Cord blood transplantation from sibling donors in Japan. Report of the national survey

A joint national survey on cord blood transplantation (CBT) was conducted in Japan and 18 sibling CBTs were reported. Diseases of the patients were leukemia (ten), neuroblastoma (one), bone marrow failure (four) and inborn errors of metabolism (three). A volume of 50-141 ml of cord blood containing 27-197 x 10(7) nucleated cells was collected from sibling infants soon after delivery. HLA antigens were identical in 14 and one to three antigens mismatched in four. Engraftment of donor cord blood was achieved in 17 cases. Autologous hematopoiesis was recovered in one case. Days of engraftment were 13-29 days (median 19 days) for neutrophils (500/microliter), 18-67 days (median 30 days) for reticulocytes (2%) and 21-96 days (median 46 days) for platelets (50 x 10(3)/microliter). Acute GVHD was grade 0 in seven cases, grade I in five cases and grade II in one case in HLA-identical pairs, but became grade II in two cases and grade III in two cases in HLA-mismatched pairs. Chronic GVHD of limited type developed in two out of 17 evaluable cases, however both responded to immunosuppressive therapy. Altogether, 14 out of 18 patients are currently surviving 4-27 months following transplantation. Probabilities of overall survival and disease free survival were estimated to be 77.0 and 71.8% using Kaplan-Meier tests. These findings suggest the feasibility of cord blood transplantation from sibling donors and the possibility of unrelated cord blood transplantation. A cord blood banking system is necessary for the universal use of cord blood stem cells from unrelated donors.

CD34+ progenitor cell transplantation from two HLA-mismatched healthy fathers to two infants with severe aplastic anemia

Pluripotent stem cells of hematopoiesis are included among CD34+ cells in the blood and bone marrow. After granulocyte-colony stimulating factor (G-CSF) mobilization, 1-2% of the mononuclear cells in the blood are CD34+ cells, which can be obtained by leukapheresis. We performed CD34+ progenitor cell transplantation in two children with severe aplastic anemia (SAA) who lacked HLA-matched donors. The donors were treated with G-CSF, 600 micrograms/body/day subcutaneously, for 4-5 days. CD34+ cell selection was performed from the apheresis concentrate with mouse anti-CD34 antibody 9C5 and magnet beads coated with sheep anti-mouse IgG1. After the transplantation, the patients received tacrolimus to prevent graft-versus-host disease (GVHD). G-CSF was given to both patients. A mean number of 4.96 x 10(6) CD34+ cells per kilogram of body weight were transplanted. The hematopoietic recovery after the CD34+ cell transplantation was rapid, except for platelets, and acute GVHD was less than or equal to grade I. Case 1, who demonstrated mixed chimerism, anemia and thrombocytopenia after the graft, received a second transplant with intensified preconditioning, and now sustains complete and stable hematopoiesis after a follow-up of 314 days posttransplant. Although Case 2 showed early rejection and received a second transplant, sustained engraftment was never achieved. However, the patient's own hematopoiesis appeared. For SAA patients who do not have HLA-matched donors, this type of approach seems to be a feasible and useful method. However, an intensified preconditioning regimen to overcome the high likelihood of rejection should be employed.

Ophthalmic findings in a case of hemophagocytic syndrome

PURPOSE

To report a case of hemophagocytic syndrome, which is characterized by hemophagocytosis of histiocytes; optic nerve involvement, and unusual retinal white patches.

METHOD

Case report. A 10-year-old boy had repeated relapses of hemophagocytic syndrome. He complained of swelling of the right upper eyelid and bilateral visual disturbance.

RESULTS

Ophthalmoscopic examination disclosed bilateral optic disk edema, retinal hemorrhages, and multiple perivenous white patches in the retina. Magnetic resonance imaging demonstrated enlargement of both optic nerves. After chemotherapy and bone marrow transplantation, his visual acuity improved in both eyes, and retinal patches changed to inactive-appearing scars.

CONCLUSION

Hemophagocytic syndrome may manifest with ophthalmic findings such as optic nerve involvement, retinal hemorrhages, and multiple white perivenous retinal patches.

