Multihormonal Control of the Interrenal of Coho Salmon, Oncorhynchus Kisutch.
In vitro studies have demonstrated that thyroid hormones and growth hormone (GH) exertregulatory effects on the salmonid interrenal (Young and Lin, 1988, Young, 1988). Increased plasma levels of cortisol may occur in hypophysectomized (Hx) Coho salmon after stress or change in salinity (Bjijrnsson et al., 1987). The effects of Hx on interrenal function were examined using yearling Coho salmon which were hypophysectomized 10 weeks prior to experimentation and maintained in l/4 seawater. Animals were given three injections (every other day) of 5 pg/g body wt. ovine GH or 1 pug/gb ody wt. triiodothyronine ( T3). Controls consisted of sham-Hx and Hx animals injected with vehicle. Interrenal tissue was incubated with ACTH or pregnenolone for 3 h, and cortisol was measured in
incubation media. Hx caused a moderate reduction in the sensitivity of the interrenal to ACTH but did not affect the maximal steroidogenic response; neither GH nor Ts caused a significant increase in sensitivity. In contrast, interrenal tissue from vehicle-injected Hx fish displayed a twofold greater ability to utilize pregnenolone as a substrate, and both GH and T, increased the ability
of interrenal tissue to utilize the substrate. These results indicate that (1) the long-term maintenance of steroidogenic capability is not dependent on pituitary hormones; (2) the maintenance of sensitivity of tissue to ACTH appears to be pituitary-dependent; and (3) both GH and T, probably act by enhancing the steroidogenic pathway between pregnenolone and cortisol.