Structures of the somatotropin receptor and prolactin receptor on rat hepatocytes characterized by affinity labelling

The phospholipase C-InsP3 pathway is involved in calcium mobilization induced by growth hormone in hepatocytes

We investigated the effects of bovine GH (bGH) on Ca(2+) handling, phospholipase C (PLC) activation and inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)] formation in hepatocytes. bGH generates oscillations in cytosolic free Ca(2+) concentration ([Ca(2+)](i)) in single male rat hepatocytes microinjected with the photoprotein aequorin. In the absence of extracellular Ca(2+) these transients persisted for more than 10 min indicating a requirement for intracellular Ca(2+). Treatment of the hepatocyte with the phosphatidylinositol-specific phospholipase C (PI-PLC) inhibitor U-73122 removed the oscillations. These results suggest bGH-induced oscillations are due to PLC activation and generation of Ins(1,4,5)P(3). We measured the mass of Ins(1,4,5)P(3) in freshly isolated hepatocyte suspensions in response to bGH, and vasopressin as a control. Both agonists rapidly increased the levels of Ins(1,4,5)P(3). This is the first study to indicate that early events in the signal transduction pathways mediated by GH in hepatocytes involve intracellular Ca(2+) mobilization via activation of a PI-PLC and subsequent Ins(1,4,5)P(3) production.

Biochemistry and pharmacology of rabbit cardiac growth hormone (GH) receptors

In this report we present the first in-depth description of the biochemical and pharmacological properties of rabbit cardiac GH receptors. The apparent M(r)'s of the [125I]human (h) GH-receptor complexes were 380, 205, 90, 62, 52 and 38 kDa as demonstrated by an autoradiograph of affinity-labelled cardiac GH receptors separated under non-reducing conditions by SDS PAGE. The [125I]hGH-cardiac GH receptor complexes were disulfide-linked since the M(r)s of the complexes diminished to 170, 116, 97, 71, 45 and 38 kDa under reducing conditions, indicating the presence of multiple receptors, receptor-associated macromolecules or receptor and ligand in various ratios. The pharmacology of cardiac GH receptors is not typical of GH receptors present in other tissues. In radio receptor assays, both bovine GH and ovine prolactin were 50-fold and 100-fold less potent, respectively, than unlabelled hGH, in blocking the binding of [125I]hGH to cardiac binding sites and were, therefore, extremely weak antagonists. Similarly, neither bovine GH nor ovine prolactin blocked the [125I]hGH affinity-labelling of cardiac GH receptors compared to equivalent doses of unlabelled hGH. Parameters which characterize the kinetics for the association, dissociation and equilibrium binding of [125I]hGH to cardiac GH receptors were ascertained. Association kinetics for the binding of [125I]hGH to heart GH receptors exhibited a maximum specific binding at 17 h and 25 degrees C. The association of [125I]hGH to heart GH receptors was reversible with approximately 15 h required for half of the specifically bound [125I]hGH to dissociate. The coupling of [125I]hGH to heart GH receptors was optimum at pH 6 and the strength of the equilibrium binding, as measured by the ED50, was approximately 2 ng/ml. These data indicate that the cardiac GH receptors are pharmacologically distinct and that there is a M(r) heterogeneity in the [125I]hGH receptor complexes.

Differential expression of the growth hormone receptor and its transcript in bovine uterus and placenta

By reverse transcription-polymerase chain reaction (RT-PCR) the transcript of the growth hormone receptor (GHR) was proved in bovine placentae of different gestational stages. Using non-radioactive in situ hybridization (ISH) the mRNA encoding GHR was localized in the uterine epithelium, glands, vessels and amniochorion of the placenta from the sixth week until the ninth month of gestation. Production of mRNA was low during the first trimester, significantly increased in the second trimester and peaked at the end of the sixth month of pregnancy. No transcript of GHR was found in the endometrium of non-pregnant uteri. The colocalization of the protein using a monoclonal antibody showed the same distribution pattern as the mRNA. Immunoblotting revealed a protein with a molecular weight of 120 kDa in the endometrium of pregnant cows which was not found in non-pregnant uteri. Our results imply that growth hormone is involved in placental metabolism and embryonic development from the early beginning of pregnancy until birth.