[Epstein-Barr virus-infected T-cell malignancy in an adult patient with Behçet's disease-like symptoms]

A 20-year-old woman was hospitalized on November 11, 1994 with Behçet's disease-like symptoms (fever, genital ulcer and aphtha in the oral cavity). Bilateral cervical lymph node swelling was also noted and diagnosed as lymphadenitis on biopsy. Chronic active Epstein-Barr virus infection (CAEBV) was diagnosed based on the high titer of antibodies to the EBV capsid antigen, early antigen, and nuclear antigen. She was treated with prednisolone and acyclovir and all symptoms improved. However, ten months after onset of symptoms, T-cell malignancy was diagnosed on bone marrow aspiration, which revealed 34.9% blast cells that had rearrangement of TCR-beta. She died on May 8, 1995, despite anticancer therapy. In analyzing the blast cells, the monoclonal junctional DNA structure of the EBV terminal repeat was analyzed by Southern blotting and provided definitive evidence for the monoclonality of EBV-infected T cells. These findings strongly suggest that EBV plays a pathogenic role in T-cell malignancy. EBV-infected T-cell malignancy, such as this case, is very rare in Japan, especially in adult.

[Leukoencephalopathy probably caused by tacrolimus hydrate after stem cell transplantation in a girl with MDS 7 monosomy]

Leukoencephalopathy probably caused by tacrolimus hydrate after stem cell transplantation in a girl with MDS 7 monosomy is reported. The conditioning regimen consisted of thiotepa (150 mg/m2 x 4), melphalan (70 mg/m2 x 2) and 12 Gy total body irradiation. She received peripheral blood CD34 positive cells (4.17 x 10(6)/kg) from her HLA-mismatched father and tacrolimus hydrate was used for GVHD prophylaxis. Engraftment was rapid and grade 1 acute GVHD of the skin responded well to pulse therapy. From day 27 she became irritable and sleepless, and right facial convulsions developed on day 37. No abnormality was found in the cerebrospinal fluid. Cranial CT findings showed no abnormalities except for low density lesions around the bilateral ventricle. Leukoencephalopathy was suspected and tacrolimus hydrate was discontinued. Thereafter psychosomatic symptoms improved, temporarily however, similar symptoms again developed following cyclosporine administration. Therefore we had to halt the administration of both tacrolimus and cyclosporine. She died on day 104 because of GVHD and fungal infection without recovering from leukoencephalopathy.

[Current diagnosis and prognostic importance of EBV-associated neoplasms]

In addition to endemic Burkitt's lymphoma and nasopharyngeal carcinoma a variety of other epithelial-and lymphoid-derived proliferative diseases has been shown to closely link with Epstein-Barr virus (EBV) infection. The former include thymic lymphoepithelial carcinoma, oral hairy leukoplakia and gastric carcinoma. The latter include B-cell lymphoproliferative disorders arising in individuals with primary or secondary immunodeficiency, Hodgkin's disease and T/NK lymphoma. The significance of serological, immunohistochemical and molecular biological methods such as PCR, in situ hybridization and Southern blotting is described. And also the possible prognostic impact of EBV in T/NK-lymphoma is discussed.

[Clinical characteristics of Epstein-Barr virus-associated natural killer cell lymphoma/leukemia]

Granular lymphocytes proliferative disease(GLPD) is a heterogeneous disorder and the pathogenesis is likely to be complex. More recently, however, it has become obvious that about one third of GLPD belongs to NK-lineage and nearly one half of the NK-GLPD contains EBV-DNA. In addition, lethal midline granuloma(a subtype of nasal lymphoma) is also classified as NK/T-lineage and exclusively contains EBV-DNA. These NK-GLPD likely represent fever, splenomegaly and extranodal involvement, such as skin, lung and G-I tract. Although some elderly patients show a chronic clinical course, most of the patients present with an aggressive course and sometimes hemophagocytic syndrome is observed in the clinical course.

[The clinical significance of minimal residual disease of acute leukemia with t(4;11) (q21;q23)]

Hybrid fusion genes are specific tumor markers of several leukemic subtypes. The use of reverse transcription-polymerase chain reaction (RT-PCR) to amplify chimeric cDNAs allows sensitive detection of the leukemia clone. The clinical relevance of minimal residual disease (MRD) remains controversial. In this report, an infantile acute lymphoblastic leukemia with t(4;11) (q21; q23) was analyzed after each treatment for the presence of MRD by RT-PCR amplification of the MLL/LTG4 fusion gene which became available recently. The patient soon achieved a hematological CR, after induction therapy, and underwent autologous BMT following consolidation chemotherapy for 9 months. However, he relapsed three months after the BMT. MRD was always detectable during his clinical course. These findings suggest that the detection of MRD of the MLL/LTG4 fusion transcript is a useful tool for monitoring MRD and selecting treatment.

