Interrelationship between estradiol and thyroxine on the release of thyrotropin and prolactin in...

The effects of estradiol and thyroxine (T4) on the spontaneous and thyrotropin-releasing hormone (TRH)-stimulated release of thyroid-stimulating hormone (TSH) and prolactin (PRL) in vitro from rat anterior pituitary glands (APs) were studied. Rats were ovariectomized (Ovx) and thyroidectomized (Tx) following injection of either thyroxine (20 micrograms/kg, sc) or saline and either estradiol benzoate (EB, 0.4 micrograms/kg, sc) or peanut oil daily for 42 days. Twenty hours after the last injection, all rats were decapitated. The blood samples were collected and the APs were bisected and incubated with or without TRH (1 or 1000 ng/ml) at 37 degrees C for 4 h. Serum TSH concentration was reduced by T4 injection but not altered by EB replacement. The concentration of serum PRL was increased by EB. Injection of both T4 and EB reduced the serum PRL level as compared with the PRL levels in Ovx-Tx rats injected with T4 or EB alone. The spontaneous release of TSH in vitro from APs of OVx-Tx rats was not altered by the replacement of T4, EB, or both. The TSH response to TRH (1000 ng/ml) was not influenced by EB treatment, but higher in T4-treated than in vehicle-treated group. Injection of both EB and T4 prevented the stimulatory effect of T4 alone on TSH response. Both spontaneous and TRH-stimulated releases of PRL in vitro were enhanced by EB injection. Administration of T4 did not affect the basal release of PRL, but stimulated the PRL response to TRH (1000 ng/ml). Injection of EB plus T4 caused a higher PRL response to TRH as compared with that in the rats injected with EB or T4 alone. These results suggest an antagonistic interaction between estradiol and thyroxine on the in vitro release of TSH in response to TRH and a synergetic interaction between these two hormones on PRL response to TRH. The interaction between estradiol and thyroxine on serum prolactin and TSH levels was not reflected by the interaction between these two hormones on the anterior pituitary gland.