Growth hormone activation of Stat 1, Stat 3, and Stat 5 in rat liver. Differential kinetics of hormone desensitization and growth hormone stimulation of both tyrosine phosphorylation and serine/threonine phosphorylation

Intermittent plasma growth hormone (GH) pulses, which occur in male but not female rats, activate liver Stat 5 by a mechanism that involves tyrosine phosphorylation and nuclear translocation of this latent cytoplasmic transcription factor (Waxman, D. J., Ram, P. A., Park, S. H., and Choi, H. K. (1995) J. Biol. Chem. 270, 13262-13270). We demonstrate that physiological levels of GH can also activate Stat 1 and Stat 3 in liver tissue, but with a dependence on the dose of GH and its temporal plasma profile that is distinct from Stat 5 and with a striking desensitization following a single hormone pulse that is not observed with liver Stat 5. GH activation of the two groups of Stats leads to their selective binding to DNA response elements upstream of the c-fos gene (c-sis-inducible enhancer element; Stat 1 and Stat 3 binding) and the beta-casein gene (mammary gland factor element; liver Stat 5 binding). In addition to tyrosine phosphorylation, GH is shown to stimulate phosphorylation of these Stats on serine or threonine in a manner that either enhances (Stat 1 and Stat 3) or substantially alters (liver Stat 5) the binding of each Stat to its cognate DNA response element. These findings establish the occurrence of multiple, Stat-dependent GH signaling pathways in liver cells that can target distinct genes and thereby contribute to the diverse effects that GH and its sexually dimorphic plasma profile have on liver gene expression.

Bovine growth hormone induces oscillations in cytosolic free Ca2+ in single rat hepatocytes

Single rat hepatocytes microinjected with the photoprotein aequorin generate oscillations in the cytosolic free Ca2+ concentration ([Ca2+]i) when stimulated with agonists acting through the phosphoinositide signalling pathway. We show here that, in single rat hepatocytes, bovine growth hormone (bGH) is able to induce [Ca2+]i oscillations which display similarities with oscillations induced by phenylephrine. Thus the rate of rise of intracellular Ca2+ in each oscillation closely resembles that induced by Ins(1,4,5)P3-mediated agonists. However, the duration of bGH-induced oscillations increases with agonist concentration, in contrast to phenylephrine-induced oscillations, which undergo an increase in frequency as the agonist concentration is raised, without any increase in the duration of individual oscillations.

Intermittent plasma growth hormone triggers tyrosine phosphorylation and nuclear translocation of a liver-expressed, Stat 5-related DNA binding protein. Proposed role as an intracellular regulator of male-specific liver gene transcription

Growth hormone (GH) exerts sexually dimorphic effects on liver gene transcription that are regulated by the temporal pattern of pituitary GH release, which is intermittent in male rats and nearly continuous in females. To investigate the influence of these GH secretory patterns on intracellular hepatocyte signaling, we compared the pattern of liver nuclear protein tyrosine phosphorylation in male and female rats. An M(r) approximately 93,000 polypeptide, p93, was found to be tyrosine phosphorylated to a high level in male but not female rats. GH, but not prolactin, rapidly stimulated p93 tyrosine phosphorylation in hypophysectomized rats. Intermittent plasma GH pulses triggered repeated p93 phosphorylation, while continuous GH exposure led to desensitization and a dramatic decline in liver nuclear p93. p93 was cross-reactive with two monoclonal antibodies raised to mammary Stat 5, whose tyrosine phosphorylation is stimulated by prolactin. Intermittent GH pulsation translocated liver Stat 5/p93 protein from the cytosol to the nucleus and also activated its DNA binding activity, as demonstrated using a mammary Stat 5-binding DNA element derived from the beta-casein gene. p93 is thus a liver-expressed, Stat 5-related DNA binding protein that undergoes tyrosine phosphorylation and nuclear translocation in response to intermittent plasma GH stimulation and is proposed to be an intracellular mediator of the stimulatory effects of GH pulses on male-specific liver gene expression.

