Occurrence and binding affinity of prolactin receptors in amphibian tissues

Distribution of prolactin receptor in frog (Rana ridibunda) dorsal skin during hibernation

The role of prolactin in the regulation of frog skin functions is still unclear particularly during environmental changes. In this study, prolactin receptor (PRLR) was detected in active and hibernating frog dorsal skin using immunohistochemical method. PRLR immunoreactivity in active frogs was observed in the epidermis, in the secretory epithelium of granular glands and the secretory channel cells of the glands. Myoepithelial cells of granular glands that started accumulating secretory material or those with a full lumen were PRLR immunoreactive, while some myoepithelial cells of empty granular glands were negative for PRLR. In hibernating frogs, this immunoreactivity was observed in the same regions; however, immunoreactivity was more intense than that in active frogs. PCNA was employed for detection of proliferative activity of PRL in the dorsal skin, and immunoreactivity was detected in the nuclei of a few epidermis cells and in the duct of glands of active frogs. The number of immunoreactive nuclei in these regions increased in hibernating and in prolactin injected groups. We conclude that prolactin provides morphological and functional integrity of skin stimulating the proliferation and regulating the function of granular glands and plays an important role in the adaptation of amphibians to the long winter period.

Cloning of a cDNA for Xenopus prolactin receptor and its metamorphic expression profile

A pituitary hormone, prolactin (PRL) shows various effects on cellular metabolism in amphibians, such as stimulation of larval tissue growth and inhibition of metamorphic changes. All these effects are mediated by its cell surface receptor. However, lack of information on PRL receptor (PRL-R) gene expression has made the physiological importance of the PRL/PRL-R system obscure in amphibian metamorphosis. Hence, a Xenopus PRL-R cDNA was cloned, its structure was characterized, and specific binding of PRL to Xenopus PRL-R expressed in COS-7 cells was confirmed. In adult tissues, high level expression was found in the lung, heart, brain, thymus and skin, and low level in the oviduct, kidney and spinal cord. The developmental expression pattern showed that PRL-R messenger ribonucleic acid (mRNA) was expressed in the brain and tail from premetamorphosis and the level increased toward late metamorphosis, suggesting that PRL may inhibit the metamorphic changes in those organs. The level of brain PRL-R mRNA reached a peak just at the start of the metamorphic climax stages and then decreased, whereas in the tail, mRNA expression peaked at late metamorphosis. In the kidney, mRNA expression increased and reached a maximum level at the end of metamorphosis. The results obtained were discussed in relation to metamorphosis.

Visualization of gene expression of prolactin receptors (PRL-R) by in situ hybridization, in Typhlonectes compressicaudus, a gymnophionan amphibian

The expression of the short and long forms of prolactin receptors (PRL-R) mRNA was studied in various types of tissue from Typhlonectes compressicaudus, an amphibian, by quantitative in situ hybridization. Both forms were expressed in all the types of tissues studied. In the liver, small intestine and hypophysis, the mRNA coding for the short form of PRL-R was more strongly expressed than the mRNA coding for the long form and vice-versa for the stomach, spleen and kidneys. In the female liver, quantification showed a higher value of mRNA expression mid-way through pregnancy than during the sexual inactivity period. This result was found to be correlated with the reserve function of the liver. In the kidney and small intestine, the presence of PRL-R was correlated with the hydromineral function. A comparison with certain mammals was also established. These results confirm the ubiquity of PRL effects on metabolic regulation, and suggest a phylogenic conservation of its receptors.

Hormonal effect on the osmotic, electrolyte and nitrogen balance in terrestrial Amphibia

Two main hormones regulate water balance in amphibian. First, mesotocin (MT) acting as a diuretic agent, and second arginine vasotocin (AVT) being an anti-diuretic hormone. In addition, prolactin (PRL), aldosterone, corticosterone, angiotensin II and atriunatriuretic hormones, play a role too in regulating water and ion balance. The hormones affect the epidermis and bladder permeability to water and ions as well as the kidney through the control of the glomerular filtration rate (GFR). The main questions concern the presence and action of these hormones during the amphibian's life history. Are they present in both larval and adult stages? Are these hormones being synthesized in both aquatic and terrestrial adult phases? Under what circumstances are they being stored or released? Would the target organs (epidermis, bladder, kidney) respond in a similar way during all periods? The problem is the fact that under most circumstances an amphibian while in an aquatic environment responds physiologically differently than when on land. Only partial information concerning hormone presence, release and control of water balance is available at the moment, and even that is fragmentary and based on only a very small number of amphibian species.

