Penetration of model hormones through the pericardium in simulated conditions in vivo.

The process of penetration of selected protein-peptide substances including insulin (INS), corticotropin (ACTH), prolactin (PRL) and albumin (reference protein) through the model membrane - pig pericardium was traced. These substances show a wide spectrum of therapeutic effects and diverse physicochemical properties (molecular weight, pI). The model substances penetrated the pericardium in simulated in vivo conditions from 1.0 mg / ml solutions. Based on the results obtained, pharmacokinetic parameters of the permeation process were determined - permeation rate (k), half-life (t50%) and their pharmaceutical availability (AUC [0-24 h]). All tested model substances penetrate the pericardium to different degrees. Within 24 h, they penetrate from 16.8% of albumin to 98.9% of insulin. Corticotropin penetrates 43.8% and PRL 34.2%. The highest availability is achieved with insulin, followed by ACTH, PRL and the lowest content of albumin. The results obtained suggest that the higher molecular weight of model protein-peptide substances, the lower the pericardial penetration (R2 = - 0.700) and availability (R2 = - 0.600), and the longer the half-life (R2 = 0.948).

Effectiveness of absorption and passage through the small intestine, as a model of oral prolactin administration.

The process of absorption and permeation of PRL through the small intestine of 1-day-old piglet from the different compositions of solutions prepared for oral administration was investigated. This was achieved by determining the effect of hormone concentration (0.25 mg / ml or 0.5 mg / ml or 0.75 mg / ml), the concentration of stabilizing substances - trehalose (6 mg / ml or 12 mg / ml or 18 mg / ml) and mannitol (6 mg / ml or 12 mg / ml or 18 mg / ml) and the pH of the solution (2.5 or 3.0 or 3.5) on the degree of absorption and permeation of the PRL. The conditions for the absorption and penetration of PRL from solutions of various compositions for oral administration through the natural membrane (small intestine of the 1-day-old sucking piglet) in the in vivo conditions were simulated. The studies used an in vivo model in which the enzymatic profile in the body is not yet fully developed (no pepsin). It was found that in the studied range the absorption of PRL in the small intestine of the 1-day-old sucking piglet is significantly related to the concentration of the hormone and trehalose in the solution from which it is absorbed. In contrast, all factors studied (hormone concentration, trehalose and mannitol concentration, pH value of the solution) influence the process of penetration of the PRL in the studied range. It was also found that the hormone concentration significantly influences the rate of its absorption and permeation (the fastest occurs at a concentration of 0.5 mg/mL). The results suggest possibility of oral prolactin administration in order to ensure proper growth, development and increase the resistance and survival of sucking piglets.

An in vivo half-life extended prolactin receptor antagonist can prevent STAT5 phosphorylation.

Increasing evidence suggests that signaling through the prolactin/prolactin receptor axis is important for stimulation the growth of many cancers including glioblastoma multiforme, breast and ovarian carcinoma. Efficient inhibitors of signaling have previously been developed but their applicability as cancer drugs is limited by the short in vivo half-life. In this study, we show that a fusion protein, consisting of the prolactin receptor antagonist PrlRA and an albumin binding domain for half-life extension can be expressed as inclusion bodies in Escherichia coli and efficiently refolded and purified to homogeneity. The fusion protein was found to have strong affinity for the two intended targets: the prolactin receptor (KD = 2.3±0.2 nM) and mouse serum albumin (KD = 0.38±0.01 nM). Further investigation showed that it could efficiently prevent prolactin mediated phosphorylation of STAT5 at 100 nM concentration and above, similar to the PrlRA itself, suggesting a potential as drug for cancer therapy in the future. Complexion with HSA weakened the affinity for the receptor to 21±3 nM, however the ability to prevent phosphorylation of STAT5 was still prominent. Injection into rats showed a 100-fold higher concentration in blood after 24 h compared to PrlRA itself.

