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Search for other papers by S. V. SMITH in
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The response of. prolactin receptor and lactose synthetase to suppression of plasma concentrations of prolactin was examined in normal and occluded (teat-sealed) mammary glands of Sprague–Dawley rats. Rats, with mammary glands unilaterally occluded, were given bromocriptine (2·5 mg/kg per 12 h) between days 5 and 8 post partum. Bromocriptine reduced plasma prolactin concentrations from 460·4±120·8 (mean ±s.e.m.) to 2·56 ± 0·89 ng/ml within 12 h whilst concentrations in control rats were 553·4± 110·25 ng/ml. Lactose synthetase activity declined rapidly, within 24 h, in occluded glands of both groups but was maintained for 24 h in normal glands of bromocriptine-treated rats and decreased thereafter. Prolactin receptors also declined significantly within 24 h in occluded glands. Desaturation of the prolactin receptor by bromocriptine treatment in vivo was compared with desaturation by exposure of membranes to MgCl2 in vitro. Both treatments enhanced prolactin binding but the increase after treatment with MgCl2 may have been partly artefactual since there was a selective loss of protein from the membranes. These results indicate that the prolactin receptor in rat mammary gland may be maintained after acute suppression of prolactin secretion.
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The possible role of K+ as a 'second messenger' for prolactin was investigated. Explants obtained from mammary glands of pseudopregnant rabbits were cultured (a) in media with varying potassium concentrations and (b) in the presence of the potassium-specific ionophore, nigericin. The explants were examined histologically and the rate of protein synthesis was measured to determine the viability of the tissue. Increase in the rate of fatty acid synthesis in the presence of prolactin was used as a marker of prolactin stimulation.
At low K+ concentrations prolactin-stimulated fatty acid synthesis decreased with the decrease in K+ concentration of the media, whereas there was no similar trend in the rate of protein synthesis. Nigericin (0·01–0·1 μmol/l) inhibited fatty acid synthesis and protein synthesis in the presence or absence of prolactin without significantly affecting tissue K+ content. The mechanism of nigericin inhibition is hence unclear and the results do not provide support for K+ as a 'second messenger' for prolactin.
Search for other papers by A. V. Capuco in
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Search for other papers by J. J. Smith in
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ABSTRACT
The effect of administration of bovine somatotrophin (bST) on peripheral conversion of thyroxine (T4) to tri-iodothyronine (T3) was studied in non-pregnant lactating Holstein cows. Six cows were injected daily for 5 days with 40 mg recombinantly derived bST, while six control cows received excipient alone. Blood samples were collected hourly from 08.00 to 19.00 h on a single day the week before treatment and on days 4–5 of treatment. All other tissue samples were obtained at slaughter, 20–23 h after the last injection.
Administration of bST increased milk production and caused a 9% increase in hepatic DNA. Consumption of feed did not differ between control and bST-treated cows. Treatment did not alter serum concentrations of T4 or T3, although concentrations of thyroid hormones in the serum increased from 08.00 to 19.00 h. Activity of thyroxine-5′-monodeiodinase (5′-D) in liver and kidney was similarly unaffected. However, activity of 5′-D in mammary tissue increased approximately twofold in response to bST administration. We suggest that an increase in mammary conversion of T4 to the more biologically potent thyroid hormone T3 plays a role in mediating the galactopoietic response of dairy cattle to bST.
Journal of Endocrinology (1989) 121, 205–211
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Search for other papers by I. A. Forsyth in
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Search for other papers by B. T. Donovan in
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A radioimmunoassay for canine prolactin has been used to measure prolactin in the ferret. Serial dilutions of extracts of ferret pituitary glands and of ferret plasma yielded curves that were parallel with the canine prolactin standard curve. The sensitivity, accuracy, reproducibility and precision of the assay were within acceptable limits. Plasma prolactin levels increased after the administration of thyrotrophin releasing hormone (TRH) or chlorpromazine, but not after giving luteinizing hormone releasing hormone. Female ferrets, which were anoestrous, oestrous or spayed, and male ferrets had similar basal prolactin levels when sampled under sodium pentobarbitone anaesthesia. These basal levels were higher than in conscious males and the latter also showed a lesser response to TRH. Hypophysectomy significantly reduced basal prolactin levels in female ferrets by 2 h postoperatively and abolished the response to TRH.
