Evidence that two tilapia (Oreochromis niloticus) prolactins have different osmoregulatory functions during adaptation to a hyperosmotic environment

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Inglés. Two forms of prolactin (tiPRL(I) and tiPRL(II)), with only 69% sequence identity, have been previously described in the cichlid fish tilapia (Oreochromis species). In the present study we have attempted to investigate the biological activity of these two prolactin forms during adaptation to a hyperosmotic environment. For this purpose, we have developed two highly sensitive (sensitivity: 0.05 ng/ml) and specific (cross-reactivity <0.04%) radioimmunoassays for tiPRL(I) and tiPRL(II), using recombinant hormones. When fish were directly transferred from fresh to brackish water, the measured levels of plasma tiPRL(I) and tiPRL(II) dropped abruptly until 12h after transfer. Thereafter, plasma tiPRL(II) remained stable (around 0.5 ng/ml) until the end of the experiment, whereas plasma tiPRL(I) continued to decrease to undetectable levels. These different patterns of change are reflected in the calculated ratio of plasma tiPRL(II) to tiPRL(I), which increased from 2-3 in fresh water-adapted fish to over 10 in fish which had spent 3 days or more in brackish water. The pituitary contents of tiPRL(I) and tiPRL(II) varied in a qualitatively similar fashion after transfer to brackish water. The tiPRL(I) content dropped continuously after twelfth of its initial level pituitary tiPRL(II) content, on the other hand, did not decrease significantly until day 7, and after a %-week exposure to brackish water it had only decreased by 50%. When injected into tilapia adapted to brackish water, both ovine prolactin and recombinant tiPRL(I) induced a clear dose-dependent ion-retaining effect. In contrast, the effect induced by tiPRL(II) treatment was markedly smaller and not dose-dependent. Northern blot analysis of tiPRL mRNAs using either a tiPRL(I) or a tiPRL(II) cDNA probe indicated the presence of two mRNAs differing in size: a 1.7 kb mRNA coding for tiPRL(I) and a 1.3 kb mRNA coding for tiPRL(II). After transfer to brackish water, levels of the two mRNAs decreased similarly. The present study indicates that, in O. niloticus, the two forms of prolactin have different osmoregulatory roles during adaptation to brackish water. Accordingly, their syntheses are differentially regulated after transfer to a hyperosmotic environment, presumably at a post-transcriptional level.