Schneider M J, Galton V A
Department of Physiology, Dartmouth Medical School, Lebanon, New Hampshire 03756-0001.
Endocrinology. 1995 Apr;136(4):1435-40. doi: 10.1210/endo.136.4.7895654.
Metamorphosis in anuran amphibia requires thyroid hormone (TH) and can be induced prematurely by the administration of TH. There is also evidence that the developmental effects of TH in these forms are modified by other hormones. For example, PRL has been shown to retard and corticosterone (B) to accelerate some, but not all, components of TH-induced metamorphosis. Red blood cells (RBCs) of Rana catesbeiana tadpoles exhibit a 4- to 5-fold increase in thyroid hormone receptor (TR) number (sites per nucleus) in vivo during either spontaneous or TH-induced metamorphosis. In the present study this TH-induced effect on RBC TR number was examined in an in vitro culture system. RBC TR number was increased by T3 in vitro; the maximum effect (2-fold increase) was obtained after exposure to 0.3 nM T3 for 60 h. This T3-induced increase in TR number was completely abolished in the presence of either 34 nM B or 10 nM dexamethasone, whereas basal TR number was unaffected. The effect appears to be a specific effect of glucocorticoid (GC), because it was not mimicked by the sex steroid, testosterone, and it was not obtained when RU-486, a glucocorticoid antagonist, was included with B in the medium. Other experiments demonstrated that the T3-induced increase in RBC TR was associated with an increase in the TR alpha messenger RNA level. This increase in TR alpha messenger RNA was reduced, but not eliminated, in the presence of concentrations of GC that abolished the TH-induced increase in TR, suggesting that the effects of GC occur in part at a pretranslational level. Using a GC binding assay, tadpole RBCs were found to contain approximately 10(4) GC receptors/cell. These findings indicate that B may be a physiological modulator of TH action in tadpole RBCs. This inhibitory effect of GC contrasts with previous reports that GC accelerates some of the morphological effects of TH in developing tadpoles, indicating that the nature of this modulating effect on TH action is tissue specific.
无尾两栖动物的变态需要甲状腺激素（TH），并且通过给予TH可提前诱导变态。也有证据表明，这些动物中TH的发育效应会受到其他激素的调节。例如，已表明催乳素（PRL）会延缓而皮质酮（B）会加速TH诱导的变态的一些（但不是全部）成分。在自发或TH诱导的变态过程中，牛蛙蝌蚪的红细胞（RBC）在体内甲状腺激素受体（TR）数量（每个细胞核的位点）会增加4至5倍。在本研究中，在体外培养系统中检测了TH对RBC TR数量的这种诱导效应。体外T3可增加RBC TR数量；在暴露于0.3 nM T3 60小时后获得最大效应（增加2倍）。在存在34 nM B或10 nM地塞米松的情况下，这种T3诱导的TR数量增加完全被消除，而基础TR数量不受影响。这种效应似乎是糖皮质激素（GC）的特异性效应，因为它未被性类固醇睾酮模拟，并且当糖皮质激素拮抗剂RU - 486与B一起包含在培养基中时未观察到该效应。其他实验表明，T3诱导的RBC TR增加与TRα信使RNA水平的增加相关。在存在消除TH诱导的TR增加的GC浓度的情况下，TRα信使RNA的这种增加减少但未消除，这表明GC的作用部分发生在翻译前水平。使用GC结合测定法，发现蝌蚪RBC含有约10⁴个GC受体/细胞。这些发现表明B可能是蝌蚪RBC中TH作用的生理调节剂。GC的这种抑制作用与先前的报道形成对比，先前报道GC会加速发育中的蝌蚪中TH的一些形态学效应，表明这种对TH作用的调节效应的性质是组织特异性的。
