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Using high-performance liquid chromatography (HPLC) in combination with radioimmunoassay, three forms of -MSH (des-acetyl, mono-acetyl and di-acetyl -MSH) were separated and identified in tilapia neurointermediate lobes and plasma, and in medium from lobes superfused in vitro. The presence of acetylated forms in lobe extracts indicated that the peptides are acetylated intracellularly. Di-acetyl -MSH was, especially in comparison with monoacetyl -MSH, relatively more abundant in lobe extracts than in plasma. This suggests that the three forms of -MSH are not released according to their relative intracellular abundances. The possibility of regulation of this differential release by dopamine and TRH was investigated, using a microsuperfusion system. Dopamine was a potent inhibitor of -MSH release, but did not modulate the relative abundance of the different forms of -MSH released from the MSH cells. TRH was a potent stimulator of -MSH release. It enhanced in vitro the release of di-acetyl -MSH more than the release of mono-acetyl -MSH. Thus tilapia may be able to modulate not only the quantitative but also the qualitative signal from the MSH cells. This might enhance the flexibility of the animals to respond to environmental challenges.

Journal of Endocrinology (1991) 129, 179–187

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