Effects of prolactin, growth hormone, and triiodothyronine on prolactin receptors in larval and adult tiger salamanders (Ambystoma tigrinum)

The effects of porcine growth hormone (pGH) or ovine prolactin (oPRL) alone and in combination with triiodothyronine (T3) on renal PRL receptors were determined in both pre- and post-metamorphic tiger salamanders (Ambystoma tigrinum). The protein hormones were given at a dose of 1.0 micrograms/gm body weight/day and the T3 was given at 10.0 ng/gm body weight/day. The duration of treatment was 7 days. Effects on growth, and plasma thyroid hormone levels were also determined. Ovine PRL increased growth in both larvae and adults and reversed metamorphic changes. Administration of T3 increased the plasma T3 concentration, as measured by radioimmunoassay, and when given alone caused weight loss at both stages. The GH decreased plasma T4 and increased plasma T3 concentrations, indicating that it caused an increase in T4 deiodination. In adults the renal PRL receptor affinity of 2.9 +/- 0.7 x 10(10) L/mol and capacity of 160 +/- 22 fmol/mg protein were higher than the corresponding values of 1.8 +/- 0.3 x 10(10) L/mol and 29.2 +/- 3.8 fmol/mg in larvae. In adults only, there is an additional low-affinity, high capacity PRL binding site. The oPRL treatment decreased the binding capacity of 33.2 +/- 1.2 and 5.9 +/- 4.9 fmol/mg in adults and larvae, respectively. By contrast, pGH increased the capacities to 249 +/- 18 and 62.1 +/- 6.8 fmol/mg in adults and larvae, respectively. Treatment with T3 alone doubled the oPRL binding capacity to 58.3 +/- 4.7 fmol/mg in larvae, but there was no effect in adults. In both developmental stages the effects of oPRL and pGH on the receptors were not changed by the simultaneous T3 treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Growth hormone binding sites in tilapia (Oreochromis mossambicus) liver

125I-labeled bovine and tilapia growth hormones were used to assess the presence of growth hormone receptors in membranes prepared from tissues of the tilapia Oreochromis mossambicus. The highest level of specific binding was detected in liver membranes from animals of both sexes and the binding was protein-dependent. Tilapia growth hormone, bovine growth hormone, and ovine prolactin, but not tilapia prolactin, potently inhibited the hepatic binding of 125I-labeled bovine growth hormone. Scatchard analysis of the 125I-labeled bovine growth hormone binding data revealed a Bmax (maximum binding) value of 180 fmol/mg protein and a Kd (dissociation constant) value of 13 nM. Tilapia growth hormone potently inhibited hepatic binding of 125I-labeled tilapia growth hormone. Scatchard analysis revealed a single class of binding sites with Bmax and Kd values of 390 fmol/mg protein and 2.5 nM, respectively. Bovine growth hormone and ovine prolactin were less potent while tilapia prolactin was inactive in inhibiting hepatic 125I-labeled tilapia growth hormone binding.

Prolactin receptor gene expression in the rabbit: identification, characterization and tissue distribution of several prolactin receptor messenger RNAs encoding a unique precursor

The expression of the prolactin (PRL) receptor gene was studied in rabbit tissues by Northern blot and S1 mapping analysis of mRNA preparations. Rabbit mammary gland contained three major (10.5, 3.4, and 2.7 kb) and one minor (6.2 kb) prolactin receptor poly(A)+ RNA transcripts all of which contain the entire coding sequence of the long form of PRL receptor. Each of these mammary mRNAs hybridized equally well with cDNA sequences encoding either the NH2 terminal, middle, or COOH terminal part of the rabbit mammary PRL receptor. The four mRNAs differed only in their 5'- and 3'-untranslated regions. The 10.5 kb mammary transcript was further shown to represent a primary transcript of nuclear origin. Among the various rabbit tissues tested, male and female adrenals, mammary gland, ovaries, and jejunum contained the highest level of prolactin receptor mRNA. The prolactin receptor gene was also expressed at moderate to weak abundance in uterus, liver, kidney, pancreas, testis and seminal vesicles. No prolactin receptor mRNA species were detected in adult muscle, lung, total brain, placental cotyledons and spleen, and in thymus from young animals. In all the rabbit tissues examined, the same four PRL receptor poly(A)+ RNA transcripts identified in the mammary gland were expressed and no additional transcript(s) were detected. Variations in the relative proportion of the 10.5 kb transcript and the two smaller transcripts were observed, while the ratio of the 3.4 and 2.7 kb mRNAs remained unchanged. These findings ask for the role of these different transcripts generated in the rabbit, all of which encode the same long form of PRL receptor precursor but have heterogenous 5'- and 3'-untranslated regions. Moreover, they suggest that the various forms of PRL receptor mRNA originate through differential splicing of a single PRL receptor gene.

