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J Mizuki

Publications and source records attributed to J Mizuki.

At least 19 recordsLinked to original sources
PubMed2005

Momentum dependence of charge excitations in the electron-doped superconductor Nd1.85 Ce0.15 CuO4: a resonant inelastic x-ray scattering study.

We report a resonant inelastic x-ray scattering (RIXS) study of charge excitations in the electron-doped high-T(c) superconductor Nd1.85 Ce0.15 CuO4. The intraband and interband excitations across the Fermi energy are separated for the first time by tuning the experimental conditions properly to measure charge excitations at low energy. A dispersion relation with q-dependent width emerges clearly in the intraband excitation, while the intensity of the interband excitation is concentrated around 2 eV near the zone center. The experimental results are consistent with theoretical calculation of the RIXS spectra based on the Hubbard model.

Journal Article↗
PubMed2005

Mott gap excitations in twin-free YBa2Cu3O7-delta (Tc=93 K) studied by resonant inelastic x-ray scattering.

Mott gap excitations in the optimally doped high-T(c) superconductor YBa(2)Cu(3)O(7-delta) (T(c)=93 K) have been studied by the resonant inelastic x-ray scattering method. Anisotropic spectra in the ab plane are observed in a twin-free crystal. The excitation from the one-dimensional CuO chain is enhanced at 2 eV near the zone boundary of the b* direction, while the excitation from the CuO2 plane is broad at 1.5-4 eV and almost independent of the momentum transfer. Theoretical calculations based on the one-dimensional and two-dimensional Hubbard model reproduces the observed spectra when different values of the on-site Coulomb energy are assumed. The Mott gap of the CuO chain site is found to be much smaller than that of the CuO2 plane site.

Journal Article↗
PubMed2003

Charge-ordered state in single-crystalline CaFeO3 thin film studied by x-ray anomalous diffraction.

X-ray anomalous diffraction, together with a band-structure calculation, was employed to obtain a quantitative understanding of the charge-ordering state in a single-crystalline CaFeO3 thin film. The experimental result shows a characteristic energy dispersion of the nearly inhibited reflection at 150 K, implying Fe atoms split into two distinct states. The energy dispersion is in good agreement with the calculated spectrum based on the LDA+U scheme. The calculation also reveals an electronic structure of the system where holes in the oxygen orbital surround one of the distinct Fe atoms, in spite of the total electron number in both Fe atoms remaining unchanged.

Journal Article↗
PubMed2002

Self-regeneration of a Pd-perovskite catalyst for automotive emissions control.

Catalytic converters are widely used to reduce the amounts of nitrogen oxides, carbon monoxide and unburned hydrocarbons in automotive emissions. The catalysts are finely divided precious-metal particles dispersed on a solid support. During vehicle use, the converter is exposed to heat, which causes the metal particles to agglomerate and grow, and their overall surface area to decrease. As a result, catalyst activity deteriorates. The problem has been exacerbated in recent years by the trend to install catalytic converters closer to the engine, which ensures immediate activation of the catalyst on engine start-up, but also places demanding requirements on the catalyst's heat resistance. Conventional catalyst systems thus incorporate a sufficient excess of precious metal to guarantee continuous catalytic activity for vehicle use over 50,000 miles (80,000 km). Here we use X-ray diffraction and absorption to show that LaFe(0.57)Co(0.38)Pd(0.05)O(3), one of the perovskite-based catalysts investigated for catalytic converter applications since the early 1970s, retains its high metal dispersion owing to structural responses to the fluctuations in exhaust-gas composition that occur in state-of-the-art petrol engines. We find that as the catalyst is cycled between oxidative and reductive atmospheres typically encountered in exhaust gas, palladium (Pd) reversibly moves into and out of the perovskite lattice. This movement appears to suppress the growth of metallic Pd particles, and hence explains the retention of high catalyst activity during long-term use and ageing.

