ABSTRACT

The present study investigated possible sites through which ACTH or corticosterone inhibit progesterone secretion in pregnant rats, and the role of placental factors in blocking the inhibitory effect. The number of conceptuses was adjusted to one (1C group) or more than ten (FC group) on day 7 of pregnancy by aspirating the desired number. Serum concentrations of progesterone, testosterone and oestradiol were significantly (P<0·01) lower on day 15 in the 1C group than in the FC group. Corpora lutea (CL) obtained on day 15 were incubated for 6 h with corticosterone or ACTH. Corticosterone (1 μmol/l) significantly (P<0·05) inhibited progesterone secretion in the IC group but not in the FC group. The inhibitory effect of corticosterone in the IC group was completely blocked by co-addition of 1 μmol testosterone/l or 1 μmol oestradiol/l but not by 1 μmol dihydrotestosterone/l. ACTH (1 μg/l–1 mg/l) had no direct effect on progesterone secretion in either the IC or the FC groups, although ACTH apparently decreases progesterone secretion in vivo. Placentae obtained from rats of the FC group on day 15 were incubated for 24 h with or without ACTH (1 mg/l). The supernatant after placental incubation without ACTH significantly (P<0·01) increased progesterone secretion by the CL in both the IC and FC groups, and also eliminated the inhibitory effect of corticosterone in the IC group. The supernatant after placental incubation with ACTH also increased progesterone secretion in the FC group as effectively as the supernatant from the control incubation, but it had no effect in the IC group. It is concluded that corticosterone directly inhibits progesterone secretion by the CL, whereas the inhibitory effect of ACTH is mediated through the placenta. The results indicate that these inhibitory effects of corticosterone or ACTH are eliminated if the CL has been exposed to enough placental hormones before day 15 of pregnancy.

Journal of Endocrinology (1991) 129, 405–410

Abstract

The purpose of this study was to examine the possible mechanism through which RU486 induces luteolysis during the late-luteal phase in pseudopregnant (PSP) rats. PSP rats received a subcutaneous injection of RU486 in sesame oil (5 mg/kg body weight) or sesame oil alone once a day between day 9 and day 11 of pseudopregnancy. Serial blood samples were collected on days 5, 9, 10, 11 and 12 and assayed for progesterone content. To examine the possible action of RU486 through a uterine and/or a pituitary (prolactin-dependent) mechanism, PSP rats and chronic hysterectomized PSP rats which had been hysterectomized before PSP induction received a subcutaneous injection of RU486 in sesame oil (5 mg/kg body weight), sesame oil alone, prolactin in 50% polyvinylpyrrolidone (15 IU/day), or RU486 and prolactin once a day between day 9 and day 11 of pseudopregnancy. Serial blood samples were collected on days 5, 9, 10 and 11 and assayed for progesterone content. Blood samples were also collected at 0400 h on day 12 and used for prolactin and progesterone determinations. To examine the direct effect of RU486 on corpus luteum and/or pituitary, hysterectomized rats underwent hypophysectomy and pituitary autotransplantation on dioestrus 1 and received a subcutaneous injection of RU486 in sesame oil or sesame oil alone for 3 days between day 21 and day 23 after surgery. Serial blood samples were collected on days 10, 21, 22, 23 and 24 and assayed for progesterone and prolactin contents.

In ordinary PSP rats, serum progesterone levels were significantly (P<0·01) lower in the RU486-treated group than in the control group (9 ± 1 vs 53 ± 7 ng/ml; mean ± s.e.m.) on day 11. Serum prolactin levels at 0400 h on day 12 of pseudopregnancy were significantly (P<0·05) lower in the RU486-treated group than in the control group (16 ±4 vs 154 ±44 ng/ml; mean ± s.e.m.). The concomitant prolactin treatment reversed the luteolytic effects of RU486 on day 11 of pseudopregnancy. In hysterectomized PSP rats, RU486 also suppressed serum prolactin levels, and the concomitant prolactin treatment again reversed the luteolytic effects of RU486. In hysterectomized rats which were hypophysectomized and pituitary autotransplanted, RU486 treatment did not induce any significant changes in serum progesterone and prolactin levels.

These results indicated that RU486 induced luteolysis during the late-luteal phase in PSP rats by suppressing prolactin secretion via a hypothalamic mechanism.

