Effect of vincristine on platelet production in mice

Protection of survival and hematopoiesis in irradiated mice by 5-fluorouracil

The effects of whole-body irradiation on survival and the hematopoietic system were studied in mice treated with 5-fluorouracil (5-FU). Animals (ddY-SLC male mice, 8-10 weeks old) were injected with 5-FU (i.p.) as a single dose (150 mg/kg) at various times before or after irradiation with X-rays at graded doses (4.8 to 7.6 Gy). The treatment of mice with 5-FU 5 days before irradiation was the most effective for the reduction of radiation lethality, having a radioprotective effect. The dose reduction factor (DRF) was 1.24. However, treatment with 5-FU at 1 day and 2 hours before, or at various times after irradiation significantly increased the radiation lethality compared to the untreated controls, creating a radiosensitizing effect. The decrease or the increase of radiation lethality exhibited by 5-FU was similar to the radiation-dose relationship pattern shown by endogenous and exogenous CFU-S. The pattern of change of thrombocyte counts in the circulating blood after irradiation was greatly modified by pretreatment with 5-FU 5 days before irradiation, effectively lessening the radiation-induced depression. In contrast, the post-irradiation patterns of leukocyte and erythrocyte variation did not show any significant change due to pretreatment with 5-FU.

Megakaryocytes in cubital venous blood in patients with chronic myeloproliferative diseases

33 patients with chronic myelo-proliferative disease were investigated for megakaryocytes in cubital venous blood. 6 Patients had chronic myeloid leukaemia, 11 polycythaemia vera and 16 myelofibrosis. In all patients, a significantly higher number of megakaryocytes than in normal adult humans was found. In patients with chronic myeloid leukaemia and myelofibrosis, an increased number of intact megakaryocytes was found, indicating thrombopoiesis in the peripheral part of the upper extremity. 2 patients with myelofibrosis showed a % of intact megakaryocytes greater than 25, as found in blood from the inferior vena cava in normal adults. In patients with chronic myeloid leukaemia, some of the megakaryocytes were smaller than normal, whereas in myelofibrosis, some of the megakaryocytes were larger than normal, with increased lobulation of nuclei. Active thrombopoiesis in the peripheral part of the extremities can be of great importance, when splenectomy is considered in treatment of myelofibrosis.

Effects of cyclophosphamide on murine bone marrow and splenic megakaryocyte-CFC, granulocyte-macrophage-CFC, and peripheral blood cell levels

The effect of cyclophosphamide (CY) on megakaryocytopoiesis in mice was examined with assays of megakaryocyte colony-forming cells (Meg-CFC) in bone marrow and spleen and simultaneous determinations of peripheral blood counts, after a single intraperitoneal dose (200 mg/kg) of CY. Significant rebound thrombocytosis (170% of normal) occurred at day 11 after injection with CY, although only modest preceding thrombocytopenia (70% of normal) was observed. After an initial 3-5 day period of suppression, total megakaryocyte colony-forming cells (Meg-CFC) in both bone marrow and spleen of CY-treated mice demonstrated rebound increases at 5 and 7 days, respectively, after administration of the drug. Granulocyte-macrophage colony-forming cells (GM-CFC) exhibited alterations which were similar to those of Meg-CFC, suggesting similar sensitivities of Meg-CFC and GM-CFC to CY. The increase in Meg-CFC in both bone marrow and spleen preceded development of thrombocytosis by 4-6 days. This suggests that increased platelet counts in CY-treated mice are attributable, at least in part, to alterations in feedback mechanisms which control megakaryocytopoiesis, with resultant stimulation of the megakaryocyte progenitor compartment.

Platelet size distribution in mice following immunothrombopenia and vincristine administration

In the present study platelet size distribution was investigated after induction of immunothrombocytopenia by rabbit-anti-mouse-platelet-serum (RAMPS) and after vincristine-induced thrombocytopenia. The platelet size distribution after a single dose of RAMPS showed a shift to larger volumes at day 1 and 2, and a decrease to slightly smaller volumes than normal at day 8. These differences, however, were not statistically significant. After vincristine administration, a dose-dependent increase of the platelet size distribution was demonstrated, which lasted from day 1-7. It is suggested that in immune-induced thrombocytopenia the young platelets released from bone marrow megakaryocytes are not exclusively large platelets. On the other hand the early appearance of large platelets after vincristine administration points to a toxic or segregating effect of vincristine on circulating platelets. Therefore, in our experiments, the platelet size is not suitable for the differentiation of young and old platelets.

