Estrogen responsiveness of vitellogenin gene expression in rainbow trout (Oncorhynchus mykiss) kept at different...

We have examined the effect of ambient water temperature on the ability of juvenile rainbow trout to respond to estradiol (E2) injection with vitellogenin (Vg) synthesis. Vg appeared in serum within 24 hr of E2 injection in fish kept at 15 degrees and rose to 70 mg/ml over a 10-day treatment period during which four injections of E2 were given. A group of fish kept at 9 degrees responded more slowly to the same multiple injection protocol and showed Vg accumulation of only 8.9 mg/ml on the 10th day. Hepatic Vg mRNA levels accumulated more rapidly and extensively in animals treated and kept at 15 degrees than at 9 degrees; however, differences in serum Vg concentrations could not simply be attributed to differences in Vg mRNA levels. The ratio of serum Vg:Vg mRNA increased steadily over the treatment period, especially in the 15 degrees group, suggesting greater efficiency or capacity for translation and/or processing of the Vg protein at the higher temperature. Examination of hepatic nuclear estrogen receptor (ER) concentrations revealed a three- to five-fold increase in high-affinity E2-binding activity within the first 24 hr after injection in both temperature groups. Nuclear ER levels remained elevated to roughly the same extent in both groups throughout the 10-day period. Differences in nuclear ER concentrations and serum E2 concentrations could not account for the large differences in Vg mRNA and protein levels between the two temperature groups. Furthermore, a single injection of E2 at 15 degrees was able to induce higher levels of Vg mRNA and protein than multiple injections at 9 degrees. We suggest that temperature modulates the responsiveness of the liver to E2 at stages which are independent of E2 or ER concentrations.