Recombinant bovine growth hormone treatment of tilapia: Growth response, metabolic clearance, receptor binding and immunoglobulin production

Experiments were performed to examine the growth-promoting effects of recombinant bovine growth hormone (rbGH) in the euryhaline tilapia (Oreochromis mossambicus). A radioreceptor assay using a crude membrane preparation of tilapia liver revealed that rbGH was about 100-fold less potent than native tilapia GH (tGH) in displacing ¹²⁵I-labeled tGH. Bovine prolactin (bPRL) was equipotent to bovine GH indicating that the GH receptor of tilapia does not distinguish mammalian GH from mammalian PRL. When juvenile tilapia, weighing 1 g, were maintained at 28°C and received intraperitoneal injection of rbGH at doses of 0.1, 1 or 10 μg/g weekly for 8 weeks, no significant effect on growth was observed. A second experiment examined weekly doses ranging from 1 to 50 μg/g for 16 weeks, using 1 g fish maintained at 23°C. rbGH (50 μg/g) significantly increased growth after 14 and 16 weeks, although the growth rate was significantly less than those held at 28°C. More pronounced growth-promoting effects were observed, however, when fish weighing 5 g and held at 29°C were injected with rbGH at doses of 100 and 1000 μg/g once a week for 4 weeks. A single injection of a sustained-release formulation of rbGH (Posilac®, 100 and 1000 μg/g) also elicited growth-promoting effects in fish weighing 4 g and kept at 29°C. Treatment with rbGH, Posilac® or bovine serum albumin (BSA) elicited significant increases in plasma levels of immunoglobulin (IgM) in a dose-dependent manner. By contrast, there was no change in plasma levels of lysozyme activity in rbGH- or Posilac®-injected fish compared with controls. An uptake and clearance study confirmed a slower decline in circulating levels of rbGH following Posilac® injection compared with rbGH in saline. There was no change in plasma concentration of tGH after rbGH treatment, indicating that GH secretion from the tilapia pituitary was unaffected by high plasma levels of rbGH. The relative refractoriness of juvenile tilapia to growth-promoting effects of rbGH compared with that of other species may be due to the specific nature of the tGH receptor in recognizing the homologous hormone.