Enzyme activities in endothelial cells and smooth muscle cells from swine aorta

Intracellular adenosine formation and release by freshly-isolated vascular endothelial cells from rat skeletal muscle: effects of hypoxia and/or acidosis

Previous studies suggested indirectly that vascular endothelial cells (VECs) might be able to release intracellularly-formed adenosine. We isolated VECs from the rat soleus muscle using collagenase digestion and magnetic-activated cell sorting (MACS). The VEC preparation had >90% purity based on cell morphology, fluorescence immunostaining, and RT-PCR of endothelial markers. The kinetic properties of endothelial cytosolic 5'-nucleotidase suggested it was the AMP-preferring N-I isoform: its catalytic activity was 4 times higher than ecto-5'nucleotidase. Adenosine kinase had 50 times greater catalytic activity than adenosine deaminase, suggesting that adenosine removal in VECs is mainly through incorporation into adenine nucleotides. The maximal activities of cytosolic 5'-nucleotidase and adenosine kinase were similar. Adenosine and ATP accumulated in the medium surrounding VECs in primary culture. Hypoxia doubled the adenosine, but ATP was unchanged; AOPCP did not alter medium adenosine, suggesting that hypoxic VECs had released intracellularly-formed adenosine. Acidosis increased medium ATP, but extracellular conversion of ATP to AMP was inhibited, and adenosine remained unchanged. Acidosis in the buffer-perfused rat gracilis muscle elevated AMP and adenosine in the venous effluent, but AOPCP abolished the increase in adenosine, suggesting that adenosine is formed extracellularly by non-endothelial tissues during acidosis in vivo. Hypoxia plus acidosis increased medium ATP by a similar amount to acidosis alone and adenosine 6-fold; AOPCP returned the medium adenosine to the level seen with hypoxia alone. These data suggest that VECs release intracellularly formed adenosine in hypoxia, ATP during acidosis, and both under simulated ischaemic conditions, with further extracellular conversion of ATP to adenosine.

Biomarkers of oxidative stress and endothelial dysfunction in glucose intolerance and diabetes mellitus

OBJECTIVES

To evaluate biomarkers of endothelial dysfunction and oxidative stress in glucose intolerance (GI) compared to overt diabetes (DM2).

DESIGN AND METHODS

140 volunteers including 96 with DM2, 32 with GI and 12 controls (C) were studied. (*)NO metabolites, (*)NO synthase inhibitors, thiols and N-acetyl-beta-glucosaminidase (NAGase) activity were analyzed by chemiluminescence, capillary electrophoresis, ELISA and colorimetric assay, respectively.

RESULTS

(*)NO metabolites were higher in GI (NOx: p=0.03; S-nitrosothiols: p=0.001) and DM2 (p=0.006; p=0.0006) groups in relation to group C, while nitrotyrosine was higher only in the DM2 group in comparison to the other groups. NAGase activity was elevated in GI (p=0.003) and DM2 (p=0.0004) groups in relation to group C, as well as, ADMA (p=0.01; p=0.003) and GSSG (p=0.01; p=0.002).

CONCLUSIONS

(*)NO metabolites, (*)NO synthase inhibitors, thiols and NAGase are biomarkers suitable to indicate endothelial dysfunction and oxidative stress in the early stages of impaired response to insulin.

The effect of systemic hypoxia on interstitial and blood adenosine, AMP, ADP and ATP in dog skeletal muscle