Abnormal platelet glycogen metabolism in multiple myeloma patients

This study presents evidence on the occurrence of an inverse relationship between activation of glycogen synthetase and inactivation of phosphorylase in a platelet preparation in vitro. The activities of glycogen synthetase and phosphorylase and the pattern of changes in these activities in platelets from controls and multiple myeloma patients were compared. Platelets obtained from multiple myeloma patients were shown to have an increased glycogen content, accompanied by an elevated level of glycogen synthetase a and a decreased activity of phosphorylase a. The pattern of changes in these enzyme activities during incubation was also different in platelets of controls and multiple myeloma patients. Extracts from patients' platelets prevented glycogen synthetase activation and phosphorylase inactivation of control platelets. Preincubation of platelets from multiple myeloma patients in control plasma resulted in an increased rate of glycogen synthetase activation, and abolished activation of phosphorylase which was found after preincubation in autologous plasma.

Serum thrombopoietic activity following administration of vinblastine

A possible role of humoral factors in the pathogenesis of vinblastine-induced thrombocytosis was examined. The thrombopoietic activity in serum of experimental animals was tested for its ability to stimulate the incorporation of 75-Se-selonemethionine into platelets of thrombocythaemic mice. The administration of low doses (0.1--0.5 mg/kg body wt.) of vinblastine to rabbits caused a significant increase in serum thrombopoietic activity. Higher doses of vinblastine (1--5 mg/kg body wt.) also increased the serum thrombopoietic activity, but this increase was preceded by a transient drop in the platelet count of peripheral blood. This thrombocytopenia could have been a stimulus for an increase in thrombopoietic activity, through a compensatory feedback mechanism. The vinblastine-induced increase in thrombopoietic activity was abolished by bilateral nephrectomy but not by bilateral ureteral ligation. These data suggest that kidney tissue may be a major source of the serum thrombopoietic factors.

Evidence that stimulation of megakaryocytopoiesis by low dose vincristine results from an effect on platelets

Appropriate low dosages of vincristine stimulate megakaryocytopoiesis and produce thrombocytosis. In this study of the thrombocytotic action of vincristine, administration of a single dose of 0.1 mg/kg to rats produced an increase in megakaryocyte concentration, diameter and 24 h [3H]thymidine labelling index. Transfusion of one body equivalent of platelets from normal donors prevented stimulation of megakaryocytopoiesis by vincristine whereas platelets from vincristine-treated donors did not. These results suggest that vincristine stimulates megakaryocytopoiesis by altering the functional role of circulating platelets in the regulation of thrombopoiesis.

The effects of colcemid on hematopoiesis in the mouse

Colcemid was found to induce a dose and schedule dependent marrow magakaryocytosis and peripheral thrombocytosis. The response could be divided into early and late components. The early component appears to have been due to a direct stimulatory effect, probably by enhancement of endoreduplication in metaphase arrested megakaryocyte precursors. The ealy stimulatory response was blunted on toxic drug schedules. In contrast, the late component of the thrombopoietic response was demonstrated best on the most toxic drug schedules. It coincided temporally with the reactive restoration of the mononuclear marrow and blood cell elements, respectively. Thus, the late component appears to be a nonspecific rebound phenomenon. On comparing the thrombopoietic properties of Colcemid with those of the vinca alkaloids in experimental systems, the former appears to have a more favorable therapeutic index. The data suggest that colchicine and its derivatives may be useful agents in the treatment of clinical thrombocytopenic states.

The effects of colcemid on mouse bone marrow

Following Colcemid administration, mitoses accumulate preferentially in the subendosteal region of the bone marrow of the mouse. This finding suggests that the most rapidly proliferating cells are localized to the subendosteal region, and complements previous radioautographic studies which have demonstrated a corresponding labelling gradient in the marrow. Quantitative estimates of cell cycle time by the stathmokinetic method were precluded by the presence of significant Colcemid induced interphase cell loss. Colcemid also affected cell differentiation in the marrow. Following Colcemid administration there was a fall in mature granulocytes in the marrow, and a concommitant rise in marrow megakaryocytes.

Effects of vincristine on normal and stimulated megakaryocytopoiesis in the rat

Vincristine was given to rats in which thrombocytopoiesis was either normal or acutely or chronically stimulated by injections of heterologous antiplatelet serum. A single dose of 0.3 mg/kg was given intravenously. The drug produced an early and a delayed megakaryocytopenia suggesting that it was toxic to differentiated megakaryocytes as well as to proliferating stem cells. The results support the hypothesis that vincristine-induced thrombocytosis may be due to homeostatic adjustments which, in turn, are activated as a result of drug-induced cytotoxicity.