Effects of acclimation salinity and in vitro medium osmotic pressure on the incorporation of3H-leucine into the two prolactins of the tilapia, Oreochromis mossambicus

The control of synthesis of the two prolactins (tPRL188 and tPRL177) in the anterior‐most portion of the pituitary, the rostral pars distalis (RPD), was investigated in freshwater (FW)‐ and seawater (SW)‐ acclimated tilapia. Individual RPD of FW and SW male tilapia were incubated in either hyposmotic or hyperosmotic medium containing3H‐leucine for various intervals, up to 360 min. The total quantity of the two tPRLs in media and RPD was assessed by gel electrophoresis and scanning densitometry of the stained bands. Synthesis was characterized as the uptake of3H‐leucine into the tPRL molecules. Results showed greater synthesis and release of both3H‐tPRLs in the RPD from FW tilapia than in those from SW‐ acclimated tilapia. The in vitro synthesis of both prolactins tPRLl88 and tPRL177 in RPD of FW tilapia and tPRLl77 in RPD of SW fish was also augmented by exposure to hyposmotic medium. The ratio of the appearance of newly synthesized3H‐ tPRLl88:3H‐tPRL177was approximately 1.3 in FW tilapia RPD and approximately 0.5 in SW tilapia RPD. The delay between effect of osmotic pressure on tPRL release and its effect on tPRL synthesis suggests that the rapid effect of osmotic pressure upon PRL release is at least partially independent of its effect upon tPRL synthesis. Moreover, the altered ratios of tPRL188 and tPRL177 observed in our studies suggest that the synthesis rates of the two tPRLs are differentially sensitive to environmental salinity. © 1995 Wiley‐Liss, Inc.