1. We investigated the effect of moderate systemic hypoxia on the arterial, venous and interstitial concentration of adenosine and adenine nucleotides in the neurally and vascularly isolated, constant-flow perfused gracilis muscles of anaesthetized dogs. 2. Systemic hypoxia reduced arterial PO2 from 129 to 28 mmHg, venous PO2 from 63 to 23 mmHg, arterial pH from 7.43 to 7.36 and venous pH from 7.38 to 7.32. Neither arterial nor venous PCO2 were changed. Arterial perfusion pressure remained at 109 +/- 8 mmHg for the first 5 min of hypoxia, then increased to 131 +/- 11 mmHg by 9 min, and then decreased again throughout the rest of the hypoxic period. 3. Arterial adenosine (427 +/- 98 nM) did not change during hypoxia, but venous adenosine increased from 350 +/- 52 to 518 +/- 107 nM. Interstitial adenosine concentration did not increase (339 +/- 154 nM in normoxia and 262 +/- 97 nM in hypoxia). Neither arterial nor venous nor interstitial concentrations of adenine nucleotides changed significantly in hypoxia. 4. Interstitial adenosine, AMP, ADP and ATP increased from 194 +/- 40, 351 +/- 19, 52 +/- 7 and 113 +/- 36 to 764 +/- 140, 793 +/- 119, 403 +/- 67 and 574 +/- 122 nM, respectively, during 2 Hz muscle contractions. 5. Adenosine, AMP, ADP and ATP infused into the arterial blood did not elevate the interstitial concentration until the arterial concentration exceeded 10 microM. 6. We conclude that the increased adenosine in skeletal muscle during systemic hypoxia is formed by the vascular tissue or the blood cells, and that adenosine is formed intracellularly by these tissues. On the other hand, adenosine formation takes place extracellularly in the interstitial space during muscle contractions.

Roles of vasodilatory prostaglandins in mitogenesis of vascular smooth muscle cells

Vasodilatory prostaglandins (PGI2, PGE1) and synthetic prostacyclin mimetics inhibit smooth muscle cell proliferation in vitro after stimulation by growth factors. Similar results are obtained in vivo after endothelial injury, suggesting that vasodilatory prostaglandins might also control smooth muscle cell proliferation in vivo. However, available data from clinical trials are conflicting and currently do not support the concept that these compounds might be successfully used to suppress excessive smooth muscle cell growth in response to tissue injury, specifically restenosis after PTCA. One possible explanation for these different results is an agonist-induced down-regulation of prostacyclin receptors in vascular smooth muscle cells. It is possible that enhanced endogenous prostacyclin biosynthesis, subsequent to induction of COX-2 and/or in relation to the formation of a neointima from media smooth muscle cells, might have a similar effect. There is still uncertainty regarding the cellular signal transduction pathways and their possibly complex interaction, although cAMP-dependent reactions are probably involved. In addition, vasodilatory prostaglandins might also interfere with the generation and action of other growth modulating factors, including PDGF, hepatocyte growth factor and nitric oxide. In conclusion, vasodilatory prostaglandins might be considered as growth modulating endogenous mediators in vascular smooth muscle cells.

Extracellular adenine compounds, red blood cells and haemostasis: facts and hypotheses

Previously, the role of adenine nucleotides was thought to be confined to the intracellular space of the cell. Research of the last decades has revealed that nucleotides also occur in the extracellular milieu. This survey deals with extracellular adenine compounds in the blood, focussing on their role as chemical mediators in the haemostatic effect of red cells. Erythrocytes may act as pro-aggregatory cells by at least two chemical mechanisms. Firstly, they can enhance platelet aggregation by releasing adenosine diphosphate (ADP), a well known platelet stimulatory substance. ADP is set free when red cells are stressed mechanically, for instance by shear forces generated in the blood stream; ample experimental evidence supporting this view is summarized. Secondly, erythrocytes efficiently take up extracellular adenosine via their nucleoside transporters, thereby removing a potent inhibitor of platelet function. Extracellular adenosine occurs in the blood stream, either directly released from various tissues or as the end product of extracellular adenine nucleotide metabolism, e.g. after degradation of red cell-born ADP or ATP. Finally, a novel mechanism of action of the antithrombotic drug dipyridamole, which has very recently been put forward, is demonstrated. Dipyridamole inhibits platelet function indirectly by blocking the uptake of extracellular adenosine via the nucleoside transporter of red cells; increased adenosine levels in turn are responsible for the antiaggregatory effect of dipyridamole.