Activation of JAK2 tyrosine kinase by prolactin receptors in Nb2 cells and mouse mammary gland explants

One of the earliest cellular responses to prolactin (PRL) binding in Nb2 cells, a rat pre-T lymphoma cell line, is an increase in tyrosine phosphorylation of cellular proteins. In this work, immunologic techniques have been used to demonstrate that in Nb2 cells and in mouse mammary gland explants, JAK2, a non-receptor tyrosine kinase, is activated following stimulation with PRL. PRL stimulated tyrosine phosphorylation of JAK2 at times as early as 30 sec and concentrations of PRL as low as 0.5 ng/ml (2.5 pM) in Nb2 cells and 100 ng/ml (5 nM) in mammary gland explants. When JAK2 was immunoprecipitated from solubilized Nb2 cells or mammary gland explants and incubated with [gamma-32P]ATP, 32P was incorporated into a protein migrating with an apparent molecular weight appropriate for JAK2 only when cells had been incubated with PRL, indicating that JAK2 tyrosine kinase activity is exquisitely sensitive to PRL. In Nb2 cells, JAK2 was found to associate with PRL receptor irrespective of whether or not the cells had been incubated with PRL. These results provide strong evidence that JAK2 is constitutively associated with the PRL receptor and that it is activated and tyrosine phosphorylated upon PRL binding to the PRL receptor. These results are consistent with JAK2 serving as an early, perhaps initial, signaling molecule for PRL.

Identification of somatogenic binding sites in liver microsomes from normal mice and transgenic mice expressing human growth hormone gene

Somatogenic binding sites were detected and characterized in microsomal preparations from livers of normal mice and mice expressing metallothionein-I/hGH (mMT/hGH) hybrid gene, using 125I-labelled bovine or human GH, or a photoreactive derivative of hGH (125I-AP-hGH1). Specific binding of 125I-bGH was detected in liver microsomes from both normal and transgenic mice with an apparent Kd of 2 nM. 125I-hGH was partially displaced by bGH. 125I-AP-hGH1 was covalently bound to the microsomal preparations, and bGH prevented the formation of the 130 kDa species with no appreciable effect on 63 kDa and 70 kDa lactogenic complexes.

Binding and cross-linking of iodinated rat prolactin to rat hepatic prolactin receptor

Basal parameters for binding and cross-linking of 125I-rat prolactin (rPRL) to lactogenic (PRL) binding species present in crude membrane fraction (CMF) or detergent-solubilized preparations of rat liver have been investigated. (1) The highest specific binding to CMF was obtained with an incubation time of 50 h at 20 degrees C and with a 50 mM potassium phosphate buffer adjusted to pH 8.5. (2) Cross-linking of 125I-rPRL to binding sites in CMF with disuccinimidyl suberate (DSS) showed the autoradiographic appearance of an Mr 40,000 binding species. (3) No specific binding or cross-linking of rPRL was seen in Triton X-100-solubilized CMF. This is probably due to Triton X-100-induced changes in the physical properties of rPRL. (4) Specific binding of 125I-rPRL was detected in CHAPS-solubilized CMF. Following cross-linking the autoradiographic appearance of a binding species with an Mr value of 40,000 was shown. 125I-hGH was cross-linked to three PRL binding species with Mr 82,000, 40,000 and 35,000 in CHAPS-solubilized preparations. (5) In Golgi-enriched low-density membrane preparation 125I-rPRL was cross-linked to Mr 82,000, 40,000 and 35,000 species. It is proposed that the inability of rPRL to be cross-linked to Mr 82,000 and 35,000 species present in CHAPS-solubilized preparation is the result of CHAPS-induced changes of rPRL binding properties and low solubilizing capacity of CHAPS. (6) In conclusion, this study shows that also the iodinated endogenous hormone, rat prolactin, and not only hGH identifies high and low molecular forms of the rat liver prolactin receptor.

