Dotman C H, van Herp F, Martens G J, Jenks B G, Roubos E W
Department of Cellular Animal Physiology, Nijmegen Institute for Neurosciences and Institute for Cellular Signalling, University of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands.
J Endocrinol. 1998 Nov;159(2):281-6. doi: 10.1677/joe.0.1590281.
The toad Xenopus laevis is able to adapt its skin color to background light intensity. In this neuroendocrine reflex, the proopiomelanocortin (POMC)-derived peptide alpha-melanophore-stimulating hormone (alphaMSH) is a key regulatory factor. In animals adapting to a black background, release of alphaMSH from the pituitary pars intermedia causes dispersal of melanin in skin melanophores. To investigate the long-term in vivo dynamics of alphaMSH production during black background adaptation, the biosynthetic rate of POMC and the contents of POMC, alphaMSH and the POMC processing enzyme precursor convertase 2 (PC2) have been studied in the pars intermedia using pulse-labeling, Western blot and radioimmunoassay. In control animals, adapted to a white background, the rate of POMC biosynthesis and the POMC content were low, while high alphaMSH and PC2 contents were found. After 1 week of adaptation to a black background, the rate of POMC biosynthesis and the POMC protein content had increased 19- and 3.7-fold respectively. These parameters attained a maximum level (28- and 5. 8-fold higher than control) after 3 weeks and remained at these elevated levels for at least 12 weeks. After 1 week, the pars intermedia content of alphaMSH was only 30% of the control level, but after 6 and 12 weeks, the alphaMSH level had increased to the control level. The PC2 content decreased to 52% of control after 1 week and stabilized after 3 weeks at a level slightly lower than the control value. The results show that during long-term background adaptation a steady-state situation is reached, with a balance between the biosynthesis, enzymatic processing and release of alphaMSH. The in vivo dynamics of the processing enzyme PC2 suggest a parallel storage and release of alphaMSH and mature PC2 in the Xenopus pituitary pars intermedia.
非洲爪蟾能够使其皮肤颜色适应背景光强度。在这种神经内分泌反射中，源自阿黑皮素原（POMC）的肽α-促黑素细胞激素（αMSH）是关键调节因子。在适应黑色背景的动物中，垂体中间部释放的αMSH会导致皮肤黑素细胞中的黑色素扩散。为了研究黑色背景适应过程中αMSH产生的长期体内动态变化，利用脉冲标记、蛋白质免疫印迹和放射免疫测定法，对中间部中POMC的生物合成速率以及POMC、αMSH和POMC加工酶前体转化酶2（PC2）的含量进行了研究。在适应白色背景的对照动物中，POMC生物合成速率和POMC含量较低，而αMSH和PC2含量较高。在适应黑色背景1周后，POMC生物合成速率和POMC蛋白含量分别增加了19倍和3.7倍。3周后这些参数达到最高水平（比对照高28倍和5.8倍），并在至少12周内保持在这些升高水平。1周后，中间部αMSH含量仅为对照水平的30%，但在6周和12周后，αMSH水平已升至对照水平。PC2含量在1周后降至对照的52%，并在3周后稳定在略低于对照值的水平。结果表明，在长期背景适应过程中达到了一种稳态，αMSH的生物合成、酶促加工和释放之间达到平衡。加工酶PC2的体内动态变化表明，αMSH和成熟PC2在非洲爪蟾垂体中间部平行储存和释放。
