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Polychlorinated dibenzo-p-dioxins, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) have been recognized as highly potent developmental and reproductive toxins. We have previously demonstrated effects of TCDD in modulating the expression of rat Sertoli cell secretory products and markers for cell–cell interaction. In this study, we examined the direct biological effects of TCDD in rat Leydig cell primary cultures. Mature rat Leydig cells were purified by Percoll gradient centrifugation and the cell purity was determined by 3β-hydroxysteroid dehydrogenase (3β-HSD) staining and a testosterone induction assay. To examine TCDD-induced biological consequences, we measured the changes in the secretion of progesterone and testosterone, as well as transcript levels of some selected steroidogenic enzymes (i.e. StAR, P450scc, 3β-HSD and CYP17α), in TCDD/human chorionic gonadotropin (hCG) co-treated cells. Our results indicated that TCDD (0.2 or 2 ng/ml) treatment significantly suppressed hCG (5 or 10 ng/ml)-induced testosterone secretion. The suppressive effect aligned with a reduction of progesterone secretion (P<0.05), as well as a decrease of P450scc mRNA and protein expression (P<0.05). The mechanistic action of TCDD was found to be via the reduction of cellular cAMP levels in the hCG-treated cells. This observation was further confirmed, as the TCDD-mediated suppressive effect could be reversed by dibutyryl cAMP co-treatment. The data indicate that TCDD can modulate cAMP signaling in rat Leydig cells to affect the process of steroidogenesis.
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Replacement of the 3′-halogen of the tri-iodothyronine (T3) molecule by a propyl-group produces a thyromimetic analogue, 3′-isopropyl-3,5-di-iodo-l-thyronine (T2iPr), with high biological potency. A serum thyroid-stimulating hormone (TSH) suppression test with one single intraperitoneal injection of 3 or 30 nm-T3 or T2iPr or with 30 or 300 nm-thyroxine (T4) per kg body weight was performed on 56 adult male Lewis rats which were maintained for 3 weeks on an iodine-deficient diet containing 0·2% 6n-propyl-2-thiouracil (PTU). Blood was withdrawn from each rat by cardiac puncture 24 h before and 3, 7, 24 and 48 h after application of the iodothyronines.
Raised serum levels of TSH, due to the treatment with PTU, were significantly reduced within 3 h of treatment with 30 nm-T3, 300 nm-T4, 3 or 30 nm-T2iPr and they remained low throughout the observation period. Treatment with 3 nm-T3 or 30 nm-T4 per kg body weight was less effective. Pituitary concentrations of growth hormone, TSH, prolactin and FSH were significantly reduced by the treatment with PTU. There was also a slight, but insignificant reduction of pituitary concentrations of LH. Treatment with T3, T4 or T2iPr stimulated the reaccumulation of growth hormone, TSH, prolactin, LH and FSH in the pituitary gland.
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Search for other papers by URSULA DÖHLER in
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Abteilung für Klinische Endokrinologie, Department für Innere Medizin und Zentrales Tierlabor, Medizinische Hochschule, 3000 Hannover 61, Federal Republic of Germany
(Received 8 May 1978)
The application of a blood sampling technique which by itself has little or no influence on the concentrations of hormones in the circulation of the rat is of utmost importance in experimental hormone research. It has recently been demonstrated that the type of anaesthesia and probably also the site of withdrawal of blood from the body may become critical parameters when the determination of unaltered hormone levels is required (Döhler, von zur Mühlen, Gärtner & Döhler, 1977b). That the site of blood sampling may act as an interfering parameter is questionable, since cage transport, handling and possible differences in the intensity of anaesthesia may also influence the variability of individual hormone levels (Baldwin, Colombo & Sawyer, 1974; Krulich, Hefco, Illner & Read, 1974; Euker, Meites &
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Department of Physiology, Faculty of Medicine, University of Hong Kong, Li Shu Fan Building, Sassoon Road, Hong Kong
(Received 22 November 1977)
The rat testis secretes fluid (Tuck, Setchell, Waites & Young, 1970; Cheung, Hwang & Wong, 1977) which is reabsorbed by the epididymis (Crabo & Gustafsson, 1964; Levine & Marsh, 1971). In the cauda epididymidis, sodium chloride is reabsorbed isotonically with water and potassium is secreted into the ductal lumen (Wong & Yeung, 1977, 1978); these transport processes seem to have many characteristics typical of the processes occurring in the distal tubule of the kidney. Apart from electrolyte and water transport, proteins are also secreted into the ductal lumen. The epididymal epithelium actively maintains a definite milieu within the tubule in which the spermatozoa are maturing. In several transporting epithelia such as those of the toad bladder, frog skin, salivary and sweat glands, intestine and renal tubule (for references,
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Abstract
Sex hormone binding globulin (SHBG) is a homodimeric plasma protein found in mammals that binds sex steroids with high affinity and regulates their bioavailability. The protein is identical in structure and properties to the androgen binding protein (ABP) found in the male reproductive tract. We have isolated a 1245-base pair rabbit SHBG cDNA encoding a reading frame for a signal peptide followed by a protein of 367 amino acids, which shares 79·0, 68·1 and 63·2% amino acid identity with the corresponding human, rat and mouse proteins respectively. Northern blot and hot-nested PCR analyses indicated that rabbit SHBG is produced from a 1·6 kilobase mRNA in the liver of both sexes and in the testis. The rabbit SHBG cDNA was inserted into pGEX-1λT for expression of a glutathione S-transferase/SHBG fusion protein in Escherichia coli. The bacterial product bound 5α-dihydrotestosterone (DHT) in the same manner as the corresponding protein in serum. The dissociation constants (Kd) for rabbit and human SHBGs produced in E. coli were 11·1 ± 1·1 nm and 2·1 ± 0·6 nm respectively, and rabbit SHBG formed a less stable protein-steroid complex (t½=5 min) than human SHBG (t½>60 min). Unlike human SHBG, rabbit SHBG does not bind estradiol with high affinity. To aid in the identification of differences in the sequences of rabbit and human SHBG, which determine species differences in steroid-binding affinity and specificity, chimeras containing the 5′-terminal half of SHBG from one species and 3′-terminal half of SHBG from the other species were constructed and expressed. It was found that the chimeric proteins assumed similar steroid-binding affinity and specificity as the wild-type proteins when the amino (N)-terminal half of SHBG was derived from the same species. Replacement of the carboxyl (C)-terminal half of rabbit SHBG by the corresponding region of the human molecule increased the integrity of its steroid-protein complex. This supports the concept that amino acids within the N-terminal half of SHBG constitute the steroid-binding domain while the C-terminal half of the molecule may provide structural stability to the protein and its steroid-binding site.
Journal of Endocrinology (1997) 153, 373–384
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Seasonal effects were studied on basal levels of hormones in the serum of adult male Sprague–Dawley rats, which were born and raised under rigorously controlled laboratory conditions. Groups of 90-day-old rats were killed at monthly intervals by rapid decapitation. Significant fluctuations were observed throughout the observation period of 19 months in serum levels of TSH, prolactin, androgens, tri-iodothyronine and LH. Minor fluctuations were observed in serum levels of FSH, corticosterone, parathyroid hormone and thyroxine. The results indicate that male laboratory rats exhibit circannual and semi-annual fluctuations in serum levels of several hormones even though the animals were born, raised and maintained in constant laboratory conditions.