[In vitro evaluation of the gels properties prepared on Pluronic F-127 as vehicles for administration prolactin by injection]. / Ocena wlasciwosci zeli otrzymywanych na bazie Pluronicu F-127 jako nosników dla prolaktyny podawanej w postaci iniekcji.

The aim of this study was to prepare a thermoresponsive formulations, which are a carrier for proteins--prolactin administered directly into solid tumor and which obtain sol-gel transitions at physiological ranges of temperature. Prolactin (PRL) is a hormone that in vivo and in vitro exhibits antiangiogenic properties. Application of this protein in the proposed formulations can be particularly advantageous because of its relatively low stability and limited ability to transmembrane penetration. The paper prepared thermoresponsive carriers, based on nonionic polymer Pluronic F-127 with selected excipients such as dextran 7000, PEG 400, Tween 20 and Tween 80. The sol-gel transition temperature of the formulations was investigated and their physicochemical properties such as pH, density, osmotic pressure were studied. In the remainder of the work carried out tests of prolactin release from the proposed media. The results obtained indicate that a significant influence on the theological parameters obtained carriers and the availability of pharmaceutical composition of prolactin was developed formulation.

Improving the pharmacokinetics/pharmacodynamics of prolactin, GH, and their antagonists by fusion to a synthetic albumin-binding peptide.

To prolong the circulation half-life of human prolactin (hPRL), human GH (hGH), and their competitive antagonists, hPRL-G129R and hGH-G120R, we examined the effects of fusing a serum albumin-binding peptide (SA20) to their amino- or carboxyl-terminus. Fusion of the SA20 peptide to the amino-terminus of the ligands was less detrimental upon their ability to induce or inhibit signal transduction and cell proliferation in vitro than fusion to the carboxyl-terminus. Pharmacokinetic (PK) studies in mice revealed that the half-life of SA20-hPRL and SA20-hGH was prolonged and their clearance was reduced in comparison with hPRL and hGH. Pharmacodynamic (PD) studies in 8-week-old female mice revealed that lobuloalveolar development in mammary glands was greater in all three groups (daily, every 2 days, or every third day over a 12-day period) of mice treated with SA20-hPRL (4 mg/kg) compared with hPRL (3.59 mg/kg). Similarly, daily administration (i.p.) of SA20-hGH (8 mg/kg) or hGH (7.15 mg/kg) to 23-day-old female mice over a 40-day period revealed the superiority of SA20-hGH over hGH as measured by weight gain, body length, and lobuloalveolar development in the mammary glands. These findings indicate that SA20 modification of hPRL, hGH, and their respective antagonists improves their PK/PD properties.

Mechanisms of membrane estrogen receptor-alpha-mediated rapid stimulation of Ca2+ levels and prolactin release in a pituitary cell line.

The role of membrane estrogen receptor-alpha (mERalpha) in rapid nongenomic responses to 17beta-estradiol (E(2)) was tested in sublines of GH3/B6 rat prolactinoma cells selected for high (GH3/B6/F10) and low (GH3/B6/D9) mERalpha expression. E(2) elicited rapid, concentration-dependent intracellular Ca(2+) concentration ([Ca(2+)](i)) increases in the F10 subline. Lack of inhibition by thapsigargin depletion of intracellular Ca(2+) pools, together with abrogation of the response in Ca(2+)-free medium, suggested an extracellular source of Ca(2+) for this response. The participation of voltage-dependent channels in the E(2)-induced [Ca(2+)](i) increase was confirmed by the specific L-type Ca(2+) channel inhibitor nifedipine. For comparison, the D9 mERalpha-depleted subline was insensitive to steroid action via this signaling mechanism. [Ca(2+)](i) elevation was correlated with prolactin (PRL) release in the F10 cell line in as little as 3 min. E(2) caused a much higher PRL release than KCl treatment (which caused maximal Ca(2+) elevation), suggesting that secretion was also controlled by additional mechanisms. Participation of mERalpha in these effects was confirmed by the ability of E(2)-peroxidase (a cell-impermeable analog of E(2)) to cause these responses, blockage of the responses with the ER antagonist ICI 182 780, and the inability of the E(2) stereoisomer 17alpha-E(2) to elicit a response. Thus rapid exocytosis of PRL is regulated in these cells by mERalpha signaling to specific Ca(2+) channels utilizing extracellular Ca(2+) sources and additional signaling mechanisms.

