An investigation of sites that bind human somatotropin (growth hormone) in the liver of the pregnant rabbit.

The binding of 125I-labelled human somatotropin (growth hormone) to a crude membrane preparation from the liver of pregnant rabbit, and to receptors solubilized from this fraction by Triton X-100, was dependent on time, temperature and receptor concentration. At 4 degrees C a steady state was reached after 20 h, and maximum specific binding (as a percentage of total tracer added) was approx. 50% for both membrane-bound and solubilized receptors. Solubilization did not significantly affect the binding properties of the receptor at low concentrations of Triton X-100 (less than 0.05%, v/v, in the assay tube). However, at higher concentrations (approx. 0.1%, v/v), the detergent lowered the ability of some hormones, for example ovine prolactin, to displace 125I-labelled human somatotropin, but did not affect other hormones such as bovine somatotropin. Some somatogenic hormones, such as bovine somatotropin, and some lactogenic hormones, such as ovine prolactin, displaced 125I-labelled human somatotropin from membrane-bound and solubilized receptor preparations. Furthermore, 85% of 125I-labelled bovine somatotropin was displaced from membrane-bound receptors by ovine prolactin, and 125I-labelled ovine prolactin was almost completely displaced by bovine somatotropin. Scatchard analysis of the binding data for human somatotropin suggested a single class of binding sites in the membrane-bound receptor preparation, with an affinity (Ka) of 1.9 X 10(9) M-1 and a capacity of 1726 fmol/mg of protein; these values were slightly increased by solubilization (Ka = 3.2 X 10(9) M-1, capacity = 2103 fmol/mg of protein). Scatchard analysis of binding to membrane-bound receptors also indicated a single class of high-affinity binding sites for bovine somatotropin (Ka = 4.8 X 10(9) M-1, capacity = 769 fmol/mg) and for ovine prolactin (Ka = 6.1 X 10(9) M-1, capacity = 187 fmol/mg).     

Effects of photoperiod, hypophysectomy, and follicle-stimulating hormone on testicular follicle-stimulating hormone binding sites in golden hamsters

Experiments were conducted to partially characterize and to examine the regulation of unoccupied testicular follicle-stimulating hormone (FSH) binding sites in adult golden hamsters. Testicular FSH binding sites were measured in the 1800 X gav fraction of whole testicular homogenates using iodinated bovine FSH. Binding of FSH was highly specific for FSH, located primarily in the testes, was time- and temperature-dependent, initially reversible, saturable, and consistent with a model consisting of a single class of high-affinity binding sites (range of equilibrium association constants (Ka) 2-12 X 10(10) M-1). Exposure of hamsters to a short photoperiod consisting of 5L:19D was associated with an increase in concentration (fmol/mg protein), but a reduction in total content (fmol/testes) of testicular FSH binding sites. There was no appreciable 5L:19D-associated alteration in receptor affinity (average Ka = 7.83 X 10(10) M-1). Injections of ovine prolactin (oPRL), ovine luteinizing hormone (oLH), or ovine FSH (oFSH) for 3 days into hamsters housed in 5L:19D for 12 wk had no effect on photoperiod-induced changes in testicular FSH binding sites. On Days 5 and 6 post hypophysectomy, a dramatic increase in FSH binding site concentration occurred, with but marginal effects on binding site affinity. Injections of 5 micrograms oFSH on Days 2, 3, and 4 after hypophysectomy prevented the increase in binding site concentrations measured on Day 5. Injection of a combination of 5 micrograms oFSH, 50 micrograms oPRL, and 25 micrograms oLH also reduced testicular FSH binding site concentrations in hypophysectomized hamsters, but oPRL or oLH by themselves were ineffective. The data indicate a homologous down-regulation of testicular FSH binding sites, but do not exclude the involvement of other hormones.     

