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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.
