EVIDENCE THAT 2 TILAPIA (OREOCHROMIS-NILOTICUS) PROLACTINS HAVE DIFFERENT OSMOREGULATORY FUNCTIONS DURING ADAPTATION TO A HYPEROSMOTIC ENVIRONMENT

Two forms of prolactin (tiPRL(I) and tiPRL(II)), with only 69% sequenc e identity, have been previously described in the cichlid fish tilapia (Oreochromis species). In the present study we have attempted to inve stigate the biological activity of these two prolactin forms during ad aptation to a hyperosmotic environment. For this purpose, we have deve loped two highly sensitive (sensitivity: 0.05 ng/ml) and specific (cro ss-reactivity <0.04%) radioimmunoassays for tiPRL(I) and tiPRL(II), us ing recombinant hormones. When fish were directly transferred from fre sh to brackish water, the measured levels of plasma tiPRL(I) and tiPRL (II) dropped abruptly until 12h after transfer. Thereafter, plasma tiP RL(II) remained stable (around 0.5 ng/ml) until the end of the experim ent, whereas plasma tiPRL(I) continued to decrease to undetectable lev els. These different patterns of change are reflected in the calculate d ratio of plasma tiPRL(II) to tiPRL(I), which increased from 2-3 in f resh water-adapted fish to over 10 in fish which had spent 3 days or m ore 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 br ackish 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, t he effect induced by tiPRL(II) treatment was markedly smaller and not dose-dependent. Northern blot analysis of tiPRL mRNAs using either a t iPRL(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 synthese s are differentially regulated after transfer to a hyperosmotic enviro nment, presumably at a post-transcriptional level.