Solubilization and characterization of a lactogenic receptor from human placental chorion membranes

Prolactin has a wide range of actions, including osmoregulation and the control of mammary gland development and lactation. These effects are mediated through a high-affinity cell surface receptor, which has been well characterized in a number of animal tissues. The molecular characteristics of the human receptor are unknown, however. The present studies were initiated, therefore, to determine the binding and molecular characteristics of the lactogenic receptor of human placental chorion membranes. Subcellular fractionation studies showed that the bulk of the receptor sedimented in the microsomal fraction at 45,000gav. Endogenous ligand was dissociated from the receptor with 3.5 M MgCl2 or 0.05 M acetate buffer (pH 4.8) with preservation of binding activity. The microsomal receptor bound human growth hormone (hGH), human prolactin (hPRL), ovine prolactin (oPRL), and human placental lactogen (hPL) but not non-primate growth hormones, indicating a narrow specificity for lactogenic hormones. The binding was only partially reversible in agreement with the known binding kinetics of animal lactogenic receptors. The receptor was solubilized with 45% yield from the microsomes using 16 mM 3-[(3-cholamidopropyl)dimethylammonio]-1-propane sulphonate (CHAPS) detergent-250 mM NaCl, and the binding activity was fully restored by a two-fold dilution in the binding reaction to reveal a KD of 0.8 nM for hGH and a binding capacity of 200 fmol of specifically bound hGH per mg of microsomal protein. Gel filtration chromatography indicated the minimum molecular weight of the ligand-receptor complex was approximately 60,000 daltons, and sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) of covalently cross-linked 125I-hGH-receptor complexes revealed a molecular size of 58,000 daltons. When account was taken of the contribution of the ligand, a molecular weight of 36,000 for the receptor's binding domain was obtained. These data indicate that the chorion lactogenic receptor has very similar binding and molecular characteristics to the lactogenic receptors from other mammalian species. Chorion membranes are thus a convenient source of material for the further purification and characterization of the human lactogenic receptor.

Decreased expression of the growth hormone receptor and growth hormone binding protein in rat liver nodules

The growth hormone receptor (GHR) and growth hormone-binding protein (GH-BP) expression were characterized in liver nodules and hepatomas from male Wistar rats. The mRNA levels of GHR and GH-BP, studied by northern blot analysis and solution hybridization, were 35-50% (in nodules) and 2-6% (in hepatomas) of the level found in liver from untreated, age-matched rats. The binding of 125I-labeled human growth hormone to a low-density membrane fraction (LDMF) containing Golgi and endosome membranes in the presence of excess ovine prolactin was 75-80% lower in nodules than in liver. When endogenous ligand was removed, the binding increased in nodules but not in liver. Affinity cross-linking experiments revealed identical specific receptor-binding protein complexes at Mr of 95,000, 55,000, and 43,000 in both nodules and liver, assuming stochiometric binding of ligand. The in vivo endocytosis of 125I-labeled bovine growth hormone correlated with the level of GH binding and was thus reduced 75-80% in nodules, compared with liver. The level of insulin-like growth factor-I (IGF-I) mRNA was reduced by 50% in nodules; however, GH administration resulted in a twofold induction of IGF-I mRNA in both nodules and liver. It is concluded that the greater proportion of occupied GHR in nodules could result from an impaired dissociation of endogenous GH, which might explain the reduced GHR mRNA expression. Furthermore the low GHR mRNA expression may reflect a de-differentiated phenotype in nodules.

