Studies on Autoregulation of Prolactin Secretion from Perifused Rat Pituitary Glands in the Basal and Thyrotropin-Releasing Hormone-Stimulated States*

In order to evaluate autoregulation of PRL secretion at the pituitary gland, spontaneous and TRH-stimulated PRL secretion was determined in an isolated anterior pituitary (AP) perifusion system utilizing normal and chronically hyperprolactinemic (MtTW 15 tumor-bearing) male rat pituitaries. Decreased AP weight in chronically hyperprolactinemic rats [5.7 ± 0.1 (SE) VS. 7.2 ± 0.2 mg in controls, P < 0.001] was associated with lower mean basal PRL secretion compared to controls (9.9 ± 1.8 vs. 20.6 ± 2.7 ng/min/AP, P < 0.01). PRL secretion from normal AP was not altered by the addition of ovine PRL (oPRL) at 10 or 100 ng/ml (19.9 ± 3.8 and 19.2 ± 2.9 ng/min/AP, respectively). TRHstimulated PRL secretion from normal AP in the absence of oPRL was 133 ± 7% of baseline (designated as 100%) and this did not differ from the response observed in the presence of 10 or 100 ng/ml oPRL (134 ± 9 and 133 ± 7%, respectively). Basal PRL secretion from tumor rat AP was likewise unaffected by the presence of medium oPRL (9.9 ± 1.8 in the absence of oPRL and 10.0 ± 1.9 ng/min/AP in the presence of 100 ng/ml oPRL). TRH-stimulated PRL secretion from tumor rat AP was 149 ± 10% of baseline in the absence of medium oPRL and 140 ± 7% in the presence of 100 ng/ml oPRL (P > 0.10). The data suggest that autoregulation of PRL secretion exists, and this regulation may not be mediated directly at the pituitary gland but rather at higher, possibly hypothalamic, centers.