Abstract
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 Ca 2+ 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 Ca 2+ only in a later phase of platelet activation, in most cases after the earlier release of Ca 2+ into the cytoplasm from Ca-storing 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.
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