Search for other papers by G. Thordarson in
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Search for other papers by G. H. McDowell in
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ABSTRACT
Continuous intravenous infusions of saline or of a placental extract containing ovine placental lactogen were given to three non-pregnant, non-lactating ewes over periods of 36 h, 1 week apart. During saline infusion no placental lactogen was detected in jugular vein plasma, but infusion of the placental extract raised the placental lactogen concentration from undetectable to 40-50 μg/l, similar to concentrations in ewes with one fetus on day 90 of pregnancy. By comparison with the saline control period, infusion of the placental extract consistently increased both plasma concentrations and irreversible loss of non-esterified fatty acids. Plasma concentrations of glucose and urea, but not irreversible loss of these metabolites, were consistently increased. Although the placental extract was not subjected to extensive purification, it was enriched in placental lactogen and contained no detectable contamination with insulin, prolactin or growth hormone. The results are suggestive of a role for placental lactogen in modifying metabolism and acting during pregnancy to provide nutrients for fetal metabolism.
J. Endocr. (1987) 113, 277–283
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Search for other papers by V. Smith in
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ABSTRACT
Thyrotrophin-releasing hormone (TRH) occurs in high concentrations in the rat ventral prostate and its concentration is regulated in a positive dose–response manner by testosterone in castrated rats. α-Amidation of the tetrapeptide precursor, TRH-Gly, is a rate-limiting step in TRH biosynthesis. To investigate further the hormonal regulation of TRH biosynthesis in prostatic tissue, Sprague–Dawley rats of approximately 250 g were injected s.c. with either physiological saline or 3 mg propylthiouracil (PTU) daily for 5 days. The reproductive tissues were boiled in acetic acid (1 mol/l), dried and extracted with methanol. The methanol extracts were measured for TRH immunoreactivity (TRH-IR) and TRH-Gly-IR by radioimmunoassay. Hypothyroidism induced by PTU significantly increased TRH-IR and TRH-Gly-IR levels in prostate and testis and reduced these levels in epididymis but did not affect the serum concentrations of testosterone compared with those of controls. Corresponding changes in TRH and TRH-Gly in the rat prostate were established by high-pressure liquid chromatography. To control for possible pharmacological effects of PTU on TRH biosynthesis, additional experiments were carried out on castrated rats receiving testosterone replacement and treatment with PTU plus methimazole. Treatment with thyroxine (T4) significantly reduced the increase in prostatic TRH levels due to hypothyroidism, despite the drug-induced blockade of the conversion of T4 to tri-iodothyronine. These effects parallel similar observations made in rat spinal cord and pancreas. This study demonstrates that in the male rat reproductive system the levels of TRH and its immediate biosynthetic precursor, TRH-Gly, are regulated by thyroid hormones.
J. Endocr. (1987) 114, 271–277
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ABSTRACT
Developmental variation in the expression of the prolactin receptor in the ruminant mammary gland was investigated. Affinity chromatography revealed that bovine prolactin and human GH each bound to the same mammary gland proteins, yielding fractions enriched in binding activity and a protein of M r 36 000, assumed to be a bovine prolactin receptor. Affinity cross-linking of 125I-labelled human GH to mammary microsomes confirmed that the M r 36 000 protein was a bovine prolactin receptor. Binding assays of receptors in microsomes from the mammary tissue of cows and ewes at various stages of the lactational/reproductive cycle indicated developmental regulation of receptor concentration, but not receptor type, as no other bovine prolactin receptor type was detected by affinity cross-linking. These results suggest that differences in the response to prolactin in the mammary gland at various developmental stages in ruminants are not due to the expression of different forms of the prolactin receptor, and the lack of a prolactin effect on established lactation in ruminants is not due to the absence of the M r 36 000 form of the prolactin receptor.
