Age-Related Differences in the Spontaneous and Thyrotropin-Releasing Hormone-Stimulated Release of Prolactin and Thyrotropin in Ovariectomized Rats

Abstract

Effect of estradiol on the spontaneous and thyrotropin-releasing hormone (TRH)-stimulated release of prolactin (PRL) and thyrotropin (TSH) in young and aged ovariectomized (Ovx) rats was investigated. Old (22–26 months) and young (3 months) female rats were Ovx 3 weeks before use. They were injected subcutaneously with estradiol benzoate (EB, 25 µg/kg) or sesame oil for 3 days and were catheterized via the right jugular vein. Twenty hours after the last administration of EB, rats were injected with TRH (10 µg/kg) through the catheter. Blood samples were collected before and 5, 10, 20, 40 and 60 min after TRH injection. On the day following blood sampling, all rats were decapitated. The anterior pituitary glands (APs) were excised, and incubated with or without TRH (10 ng/ml) at 37 ° C for 30 min. The basal level of PRL concentration in plasma samples was 5-fold higher in old Ovx rats than in young Ovx rats. Five min after TRH injection, the increase in plasma PRL was greater in old animals than in young animals. Plasma PRL remained higher in old animals than in young animals at 10, 20, 40 and 60 min following TRH challenge. Administration of EB to old and to young Ovx rats produced increases in both basal and TRH-stimulated secretions of PRL, but did not affect the difference in plasma PRL patterns between old and young animals. The release of PRL from APs was increased significantly in all rats after a 30-min incubation with TRH. In Ovx rats injected with oil, the basal release of PRL in vitro was increased with age. The TRH-stimulated release of PRL from APs of oil- and EB-primed rats was also higher in aged rats than in the corresponding young rats. The concentration of plasma TSH in response to TRH was less in old rats than in young rats. Estradiol seemed to have no effect on the secretion of TSH in vivo. Neither aging nor EB injection appeared to alter the spontaneous release of TSH in vitro. However, the release of TSH from APs in vitro in response to TRH was significantly reduced by age. These results suggest that the increased capability of the basal and TRH-stimulated secretion of PRL as well as the decreased capability of the secretion of TSH in response to TRH in old rats was due at least in part to intrinsic changes in anterior pituitary function, including the discordant patterns of lactotrophs and thyrotrophs responding to TRH during the aging process.

Inhibitory effect of digoxin on testosterone secretion through mechanisms involving decreases of cyclic AMP production and cytochrome P450scc activity in rat testicular interstitial cells

“…Male rats were anaesthetized with ether and then catheterized via the right jugular vein (Wang et al, 1989(Wang et al, , 1994Tsai et al, 1996a). Twenty hours later, the conscious rats were injected intravenously with vehicle (saline, 1 ml kg 71 ), hCG (5 iu ml 71 kg 71 ), digoxin (1 mg ml 71 kg 71 ), or hCG plus digoxin, via the jugular catheter.…”

Inhibition by amphetamine of testosterone secretion through a mechanism involving an increase of cyclic AMP production in rat testes

“…Male rats were catheterized via the right jugular vein (Wang et al, 1989;1994;Hwang et al, 1990). Twenty hours later, they were injected with hCG (5 iu ml-' kg-, Sigma), amphetamine (0.4 yg ml-1 kg-', Sigma) or hCG plus amphetamine via the jugular catheter.…”

Age-Related Differences in the Release of Luteinizing Hormone and Gonadotropin-Releasing Hormone in Ovariectomized Rats

“…Ovx rats were injected subcutaneously with EB (25 yig/kg) or sesame oil once daily for 3 days, and calheterized via the right jugular vein using the method described by Harms and Ojeda [9) with minor modification [10], They were then kept I/cage. Twenty hours after the last injection of EB.…”