Origin, metabolism and function of extracellular adenine nucleotides in the blood

In previous views the role of adenine nucleotides was thought to be confined to the intracellular space of the cell. However, research of the last decades has revealed that nucleotides also occur in the extracellular space. This survey deals with the sources, metabolism and the role in blood of the extracellular adenine mononucleotides ATP, ADP, AMP and the dinucleotides diadenosine tetraphosphate (Ap4A) and diadenosine triphosphate (Ap3A). The latter two are novel compounds, which have recently been discovered in human platelets. The mononucleotides originate from damaged tissues, from red blood cells during haemolysis, from activated platelets, the working muscle and from the nervous system, whereas the dinucleotides are exclusively released from stimulated platelets. Both the adenine mono- and the dinucleotides act as signal molecules on blood cells as well as on cells of the vascular wall, thereby modulating physiological processes such as platelet aggregation, histamine release from mast cells, regulation of vascular tone and white cell functions. In order to limit the signal effects of extracellular nucleotides, blood cells, plasma and the interior of the vessel walls are provided with nucleotide splitting enzymes: ATP, ADP and AMP are mainly degraded by ectoenzymes present on blood cells, endothelial and on smooth muscle cells, whereas dinucleotides are primarily metabolized by plasma enzymes. This review closes with the presentation of the clinical utility of Ap3A and Ap4A as tools for the diagnosis of platelet storage pool defects.

Elastase-like activity in cultured aortic endothelial cells

Cultured porcine aortic endothelial cells were studied for cellular and secreted elastase activity. We describe an activity hydrolyzing the synthetic elastase substrate, succinyl(alanine)3 nitroanilide, but not elastin, which was shown to be membrane located and was not secreted to the culture medium. A different neutral proteinase activity degrading insoluble elastin was demonstrated in the culture medium following its fractionation by gel filtration high performance liquid chromatography (HPLC). Since no elastinolytic activity could be directly detected in the conditioned medium, it is likely that the chromatographic separation removed an endogenous inhibitor.

Specific inhibition of endothelial cell proliferation by isolated endothelial plasma membranes

Cultures of human vascular endothelial cells were used to study the phenomenon of density-dependent inhibition of cell growth. Endothelial cells were disrupted by nitrogen cavitation, and a plasma membrane-enriched fraction was prepared by differential centrifugation followed in some cases by sucrose density gradient fractionation. Membrane suspension was added to low-density early-passage endothelial cultures grown in microwells. Hemocytometer cell counts and 6 hr 3H-thymidine pulses were performed in triplicate wells at varying intervals. Plasma membranes suppressed cell proliferation in a reversible, dose-dependent fashion. Increasing the ambient concentration of endothelial cell growth factor did not alter the inhibitory effect. The antiproliferative effect was sensitive to heat and trypsin and to incubation with 0.1 M sodium carbonate, pH 11.5. Membrane vesicles selectively derived from the apical cell surface also suppressed proliferation. This phenomenon showed at least some specificity for cell type and species in both human and bovine models. Therefore, cell-cell contact is capable of regulating endothelial cell proliferation in vitro despite the presence of available growth surfaces and of optimally supportive culture medium.