[LGL lymphoma]

We reported in this article two patients with large granular lymphocytic lymphoma (abbreviated as LGL lymphoma). One was the patient with LGL leukemia/lymphoma (patient 1) and other was the patient with NK-LGL lymphoma (patient 2). Because the gene of TCR delta was rearranged in the patient 1, the clonality of the LGL leukemia/lymphoma was confirmed. However, it is not determined yet whether the lineage of tumor cells is T cells or NK cells. The cytochemical features of the lymphoma cells of the patient 2 were studied. It was found that NK cell-derived lymphoma cells of the patient were positively stained with these two monoclonal antibodies that are reactive with T cell; one is anti-CD45RO (UCHL-1) and other is anti-CD3. Judging from the result, malignant NK-LGL cells in some patients are cytoplasmic CD3+ and UCHl-1+. It is emphasized that May-Grünwald-Giemsa stain of biopsied specimen of the lymphoma is required for making the diagnosis of LGL lymphoma.

ADP-induced rapid inward currents through Ca(2+)-permeable cation channels in mouse, rat and guinea-pig megakaryocytes: a patch-clamp study

1. The rapid inward currents in mouse megakaryocytes evoked by adenosine diphosphate (ADP), a ubiquitous platelet-activating substance, were studied. Time and current resolution were improved by using patch-clamp recording and an extracellular fast perfusion ("Y tube') technique. 2. Application of ADP (40 microM) to megakaryocytes immersed in physiological saline evoked rapid inward currents (80-340 pA at -42 mV). The cellular responses to a second ADP application were markedly reduced, but in the absence of external Ca2+, responses to repeated ADP application were maintained and did not deteriorate. 3. The ADP-induced current recorded in Ca(2+)-free external media showed short latency (less than 20 ms) and approximately exponential decay (time constant, 300-500 ms), which was independent of the holding potential and seemed to be caused mainly by receptor desensitization; it took over 5.5 min for complete recovery. 4. The ADP concentration response relationship of the megakaryocytes revealed that the half-maximal concentration and the Hill coefficient were 12.6 microM and 1.4, respectively. 5. An ion replacement experiment showed that the ADP-induced currents could be carried by Na+, Cs+ and K+, but not Cl-, and the cation channels were permeable to Ca2+, Ba2+ and Mg2+. 6. Neither Ca2+ chelators (10 mM EGTA and 10 mM BAPTA) nor hydrolysis-resistant guanine nucleotides (2 mM GDP-beta-S and 0.4 mM 5'-guanylylimidodiphosphate) in the internal saline affected the rapid responses to ADP, and ADP-induced currents were recorded in excised membrane patches, suggesting that the ADP receptor site and the molecular structure forming the cation channel are tightly coupled and/or parts of the same molecule. 7. In rat and guinea-pig megakaryocytes, ADP-induced rapid inward currents showed the same properties as in mouse megakaryocytes.

[Natural killer cell lymphoma having a nodular shadow in the lung as an initial finding, developed to leukemia complicated with hemophagocytic syndrome at the time of relapse]

A 57-year-old female was admitted to Uji hospital for the further evaluation of nodular shadow on her right lung. During the period of admission, she developed cervical lymph node swelling. She was diagnosed as having malignant lymphoma (diffuse, small cleaved cell) by lymph node biopsy. She received combined chemotherapy and obtained partial remission for seven months until she developed fever and pancytopenia. Laboratory data showed increased number of large granular lymphocytes (LGLs) in blood. Bone marrow revealed increased number of LGLs with hemophagocytosis by macrophage. Surface marker analysis revealed LGLs were positive for CD2 CD16, and CD56 and negative for CD3, CD4, CD8, and CD20. T-cell receptor genes beta and gamma were in germ line configuration. Analysis of Epstein-Barr virus genome using termini probe indicated a monoclonal proliferation of LGLs. Reexamination of the biopsy specimen of lymph node revealed LGLs which was negative for CD3 and CD20. The patient was diagnosed as a leukemic phase of natural killer (NK) cell lymphoma complicated with hemophagocytic syndrome (HPS). Serum levels of interferon-gamma, macrophage colony-stimulating factor, granulocyte colony-stimulating factor, and interleukin-6 increased, which might be related to HPS.

