Somatotropic actions of the homologous growth hormone and prolactins in the euryhaline teleost, the tilapia, Oreochromis mossambicus

It is increasingly clear that growth hormone (GH) has growth-promoting effects in fishes, which are mediated in part by the insulin-like growth factor (IGF)-I. Growth-promoting actions of prolactin (PRL) have been reported in higher vertebrates, but are less well established in teleosts. We examined the effects of injecting homologous GH or the two homologous tilapia PRLs (tPRL₁₇₇ and tPRL₁₈₈) on the in vitro incorporation of [³⁵S] sulfate (extracellular matrix synthesis) and [³H]thymidine (DNA synthesis) by ceratobranchial cartilage explants and on IGF-I mRNA levels in tilapia liver. Tilapia GH (tGH) and tPRL₁₇₇ stimulated sulfate uptake at the highest doses examined. Thymidine incorporation was stimulated by tPRL₁₇₇. tPRL₁₈₈ was without these effects. Consistent with its somatotropic actions, tGH elevated IGF-I mRNA levels in the liver. tPRL₁₇₇ also elevated liver IGF-I levels. Consistent with the previously described osmoregulatory actions of GH and PRL in teleosts, we observed that tGH elevated and tPRL₁₇₇ and tPRL₁₈₈ lowered levels of gill Na⁺,K⁺- ATPase activity. High-affinity, low-capacity binding sites for tGH in the tilapia liver were identified. tPRL₁₇₇ binds with lower affinity than tGH to these sites but can displace ¹²⁵I-labeled tGH from its receptor. The ability of tPRL₁₇₇ to displace tGH was similar to that of ovine GH. tPRL₁₈₈ did not displace ¹²⁵I-labeled tGH binding. Collectively, this work suggests that tPRL₁₇₇ may possess somatotropic actions similar to tGH, but only in freshwater tilapia where tPRL₁₇₇ levels are sufficiently high for it to act as a competitive ligand for GH receptors.