Changes in the composition and metabolism of arterial collagens during the development of pulmonary hypertension in rabbits.

Increased pulmonary artery pressure is known to result in enhanced collagen deposition in the pulmonary artery. Here we investigate how changes in collagen metabolism may bring about this increased deposition in the pulmonary artery of animals with pulmonary hypertension induced by bleomycin. Rabbits were injected intratracheally with bleomycin sulfate or with saline. After 14 days the animals were injected with L-[U-14C]proline plus a "flooding" dose of unlabeled proline. Uptake into arterial collagens and release of labeled hydroxyproline were then measured after 2.5 h. The relative amounts of types I and III collagens were assessed from the levels of cyanogen-bromide-derived peptides alpha 1(I)CB8 and alpha 1(III)CB5, respectively, after sodium dodecyl sulfate polyacrylamide gel electrophoresis. Collagen synthesis rates of about 3%/day were found in the control pulmonary artery and aorta, and about one-half of the newly synthesized collagen was degraded rapidly. Fourteen days after bleomycin, there was a fivefold increase in collagen synthesis rate (p less than 0.01) and a marked decrease in the percentage of newly synthesized collagen degraded rapidly. There was no change in collagen metabolism in the aorta of these animals. Pulmonary artery collagen from control rabbits consisted of 26.5 +/- 1.0% type III collagen. There was no change in composition in bleomycin-treated animals. This study demonstrates quite rapid turnover rates for collagen in normal blood vessels. Our results also indicate that remodeling of arterial connective tissue matrix during pulmonary hypertension involves marked but commensurate increases in type I and III collagens brought about by changes in both synthesis and degradative processes.

Characterization and distribution of P2-purinoceptor subtypes in rat pulmonary vessels.

To characterize P2-purinoceptors in pulmonary vessels we have examined the effects of ATP analogs on rat isolated pulmonary artery and vein in vitro. The rank order of potency for causing vasoconstriction was: alpha,beta-methylene-ATP (alpha,beta-meATP) greater than beta,gamma-methylene-ATP (beta,gamma-meATP) greater than 2-methylthio-ATP (2m.S.ATP) greater than ATP for arteries; and alpha,beta-meATP much greater than beta,gamma-meATP = 2m.S.ATP greater than ATP for veins, indicating that a P2x receptor was involved. The contractile response to these analogs was virtually abolished after desensitization of P2x-receptors by alpha,beta-meATP. Removal of the endothelial cells enhanced the contractile responses to all of the ATP analogs in both arteries and veins. The rank order of potency for vasodilatation was 2m.S.ATP much greater than ATP = beta,gamma-meATP much greater than alpha,beta-meATP for arteries and 2m.S.ATP much greater than ATP = beta-gamma-meATP, with alpha,beta-meATP being no effect for veins, indicating a P2y receptor. Pretreatment of the pulmonary arteries with the P2y-antagonist reactive blue 2 caused a rightward shift of the dose-response curves to 2m.S.ATP, ATP and beta,gamma-meATP. Reactive blue 2 was only used with the pulmonary arteries. Removal of the endothelium converted the relaxant responses to all the ATP analogs (except to ATP in pulmonary artery) to further contraction. In the pulmonary artery, the small endothelium-independent relaxation induced by ATP was abolished completely by pretreating the vessels with 100 microM theophylline (a P1-purinoceptor antagonist), suggesting that it was due to conversion of ATP to adenosine.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution of prostacyclin synthase, 6-keto-prostaglandin F1 alpha, and 15-hydroxy-prostaglandin dehydrogenase in the normal and persistent ductus arteriosus of the dog.