Quantitative autoradiographic determination of angiotensin-converting enzyme binding in rat pituitary and adrenal glands with 125I-351A, a specific inhibitor

We describe a quantitative autoradiographic technique which allows measurement of angiotensin-I-converting enzyme [ACE] (kininase II, peptidyldipeptide hydrolase, EC 3.4.15.1) levels in discrete areas of pituitary and adrenal glands in individual animals. Tissue sections were incubated with 125I-351A, a specific ACE inhibitor, and results were obtained with computerized densitometry and comparison to 125I standards. There were high levels of ACE in both the anterior and posterior lobes of the pituitary, with no detectable binding in the intermediate lobe. The maximum binding capacity (Bmax) was 920 +/- 62 fmol/mg protein for the anterior pituitary and 1162 +/- 67 fmol/mg protein for posterior pituitary. The binding affinity constant (Ka) was 0.95 +/- 0.11 X 10(9) M-1 and 1.20 +/- 0.19 X 10(9) M-1 for the anterior and posterior lobes, respectively. In the adrenal gland, there were two distinct areas of specific binding, the adrenal medulla and the adrenal capsule-zona glomerulosa area. The Bmax for the adrenal medulla was 652 +/- 80 fmol/mg protein and 294 +/- 53 fmol/mg protein for the adrenal capsule-zona glomerulosa. The Ka for 351A was 1.04 +/- 0.19 X 10(9) M-1 and 1.74 +/- 0.40 X 10(9) M-1 for medulla and adrenal capsule-zona glomerulosa respectively. The results support the existence of local ANG systems active in both the pituitary and adrenal glands.     

Prolactin receptors in the hypoprolactinemic male rat (IPL nude rat): measurement in testis and induction in liver

In order to evaluate the importance of PRL in the regulation of its own receptors, characteristics of specific binding for PRL were studied in membrane preparations from liver and testis of a new hypoprolactinemic male rat, the IPL nude male rat, and this was compared to those found for normal male rats. Under basal conditions, hepatic specific binding of PRL in IPL nude rats, as in normal rats was not detectable. Following castration, it became detectable in both groups, and was 6.99 +/- 0.78% and 6.34 +/- 0.87% for IPL nude and normal rats respectively. Under such conditions, the apparent affinity constant (Ka) and the binding capacity (Nmax) obtained were also similar for both groups (Ka) = 1.36 +/- 0.14.10(9) M-1, Nmax = 102 +/- 14 fmol/mg protein in IPL and Ka = 1.34 +/- 0.28.10(9) M-1, Nmax = 97 +/- 11 fmol/mg protein in normal rats) although a decrease in serum levels of PRL was observed in both groups. This decrease was greater for IPL nude rats. As already reported, estradiol injection following castration was able to further increase the percentage of PRL hepatic specific binding (4 times). Furthermore, our results demonstrated that the affinity constant was significantly increased by estradiol injection in both groups. On the other hand, for testicular PRL binding characteristics, a statistically significant difference was found between IPL nude and normal rats. The PRL specific binding percentage was 7.01 +/- 0.85% for the IPL nude rat and 10.07 +/- 0.64% for the normal rat. By Scatchard analysis, the Ka of testicular membranes for labelled oPRL was similar in both groups, while the capacity differed (Nmax = 9.82 +/- 1.25 fmol/mg protein for IPL nude rat and Nmax = 26.06 +/- 4.39 fmol/mg protein for normal rats). These data established the fact that IPL nude male rats presented characteristics of hepatic PRL receptors similar to those of normal rats, while their testicular oPRL binding significantly differed. These findings therefore suggest that in genetic hypoprolactinemic rats (IPL nude rats), PRL might be more involved in the regulation of testicular PRL receptors than in that of hepatic receptors.     

Radioreceptor assay for lactogenic hormones based on membranes from rat mammary tumour

A highly sensitive radioreceptor assay (RRA) for human prolactin (hPRL) based on membrane preparations obtained from chemically induced rat mammary tumour is described. The binding of 125I-labelled, highly purified pituitary human prolactin was specific for lactogenic hormones and depending on time, temperature, and concentration of receptor protein. Optimal specific receptor binding (18-20%) was obtained by incubation at 21 degrees C for 18 h. The prolactin receptor was shown to have a single "class" of binding sites with an affinity constant (Ka) of 6.0 X 10(10) mol-1. The binding capacity was 8-33 fmol/mg membrane protein. The sensitivity of the radioreceptor assay was 0.5 ng/ml ovine prolactin (NIH-PS-10) or 0.84 ng/ml human prolactin (NIH-VLS-4). The receptor binding activity of various purified prolactin preparations from different species was comparable to the biological hormone activities, indicating that this in vitro assay system measures values which are biologically relevant.     

