acquire acclimatization, but not those exposed to the equivalent of the upper limit for short-term human survival.

Chronic hypoxia impairs pulmonary venous smooth muscle contraction

Chronic hypoxia increases total pulmonary vascular resistance and causes pulmonary hypertension. Although the effect of chronic hypoxia on pulmonary arterial tissue has been extensively studied, very little is known about the effects on the pulmonary vein. The purpose of the present investigation was to determine the effect of chronic hypoxia on pulmonary venous reactivity to several vasoactive agonists and on the venous response to acute hypoxia. Isolated pulmonary venous rings were taken from rats exposed to 2, 7, and 14 days of hypoxia (FIO2 = 0.1). A decrease in the response of the pulmonary vein to KCl was observed after 14 days of hypoxia. The reactivity (maximum active force produced) of the pulmonary vein in response to phenylephrine (PE) was reduced after 7 days of hypoxia. The response of the pulmonary vein to angiotensin II (AII) was more sensitive to the effects of chronic hypoxia since decreased reactivity to angiotensin II occurred after only 2 days of hypoxia. Prolonged hypoxia (14 days) had no further effect on the decreased reactivities to PE and AII. The sensitivities of pulmonary venous muscle to PE and AII were decreased (increased ED50 values) by 2 days of chronic hypoxia, but tended to return to control levels after 7 and 14 days of hypoxia. However, the contractile response of the pulmonary vein to acute hypoxia was not changed even after 14 days of chronic hypoxia. These results suggest that chronic hypoxia: (1) impairs pulmonary venous smooth muscle contractility; (2) reduces pulmonary venous reactivity and sensitivity to phenylephrine and angiotensin II; and (3) does not alter the pulmonary venous contractile response to acute hypoxia.

Ultrastructure of the lung in chronic hypoxia

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Exploration of the pulmonary circulation. Festschrift to Professor Donald Heath

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Endothelial cell protrusions in the rat aortic wall. Immunocytochemical evidence for an alternative transendothelial passage of plasma proteins

Focal morphological changes in the endothelial lining were observed in the aortic wall of control rats. They consisted of endothelial cytoplasmic projections and vacuolar structures protruding towards the luminal space and containing electron-dense material. Some of these structures were observed to open into the subendothelial space. Endogenous albumin was detected in these compartments by applying protein A-gold immunocytochemistry to thin tissue sections of glutaraldehyde-fixed, Lowicryl-embedded aortic segments. The labelling was mainly distributed along the plasma membrane of the projections as well as over the dense content of the endothelial protrusions. The presence of endogenous albumin in these endothelial structures, together with their opening into the subendothelial space, suggests a role for these structures in an alternative transendothelial transport of albumin.

The physiological closure of ductus arteriosus in the rat. An ultrastructural study

The evolution of morphological changes in the wall of the ductus arteriosus during its physiological closure in newborn rats was examined by electron microscopy. The contraction of smooth muscle cells in the tunica media seems to be the primary mechanism which leads to the physiological closure of the ductus arteriosus. For this reason our attention was centred mainly on the morphology of the tunica media. No important changes in the ultrastructure of smooth muscle cells can be observed in the early phases of the closure. Most of them exhibit ultrastructural features of cells with enhanced synthetic activity during all phases of the closure. The permanent contraction of smooth muscle cells results in their morphological changes. The most striking is the herniation of smooth muscle cell cytoplasm into the endothelial and later into adjoining muscle cells. These changes together with signs of degeneration of the smooth muscle cells are already clearly discernible 120 min after birth. The elastic component of the tunica media exhibits surprisingly fast changes. As soon as 60 min after birth, the fragmentation of elastic membranes and their structural changes provided evidence about the degradation of elastic material. The matrix vesicles, probably derived from the lysosomal apparatus of the muscle cells, may play an essential role in this process.

The structure of the wall of the rat intraacinar pulmonary artery: an electron microscopic study of microdissected preparations

Rat intraacinar arterial segments that by light microscopy lack a medial muscle layer are capable of constriction and, during pulmonary hypertension, acquire morphologically differentiated smooth muscle. These facts suggest that effector cells of smooth muscle type are present in the normal vessel wall. Studies in the literature, however, fail to agree on their location or even existence. By combining microdissection, step sectioning, and electron microscopy, we have now performed a serial study of six arterial pathways. At its proximal end, the artery has a circumferentially continuous single layer of smooth muscle cells, separated from the endothelium by a fenestrated internal elastic lamina. Myoendothelial junctions are frequent and incorporate basal laminae of both cell types. More distally, the internal elastic lamina is discontinuous and the smooth muscle cells lose myofilaments and dense bodies so as to resemble intermediate cells. They still form a continuous layer, however. At mid-alveolar-duct level, this layer is discontinuous, and in the most distal arteries investigated, the cells are often solitary. They lie close to the endothelial cell, but, except for localized regions of contact, are separated from it by a single basal lamina that is continuous with one covering their abluminal surface.