The presence of prostacyclin synthase (PGI2 synthase), 6-keto-prostaglandin F1 alpha (6k-PGF1 alpha), and the stable hydrolysis product of prostacyclin (PGI2), prostaglandin E2 (PGE2), as well as the activity of 15-hydroxy-prostaglandin dehydrogenase (PGDH) were studied in the aorta, pulmonary artery, the normal ductus arteriosus (DA), and persistent DA (PDA) of the dog using histochemical and immunohistochemical techniques. The normal DA is characterized by the development of intimal thickening, a process that does not occur in the persistent DA. Distribution of PGI2 synthase was identical in the aorta, pulmonary artery, and persistent DA. In these vessels endothelial cells contained higher levels of PGI2 synthase as compared with medial smooth muscle cells. In the normal DA, levels of PGI2 synthase were clearly higher in smooth muscle cells at the sites of intimal thickening than at other sites. Distribution of 6-keto-PGF1 alpha resembled the localization of PGI2 synthase. Presence of PGE2 and activity of PGDH could not be demonstrated. The results demonstrated existence of a clear relationship between ductal morphology and the presence of PGI2 synthase. This finding suggests a more important role for PGI2 in regulating ductal patency than has heretofore been appreciated. It was assumed that the role of PGI2 in regulating ductal patency is, at birth, at least overruled by the constrictive effect of the cytochrome P450 mono-oxygenase mechanism. It is still possible to attribute a role to PGI2 in the regulation of cushion formation. Once smooth muscle cell activity has been enhanced by the presence of a glycosaminoglycan rich environment, increase in PGI2 may produce a concurrent inhibition of smooth muscle cell growth.

Heparitinases facilitate separation of disaccharides of heparan sulfate isomers in human arteries using high-performance liquid chromatography.

The constituents of heparan sulfate isomers in human arteries were analysed at the disaccharide unit by high-performance liquid chromatography. Heparitinases I and II facilitated differentiation of six unsaturated disaccharides from heparan sulfate isomers. The variously sulfated disaccharide components of these heparan sulfate isomers were detected after digestion with heparitinases I and II. The heparan sulfate isomers in the aorta and pulmonary arteries were found to consist of various disaccharide units. These heparan sulfate isomers in the arteries are apparently formed during the process of aging and may influence arterial matrix components.

[Identification and characteristics of the beta-adrenergic receptors in the membranes of cultured endothelial cells of the human pulmonary artery]. / Identifikatsiia i kharakteristika beta-adrenergicheskikh retseptorov v membranakh kul'tiviruemykh kletok éndoteliia legochnoi arterii cheloveka.

The properties of beta-adrenoreceptors (beta-AR) have been studied in endothelial cells from the human pulmonary artery (EPA) and umbilical vein (EUV). [125I] Iodosyanopindolol binding assay revealed the amount of 22 and 12 fmol/10(6) cells as well as Kd = 92 and 52 pM for EPA and EUV, respectively. Adrenergic agonists increased the cAMP levels in EPA in the order of potency characteristic for beta 2-AR: isoproterenol greater than epinephrine greater than norepinephrine. Basing on the results obtained in the present study and the literature data a conclusion is made that the properties of beta-AR in vascular endothelial cells do not practically depend on their localization, suggesting universality of their function within the vascular system.

Angiotensin receptors in pulmonary arterial and aortic endothelial cells.

Angiotensin II (ANG II) is formed from angiotensin I by the action of angiotensin-converting enzyme located on the luminal surface of vascular endothelial cells. We determined whether binding sites specific for ANG II exist on pulmonary artery and aortic endothelial cells. The binding of 125I-ANG II to pulmonary artery and aortic endothelial cells was time dependent, saturable, and reversible. Scatchard analysis indicated a single class of high-affinity binding sites with equilibrium dissociation constants (Kd) of 0.85 and 0.81 nM and total binding capacities of 70 and 73 fmol/mg protein in pulmonary artery and aortic endothelial cells, respectively. Angiotensin analogues [Sar1,Ile8]ANG II and [Sar1,Ala8]ANG II, as well as angiotensin I and angiotensin III, competitively displaced 125I-ANG II in both pulmonary artery and aortic endothelial cells. The degree of inhibition of 125I-ANG II binding by these angiotensin analogues and antagonists was comparable except that [Sar1,Ala8]ANG II was 65% less potent than the other antagonists in both cell types. The binding of 125I-ANG II in pulmonary artery and aortic endothelial cells was not affected by vasopressin, substance P, or insulin, suggesting the presence of specific angiotensin receptors on these cells. These receptors appear to recognize the general configuration of angiotensin peptide rather than being specific to ANG II with no major differences between endothelial cells from pulmonary arterial or aortic vessels.

Canine pulmonary artery contains a homogeneous population of alpha-1 adrenoceptors.

