Growth hormone and prolactin gene expression and release in the pituitary of rainbow trout in serum-free culture

The trout pituitary continuously released a large amount of GH for over a week in serum-free culture, but PRL secretion declined by day 2 in culture. The total amount of GH released during the 8 days exceeded the initial content. The pituitary content of PRL remained unchanged after 8 days of culture. The number of weakly stained GH-immunoreactive (ir) cells was increased by day 4 in culture. The GH mRNA levels in individual cells as revealed by in situ hybridization were maintained at around the same level throughout culture. In contrast, the stainability of PRL-ir cells remained unchanged during culture, although the levels of PRL mRNA decreased by day 8. Electron microscopic immunocytochemistry showed a decrease in the numbers of secretory granules and dilation of rough endoplasmic reticulum in GH-ir cells. GH synthesis, measured by incorporated [3H]leucine after GH immunoprecipitation, was elevated 120% on day 8 over day 0. The dominant control of GH and PRL secretion in rainbow trout seems to be the converse of those seen in mammals; inhibition for GH and stimulation for PRL.