Journal Article↗
PubMed2001

EXAFS spectra above Pb and Pt K edges observed at low temperature.

X-ray absorption spectra near K edges of Pb (88.0keV) and Pt (78.4keV) for the foils were measured in transmission mode at SPring-8. We succeeded for the first time in observing the EXAFS oscillation of the Pb foil at 12K, although it was failed at room temperature in a previous experiment. The amplitude of the EXAFS oscillation of the Pt foil at 12K considerably increased in comparison with that at room temperature, indicating that the amplitude is very sensitive to the Debye-Waller factor at high-k values.

Journal Article↗
PubMed1998

An additional axis for the surface X-ray diffractometer.

A new surface X-ray diffractometer based on a kappa-type diffractometer will be installed in BL14B1, SPring-8. This diffractometer has an additional axis on its detector arm for rotating the receiving slit about the normal of the slit plane, in addition to two axes for positioning the detector. This additional axis is founded on the consideration of the correction factor which has been derived so as to be valid for the z-axis mode measurement using any incoming and outgoing angles of the X-ray beam. The rotational slit allows accurate measurement of the surface structure factor up to large perpendicular momentum transfer.

Journal Article↗
PubMed1998

Rab3B is essential for GnRH-induced gonadotrophin release from anterior pituitary cells.

Gonadotrophin-releasing hormone (GnRH) induces the release of gonadotrophins via an increase in cytosolic Ca2+ concentration ([Ca2+]). Rab3B, a member of the small GTP-binding protein Rab family, is known to be involved in Ca(2+)-regulated exocytosis in pituitary cells. However, it is not known whether Rab3B functions in the physiological process regulated by GnRH in gonadotrophs. In this study using antisense oligonucleotide against Rab3B (AS-Rab3B) we determined that Rab3B is involved in GnRH-induced gonadotrophin release. Rab3B immunopositive cells were reduced in 24% of pituitary cells by AS-Rab3B. This treatment did not affect the population of gonadotrophs or the intracellular contents of gonadotrophins. However, AS-Rab3B significantly inhibited the total amount of basal and GnRH-induced gonadotrophin released from pituitary cells. These results show that Rab3B is involved in basal and GnRH-induced gonadotrophins release but not the storage of gonadotrophins. Next, the changes in [Ca2+] and exocytosis in gonadotrophs treated with AS-Rab3B were compared among Rab3B-positive and -negative cells. The change in [Ca2+] was not different in the two groups, but exocytosis was significantly inhibited in Rab3B-negative cells. These results suggest that Rab3B is essential for GnRH-regulated exocytosis downstream of cytosolic Ca2+ in gonadotrophs.

Animals↗
PubMed1997

Bihormonal cells secreting both prolactin and gonadotropins in normal rat pituitary cells.

The present study was designed to characterize bihormonal cells in rat pituitary cells which secrete PRL and gonadotropins. This was done by using sandwich cell immunoblot assay (CIBA) and reverse hemolytic plaque assay (RHPA) in combination with immunocytochemistry (ICC) and by measuring the intracellular free calcium concentration ([Ca2+]i). The result of the experiment with sandwich CIBA revealed that the populations of LH- and FSH-secreting cells in the PRL-secreting cells were 6.23% and 5.91%, respectively, and the populations of the PRL secreting cells in the LH- and FSH-secreting cells were 18.4% and 15.5%, respectively. Additional studies by the combined techniques of RHPA with ICC revealed that the populations of LH- and FSH-containing cells in the PRL secreting cells were 4.43% and 2.40%, respectively, which were consistent with the results of Sandwich CIBA. Some of the PRL-secreting cells determined by RHPA showed responsiveness to TRH and GnRH in [Ca2+]i. These results suggest that bihormonal cells which secrete both PRL and LH, or both PRL and FSH are present in the normal rat pituitaries.

Animals↗
PubMed1995

Simultaneous measurements of exocytosis and intracellular calcium concentration with fluorescent indicators in single pituitary gonadotropes.

