APPLICATION OF A RECOMBINANT CICHLID GROWTH-HORMONE RADIOIMMUNOASSAY TO MEASURE NATIVE GH IN TILAPIA (OREOCHROMIS-NILOTICUS) BRED AT DIFFERENT TEMPERATURES

This work describes the application of a homologous radioimmunoassay ( RIA) in the measurement of plasma and pituitary growth hormone levels of tilapia maintained at 20 and 26 degrees C, using recombinant tilapi a growth hormone (rtiGH). RlA sensitivity (ED(90)) was 0.08 ng/ml and ED(50) was 0.62+/-0.02 ng/ml. Intra- and inter-assay coefficients of v ariation were 3.3 and 10.6% respectively, for a plasma sample of 1 ng/ ml of tiGH. Mammalian or salmonids growth hormones, thyrotropins or ti lapia prolactins did not show cross-reactivity with rtiGH antiserum. P lasma and pituitary extract of Oreochromis niloticus gave inhibition c urves parallel to the rtiGH standard curve without interaction with se rial dilutions either of tissue extracts or plasma from hypophysectomi zed tilapia. Pituitary extracts from other cichlid fish showed paralle l competitive binding curves. Pituitary extracts of perciform fish exh ibited a lower affinity to anti-rtiGH serum, while extracts of species from more distant families showed only negligible cross-reactivity or none at all. The effects of water temperature on plasma concentration s and pituitary contents of GH were investigated in tilapia acclimatiz ed to 26 and to 20 degrees C. Measurements have been performed at the beginning and at the end of a two-week experiment. Whereas in fish fro m colder water plasma GH mean values were decreased, only a similar te ndency was observed in pituitary GH contents. These results emphasize the hypothesis wich reports that the increase of growth with temperatu re is, at least in part, controlled by GH.