Effects of hypothalamic factors on somatolactin secretion from the organ-cultured pituitary of rainbow trout.

The profile of daily release of somatolactin (SL) and effects of hypothalamic factors on SL secretion from the organ-cultured pituitary of rainbow trout were examined. The daily release of SL was relatively high (340-380 ng/pituitary/day) for the first 2 days and then decreased. After Day 5, the SL release was maintained at a low level (30-50 ng/pituitary/day) until the end of the experiment at Day 7. The secretory patterns of SL differed from those of growth hormone (GH) and prolactin (PRL); GH secretion was consistently high (15-20 microg/pituitary/day), whereas PRL secretion was low (10 ng/pituitary/day) during the experiment. SL release was not stimulated by calcium ionophore on Days 2 and 6, suggesting that SL release was maximal. Dopamine and epinephrine, added separately to the medium, inhibited SL release. In contrast, serotonin, corticotropin-releasing factor, and gonadotropin-releasing hormone stimulated the dopamine-inhibited SL release. Thus, SL secretion is concluded to be under hypothalamic control and regulated by mechanisms different from those affecting PRL and GH secretion.