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Biomedical subjects

A J Marcus

Publications and source records attributed to A J Marcus.

At least 91 records · Page 5Linked to original sources
PubMed1981

Interactions between stimulated platelets and endothelial cells in vitro.

Prostaglandins and hydroxy acids are synthesized mainly from the essential polyunsaturated fatty acid arachidonate, and these substances have been identified in almost all mammalian tissues. Prostaglandins, thromboxane A2 (TXA2) and prostacyclin (PGI2) are autocoids that appear to function in the regulation of vascular tone, cell secretion and contractile processes. So far, hydroxy acids have been found to function as chemotactic agents and in the formation of slow-reacting substances. Other actions of hydroxy acids will certainly be defined in future research. The endoperoxides PGG2 and PGH2 represent common precursors of all prostaglandin end-products. In studying the prostaglandin metabolism of a specific tissue, the total profile of endoperoxide transformation should be determined. In platelets the endoperoxides are transformed mainly into TXA2, a potent vasoconstrictor and inducer of platelet aggregation. Endothelial cells convert endoperoxides to PGI2, a vasodilator and inhibitor of platelet aggregation. In addition, endothelial cells can utilize endoperoxides from stimulated plates to form PGI2. The concept that platelets and endothelial cells can share common precursors for the production of modulating substances may be applicable to other cell types.

Arachidonic Acids↗
PubMed1981

Effects of acetyl glyceryl ether phosphorylcholine on human platelet function in vitro.

AGEPC (PAF), at 1.9 x 10(-8) M or higher, induced concentration-dependent aggregation and release in human platelet-rich plasma. Comparative studies with arachidonate, collagen, ionophore, and ADP suggested that AGEPC was a strong stimulus for platelet aggregation and probably a moderate agonist for release, as well as a relatively weak inducer of TXA2 production. The initial phase of AGEPC-induced aggregation was independent of ADP release and TXA2 formation, since it was not inhibited by ASA, apyrase, or CP/CPK. Whereas irreversible aggregation always required ADP release, TXA2 formation was not essential in each instance. Thus, in several experiments, full aggregation responses took place in AGEPC-stimulated platelets that had been pretreated with ASA. AGEPC-induced release of 5-HT, beta -thromboglobulin and PF-4 occurred in parallel and were inhibited by both apyrase and ASA. Washed human platelets did not respond to exogenous AGEPC in the absence of ADP and did not appear to generate significant quantities of AGEPC upon stimulation with thrombin or ionophore.

Adenosine Diphosphate↗
PubMed1980

Phospholipid metabolism in stimulated human platelets. Changes in phosphatidylinositol, phosphatidic acid, and lysophospholipids.

Endogenous phospholipid metabolism in stimulated human platelets was studied by phosphorus assay of major and minor components following separation by two-dimensional thin-layer chromatography. This procedure obviated the use of radioactive labels. Extensive changes were found in quantities of phosphatidylinositol (PI) and phosphatidic acid (PA) as a consequence of thrombin or collagen stimulation. Thrombin addition was followed by rapid alterations in the amount of endogenous PI and PA. The decrease in PI was not precisely reciprocated by an increase in PA when thrombin was the stimulus. This apparent discrepancy could be explained by removal of a transient intermediate in PI metabolism, such as diglyceride, formed by PI-specific phospholipase C (Rittenhouse-Simmons, S., J. Clin. Invest.63: 580-587, 1979). Diglyceride would be unavailable for PA formation by diglyceride kinase, if hydrolyzed by diglyceride lipase (Bell, R. L., D. A. Kennerly, N. Stanford, and P. W. Majerus. Proc. Natl. Acad. Sci. U. S. A.76: 3238-3241, 1979) to yield arachidonate for prostaglandin endoperoxide formation. Thrombin-treated platelets also accumulated lysophospho-glycerides. Specifically, lysophosphatidyl ethanolamines accumulated within 15s following thrombin addition. Fatty acid and aldehyde analysis indicated phospholipase A(2) activity, with an apparent preference for diacyl ethanolamine phosphoglycerides. In the case of collagen, these changes occurred concomitantly with aggregation and consumption of oxygen for prostaglandin endoperoxide formation.THESE STUDIES OF ENDOGENOUS PHOSPHOLIPID METABOLISM PROVIDE INFORMATION SUPPORTING THE EXISTENCE OF TWO PREVIOUSLY POSTULATED PATHWAYS FOR LIBERATION OF ARACHIDONIC ACID FROM PLATELET PHOSPHOLIPIDS: (a) the combined action of PI-specific phospholipase C plus diglyceride lipase yielding arachidonate derived from PI; and (b) a phospholipase A(2) acting primarily on diacyl ethanolamine phosphoglyceride.

Arachidonic Acids↗
PubMed1980

Synthesis of prostacyclin from platelet-derived endoperoxides by cultured human endothelial cells.

