DIFFERENTIAL PROCESSING OF THE 2 PROLACTINS OF THE TILAPIA (OREOCHROMIS-MOSSAMBICUS) IN RELATION TO ENVIRONMENTAL SALINITY

Prolactin (PRL) is thought to play a significant role in the freshwater (FW) adaptation of euryhaline teleost fishes, including the tilapia, Oreochromis mossambicus. Two PRLs (tPRL188 and tPRL177) have been characterized in the anteriormost portion of the tilapia pituitary, the rostral pars distalis (RPD; Specker et al., '85a,b; Yamaguchi et al., '88). Our studies were undertaken to determine whether the in vivo RPD content of tPRL188 and tPRL177 varied with environmental salinity. To this end, tilapia were reared from the period of yolk-sac absorption for 7 months or acclimated for 21 and 35 days in FW or seawater (SW). In a parallel study, we also examined whether the release of the 2 tilapia PRLs from the RPD of FW or SW tilapia might be differentially sensitive to changes in medium osmotic pressure during 18-20 hr incubations. In agreement with earlier work, we found that the smaller tPRL177 molecule was present in greater quantities in the RPD of FW tilapia compared to the content in RPD of SW fish. For the first time, we show here that the content of tPRL188, like that of tPRL177, is also significantly greater in the RPD of FW fish compared to the quantity in RPD of SW tilapia. Nevertheless, the relative content or ratio of the 2 PRLs in the pituitary was dependent on rearing salinity. Thus, the relative content or ratio of the larger tilapia PRL to smaller PRL (tPRL188/tPRL177) in the RPD of FW tilapia was significantly higher (1.5:1) than that seen in SW fish (0.75:1). This ratio was not altered after 18-20 hr in vitro incubations in hyposmotic or hyperosmotic medium, suggesting that the shift in ratios did not result from differential release by osmotic pressure. During 18-20 hr incubation in either hyposmotic or hyperosmotic medium, the RPD of FW tilapia released more of both forms of PRL than did the RPD of SW fish. Consistent with previous studies, reductions in medium osmotic pressure within the physiological range of the tilapia stimulated the release of both tPRL188 and tPRL177 from RPD of FW fish over levels observed during exposure to hyperosmotic medium. We report here, for the first time, that the reduction of medium osmotic pressure also stimulates the release of both tPRL188 and tPRL177 from RPD of SW fish. Throughout the experiments we did not observe differences between the release of tPRL188 and tPRL177 under our incubation conditions. The transfer of FW-reared fish to SW and vice versa for 49 days altered the ratio of the 2 PRLs (tPRL188/tPRL177) to that observed in the RPD of SW-reared and FW-reared fish, respectively. Overall, our studies indicate that the processing of the 2 PRLs may be differentially sensitive to environmental salinity.