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 (tPRL 177 and tPRL 188 ) on the in vitro incorporation of [ 35 S]sulfate (extracellular matrix synthesis) and [ 3 H]thymidine (DNA synthesis) by ceratobranchial cartilage explants and on IGF-I mRNA levels in tilapia liver. Tilapia GH (tGH) and tPRL 177 stimulated sulfate uptake at the highest doses examined. Thymidine incorporation was stimulated by tPRL 177 . tPRL 188 was without these effects. Consistent with its somatotropic actions, tGH elevated IGF-I mRNA levels in the liver. tPRL 177 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 tPRL 177 and tPRL 188 lowered levels of gill Na ؉ ,K ؉ -ATPase activity. High-affinity, low-capacity binding sites for tGH in the tilapia liver were identified. tPRL 177 binds with lower affinity than tGH to these sites but can displace 125 I-labeled tGH from its receptor. The ability of tPRL 177 to displace tGH was similar to that of ovine GH. tPRL 188 did not displace 125 I-labeled tGH binding. Collectively, this work suggests that tPRL 177 may possess somatotropic actions similar to tGH, but only in freshwater tilapia where tPRL 177 levels are sufficiently high for it to act as a competitive ligand for GH receptors.

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