DYNAMICS OF PROOPIOMELANOCORTIN AND PROHORMONE CONVERTASE-2 GENE-EXPRESSION IN XENOPUS MELANOTROPE CELLS DURING LONG-TERM BACKGROUND ADAPTATION

The toad Xenopus laevis is able to adapt its skin color to background light intensity. In this neuroendocrine reflex, the proopiomelanocorti n (POMC)-derived peptide alpha-melanophore-stimulating hormone (alpha MSH) is a key regulatory factor. In animals adapting to a black backgr ound, release of alpha MSH front the pituitary pars intermedia causes dispersal of melanin in skin melanophores. To investigate the long-ter m in vivo dynamics of aMSH production during black background adaptati on, the biosynthetic rate of POMC and the contents of POMC, alpha MSH 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, t he rate of POMC biosynthesis and the POMC content were low, while high alpha MSH 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 37-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. A fter 1 week, the pars intermedia contest of alpha MSH was only 30% of the control level, but after 6 and 12 weeks, the alpha MSH level had i ncreased to the control level. The PC2 content decreased to 52% of con trol after 1 week and stabilized after 3 weeks at a level slightly low er than the control value. The results show that during long-term back ground adaptation a steady-state situation is reached, with a balance between the biosynthesis, enzymatic processing and release of alpha MS H. The in vivo dynamics of the processing enzyme PC2 suggest a paralle l storage and release of alpha MSH and mature PC2 in the Xenopus pitui tary pars inter-media.