Presence of prolactin receptors in kidney and large intestine of the snake Ptyas mucosa

125I-labeled ovine prolactin was used to test for the presence of prolactin receptors in membranes prepared from tissues of the common rat snake Ptyas mucosa. High levels of specific binding were found in kidney and large intestine membranes prepared from snakes of both sexes. The binding was time, temperature, and protein dependent. The presence of Ca2+ in the incubation medium enhanced the binding, with optimal concentrations at 10-20 mM. Specificity of binding was established by employing different hormones as the displacing species in the radioreceptor assay. The results suggested the presence of lactogenic receptors in snake kidney and large intestine membranes. Scatchard analysis of the binding data indicated the presence of a single class of binding sites in snake kidney membranes with a dissociation constant of 0.83 nM. The present study is the first report of the presence of prolactin receptors in snake kidney and large intestine membranes.

Insulin-like growth factor-I and insulin-like growth factor-binding protein in the toad, Bufo woodhousei

Molecular weight characteristics and plasma concentrations of insulin-like growth factor-I (IGF-I) and its binding protein (IGF-BP) were investigated in the toad, Bufo woodhousei. IGF-I and IGF-BP were measured by radioimmunoassay (RIA, Kd = 0.37 +/- 0.04 ng/ml) and charcoal-separated ligand binding assay, respectively, in male toad plasma and adult male human donor plasma using a synthetic human IGF-I standard. Prior to the IGF-I RIA, samples were acid-ethanol extracted. Molecular weight characteristics were determined using size exclusion chromatography. At neutral pH (pH = 7.4), IGF-I immunoreactivity and IGF-BP eluted at molecular weight greater than 66 kDa in both toad and human plasma. Acid chromatography (pH approximately 3) resulted in the separation of IGF-I from its binding protein and consequently a shift of IGF-I immunoreactivity to the low molecular weight fractions (approximately 8 kDa) for both toad and human. IGF-BP activity shifted to molecular weight approximately 50 kDa. Toad plasma IGF-I and IGF-BP activity exhibited differences according to season: IGF-I levels were low in the spring (March = 0.48 +/- 0.11 ng eq/ml), increased progressively to reach a peak in July (5.84 +/- 2.5 ng eq/ml), and decreased to low levels again in the fall (October = 0.60 +/- 0.08, November = 0.45 +/- 0.09 ng eq/ml). Plasma IGF-BP activity demonstrated a similar pattern (March = 17.4 +/- 2.5, July = 35.0 +/- 2.4, November = 12.6 +/- 3.2% specific binding). IGF-I was produced for at least 72 hr when toad liver explants were cultured in serum-free medium, indicating that the liver is a source of IGF-I in anurans.

Prolactin receptors in liver, kidney, and gill of the tilapia (Oreochromis mossambicus): characterization and effect of salinity on specific binding of iodinated ovine prolactin

Specific binding of 125I-ovine prolactin (oPRL) to microsomal fractions from gill, kidney, and liver of adult tilapia was determined. Specific binding varied among tissues, the highest values being displayed by kidney membranes. In the liver, the binding of oPRL was not strongly displaced by tilapia prolactins (tPRL177 and tPRL188), although tPRL177 was six times more potent than tPRL188. On the other hand, in kidney and gill membranes, the two tPRLs were equipotent. Tilapia PRLs showed low potency in competing for oPRL-binding sites when pregnant rat liver membranes were utilized. Tilapia growth hormone (tGH) and human growth hormone (hGH) displaced 125I-oPRL from liver as well as did tPRL177 but were not recognized well by renal or branchial receptors. Two 125I-oPRL-binding sites were detected in every tissue tested. These binding sites are subject to physiological regulation since adaptation to seawater resulted in a significant decrease in specific binding.