Characterization of ectonucleotidases on vascular smooth-muscle cells

We compared the properties of the ectonucleotidases (nucleoside triphosphatase, EC 3.6.1.15; nucleoside diphosphatase, EC 3.6.1.6; 5'-nucleotidase, EC 3.1.3.5) in intact pig aortic smooth-muscle cells in culture with the properties that we previously investigated for ectonucleotidases of aortic endothelial cells [Cusack, Pearson & Gordon (1983) Biochem. J. 214, 975-981]. In experiments with nucleotide phosphorothioate diastereoisomers, stereoselective catabolism of adenosine 5'-[beta-thio]triphosphate, but not of adenosine 5'-[alpha-thio]triphosphate, by the triphosphatase and stereoselective catabolism of adenosine 5'-[alpha-thio]diphosphate by the diphosphatase were found, as occurs in endothelial cells. In contrast with endothelial ecto-5'-nucleotidase, the smooth-muscle-cell enzyme catabolized adenosine 5'-monophosphorothioate (AMPS) to adenosine: the affinity of the enzyme for AMPS was greater than for AMP, and Vmax for AMPS was about one-sixth that for AMP. In both cell types AMPS was an apparently competitive inhibitor of AMP catabolism by 5'-nucleotidase. The relative rates of catabolism of nucleotide enantiomers in which the natural D-ribofuranosyl moiety is replaced by an L-ribofuranosyl moiety were similar to those in endothelial cells. No ectopyrophosphatase activity was detected in smooth-muscle cells, in contrast with endothelial cells, where modest activity is present.

Monoamine oxidase activity in the cerebral vasculature: comparison between fresh microvessels from different structures and cell cultures derived from microvessels

Monoamine oxidase (MAO) activity was studied in various preparations of porcine brain microvessels to explore further the role of this enzyme in the blood-brain barrier to catecholamines. No difference was noted (Vm and Km) between microvessels isolated from three structures (caudate nucleus, thalamus, and cerebral cortex) in which the responses to circulating catecholamines in vivo are markedly different. Large and small microvessels from the caudate nucleus and the thalamus presented the same specific activity. Cell cultures obtained from small microvessels were rich in endothelial cells as identified by the presence of Factor VIII-related antigen. These preparations displayed an MAO activity about ninefold less than freshly isolated microvessels, although their prostaglandin synthetase activity appeared normal. These results suggest that MAO activity is not the main factor determining the regional differences in the cerebrovascular reactions to catecholamines, that MAO is not specifically localized in the endothelium but must be also present in the smooth muscle, and that the MAO activity is greatly decreased during cell culture.

Regulation of prostaglandin production and ectoenzyme activities in cultured aortic endothelial cells

Prostaglandin production, angiotensin-converting enzyme, and 5'-nucleotidase were measured in porcine aortic endothelial cells in situ (with a multi-well template on an opened aorta), in primary culture and in subcultures. Changes during culture were monitored and the effects of culture conditions were investigated by growing cells on a biological matrix or on plastic, by adding different sera to the growth medium, and by harvesting cells enzymically or mechanically. Prostacyclin production by endothelium in primary culture is highest immediately after cell isolation and subsequently declines; this pattern is repeated each time the cells are subcultured. The level at which production stabilises is approximately 200 pg X 10(6) cells-1 X h-1. Detaching cells by physical means stimulates production much more than enzymic dispersion; the type of serum or the presence of a biological matrix does not alter prostaglandin production. The relative amount of prostaglandin E produced increases with time, from approximately 20% of the prostacyclin production shortly after isolation to greater than 100% in subcultured cells. None of the culture conditions that we tested altered this trend. Angiotensin-converting enzyme activity decreases during primary culture, but activity can be sustained by including homologous serum (from whole blood or from platelet-free plasma) in the culture medium. The method of harvesting cells, or the presence of a matrix, did not affect enzyme activity. 5'-Nucleotidase also declines during culture, with a progressive decrease in both Km and Vmax from template to primary culture to subcultures. None of the variations in culture conditions prevented this change. Ecto-adenosine-deaminase activity, not detectable in cultured cells, can be measured in the template. Part of this activity was released by the vascular wall and could be due to plasma diffusing from the interstitial space.

