Growth hormone as an in vitro phagocyte-activating factor in the gilthead sea bream (Sparus aurata)

The stimulatory action of growth hormone on gilthead sea bream phagocyte-enriched cultures was demonstrated in vitro for the first time in a fish species. Phagocytes consisted mainly of macrophages, with a small number of neutrophils and eosinophils. Macrophages were unequivocally identified by their esterase staining and the lack of myeloperoxidase staining. Cells primed with recombinant rainbow trout GH showed clear morphological (light- and scanning electron-microscopic) and functional differences from non-primed cells. Stimulated phagocytes exhibited numerous branched lamellipodia, abundant membrane ruffles, increased spreading, and cell size. When incubated with sheep red blood cells, the phagocytic index and phagocytic capacity was also enhanced in primed cells. A bell-shaped dose-response curve (1.5-500 nM) was obtained when the metabolic activity of growth-hormone-activated cells was measured. This finding suggests that the homodimerization of the growth hormone receptor is a characteristic feature both in mammals and fish.