Previously, we established a method for the estimation of exocytosis in single gonadotropes using an impermeable fluorescent membrane probe, TMA-DPH. In this study, we have developed a method for the simultaneous measurement of exocytosis and intracellular free Ca2+ concentration ([Ca2+]i) by double-labeling with TMA-DPH and the intracellular Ca2+ probe, Fura-2/AM, using a fluorescence microscope with a 3-wavelength excitation and 2-wavelength emission system. We, therefore, clarified the relationship between spontaneous [Ca2+]i oscillation or gonadotropin releasing hormone (GnRH)-induced intracellular Ca2+ mobilization and exocytosis in gonadotropes. Under resting conditions, some gonadotropes showed various types of spontaneous [Ca2+]i oscillations, while others did not, but all showed basal exocytosis. Each [Ca2+]i peak oscillation did not cause Ca(2+)-regulated exocytosis, and even complete blockage of the [Ca2+]i increase by the intracellular Ca2+ chelator BAPTA/AM had no effect on basal exocytosis. Both GnRH-induced intracellular Ca2+ mobilization and regulated exocytosis showed a similar pattern of peaks and plateaus. Blockage of the [Ca2+]i increase by BAPTA/AM almost completely inhibited the GnRH-stimulated exocytosis. These results show that spontaneous [Ca2+]i oscillations under resting conditions are not linked to regulated or basal exocytosis, and that intracellular Ca2+ mobilization is essential for GnRH-stimulated exocytosis.

Animals↗
PubMed1995

Scanning electron microscope assessment of exocytotic changes in purified gonadotropes.

These studies were undertaken to characterize the exocytotic changes in purified gonadotropes by three-dimensional imaging using scanning electron microscopy. Rat gonadotropes were purified using a fluorescence-activated cell sorter and an argon laser treatment system. The purified gonadotropes were stimulated with GnRH under various conditions and fixed for scanning electron microscopy. After the GnRH stimulation, many 'hole' structures (diameter 0.1-0.5 micron) were observed on the cell surface, and notably the population of cells with 10 or more holes was clearly increased. The pattern of the time-course of the changes in this population was perfectly consistent with the LH secretory profile of pituitary cells, and their formation of the cells with 10 or more holes was completely inhibited by pretreatment with a GnRH antagonist. Our data suggest that the hole structure represents an exocytotic opening site and that regulated exocytosis in purified gonadotropes can be evaluated by scanning electron microscopy. This method may be widely applicable to other endocrine cells.

Animals↗
PubMed1995

Activin inhibits but inhibin activates mouse placental lactogen-II secretion.

The regulation of mouse placental lactogen (mPL)-I and mPL-II secretion by activin and inhibin and the expression of activin and inhibin subunit mRNAs in the mouse decidua were examined. Activin-A at a concentration of 10 nM/l significantly inhibited mPL-II secretion by placental cells from days 9 and 12 of pregnancy. However, activin-A did not affect mPL-I secretion by cells from days 7 and 9 of pregnancy nor mPL-II secretion by cells from day 7 of pregnancy. By contrast, 10 nM/l inhibin activated mPL-II secretion by cells from day 12 of pregnancy. These effects of activin and inhibin on mPL-II secretion were dose-dependent. Follistatin, which binds to activin and blocks its bioactivity, completely eliminated the inhibitory effect of activin on mPL-II secretion. Incubation of placental cells from day 12 of pregnancy with activin-A resulted in a significant reduction of the mPL-II mRNA level assessed by Northern blot analysis. Northern blot analysis using poly(A)+RNA extracted from the decidua indicated that mouse decidua, as well as the placenta, express all activin and inhibin subunits and that their gene expressions increased during gestation. The expression of these mRNAs in the decidua was much higher than those in the placenta. These findings suggest that activin and inhibin regulate mPL-II secretion and suggest the presence of an autocrine or paracrine regulation of mPL-II secretion in mouse placenta by activin and inhibin after mid-pregnancy in vivo.