Prolactin binding in the developing rat fetal liver

The binding of prolactin by fetal rat liver cell membrane fractions from 17 to 21 days gestation was studied. Particulate liver membranes were prepared in Dulbecco's Phosphate Buffered Saline (PBS) by ultracentrifugation and incubated at 22 degrees C for 16 hours with [125I] iodo-human growth hormone (hGH). Non-specific binding was assessed by parallel incubations in the presence of a 2000-fold excess ovine prolactin. Specific prolactin binding sites were detected only at 21 days gestation (2932 +/- 401 cpm/mg protein) in freshly prepared membranes. On freezing at -20 degrees C for 24 to 48 hours, the membranes of 20 days gestation animals were able to specifically bind prolactin (1295 +/- 239 cpm/mg protein). Freezing led to a 45 +/- 7% increase (4270 +/- 701 cpm/mg protein) in prolactin binding at 21 days gestation. No hormonal binding was detected from 17 through 19 days gestation in either fresh or freeze-thawed membranes. Scatchard analysis revealed a high affinity binding site with a Ka of approximately 1.4 X 10(8)M-1 in both fresh and freeze-thawed membrane preparations. The data show that 1) prolactin receptors appear in liver only during late fetal life and that 2) freezing of membranes may unmask binding sites that are initially unavailable to specifically bind prolactin.     

Prolactin receptor expression in monolayer cultures of rabbit mammary epithelial cells: pre- and postpartum [125I]-prolactin binding activity

Expression of specific [125I]-prolactin-binding sites under culture conditions has been investigated for isolated mammary epithelial cells from virgin, pregnant, and lactating rabbits. Primary monolayer cultures were obtained by sequential enzymatic dispersion of mammary tissue followed by 48 hr incubation in a medium selective for epithelial cells. Scatchard analyses of binding data obtained from these cultures indicated a single class of receptor sites, the affinity constant of which (2.5 X 10(9) M-1) did not vary significantly during mammary development. The number of prolactin receptors, however, expressed by virgin and early pregnant epithelial cells was significantly increased over those from late pregnancy or lactation. Less differentiated cells also respond to growth in pregnant rabbit serum with an increase in specific [125I]-prolactin binding. The diminished receptor expression by cells obtained after 17 days of pregnancy coincides with the attainment of secretory capacity in the animal, and may reflect the influence of the low serum prolactin or high progesterone levels circulating during the last trimester in the rabbit, or be the cultural expression of secretory differentiation.     

Effects of the intensity of the suckling stimulus and ovarian steroids on pituitary gonadotropin-releasing hormone receptors during lactation

These studies examined whether the decrease in pituitary responsiveness to gonadotropin-releasing hormone (GnRH) observed during lactation in the rat results from a change in pituitary GnRH receptors. GnRH binding capacity was determined by saturation analysis using D-Ala6 as both ligand and tracer. During the estrous cycle, the number of GnRH binding sites increased from 199 +/- 38 fmol/mg protein on estrus to 527 +/- 31 fmol/mg protein on the morning of proestrus, whereas there was no change in receptor affinity (Ka, 6-10 X 10(9) M-1), During lactation, females nursing 8 pups on Days 5 or 10 postpartum had 50% fewer GnRH receptors (109-120 fmol/mg protein) than observed during estrus or diestrus 1 (199-242 fmol/mg protein) although receptor affinity was similar among all the groups. No deficits in pituitary GnRH receptors were observed in females nursing 2 pups on Day 10 postpartum. Removal of the 8-pup suckling stimulus for 24 or 48 h resulted in a dramatic increase in GnRH receptor capacity by 24 h from 120 +/- 16 to 355 +/- 39 fmol/mg protein. The rise in GnRH receptors after pup removal was accompanied by an increase in serum luteinizing hormone (LH) and estradiol concentrations. To assess the role of ovarian steroids in determining GnRH receptor capacity during lactation, females were ovariectomized (OVX) on Day 2 postpartum. Suckling of a large litter (8 pups) completely blocked the postcastration rise in serum LH and in pituitary GnRH receptors on Day 10 postpartum (OVX+ 8, 77 +/- 12 fmol/mg protein; OVX+ 0, 442 +/- 38 fmol/mg protein).(ABSTRACT TRUNCATED AT 250 WORDS)     

Prolactin binding to dissociated cells from rat mammary tumors and mammary gland.

