SummaryWhen blood platelets are centrifuged at elevated pH they release serotonin. This reaction is similar to the release reaction which makes part of the complex called viscous metamorphosis. The experiments support the hypothesis that contact between platelets forms the stimulus which initiates the release reaction.

Summary1. The respiratory activity of human blood platelets has been investigated quantitatively. It is not affected by the normal procedures for isolating washed platelets from plasma, nor by the absence of plasma proteins. Optimal synthetic media for platelet respiration are described; they contain EDTA in a suitable buffered system. The addition of thrombin always causes a depression of oxygen uptake. No direct relationship exists between respiratory activity and clot retraction. Respiration may even be inhibited without affecting clot retraction.2. Under the conditions of clot retraction and with glucose present, a remarkable stimulation of the glycolytic activity is observed. This increase in the production of lactic acid and of glucose consumption is of short duration and is followed by a total breakdown of glycolysis. The maximum of glycolytic activity coincides with the onset of clot retraction initiated by the addition thrombin. Inhibition of glycolysis leads to a total suppression of glucose utilisation and to impaired retraction activity.3. In glucose containing systems and with thrombin the ATP-level in platelets closely follows the pattern of glycolytic activity, i.e. a sharp initial rise, followed by a rapid decay of the triphosphate. Without glucose the addition of thrombin to suitably buffered platelets causes a rapid decrease of ATP.4. The retraction activity of the blood platelets is found to be directly dependent on their ATP-level. The synthesis of ATP during the process of clot retraction is effected by the glycolytic system, provided glucose is present as a substrate5. Most probably the major part of the ATP consumed during the retraction period is active in the contraction of an actomyosin-like protein contained in the platelets.

SummaryPlatelet functions have been studied in relation to hemostasis in two patients with congenital afibrinogenemia.Neither in the plasma nor in the aqueous platelet extracts of these patients was fibrinogen detectable by immunoelectrophoresis or with the aid of the Ouchterlony technique. ADP-induced platelet aggregation, adhesion to connective tissue particles, viscous metamorphosis under the influence of thrombin, clot retraction activity of the platelets, as well as their factor 3 activity were all found normal. Abnormal was the behaviour of the patient’s platelets on glass surfaces : they were unable to adhere to glass and the typical spreading on such surfaces was equally missing. This defect was normalized in vitro by the addition of small amounts of fibrinogen and correspondingly the patients platelets showed normal adhesiveness after fibrinogen transfusions. Normal platelets, suspended in afibrinogénémie plasma lost their adhesiveness toward glass surfaces.After transfusion of Cohn fraction I the prolonged bleeding time of the patients was normal and the clinical improvement presisted for a period of about 3 weeks, this inspite of the fact, that no fibrinogen was detectable by the usual methods 10 days after the transfusion.The significance of these results as well as their implications for the role of fibrinogen in hemostasis are discussed.

SummaryHomogenized human blood platelets have been fractionated by centrifugation in Ficoll and sucrose density gradients. The different fractions were examined by electron microscopy.Although Ficoll allows for the separation of very distinct zones, its ability to form complexes with cellular components made sucrose the preferable gradient. Sucrose, in spite of its unfavorable osmotic effect, allows for an acceptable fractionation of platelet components.

Summary1. Thrombosthenin A, the actin-like component of the contractile protein of blood platelets, was prepared in its monomeric or globular form and compared by gel electrophoresis with actin from rabbit striated muscle. Both proteins migrate at the same speed.The ultrastructure of the fibrillar form of thrombosthenin A was studied by negative contrast technique in the electron microscope. The pictures reveal a helicoidal, double-stranded, beaded structure similar to the fibrillar form of muscle actin. From measurements of periodicity it was concluded that the protein unit must have a diameter of approximately 35 Å.2. The structure of thrombosthenin in its superprecipitated form, also on negative contrast pictures, shows the presence of the fibrillar form of thrombosthenin A and of large, spindle-like aggregates of thrombosthenin M.3. Thrombosthenin in the precipitated and superprecipitated form was fixed and examined on ultra-thin sections. The precipitate is built up of fine, evenly distributed strands due mainly to the presence of the fibrillar form of thrombosthenin A; the super-precipitate shows an obvious contraction of the protein material made up of thin filaments of thrombosthenin A and spindle-like, well contrasted large needles of thrombosthenin M.4. Glycerol-extracted normal, human platelets were exposed to conditions favoring precipitation and superprecipitation of thrombosthenin : it was possible to localize the contractile material in the platelet cytoplasm on the basis of its characteristic structure in the contracted state.5. Normal platelets were fixed and examined by means of electron microscopy: the complex structure of the marginal region is described and possible relationships between its structure and the contractile protein are discussed.