The adrenergic innervation of pulmonary vasculature in the normal and pulmonary hypertensive rat

It would appear that susceptibility to chronic proliferative pulmonary hypertension in response to chronic alveolar hypoxia is most severe in species in which adrenergic innervation of pulmonary arteries is reduced or lacking. Intrapulmonary arteries of the rat have been reported to lack adrenergic innervation by some workers but not others. Since the rat develops severe proliferative pulmonary hypertension in response to prolonged alveolar hypoxia, the different divisions of the lung vasculature of Sprague-Dawley rats were thoroughly examined to determine the presence or absence of an adrenergic innervation. The degree of innervation in normal rats was compared with that of rats developing pulmonary hypertension. Both in normal and experimental pulmonary hypertensive rats the pulmonary arteries, all their branches and the small pulmonary veins with a smooth muscle media were found to be devoid of adrenergic innervation. In contrast, the cardiac-like muscle in the media of large pulmonary veins, the bronchial arteries and the vasa vasorum of larger vessels were richly innervated by adrenergic nerves. Thus the increase in medial smooth muscle which occurs in pulmonary arteries during chronic alveolar hypoxia is independent of a pre-existing adrenergic innervation or of such an innervation newly derived from that of adjacent vessels or structures. This is in contrast to systemic vessels where it has been suggested that increased adrenergic activity and density of innervation may augment hypertrophy of the media in hypertensive animals. Adrenergic nerves are suggested to have a protective action on pulmonary vessels.

Ultrastructural pathology of intrapulmonary arteries in 3-methylindole-induced pneumotoxicity in cattle: II. Glycogen accumulation in the smooth muscle cells and intimal changes

3-Methylindole (3MI) was orally administered as a single dose of 0.1-0.2 g/kg which then produced acute pulmonary oedema in cattle 72 h post-treatment. Electron microscopic examination of intrapulmonary arteries ranging between 35-120 microns in the control animals, revealed microsequestration of rough endoplasmic reticulum (RER) and the storage of proteinaceous material in the dilated cisternae of RER of the endothelial cells. Lung tissue samples collected after 72 h of 3MI administration, showed denudation and hyperplasia of the endothelial cells. Massive storage of glycogen particles and the lamellar bodies in association with glycogen, were observed in the smooth muscle cells of the media. The importance of glycogen accumulation in vascular smooth muscle cells of the pulmonary arteries is discussed in terms of energy utilization and vascular energetics to meet the metabolic requirement of haemodynamic stress arising from alveolar hypoxia and possible vasoconstriction. Furthermore, the suggestion is made, that the alterations may signify the early changes of pulmonary hypertension induced by the toxic effect of 3MI in cattle.

Smooth muscle myosin in precursor and mature smooth muscle cells in normal pulmonary arteries and the effect of hypoxia

Exposure to hypoxia increases pulmonary arterial muscularity-in the intra-acinar arteries "new" muscle appears in the normally nonmuscular regions and in the preacinar arteries, medial thickness increases. In the present study by immunofluorescence techniques, the myosin content of the pulmonary arterial walls at two levels of the circulation (intra-acinar and preacinar) were studied in control rats and those exposed to hypobaric hypoxia of 380 torr for 3,7, 10, or 14 days. In control animals, we show that the precursor smooth muscle cells, pericytes and intermediate cells normally present in the nonmuscular regions of the intra-acinar arteries, contain smooth muscle myosin. With exposure to hypoxia, smooth muscle myosin in the intra-acinar arteries increases to Day 10, both in area of staining and fluorescent intensity. This is in contrast to the preacinar arteries were only the area of myosin increases. Antihuman platelet (non-muscle) myosin shows a little faint staining in both control and hypoxic animals. Adaptations to hypoxia by the intra-acinar precursor and preacinar mature smooth muscle cells is different, and suggests that the functions subserved by the myosin filaments at each of the two levels differs.