Prolactin binding activity was studied in suspensions of cells which had been enzymatically dissociated from R3230AC mammary tumors, 7,12-dimethylbenz(a)anthracene (DMBA)-induced mammary tumors and lactating rat mammary glands. Prolactin bound specifically with high affinity (apparent binding affinity = 4.0 X 10(9) M-1) to R3230AC tumor cells. Hormone binding at room temperature was proportional to cell number and increased with time of incubation up to 120-180 min. Prolactin binding to R3230AC tumor cells from diabetic animals was reduced by about 50%. Specific prolactin binding activity was also demonstrated in preparations of cells from DMBA-induced tumors and lactating mammary gland. The levels of hormone binding in both dissociated cells and subcellular particles prepared from these tissues varied as follows: DMBA-induced tumors > lactating mammary gland > R3230AC mammary adenocarcinoma.     

Solubilization, gel filtration and sedimentation behaviour of prolactin receptors from human ovarian tissue

Receptors for 125I-labelled human prolactin have been identified in the crude membrane fraction isolated from human ovarian tissue. The non-ionic detergent Triton X-100, has been used to solubilize the membrane fraction. The presence of the receptor in the detergent extract was demonstrated by gel filtration and sucrose density gradient centrifugation. The binding was time-temperature dependent, being maximal at 23 degrees C after 15 h of incubation. Large amounts of other peptide hormones did not inhibit the binding of 125I-labelled human prolactin. The binding Scatchard analysis demonstrated that the affinity of the soluble receptor (Ka 1.13 +/- 0.15 X 10(10) M-1) for the labelled hormone was slightly greater than that of the crude membrane fraction (Ka 0.91 +/- 0.12 X 10(10) M-1). The binding capacity of the solubilized receptor was also significantly greater than that seen in the particulate before solubilization. The apparent Stokes radius of the solubilized receptor was estimated to be 57 A and that the hormone-receptor complex 60 A. The sedimentation coefficient of the solubilized receptor was 7.0 +/- 0.1 s, whereas that of the hormone-receptor complex was 7.5 +/- 0.2 s.     

The porcine LH/hCG receptor. Characterization and purification

Porcine luteal LH/hCG receptor (LH/hCG R) was solubilized with 70-80% recovery from the crude plasma membrane fraction by Triton X-100 in the presence of 25% glycerol and protease inhibitors. The solubilized receptor maintained 90% of original activity at -60 degrees C for 90 days. Equilibrium association constant (Ka) values of 1.92, 2.22, and 2.03 X 10(10) M-1 were observed for the whole homogenate, plasma membrane fraction, and solubilized LH/hCG R preparations, respectively. The specific binding capacity for the same fractions were 49, 70, 55 fmol/mg protein, respectively. Complexes of LH/hCG R and Triton X-100 were resolved into two components with approximate Mr = 2.7 X 10(5) and 5.4 X 10(5) by gel filtration on Sepharose 6B and two glycoprotein components by chromatography on concanavalin A-Sepharose. Solubilized porcine LH/hCG R was purified by two cycles of affinity chromatography on highly purified hCG-Sepharose with an overall recovery of 30-35% of the initial activity in the Triton extract. Purified porcine LH/hCG R had a specific binding capacity of 2300 pmol/mg protein and a Ka = 1.5 X 10(10) M-1. Silver staining of sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels demonstrated that the major protein in porcine LH/hCG R preparations has Mr = 68,000. A weakly staining band at Mr = 45,000 was also observed in the purified receptor preparation. Analysis of iodinated purified LH/hCG R by autoradiography has confirmed these results. Porcine LH/hCG R was purified 40,000-fold by this method.     

A receptor site for prolactin in lactating mouse mammary tissues.

Prolactin iodinated by lactoperoxidase method showed immunologically, electrophoretically and biolo9gically similar properties to native prolactin and possessed enough specific radioactivity for receptor studies. 1251-prolactin was incubated with mouse mammary tissues at 8 days of lactation. Both binding and release of 1251-prolactin depended on incubation time and temperature and were maximal at 37 degrees C. Michaelis constant was estimated to be 1.4 X 10(-9) M from Lineweaver-Burk plot and to be 1.2 X 10(-9) M from id-value of the dose-response curve for displacement with native prolactin. Total number of binding sites for prolactin was 1.38 X 10(-15) mole per mg weight of tissue. Ovine prolactin, human growth hormone and human placental lactogen complete with 1251-prolactin and dose-response curves for these three hormones were all parallel. These results suggest the existence of a specific receptor site with high affinity for prolactin in lactating mouse mammary glands.     

