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Biomedical subjects

M A Packham

Publications and source records attributed to M A Packham.

At least 109 records · Page 6Linked to original sources
PubMed1983

Factors influencing the deaggregation of human and rabbit platelets.

The mechanisms involved in platelet deaggregation are unclear. Washed platelets from rabbits or humans aggregated by ADP can be deaggregated by EDTA or PGI2 if the release reaction has not occurred; during deaggregation 125I-fibrinogen dissociates from the platelets. Human platelets suspended in a medium without calcium undergo the release reaction during ADP-induced aggregation; EDTA, PGE1 or PGI2 do not deaggregate these platelets although EDTA displaces much of the 125I-fibrinogen that associates with them during aggregation. Rabbit platelets aggregated by low concentrations of release-inducing stimuli (sodium arachidonate, collagen or thrombin) can be deaggregated by EDTA, PGI2 or PGE1 and 125I-fibrinogen dissociates from them; with high concentrations of collagen or thrombin, deaggregation and dissociation of 125I-fibrinogen is slower. Human platelets that have undergone the release reaction in response to thrombin, collagen or a combination of sodium arachidonate and ADP are not readily deaggregated by EDTA or PGE1. Since aggregation and fibrinogen binding involving the glycoprotein IIb/IIIa complex are readily reversed by EDTA, and since Ca2+ is required for thrombospondin binding to activated platelets, there may be a third type of platelet-platelet adherence that is not disrupted by EDTA; this type of binding plays a greater role with human than with rabbit platelets.

Adenosine Diphosphate↗
PubMed1983

Factors influencing the deaggregation of chymotrypsin-treated human platelets aggregated by fibrinogen.

Washed human platelets aggregated by ADP can be deaggregated by EDTA or PGE1, provided the release reaction does not occur; when the release reaction occurs platelets deaggregate less readily. Platelets treated with chymotrypsin are aggregated by fibrinogen indicating that fibrinogen receptors may be permanently exposed by this treatment. Fibrinogen-induced aggregation of chymotrypsin-treated platelets provides another method of bringing platelets into close contact with each other. Although EDTA deaggregated chymotrypsin-treated platelets aggregated by fibrinogen in a medium containing a physiological concentration of Ca2+, EDTA did not deaggregate these platelets if they were in a medium without calcium in which the release reaction occurs. In this medium, when ASA was added to prevent the release reaction, EDTA caused deaggregation. More fibrinogen associated with platelets in the medium without calcium than in the calcium-containing medium. In both media, EDTA displaced approximately half of the fibrinogen indicating that deaggregation is not solely dependent on dissociation of fibrinogen from its receptors. Thus when platelets undergo the release reaction, a form of platelet-to-platelet adhesion occurs that is not disrupted by chelation of divalent cations and is therefore not likely to involve only fibrinogen or thrombospondin and fibrinogen since the association of fibrinogen with its receptor requires Ca2+ and the binding of thrombospondin to platelets that have undergone the release reaction is also dependent on Ca2+.

Alprostadil↗
PubMed1983

Aspirin in the treatment of cardiovascular disease: a review.

Large-scale clinical trials of the use of aspirin in post-myocardial infarction patients were based on the assumption that inhibition of platelet activity would reduce thromboembolism associated with atherosclerosis, and that thromboembolism is a major cause of the clinical complications of atherosclerosis. However, spasm and occlusive thrombi may also contribute to this picture, and thus thromboembolism is probably only one of the mechanisms that cause the clinical complications. Aspirin inhibits thrombosis only if thromboxane A2 formation by platelets plays a major part in the growth of thrombi; aspirin has little effect on thrombosis when thrombin generation and fibrin formation are dominant factors. Nevertheless, analysis of the combined data from the six clinical trials indicates a highly significant (21 percent) reduction in reinfarction rate and a 16 percent reduction in cardiovascular mortality rate in patients treated with aspirin. Aspirin may be most useful in treating an as-yet-unidentified subgroup of patients.

Aspirin↗
PubMed1983

Effect of aspirin and sodium salicylate on thrombosis, fibrinolysis, prothrombin time, and platelet survival In rabbits with indwelling aortic catheters.

