CHARACTERIZATION OF A SINGLE PROLACTIN (PRL) RECEPTOR IN TILAPIA (OREOCHROMIS-NILOTICUS) WHICH BINDS BOTH PRL(I) AND PRL(II)

In tilapia, there are two forms of prolactin (PRL) whose effects on so dium and chloride movements differ and depend on the living environmen t of the fish. To see whether different receptors or the same receptor mediates these different effects, we have characterized the specific binding of both forms of tilapia (ti)PRL in two osmoregulatory organs, the gill and kidney. Two recombinant tiPRLs were used for this analys is. The recombinant hormones had the same properties as the native hor mones in a tilapia gill radioreceptor assay. Specific binding to gill and kidney membranes was increased by optimizing the quality of the ti ssue preparations (physiological state of fish, membrane preparation) and the incubation conditions (pH, salt concentrations, temperature, t ime). Under these optimized conditions, we detected only one class of high affinity PRL receptor in gill and kidney. Its binding affinity wa s higher for tiPRL(I) than for tiPRL(II) in both gill and kidney (for tiPRL(I) the respective affinity values were 2.9 and 2.3 x 10(10) per M, for tiPRL(II) they were 1.9 and 0.5 x 10(10) per M). In competition studies, tiPRL(I) was more potent, followed by tiPRL(II) and ovine (o )PRL. tiGH and oGH did not significantly displace either tiPRL. The re ceptor we have characterized thus recognizes quite specifically both t iPRLs.