EVIDENCE FOR A DIRECT ACTION OF GH ON HEMATOPOIETIC-CELLS OF A MARINEFISH, 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 membr ane concentration. Salmon prolactin, salmon gonadotrophin and carp gon adotrophin did not compete for I-125-labelled rsbGH-binding sites. Unl abelled rsbGH competitively displaced I-125-labelled rsbGH bound to he ad kidney membranes. Scatchard plots were always linear, denoting the presence of a single class of binding sites. The binding affinity (K-a = 2.7 x 10(9) M(-1)) was equivalent to that found in liver membrane p reparations, but the binding capacity (2.5 +/- 0.30 fmol/mg protein) w as 50- to 75-fold lower. To identify the cells which express the GH re ceptor, head kidney smears were incubated with biotinylated rsbGH, fol lowed by incubation with an avidin-biotin complex conjugated to alkali ne phosphatase. The reaction with the new-fuchsin substrate gave a red precipitate, showing a specific and intense labelling in erythroblast s, polychromatophilic erythroblasts and myeloblasts. Noticeable bindin g was observed in myelocytes and immature granulocytes, tending to dis appear at the latter stages of granulocyte maturation. Light but appre ciable binding was also observed in monocytes, lymphocytes and acidoph ilic erythroblasts, whereas it was completely absent in proerythrocyte s 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 sp ecies.