We have investigated systemically the hypothesis that there exist subtypes of the alpha-1 adrenoceptor in the canine pulmonary artery. The affinity of the alpha-1 adrenoceptor antagonist, prazosin, was assessed by Schild Regression Analysis against the agonists, phenylephrine, norepinephrine and methoxamine, in rings prepared from canine pulmonary artery using the classical technique in which control and treatment concentration-response curves are obtained in every tissue. This technique allows for appropriate corrections to be made in tissue-dependent differences in agonist potency, time-dependent changes in agonist sensitivity and differences in maximum responses that result from nonspecific effects of agonists occurring at higher concentrations, as was found to be the case for methoxamine. Two cumulative concentration-response curves were constructed for each agonist at 2-hr intervals, with the first (control) concentration-response curve being constructed before the addition of prazosin, and the second concentration-response curve obtained 1 hr after exposure to prazosin. Prazosin had similar pA2 values against phenylephrine (9.56), norepinephrine (9.32) and methoxamine (9.35), with slopes of the Schild Regressions not differing significantly from the theoretical value of unity, suggesting that blockade was competitive in all cases. The results provide no evidence for heterogeneity in alpha-1 adrenoceptors in canine pulmonary artery when assessed using the classical method involving two concentration-response curves in a single tissue and allowing for appropriate corrections in tissue-dependent differences and time-dependent changes in agonist response.

Myosin heavy-chain isoforms in human smooth muscle.

The myosin heavy-chain composition of human smooth muscle has been investigated by sodium dodecyl sulfate/polyacrylamide gel electrophoresis, enzyme immunoassay, and enzyme-immunoblotting procedures. A polyclonal and a monoclonal antibody specific for smooth muscle myosin heavy chains were used in this study. The two antibodies were unreactive with sarcomeric myosin heavy chains and with platelet myosin heavy chain on enzyme immunoassay and immunoblots, and stained smooth muscle cells but not non-muscle cells in cryosections and cultures processed for indirect immunofluorescence. Two myosin heavy-chain isoforms, designated MHC-1 and MHC-2 (205 kDa and 200 kDa, respectively) were reactive with both antibodies on immunoblots of pyrophosphate extracts from different smooth muscles (arteries, veins, intestinal wall, myometrium) electrophoresed in 4% polyacrylamide gels. In the pulmonary artery, a third myosin heavy-chain isoform (MHC-3, 190 kDa) electrophoretically and antigenically distinguishable from human platelet myosin heavy chain, was specifically recognized by the monoclonal antibody. Analysis of muscle samples, directly solubilized in a sodium dodecyl sulfate solution, and degradation experiments performed on pyrophosphate extracts ruled out the possibility that MHC-3 is a proteolytic artefact. Polypeptides of identical electrophoretic mobility were also present in the other smooth muscle preparations, but were unreactive with this antibody. The presence of three myosin heavy-chain isoforms in the pulmonary artery may be related to the unique physiological properties displayed by the smooth muscle of this artery.

Chronic hypoxia alters structure and transmitter dynamics in dog pulmonary artery.

Confinement of dogs to 10% oxygen for 14 days caused erythropoiesis and pulmonary hypertension. Histological sections of the lung tissue showed thickening of the smooth muscle component of muscular arteries and arterioles. Segments of pulmonary artery from dogs exposed to hypoxia were superfused under continuation of hypoxic conditions or after return to oxygenated conditions. Parallel segments of pulmonary artery from normal dogs were also studied. Norepinephrine stores were labeled with [3H]norepinephrine and measurements were made of [3H]norepinephrine and its radiolabeled metabolites (separated by column chromatography) in superfusates using liquid scintillation spectrometry. Chronic hypoxia (1) reduced neuronal uptake of NE from synaptic clefts, (2) reduced the content of DOPEG in superfusate from tissues studied during continuation of hypoxic conditions and in tissues studied after return to oxygenated conditions, (3) increased extraneuronal uptake of NE and (4) increased overflow of NE from synaptic clefts. In similar segments of pulmonary artery removed from the same lung, endogenous free and conjugated norepinephrine and dopamine were measured in pulmonary artery by liquid chromatography with electrochemical detection. The tissue content of free norepinephrine after stimulation was reduced, which was compatible with the reduction in neuronal uptake. Conjugated norepinephrine was a minor metabolite and was increased modestly compared to concentrations reported previously in pulmonary artery from normal dogs.

Polyamine content in pulmonary arteries from rats with monocrotaline-induced pulmonary hypertension.