We have studied the effect of different doses of aspirin on platelet function, PGI2 formation, platelet survival, thrombosis, fibrinolysis, and prothrombin time in rabbits with indwelling aortic catheters. The thrombi formed around indwelling aortic catheters were found to have a large fibrin component, and their formation was inhibited by heparin administration. Thus, in these experiments we examined the effect of aspirin (a weak inhibitor of thrombin-mediated platelet aggregation) under conditions in which thrombin was a major factor in the initiation and growth of the thrombi. Only very high doses of aspirin tended to inhibit thrombus formation over the 5-day period of observation, and a statistically significant inhibition of thrombus formation was produced by equivalent concentrations of sodium salicylate. The failure of high doses of aspirin to achieve a significant inhibition of thrombosis under the conditions of these experiments (whereas an equivalent dose of sodium salicylate was inhibitory) could be due to aspirin inhibition of PGI2 formation. Shortened platelet survival was not affected by aspirin treatment or the dose sodium salicylate that inhibited thrombus formation. The tendency to inhibit thrombus formation appeared to be unrelated to an effect on platelets but was associated with prolongation of the one-stage prothrombin time and increased whole blood fibrinolytic activity; doses of aspirin that inhibited platelet aggregation in response to sodium arachidonate or collagen, and PGI2 formation by the vessel wall, did not have a significant effect on the amount of thrombus present at 5 days. However, the high doses of aspirin that inhibited PGI2 formation were associated with a tendency to increased thrombus formation during the first 3 hr after insertion of the catheter. The results of these experiments show that when thrombin is an important factor in the formation of thrombi, aspirin is a weak inhibitor of thrombosis unless doses are used that provide sufficient salicylate to interfere with blood coagulation and promote whole blood fibrinolytic activity. These results also show that thrombus formation can be inhibited without an apparent change in platelet survival.

Animals↗
PubMed1983

Survival of density subpopulations of rabbit platelets: use of 51Cr-or 111In-labeled platelets to measure survival of least dense and most dense platelets concurrently.

The origin of the density heterogeneity of platelets was studied by measuring the survival of density subpopulations of rabbit platelets separated by discontinuous Stractan density gradient centrifugation. When a total population of 51Cr-labeled platelets was injected into recipient rabbits, the relative specific radioactivity of the most dense platelets decreased rapidly. In contrast, that of the least dense platelets had not changed 24 hr after injection, and then decreased slowly. To distinguish between the possibilities that most dense platelets are cleared from the circulation more quickly than least dense platelets or that platelets decrease in density as they age in the circulation, the concurrent survival of least dense and most dense platelets, labeled with either 51Cr or 111In-labeled total platelet populations, determined concurrently in the same rabbits, were identical, calculated from 1 hr values as 100%. However, the 1-hr recovery of 111In-labeled platelets was slightly but significantly less than that of 51Cr-labeled platelets. Therefore, we studied the survival of 51Cr-labeled least dense and 111In-labeled most dense platelets as well as that of 111In-labeled least dense and 51Cr-labeled most dense platelets. Mean 1-hr recovery of least dense platelets, labeled with either isotope (78% +/- 7%, SD) was similar to that of most dense platelets, labeled with either isotope (77% +/- 8%; SD). Mean survival of least dense platelets was 47.3 +/- 18.7 hr (SD), which was significantly less than that of most dense platelets (76.1 +/- 21.6 hr; SD) (p less than 0.0025). These results indicate that platelets decrease in buoyant density as they age in the circulation and that most dense platelets are enriched in young platelets, and least dense in old. Thus, the events that affect platelets as they age in the circulation contribute to platelet density heterogeneity, although they may not be the sole cause of it.

Animals↗
PubMed1983

Changes in the properties of platelets from rats with experimentally induced shortened platelet survival.

Platelet survival is shortened in experimental animals in which indwelling aortic catheters have been placed. We have examined the properties of platelets harvested from rats on day 1 or day 6 after insertion of indwelling aortic catheters. Platelets taken from these animals at 6 days (but not at 1 day) survived in normal rats or in rats with indwelling catheters for a significantly longer time than platelets from sham-operated rats. The catheters caused a persistent fall in the platelet count, an increase in the proportion of platelets in the most dense fraction after separation on discontinuous Stractan density gradients, and a decrease in mean sialic acid and protein per 10(9) platelets, but not change in modal size as determined with a Coulter Counter Channelyzer. No significant differences in the sensitivity to aggregating and release-inducing agents (ADP, thrombin, or collagen) were observed. It seems likely that chronic damage of the vessel wall shortened platelet survival and increased platelet turnover, although the increase in platelet production was not sufficient to maintain the platelet counts at the same values as in the sham-operated animals. The prolonged survival times in recipient rats of platelets from rats with indwelling aortic catheters indicate that a population of predominantly young platelets exists in these animals. Their increased density supports this conclusion.