Distribution of prolactin in selected rat organs and tissues.

A single dose of 25 microg prolactin (PRL)/kg of rat body weight was administered to rats subcutaneously. At 1, 2, 3, 4 and 5 h after the injection, selected organs and tissues were taken for analysis. It was found that 1 h after administration, the highest amount of PRL accumulated in the milk (lactiferous) gland, the blood, the ovaries, the pituitary and the liver. Over time, the prolactin content in the selected organs and tissues decreased. PRL is selectively captured by the milk gland, the pituitary, the ovaries, the liver and the heart. Based on the value of the organ or tissue capacity index for PRL, the following order was established for the organs and tissues to which the hormone binds: milk gland > blood > pituitary > ovaries > lungs > liver > cranial bone > spleen > heart > kidneys > muscular tissue > adrenals > adipose tissue > brain.

Initial studies on the administration route of prolactin.

The influence of administration route of prolactin on its biological availability in rats was determined. Prolactin was administered intraperitoneally, intramuscularly and intranasally (intranasal inhalation) in single doses of 2.5 mg/kg body weight. Within 5 hours after the administration, there were noticed evident differences in prolactin concentrations in blood dependening on the route of administration. The maximal prolactin concentration after its intraperitoneal administration was 1.5 times as high as the maximal concentration observed for intramuscular or intranasal administration. The area under the curves: concentration: intraperitoneal administration time and concentration: intramuscular administration time was 132.99 ng/cm3/h and 136.28 ng/cm3/h, respetively. The area under the curve (AUC0-&) for the intranasal administration of prolactin was not much different than for the intraperitoneal and intramuscular ones, and was 111.30 ng/cm3/h.

Prolactin signalling to milk protein secretion but not to gene expression depends on the integrity of the Golgi region.

Prolactin added to the incubation medium of lactating mammary epithelial cells is transported from the basal to the apical region of cells through the Golgi region and concomitantly stimulates arachidonic acid release and protein milk secretion. We report that when PRL is added after disorganisation of the Golgi apparatus by brefeldin A treatment, prolactin signalling to expression of genes for milk proteins and prolactin endocytosis are not affected. However, prolactin transport to the apical region of cells (transcytosis), as well as prolactin-induced arachidonic acid release and subsequent stimulation of the secretion of caseins, which are located in a post-Golgi compartment, are inhibited. This inhibition was not a consequence of damage to the secretory machinery, as under the same conditions, protein secretion could be stimulated by the addition of arachidonic acid to the incubation medium. Thus, it is possible to discriminate between prolactin-induced actions that are dependent (signalling to milk protein secretion) or independent (signalling to milk gene expression) on the integrity of the Golgi apparatus. These results suggest that these two biological actions may be transduced via distinct intracellular pathways, and support the hypothesis that prolactin signals may be emitted at various cellular sites.

Biologic activity and plasma clearance of prolactin-IgG complex in patients with systemic lupus erythematosus.

