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series of in vivo and in vitro studies, we have shown that the activation of prolactin receptors by lactogenic hormones, either placental lactogen or prolactin, induces all of the known up-regulatory changes in islet function, which are known to be
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Department of Physiology, Middlesex Hospital Medical School, London W1P 6DB, and *Institut für Physiologie, Technische Universität München-Weihenstephan, West Germany
(Received 6 August 1974)
The release of both prolactin and oxytocin during lactation and machine milking in the cow is well established. Benson & Folley (1956) first suggested that oxytocin might initiate the release of prolactin. Bryant & Greenwood (1968), using a specific immunoassay for prolactin, were able to demonstrate this relationship. However, subsequent workers (Koprowski & Tucker, 1971; Schams, 1972) using exogenous oxytocin were unable to confirm these observations.
Experiments involving the use of exogenous oxytocin are inconclusive as it may not be possible to produce appropriate concentrations of oxytocin in the portal vessels to the hypophysis. Observations, therefore, were made on plasma samples taken frequently during milking and teat stimulation to elucidate the relationship between oxytocin and prolactin release. Release of the second neurohypophysial hormone, arginine-vasopressin (AVP), was also
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SUMMARY
Prolactin was isolated from frozen goat pituitary glands by a simple procedure involving gel filtration and chromatography on DEAE-cellulose. The major product (yield, 2·5 mg/g pituitary tissue) had high pigeon crop sac-stimulating activity (27 i.u./mg) and was free of growth hormone and other pituitary hormones. The molecular weight was similar to that of ovine prolactin.
Caprine prolactin was immunologically indistinguishable from ovine prolactin in radioimmunoassays, in which ovine prolactin antiserum and either ovine or caprine prolactin labelled with 125I were used.
The results indicate that caprine and ovine prolactin are closely related and that radioimmunoassay for ovine prolactin may be used to estimate caprine prolactin in serum.
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SUMMARY
A solid phase-antibody system is described for the radioimmunoassay of ovine prolactin. The sensitivity of the method is equal to 0·5 ng of prolactin/ml. Prolactin was measured in the serum of pregnant and lactating ewes; the levels increased at 5 weeks before parturition and then decreased during the following 3 weeks. A second peak of prolactin was observed at parturition and then the levels gradually decreased during the first 3 weeks post partum.
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Introduction Regulation of prolactin secretion from the anterior pituitary in lactating females is not fully recognized. It is accepted that the hypothalamic neuroendocrine dopaminergic (NEDA) system exerts a dominant and inhibitory effect on
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ABSTRACT
The intracellular localization of different molecular forms of prolactin was studied in various experimental models covering a wide range of secretory states.
By correlating electron microscopy, morphometry and quantification of monomeric (small) and polymeric (big) prolactin after differential extraction procedures, big prolactin was found stored in secretory granules while small prolactin was loosely associated with all organelles involved in hormone synthesis and processing. No correlation with levels of lactotrophic secretory activity was detected by either the number of secretory granules or prolactin content in lactotrophs.
J. Endocr. (1985) 105, 219–225
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Specific binding of 125I-labelled ovine prolactin iodinated by a lactoperoxidase method was demonstrated in crude membrane preparations of kidneys and adrenals of male Sprague–Dawley rats and livers from female rats. Membrane preparations derived from the 100 000 g fractions of tissue homogenates contained most of the specific prolactin binding. Kinetic and affinity characteristics of prolactin binding to kidney membranes were examined in detail. Maximal specific binding occurred after incubation for 30 h at room temperature. Scatchard analysis indicated that prolactin binding to kidney membranes was of high affinity (dissociation constant = 1·4 × 10−10 mol/l) and similar to that for liver membranes, although kidney membranes from male rats bound approximately sixfold less prolactin/mg membrane protein than did liver membranes from female rats. Specific prolactin binding was demonstrated in both renal medulla and cortex. Autoradiography showed maximal prolactin binding activity in the epithelial cells of the proximal tubule and faint activity in the tubular cells throughout the nephron. Specificity of uptake by proximal tubular cells was indicated by the gross reduction in prolactin activity when excess ovine prolactin was administered simultaneously. The demonstration of specific binding sites for prolactin localized primarily in the proximal tubules was consistent with renal action of prolactin, predominantly on sodium metabolism.
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Abstract
125I-Labelled ovine prolactin was infused for 15 min into a pudic artery supplying one mammary gland of lactating goats (n=17). Between 0 and 4·25 h significandy more total (P<0·01) and trichloroacetic acid (TCA)-precipitable (P<0·001) radioactivity appeared in the milk of the infused compared with the non-infused gland. Gel chromatography and antibody precipitation indicated the presence of undegraded 125I-labelled prolactin in milk whey. Maximum transfer occurred 60–80 min after the end of infusion suggesting passage via a transcellular route. High plasma prolactin concentrations, resulting from infusion of cold prolactin with labelled prolactin in late lactation or from seasonally elevated prolactin at peak lactation, reduced the specific activity of infused prolactin and depressed the difference in secretion of 125I-labelled prolactin into milk of infused and non-infused glands. This suggests the operation of a competitive and saturable mechanism. Together with the increase in the milk to blood ratio of prolactin in goats given long-term (3 week) bromocriptine treatment, the results suggest that the goat mammary gland has a high avidity for prolactin especially when circulating prolactin is low. There was also evidence from TCA precipitation that prolactin may be protected from degradation in these circumstances. These mechanisms may contribute to the resistance of ruminant lactation to reduction in plasma prolactin and protect lactation from seasonal prolactin fluctuations.
Journal of Endocrinology (1995) 146, 411–420
Department of Endocrinology, Kobe City General Hospital, Kobe 650-0046, Japan
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Department of Endocrinology, Kobe City General Hospital, Kobe 650-0046, Japan
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Department of Endocrinology, Kobe City General Hospital, Kobe 650-0046, Japan
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Department of Endocrinology, Kobe City General Hospital, Kobe 650-0046, Japan
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Department of Endocrinology, Kobe City General Hospital, Kobe 650-0046, Japan
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Department of Endocrinology, Kobe City General Hospital, Kobe 650-0046, Japan
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of hyperprolactinemia in which serum prolactin (PRL) consists mainly of big–big PRL (macroprolactin) with a molecular mass greater than 100 kDa, in contrast to the predominant 23-kDa monomeric PRL as seen in other hyperprolactinemic states ( Sinha
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Prolactin is the hormone principally involved with stimulating milk production. Therefore, at first sight, a role for prolactin in controlling gonadotrophin secretion may seem strange, especially as its involvement at the ovarian level has only been irrevocably established in rodents, particularly the rat (see McNeilly, 1984). However, in almost all species studied so far, the high levels of prolactin induced by suckling and essential for lactation are also associated with a reduction in normal gonadotrophin secretion, principally the pulsatile secretion of luteinizing hormone (LH), and as a consequence, a suppression of ovarian activity. A similar association between increased levels of prolactin and a reduction in both LH and follicle-stimulating hormone (FSH) occurs during seasonal infertility in male and female ungulates and in pathological hyperprolactinaemia in both men and women.
In view of this apparent relationship between increased prolactin and decreased gonadotrophin secretion, a specific role for prolactin itself in suppressing