Effects of isolation and culture on prostaglandin synthesis by porcine aortic endothelial and smooth muscle cells

Freshly isolated neonatal porcine aortic tissue (smooth muscle with or without endothelium present) produced approximately 30 ng/mg wet tissue of 6-oxo-prostaglandin F1 alpha (the stable hydrolysis product from prostacyclin) and approximately 15 ng/mg of prostaglandin E2, as measured by radioimmunoassay after 24 h incubation in culture medium. Primary cultures of porcine endothelial and smooth muscle cells (isolated by enzymic digestion of aortic tissue) exhibited the same pattern of prostaglandin production, but absolute values were greater than for fresh tissue, particularly in the case of endothelium. Subcultures of endothelium produced smaller amounts of prostaglandins, although the pattern remained similar. In contrast, subcultures of smooth muscle cells produced a greater total amount of prostaglandins than did primary cultures, and the main product was prostaglandin E2. Experiments with [14C] prostaglandin H2 or [14C]arachidonic acid confirmed that aortic tissue, cultured endothelium, and primary cultures or aortic smooth muscle cells synthesized prostacyclin, and demonstrated that subcultured smooth muscle cells enzymically isomerised prostaglandin H2 to prostaglandin E2. Kinetic studies showed that prostaglandin production by cultured vascular cells was transiently increased by subculture or changing the growth medium, and that production per cell declined with increasing cell density. The change in pattern of prostaglandin production during culture was shown to be due to a rapid decline in the rate of prostacyclin production (which apparently began immediately after tissue isolation), together with a more gradual rise in prostaglandin E2 production. These results indicate that the amounts and ratios of prostaglandins produced by vascular endothelial and smooth muscle cells are greatly affected by the conditions used to isolate and culture the cells; vascular cells in vivo may similarly alter their pattern of prostaglandin production in response to local changes in their environment.

Analytical cell fractionation of isolated rabbit renal proximal tubules

Proximal tubules were isolated in highly pure form from rabbit cortices by a mechanical procedure that is known to preserve the structural and metabolic aspects of the tubular cells. Postnuclear supernates prepared from the isolated tubules were subjects to isopycnic centrifugation in linear sucrose gradients. The enzyme activities associated with the plasma membrane (gamma-glutamyl transpeptidase, amino-peptidase M, alkaline phosphatase, Na-K-ATPase, and phosphodiesterase I) exhibited sharp unimodal frequency-density profiles with a median density near 1.16 g/ml, which shifted to a heavier density when treated with digitonin. The lysosomal enzymes, N-acetyl-beta-glucosaminidase, alpha-mannosidase, and cathepsin B, and the peroxisomal enzyme catalase exhibited particle-associated activity near a density of 1.22 g/ml. Disruption of these particles by freezing and thawing resulted in these activities appearing in the rho = 1.10 g/ml region of the gradient where the soluble cytosolic enzyme, phosphoglucomutase, exhibited activity. Cytochrome oxidase activity typical of mitochondria gave a sharp unimodal profile at rho = 1.18 g/ml. Microsomal glucose-6-phosphatase and NADPH: cytochrome c reductase activities gave median densities near 1.16 g/ml, which did not change after incubation with digitonin. Galactosyl transferase activity gave a skewed profile at rho = 1.16 g/ml and showed a slight shift to heavier density after digitonin. This study of the enzymatic activities and density gradient distribution of the components of the proximal tubule cells provides the methodology for the further study of the cellular processing of endogenous and exogenous substances by this vital cell type.

Metabolism of adenine nucleotides by ectoenzymes of vascular endothelial and smooth-muscle cells in culture

1. Pig aortic endothelial and smooth-muscle cells in culture rapidly catabolize exogenous ATP, ADP or AMP. 2. In both cell types catabolism is due to Mg2+-stimulated ectoenzymes. 3. Inhibition and substrate-specificity studies suggest that both cell types possess three distinct ectonucleotidases, namely nucleoside triphosphatase (EC 3.6.1.15), nucleoside diphosphatase (EC 3.6.1.6) and 5'-nucleotidase (EC 3.1.3.5), as well as nucleoside diphosphate kinase (EC 2.7.4.6). 4. These ectonucleotidase systems could be of importance in the regulation of neurotransmission, blood platelet function and vasodilation.