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

Abstract

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.

Immunohistochemically detected ontogeny of prolactin and growth hormone cells in the African catfish Clarias gariepinus

“…Recombinant tPRL I is more sensitive to the transfer from fresh to salt water in comparison to recombinant tPRL II [3,13] while it may possess somatotropic actions similar to GH [39]. The tilapia PRL II hormone comprises 177 amino acids and is known to be present in the same granules as PRL I [40].…”

“…While classical techniques such as experimental endocrinology, histology and immunohistochemistry have taught us much about the location and possible functions of the growth hormone (GH) -prolactin (PRL) family [11,18], more recent studies on gene regulation provide evidence for specific and defined roles of hormones [33,39]. GH and PRL, originating from the same stem cells [7], are expressed in specific cells located in the pituitary.…”

“…For example the pituitary-specific Pit-1 protein regulates the temporal and spatial transcription of PRL, GH and SL [43]. Insulin-like growth factor (IGF)-I mediates the action of GH and PRL [34,39]. In addition, it has become clear that the molecular evolution of fish GH, PRL and IGF-I largely follows the morphological evolution in several sustained bursts of evolution [34,48].…”

Prolactin177, prolactin188 and prolactin receptor 2 in the pituitary of the euryhaline tilapia, Oreochromis mossambicus, are differentially osmosensitive

“…While the existence of two distinct Prl molecules, designated Prl 177 and Prl 188 , suggests an evolution of differing actions (Chen et al 1994), a bioassay failed to identify any clear differences in their osmoregulatory activities (Specker et al 1985). On the other hand, it has been reported that Prl 177 exhibits somatotropic activity while Prl 188 does not (Shepherd et al 1997). The tilapia Prls have also been found to respond differently to acclimation salinity.…”

Associations between single nucleotide polymorphisms in candidate genes and growth rate in Arctic charr (Salvelinus alpinus L.)

“…GH plays a major role in stimulating somatic growth in teleosts, including salmonid fishes (Chen et al, 1994;Shepherd et al, 1997;Devlin et al, 1995), and also has an osmoregulatory function to efficiently promote fresh water fish to adapt to seawater (Sakamoto et al, 1991).…”