Based on the documented role of polyamines in regulation of cell growth and differentiation, we have proposed that these organic cations are essential for hypertrophic and hyperplastic responses of pulmonary vascular cells which underlie development of hypertensive pulmonary vascular disease. In support of this contention, we have shown in rat models of monocrotaline (MCT)- and chronic hypoxia-induced pulmonary hypertension that whole-lung polyamine contents are elevated and that blockade of polyamine synthesis forestalls development of hypertensive pulmonary vascular remodeling and sustained pulmonary hypertension. To determine if the involvement of polyamines in pulmonary hypertension could be ascribed to events occurring in cells of the vascular wall, as opposed to parenchymal or airways cells, the present study measured polyamine contents as a function of time in macroscopic pulmonary arteries from MCT-treated rats. Rats were given MCT or its vehicle and, at 1,4,7, and 21 days post-treatment, the contents of putrescine, spermidine, and spermine were assessed in whole right lung and in an arterial segment from the left lung consisting of the first and extending to the sixth intrapulmonary branch. Relative to control animals, contents of putrescine and spermidine were elevated in whole lung at 4, 7 and 21 days post MCT while the content of spermine was elevated above control at 21 days. In pulmonary arterial segments, contents of putrescine and spermidine were augmented significantly at 7 and 21 days after MCT treatment. Spermine content in arterial segments was increased at 21 days. Right ventricular hypertrophy indicative of sustained pulmonary hypertension was not evident until 21 days post-treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Concurrent separation of catecholamines, dihydroxyphenylglycol, vasoactive intestinal peptide, and neuropeptide Y in superfusate and tissue extract.

A method is described for separation and quantification of 3,4-dihydroxyphenylglycol (DO-PEG), norepinephrine (NE), dopamine (DA), vasoactive intestinal peptide (VIP), and neuropeptide Y (NPY) from single samples of tissue homogenate and from superfusate from in vitro dog blood vessel preparations using cartridges containing 0.4 g of octadecylsilane (Sep-Pak C-18). Samples were passed through the cartridge at pH 7.4. A step-gradient system was used to first selectively desorb the catechols (DOPEG, NE, DA) with a moderately polar eluent; subsequently VIP and NPY were eluted with 2.5 ml of a mixture of 1% trifluoroacetic acid, 80% acetonitrile. Five Sep-Pak catechol eluents were tested. Catechols were quantified by HPLC with electrochemical detection and peptides by radioimmunoassay. An HPLC solvent system is described which is particularly useful for chromatography of the more hydrophilic catechols DOPEG, 3,4-dihydroxymandelic acid, and 3,4-dihydroxyphenylalanine concurrently with catecholamines. For superfusion studies, sample cleanup time was reduced to about 4 min per sample by attachment of the cartridges directly to the bottom of the superfusion chamber. Superfusate was subsequently pulled through the cartridges immediately after they were passed over the tissue. Batches of 12 high-speed tissue supernates were processed through the method in about 30 min. The method was used to analyze DOPEG, NE, DA, VIP, and NPY in various rat and dog tissues. The values obtained were similar to values obtained previously by other methods. Because the catechols and peptides are separated from a single sample, the method has several advantages over those described previously; e.g., it is rapid, simple, and more sensitive.

Altered elastin and collagen synthesis associated with progressive pulmonary hypertension induced by monocrotaline. A biochemical and ultrastructural study.

Changes in elastin and collagen synthesis in the pulmonary artery wall, assessed both biochemically and ultrastructurally, were related to the development of progressive pulmonary hypertension induced by the toxin monocrotaline. Male Sprague-Dawley rats (200 to 225 gm) were injected subcutaneously in the hind flank with either monocrotaline (60 mg/kg) or an equivalent volume of saline and studied 8, 16 and 28 days later. At each time point, the right ventricle and left ventricle with septum were separated and weighed to follow the development of right ventricular hypertrophy. The hilar pulmonary artery was assessed by light microscopy for medial hypertrophy and by electron microscopy for changes in the endothelium, subendothelium and media. The mainstem pulmonary artery was used to determine synthesis of elastin and collagen by in vitro incorporation of [14C]proline into nonelastin, [14C]hydroxyproline into collagen, and [3H]valine into cyanogen bromide-insoluble elastin. In addition, total content of insoluble elastin was determined by weight of the residue after cyanogen bromide digestion and of collagen by total hydroxyproline content in sodium dodecyl sulfate and cyanogen bromide extracts. Eight days after monocrotaline injection, there was pulmonary artery endothelial swelling and a significant decrease in the number of myoendothelial junctions (p less than 0.05) associated with a decreased proportion of amorphous elastin in the media (p less than 0.01). Sixteen days after monocrotaline injection, a decrease in the proportion of elastin in the media was still evident (p less than 0.01) despite an apparent increase in insoluble elastin synthesis. Moreover, the amorphous elastin was distributed preferentially in small islands rather than in laminae (p less than 0.05). Twenty-eight days after monocrotaline injection, medial and right ventricular hypertrophy had developed (p less than 0.05 and p less than 0.01, respectively). At the same time, there was a striking increase in both insoluble elastin synthesis and total insoluble elastin content (p less than 0.01 for both) and an increase in collagen synthesis and total collagen content (p less than 0.05 for both). In addition, the ratio of insoluble elastin synthesis to collagen synthesis was greater than in controls (p less than 0.01), whereas the ratio of total insoluble elastin to total collagen did not change. On ultrastructural analysis, the proportion of amorphous elastin in the vessel wall relative to other elements remained low (p less than 0.01) and was distributed throughout the media as increased numbers of small islands (p less than 0.01).(ABSTRACT TRUNCATED AT 400 WORDS)

