Determination of circulating maturational gonadotropin in rainbow trout (oncorhynchus mykiss) using a heterologous

A radioimmunoassay (RÍA) for the measurement of maturational gonadotropin (GtH II) in the plasma of rainbow trout was developed using a rabbit antiserum to b subunit of chum salmon GtH II. Intact GtH II was used as standard and radioactive competitor. RIA was performed using a double-antibody method under non-equilibrium conditions, and was used to evaluate plasma GtH II release stimulated by a gonadotropin releasing-hormone (GnRH) analogue: des-GlyGnRH (1-9) ethylamide. The GnRH analogue was administrated during 6 days either orally or parenterally (one single ip injection or two ip injections with a 6- day interval). Plasma concentrations of GtH II in control (without hormone induction) and with oral GnRH were low (less than 2 ng/ml) in spite of a higher ovulatory rate. In contrast, both injections of GnRH (15 \ig/kg body weight) induced production and secretion of GtH II (16.6 ± 9.06 ng/ml). These data suggest that GnRH injection, but not oral administration, is an appropriate method to induce GtH II and higher ovulation rates in rainbow trout. The heterologous RIA was effective for detecting plasma GtH II from rainbow trout at concentrations higher than 2 ng/ml.