OBJECTIVE: To characterize the clinical findings in hyperprolactinemic systemic lupus erythematosus (SLE) patients with or without macroprolactinemia (big, big prolactin [PRL]) due to anti-PRL autoantibodies (PRL-IgG complex), and to assess the bioactivity and structure of big, big PRL. METHODS: Twenty-seven SLE patients with hyperprolactinemia (HPRL) were studied. Patients with (n = 8) or without (n = 19) big, big PRL were identified by gel filtration chromatography and affinity chromatography for IgG. PRL concentrations in serum and fractions by gel filtration chromatography and affinity chromatography were characterized by immunoradiometric assay (IRMA), Nb2 bioassay, sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting, and clearance studies. RESULTS: SLE patients without big, big PRL had significantly higher levels of disease activity, cause-proven HPRL, and menstrual disturbances compared with patients with big, big PRL (P < or = 0.05). The big, big PRL fractions by Western blotting revealed a single 23-kd nonglycosylated PRL. The Nb2:IRMA ratio of the samples with big, big PRL was significantly higher than that of the samples without big, big PRL (P < or = 0.02). However, bioactivity of big, big PRL in the Nb2 cells was very similar to that of 23-kd nonglycosylated PRL. Clearance studies in rats demonstrated that the PRL-IgG complex was eliminated more slowly than monomeric PRL (little PRL). CONCLUSION: We demonstrated that the PRL-IgG complex was formed by 23-kd nonglycosylated PRL that was noncovalently bound to IgG and showed that the complex was fully active in vitro. This result suggests that the absence of symptoms of HPRL or lower levels of lupus activity in these patients is not explained by lower bioactivity of the complex. Instead, because of the large molecular size of the complex, the PRL does not easily cross the capillary walls. Delayed clearance of the PRL-IgG complex may account for increased serum levels of PRL in SLE patients with anti-PRL autoantibodies.

Uptake of prolactin and tyroliberin by the heart.

The aim of this study was to observe the specificity of prolactin and tyroliberin uptake and thus to determine their affinity for the heart ventricles and atria. Comparison of the uptake of the examined substances revealed that more of these hormones reached the atria than the ventricles. The contents of prolactin and tyroliberin in the atria were statistically significant compared with 125J. The results observed provide evidence for nonuniform prolactin uptake by the heart.

Growth hormone-binding protein enhances growth hormone activity in vivo.

The decay curve of labeled growth hormone (GH) in the plasma followed a three-compartment model and could be described by the equation: concentration = Ae-alpha t + Be-beta t + Ce-gamma t, where A, B, and C are y-intercepts and alpha, beta, and gamma are compartments. When 125I-labeled ovine prolactin (oPRL) was injected, the decay curve could be described by the equation: concentration = Ae-alpha t + Ce-gamma t. Formation of 125I-labeled bovine-GH-binding protein (GHBP) complexes with somatogenic characteristics was demonstrated in the serum of both normal and GH transgenic mice. In contrast, 125I-oPRL was unable to form complexes of this type in any of the mice studied. Receptor-mediated liver uptake was found to be faster for PRL than for GH (5-6 min vs. 15-20 min). Liver uptake of radioactivity was significantly lower for PRL than for GH [liver to blood ratio (L/B) of 1.7 +/- 0.3 at 6 min vs. L/B of 3.7 +/- 0.6 at 20 min, respectively]. The presence of binding proteins for GH substantially reduces the clearance of this hormone and consequently increases the liver uptake of GH (mediated by GH receptors). This suggests that GHBPs act to increase the biological activity of GH in vivo.

Alterations in plasma prolactin and glucose levels induced by surgical stress in hyperprolactinemic female rats