High hyaluronic acid and low dermatan sulfate contents in human pulmonary arteries compared to in the aorta.

We examined the glycosaminoglycan (GAG) components in human pulmonary arteries compared to those in the aorta. Definite changes in GAG contents and components were found between the pulmonary arteries and the aorta. The GAG content of the aorta was constantly higher than that of the pulmonary artery. As to the components of the arterial GAGs, however, hyaluronic acid comprised significantly higher proportions of total GAGs in the pulmonary arteries than in the aorta, whereas dermatan sulfate showed lower proportions in the pulmonary arteries than in the aorta. These significant differences are possibly due to either the effect of the continuously different blood pressure on the wall of these two types of arterial vessels or to constitutional changes of their fundamental structures associated with atherosclerosis.

Postnatal development of the innervation and paraganglia in the porcine pulmonary arterial bed.

The innervation of the pulmonary trunk and pulmonary arterial bed was studied in 17 pigs from birth to 6 months of age. After birth, the pulmonary trunk and extra- and intra-pulmonary arteries contained neurofilament and protein gene-product-immunoreactive nerve fibres in both the adventitia and media. The density of nerve fibres increased from birth to 2 months, this being most marked during the first 2 weeks of life. Most of the fibres in the media were presumed to be sympathetic in origin as they contained both neuropeptide tyrosine and tyrosine hydroxylase immunoreactivity. Fibres were associated with the vasa-vasorum and vascular smooth muscle running around the vessel, between the elastic laminae and smooth muscle cells, in the outer two-thirds of the media. Vasoactive intestinal polypeptide and calcitonin gene-related peptide-immunoreactive nerve fibres were found to be associated with the pulmonary trunk and extra-pulmonary artery, but generally not with the intra-pulmonary arteries. Tyrosine hydroxylase immunoreactivity was detected in the glomus cells at birth, but peptide immunoreactivity (enkephalin) was not demonstrated in paraganglia until 14 days of age. Adaptation to extra-uterine life is associated with rapid development changes in the innervation of the pig pulmonary trunk, extra- and intra-pulmonary arteries and in the expression of peptide immunoreactivity in both nerve fibres and glomus cells. These changes may have a role in the postnatal adaptation of the pulmonary circulation.

Endothelium-derived relaxing factor from pulmonary artery and vein possesses pharmacologic and chemical properties identical to those of nitric oxide radical.

The objective of this study was to elucidate the close similarity in properties between endothelium-derived relaxing factor (EDRF) and nitric oxide radical (NO). Whenever possible, a comparison was also made between arterial and venous EDRF. In vascular relaxation experiments, acetylcholine and bradykinin were used as endothelium-dependent relaxants of isolated rings of bovine intrapulmonary artery and vein, respectively, and NO was used to relax endothelium-denuded rings. Oxyhemoglobin produced virtually identical concentration-dependent inhibitory effects on both endothelium-dependent and NO-elicited relaxation. Oxyhemoglobin and oxymyoglobin lowered cyclic guanosine monophosphate (cGMP) levels, increased tone in unrubbed artery and vein, and abolished the marked accumulation of vascular cGMP caused both by endothelium-dependent relaxants and by NO. The marked inhibitory effects of oxyhemoglobin on arterial and venous relaxant responses and cGMP accumulation as well as its contractile effects were abolished or reversed by carbon monoxide. These observations indicate that EDRF and NO possess identical properties in their interactions with oxyhemoproteins. Both EDRF from artery and vein and NO activated purified soluble guanylate cyclase by heme-dependent mechanisms, thereby revealing an additional similarity in heme interactions. Spectrophotometric analysis disclosed that the characteristic shift in the Soret peak for hemoglobin produced by NO was also produced by an endothelium-derived factor released from washed aortic endothelial cells by acetylcholine or A23187. Pyrogallol, via the action of superoxide anion, markedly inhibited the spectral shifts, relaxant effects, and cGMP accumulating actions produced by both EDRF and NO. Superoxide dismutase enhanced the relaxant and cGMP accumulating effects of both EDRF and NO. Thus, EDRF and NO are inactivated by superoxide in a closely similar manner. We conclude, therefore, that EDRF from artery and vein is either NO or a chemically related radical species.

