title: Increases in circulating megakaryocyte growth-promoting activity in the plasma of rats following whole body irradiation.
abstract: To gain insight into the regulation of megakaryocyte precursors in vivo, we assayed (in vitro) megakaryocyte growth-promoting activity (Meg-GPA) in plasma of rats in which both marrow hypoplasia and thrombocytopenia had been induced by irradiation. Rats received whole body irradiation of 834 rad from a 137Cs source. Plasma was collected at intervals of hours to days, up through day 21 postirradiation, and was tested, at a concentration of 30%, for Meg-GPA on bone marrow cells cultured in 1.1% methylcellulose with 5 X 10(-5) M 2-mercaptoethanol. With normal rat plasma, no megakaryocyte colonies (defined as greater than or equal to 4 megakaryocytes) were seen and only a few single megakaryocytes and clusters (defined as 2 or 3 megakaryocytes) were formed. Two peaks of plasma Meg-GPA were observed after irradiation. The first appeared at 12 hr, before any decrease in marrow megakaryocyte concentration or platelet count. The second occurred on days 10-14 after irradiation, after the nadir in megakaryocyte concentration and while platelet counts were at their lowest levels. A dose-response study of plasma concentration and megakaryocyte growth, using plasma collected 11 days postirradiation, demonstrated that patterns of megakaryocyte growth were related to plasma concentration; formation of single megakaryocytes was optimal over a range of 20%-30% plasma concentration, while cluster and colony formation were optimal at a plasma concentration of 30%. All forms of megakaryocyte growth were decreased with 40% plasma. There was a linear relationship between the number of bone marrow cells plated and growth of single cells, clusters, and colonies using a concentration of 30% plasma collected 11 days after irradiation. We conclude that irradiation causes time-related increases in circulating megakaryocyte growth-promoting activity. We suggest that the irradiated rat is a good model for studying the relationships between Meg-GPA and megakaryocyte and platelet concentration in vivo.
mesh_terms: D000818:Animals; D001853:Bone Marrow / Q000187:drug effects / Q000528:radiation effects; D001854:Bone Marrow Cells; D003115:Colony-Stimulating Factors / Q000096:biosynthesis* / Q000502:physiology / Q000528:radiation effects; D004305:Dose-Response Relationship, Drug; D006400:Hematocrit; D006410:Hematopoiesis / Q000187:drug effects / Q000528:radiation effects; D008297:Male; D008533:Megakaryocytes / Q000166:cytology* / Q000187:drug effects / Q000528:radiation effects; D008623:Mercaptoethanol / Q000494:pharmacology; D010976:Platelet Count; D051381:Rats; D011919:Rats, Inbred Strains; D014916:Whole-Body Irradiation
chemical_list: D003115:Colony-Stimulating Factors; D008623:Mercaptoethanol