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In the present study, we examined the long-term effects of environmental salinity, diet (35% and 25% crude protein) and 17α-methyltestosterone (MT) on corresponding levels of pituitary and serum growth hormone (GH) and prolactins (tPRL177 and tPRL188) in the tilapia (Oreochromis mossambicus). We observed no discernible patterns in serum GH that would suggest an effect of salinity, diet or MT. However, serum GH levels in all treatments declined at 1 and 3h after first feeding. Serum tPRL177 and tPRL188 were significantly higher in freshwater (FW) than in seawater (SW) and levels were significantly affected by dietary protein. tPRL177 levels were higher in all groups fed a 35% protein diet, but tPRL188 levels were higher only in the groups fed the MT-treated 35% protein diet; only serum tPRL188 levels were affected by MT. Moreover, serum tPRL177 and tPRL188 increased throughout the sampling time-course. Subsequent work using fasted tilapia suggests that first feeding is likely to initiate the post-prandial suppression of serum GH levels. In contrast with the picture observed in blood, pituitary glands of SW animals showed higher levels of GH than FW fish. Pituitary GH was elevated by MT in both FW and SW. We also observed that pituitary tPRL177 and tPRL188 levels were higher in FW fish than in SW fish; tPRL177 and tPRL188 levels were elevated by MT only in FW animals. To assess the somatomedin activity of plasma from FW- and SW-reared tilapia, we measured [35S]-sulfate incorporation into ceratobranchial cartilage explants in vitro. Plasma from SW-adapted tilapia showed greater activity in this assay than plasma from FW-reared tilapia, suggesting that the GH-dependent IGF bioactivity of plasma is higher in SW-reared tilapia. Collectively, these studies suggest that the growth-promoting actions of SW rearing and of MT administration in tilapia may be linked to elevations in GH and/or prolactin (tPRL177 and tPRL188)levels.

Plasma volumes in male tilapia (Oreochromis mossambicus) of different size were estimated following intracardial injection of radioiodinated human serum albumin ((125)I-HSA), coupled with short-term, early sampling transient response analysis of 1251-HSA disappearance from the plasma pool. This approach circumvents vascular marker leakage problems associated with constant steady state indicator dilution methods, minimizes some sampling and mixing problems, and simplifies analysis of the data. Changes in hematological parameters due to experimental stress were also studied, because the fish were not chronically cannulated. Results were used in a novel way to correct estimates of plasma volume upward by 15%, thereby providing a potentially useful alternative approach to vascular volume measurement in species where stress-eliminating or reducing techniques, e.g., cannulation, are impractical or infeasible. Hematrocrits increased 38% at the onset, from 24.9% to 34.4%, and remained essentially constant during the 60 minute kinetic study, and plasma osmolalities increased 7%. Corrected plasma volumes Vp (ml) were a linear function of body weight (BW). The group mean Vp was 2.93% of BW and corresponding blood volumes were 3.9% of BW.