Salmon plasma contains at least three IGF-binding proteins (IGFBPs) with molecular masses of 41, 28 and 22 kDa. The 41 kDa IGFBP is similar to mammalian IGFBP-3 in size, type of glycosylation and physiological responses. In this study, we developed an RIA for the 41 kDa IGFBP. The 41 kDa IGFBP purified from serum was used for antibody production and as an assay standard. Binding of three different preparations of tracer were examined: (125)I-41 kDa IGFBP, (125)I-41 kDa IGFBP cross-linked with IGF-I and 41 kDa IGFBP cross-linked with (125)I-IGF-I (41 kDa IGFBP/(125)I-IGF-I). Only binding of 41 kDa IGFBP/(125)I-IGF-I was not affected by added IGFs, and therefore it was chosen for the tracer in the RIA. Plasma 41 kDa IGFBP levels measured by RIA were increased by GH treatment (178.9+/-4.9 ng/ml) and decreased after fasting (95.0+/-7.0 ng/ml). The molarities of plasma 41 kDa IGFBP and total IGF-I were comparable, and they were positively correlated, suggesting that salmon 41 kDa IGFBP is a main carrier of circulating IGF-I in salmon, as is mammalian IGFBP-3 in mammals. During the parr-smolt transformation (smoltification) of coho salmon, plasma 41 kDa IGFBP levels showed a transient peak (182.5+/-10.3 ng/ml) in March and stayed relatively constant thereafter, whereas IGF-I showed peak levels in March and April. Differences in the molar ratio between 41 kDa IGFBP and IGF-I possibly influence availability of IGF-I in the circulation during smoltification.
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M Shimizu, A Hara, and WW Dickhoff
Christine A Beamish, Sofia Mehta, Brenda J Strutt, Subrata Chakrabarti, Manami Hara, and David J Hill
The presence and location of resident pancreatic β-cell progenitors is controversial. A subpopulation of insulin-expressing but glucose transporter-2-low (Ins+Glut2LO) cells may represent multipotent pancreatic progenitors in adult mouse and in human islets, and they are enriched in small, extra-islet β-cell clusters (<5 β cells) in mice. Here, we sought to identify and compare the ontogeny of these cells in mouse and human pancreata throughout life. Mouse pancreata were collected at postnatal days 7, 14, 21, 28, and at 3, 6, 12, and 18 months of age, and in the first 28 days after β-cell mass depletion following streptozotocin (STZ) administration. Samples of human pancreas were examined during fetal life (22–30 weeks gestation), infancy (0–1 year), childhood (2–9), adolescence (10–17), and adulthood (18–80). Tissues were analyzed by immunohistochemistry for the expression and location of insulin, GLUT2 and Ki67. The proportion of β cells within clusters relative to that in islets was higher in pancreas of human than of mouse at all ages examined, and decreased significantly at adolescence. In mice, the total number of Ins+Glut2LO cells decreased after 7 days concurrent with the proportion of clusters. These cells were more abundant in clusters than in islets in both species. A positive association existed between the appearance of new β cells after the STZ treatment of young mice, particularly in clusters and smaller islets, and an increased proportional presence of Ins+Glut2LO cells during early β-cell regeneration. These data suggest that Ins+Glut2LO cells are preferentially located within β-cell clusters throughout life in pancreas of mouse and human, and may represent a source of β-cell plasticity.
K Ichikawa, T Miyamoto, T Kakizawa, S Suzuki, A Kaneko, J Mori, M Hara, M Kumagai, T Takeda, and K Hashizume
The thyromimetic compound SK&F L-94901 shows more potent thyromimetic activity in the liver than in the pituitary gland or heart when administered to rats. The mechanisms of liver-selectivity of SK&F L-94901 were examined using cultured rat hepatoma cells (dRLH-84) and rat pituitary tumor cells (GH3), both of which showed saturable cellular uptake of tri-iodothyronine (T(3)). When isolated nuclei with partial disruption of the outer nuclear membrane were used, SK L-94901 competed for [(125)I]T(3) binding to nuclear receptors almost equally in dRLH-84 and GH3 cells. SK L-94901 also did not discriminate thyroid hormone receptors (TR) alpha1 and beta1 in terms of binding affinity and activation of the thyroid hormone responsive element. In intact cells, however, SK L-94901 was a more potent inhibitor of nuclear [(125)I]T(3) binding in dRLH-84 cells than in GH3 cells at an early phase of the nuclear uptake process and after binding equilibrium. These data suggest that SK L-94901 is more effectively transported to nuclear TRs in hepatic cells than in pituitary cells and therefore shows liver-selective thyromimetic activity. In conclusion, SK L-94901 discriminates hepatic cells and pituitary cells at the nuclear transport process. The cellular transporters responsible for this discrimination were not evident.
T Nagasawa, K Ichikawa, K Minemura, M Hara, H Yajima, A Sakurai, H Kobayashi, K Hiramatsu, S Shigematsu, and K Hashizume
Cellular and nuclear uptake of tri-iodothyronine (T3) and thyroxine (T4) was examined using the cultured cell line derived from rat liver, clone 9, and rat hepatoma, dRLH-84. The saturable cellular uptake of T3 and T4 was demonstrated in these cells. First we examined the cell cycle-dependent alteration of thyroid hormone uptake. Cellular T3 uptake was minimal in the early G1 phase and increased in the late G1 phase, reaching a maximal level in the S phase. Alterations in nuclear T3 uptake were in accordance with the changes in cellular T3 uptake. On the other hand, cellular and nuclear T4 uptake was unchanged throughout the cell cycle, suggesting the T3 specificity of the cell cycle-dependent alteration of cellular hormone transport. Next we examined the effect of sodium butyrate on the cellular transport of thyroid hormones. After treatment with 5 mm sodium butyrate, cellular and nuclear uptake of T3 was increased, reaching a maximal level (four- to sevenfold increase) after 48 h. When cells were incubated for 48 h with various concentrations of sodium butyrate, T3 uptake was enhanced by 1 mm sodium butyrate, reaching a maximal level with 5 mm. Although cellular T4 uptake was also increased after treatment with sodium butyrate, the degree and time-course of the increase were different from those of T3. The maximal increase in cellular T4 uptake (two- to threefold increase) was attained 20 h after treatment. Despite the increase in cellular T4 uptake, nuclear T4 uptake was decreased after treatment with sodium butyrate. For both T3 and T4, the enhanced cellular uptake was due to the increased Vmax without changes in the Michaelis–Menten constant. These data indicate that cellular transport of T4 is different from that of T3 in rat hepatic cells.
Journal of Endocrinology (1995) 147, 479–485