Subcellular localization of the heparin‐neutralizing factor in blood platelets.

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Abstract
1. The distribution of the heparin‐neutralizing factor (platelet factor 4, PF4) in subcellular organelles of blood platelets of rabbits and man was investigated. 2. In both species the organelles storing 5‐hydroxytryptamine (5‐HT storage organelles) contained only trivial amounts of PF4. 3. In contrast, the content of PF4 was highest in the subcellular fractions rich in alpha‐granules. 4. In conclusion, PF4 is probably localized in the alpha‐granules and therefore the platelets contain at least two types of organelles (5‐HT organelles and alpha‐granules) capable of releasing their contents in response to the same stimuli, such as exposure to collagen, thrombin, etc.