Proteins associated with somatogenic and lactogenic receptors in microsomal membranes and intact rat hepatocytes

Lactogenic and somatogenic receptors present in rat liver have been examined by cross-linking with a derivative of human somatotropin (AP-hGH1) followed by SDS-polyacrylamide gel electrophoresis. AP-hGH1, which has a content of 2.2 azidophenacyl groups per molecule, mainly linked to half Cys-182 and half Cys-189, exerted a specificity similar to that of the native hormone (hGH), with an ability of 46% with respect to hGH to compete with the radiolabelled hormone for the binding sites of microsomal preparations. Photolysis of the 125I-labelled derivative bound to the lactogenic receptors present in either microsomal membranes or Triton X-100 solubilized preparations gave rise to a 63 kDa species. In addition, 30% of the covalent complexes formed in microsomal membranes belonged to a species with a molecular mass of 70 kDa. Incubation of viable rat hepatocytes with the radiolabelled derivative at either 0 degrees C for 3 h or 15 degrees C for 1.5 h and subjection to irradiation, yielded covalent complexes of molecular masses estimated at 130, 73, 63, 45 and 35 kDa. Experiments performed in the presence of 1 mM NaCN, gave rise to the previous species in a similar yield as that obtained in the absence of cyanide. The 130 kDa complex is related to the somatogenic binding sites, since it was not visualized in the presence of unlabelled bovine somatotropin, while the 70-73, 63, 45 and 35 kDa bands disappeared when the incubations were performed in the presence of unlabelled ovine prolactin.

Monoclonal antibody detection of prolactin-binding subunits in the rabbit mammary gland

The structure of prolactin (PRL) receptor in the rabbit mammary gland was examined using a receptor-specific monoclonal antibody (MAb). The PRL receptor preparation used was purified by making use of a PRL-affinity column. MAb inhibited the binding of PRL to the receptor, in a dose-dependent manner and completely at a high concentration. Using the receptor directly labelled by 125I, the preparation was incubated with MAbs and the immune complex was collected by Pansorbin and examined by SDS/polyacrylamide-gel electrophoresis. The autoradiography showed that three species with apparent Mr values of 77,000, 41,000 and 25,000 specifically reacted with MAbs. The pattern changed little in the presence or absence of dithiothreitol. Western blot analysis showed that two species (Mr 77,000 and 41,000) reacted with MAb. Affinity labelling of the receptor with labelled PRL revealed three bands with Mr values of 96,000, 60,000 and 43,000 on SDS gels. The high-Mr complex (Mr greater than 200,000) was always present at the top of the gel. These results show that the mammary gland contains at least three PRL-binding subunits. The differences in Mr before and after PRL binding were close to the Mr of PRL. This would suggest that each PRL binding subunit reacts with one PRL molecule.

Receptor-mediated endocytosis and degradation of bovine growth hormone in rat liver

Receptor-mediated endocytosis of radiolabelled bovine growth hormone (125I-bGH) via somatogenic receptors in the liver was studied following in vivo intraportal injection. At different times after injection, subcellular membrane fractions involved in binding (plasma membranes), endocytosis (endocytic vesicles) and degradation (lysosomes) of peptide hormones were isolated by sucrose density gradient centrifugation. These fractions were evaluated for the time-course accumulation of radiolabelled bGH and for the presence of internalized 125I-bGH-receptor complexes. These uptake studies indicate that after initial plasma membrane association of 125I-bGH, the ligand is transported in two successive endocytic compartments prior to arrival in lysosomes. The molecular weight of the somatogenic binders of male and female rat livers involved in internalization of 125I-bGH was determined to 95,000, 64,000, 55,000, 43,000 and 35,000, assuming a 1:1 binding of the hormone to the binder. These binders were seen in both endosomes and lysosomes, which suggests that growth hormone is transported to the lysosomes in a complex with its receptor. Binding and uptake of 125I-bGH was also compared in male and female rat livers, and endocytosis of 125I-bGH was compared to that of radiolabelled ovine prolactin (125I-oPrl). The specific uptake of 125I-bGH appeared not to be sexually differentiated in contrast to that of 125I-oPrl which showed a 35-fold higher uptake in female rat liver. Degradation of 125I-bGH was studied under in vitro binding assay conditions. A distinct 15,000 Da fragment was generated by plasma membrane, endosomal and lysosomal fractions. Based on protease inhibitor studies, a non-trypsin-like serine protease is suggested to be involved in the degradation of bGH. The 15,000 Da proteolytic fragment of GH can be affinity cross-linked to somatogenic binders of similar molecular weights as those involved in the binding of intact GH.

