Ca2+ ionophores and the activation of human blood platelets. The effects of ionomycin, beauvericin, lysocellin, virginiamycin S, lasalocid-derivatives and McN 4308.

Platelet activation is linked to an increase in the cytoplasmic Ca2+ concentration and consequently can also be induced by ionophores which mobilize Ca2+ from intracellular storage sites or transport it through the plasma membrane. The ionophores mostly used in studies on platelet activation are A 23187 and lasalocid (X-537A). The effects of eight compounds with known Ca2+-ionophoric activity in synthetic or natural membrane systems were studied in order to investigate the relationship between transport Ca2+ and activation of platelets. Inomycin acts as a true Ca2+ ionophore: it elicits rapid shape change, aggregation, the release reaction (secretion) and clot retraction (contraction). Beauvericin activates platelets too, but probably not by increasing the cytoplasmic Ca2+ concentration. Lysocellin does not activate platelets but induces a passive loss of serotonin.

On the significance of the influx of calcium ions into stimulated human blood platelets.

Blood platelets, upon stimulation with various substances, take up calcium ions from the suspending medium. This influx occurs simultaneously with the release reaction, i.e. the specific secretion of a variety of substances from storage organelles and the second wave of aggregation. Various inhibitors of the release reaction inhibit this Ca2+ influx. Platelets previously loaded with 45Ca show an increased efflux of the cation upon stimulation by thrombin. These results suggest that the plasma membrane acquires an increased permeability to Ca2+ only in a later phase of platelet activation, in most cases after the earlier release of Ca2+ into the cytoplasm from Castoring organelles. Rapid shape change and release proceed independently of external calcium, whereas clot retraction depends upon a prolonged increased permeability of the plasma membrane to this cation.

Movement of calcium ions and their role in the activation of platelets.

The increase of the cytoplasmic Ca-concentration plays a central role in the initiation of platelet activation. Four kinds of movements of Ca-ions are presumed to occur during this process: a) Ca-ions liberated from membranes induce the rapid shape change. b) Vesicular organelles release Ca-ions into the cytoplasm which initiate the release reaction. c) The storage organelles called dense bodies, secrete their contents including Ca-ions to the outside during the release reaction. d) At the same time a rearrangement of the plasma membrane occurs, resulting in an increase in its permeability for Ca-ions as well as in an increase in the number of Ca-binding sites. Since most processes occurring during platelet activation are reversible, the platelet must be equipped with a mechanism which removes Ca-ions from the cytoplasm. A vesicular fraction obtained from homogenized platelets indeed accumulates Ca actively. This Ca-pump is stimulated by cyclic AMP and protein kinase; it may be involved in the recovery of platelets after activation.

[The thrombocyte membrane]. / Die thrombozytenmembran

The thrombocyte membrane is characterized by several pecularities, first by its morphology which includes deep invaginations into the interior of the cell and, second, by its capacity to become stimulated by a wide variety of seemingly unrelated external agents which extend from proteases to collagen, certain immune complexes and small molecular weight substances such as ADP, adrenaline, serotonin and vasopressin. The response of the membrane to stimulation consists in a drastic rearrangement of its constituents, as exemplified by the appearance on the outer surface of components which are not accessible in the resting platelet. Stimulation may either lead to morphological changes and to aggregation or to more far-reaching alterations linked to aggregation, namely the release of substances from storage organelles and manifestations of gross contractile activity. The generation of these sequential reactions involves the production, by the exited membrane of a hitherto ill-defined signal to the interior of the cell. One of the most important consequences of this signal consists in the release, from internal sources, of calcium ions. Calcium ions are directly involved in the rapid shape change of stimulated platelets, due to their depolymerizing effect on the microtubules, they furthermore trigger the release reaction, in which the prostaglandin in system seems also to be involved and, finally, they are essential for the activation of the contractile system. Simultaneous with the release reaction, the platelet plasma membrane acquires calcium permeability; hence, in a later phase, cytoplasmic calcium originates not only from internal sources, but also from the surrounding medium. It is particularly noteworthy that all these alterations of the plasma membrane are reversible, which means that not only the essential structural rearrangements which occur upon stimulation in the membrane are reversed but also that the cell is capable of removing the Ca2+-ions which have entered the cytoplasm during the activation phase.