Journal of Endocrinology (1993) 139, 37–49
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Search for other papers by A. F. SMITH in
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Perifusion experiments were performed to study the stimulatory effects of luteinizing hormone releasing hormone (LH-RH) on the release of LH from anterior pituitary tissue. Exposure of pituitary tissue from normal male rats to LH-RH (5 ng/ml for 5 min) induced a small release of LH; in tissue from ovariectomized rats receiving no pretreatment, the release was more than three times greater and in tissue from gonadectomized male or female rats pretreated with oestradiol benzoate and progesterone, the release was six times greater than that observed in normal rats. Further exposure of pituitary tissue from gonadectomized steroid-pretreated male and female rats to LH-RH (5 ng/ml) induced an increase in the level of LH even greater than that seen after the initial exposure (priming action of LH-RH); in tissue from ovariectomized rats receiving no pretreatment, less LH was released than after the first exposure to LH-RH and in tissue from normal male rats the response was unchanged.
Search for other papers by P. J. D. DAWES in
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Search for other papers by V. B. PETERSEN in
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Search for other papers by B. REES SMITH in
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Thyrotrophin (TSH) receptors have been extracted from human and porcine thyroid membranes by treatment with Triton X-100.125I-Labelled bovine TSH was used to monitor receptor activity. Analysis by gel filtration and electrophoresis on acrylamide gels containing sodium dodecyl sulphate suggested that Triton extracts of human thyroid membranes contained TSH receptors with a molecular weight in the region of 50 000 closely associated with Triton micelles of approximate molecular weight 300 000. Isoelectric focusing studies indicated that the Triton-solubilized TSH binding activity had an isoelectric point of pH 4–4·5. The soluble TSH receptors were heat-labile, showed optimum TSH binding at pH 7·4 and reduced hormone binding at high ionic strength. The TSH binding characteristics of membrane-bound and solubilized human TSH receptors were similar and both preparations gave curved Scatchard plots.
Solubilized porcine TSH receptors appeared to have a similar molecular weight to the human receptors and were also closely associated with Triton micelles of approximate molecular weight 300 000. Scatchard analysis of TSH binding to membrane-bound or solubilized porcine TSH receptors gave approximately linear plots with association constants of 2·8 ± 0·95 (s.e.m.) × 109 and 1·7 ± 0·27 × 1091/mol respectively. Comparison of the binding capacities of the solubilized and membrane-bound porcine receptors indicated that the 0·5% Triton extracts contained 40% of the original TSH binding activity and that this was present at a concentration of 25 ng/ml.
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Search for other papers by GWYNETH A. PYLE in
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The relationship between the binding of thyrotrophin (TSH) to receptors and the production of cyclic AMP has been investigated using crude human thyroid membranes. Receptor binding was studied by use of 125I-labelled bovine TSH, which had been purified by absorption to and elution from human thyroid membranes. This 125I-labelled material showed the same biological activity as the unlabelled hormone. Binding studies at equilibrium indicated that the association constant of the TSH–membrane interaction decreased as the amount of TSH bound increased. Kinetic data did not provide definite evidence that this was due to an effect of increasing receptor occupancy on the dissociation rate constant. Detectable stimulation by TSH of cyclic AMP production by the membranes was observed with as little as 30 μu. (1 ng) hormone. Increasing amounts of TSH over the range 0–250 μu. caused increases in the production of cyclic AMP, proportional to the amount of hormone bound. With larger amounts of TSH, increasingly greater amounts of bound hormone were required to give corresponding increases in cyclic AMP formation, and addition of TSH in amounts greater than 16 mu. resulted in progressive inhibition of cyclic AMP formation. Kinetic studies indicated that receptor binding was not rate-limiting in the stimulation of cyclic AMP production by TSH.
