The Blood Platelets

Platelets were first observed as structural elements of the blood by Donn6 (1842), but it was only about 40 years later that more detailed accounts became available. At that time Hayem (1878), Bizzozero (1882), and Eberth and Schimmelbusch (1886) not only gave excellent descriptions of the morphology of the blood platelet, but they also made remarkably accurate observations about their physiological and pathological functions. 

The most important physiological function of the blood platelets consists in their participation in the mechanisms leading to the arrest of hemorrhage. Bleeding from smaller vessels, such as the arterioles and venules, is almost exclusively arrested by means of the formation of a hemostatic plug which is composed of densely packed blood platelets. In larger 

Hugues (1959), in particular, was able to show convincingly that the formation of an efficient hemostatic plug consists of three phases (1) adherence of the platelets to the injured endothelium (2) growth of a loose aggregate of platelets by the addition of new platelets, (3) consolidation of this aggregate to a stable and impermeable hemostatic plug. 

On the other hand, evidence is accumulating which seems to indicate that fibrin plays a passive role in clot retraction and is simply carried along by actively contracting elements of platelet origin (Discombe, 1950 Bloom, 1955 Sokal, 1960 Castaldi et al., 1962 Rodman et al., 1963). 

Thromboslhenin, the Contractile Protein from Blood Platelets 

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The number of circulating blood platelet is markedly reduced by injection of a lethal dose of toxin-LR into mice ( 7 ). The time course of the decrease of the blood platelets is closely paralleled by the increase in fresh weight of the liver. The sharp rise in Spearman s rank correlation between platelet count and liver weight 30 minutes after injection indicates that thromocytopenia and hepatomegaly were almost concurrent. 

Bass EW, Means A, McMillen B Buspirone impairs performance of a three-choice working memory escape task in rats. Brain Res Bull 28 455-461, 1992 Bastani B, Arora RC, Meltzer HY Serotonin uptake and imipramine binding in the blood platelets of OCD patients. Biol Psychiatry 30 131-139, 1991 Bauer MS, Whybrow PC Rapid cychng bipolar affective disorder, II treatment of refractory rapid cychng with high-dose levothyroxine a prehminary study. Arch Gen Psychiatry 47 435-440, 1990... 

All are powerful alkylating agents. The development of Mitozolomide was progressed by May and Baker in Britain. Although it showed clinical activity and a marked advantage over Dacarbazine in crossing the blood-brain barrier, its development was terminated when it was found to cause severe thrombocytopenia (decrease of the blood platelet count). 

Many of the described properties of the blood platelets suggested the preseiKie of a (loutractile protein in these cells. This possibility seemed the more likely since Hoffmann-Eerling (1954, 1956) had already demonstrated that the morphological alterations of other cell types were due to the presence of actomyosin-like material. 

This difference in the effects of the two metal ions explains why the ATP loss suffered by the blood platelets in model systems containing only Mg++ ions is not as pronounced as the disappearance of the nucleotide during normal blood coagulation (Born, 1958). It has been mentioned before that ADP production during VM of the platelets was of biological importance. This production most likely is due mainly to the activation of the thrombosthenin ATPase by the influx of plasmatic Ca++ ions their pronounced activator role therefore appears of particular significance. 

The Relaxing Factor of the Blood Platelets—a Natural Inhibitor of Thrombosthenin Activities... 

Today thrombosthenin is the only contractile protein of nonmuscular origin available in larger quantities. This alone makes it an interesting material from the point of view of comparative physiology. On the other hand, the high content of thrombosthenin in the blood platelets also suggests an important function of this material in physiological platelet activity. 

Thus, thrombosthenin may prove not only interesting with respect to the functions of the blood platelets but also with respect to the general importance of the elucidation of the many remaining problems in the field of the contractile proteins. 

ChEs are widely distributed in the body. AChEs regulate excitation at cholinergic synapses by destroying the neurotransmitter ACh. These enzymes are some of the most active known, cycling within a few milliseconds. AChEs are found in excitable tissues at synapses, neuromuscular junctions, my-otendinous junctions, central nervous system (CNS) neuron cell bodies, axons, and muscles (Table 2). AChEs are also found in the erythrocytes (red blood cells (RBCs)) of mammals, in the serum of some birds and mammals, and in the blood platelets of... 

The occurrence of plaques is accepted as an intermediate end point in the development of CHD (Biesalski, 1999). The blood platelets aggregate and adhere... 

Mayer, P., Geissler, K., Ward, M. and Metcalf, D. (1993) Recombinant human leukemia inhibitory factor induces acute-phase proteins and raises the blood platelet counts in nonhuman primates. Blood 81 3226-3233. 

An abnormal decrease in number of the blood platelets Syn Thrombopenia). 

Thrombotic complications are frequently encountered when blood is exposed to the surfaces of hemodialysis devices, heart-lung machines, arterial grafts, artificial heart components and other prosthetic devices. The blood platelets are particularly vulnerable to these adverse effects which may include a decrease in platelet count, shortened platelet survival and attendant higher platelet turnover, and altered platelet function. However the interaction of platelets with an artificial surface exposed to blood must be preceded by the interaction of the molecular components of plasma, particularly the plasma proteins, with the surface (1,2). This is due to the prepon-... 

Studies on the blood platelets of dogs with hemophilia, dogs that only transmitted the disease, and normal dogs showed differences in the DEP responses between the three. 

A pronounced drop was observed in the blood platelet count of subjects given HEAR oil and margarine as essentially the sole source of dietary fat for 22 days (Table VI). Blood platelet counts dropped to values that were considered to be below normal for five of the seven subjects on this regimen. Platelet counts in what was the low normal range (120,000-150,000/mm ) also were observed in the LEAR oil group but no consistent pattern devel-... 

Thromboplastin is present in tissue juices and in platelets. In experimental studies on blood coagulation, thromboplastin is usually obtained by extracting it with saline from brain or lung. In the blood vessels, the thromboplastin that starts the coagulation process is generated by the disintegration of the blood platelets (which will be discussed in more detail later), but thromboplastin release coincides with the release of vasconstricting substances. 

Abramson HA. The electrophoresis of the blood platelets of the horse with reference to their origin and to thrombus formation. J Exp Med 1928 47 677-83. 

Blood contains only the histamine which comes from sites of synthesis or from the digestive tract this histamine is immediately taken up by the tissues or the blood platelets (Green, 1967, in ref. 8). The principal site of synthesis is the mast-cell system. 

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