Density subpopulations of rabbit platelets: size, protein, and sialic acid content, and specific radioactivity changes following labeling with 35S-sulfate in vivo

Abstract The relationships among platelet size, density, protein and sialic acid content, and age have been investigated. More than 90% of the platelets were obtained from anticoagulated rabbit blood and separated into three density subpopulations by centrifugation in isosmolar discontinuous arabinogalactan (STRactan) density gradients. The size of the most dense platelets was greater than that of the least dense platelets. Most dense platelet subpopulations contained significantly larger amounts of protein and sialic acid per platelet than least dense; however, no relationship between nanomoles sialic acid per milligram protein and platelet density was observed. 36SO was injected intravenously into rabbits as a cohort label of young platelet proteoglycans. The incorporation and loss of radioactivity in the platelet density subpopulations was determined daily for 5 days. By 24 hr, the relative specific radioactivity of the most dense subpopulation was 7 times greater than that of the least dense, although the most dense platelets were only 1. 2 times larger. The relative specific radioactivity of the most dense platelet subpopulation reached a maximum between 48 and 72 hr, and then decreased gradually, whereas that of the least dense subpopulation increased slowly, reaching its maximum after 96 hr. The relative specific radioactivity of the least dense platelets was always less than that of the most dense. Thus, it seems likely that young platelets are most dense platelets that decrease in density on aging in the circulation and that the old, least dense platelets are recognized as such and are cleared from the circulation, possibly due to loss of sialic acid or sialoglycopeptides from their surface.