Urotensin II, a peptide hormone from the caudal neurosecretory system of the teleost, Gillichthys mirabilis, was isolated by using classical chromatographic techniques and high-performance liquid chromatography (HPLC). Direct microtechniques for sequence determination were used to establish its structure. Urotensin II from Gillichthys is a 1363-dalton dodecapeptide with the amino acid sequence AlaGly-Thr-Ala-Asp-Cys-Phe-Trp-Lys-Tyr-Cys-Val. This sequence is homologous with somatostatin in positions 1 and 2 and 7-9. The sequence has been verified by the production of a bioactive synthetic urotensin II. The possible chemical and physiological significance of its homology to somatostatin is discussed.

We have experimentally elevated the triiodothyronine ("3) content of striped bass (Morone saxutilis) eggs by injecting female striped bass intramuscularly with a large dose of T3 (20 pglg body weight) shortly before the induction of spawning. The significant elevation of T3 concentrations in the eggs from the hormone-injected fish (to 180 ng per gram), compared with those of vehicleinjected controls (about 19 nglg), confirms the transfer of T3 from the maternal circulation into oocytes. There were significant differences in the development of embryos and larvae from the two groups. Within the first week after hatching, the hormone-treated cohorts showed significantly greater larval body area, length, and dry weight, despite similar patterns of yolk and oil-globule utilization. The rate of swimbladder inflation and the survival rate were also significantly enhanced in the cohorts produced by T3-treated females. There was a positive and highly significant correlation between the concentration of T3 in the unfertilized oocytes and the rate of survival within the cohort. The improvement in larval survival may be a result of the maintenance of elevated T3 levels through the first week of development: during the transition to active feeding but before the onset of larval thyroid function. Thyroid hormone supplements appear to confer a distinct advantage to larval striped bass during this early, fragile developmental stage.

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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.