Evidence for a direct action of GH on haemopoietic cells of a marine fish, the gilthead sea bream (Sparus aurata).

Receptors for GH were characterized in the head kidney of gilthead sea bream (Sparus aurata), using radioiodinated and biotinylated ligands. The specific binding of radiolabelled recombinant gilthead sea bream GH (rsbGH) to head kidney membrane preparations was dependent on membrane concentration. Salmon prolactin, salmon gonadotrophin and carp gonadotrophin did not compete for 125I-labelled rsbGH-binding sites. Unlabelled rsbGH competitively displaced 125I-labelled rsbGH bound to head kidney membranes. Scatchard plots were always linear, denoting the presence of a single class of binding sites. The binding affinity (Ka = 2.7 x 10(9) M-1) was equivalent to that found in liver membrane preparations, but the binding capacity (2.5 +/- 0.30 fmol/mg protein) was 50- to 75-fold lower. To identify the cells which express the GH receptor, head kidney smears were incubated with biotinylated rsbGH, followed by incubation with an avidin-biotin complex conjugated to alkaline phosphatase. The reaction with the new-fuchsin substrate gave a red precipitate, showing a specific and intense labelling in erythroblasts, polychromatophilic erythroblasts and myeloblasts. Noticeable binding was observed in myelocytes and immature granulocytes, tending to disappear at the latter stages of granulocyte maturation. Light but appreciable binding was also observed in monocytes, lymphocytes and acidophilic erythroblasts, whereas it was completely absent in proerythrocytes and erythrocytes. The proliferative action of rsbGH and recombinant human IGF-I on in vitro cultures of head kidney cells was demonstrated by a 5-bromo-2'-deoxy-uridine immunoassay. To our knowledge, this is the first report that provides suitable evidence for a role of GH as a haemopoietic growth and differentiation factor in lower vertebrate species.