Hormone changes during ovulation in the rainbow trout (Salmo gairdneri Richardson)

Plasma levels of 17 beta-oestradiol, testosterone, 17 alpha-hydroxy-20 beta-dihydroprogesterone, 17 alpha-hydroxyprogesterone, and gonadotrophin were measured in 14 female rainbow trout during the course of their first ovulation period. Gonadotrophin levels were rising at the beginning of the experiment (12-16 days prior to ovulation) and kept rising to reach a peak (65 ng ml-1) at 20 days after ovulation. 17 alpha-Hydroxy-20 beta-dihydroprogesterone levels rose rapidly to reach a peak (317 ng ml-1) 4 days prior to ovulation and fell gradually over a further 32 days. 17 beta-Oestradiol fell 15-17 ng ml-1 12 days prior to ovulation to basal levels (2-3 ng ml-1) at 4 days prior to ovulation, and remained low. Testosterone fell more slowly, however, from a peak value (276 ng ml-1) 8 days prior to ovulation to basal levels 28 days postovulation. 17 alpha-Hydroxyprogesterone appeared in plasma at the same time as 17 alpha-hydroxy-20 beta-dihydroprogesterone. The level of 17 alpha-hydroxyprogesterone rose more slowly, however, and stayed at a fairly constant level (about 100 ng ml-1) for 16-20 days. These results give the first clear picture of the interrelationships between some of the major hormones known to be involved in salmonid reproduction. They support the existence of feedback inhibition between the sex steroids and gonadotrophin, and provide further evidence for the important role of 17 alpha-hydroxy-20 beta-dihydroprogesterone in the processes of oocyte maturation and ovulation. It is suggested that the rapid rise in 17 alpha-hydroxy-20 beta-dihydroprogesterone prior to ovulation is due to the switching of already highly active steroid cells from C19 (androgen) to C21 (progestagen) production.