Prolactin is transported across the epithelium of the jejunum and ileum of the suckling rat

Milk prolactin is transferred from the gastrointestinal tract to the circulation of the suckling rat. To identify the site of prolactin penetration and to determine the mechanism by which the hormone traverses the mucosal barrier, we followed the uptake of prolactin from ligated loops of jejunum or ileum in vivo by three methods: autoradiography, transport of prolactin-gold conjugates, and immunocytochemistry. Autoradiographic studies demonstrated specific binding sites for 125I-prolactin on apical membranes of the jejunum and ileum. Excess cold prolactin reduced radiolabel in apical and basal compartments. Gel autoradiography of portal sera showed the presence of intact prolactin and a prolactin fragment following jejunal transport but only a prolactin fragment following ileal transport. Uptake of prolactin-gold conjugates demonstrated that, in the jejunum, label was present at the luminal surface, within endosomal compartments and lysosomes, in basal coated and smooth vesicles, within basal coated pits, and beyond the basolateral surface. In the ileum, label was found at the luminal surface; within the tubulocisternae, endosomal vesicles, lysosomes, and basal smooth vesicles; and beyond the basolateral surface. Immunoreactive prolactin was present throughout the transepithelial pathways. This study demonstrates that prolactin is selectively and nonselectively absorbed in the jejunum and ileum and that the hormone is directed either to the lysosome for degradation or across the epithelium by means of a transcellular pathway.

Further study on the changes in the concentration of prolactin-binding sites in different organs of Xenopus laevis male and female, kept under dry conditions and then returned to water (their natural habitat)

The binding of 125I-labeled ovine prolactin (125I-oPRL) to membranes from the kidney and liver of Xenopus laevis male and female specimens (Experiment I) and from the kidney, epidermis, and liver of female specimens (Experiment II) (1) kept in an aquatic environment, (2) exposed for 2 weeks to dehydrating conditions, and (3) then placed back into their aquaria after exposure to dehydrating conditions (Experiment II) was studied. No significant sex differences in PRL binding to kidney, epidermis, and liver were found. A highly significant drop in PRL specific binding to the membranes from the kidney and epidermis is brought about in the specimens from both sexes exposed to dehydrating conditions. The results obtained by MgCl2 treatment in vitro of the membranes under study for an estimation of total PRL receptor concentrations seem to point to an actual decrease in PRL specific binding sites. The values of PRL specific binding to the membranes from the liver are not affected by dehydration of the animals (Experiment I and II) or their subsequent rehydration (Experiment II). In rehydrated females (Experiment II), PRL binding values were closely related to those recorded in females permanently maintained in water (controls).

Evidence for hepatic involvement in the regulation of amphibian development by prolactin

Hormonal control of amphibian development involves thyroid hormones (TH), which promote metamorphosis, and prolactin (PRL), which antagonizes the effects of TH and promotes larval growth. Although the liver is not considered to be a regulator of developmental processes such as metamorphosis, it secretes a PRL-synergizing factor (synlactin) in response to PRL. We explored the possibility that the liver may participate in the antimetamorphic actions of PRL in Rana catesbeiana. Bullfrog tadpoles, in which release of endogenous PRL was suppressed by injections of bromocryptine to induce metamorphic changes including tail regression, received hormone-containing implants in various sites. PRL implants in the spleen to deliver hormone directly to the liver via the hepatic portal drainage not only prevented tail regression but actually caused a substantial increase in the height of the tail fin. PRL implanted in other sites or GH implanted in the spleen was much less effective. The liver of animals with intrasplenic PRL implants secreted more synlactin in vitro than that of tadpoles with subcutaneous PRL implants. Young grass frogs were injected with ovine (o) GH or oPRL to determine effects on hepatic synlactin secretion. Although the GH stimulated body growth it did not induce the liver to secrete synlactin. By contrast, PRL treatment did stimulate hepatic secretion of synlactin without stimulating body growth. These results indicate that the liver of pre- and postmetamorphic animals can be stimulated by PRL to secrete synlactin. Furthermore, the antimetamorphic actions of PRL in tadpoles appears to be mediated, at least in part, by an action on the liver. Synlactin may mediate this hepatic effect.

