Estrone and estradiol participation during exogenous vitellogenesis in the female rainbow trout, Salmo gairdneri

Abstract

Ovarian steroids were treated for their ability to induce vitellogenin synthesis in the liver of the rainbow trout, Salmo gairdneri. These steroids were chosen because they had been reported to induce vitellogenin in the plasma of teleosts, i.e., estrone, estradiol, and testosterone, or because they were synthesized during vitellogenesis, as was the case with the above steroids and androstenedione and 17α-hydroxyprogesterone. Immature trout were ovariectomized and injected daily for 7 days. The administered doses of steroid were 100, 250, and 500 ng/g body wt. Only estradiol and estrone induced vitellogenin in the plasma; estrone had about 5% of the potency of estradiol. A dose-effect curve was determined for estradiol in the range from 25 to 1000 ng/g body wt. A maximal amount of 7 mg vitellogenin/ml plasma was found following the administration of a dose of 250 ng/g estradiol. Vitellogenin was not present in the plasma of animals treated with saline, nor could it be detected in the liver, not even after the administration of estradiol. Estradiol administration increased the total plasma protein concentration from 35 to maximally 51 mg/ml and decreased the total piver protein concentration from 163 to minimally 118 mg/g. The relative weight of the liver increased from 12.9% to maximally 22.4%. Vitellogenin was not detected in the liver of any of the experimental animals, indicating a low storage and rapid secretion of vitellogenin. The other steroids influenced some of the variables, but never was the total pattern of effect comparable to that of estradiol. Estradiol is found to be the ovarian steroid that physiologically regulates the synthesis of vitellogenin in the liver of the rainbow trout; estrone is less active. Experiments undertaken to determine the effects of the combined presence of estrone and estradiol revealed that estrone was capable of boosting the vitellogenic effect of estradiol when compared to the induced vitellogenin levels following treatment with a combination of testosterone and estradiol. The vitellogenin concentrations induced by a certain dose of one of the combinations of estrone and estradiol approximated the concentrations induced by the same dose of estradiol alone. This effect was independent of the ratio in which estrone and estradiol were administered. These findings could not be explained by conversions of estrone into estradiol or by the vitellogenic activity of estrone alone. The estrone, estradiol, and vitellogenin concentrations in the plasma of the experimental animals were in the same range as determined previously in untreated mature vitellogenic females. Vitellogenin and estradiol levels were found to correlate in experimental animals treated with estradiol ( r = 0.627; N = 20). This was not the case in animals treated with combinations of estrone and estradiol. In these animals, however, the sum of estrone and estradiol levels in the plasma correlated with vitellogenin levels ( r = 0.724; N = 42). Vitellogenin was not detected in the liver of any of the experimental animals, which indicates a low storage and rapid secretion of vitellogenin. The importance of viewing the sum of estrone and estradiol plasma levels in connection to physiological studies of the regulation of exogenous vitellogenesis in the rainbow trout is discussed.