Pattern of connective tissue development in swine pulmonary vasculature by immunolocalization.

Light microscopic immunolocalization studies were carried out on lung tissue from eight newborn and four adult pigs using antibodies to six extracellular matrix components. Antibodies to fibronectin, collagen type IV, and laminin localized on the same structures in adult and newborn lungs. By contrast, antibodies to the interstitial collagens (types I, III, and V) were less extensively localized in the newborn than in the adult, particularly those to type I because the fibres on which they localized in the newborn were thin and sparse. At all ages, antibodies to collagen types III and V co-localized on type I fibres and also on thin individual fibres which formed networks, more dense in the adult than in the newborn. At all ages, anti-type III collagen antibodies also localized on smooth muscle cells. In the adult, but not in the newborn, anti-type I and type V collagen antibodies localized on the connective tissue around the smooth muscle cells in the pulmonary arterial media and vein wall. The dominance of collagen type III suggests greater plasticity in the newborn pulmonary vasculature, which helps explain the recently described rapid changes in arterial wall structure which constitute adaptation to extrauterine life. The postnatal increase in collagen type I helps explain the documented postnatal increase in structural stiffness of the pulmonary arteries.

Presence of atrial natriuretic polypeptide in the pulmonary vein and vena cava.

Striated muscle cells and storage granules observed in the atria were found in main branches of the pulmonary veins and superior and inferior venae cavae of the rat, pig, and ox. The presence of atrial natriuretic polypeptide (ANP) in these veins was examined by reverse-phase high-performance liquid chromatography coupled with a radioimmunoassay for ANP. The veins contained 0.6 to 8.0 ng ANP/mg wet tissue with the major molecular form being gamma-ANP. ANP was detected in the peripheral lung tissue in a small quantity, but was not detected in the pulmonary artery. The identification of gamma-ANP and storage granules stained with an anti-ANP antiserum in the pulmonary vein and vena cava suggest that the veins may participate in regulating volume status, blood pressure, and cardiovascular homeostasis through the release of ANP.

Viability and morphology of aortic and pulmonary homografts. A comparative study.

In view of possible clinical use of the pulmonary homograft for right ventricular outflow tract reconstruction a comparative study with the aortic counterpart was performed. Samples of aortic and pulmonary walls from 10 cadaveric hearts were assessed for viability and morphologic characteristics before and after storage in nutrient-antibiotic solution. The viability, as evaluated by an autoradiographic technique, was similar in both aortic and pulmonary specimens at the time of dissection, after 2 weeks, and after 4 weeks of storage. The histologic examination showed no changes in the structure of the media in all samples up to 4 weeks of storage. The total calcium content per gram of tissue in the pulmonary media was on an average less than half of that in the aortic counterpart. We conclude that the pulmonary homograft is preserved the same as the aortic homograft and, accordingly, it becomes available for clinical application. Moreover, a lesser content of elastic tissue and a lower amount of total calcium may, in all likelihood, make the pulmonary wall less prone to calcification.

Lack of cytochemically detectable cholesterol in rabbit vena cava endothelial plasma membrane.

The response of rabbit vascular endothelial plasma membranes to the cholesterol-binding agents filipin and tomatin was investigated by freeze-fracture electron microscopy. In both single and double labelling experiments, the endothelial plasma membranes of the posterior vena cava were remarkably resistant to the agents compared with the endothelial plasma membranes of other vessels (aorta, pulmonary artery, and pulmonary and cardiac capillaries). This suggests that vena cava endothelium differs markedly from other endothelia, either by having exceptionally low cholesterol levels in its plasma membranes, or with respect to other membrane properties that influence the reaction to filipin and tomatin.