Journal of Endocrinology (1996) 150, 93–98

Activin type II receptors (ActRIIs) including ActRIIA and ActRIIB are serine/threonine kinase receptors that form complexes with type I receptors to transmit intracellular signaling of activins, nodal, myostatin and a subset of bone morphogenetic proteins. ActRIIs are unique among serine/threonine kinase receptors in that they associate with proteins having PSD-95, Discs large and ZO-1 (PDZ) domains. In our previous studies, we reported specific interactions of ActRIIs with two independent PDZ proteins named activin receptor-interacting proteins 1 and 2 (ARIP1 and ARIP2). Overexpression of both ARIP1 and ARIP2 reduce activin-induced transcription. Here, we report the isolation of two isoforms of ARIP2 named ARIP2b and 2c. ARIP2, ARIP2b and ARIP2c recognize COOH-terminal residues of ActRIIA that match a PDZ-binding consensus motif. ARIP2 and its isoforms have one PDZ domain in the NH₂-terminal region, and interact with ActRIIA. Although PDZ domains containing GLGF motifs of ARIP2b and 2c are identical to that of ARIP2, their COOH-terminal sequences differ from that of ARIP2. Interestingly, unlike ARIP2, overexpression of ARIP2b or 2c did not affect ActRIIA internalization. ARIP2b/2c inhibit inhibitory actions of ARIP2 on activin signaling. ARIP2 is widely distributed in mouse tissues. ARIP2b/2c is expressed in more restricted tissues such as heart, brain, kidneys and liver. Our results indicate that although both ARIP2 and ARIP2b/2c interact with activin receptors, they regulate ActRIIA function in a different manner.

Activin has previously been shown to act as a nerve cell survival factor and to have neurotrophic effects on neurons. However, the role of activin in regulating neurotransmitter expression in the central nervous system and the exact mechanisms involved in this process are poorly understood. In the present study, we report that activin A and basic fibroblast growth factor (bFGF) synergistically increased the protein level of tyrosine hydroxylase (TH), and also greatly increased the TH mRNA level, in both mouse E14 striatal primary cell cultures and the hippocampal neuronal cell line HT22. Activin A and bFGF cooperatively stimulated nuclear translocation of Smad3 and specifically activated ERK1/2, but not p38 or JNK. Interestingly, a specific inhibitor for MEK, U0126, efficiently blocked the induction of TH promoter activity by activin A and bFGF, indicating that activin A collaborated with bFGF signaling to induce the TH gene through selective activation of ERK-type MAP kinase in mouse striatal and HT22 cells. These data suggest that activin A may act in concert with bFGF for the development of TH-positive neurons.

We investigated the effect of activin A on secretion of LH, FSH, and prolactin (PRL) by female cultured rat pituitary cells at the single-cell level by means of the cell immunoblot assay. Anterior pituitary cells from 8-week-old female rats were preincubated with or without activin A for 24 h, after which they were monodispersed and immediately used for cell immunoblot assay. The percentages of LH-, FSH- and PRL-immunoreactive cell blots detected were 5.5, 5.3 and 43.1%, respectively, of all pituitary cells applied to the transfer membrane. The percentage of LH-secreting cells and mean LH secretion per cell did not change after treatment with activin. In contrast, activin significantly increased the percentage of FSH-secreting cells and mean FSH secretion per cell to 136.0 and 114. 5% respectively. In addition, activin significantly decreased the percentage of PRL-secreting cells and mean PRL secretion per cell to 52.2 and 72.0% respectively. These results suggest that (1) activin A has effects on female rat pituitary cells that increase not only the amount of FSH secretion per cell but also the number of FSH-secreting cells, and (2) activin A decreases both the amount of PRL secretion per cell and the number of PRL-secreting cells.

The purpose of this study was to investigate the effects of physiologic levels of ghrelin on insulin secretion and insulin sensitivity (glucose disposal) in scheduled fed-sheep, using the hyperglycemic clamp and hyperinsulinemic euglycemic clamp respectively. Twelve castrated Suffolk rams (69.8 ± 0.6 kg) were conditioned to be fed alfalfa hay cubes (2% of body weight) once a day. Three hours after the feeding, synthetic ovine ghrelin was intravenously administered to the animals at a rate of 0.025 and 0.05 μg/kg body weight (BW) per min for 3 h. Concomitantly, the hyperglycemic clamp or the hyperinsulinemic euglycemic clamp was carried out. In the hyperglycemic clamp, a target glucose concentration was clamped at 100 mg/100 ml above the initial level. In the hyperinsulinemic euglycemic clamp, insulin was intravenously administered to the animals for 3 h at a rate of 2 mU/kg BW per min. Basal glucose concentrations (44± 1 mg/dl) were maintained by variably infusing 100 mg/dl glucose solution. In both clamps, plasma ghrelin concentrations were dose-dependently elevated and maintained at a constant level within the physiologic range. Ghrelin infusions induced a significant (ANOVA; P < 0.01) increase in plasma GH concentrations. In the hyperglycemic clamp, plasma insulin levels were increased by glucose infusion and were significantly (P < 0.05) greater in ghrelin-infused animals. In the hyperinsulinemic euglycemic clamp, glucose infusion rate, an index of insulin sensitivity, was not affected by ghrelin infusion. In conclusion, the present study has demonstrated for the first time that ghrelin enhances glucose-induced insulin secretion in the ruminant animal.