[Chronic active EB virus infection and granular lymphocytes proliferative disorders in Japan]

To clarify the characteristics of chronic active EB virus infection (CAEBV) in Japan, and to investigate the relation between granular lymphocytes proliferative disorder (GLPD) and EB virus, we conducted a survey through a questionnaire conducted throughout Japan. Among 17 registered patients with CAEBV, 9 developed various types of lymphoproliferative disorders (LPDs), and 6 patients died of LPD. Among 72 cases of GLPD, 43 were CD3-positive and 27 were CD3-negative. EB viral DNA was detected in the peripheral mononuclear cells in 6 of 7 CD3-negative and 1 of 4 CD3-positive cases. These data suggest that EB virus-associated LPDs frequently derive from patients with CAEBV. However, some GLPD patients without CAEBV, especially for CD3-negative GLPD, are associated with EB virus infection. Therefore detection of EB viral DNA is very important to understand the pathogenesis of GLPD.

Immunohistochemical detection of early myocardial infarction. An evaluation of antibodies against the terminal complement complex (C5b-9)

Antibodies (abs) against the terminal complement complex (C5b-9) were used on routinely processed post mortem myocardial tissue in parallel with conventional staining methods. Both monoclonal and polyclonal abs were tested using the avidin biotin peroxidase complex (ABC), alkaline phosphatase anti-alkaline phosphatase (APAAP) methods and an ab-bridge with alkaline phosphatase. Enhancement of the diaminobenzene (DAB) end product with cobalt-nickel (ABC method) was also done. The polyclonal ab gave the most satisfactory results and the alkaline phosphate conjugated ab-bridge had a slight advantage over the ABC method. Cobalt-nickel enhancement of DAB improved the visualization, but with higher background staining. APAAP was the least satisfactory method. Comparing the immunohistochemical method with the conventional staining methods, the former showed positive reaction in 97% of areas of coagulation necrosis and in 65% of contraction band necrosis. On the other hand coagulation necrosis was seen in 44% and contraction band necrosis in 68% of C5b-9 positive areas indicating that C5b-9 abs react with ischemically damaged myocytes before visible alterations are seen in hematoxilin-eosin staining. Moreover, using C5b-9 abs, it seems possible to exclude agonal/artefactual contraction bands which show a negative reaction. Immunohistochemical detection of C5b-9, using an adequate technique could increase the possibility to demonstrate early ischemic myocardial damage.

Distribution and functional properties of 5-HT3 receptors in the rat hippocampal dentate gyrus: a patch-clamp study

1. In dentate gyrus of rat hippocampal slices two distinct types of neurons, principal excitatory neurons (granule cells) and local inhibitory neurons (basket cells), could be identified under Nomarski microscopy; I investigated the actions of serotonin using the whole-cell patch-clamp technique. The identification of the neurons was later confirmed by intracellular staining with Lucifer yellow. 2. In both basket cells and granule cells, whole-cell current recordings revealed spontaneous synaptic currents ranging from < 10 pA to > 200 pA in symmetrical Cl- conditions at a holding potential of -63 mV. These currents were blocked by 10 microM bicuculline, indicating that they resulted from the spontaneous activation of GABAergic inputs (which had been morphologically described in both types of neurons). 3. By focal application of serotonin (2-50 microM) to basket cells under current clamp I evoked a train of action potentials superimposed on a baseline membrane depolarization. Under voltage-clamp conditions serotonin evoked an inward current at a holding potential of -63 mV (currents were detectable in approximately 90% of basket cells studied). The inward current was accompanied by a multitude of small inward currents of short duration (< 100 ms) that were found to be due to the stimulation by serotonin of nearby GABAergic presynaptic neurons innervating the recorded neuron. 4. In granule cells (total of 11 cells) serotonin did not produce any responses under conditions similar to those used for basket cells. The occurrence of bicuculline-sensitive spontaneous synaptic current events seemed to increase during the application of serotonin; this phenomenon reflected the excitatory action of serotonin exclusively on GABAergic interneurons. 5. The serotonin-induced inward currents in basket cells were mediated by the 5-HT3 receptor subtype because 1) they were blocked by either metoclopramide (10 microM) or [3-alpha-tropanyl]-1H-indolecarboxylic acid ester (2 nM), the latter being a specific blocker for the 5-HT3 receptor subtype, and 2) almost similar currents were induced by the application of the selective 5-HT3 receptor agonist 2-methyl 5-HT (2-50 microM) or 1-(m-chlorophenyl)-biguanide (0.1-10 microM). 6. Current-voltage (I-V) relations of serotonin-induced currents in basket cells showed that the reversal potential was close to 0 mV in external standard saline and depended on the concentrations of monovalent cations. I-V relations of serotonin-induced currents revealed inward rectification at the membrane potential range of +30 to -60 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

