Heterogeneity of rabbit platelets. VI. Further resolution of changes in platelet density, volume, and radioactivity following cohort labelling with 75Se-selenomethione.

Rabbits were injected intravenously with a cohort platelet label, 75Se-selenomethionine Platelet-rich plasma was separated into five different platelet density fractions on each of seven days by repetitively centrifuging the same sample of platelet-rich plasma at increasing gravitational force. The heaviest platelet sediment fraction was enriched with smaller platelets. Incorporation of isotope into the heaviest platelet fraction was considerably greater than incorporation into the highest platelet fraction, i.e. heavy--light specific activity ratio was 9.63, on day 1. The ratio of the heaviest platelet fraction specific activity to the specific activities of the platelets sedimented at higher g forces (lighter-smaller platelets), generally decreased on each day for 4--6 d by a factor of 2.8--7.0-fold. Whereas incorporation of isotope into the heaviest platelet fraction declined on days 3--4, incorporation into the lightest three fractions remained the same. Cohort platelet survival curves revealed a significant lag period of 1--2 d for the incorporation of isotope into the lightest three platelet fractions compared to the heaviest two platelet fractions. The mean platelet survival of the lightest two fractions was significantly shorter than the mean platelet survival of the heaviest three fractions. SDS-polyacrylamide gel electrophoresis of the platelet cell sap of the five platelet fractions generally revealed 10 prominent protein bands for the heaviest platelet fractions. The lightest platelet fraction, which comprised 16% of the harvested platelet population, had six absent to markedly diminished platelet proteins of modecular weights between 142 000 and 46 000 daltons. The data are compatible with two models: (1) Heavy-large platelets are, on average, young platelets which become lighter-smaller platelets while losing platelet membrane and cell sap components with time. (2) Heavy-large platelets and light-small platelets are produced independently by specific megakarocytes. The heavy-large platelets incorporate more isotope than lighter-smaller platelets (possibly because of their megakarocyte precursor). However, they are released earlier into the circulation than lighter-smaller platelets and are therefore younger platelets.