A NEW GONADOTROPIN-RELEASING-HORMONE (GNRH) SUPERAGONIST IN GOLDFISH - INFLUENCE OF DIALKYL-D-HOMOARGININE AT POSITION-6 ON GONADOTROPIN-IIAND GROWTH-HORMONE RELEASE

The two native forms of gonadotropin-releasing hormones (GnRH) present in goldfish, salmon GnRH (sGnRH) and chicken GnRH-II (cGnRH-II), stim ulate gonadotropin-II (GTH-II) and growth hormone (CH) release both in vivo and in vitro. In our previous study using perifused goldfish pit uitary fragments, many mammalian GnRH antagonists, especially those wi th D-Arg(6), showed weak to strong stimulation of GTH-II and GH releas e. In the present study, the dose-related stimulation of GTH-II and GH release by [Ac-D(2)-Nal(1), 4Cl-D-Phe(2), D-Trp(3), D-Arg(6), Trp(7), D-Ala(10)] mGnRH (analog J) and [Ac-D(2)-Nal(1), 4Cl-D-Phe(2), D-Trp( 3), D-hArg(Et(2))(6) D-Ala(10)] mGnRH (analog K) was demonstrated; the stimulatory potency of both analogs was significantly lower than that of native sGnRH. In the presence of analogs J and K, cGnRH-II stimula ted GTH-II release was significantly suppressed. Further, GTH-II and G H stimulation by 2 mu M of analog K was significantly suppressed by a 'true' GnRH antagonist, [Ac-Delta(3)-Pro(1), 4FD-Phe(2), D-Trp(3,6)] m GnRH (analog E). These results indicate that analogs J and K increase GTH-II and GH release in goldfish by acting on GnRH receptors on gonad otrophs and somatotrophs. Since analog K, having [D-hArg(Et(2))(6)], s trongly stimulated GTH-II release, the potency of [D-hArg(Et,)6] or [D -hArg(CH2CF3)(2)(6)] substituted analogs to stimulate GTH-II and GH re lease from the perifused goldfish pituitary fragments was tested. Amon g the peptides tested, [D-hArg(Et(2))(6), Pro(9)-NHEt] sGnRH had a hig her potency in stimulating GTH-II release than any other analog tested in the present or in previous studies. For stimulation of GH release, [D-hArg(Et(2))(6), Pro(9)-NHEt] sGnRH and [D-Arg(6), Pro(9)-NHEt] sGn RH were the most potent analogs tested; analogs of mGnRH were less pot ent than sGnRH, indicating the importance of Trp(7), Leu(8) residues i n the native peptide. These results suggest the importance of [D-Arg(6 )] or alkylated [D-Arg(6)] in determining the intrinsic activity and p otency of GnRH peptides in goldfish.