Dispersed mammary epithelial cells. Receptors of lactogenic hormones in virgin, pregnant, and lactating rabbits.

The number and affinity of binding sites for lactogenic hormones have been determined in dispersed mammary cells from virgin, pregnant, and lactating rabbits. Dispersed epithelial cells, prepared from mammary glands by enzyme digestion, calcium chelation, and gentle shearing, were separated from nonepithelial cells by density centrifugation. 125I-labeled ovine prolactin (oPRL) and 125I-labeled human growth hormone (/GH) were used as tracers. Association and dissociation of 125I-oPRL or 125I-hGH were time- and temperature-dependent. The rate of association followed a second order reversible reaction with a rate constant of approximately 0.5 at 4 degrees C, approximately 2.0 at 23 degrees C, and approximately 9 x 10(7) M-1 min-1 at 37 degrees C. Maximum binding was achieved after 120 h at 4 degrees C, 48 h at 23 degrees C, and 2 to 4 h at 37 degrees C. Dissociation of 125I-oPRL or hGH from cells by unlabeled oPRL was complete at 4 degrees C after 160 h, following a first order reaction (5-1 = 9.9 x 10(-5) min) and incomplete at 23 degrees C and 37 degrees C even after prolonged time. Internalization of receptor-bound 125I-oPRL was studied by quantitative electron microscope autoradiography. Grain distribution over- and volume densities of cellular organelles was analyzed as a function of time and temperature. At 37 degrees C, there was a rapid and specific translocation of lactogenic hormones to intracellular organelles. Autoradiographic grains were found associated with vesicles, Golgi elements, lysosome-like structures, and the nucleus. One class of high affinity binding sites was estimated from Scatchard plot and direct kinetic analyses at 4 degrees C. Whereas the apparent affinity constant (approximately 10(10) M-1) did not change significantly throughout pregnancy and early lactation, the number of receptors extrapolated from Scatchard plots at 4 degrees C varied in an inverse relation to serum progesterone concentration. Thus, approximately 1900 sites were detected in virgin rabbits (progesterone, approximately 200 pg/ml), and midpregnancy (progesterone, approximately 15,000 pg/ml), and approximately 1800 during early lactation (progesterone, approximately 500 pg/ml). The binding properties of lactogenic hormones to dispersed cells was compared with those to Triton X-100 solubilized microsomal membrane preparations. Good correlation between the two systems was found indicating that cell dispersion did not alter binding properties. Our results indicate that dispersed mammary cells bind lactogenic hormones in a saturable and reversible process, that the number of exposed receptors varies throughout gestation and lactation, and finally that lactogenic hormones are internalized following interaction with their membrane receptors.     

Characterization of prolactin receptors in pig mammary gland.

Prolactin receptors present in the particulate fraction of lactating pig mammary gland were solubilized by 7.5mM-3-[(3-cholamidopropyl)dimethylammonio]-1-propane-su lph onic acid (Chaps) and purified by affinity chromatography on prolactin coupled to Affi-Gel 10. Nearly 30% of the particulate receptors were solubilized by the detergent and over a 1000-fold purification from homogenates was achieved. A water-soluble fraction rich in receptors was observed during the preparation of membranes, although this fraction has not yet been purified. Prolactin binding to the receptors was a time-dependent, reversible and saturable reaction in particulate, Chaps-solubilized and purified receptors. In all forms, receptors showed the same specificity to peptide hormones. Prolactin and human growth hormone bound to the same receptors, whereas bovine growth hormone, follicle-stimulating hormone, luteinizing hormone, thyroid-stimulating hormone and insulin failed to bind. After solubilization, the dissociation constant (Kd) for prolactin was decreased 5-fold from 9.8 X 10(-11) M in the particulate receptors to 1.8 X 10(-11) M in solubilized and purified receptors, being due principally to an increase in the association rate constant from 1.0 X 10(9)M-1 X h-1 to (3.9-4.6) X 10(9)M-1 X h-1, respectively, with the dissociation rate constant remaining unchanged at (1.1-1.3) X 10(-2)h-1. Isoelectric focusing of the prolactin-receptor complex revealed two peaks, one at a pI of 5.5-5.6 and the other at 5.2-5.3. Microsomal receptors were covalently cross-linked to 125I-labelled ovine prolactin with ethylene glycol bis(succinimidyl succinate) and analysed by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. Autoradiography of the gel revealed a major subunit of Mr 28 000-35 000 and a minor one of Mr 67 000-69 000. Anti-(prolactin receptor) antibodies raised against rabbit mammary gland prolactin receptors were equally effective in inhibiting prolactin binding to particulate, solubilized and affinity-purified receptors, suggesting that purified prolactin receptors have a structure indistinguishable immunologically from particulate receptors and rabbit mammary gland prolactin receptors. The present demonstration shows that particulate prolactin receptors from a domestic animal can be solubilized and purified without losing the original properties of high affinity and binding specificity for hormones.     

