Prolactin and growth hormone secretion during long-term incubation of the pituitary pars distalis of mature chum salmon, Oncorhynchus keta.

The effects of hypotonicity of the medium on prolactin (PRL) and growth hormone (GH) release from the pars distalis (PD) of mature chum salmon pituitary were examined during culture for 4 days. Large amounts of PRL and GH were released during the first 6 hr and secretion decreased rapidly thereafter. The amounts of GH released as well as the residual content in PD were 10 times greater than those of PRL. There was no difference in PRL or GH release between the PD incubated in isotonic medium (325 mOsm) and those in hypotonic medium (250 mOsm) even during the first 6 hr. Female PD secreted more PRL than male PD during the first 6 hr, whereas a sex difference was not observed in GH release. Considerable amounts of both PRL and GH remained in the PD at the end of the culture period. The amount of PRL released during 4 days of culture from female PD (37-39%) was greater than that from male PD (23-26%), and the amount of GH released was less in females (28-32%) than that in males (53-54%). Release of PRL and GH appears to be mainly under stimulatory hypothalamic control in the mature chum salmon.