Animals↗
PubMed1982

Effect of cAMP phosphodiesterase inhibitors on ADP-induced shape change, cAMP and nucleoside diphosphokinase activity of rabbit platelets.

The effects of cAMP phosphodiesterase inhibitors on ADP-induced shape change and cAMP concentrations have been studied. Caffeine (10 mM), theophylline (8 mM), dipyridamole (0.2 mM), or papaverine (0,05 mM) prevented the shape change of washed rabbit platelets induced by 0.4 microM ADP. At these concentrations, none of these cAMP phosphodiesterase inhibitors increased 14C-cAMP in platelets in which the cytoplasmic adenine nucleotides had been labelled with 14C-adenine. By a protein binding assay, only papaverine by itself increased platelet cAMP above its basal level. These results indicate that two pools of cAMP may exist in platelets. Both methods showed that stimulation of platelet adenylate cyclase with PGE1 (1 microM) resulted in an increase in platelet cAMP and all these cAMP phosphodiesterase inhibitors potentiated this increase caused by PGE1. By themselves, some of these compounds may act through mechanisms that do not involve platelet cAMP. The effects of these cAMP phosphodiesterase inhibitors on platelet nucleoside diphosphokinase (NDK) activity were also investigated. At concentrations that prevented ADP-induced shape change, papaverine and dipyridamole had no effect on the formation of 14C-ATP from 14C-ADP by washed rabbit platelets. The methylxanthines partially inhibited NDK activity of washed rabbit platelets and of isolated platelet membranes, probably due to the structural similarity between the adenine ring of ADP and these substances. However, adenine (8 mM) inhibited ADP-induced shape change and platelet NDK activity but was a less effective inhibitor of ADP-induced platelet aggregation. Thus it seems unlikely that interference with platelet NDK or the ADP receptor is the major mechanism by which the methylxanthines inhibit platelet functions.

3',5'-Cyclic-AMP Phosphodiesterases↗
PubMed1982

The inhibitory effects of exogenous arachidonic acid on rabbit platelet aggregation and the release reaction.

Although arachidonic acid causes rabbit platelet aggregation and the release of granule contents in suspensions of washed platelets when used in concentrations of approximately 50-300 microM, higher concentrations (500 microM) cause neither aggregation nor release. Suspensions of platelets from rabbits wee exposed to arachidonic acid (250 microM) for 15 min, allowed to recover in the presence of PGE1 for 30 min, washed, and resuspended; in some experiments, the platelets were treated with aspirin before being exposed to arachidonic acid. Aggregation of platelets pretreated with arachidonic acid was inhibited in response to ADP; this effect was greater with the non-aspirin-treated platelets and persisted for at least 4 hr after resuspension. The association of 125I-fibrinogen with the platelets as a result of ADP stimulation was also inhibited. Aggregation and release of granule contents in response to collagen and low concentrations of thrombin was inhibited, but the inhibition could be overcome by higher concentrations. Thrombin induced further release of granule contents from platelets exposed to arachidonic acid without pretreatment with aspirin. Platelets that had been exposed to arachidonic acid, either with or without pretreatment with aspirin, did not aggregate or undergo further release upon stimulation with arachidonic acid after they were washed and resuspended. Inhibition of the lipoxygenase pathway with eicosatetraynoic acid (ETYA) or nordihydroguaiaretic acid (NDGA) did not affect the inhibition caused by arachidonic acid, so it is unlikely that a product of this pathway is responsible for the inhibition. Mixing experiments indicated that the pretreated platelets did not form a thromboxane-A2-like activity, and that they were unresponsive to aggregation and release induced by products formed from arachidonic acid. Experiments with 3H-arachidonic acid showed that after 45 min of incubation with platelets, only 1.1% of the 3H-arachidonic acid remained as free arachidonic acid in the platelets. Although cyclic-AMP was slightly increased 1 min after the addition of arachidonic acid, the cyclic-AMP concentration was the same as that of control platelets after the platelets were washed and resuspended, indicating that increased cyclic-AMP is not likely to be responsible for the persistent inhibitory effect. Thus, the inhibitory effect of pretreatment with arachidonic acid is a general effect on responses to a variety of aggregating agents that act through different mechanisms, and the inhibition is not related to thromboxane-A2 formation. The possibility of membrane perturbation resulting in the unavailability of receptors may explain the persistent inhibitory effect, but the responsible reactions have not been identified.