Aggressive granular lymphocyte leukemia of natural killer cell type in an elderly patient

A 75-year-old man was admitted to our hospital because of intermittent fever. His peripheral blood picture showed granular lymphocyte (GL) proliferation. The GLs were immunologically and functionally phenotyped as natural killer cells. Chromosomal analysis of peripheral lymphocytes with interleukin-2 stimulation revealed an inversion of chromosome 9 with an unusual breakpoint, showing abnormal monoclonal proliferation of the GLs. Progressive increase of GL count and hepatosplenomegaly necessitated the start of combined chemotherapy. His condition was complicated by icterus and renal failure, and he died finally of respiratory failure. Autopsy revealed disseminated intravascular coagulation and infiltration of GLs in the bone marrow, spleen, and liver.

Existence of rolipram-sensitive phosphodiesterase in rat megakaryocyte

1. The effect of rolipram (ME3176) on ADP- and IP3-induced repetitive IK(Ca) in rat megakaryocyte was investigated by use of the nystatin perforated patch and conventional whole-cell patch-clamp techniques. 2. The ADP-induced IK(Ca) was depressed by treatment with rolipram in a concentration-dependent manner. The inhibition by rolipram disappeared after treatment with a cyclic nucleotide-dependent protein kinase inhibitor, H-8. The inhibition of IK(Ca) was also observed in the presence of cyclic AMP accumulating agents such as forskolin and isobutylmethylxanthine (IBMX). 3. Rolipram enhanced the inhibitory action of forskolin, suggesting that rolipram facilitates the accumulation of cyclic AMP by blocking its breakdown. Similar results was obtained with adenosine, an endogenous adenylate cyclase activator. 4. Intracellular application of inositol trisphosphate (IP3) induced repetitive IK(Ca) in megakaryocytes. The induced IK(Ca) was also inhibited by rolipram and by other cyclic AMP accumulating agents. 5. It was concluded that megakaryocytes possess rolipram-sensitive phosphodiesterase (PDE), which was not detected in platelets, but plays a distinct modulatory role in megakaryocytes for generating ADP-induced IK(Ca).

[Chronic fatigue syndrome in school children]

Chronic fatigue syndrome (CFS) is characterized by persistent or relapsing debilitating fatigue for at least 6 months without any apparent medical diagnosis that would explain the clinical presentation. Although, most of the reported patients are over age 30, CFS also affects school children. To better understand CFS, the network of the central nervous-endocrine-immune systems should be considered, and one must be careful to distinct CFS from school absenteeism and other psychosomatic disorders often seen among them.

[Phenotypic and genotypic analysis of acute leukemia--current status of lineage specific classification]

More detailed identification and understanding of the heterogeneity of leukemias using a broad panel of markers seems to be essential for the successful design of more sophisticated and effective treatments. Based on the FAB system, immunological phenotypes using a panel of monoclonal antibodies, and rearrangements of immunoglobulin and T-cell receptor genes, acute leukemia can be divided into six subtypes such as B-lineage, T-lineage, AML, NK-lineage, AUL and mixed lineage leukemia. The definition of B-lineage and T-lineage cells, a new classification for mixed lineage leukemia, incidence of dual rearrangements and their clinical significance are discussed.

HTLV-I associated myelopathy (HAM) after blood transfusion in a patient with CD2+ hairy cell leukemia

Hairy cell leukemia complicating hemolytic anemia developed in a 46-year-old woman. Morphologically and cytochemically typical hairy cells were found to express both CD20 and CD2 antigens. Expression of surface IgG of kappa-chain type and the rearrangement of Ig but not T-cell receptor beta genes confirmed a B-cell origin of the leukemia. Blood transfusion was followed by disappearance of the hemolysis and a marked improvement of the leukemia. However, the patient developed progressive spastic spinal paraplegia about seven months after transfusion and was diagnosed as having HTLV-I associated myelopathy (HAM) by the demonstration of HTLV-I antibodies in serum and cerebrospinal fluid. HTLV-I infection via the transfusion may have been involved in the hematologic improvement seen in this patient. Autopsy showed demyelination, vacuolar degeneration, gliosis, and perivascular cuffing in the white matter of spinal cord without evidence of leukemic infiltration.

