In vivo and in vitro studies on the release of thyrotropin in aging rats

Effects of aging on the concentration of serum thyrotropin (TSH) and the release of TSH in vitro from anterior pituitary glands (APs) of male and female rats were investigated. Rats with 3-5 (young) and 24-26 (old) months of age were used. Some female rats were ovariectomized (Ovx) 10 weeks before use. After decapitation, trunk blood was collected. APs were bisected and preincubated with Krebs-Ringer phosphate buffer (KRB) at 37 degrees C for 60 min. They were then incubated with or without 10 nM thyrotropin-releasing hormone (TRH) for 30 min. The media were collected and replaced by fresh KRB. Incubations were continued and KRB media were changed three times, once every 30 min. One hemi-AP per flask and 7 or 8 flasks per group were used. The concentration of TSH in the medium and serum samples was measured by radioimmunoassay. The level of serum TSH in the diestrous, estrous and Ovx rats was reduced by aging. Thirty min after incubation with TRH, the release of TSH from APs of old intact female or Ovx rats was significantly (P less than 0.01) less than that of corresponding young rats. The concentration of serum TSH and TRH-stimulated release of TSH from APs of male rats was not altered by aging. There was no difference in the basal release of TSH from APs between old and young animals. These data suggest that the release of TSH from APs in response to TRH is decreased by aging in female but not in male rats.