Asialo platelets enhance thrombopoiesis

Because the terminal sialic acid of several glycoproteins regulates their intravascular half-life, and because asialo platelets have a shortened survival, we postulated that asialo platelets might affect the basal rate of thrombopoiesis. Thrombopoiesis was measured by the incorporation of intravenous 75Se-selenomethionine (a cohort label) into circulating rabbit platelets. Each animal acted as its own control. In one control group, saline was injected intravenously once daily for two days. 75Se-selenomethionine was then injected, 15 microCi/kgm on day 2 and platelets harvested on day 5 for measurement of specific activity (S.A.), cpm/10(7) platelets. The animals were allowed to rest for two weeks, and the experiment repeated. The ratio of the second SA over the first SA was utilized as the stimulation index. When saline was injected the second time, the stimulation index was 1.22 +/- 0.07 (SEM) in eight animals. Asialo platelets were prepared by incubating washed platelets with neuraminidase, 2 units/ml for 60 min. Injection of 2-4 X 10(9) asialo platelets/kgm (the second time) resulted in a stim. index of 2.08 +/- 0.23 (P less than 0.001) in 11 animals. Similar results were obtained with 1 mM DFP and neuraminidase (12 animals). Negative results were obtained when 11 rabbits were injected with a comparable number of untreated washed platelets, stim. index 1.07 +/- 0.13; when 10 rabbits were injected with sonicated asialo platelets, stim. index 1.10 +/- 0.12; or when four rabbits were injected with platelets treated with chymotrypsin, 3 mg/ml for 45 min, stim. index 0.80 +/- 0.04. We conclude that asialo intact platelets stimulate megakaryocytes to produce more platelets. It is suggested that the basal stimulus to thrombopoiesis may be regulated by desialidated older platelets.