Stimulation of carp interrenal function by adrenocorticotrophin

The effect of ACTH on cortisol concentration in the plasma of the carp has been studied; cortisol was determined using a competitive protein-binding radioassay. Blood was sampled at 12.00 h (the diurnal minimum) by cardiac puncture. The plasma cortisol level of undisturbed carp (blood obtained 1--3 min after netting) was 16 +/- 14 (S.E.M.) ng/ml. One hour after injection of ACTH (1-0 i.u./100 g body wt., i.m.) the level of cortisol increased to 415 +/- 62 ng/ml and reached a maximum of 656 +/- 62 ng/ml after 2 h. The level of cortisol in saline-injected control carp after 2 h was 72 +/- 33 ng/ml. A transient rise of cortisol was noted in both groups 30 min after injection (315 +/- 51, 315 +/- 62 ng/ml). This was attributed to a surge of endogenous ACTH, presumably due to handling. Carp head kidneys were superfused in vitro. They released cortisol spontaneously, but the releasing rate declined exponentially and reached a minimum after 135 min. Addition of ACTH to the medium was followed by a prompt increase in cortisol release. The rate of cortisol release was maximal 30--45 min after addition of ACTH and was identical to the rate at the beginning of the superfusion.