Abstract
The pituitary of the coho salmon (Oncorhynchus kisutch) was studied during the parr‐smolt transformation. Changes in pituitary prolactin (PRL) synthetic rate and influences of osmotic pressure (OP) and cortisol on PRL cell activity were investigated in vitro. After 20‐hr incubations in the presence of ^35^S‐methionine, tissue and medium were separated and subjected to SDS‐polyacrylamide gel electrophoresis; bands stained with Coomassie Blue R‐250 (= total PRL) and bands on X‐ray film resulting from autoradiography of the gels (= newly synthesized PRL) were quantified by soft laser densitometry. Physiological changes in medium OP exerted no detectable effect on the % release of total or newly synthesized PRL (^35^S‐PRL), whereas some differences, mostly in the % release of ^35^S‐PRL, were detected when extraphysiological medium OPs were used. Since OP effects on GH release are similar, these results suggest that OP has no specific direct effect on PRL release under physiological conditions. PRL synthetic rate during the parr‐smolt transformation increased beginning in mid‐February and peaked in mid‐March, but fell to undetectable levels in mid‐April; a second peak in PRL synthesis was observed in mid‐May. These data are in accord with reports on plasma levels of PRL, which further indicate that cortisol levels increase during the April decline in PRL levels. Our in vitro studies demonstrate that cortisol inhibits pituitary PRL synthesis and content in a dose‐related manner and also inhibits the % release of total and ^35^S‐PRL, suggesting that cortisol may be an important regulator of PRL cell activity during the parr‐smolt transformation.