Purification of PRL receptors from toad kidney: comparisons with rabbit mammary PRL receptors

The binding characteristics of the prolactin (PRL) receptors present in toad (Bufo marinus) kidneys were investigated and compared to those of PRL receptors present in rabbit mammary glands. The molecular characteristics of the Triton X-100 solubilized renal and mammary PRL receptors were assessed by gel filtration and by migration analysis on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) after affinity labeling of the binding sites with 125I-human growth hormone. Similar results were obtained for both receptors. Partial purification of the toad PRL receptor could be achieved by affinity chromatography. The molecular weight of this purified receptor could be determined by analysis on SDS-PAGE. With the use of a polyclonal antiserum raised against a purified preparation of rabbit mammary PRL receptor, one or several antigenic epitope(s) could be identified on the core of the toad renal PRL receptor. In conclusion, although the structure and the biological role(s) of PRL have substantially changed during evolution, the receptor for this hormone has retained many of its structural features as could be assessed between an amphibian and a mammalian species on functionally different target tissues.

Prolactin receptors in the male Rana esculenta

The binding of 125I-labeled ovine prolactin (oPRL) to membrane preparations of several tissues from the male green frog, Rana esculenta, collected during the year is reported. PRL binding to kidney fractions was generally high (range 5-45%). A maximum was observed in the month of October, whereas the lowest value was found during the summer season. The binding to skin fractions was equally high (range 5-25%) and the annual profile parallels that of renal fractions. In the liver, a lower specific binding (range 3-4%) occurred consistently during the year, whereas no detectable binding was found in the muscle. The 125I-oPRL binding was inhibited by oPRL and oGH but not by oFSH or oLH. Scatchard analysis gave dissociation constants of 0.4-1 x 10(-10) M and binding capacity of about 20 fmol/mg of membrane proteins was observed in both the skin and kidney fractions. No receptor sites were detectable in 30-day hypophysectomized animals. The administration of oPRL or a crude homogenate of the frog hypophysis induced the appearance of specific PRL binding. Testosterone is able to restore prolactin binding in hypophysectomized animals, as PRL treatment does.

Characterization of renal prolactin-binding sites of two amphibians (Ambystoma tigrinum and Rana catesbeiana) and a reptile (Pseudemys scripta elegans)

The specific binding of 125I-ovine prolactin (oPRL) to renal membrane preparations from Pseudemys scripta elegans, Ambystoma tigrinum, and Rana catesbeiana was characterized. All three membranes showed specific oPRL binding that was dependent upon time, temperature, pH, and membrane concentration. Scatchard analysis of the binding of 125I-oPRL revealed high-affinity sites with KA values of 2.1 X 10(10), 3.6 X 10(10), and 1.25 X 10(10) M-1 for kidney membranes of Pseudemys, Ambystoma, and Rana, respectively. In addition, there was a low-affinity site on the Ambystoma membranes. The binding capacities ranged from 31 to 70 fmol/mg of membrane protein. The hormonal specificity of these membranes was studied by competing increasing amounts of oPRL, human growth hormone (hGH) derived from recombinant DNA techniques, bovine (b) GH, and human placental lactogen (hPL) with 125I-oPRL. The oPRL standard and hGH were the most potent competitors in all three assays, although hGH was not as potent as the oPRL. Human PL was moderately active in the turtle kidney assay, weakly active in the Ambystoma radioreceptor assay, and inactive in the bullfrog assay. Bovine GH had low potency on the turtle membranes and was inactive in the amphibian assays. The results of these studies indicate that the characteristics of renal PRL receptors of Pseudemys, Ambystoma, and Rana are similar to those of lactogenic receptors throughout the vertebrates. In addition, these data provide evidence for the first time of renal PRL receptors in a reptile.

Prolactin binding sites in Xenopus laevis tissues: comparison between normal and dehydrated animals

The binding of 125I-labeled ovine prolactin (125I-oPRL) to membranes from the kidney, epidermis, liver, and testis of Xenopus laevis adult specimens either kept in an aquatic environment or exposed for 2 weeks to dehydrating conditions was studied. Prolactin binding specificity was assayed through competition with several unlabeled hormones (oPRL, hGH, rGH, rLH, and porcine insulin). In the animal exposed to dehydrating conditions a statistically highly significant reduction in prolactin binding to the membranes from the kidney and epidermis was recorded. No significant variations were revealed by the membranes from the liver and testis. The reduction detected in the binding of 125I-oPRL is not related to the dissociation constant, but to the number of PRL binding sites. Since PRL ranks among the few peptide hormones whose rise in the bloodstream promotes an increase in the number of their own receptors, the reduction of its binding sites in Xenopus specimens exposed to dehydration might lend some support to our earlier hypothesis that transfer to a dehydrating environment may bring about, in this totally aquatic species, some decrease in the blood PRL levels.

