MECHANISM OF MCH ACTION ON CHROMATOPHORES OF TELEOST FISH

1983年测定了从大马哈鱼垂体中纯化的黑色素浓缩激素（MCH）的初级结构。在研究的所有硬鱼细胞中，黑色素细胞中MCH引起色素聚集，而红细胞中的色素并不总是聚集。例如，剑尾红细胞对MCH有反应，而尼罗罗非鱼尾鳍中的红细胞则完全没有。剑尾红细胞中MCH的最小有效剂量远高于黑素细胞。在离体鱼皮肤（Corydoras paleatus）生物试验中，对5种MCH类似物的黑色素聚集活性进行了测试。四个类似物是天然序列的片段：asp - thr - met - arg - cys - met - val - clay - arg - val - tyr - arg - pro - cys - trp - glu - val，从羧基端和氨基端依次消除取代基。它们的相对有效性如下：完整的MCH >片段(5-17)*片段(1-15)>片段(5-15)>片段（1-14）。线性片段（1-14）完全没有黑色素聚集活性。根据这些结果，我们提出了S-S结合和Trp^<15>残基与MCH受体相互作用介导黑素体聚集的重要性。当使用浓度高于1mum的完整MCH时，黑素小体聚集之后是它们的分散，这表明黑素细胞膜上也存在另一种介导黑素小体分散的MCH受体。放射免疫分析表明，10 nM MCH处理10 min后，细胞内cAMP浓度下降至对照组的55%左右，而10 nM MCH处理最大聚集后再分散后，细胞内cAMP水平上升至126%左右。此外，本实验表明，蛋白激酶C可能部分参与色素聚集，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.