HYPOTHALAMIC AND THYROIDAL REGULATION OF GROWTH-HORMONE IN TILAPIA

A radioimmunoassay (RIA) for recombinant tilapia growth hormone (GH) w as established and validated. The ability of various hypothalamic fact ors to regulate GH secretion in the tilapia hybrid (Oreochromis niloti cus x Oreochromis aureus) was studied. Somatostatin(1-14) (SRIF(1-14); 10-100 mu g/kg) was found to reduce circulating GH levels in a dose-d ependent manner. SRIF(1-14) (0.1-1000 nM) inhibited GH release from pe rifused pituitary fragments (ED(50) 0.83 nM). Human growth hormone-rel easing hormone fragment 1-29 (hGHRH(1-29); 100 mu g/kg) doubled circul ating GH levels and modestly stimulated GH secretion in vitro. Carp gr owth hormone-releasing hormone (cGHRH) stimulated GH secretion in vitr o to a similar degree at the same dose (1 mu M). Injection of salmon g onadotropin-releasing hormone (sGnRH) superactive analog (10-100 mu g/ kg) increased plasma GH levels sixfold. sGnRH also stimulated GH relea se in vitro (ED(50) 142.56 nM). Dopamine (0.1-10 mu M) and the D-1 DA receptor agonist SKF 38393 increased GH secretion from perifused pitui tary fragments dose-relatedly. Thyrotropin-releasing hormone (TRH) had no effect on GH secretion from perifused pituitary fragments, but inc reased plasma GH levels, as did bovine thyroid stimulating hormone (bT SH). The increased plasma GH in the bTSH-treated fish coincided with a dramatic increase in T-4; however, TRH increased GH without changing T-4 levels. T-3 increased the synthesis of GH by isolated pituitaries (incorporation of [H-3]leucine). SRIF(1-14) seems to be a most potent hypothalamic regulator of GH secretion in tilapia; sGnRH and DA both i ncreased GH secretion, although sGnRH elicited considerably greater re sponses at lower doses. Two forms of GHRH increased GH levels, althoug h the unavailability of the homologous peptide prevented an accurate e valuation of its importance in regulating GH secretion. The thyroid ax is (TRH, TSH, and T-3) stimulates both synthesis and release of GH, al though TRH did not appear to have a direct effect on the level of the pituitary. (C) 1995 Academic Press, Inc.