Cortisol secretion in vitro by the interrenal of coho salmon (Oncorhynchus kisutch) during smoltification: relationship with plasma thyroxine and plasma cortisol.

An in vitro system for the incubation of interrenal tissue (head kidney fragments) from coho salmon, Oncorhynchus kisutch, was developed in order to examine changes in interrenal sensitivity to ACTH1-24 during smoltification, using cortisol secretion as the endpoint. Time-course studies indicated that maximal cortisol accumulation in incubation media was achieved after 3 hr exposure to ACTH. There was no correlation between head kidney weight, body weight, or sex and the response of the interrenal to ACTH1-24 in vitro. Approximately monthly or bi-weekly experiments were performed during the smoltification period (February-July): tissue was preincubated in hormone-free media for 3 hr, washed twice, and then challenged with 5 X 10(-10)-5 X 10(-7) M (1.5-1500 ng/ml) ACTH1-24 for 3 hr. The pattern of cortisol secretion was similar in February, early March, and late March in the dose range of 5 X 10(-10)-5 X 10(-8) M ACTH1-24. A marked, significant increase in sensitivity to ACTH and in the steroidogenic capacity of the tissue occurred in April, but by May the response was similar to that in the pre-April period. Enhanced sensitivity and steroidogenic capacity were found in interrenal tissue taken from coho salmon in June and July. Maximal in vitro responsiveness of interrenal tissue to ACTH in April was correlated with peak plasma thyroxine titers and enhanced hypoosmoregulatory ability, but not with peak plasma cortisol titers, which occurred in May.