Seasonal changes in oestrogen receptor affinity in the domestic rabbit, Oryctolagus cuniculus

Over the course of 1 year the Ka value of uterine oestrogen receptor for oestradiol varied 10-fold from a low of 0.259 +/- 0.065 X 10(9) M-1 in early spring to 2.21 +/- 0.21 X 10(9) M-1 in fall. There were no significant changes in receptor number. In addition, administration of oestradiol or progesterone to ovariectomized rabbits resulted in significant reductions in measureable oestrogen receptor affinity. Although these variations in oestrogen receptor Ka values parallel reproductive success in the domestic rabbit, no causal relationship has been established.     

Modulation of prolactin binding sites in vitro by membrane fluidizers. I. Effects on adult rat ventral prostatic membranes

The objective of this study was to determine if aliphatic alcohols, known fluidizers of certain membranes, could increase in vitro the apparent fluidity and prolactin binding capacity of membrane preparations obtained from ventral prostate glands of adult male rats. The degree of fluidization was monitored by a fluorescence polarization method using 1,6-diphenylhexatriene. Membrane preparations were either incubated with varying concentrations of ethanol, 1-propanol or 1-butanol and 125I-oPRL overnight at room temperature or were exposed to the alcohols for 15 min at room temperature and washed prior to the overnight incubation with ligand. Regardless of the conditions of incubation, alcohol exposure produced the same effects, a dose-dependent elevation and then decline in specific prolactin binding. Butanol produced a maximal 37-42% increase in prolactin binding at a concentration of 1.0%, propanol produced a maximal 40-56% increase in prolactin binding at a concentration of 3.8%, and ethanol produced a maximal 54-77% increase in prolactin binding at concentration of 4.8%. Scatchard analysis of the oPRL binding of ventral prostatic membranes indicated that the in vitro treatment of these membrane fluidizers increased the number of oPRL binding sites rather than the apparent affinity constant. The value of the microviscosity parameter decreased by 10-13%, 13-15% and 21-25%, after a 15 min exposure of prostatic membranes to 1.0% butanol, 3.8% propanol and 4.8% ethanol, respectively. These data suggest that in vitro fluidization of prostatic membrane modifies prolactin binding capacity and are consistent with in vivo prostatic prolactin receptor level-membrane fluidity relationships observed in earlier studies.     

Water-soluble prolactin receptors from porcine mammary gland

Two types of prolactin receptors were identified in sow mammary gland. When light membranes were prepared on a discontinuous sucrose gradient (0.3 and 1.7 M) and then diluted and washed with 0.3 M sucrose solution, a large amount (about 50%) of receptors were released from membranes and appeared in the supernatant fraction. These two forms (hydrophobic and water-soluble) of receptors were characterized as having the same binding specificity for lactogenic hormones and a similar affinity constant for ovine prolactin (K alpha approximately 10-12 X 10(9) M-1). Polyclonal antibodies and one monoclonal (mAb M110) antibody, obtained against partially purified prolactin receptors from rabbit mammary gland, cross-reacted effectively with sow mammary receptors. They completely inhibited the specific binding of [125I]oPRL to membrane and water-soluble receptors. The present studies indicate that the two types of sow prolactin receptors could represent the same molecular entity and confirm that prolactin receptors from rabbit and sow mammary gland exhibit numerous antigenic similarities.     

