Temporal Effects of Estradiol and Diethylstilbestrol on Pituitary and Plasma Prolactin Levels in Ovariectomized Fischer 344 and Holtzman Rats: A Comparison of Radioimmunoassay and Nb 2 Lymphoma Cell Bioassay

Abstract

An Nb₂ lymphoma cell bioassay (Nb₂BA) and a radioimmunoassay (RIA) were used to compare plasma and pituitary levels of prolactin in ovariectomized Fischer 344 (F344) and Holtzman rats treated with either diethylstilbestrol (DES) or estradiol for up to 8 weeks. The objectives were to determine whether there were temporal differences in prolactin responses in strains with different genetic predispositions to estrogen-induced pituitary tumor formation and to determine whether the results of the two assay methods were equivalent. All rats were ovariectomized for 7 days and all except controls received subcutaneous Silastic implants of DES or 17β-estradiol and were sacrificed at intervals from 2 days to 8 weeks later. Pituitary content and plasma levels of prolactin were determined by Nb₂BA and RIA and the ratio of these measurements was calculated. DES induced a significant increase in pituitary prolactin in F344 rats by 2 days of treatment, as measured by RIA. Pituitary content increased to a peak by Day 4, after which a gradual decline occurred until the end of the experiment. Nb₂BA measurements were similar to those obtained by RIA, except at 8 weeks, when the content determined by Nb₂BA was significantly higher than the content determined by RIA. When estradiol was given to F344 rats a pattern of increase and subsequent decrease in pituitary content similar to that seen with DES was observed and levels measured by Nb₂BA and RIA were essentially equivalent. Plasma levels of prolactin in DES-treated F344 rats increased exponentially through the 8 weeks, and the Nb₂BA measurements were significantly greater than levels determined by RIA throughout the treatment period. Estradiol treatment produced a pattern of change in plasma levels of prolactin similar to that observed with DES, except that RIA and Nb₂BA measurements were not different. Comparable results were obtained in Holtzman rats, except plasma levels were not increased to the same degree as seen in F344 rats. From these results, we conclude that DES, but not estradiol, can selectively increase the secretion of prolactin that is more bioactive than immunoreactive and that this effect of DES is observed in F344 and Holtzman rats, although F344 rats released more prolactin in response to estrogens than did Holtzman females.