Light and electron microscopic studies on rat arterial lesions induced by experimental arterial contraction

Experimental contraction was produced in the rat mesenteric arteries and the arterial segments were studied morphologically. When the rat mesenteric artery was exposed in physiological saline solution at 37 degrees C and 2-3 mg of methoxamine hydrochloride (10 mg/ml) was dripped onto it, intense contraction was observed for about 30 min but elevation in blood pressure was slight. During the contraction, numerous vacuoles were seen in the medial smooth muscle cells of the arterial segments, and these vacuoles were shown electron microscopically to have double unit membranes, indicating that they were formed by herniation of a part of the adjacent smooth muscle cell body. In the arteries 1-6 h after the end of the contraction, cellular, nuclear and vacuolar membranes and myofilaments of the medial muscle cells were partially lost. 12-24 h after the contraction the arteries exhibited necrosis and desquamation of endothelial cells and platelet adhesion. In the media, smooth muscle cells were completely deprived of cell membranes, myofilaments, nuclei, intracytoplasmic organelles other than mitochondria, and vacuolar membranes. The cytoplasm was filled with fine granular and granulo-vesicular material, and fibrin insudation was observed in these severely damaged cells. Arterial contraction may be an important factor in the induction of arterial lesions.

Electron microscopy of the plexiform lesion

Pulmonary arteries from a case of plexogenic pulmonary arteriopathy were studied with the electron microscope. Many muscular pulmonary arteries showed intimal fibrosis of concentric or "onion-skin" type. The cells embedded within this fibrosis resembled smooth muscle but since they also possessed some features of fibroblasts we refer to them as myofibroblasts. Myofibroblasts also occurred in plexiform lesions together with fibroblasts and "fibrillary cells". These fibrillary cells contained numerous, prominent filaments with a random orientation. They lined the vascular channels of the plexiform lesions as well as being present within the stroma. They appeared to phagocytose fibrin and then to organise the plexiform lesion into a fibrous scar. Fibrillary cells closely resemble vasoformative reserve cells and the cells of the cardiac myxoma and so-called "papillary tumour" of heart valves. They may, therefore, be primitive multipotential cells found throughout the entire cardiovascular system.

Ultrastructural features of the distended pulmonary arteries of the normal rat

Detailed study has been made of the structure of the normal pulmonary artery of rat by both light (1-micrometer sections) and electron microscopy. After tying the pulmonary veins at the hilum, the lungs were fixed by simultaneous injection of glutaraldehyde into the pulmonary trunk and trachea. Study of distended arteries allows precise measurement and assessment of normal lung structure. Four regions of the pulmonary artery can be identified by wall structure and are described here--muscular, partially muscular, non-muscular and the newly described thick-walled oblique muscular. Electron microscopid examination has demonstrated in the non-muscular regions of the partially muscular arteries, an "intermediate" cell and in the non-muscular arteries, a pericyte. The intermediate cell lies internal to the single elastic lamina but external to the endothelial cell, is surrounded by its own basement membrane and contains filaments mainly along the adluminal region of the cell. The pericyte also lies internal to the single elastic lamina, is within the basement membrane of the adjacent endothelial cell and has previously been reported in the lung only in the walls of alveolar capillaries. The structure of the intermediate cell and its position suggest it is a transitional stage between the pericyte and smooth muscle cell.

Evagination of vascular smooth muscle cells during the early stages of Crotalaria pulmonary hypertension

Fifteen adult female Wistar albino rats were fed on a diet containing powdered Crotalaria spectabilis seeds for periods of up to 5 wk. Electron microscopic studies were carried out on the lungs of these animals and also of three control rats. Both groups of animals showed protuberances of smooth muscle cells. In the control rats such protuberances were small and filled out spaces created by undulation of the internal elastic lamina produced by collapse of the vessel. These protuberances could be prevented by fixing the lung in distension. Evaginations of smooth muscle cells in the test rats were larger, devoid of myofilaments and organelles and arose from the parent cell between dense attachment points on the sarcolemma. Frequently they arose through a narrow cytoplasmic isthmus and had such electron-lucent contents as to resemble a cyst within the endothelium. In fact they pressed onto the undersurface of endothelial cells which fitted over them like a cap. Such evaginations are thought to arise as a result of sustained vasoconstriction.

Evagination of smooth muscle cells in the hypoxic pulmonary trunk

Six female Wistar albino rats were exposed to the hypoxia of a simulated altitude of 5500 m, three for a period of one week and three for a month. They developed ultrastructural changes in the pulmonary trunk consisting of evaginations of muscle cells of its media through gaps in the internal elastic lamina to press into the underlying endothelial cells. Such evaginations were usually devoid of myofilaments and organelles. Some appeared so electron-lucent as to be unrecognisable as muscle apart from the unequivocal connection with the parent smooth muscle cells. Elsewhere we have demonstrated that muscular evaginations in normal pulmonary blood vessels are an artefact brought about by collapse of lung tissue and that they can be avoided by distending the lung. Hence in the present investigation, in which the pulmonary trunk was fixed in distension, the evaginations are interpreted as indicating contraction of the muscle cells able to overcome the distending force. We interpret them as evidence of constriction of muscle cells in the media of the pulmonary trunk in response to hypoxia.