We have previously shown that aspirin-treated endothelial cells synthesize prostacyclin (PGI(2)) from the purified prostaglandin endoperoxide PGH(2) (1978. J. Biol. Chem.253: 7138). To ascertain whether aspirin-treated endothelial cells produce PGI(2) from endoperoxides released by stimulated platelets, [(3)H]arachidonic acid-prelabeled platelets were reacted in aggregometer cuvettes with the calcium ionophore A 23187, thrombin, or collagen in the presence of aspirin-treated endothelial cell suspensions. This procedure permitted thin-layer radiochromatographic quantitation of [(3)H]PGI(2) as [(3)H]6-keto-PGF(1alpha) and [(3)H]thromboxane A(2) (TXA(2)) as [(3)H]TXB(2), as well as analysis of platelet aggregation responses in the same sample. In the presence of aspirin-treated endothelial cells, platelet aggregation in response to all three agents was inhibited. [(3)H]6-keto-PGF(1alpha) was recovered from the supernates of the combined cell suspensions after stimulation by all three agents. The order of PGI(2) production initiated by the stimuli was ionophore > thrombin > collagen. The amounts of platelet [(3)H]TXB(2) recovered were markedly reduced by the addition of aspirin-treated endothelial cells. In separate experiments, 6-keto-PGF(1alpha) and TXB(2) were quantitated by radioimmunoassay; the results paralleled those obtained with the use of radiolabeling. The quantity of 6-keto-PGF(1alpha) measured by radioimmunoassay represented amounts of PGI(2) sufficient to inhibit platelet aggregation. These results were obtained when 200,000 platelets/mul were combined with 3,000-6,000 aspirin-treated endothelial cells/mul. At higher platelet levels the proportion of 6-keto-PGF(1alpha) to TXB(2) decreased and platelet aggregation occurred. Control studies indicated that aspirin-treated endothelial cells could not synthesize PGI(2) from exogenous radioactive or endogenous arachidonate when stimulated with thrombin. Therefore the endothelial cell suspensions could only have used endoperoxides from stimulated platelets.Thus, under our experimental conditions, production by endothelial cells of PGI(2) from endoperoxides derived from activated platelets could be demonstrated by two independent methods. These experimental conditions included: (a) enhanced platelet-endothelial cell proximity, as attainable in stirred cell suspensions; (b) use of increased endothelial cell/platelet ratios; and (c) utilization of arachidonate of high specific activity in radiolabeling experiments. Furthermore, when a mixture of platelets and endothelial cells that were not treated with aspirin was stimulated with thrombin, more than twice as much 6-keto-PGF(1alpha) was formed than when endothelial cells were stimulated alone. These results indicate that endothelial cells can utilize platelet endoperoxides for PGI(2) formation to a significant extent.

Arachidonic Acids↗
PubMed1977

Synthesis of prostaglandin I2 (prostacyclin) by cultured human and bovine endothelial cells.

Cultured endothelial cells derived from human umbilical veins or bovine aorta produce a potent inhibitor of platelet aggregation. The inhibitor is synthesized from sodium arachidonate or or prostaglandin endoperoxides by a microsomal enzyme system. Tranylcypromine, a specific antagonist of prostacyclin synthetase, suppresses production of the inhibitor by endothelial cells. The inhibitor, which is ether extractable, has been identified using a two-step thin-layer radiochromatographic procedure and a synthetic prostaglandin I2 standard. With this procedure, we have shown that human and bovine endothelial cells convert sodium [3H]arachidonate to radiolabeled prostaglandin I2 and 6-keto-prostaglandin F1alpha, as wellas prostaglandin E2. Thus, endothelial cells may be non-thrombogenic in vivo because they synthesize and release prostaglandin I2, a potent inhibitor of platelet aggregation.

Animals↗
PubMed1977

Superoxide production and reducing activity in human platelets.

Human platelets contain the cuprozinc (cytoplasmic) and manganese (mitochondrial) forms of superoxide dismutase. Nevertheless, superoxide radicals were detectable in the surrounding medium of metabolically viable platelet suspensions by using two assay systems: cytochrome c and nitroblue tetrazolium. The quantity of superoxide generated by platelets (5 X 10(5) superoxide radicals/platelet per 10 min) was constant and did not increase after aggregation by agents such as collagen and thrombin. The superoxide-generating system was present in the supernate of both aggregated and resting platelets and therefore was not platelet-bound. Platelet superoxide production was unaffected by prior ingestion of aspirin, indicating that the prostaglandin and thromboxane pathways were not involved. Both resting and aggregated platelets exhibited a reductive capacity toward cytochrome c and nitroblue tetrazolium which was unrelated to superoxide production. Furthermore, the aggregation process always resulted in a marked increase in this reduction. The nonsuperoxide reduction associated with aggregation was found to be membrane bound and to decrease with an apparent first order reaction rate (k1 = 0.067 min-1). In addition, accumulative, time-dependent nonsuperoxide-related cytochrome c reduction was also detected. Since there is no superoxide dismutase in plasma, the presence of superoxide radicals in the surrounding medium of platelets may have in vitro significance for platelet and leukocyte concentration and storage and in vivo significance for hemostasis, coagulation, and thrombosis. The nonsuperoxide-related reducing activities may represent a biochemical basis for platelet-blood vessel interactions, with particular reference to blood vessel integrity.

Aspirin↗
PubMed1976

Platelet aggregation in portal cirrhosis.

Primary and secondary platelet aggregation in response to adenosine diphosphate was studied in 24 patients with portal (Laënnec) cirrhosis and compared with platelet aggregation in 14 normal subjects. In 12 patients with cirrhosis, platelet aggregation was diminished when compared to controls. Of the 12 patients with impaired aggregation, 6 had elevated levels of fibrinogen-fibrin degradation products (FDPs), 11 had thrombocytopenia, 10 had shortened euglobulin lysis times, 11 had prolonged bleeding times, 4 had hypofibrinogenemia, and all had prolonged thrombin clotting times. The data suggest that elevated levels of serum FDPs do not explain fully the impairment of platelet aggregation or the prolongation of the thrombin clotting time that was noted in patients with advanced liver disease. A possible explanation for the prolongation of the thrombin clotting time is the presence of "altered" plasma fibrinogen.

Adenosine Diphosphate↗