Yousufzai S Y, Abdel-Latif A A
Department of Biochemistry and Molecular Biology, Medical College of Georgia, Augusta 30912-2100.
Membr Biochem. 1993 Jan-Mar;10(1):29-42. doi: 10.3109/09687689309150250.
We have shown that in bovine iris sphincter membranes G proteins are involved in coupling muscarinic-, PGF2 alpha-, endothelin- and platelet-activating factor receptors to the activation of phospholipase A2 and the release of arachidonic acid. GTP gamma S and GTP gamma S plus carbachol stimulated arachidonic acid release in the membranes in a dose- and time-dependent manner. Nucleotide stimulation was specific to GTP gamma S, since GDP, GDP beta S and ATP had no effect. The stimulatory effect of GTP gamma S plus carbachol was blocked by atropine and it required the presence of physiological concentrations of Ca2-. AIF4-, which bypasses the receptor and directly activates the G protein, induced arachidonic acid liberation in the intact iris sphincter, but was ineffective in the membranes. Addition of GTP gamma S plus carbachol to sphincter muscle membranes prelabeled with [3H]inositol or 3H-arachidonic acid resulted in the formation of lysophosphatidylinositol and the liberation of arachidonic acid, thus suggesting the involvement of phospholipase A2. In vitro treatment of the iris membranes with pertussis toxic inhibited arachidonic acid release by the agonists. This is in contrast to the pertussis toxin-insensitive G protein that activates phospholipase C in this tissue (22). These data demonstrate that in the iris sphincter a G protein is involved in the step between receptor activation and the activation of phospholipase A2, and that arachidonic acid release in this tissue is mediated by a pertussis-toxin-sensitive G protein-coupled phospholipase A2. Thus, GTP can regulate arachidonic acid release and its subsequent conversion into eicosanoids by stimulating its formation.
我们已经证明，在牛虹膜括约肌膜中，G蛋白参与将毒蕈碱、前列腺素F2α、内皮素和血小板活化因子受体与磷脂酶A2的激活及花生四烯酸的释放偶联起来。GTPγS以及GTPγS加卡巴胆碱以剂量和时间依赖性方式刺激膜中花生四烯酸的释放。核苷酸刺激对GTPγS具有特异性，因为GDP、GDPβS和ATP没有作用。GTPγS加卡巴胆碱的刺激作用被阿托品阻断，并且它需要生理浓度的Ca2+存在。AIF4-绕过受体直接激活G蛋白，在完整的虹膜括约肌中诱导花生四烯酸释放，但在膜中无效。向预先用[3H]肌醇或3H-花生四烯酸标记的括约肌肌膜中添加GTPγS加卡巴胆碱导致溶血磷脂酰肌醇的形成和花生四烯酸的释放，因此提示磷脂酶A2的参与。用百日咳毒素体外处理虹膜膜抑制了激动剂诱导的花生四烯酸释放。这与该组织中激活磷脂酶C的百日咳毒素不敏感的G蛋白形成对比。这些数据表明，在虹膜括约肌中，一种G蛋白参与受体激活和磷脂酶A2激活之间的步骤，并且该组织中花生四烯酸的释放由百日咳毒素敏感的G蛋白偶联的磷脂酶A2介导。因此，GTP可以通过刺激花生四烯酸的形成来调节其释放及其随后转化为类二十烷酸。
