Both large, acute doses of erythropoietin (EPO) and short-term hypoxia increase platelet counts in mice, but long-term hypoxia causes thrombocytopenia. Therefore, we tested the hypothesis that EPO injected in large, chronic doses (a total of 80 U of EPO over a 7-day period) might cause thrombocytopenia. EPO caused increased red blood cell (RBC) production, ie, increased hematocrits, RBC counts, mean cell volume (MCV), and reticulocyte counts (from P less than .05 to P less than .0005), and decreased thrombocytopoiesis, ie, decreased platelet counts, percent 35S incorporation into platelets, and total circulating platelet counts (TCPC) (P less than .0005). Femoral marrow megakaryocyte size was unchanged, but megakaryocyte number was significantly (P less than .005) reduced in mice treated with EPO. EPO- injected mice had increased spleen volumes (P less than .0005), but blood volumes (BV) were unchanged. In EPO-treated, splenectomized mice, RBC production was also increased (P less than .05 to P less than .0005) and platelet counts, TCPC, and percent 35S incorporation into platelets were decreased (P less than .05), but BV was not altered. Therefore, the decrease in platelet counts observed in EPO-treated mice was not due to increased BV or to an enlarged spleen. In other experiments, mice were rendered acutely thrombocytopenic to increase thrombocytopoiesis, and platelet and RBC production rates were determined. In mice with elevated thrombocytopoiesis, RBC counts, hematocrits, percent 59Fe RBC incorporation values, and MCV were decreased (P less than .05 to P less than .0005). Because 59Fe RBC incorporation and MCV were not elevated, the decrease in RBC counts and hematocrits does not appear to be due to bleeding. Therefore, we show that large, chronic doses of EPO increase erythropoiesis and decrease thrombocytopoiesis. Conversely, acute thrombocytopenia causes increased thrombocytopoiesis and decreased erythropoiesis. These findings support the hypothesis of competition between precursor cells of the erythrocytic and megakaryocytic cell lines (stem-cell competition) as the cause of thrombocytopenia in EPO-treated mice and the cause of anemia in mice whose platelet production rates were increased.

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In an effort to explain the different platelet production capabilities of both normal and hypoxic male and female C3H and BALB/c mice, megakaryocyte size and number were determined utilizing bone marrow from both normal and hypoxic mice. The results indicate that normal BALB/c female mice have increased numbers of megakaryocytes, but of smaller size compared with either BALB/c male mice or to both sexes of C3H mice. An inverse relationship between the size and number of megakaryocytes was found in both normal and hypoxic mice; therefore, to evaluate total megakaryocyte characteristics, we calculated total megakaryocyte masses (TMM). With hypoxia, megakaryocyte number decreased, whereas megakaryocyte size increased. Despite the increase in megakaryocyte size, hypoxia caused a significant decrease in TMM (P less than 0.005) in all mice, but female C3H mice had higher TMM (P less than 0.05) than did female BALB/c mice. These data show that hypoxia decreases TMM in mice, and that the effect is greater in C3H mice than in BALB/c mice.

Although anti-platelet serum is frequently used to produce thrombocytopenia, the mechanism of how platelets are removed from the circulation is poorly understood. In vitro studies indicated that anti-platelet serum caused platelet aggregation, whereas normal rabbit serum did not. The aggregation was dose related; at the lower dose, aggregated platelets were shown to deaggregate and platelets released from the aggregates responded normally to ADP but not to collagen. In mice whose platelets were previously labeled with Na₂³⁵SO₄, anti-platelet serum caused an increase in the amount of ³⁵S mucopolysaccharides (MPS) recovered from the kidneys. No increase in radioactivity was found in the lungs, livers, or spleens of similarly treated mice. The data suggest that platelet specific anti-serum causes platelets to aggregate leading to ADP (and consequently ³⁵S-MPS) release; the accumulation of ³⁵S in kidneys may be due to removal of MPS and/or platelets from the circulation.

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