Lactogenic and somatogenic binding sites in intact and detergent-solubilized membrane preparations of female rat liver

The presence of lactogenic and somatogenic binding sites in intact microsomal membranes and in detergent-solubilized microsomal membrane preparations of female rat liver has been studied by affinity cross-linking-SDS/polyacrylamide-gel electrophoresis. In microsomal membrane preparations an Mr 40,000 lactogenic binder is present which is not disulphide-linked to another protein. Triton X-100 solubilization of membranes results in the appearance of three lactogenic 125I-human growth hormone (125I-hGH) binders with Mr values of 87,000, 40,000 and 35,000, and one somatogenic 125I-hGH binder with Mr 32,000. Treatment of rats with oestrogen increased the amount of lactogenic and somatogenic binding species in liver. The lactogenic binding sites are present as one entity in Triton X-100-solubilized preparations, clearly separated from the somatogenic binder as analysed by gel chromatography. Furthermore, 125I-hGH interacts with an Mr 95,000 somatogenic binder in membrane preparations to which the hormone can be cross-linked only following Triton X-100 solubilization.

A comparison of lactogenic receptors from rat liver and Nb2 rat lymphoma cells by using cross-linking techniques

Lactogenic receptors were analysed with the use of the cross-linking agent disuccinimidyl suberate to attach covalently 125I-labelled ovine prolactin or human growth hormone to binding sites from (1) liver from pregnant rats and (2) the rat-derived Nb2 lymphoma cell line. Analysis by SDS/polyacrylamide-gel electrophoresis of the proteins cross-linked to labelled hormone in rat liver indicated a major specifically-labelled complex with an Mr of 68,000-72,000, when run under reducing or non-reducing conditions. With Nb2 cells a major specifically-labelled complex with an Mr of 97,000-110,000 was identified, but only when electrophoresis was run using reducing conditions. Assuming one hormone molecule (Mr 22,000-24,000) per hormone-receptor complex, then the receptor proteins have an Mr of 44,000-50,000 for rat liver and 73,000-88,000 for the Nb2 cells. For both cell types the receptors were of lactogenic specificity; lactogenic hormones competed for binding whereas somatogenic hormones did not. These studies suggest that the lactogenic receptors in rat liver membranes and Nb2 cells differ in two respects. Firstly, the Mr of the labelled receptor protein in Nb2 cells is greater than that of the corresponding receptor protein in rat liver membranes; secondly, the Nb2 cell receptor appears to exist as a disulphide-linked oligomer whereas the receptor in rat liver membranes does not.

Covalent coupling of bovine growth hormone to its receptor in bovine liver membranes

The structure of bovine somatotropin receptor was examined following covalent coupling of iodinated recombinant bovine growth hormone ([125I]rbGH) to bovine liver membrane receptors using ethylene glycol bis(succinimidyl succinate). Iodinated rbGH was incorporated into a complex of estimated Mr of 140,000 under reducing conditions. Excess unlabeled rbGH, but not bovine prolactin (bPRL), inhibited completely the incorporation of [125I]rbGH into the Mr = 140,000 species. In dairy bulls, the Mr = 140,000 complex was undetectable soon after birth but became predominant at 6 months of age. No evidence was found to support presence of bPRL receptors in steer liver membranes. Assuming a 1:1 stoichiometry of hormone binding to receptor, it appears that bGH binds to a major receptor subunit of Mr = 119,000 which does not recognize bPRL.