Prolactin binding in rat mammary gland during pregnancy and lactation.

Lactogenic hormones from the placenta and pituitary are primarily responsible for the growth and function of the mammary gland during pregnancy and lactation. In the present study we described the optimal conditions for the measurement of 125I-labeled ovine prolactin binding to mammary gland slices of pregnant and lactating rats. Prolactin binding is saturable (Kd approx. 2.36 - 10(-9) M), hormone specific and destroyed by proteases. The hormonal environments of pregnancy and lactation dramatically influence the availability and measurement of prolactin binding sites. Whereas binding consistently appears to be low in mammary glands removed from rats during pregnancy, binding levels rise 7--8-fold shortly after birth and remain high during the 22 days of lactation. However, the removal of the ovaries and gravid uteri at specific times during pregnancy results in a prompt 3--6-fold increase in prolactin binding. Elevated levels in potential prolactin binding capacity appear in mammary tissue coincident with the reported rise in serum rat placental lactogen between the eighth and eleventh days. We suggest that high levels of this lactogenic hormone promote the appearance of prolactin binding sites during pregnancy and mask the sites such that they are not available for measurement in vitro.     

Rat ovarian and adrenal prolactin receptors. Sizes and effects of divalent metal ions

Receptor fractions were prepared from follicle-rich ovaries (for FSH), luteal cell-rich ovaries (for LH and PRL), and adrenals (for PRL) of rats. Divalent metal ions, Mg++, Ca++, and Mn++ showed inhibitory effects on the binding of LH and FSH to their receptors. The binding of the former was more sensitive to these ions than the latter. On the other hand they showed bell-shaped promotive effects on PRL-ovarian receptor binding, the maximal effects being observed at 10-20 mM. Besides these ions, Ba++ also had a promotive effect, while other divalent metal ions such as Zn++, Cd++, Ni++, and Co++ showed inhibitory effects on PRL-ovarian receptor binding at 5 mM. Mg++ and Ca++ also promoted PRL-adrenal receptor binding, while Mn++ promoted the binding at 10 mM but inhibited it at higher concentrations. Association constant (Ka) and binding capacity (Bmax) of PRL receptors of the ovary and the adrenal were significantly different (ovary: Ka = 0.69 X 10(10) M-1, Bmax = 62 fmol/mg protein, adrenal: Ka = 0.21 X 10(10) M-1, Bmax = 99 fmol/mg protein). Ka of the ovarian PRL receptor was not influenced by these divalent ions, while that of the adrenal receptor was doubled by Ca and Mn ions, Bmax of the latter was also increased. A cooperative effect of Mg and Ca ions was observed on Ka and Bmax of the adrenal receptor. The sizes of the PRL binding sites of these organs revealed by affinity labelling were 17K and 40K in the ovary, and 40K and 110K in the adrenal. These results indicate the different properties of receptors in these different target organs.     

Ontogeny of the liver nuclear T3-receptor during the last days of incubation and posthatch in the chick embryo.

The characteristics of the nuclear T3 receptor in the liver of the chick embryo were studied from incubation day 18 until day 1 posthatching. Treatment of the nuclei with 3 mol.l-1 MgCl2, which removed the endogenously bound hormone, was used in order to determine the total amount of receptors. The affinity constant Ka decreased between incubation day 18 (0.996 +/- 0.276.10(9) M-1) and day 19 (0.247 +/- 0.072.10(9) M-1), remained the same thereafter until hatching and increased again on day 1 posthatching (1.846 +/- 0.928.10(9) M-1). The total amount of receptors tended to increase from incubation day 18 to day 20 non-pipping (np) (from 4.40 to 11.55 fmol/micrograms DNA) and decreased thereafter to 2.38 fmol/micrograms DNA on day 1 posthatching. The amount of free binding sites reached a maximum on day 19 (6.91 fmol/micrograms DNA) and then decreased drastically until posthatching (0.19 fmol/micrograms DNA). The maximal specific binding was found on day 20 (np), just prior to penetration of the air chamber. During the time at which the level of T3 remains high in the plasma, a reduction in the amount of receptor was observed, which may be the consequence of a down-regulation by T3 itself.