Adenosine Diphosphate↗
PubMed1982

The use of the synthetic peptide, Gly-Pro-Arg-Pro, in the preparation of thrombin-degranulated rabbit platelets.

The method for preparing thrombin-degranulated platelets has been modified to avoid the use of plasmin or successive treatments with small amounts of thrombin, while still achieving more than 90% release of platelet amine storage granule contents. It was necessary to prevent the fibrinogen released from the platelets during thrombin treatment from forming an insoluble fibrin mesh that could trap the platelets and hinder their deaggregation. To accomplish this we have treated rabbit platelets with 0.73 U/ml of thrombin for 1 min in the presence of the synthetic peptide, Gly-Pro-Arg-Pro, which prevents the polymerization of fibrin molecules. We have demonstrated that it also prevents 125I, initially added as 125I-fibrinogen, from associating with the platelets in a form that was not removed by centrifuging and washing during the preparation of thrombin-degranulated platelets, and we infer that products formed from the fibrinogen released from the platelets would also be prevented from associating with them. Thrombin-degranulated platelets prepared by this method have lost 92% of their granule contents and they can be washed and resuspended. These platelets aggregate normally upon stimulation with thrombin, adenosine diphosphate (ADP), or arachidonate. Thus, Gly-Pro-Arg-Pro is useful in preparing thrombin-degranulated platelets for studying platelet reactions without the complicating effects of released materials such a ADP and fibrinogen.

Animals↗
PubMed1982

An investigation into the role of coagulation factor XIII in ADP-induced aggregation and fibrinogen binding with rabbit platelets.

Because there was a possibility that activated factor XIII (factor XIIIa) might stabilize a platelet-fibrinogen aggregate through its crosslinking action, we have isolated plasma factor XIII, activated it, and studied the effect of factor XIIIa at a concentration of 3.3 micrograms/ml on aggregation and 125I-fibrinogen binding of rabbit platelets stimulated with 9 microM ADP. Factor XIIIa did not cause aggregation in the absence of ADP, nor did it enhance ADP-induced aggregation or substantially stabilize the platelet aggregate. The presence of factor XIIIa did not affect the amount of fibrinogen bound to platelets immediately after stimulation with ADP, but it appeared to cause a slow specific binding of 125I-fibrinogen to platelets whether or not they were stimulated with ADP. This binding, which was not inhibited by prostaglandin E1, did not lead to aggregation and was accompanied by crosslinking of fibrinogen through its A alpha and gamma chains, either to other fibrinogen molecules or to a platelet protein or proteins.

Adenosine Diphosphate↗
PubMed1981

Inhibitors of ADP-induced platelet aggregation prevent fibrinogen binding to rabbit platelets and cause rapid deaggregation and dissociation of bound fibrinogen.

125I-fibrinogen binds to washed rabbit platelets when they ar stimulated wit ADP, and it has previously been observed that fibrinogen binding is prevented by several inhibitors of ADP-induced aggregation. We have now shown that other inhibitors of aggregation, the phosphodiesterase inhibitors caffeine and dipyridamole, and colchicine and cytochalasin B which affect the platelet cytoskeleton, also inhibit specific 125I-fibrinogen binding. A positive correlation was observed between ADP-induced aggregation and fibrinogen binding at limiting concentrations of these inhibitors. Colchicine and cytochalasin B appear to act independently, with no indication of synergism. When any of these inhibitors, as well as those previously tested (EDTA, EGTA, PGE1 and PGI2) was added to platelets that had already been stimulated with ADP and undergone considerable aggregation and fibrinogen binding, it caused rapid deaggregation of the platelets and dissociation of bound fibrinogen, although in some cases the inhibitory effects were not as pronounced as when the inhibitor was added before ADP stimulation. These observations reinforce the concept that fibrinogen plays an essential role in the formation of ADP-induced platelet aggregates.

Adenosine Diphosphate↗