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Biomedical subjects

J Mizuki

Publications and source records attributed to J Mizuki.

31 records · Page 2Linked to original sources
PubMed1993

High magnesium concentration inhibits ligand-stimulated calcium influx and hormone secretion in rat pituitary lactotropes with involvement of intracellular free magnesium.

The effects of extracellular magnesium concentration ([Mg2+]ex) on thyrotropin-releasing hormone (TRH)-stimulated intracellular free calcium mobilization and prolactin secretion were investigated concomitantly with measurement of the intracellular free magnesium concentration ([Mg2+]i). TRH-stimulated intracellular free calcium mobilization was significantly inhibited when the medium was replaced by high Mg2+ medium ([Mg2+]ex = 10 mM) in normal Ca2+ medium. The inhibitory effects of high Mg2+ became apparent concomitantly with an increase in [Mg2+]i from 0.7 to 1.3 mM. High Mg2+ significantly inhibited TRH-induced PRL secretion in a dose-dependent manner in normal Ca2+ medium. TRH-stimulated inositol triphosphate (IP3) production was rather augmented by the replacement with high Mg2+ medium. In summary, high Mg2+ inhibits Ca2+ influx stimulated by TRH in the rat pituitary lactotropes, possibly with the involvement of [Mg2+]i increase. These results have general importance in relation to high Mg(2+)-induced suppression of the biological functions of cells.

Animals↗
PubMed1993

Dynamics of exocytosis, endocytosis and recycling in single pituitary gonadotropes.

The dynamic changes in exocytosis, endocytosis and recycling in single gonadotropes induced by gonadotropin-releasing hormone (GnRH) were visualized and estimated with an impermeable fluorescent membrane probe, 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatrien (TMA-DPH), using a digital imaging and confocal laser scanning microscope. 10(-7) M GnRH induced exocytosis and endocytosis within 10 sec and 60 sec, respectively. Recycling of the plasma membrane started at 180-300 sec. Exocytosis and endocytosis in purified gonadotropes changed dose-dependently with 10(-10)-10(-7) M GnRH. These results show that GnRH-induced exocytosis, endocytosis and recycling in gonadotropes maintain dynamic equivalence. The procedure we established will be very useful in studies of the function of secretory cells.

Animals↗
PubMed1993

Magnesium sulfate inhibits oxytocin-induced calcium mobilization in human puerperal myometrial cells: possible involvement of intracellular free magnesium concentration.

OBJECTIVE: Our purpose was to elucidate the mechanisms of the tocolytic action of Mg2+ on the human myometrium. STUDY DESIGN: The effects of extracellular Mg2+ on oxytocin-induced increases in the intracellular free Ca2+ concentration in human puerperal myometrial cells were examined by means of indo-1-AM. The changes in the intracellular free Mg2+ concentration under various conditions were also measured with Mg(2+)-fura-2-AM. RESULTS: The increase in intracellular free Ca2+ concentration induced by oxytocin was reduced to 26% of that in the normal solution 20 minutes after replacement of the normal solution with an extracellular Mg2+ solution, 10 mmol/L. When extracellular Ca2+ was removed, the increase in intracellular free Ca2+ concentration was not suppressed even 20 minutes after the replacement. A solution of extracellular Mg2+ concentration, 10 mmol/L, raised the intracellular free Mg2+ concentration gradually, by approximately 150% in 20 minutes, concomitant with the suppression of the response to oxytocin in the intracellular free Ca2+ concentration. CONCLUSION: High intracellular free Mg2+ concentration, caused by high extracellular Mg2+, is essential for suppression of oxytocin-induced Ca2+ influx across the cell membrane; this presumably results in inhibition of uterine contractions.

Calcium↗
PubMed1992

Activin A increases cytosolic free calcium concentration in rat pituitary somatotropes.

The effect of activin A on the cytosolic free calcium concentration ([Ca2+]i) in normal rat pituitary cells was examined using a calcium sensitive fluorescent dye, indo 1 AM, and a digital imaging fluorescent microscope system. The cells showing an increase in [Ca2+]i in response to activin A were then characterized by comparison with cells responding to growth hormone releasing hormone (GRH), thyrotropin releasing hormone (TRH), corticotropin releasing hormone (CRH), and gonadotropin releasing hormone (GnRH) in monolayer cultures of normal rat pituitary cells. Activin A increased [Ca2+]i in some cells in a mixed population of normal rat pituitary cells. The cells that responded to activin A also responded to GRH. Most of these cells were not affected by other tropic hormones (CRH, TRH, and GnRH), but a few cells responded to both GRH and TRH. None of the activin A-responding cells responded to CRH or GnRH, and none of the CRH- or GnRH-responding cells responded to activin A. In a preparation of somatotropes purified 80-90% by fluorescence-activated cell sorting, activin A increased [Ca2+]i in 30% of the cells that shows a [Ca2+]i-response to GRH. These findings suggest direct involvement of somatotropes in activin A-induced biological events in the rat pituitary gland.

Activins↗
PubMed1992

Regulation of intracellular Mg2+ by superoxide in amnion cells.

Changes of intracellular free Mg2+ concentration ([Mg2+]i) in human amnion cells induced by superoxide anion were determined using a highly Mg(2+)-sensitive fluorescent dye Mg(2+)-fura2 or Mg(2+)-indol. Superoxide anion, produced by addition of xanthine oxidase to hypoxanthine, induced decrease of [Mg2+]i. The decrease was significantly inhibited by an anion channel blocker, 4,4'diisothiocyano-2,2' disulfonic acid stilbene (DIDS). Superoxide dismutase (SOD), injected into cells by cell fusion, also inhibited the change of [Mg2+]i, but catalase did not. Superoxide anion induced prompt increase of intracellular pH (pHi) as well as decrease of [Mg2+]i and subsequently activated the increase of intracellular free Ca2+ ([Ca2+]i) and the release of arachidonate. In contrast to superoxide anion, NH4Cl which induces increase of pHi in amnion cells increased [Mg2+]i. The elevation of basal level of [Mg2+]i by Mg(2+)-ionophore inhibited the change of [Ca2+]i and the release of arachidonate induced by superoxide anion. These results suggest that superoxide anion, transported through anion channels into cells, decreases [Mg2+]i directly, not due to a pH-effect and that the decrease of [Mg2+]i may regulate biological functions of the cells via increase of [Ca2+]i.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