Application of a sensitive radioimmunoassay for the measurement of growth hormone in gilthead sea bream (Sparus aurata) and other sparid fish

The development of a homologous radioimmunoassay (RIA) for gilthead sea bream growth hormone (sbGH) is described. RIA sensitivity was 0.4 ng/mL, and ED50 was 1.74 ± 0.05 ng/mL. Intra- and inter-assay coefficients of variation were 3.4 and 8.8%, respectively, at ED50 levels. Mammalian growth hormones (GH), carp gonadotropin, chinook salmon gonadotropin, ovine porlactin, and recombinant tilapia prolactin did not show cross-reactivity. Serial dilutions of recombinant trout GH indicated a low but significant cross-reactivity. The displacement curves for plasma and pituitary homogenates from sparid fish (gilthead sea bream, blacktail, white sea bream, Couch's sea bream, and marmor-brassen) were parallel to that of the sbGH standard. Pituitary homogenates from other perciform fish tested (gaper, grey mullet, red mullet, and sea pike) also showed parallel slopes of inhibition. Pituitary homogenates from common sole, sea scorpion, forked hake, goldfish, rainbow trout, and European eel showed low or negligible cross-reactivity. These results suggest that the GHs of perciform fish have a number of similarities in structure. RIA of sea bream GH can be used to quantify the GH of sparid fish and perciform fish, provided a validation has been carried out.