Prolactin binding sites in the brain and kidneys of the toad, Bufo arenarum Hensel

(125I)-ovine prolactin (oPRL) binding was found in several brain areas of the toad, Bufo arenarum Hensel. The olfactory bulb, cerebral hemispheres, and both dorsal and ventral mesencephalic regions showed saturable, high affinity, (125I)-oPRL binding, ranging between 5.6 to 29.9 fmol/mg protein, while the association constant (Ka) by Scatchard analysis was between 4.0 to 8.7 x 10(9) M-1. This binding was compared with the Scatchard plot of the kidney, which has been already described by other groups, and gave 41.7 fmol/mg protein and Ka 2.5 x 10(9) M-1. Liver showed no binding and in the cerebral hemispheres (125I)-oPRL was not displaced by non-lactogenic hormones, indicating that binding was hormone and tissue specific.

Specific binding sites for ovine prolactin in three amphibian cell lines

Established cell lines (TB-6c and TB-M) obtained by continuous culture of epithelial cells from toad Bufo marinus urinary bladder, which, in culture, maintained a high degree of functional differentiation, exhibited a significant number of high-affinity (KA = 1-2 X 10(10) M-1) binding sites detected both with radioiodinated (125I) ovine prolactin (oPRL) and human growth hormone (hGH). Binding capacity was higher in the case of TB-6c cells (7,573 +/- 581 sites/cell) than with the TB-M cells (1,160 +/- 87). Similarly, binding sites for oPRL were characterized on Xenopus laevis kidney-derived cell line A6. With oPRL used both as tracer and standard, significant cross-reaction was observed with hGH, less with human or rat prolactin (PRL), and none with human chorionic somatomammotropin, bovine growth hormone, and rat luteinizing hormone or follicle-stimulating hormones. B. marinus pituitary extracts completely displaced the binding of 125I-oPRL to toad bladder binding sites. This finding of specific sites for PRL on amphibian bladder and kidney cells confirms that PRL exerts specific biological actions for the control of electrolyte and water metabolism in the amphibians.

Prolactin receptors in tilapia (Sarotherodon mossambicus) tissues: binding studies using 125I-labeled ovine prolactin

The binding of 125I-labeled ovine prolactin (oPRL) to membrane preparations of tissue from freshwater-adapted tilapia (Sarotherodon mossambicus) was examined. Liver, ovary, and testis showed a relatively high specific binding (5-10%). A lower specific binding occurred consistently in intestine and gill tissue, and inconsistently in urinary bladder and kidney preparations. Desaturation experiments with MgCl2 indicated that a majority of the PRL receptors were already occupied by endogenous PRL. Scatchard analysis of liver binding gave a dissociation constant of 0.6 X 10(-9) M and a capacity of 207 fmol/mg protein.

Properties of hepatic binding sites for prolactin in ring doves (Streptopelia risoria)

Specific binding of 125I-labeled ovine prolactin (125I-oPRL) was detected in crude membrane fractions prepared from ring dove (Streptopelia risoria) liver homogenates. In characterization studies, specific binding was found to depend upon pH, incubation time, incubation temperature, and membrane protein concentration. Competitive inhibition of specifically bound 125I-oPRL was observed with human growth hormone, human and rat prolactin, and dove pituitary extract but not with turkey prolactin, human placental lactogen, and several nonlactogenic hormone preparations. Dove liver membranes showed high affinity (Kd = 3 X 10(-10) M) for binding to oPRL but had relatively low binding capacity (Bmax less than 20 fmol/mg protein). PRL binding activity in pooled liver fractions from breeding doves during early stages of incubation prior to crop sac growth did not differ markedly from that observed in doves sampled at the end of incubation when crop sac weight and serum PRL were elevated. However, binding activity was higher in pooled male liver fractions than in pooled female liver fractions at both reproductive stages. A two- to threefold increase in binding capacity was observed in pooled liver fractions from late-incubating doves following MgCl2-induced binding site desaturation. The MgCl2 treatment did not eliminate the differences in specific binding observed between male and female liver fraction pools, thus suggesting the possibility of sex-specific mechanisms of hepatic PRL binding site regulation in this species. It is concluded that the dove liver possesses specific binding sites for PRL with properties similar, but not identical, to those found in other vertebrate target tissues.