Growth hormone secretion during longterm incubation of the pituitary of the Japanese eel, Anguilla japonica.

Growth hormone (GH) secretion from organ-cultured pituitaries of the eel (Anguilla japonica) was studied during incubation in a defined medium for 2 weeks, using a homologous radioimmunoassay which does not distinguish between the two molecular forms of eel GH. The total amount of GH secreted increased gradually during the incubation period; so that the amount of GH released on day 14 was about 30 times greater than that on day 1. On day 14, the proportion of GH released relative to the total amount of GH present (the sum of GH released into the medium and residual content in the pituitary) was 96% and the amount produced on day 14 was 4 times greater than the content in the unincubated pituitary. Somatostatin (SRIF, 1.8 × 10(-7) M) inhibited the increase in GH release. On day 7, the proportion of GH released by pituitaries treated with SRIF (28%) was less than that released by the control pituitary (91%). There was no significant difference in GH release between the pituitaries incubated in isotonic medium (300 mOsm) and those in hypotonic medium (240 mOsm) for 2 weeks except for the first 3 days, when the pituitaries in hypotonic medium secreted significantly greater amounts of GH than those incubated under isotonic condition. Hypertonic medium (350 mOsm) had no effect on GH release except for significant inhibition on days 6 and 14. When secretion of the two forms of GH (GH I and II) was examined after separation by polyacrylamide gel electrophoresis followed by densitometry, slightly more GH I tended to be secreted than GH II during the culture period, although the effects of SRIF and osmolality of the media on GH I release were similar to those on GH II. It is concluded that GH secretion and production in the eel is mainly under the inhibitory control of hypothalamus, and that osmolality has a minimum influence on the GH release.