Separation of rabbit mammary-gland prolactin receptors by ion-exchange chromatography, h.p.l.c.-gel filtration and ultracentrifugation

Rabbit mammary-gland prolactin (Prl) receptors in the microsomal fraction were solubilized in 7.5 mM-Chaps) or 1% Triton X-100 and analysed by ion-exchange chromatography using DEAE-Bio-Gel A. Prl receptors in the presence of 7.5 mM-Chaps were separated into two different fractions (Fr. A and B), both of which showed identical specificity of binding to peptide hormones as those in the Chaps or Triton extract. oPrl and human growth hormone (hGH) bound to the same site, but other non-lactogenic hormones (follicle-stimulating hormone, oGH, luteinizing hormone and insulin) failed to bind to the Prl receptors. The dissociation constant (Kd) for Prl binding to the receptors in Fr. A was about 50% of those in Fr. B, suggesting that the rabbit mammary gland contains two types of Prl receptors, one with a high, and one with a low, Kd for Prl binding. A decrease in the concentration of Chaps in the column buffer to 4 mM caused aggregation of the receptors in Fr. A. H.p.l.c.-gel filtration, using Shim pack 150 and 300 columns connected in series, separated the receptor as a protein with an Mr of 74,000 +/- 4,900 (mean +/- S.D.) in the presence of 5 mM-Chaps, or of 36,800 +/- 2,100 in the presence of 7.5 mM-Chaps. Sucrose-gradient-centrifugation analysis showed that the Prl-receptor complexes in the presence of 5 mM-Chaps were sedimented between gamma-globulin and bovine serum albumin (5.56 +/- 0.22 S). As the Chaps concentration was increased to 7.5 mM, a further peak of the Prl-receptor complexes (4.01 +/- 0.23 S) appeared below ovalbumin. The present data suggest that the binding subunit causes the monomeric subunit to aggregate with itself or with another specific associated protein of similar Mr.

Evidence that non-covalent forces, thiol and disulphide groups affect the structure and binding properties of the prolactin receptor on hepatocytes from pregnant rats

Incubation of hepatocytes from pregnant rats with dithiothreitol decreased specific 125I-prolactin (125I-prl) binding to such cells by about 20% relative to control. This was not due to a non-specific effect of dithiothreitol on the cell membrane, since reduction also altered the binding of prl to solubilized partially purified receptor. Exposure of hepatocytes to N-ethylmaleimide (6 mM) for periods as brief as 1 min decreased the subsequent specific binding of 125I-prl by more than 50%. N-Ethylmaleimide was less effective as an inhibitor of binding when applied after hepatocytes had been exposed to 125I-prl, binding being decreased by about 15%. Scatchard analysis demonstrated that the effect of N-ethylmaleimide resulted from loss of receptor-binding capacity without any substantial effect on the affinity of the prl receptor for hormone. Dithiothreitol diminished the affinity of lactogenic sites for prolactin without altering cellular binding capacity. These observations suggest that thiol and disulphide groups are present in the prl receptor and that these functional moieties regulate the formation and properties of prl receptor complexes. The species to which 125I-prl had bound were identified by affinity labelling. 125I-prl was covalently coupled into saturable complexes of Mr 65000 and 50000. 125I-human growth hormone (125I-hGH) was covalently incorporated into complexes of Mr 300 000, 220 000, 130 000, 65 000 and 50 000. Bovine growth hormone (bGH), but not prl, competed for 125I-hGH uptake into the 300 000-, 220 000- and 130 000-Mr complexes, indicating that these species were somatogenic. Prl, but not bGH, inhibited 125I-hGH uptake into 65 000- and 50 000-Mr complexes. This demonstrated that 125I-hGH in the presence of bGH could affinity-label lactogenic receptors. 125I-prl aggregates in Triton X-100, whereas 125I-hGH does not. Therefore lactogenic complexes to which 125I-hGH was bound in the presence of excess bGH were solubilized in Triton X-100 and characterized sequentially by gel filtration and affinity labelling. Prl receptors were eluted from columns of Sepharose 6B as a species of Mr380 000. Fractionation of the 380 000-Mr species on sodium dodecyl sulphate polyacrylamide gels resulted in the isolation of complexes of Mr 65 000 and 50 000. Thus non-covalent forces stabilize aggregates of the monomeric prolactin receptor.