The effects of hyperprolactinemia on plasma prolactin (PRL) and glucose were investigated in female rats submitted to surgical stress (laparotomy under ether anesthesia). Wistar rats weighing 250-280g received pituitary grafs under the kidney capsule three weeks before the experiments (N = 15) while a control group underwent sham transplantation (N = 14). The sham-operated rats presented a threefold increase of PRL levels as early as after 5 min of surgical stress (P<0.01); the PRL levels reached a peak at about 15 min and returned to baseline at 40 min. The PRL levels of the grafted rats were increased 3.5-fold compared to the sham-operated controls before stress (20.2 + 5.6 ng/ml vs 5.8 + 0.9 ng/ml, respectively; P<0.05), but did not change significantly during the experimental period. Plasma glucose was already significantly increased at 5 min in sham-operated control and grafted rats (P<0.01) and reached maximal concentrations at about 15 min. The grafted rats presented higher glucose levels than sham-operated controls before stress (122.2 + 3.3 vs 100.5 + 4.2 mg/dl; P<0.01) and at 40 min (182.6 + 13.6 vs 146.7 + 8.4 mg/dl; P<0.05). The hyperprolactinemic rats showed impaired surgical stress-induced PRL release and higher glucose levels both at rest and during the first postoperative hour. These results indicate that chronic hyperprolactinemia inhibited PRL secretion and enhanced the hyperglycemic stress response in the female rat.

Blood prolactin concentrations affect prolactin transfer into goat milk: implications for maintenance of lactation.

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 significantly 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.

Release of a prolactin-like substance by bovine granulosa cells in vitro: effects of ovarian steroids.

The time-dependent accumulation of prolactin (PRL) immunoreactivity in serum-free and in serum-supplemented cultures of bovine granulosa cells was observed. Blockade of endogenous progesterone by specific antiserum had no significant influence on PRL immunoreactivity, but progesterone addition (10-10,000 ng/ml) lead to a dose-dependent decrease. Antisera against testosterone and against estradiol-17 beta were inhibitors, and additions of these hormones (100-10,000 ng/ml) stimulated the release of PRL immunoreactivity. The results obtained suggest the secretion of a PRL-like peptide by bovine ovarian cells. Growth hormone, testosterone and estradiol may be stimulators, and progesterone a possible inhibitor of this process.

Specific somatotropic and lactogenic uptake in vivo in the livers of transgenic mice expressing bovine growth hormone gene.

The distribution of labeled hGH, oGH and mPRL in different tissues of MT-bGH transgenic and normal mice was investigated using an in vivo technique. This technique allows comparisons of tissue uptake of radioactivity after the labeled hormone was injected alone (20 ng/50 g BW) or together with an excess (300 micrograms/50 g BW) of unlabeled hormone. Liver, kidney and spleen are the organs that concentrate a significant amount of radioactivity 20 min after the injection of labeled hormones, but the uptake of radioactivity decreased in the presence of unlabeled hormones only in the liver. Graphical analysis showed that the disappearance curves were described by the sum of 3 compartments alpha, beta and gamma. The first two are similar in transgenic and in normal mice but the third had a t1/2 of 56 +/- 9 min in transgenic and 71 +/- 8 min in normal mice. The inhibition of liver uptake was related to the dose of unlabeled hormone injected and a half maximal displacement was obtained with 4 micrograms and 10 micrograms of hGH per 50 g of body weight for normal and transgenic mice, respectively. The 125I-hGH taken up in vivo by the liver of transgenic mice was bound to a molecular species with Stokes radius of approximately 64 A (which is consistent with the molecular size reported for the hormone-receptor complex).(ABSTRACT TRUNCATED AT 250 WORDS)

Prolactin and growth hormone clearance in neonatal boars.

The pharmacokinetics of prolactin (PRL) and growth hormone (GH) are not known in neonatal pigs. In this study, six boars, ages 13 and 26 d, were used to determine metabolic clearance rate (MCR), half-life (t1/2), and volume of distribution (Vd) of immunoreactive porcine PRL (ipPRL) and immunoreactive porcine GH (ipGH). Blood samples were collected through indwelling jugular catheters at 0, 1, 2.5, 5, 10, 15, 30, 45, and 60 min after a 20 ng/g of BW (per hormone) cocktail of porcine PRL and GH. Analysis of the ipPRL disappearance curves indicated an apparent MCR of 13.79 +/- 2.45 and 23.79 +/- 4.10 mL/min for boars at 13 and 26 d of age, respectively (P = .0008). The t1/2 of ipPRL did not differ between ages (P = .9201). The apparent MCR of ipGH at 13 and 26 d were 10.60 +/- 2.09 and 27.30 +/- 4.55 mL/min, respectively, and this difference was still evident after adjustment for BW (P = .0465). The ipGH t1/2 decreased with age (P = .0232), 21.32 +/- 2.55 vs 13.57 +/- 3.35 min for 13 and 26 d, respectively. The apparent distribution volume for ipPRL was higher than that for ipGH at both ages (P = .0021). Within each age, the apparent MCR of ipGH and ipPRL did not differ significantly. These data represent the first clearance measures reported for ipPRL and ipGH in neonatal pigs and indicate that under the conditions of this study the apparent MCR of ipPRL and ipGH are similar but change significantly during early development.

