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K Ohtani, H Shimizu, Y Tanaka, N Sato and M Mori

Abstract

Pioglitazone hydrochloride (AD-4833), one of the thiazolidinedione analogs, is a new anti-diabetic agent which improves peripheral insulin resistance in diabetic patients. We determined the direct effect of AD-4833 on insulin secretion in HIT-T 15 cells. The effects of AD-4833 (10−7 m to 10−5 m) on insulin secretion were examined in 3 and 7 mm glucose-containing F-12 K media. The addition of 10−5 m AD-4833 significantly increased insulin secretion in both media, but its stimulatory effect was more potent in the medium containing 7 mm glucose. The addition of 10−5 m AD-4833 caused an immediate, significant increase in cytosolic free Ca2+ concentration ([Ca2+]i). Nifedipine at all concentrations from 10 to 1000 nm significantly attenuated insulin secretion by 10−5 m AD-4833. In addition, 10−5 m AD-4833 failed to stimulate insulin secretion in the Ca2+-free Kreb's-Ringer bicarbonate buffer. These data indicated that AD-4833 stimulates in vitro insulin secretion in HIT-T 15 cells, perhaps by inducing Ca2+ influx.

Journal of Endocrinology (1996) 150, 107–111

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H Shimizu, K Ohtani, Y Kato and M Mori

Interleukin (IL)-6, one of the cytokines released from inflammatory cells, stimulates insulin secretion in a physiological concentration (1-100 pg/ml), but the exact mechanism is still unknown. The present studies were undertaken to investigate the mechanism of IL-6-induced stimulation of insulin secretion in HIT-T 15 cells. The effects of the addition of nifedipine on the IL-6 (100 pg/ml)-induced stimulation of insulin secretion were investigated. We also examined the possibility that IL-6 (1-100 pg/ml) may stimulate insulin messenger ribonucleic acid (mRNA) expression, using the reverse transcription-polymerase chain reaction method. The addition of 100 and 1000 nM nifedipine significantly attenuated the stimulatory effects of 100 pg/ml IL-6 on insulin secretion. The addition of 1-100 pg/ml IL-6 dose-dependently increased preproinsulin mRNA expression relative to beta-actin mRNA. IL-6 increased insulin gene promoter activity of fragments A (-2188 to +337 bp) and B (-1782 to +270 bp) but not fragments C (-1275 to +270 bp), D (-1138 to +270 bp), E (-880 to +236 bp) or F (-356 to +252 bp). The addition of 10 nM nifedipine completely abolished the stimulatory effect of 10-100 pg/ml IL-6 on relative preproinsulin mRNA expression. These data raised the possibility that IL-6 increased preproinsulin mRNA expression via the stimulation of Ca(2+) influx which enhances insulin gene expression.

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H Shimizu, Y Shimomura, Y Nakanishi, T Futawatari, K Ohtani, N Sato and M Mori

Abstract

The decrease in estrogen in menopausal women increases body fat. The present studies were undertaken to investigate the involvement of estrogen in leptin production in vivo. In the first study, expression of ob gene mRNA in white adipose tissue was measured at 2 and 8 weeks after ovariectomy in rats. In the second, serum leptin concentration was measured in total body fat of 87 weight-matched human subjects (29 men, 29 premenopausal and 29 postmenopausal women). In the third, changes in serum leptin concentration with the menstrual cycle were determined, ob gene expression decreased in subcutaneous and retroperitoneal white adipose tissue of ovariectomized rats 8 weeks after the operation, while ovariectomy increased ob gene expression in mesenteric white adipose tissue. Serum leptin concentration was decreased by ovariectomy. Estradiol supplement reversed the effect of ovariectomy on ob gene expression and circulating leptin levels. In humans, serum leptin concentration was higher in premenopausal women than in men, and in postmenopausal women it was lower than in premenopausal women, but still higher than in men. In 13 premenopausal women, serum leptin levels were significantly higher in the luteal phase than in the follicular phase. The present studies strongly indicate that estrogen regulates leptin production in rats and human subjects in vivo. Regional variation in the regulation of ob gene expression by estrogen was found.

Journal of Endocrinology (1997) 154, 285–292

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T Takahashi, K Sato, S Kato, T Yonezawa, Y Kobayashi, Y Ohtani, S Ohwada, H Aso, T Yamaguchi, S G Roh and K Katoh

Ghrelin is a multifunctional peptide that promotes an increase of food intake and stimulates GH secretion. Ghrelin secretion is regulated by nutritional status and nutrients. Although a high-protein (HP) diet increases plasma ghrelin secretion in mammals, the mechanisms and the roles of the elevated ghrelin concentrations due to a HP diet have not been fully established. To clarify the roles of elevated acylated ghrelin upon intake of a HP diet, we investigated the regulation of ghrelin concentrations in plasma and tissues in wethers fed with either the HP diet or the control (CNT) diet for 14 days, and examined the action of the elevated plasma ghrelin by using a ghrelin-receptor antagonist. The HP diet gradually increased the plasma acylated-ghrelin concentrations, but the CNT diet did not. Although the GH concentrations did not vary significantly across the groups, an injection of ghrelin-receptor antagonist enhanced insulin levels in circulation in the HP diet group. In the fundus region of the stomach, the ghrelin levels did not differ between the HP and CNT diet groups, whereas ghrelin O-acyltransferase mRNA levels were higher in the group fed with HP diet than those of the CNT diet group were. These results indicate that the HP diet elevated the plasma ghrelin levels by increasing its synthesis; this elevation strongly suppresses the appearance of insulin in the circulation of wethers, but it is not involved in GH secretion. Overall, our findings indicate a role of endogenous ghrelin action in secretion of insulin, which acts as a regulator after the consumption of a HP diet.

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A. Nagasaka, S. Yoshida, A. Nakai, T. Ohyama, K. Iwase, S. Ohtani, H. Nakagawa, R. Masunaga, S. Kato, T. Kawabe and K. Kataoka

ABSTRACT

Using hypophysectomized rats, it has been shown that DNA polymerase-β activity in the adrenal gland and testis is largely influenced by pituitary trophic hormones. Sucrose gradient centrifugation of thyroid extracts revealed three peaks of DNA polymerase-β activity sedimenting at 3·3S, 7·3S and 12S. Of these, hypophysectomy induced a decrease in the 3·3S DNA polymerase-β, whereas other molecular forms were affected only slightly. DNA polymerase-α and -γ activities were unaffected by hypophysectomy. These changes in DNA polymerase-β caused by hypophysectomy were reversed by daily i.p. injection of TSH. Furthermore, stimulation of the thyroid by excess TSH induced by the administration of 1-methyl-2-mercaptoimidazole resulted in an increase of all forms of thyroid DNA polymerase-β.

These results show that the level of DNA polymerase is relatively constant after hypophysectomy but that DNA polymerase-β in the rat thyroid gland is also modulated by TSH mainly through the change of activity of the polymerase-β which sediments at 3·3S.

J. Endocr. (1988) 119, 303–308