Activins↗
PubMed1994

Characterization of activin A-, activin AB- and activin B-responding cells by their responses to hypothalamic releasing hormones.

Activin A-, AB- and B-responding cells were characterized by their responsiveness in cytosolic free calcium ([Ca2+])i) to four hypothalamic releasing hormones, CRH, GHRH, TRH and GnRH. First, rat pituitary cells responding to activin A, AB and B in [Ca2+]i were determined in a mixed population of pituitary cells. The populations of the activin A-, AB-, and B-responding cells were 13.9%, 9.3% and 13.2%, respectively. Overlapping of response among each population of activin- responding cells was present in some of the cells. The cells responding to activin A, AB, and B were then characterized by their responses to CRH, GHRH, TRH and GnRH. Most of the cells responding to activin A, AB, and B also responded to GHRH or TRH. These results reveal that there are distinct differences among each population of activin A-, AB- and B-responding cells and that there is still functional overlapping of responsiveness among these populations. The characterization of activin-responding cells suggests involvement of somatotropes and lactotropes in activin-induced biological events in the pituitary.

Activins↗
PubMed1994

Characterization of rat pituitary cells by their responses to hypothalamic releasing hormones.

Rat pituitary cells in monolayer culture were characterized by their [Ca2+]i responses to hypothalamic releasing hormones, growth hormone (GHRH), thyrotropin-releasing hormone (TRH), gonadotropin-releasing hormone (GnRH) and corticotropin-releasing hormone (CRH). The percentages of the cell population responding to GHRH, TRH, GnRH, CRH and non-responding cells were 27.3%, 47.6%, 13.8%, 6.2% and 35.3%, respectively. Some of the cells responded to two or more of those hormones. In the GHRH-responding cells, the population of TRH-responding cells was 51.4%, In the TRH-responding cells, the population of GHRH-responding cells was 30.8%. Some of the GHRH-responding cells also responded to CRH and GnRH. In the GnRH-responding cells, the population of TRH-responding cells was 61.8%. In summary, GHRH-responding cells have an especially close relationship with TRH-responding cells, and GnRH-responding cells also have close relationship with TRH-responding cells. There is also some relationship between the populations responding to other pairs of releasing hormones. These findings suggest functional overlapping among each population of pituitary cells.

Animals↗
PubMed1994

Roles of prostaglandins and intracellular free calcium mobilisation in epidermal growth factor-induced proliferation of human amnion cells.

OBJECTIVE: To investigate the mechanisms which regulate the growth of human amnion cells. DESIGN: A prospective descriptive study. SUBJECTS: Women undergoing caesarean section at term before the onset of labour. INTERVENTIONS: Amnion cells were cultured in monolayer. MAIN OUTCOME MEASURES: Cell cycle analysis, intracellular calcium levels, prostaglandin (PG) production rates. RESULTS: Epidermal growth factor (EGF) stimulated intracellular Ca2+ mobilisation and PGE2 production in cultured amnion cells. The addition of a Ca2+ channel blocker (cobalt) or a Ca2+ chelator (EGTA) into the culture medium inhibited intracellular Ca2+ mobilisation and PGE2 production induced by EGF. The analysis of cell cycles showed that EGF induced the initiation of DNA synthesis and that the addition of cobalt or EGTA into the culture medium inhibited EGF-induced DNA synthesis. The addition of a cyclo-oxygenase inhibitor (indomethacin) inhibited PGE2 production and DNA synthesis induced by EGF without the effect on intracellular Ca2+ mobilisation. Moreover, the inhibitory effect of indomethacin on EGF-induced DNA synthesis was attenuated by the addition of exogenous PGE2 or PGF2 alpha. CONCLUSIONS: These data suggest that EGF induces an increase in intracellular Ca2+ levels and the rate of prostaglandin production which leads to proliferation of human amnion cells.

Amnion↗