The Yin-Yang Model of TSH Regulation: Disappearance of Triiodothyronine Inhibition on the Response of Plasma Thyrotropin to Thyrotropin-Releasing Hormone in Aged Male Rats

The inhibitory effects of thyroid hormones on the release of thyrotropin (TSH) in response to thyrotropin-releasing hormone (TRH) in old and young male rats were examined. Old (21 months) and young (3 months) male rats were challenged with TRH (10 μg/kg) and/or triiodothyronine (0-1 μg/kg) via right jugular vein catheters. Blood was collected at 0, 10, 40 and 60 min following intravenous injection of TRH. One day after in vivo study, all rats were decapitated. The anterior pituitary glands (APs) were excised, bisected and incubated with TRH (10 nM) and T3(10^(-8)-10^(-5)M) at 30 min intervals. Finally, all APs were incubated with 60mM KC1 for checking the viability. At the end of incubation, the medium samples were collected and the tissues were weighed. The concentration of TSH in plasma and medium samples was measured by radioimmunoassay, The spontaneous and TRH-stimulated levels of TSH both in vivo and in vitro were not altered by aging. The inhibitory effect of T3 on the plasma TSH levels in response to TRH was dose-dependent in young, but was not significant in old rats. The in vitro release of TSH in response to TRH was dose-dependently inhibited by the administration of T3 in both old and young male rats. These results suggest that the disappearance of T3 inhibition on the response of plasma TSH to TRFI in old male rats is independent of T3 action on the anterior pituitary gland.