著者

藤井 博

出版者

九州大學農學部

雑誌

九州大学農学部学芸雑誌 (ISSN:03686264)

巻号頁・発行日

vol.33, no.2, pp.p155-166,図1枚, 1979-01

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Effect of ecdysterone on RNA and DNA synthesis in the midgut of Bombyx mori was studied. It was found that ecdysterone stimulated in vivo RNA and DNA synthesis in the midgut of the I-day-old larva of the fifth instar. At first, stimulation of incorporation of ^3H-uridine into RNA occurred at the 6th hour after ecdysterone injection and continued from then. On the other hand, stimulation of incorporation of ^3H-thymidine into DNA occurred from the 18th hour after ecdysterone injection and silver grains were observed only in the nuclei of generative cells in autoradiography, and at the 48th hour after ecdysterone injection, new epithelium is formed as generative cell divided and multiplied. This indicates that RNA synthesis occurs in the nuclei of generative cell in the midgut of the larvae injected with ecdysterone, suggesting that ecdysterone stimulates first RNA synthesis for DNA synthesis. It may be understood that the action of ecdysterone on the midgut of the fifth instar larva as follow, ecdysterone acts on the generative cells which exists in the undifferentiated cells in the midgut tissue, induces the stimulation of RNA synthesis in the cell, the synthesized RNA induces it to synthesize DNA and subsequently the cell divides, multiplies and forms the new pupal epithelium. In order to examine the primary action of ecdysterone, one should try to use in vitro RNA synthesis in isolated nuclei. The isolated nuclei had the ability of RNA synthesizing activity. RNA synthesis elevated in the isolated midgut nuclei from 24th hour after ecdysterone injection. The result is consistent with similar effect observed with ecdysterone in the midgut nuclei in vivo RNA synthesis. The rate of RNA synthesis in the isolated midgut nuclei from various time after ecdysterone injection was examined. RNA synthesis enhanced greater in the isolated nuclei at 48th hour after ecdysterone injection than that at 1 hour. The rate of RNA synthesis in the isolated nuclei at 48th hour after ecdysterone injection increased linearly with incubation time, whereas the rate of RNA synthesis in the isolated nuclei after ecdysterone injection is lower than at the 48th hour and continued at lower level until 120 minutes after incubation. The result indicates that a physiological change, such as RNA synthesis elevated in the nuclei of the midgut, is developed by the action of ecdysterone. The direct effect of ecdysterone on RNA synthesis in the isolated nuclei was examined, by means of the nuclei isolated from the midgut of the 1-, 3- and 6-day-old larvae and incubated with ecdysterone. Each of the larvae were not injected with ecdysterone. There was little stimulation effect of ecdysterone in the isolated nuclei from the 1 and 3-day-old larvae, so-called the stage of feeding. On the other hand, in the isolated nuclei from the 6-day-old larvae, the stage of larval maturation, stimulation effect of ecdysterone on RNA synthesis increased by 30 per cent. It was found that there is a degree of responsiveness of nuclei from different stages to ecdysterone, and the nuclei in the midgut of the matured larva have more responsiveness to ecdystrone that that of the feeding larva.RNA合成におよぼすエクジステロン(ED)の影響としては,ED注射後24時間後に,核において顕著なRNA合成の昂進がみられるが,細胞質においてはみられなかつた.DNA合成におよぼすEDの影響をみると,ED注射24時間後に顕著な合成がみられた.さらにDNA合成をオートラジオグラフィー法で調べたところED注射18時間後に,新生細胞の核にのみ^3H-thymidineのとりこみがみられた.48時間後には新生細胞による新しい皮膜形成がみられ,この細胞核に顕著な^3H-thymidineのとりこみがみられた.ED注射後のRNAとDNAとの合成促進効果の時間関係をみると,ED注射6時間後にRNA合成の促進効果が現われ,DNA合成は注射18時間後に促進効果がみられた,すなわちED注射によりまずRNA合成が高まり,ついでDNA合成が起こると解される.さらにDNA合成を詳しくみると.アクチノマイシンDでRNA合成を阻害した場合,DNA合成が低下することからDNA合成にはRNA合成が必要であると考えられる.ED注射24時閻後にみられるDNA合成には,アクチノマイシンDを用いた実験から,ED注射後12~24時間(DNA合成が顕著にみられる以前)につくられたRNAが大きな影響をおよぼすことがわかつた.したがつて,EDの作用は中腸細胞,特に新生細胞に働き,まずその細胞のRNA合成を誘起し,ついでこのRNAがDNA合成の開始を導き,ついには細胞分裂を引き起こすものと考えられる.つぎにはRNA合成におよぼすEDの影響を中腸単離核を用いて調べた.ED注射を受けた中腸の単離核でのRNA合成は注射後24~28時間で高くなつた.さらにこの核ではRNA合成速度においても違いがみられた.すなわちED注射1時間後の核ではRNA合成開始後,反応時間が長くなつてもRNAへの^3H-uridineのとりこみは変わらなかつたが,48時間後の核では反応時間に比例して,直線的に増加していつた.したがつてEDの作用により生体内の中腸細胞核においてはRNA合成を高めるような生理的変化が起こつていると言える.単離核のRNA合成におよぼすEDの直接作用をin vitroで調べると,5令1日目と3日目との摂食期の中腸単離核ではホルモンによるRNA合成促進効果はみられなかつたが,6目日の閉鎖系に入つたものでは約27%の促進効果がみられた.すなわち発育時期によつて核のホルモンに対する反応性に差異がみられた.以上の結果を総合すると,中腸組織は閉鎖系に入ると消化機能が低下するが,単に退化してゆく組織ではなく,全く新しい機能を持つように変化していくものであると結論される.