Subcutaneous metoclopramide in the treatment of symptomatic gastroparesis: clinical efficacy and pharmacokinetics.

We investigated the safety and efficacy of short-term s.c. administration of metoclopramide in the treatment of symptomatic gastric stasis. Ten patients with gastroparesis, documented by abnormal solid phase radionuclide gastric emptying study, were treated with 10 mg (2 ml) of s.c. metoclopramide every 6 hr for 3 days. Patients gave themselves the injections as outpatients. Questionnaires were then completed concerning symptom relief, local side effects and adverse reactions. A repeat gastric emptying study was obtained immediately after the last dose of metoclopramide. Serum metoclopramide concentrations were obtained at trough, 1, 2, 3, 4 and 5 hr postadministration and serum prolactin levels at trough, 1 and 3 hr. Pharmacokinetic analysis showed mean peak metoclopramide concentration at 30 min of 99.7 +/- 47.1 ng/ml with measured levels of 93.9 +/- 106.83 ng/ml at 60 min and return to trough values by 4 hr; trough prolactins remained elevated above normal values. Gastric stasis improved from a base-line retention of 78.7% of radioisotope at 2 hr to 72.5% after 3 days of therapy (P = .65). Eight patients reported significant improvement in symptomology and two patients reported lessening of symptoms such as nausea, vomiting, bloating, abdominal pain, heartburn and vomiting. The side effects were minimal and did not interfere with completion of the protocol. We demonstrated that s.c. administration of metoclopramide was well accepted by patients and resulted in subjective and objective improvement of gastric stasis. In addition, serum metoclopramide concentrations were comparable with other parenteral routes of administration. Furthermore, serum prolactin levels may provide both a bioassay of efficacy and a marker for monitoring compliance.

Requirement of nuclear prolactin for interleukin-2--stimulated proliferation of T lymphocytes.

Prolactin (PRL) is necessary for the proliferation of cloned T lymphocytes in response to interleukin-2 (IL-2). Translocation of PRL into the nucleus occurs during IL-2--stimulated mitogenesis. Therefore, the function of intranuclear PRL in T cell proliferation was tested. Eukaryotic expression vectors were prepared to express wild-type PRL [PRL(WT)], PRL that lacks the signal sequence for translocation into the endoplasmic reticulum [PRL(ER-)], and chimeric PRL in which the signal peptide was replaced with the sequence that directs the nuclear translocation of the SV40 large T antigen [PRL(NT+)]. Expression of these constructs in a T cell line (Nb2) responsive to PRL and IL-2 resulted in localization of PRL in the extracellular milieu, cytoplasm, or nucleus, respectively. Stimulation with IL-2 alone resulted in a five- to tenfold increase in the incorporation of [3H]thymidine by cells expressing PRL(NT+) or PRL(WT) as compared to PRL(ER-) or the parental Nb2 cells. Only the PRL(NT+) clone proliferated continuously with IL-2 stimulation in the presence of antiserum to PRL. These results demonstrate that nuclear PRL is necessary for IL-2--stimulated proliferation and suggest that a peptide hormone can function in the nucleus without binding to its cell surface receptor.

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.