Effects of biological response modifiers on childhood ALL being in remission after chemotherapy

Of 125 children with acute lymphoblastic leukemia (ALL), who had been in continuous remission for three years on chemotherapy, 108 patients received biological response modifiers (BRM) such as Bestatin, N-CWS, OK-432 and/or PSK in order to prevent relapse after treatment suspension. From 20 patients who were treated with PSK, 6 relapsed within 13 months. This relapse rate was quite similar to the rate observed with those children who were off therapy (4 relapses in 17 patients within 13 months). In contrast to these 37 patients, only 3 out of 31 patients who received Bestatin (p less than 0.05) and 8 out of 57 patients who received N-CWS or OK-432 relapsed. Based on these findings, BRMs used in the present study seems to be effective to prevent relapse of leukemia among childhood ALL who have electively stopped chemotherapy.

Guinea-pig megakaryocytes can respond to external ADP by activating Ca2(+)-dependent potassium conductance

1. The responses of megakaryocytes to adenosine diphosphate (ADP) were studied using whole-cell patch electrodes and a Ca2(+)-sensitive fluorescent dye, Fura-2. Megakaryocytes (diameter, 17-42 microns) were mechanically dissociated from the bone marrow of adult guinea-pigs and ADP (1-10 microM) was pressure-applied to megakaryocytes under recording. 2. In megakaryocytes immersed in standard saline, ADP evoked an obvious outward current at a membrane potential of -63 mV. The current was identified as a K(+)-carried current, since the reversal potential depended distinctly on the external K+ concentration, but it showed no changes after removal of external Na+. The amplitude of evoked K+ currents showed considerable intercell variation, which is presumably due to differences of current density in the membrane. 3. During application of ADP, the evoked K+ current was not sustained but slowly decayed to become negligible within 10-20 s, suggesting the appearance of desensitization. The response of the megakaryocyte to ADP recovered slowly and returned to an original level after 4-5 min of continuous washing. 4. When the intracellular free Ca2+ concentration ([Ca2+]i) was measured using the Ca2(+)-sensitive fluorescent dye, Fura-2, application of 10 microM-ADP induced an increase of [Ca2+]i by about 5-fold, which was followed by a gradual decay to the original level within 30-50 s. Roles of internal Ca2+ for activating the K+ current were confirmed by observing (1) enhancement of evoked currents by the use of internal saline containing no Ca2+ chelators and (2) generation of prolonged K+ current by application of a Ca2+ ionophore, A23187, to the megakaryocyte. 5. In a fraction of the megakaryocytes, spontaneous hyperpolarization of the resting membrane potential was observed. The hyperpolarization seemed to result from the activation of K+ channels in the membrane, which was caused by spontaneous release of Ca2+ from the internal storage site. 6. It was concluded that megakaryocytes of the guinea-pig can respond to external ADP by increasing [Ca2+]i and consequently by activating Ca2(+)-dependent K+ channels in the membrane.

Voltage-gated calcium and potassium currents in megakaryocytes dissociated from guinea-pig bone marrow

1. The electrophysiological properties of the cell membrane of guinea-pig megakaryocytes were studied using the whole-cell patch-clamp technique. The megakaryocytes (diameter, 17-42 microns) were dissociated mechanically from the bone marrow of adult guinea-pigs. 2. In a proportion of cells, spike-like action potentials were generated in response to depolarization when the cells were immersed in standard saline containing 10 mM-Ca2+. Under voltage clamping, a transient inward current followed by a slowly Ca2+. Under voltage clamping, a transient inward current followed by a slowly developing outward current was produced when the membrane potential was made more positive than -55 mV. 3. The inward currents were identified as Ca2(+)-carried current, since the amplitude depended distinctly on external Ca2+ concentration and since replacement of external Ca2+ with Mn2+ reversibly diminished the current. The Ca2+ channels involved are most probably of the transient type (T-type). 4. The reversal potential of the outward current changed from -87 to -46 and -7 mV when the external K+ concentration was raised from 5 to 25 and 125 mM. 5. The outward current was insensitive to chelation of internal Ca2+ but was blocked by external application of quinine, 4-aminopyridine and tetraethylammonium, and was thus very probably a membrane potential-dependent K+ current. The dependence of the current activation and inactivation on the membrane potential was consistent with that of a delayed K+ rectifier. 6. The amplitudes of the Ca2+ currents and K+ currents showed considerable intercell variation. However, the density of the Ca2+ current showed a tendency to increase with megakaryocyte size, presumably accompanying maturation. The roles of these currents in cellular function remain to be elucidated.