MECHANISM OF MCH ACTION ON CHROMATOPHORES OF TELEOST FISH

1983年，从大麻哈鱼脑垂体中提纯的黑色素浓缩激素（MCH）的一级结构被确定。在黑色素细胞，MCH引起的色素聚集在所有硬骨鱼研究，而色素内红细胞并不总是聚集。例如，剑尾鱼的红细胞对MCH有反应，而尼罗罗非鱼尾鳍的红细胞则完全没有反应。此外，MCH在剑尾鱼红细胞中的最低有效剂量远高于黑色素细胞。四种类似物是天然序列的片段：Asp-Thr-Met-Arg-Cys-Met-Val-Cly-Arg-瓦尔-Tyr-Arg-Pro-Cys-Trp-Glu-瓦尔，其羧基和氨基末端的取代基依次消除。它们的相对效果为：完整MCH >片段（5-17）* 片段（1-15）>片段（5-15）>片段（1-14）。线性片段（1-14）根本不具有黑色素聚集活性。从这些结果，我们建议的重要性，S-S结合和色氨酸<15>残基与MCH受体介导的黑素体聚集的相互作用。当施加浓度高于1 μ M的完整MCH时，黑素体聚集后分散，这表明黑素细胞膜上也存在另一种类型的MCH受体介导的黑素体分散。放射免疫测定表明，当用10 nM MCH处理黑素细胞10分钟时，细胞内cAMP浓度降低至对照值的约55%，而10 μ M MCH处理黑素体使其最大聚集后再分散时，胞内cAMP水平增加到约126%。此外，本实验表明，蛋白激酶C可能部分参与了色素聚集，尽管钙离子可能不参与反应。

The primary structure of melanin-concentrating hormone (MCH) purified from chum salmon pituitary was determined in 1983. In melanophores, MCH caused pigment aggregation in all teleosts studied, whereas pigment within erythrophores was not always aggregated. For example, swordtail erythrophores responded to MCH,but erythrophores present in tail fin of Nile tilapia did not at all. Moreover, the minimum effective does of MCH in swordtail erythrophores was rather higher than that in melanophores.Five analogues of MCH were tested for their melanin-aggregating activities in the in vitro fish skin (Corydoras paleatus) bioassay. Four analogues were fragments of the native sequence : Asp-Thr-Met-Arg-Cys-Met-Val-Cly-Arg-Val-Tyr-Arg-Pro-Cys-Trp-Glu-Val with sequential elimination of substituents from both the carboxy- and amino- termini. Their relative effectiveness as follows : intact MCH > fragment (5-17) * fragment (1-15) > fragment (5-15) > fragment (1-14). Linear fragment (1-14) did not have the melanin-aggregating activity at all. From these results, we suggest the importance of S-S binding and the Trp^<15> residue for interaction with MCH receptors mediating melanosome aggregation. When intact MCH at concentrations higher than 1 muM was applied, the aggregation of melanosomes was followed by dispersion of them, suggesting that another type of MCH receptor mediating melanosome dispersion is also present on the melanophore membrane.Radio Immuno Assay indicated that the intracellular concentration of cAMP was decreased to about 55% of control value, when melanophores were treated with 10 nM MCH for 10 min, whereas intracellular level of cAMP was increased to about 126% when melanosomes were redispersed after the maximum aggregation by the treatment with 10 muM MCH.Moreover, it was shown in the present experiments, that Protein kinase C may be partly involved in the pigment aggregation, although calcium ions may not take part in the response.