A MAMMALIAN GROWTH HORMONE-RELEASING HORMONE INCREASES SERUM GROWTH-HORMONE LEVELS AND SOMATIC GROWTH AT SUBOPTIMAL TEMPERATURES IN TILAPIA

The effects of a mammalian growth hormone-releasing hormone (GHRH), Pr ebGRF(1-78)OH (bGHRH), on growth and serum growth hormone (GH) levels were investigated in tilapias Oreochromis mossambicus and O. niloticus x O. aureus. Fish were injected intra-muscularly or implanted intrape ritoneally (Silastic or cholesterol implants) with distilled water, 0. 1 mu g/kg bGHRH, 10.0 mu g/kg bGHRH, 100.0 mu g/kg bGHRH and compared to untreated controls, fish implanted with 60 mg/kg 17 alpha-methyltes tosterone (MT), or a combination of bGHRH concentrations plus either M T, 0.01 mu g/kg of a thyroid hormone (T-3), or 0.01 mu g/kg of a gluco corticoid (DEX). The bGHRH increased serum GH levels in tilapia mainta ined at suboptimal temperatures (18 C). Serum GH levels were highest ( 5.3+/-0.45 ng/mL) for fish injected with 10.0 mu g/kg bGHRH. Fish impl anted with a Silastic implant containing 10.0 mu g/kg bGHRH had signif icantly higher (4.35+/-0.35 ng/mL) serum GH levels than those with an equivalent dosage in a cholesterol implant. The addition of MT, thyroi d hormones, and glucocorticoids did not increase serum GH levels above those obtained for fish receiving bGHRH alone. Tilapia reared at subo ptimal temperatures and implanted with 10.0 mu g/kg and 100.0 mu g/kg bGHRH had significantly greater increases in weight and length than co ntrol fish. Fish implanted with bGHRH, MT or bGHRH plus MT has signifi cantly higher moisture and protein content, while fat and ash contents were significantly lower than controls or sham-implanted fish. Fish i mplanted with bGHRH, MT or bGHRH plus MT had significantly higher gona dosomatic indices than fish implanted with MT alone, shams or non-trea ted controls. This study demonstrates that a mammalian GHRH stimulates release of GH, promotes somatic and gonadal growth and may affect rep roductive performance in tilapia.