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R C Baxter

Publications and source records attributed to R C Baxter.

At least 217 records · Page 12Linked to original sources
PubMed1987

Comparison of receptors for insulin-like growth factor II from various rat tissues.

Receptors for insulin-like growth factor II (IGF-II) have been compared in solubilized microsomal membranes from rat lung, brain, kidney, heart, epididymal and subcutaneous fat, ovary, testis and adrenals. Highest binding/microgram protein was seen with testicular membranes. Receptors from all tissues showed high affinity for human IGF-II (mean association constant = 65 litres/nmol) and a high degree of specificity (mean IGF-I cross-reactivity 0.3%; no cross-reactivity with insulin). Affinity labelling followed by sodium dodecyl sulphate polyacrylamide gel electrophoresis showed binding was only to a type-II IGF receptor, with a major band seen at a molecular weight of about 230,000 in lung, brain, kidney and testis, and 240,000 in heart, fat and adrenal gland. All tissues showed broad or bimodal pH dependence of binding, with optima seen at about pH 6 and pH 9. Mild stimulation of IGF-II binding by low calcium concentrations (1-2 mmol/l) was seen in all tissues, although higher concentrations were inhibitory in the brain. It was concluded that IGF-II receptors from different rat tissues, when studied under uniform conditions, show similar binding affinities but differences in size and regulation which might be missed if receptors are examined in separate studies.

Adipose Tissue↗
PubMed1987

Natural and recombinant DNA-derived human insulin-like growth factor-I compared for use in radioligand assays.

We compared two preparations of human insulin-like growth factor (IGF)-I, one isolated from human plasma and the other prepared by recombinant DNA technology, for use in three radioimmunoassays and a radioreceptor assay for IGF-I, two radioreceptor assays for IGF-II, and a protein-binding assay for both IGFs. In the IGF-I assays, iodinated natural IGF-I consistently showed higher binding than did iodinated biosynthetic IGF-I, whether or not the iodopeptides were purified before use by hydrophobic interaction chromatography. Comparing the potency of the unlabeled peptides in the eight assay systems, we found the natural and the biosynthetic IGF-I to be equipotent in every case except for a radioimmunoassay involving a monoclonal IGF-I antibody (the synthetic peptide had 2.4 times greater activity), and an IGF-II radioreceptor assay involving ovine placental membrane receptors (the natural peptide had 10-fold greater activity). We conclude that recombinant DNA-derived IGF-I is suitable for use, both as a standard and as a radioligand, in a wide variety of IGF assays.

Animals↗
PubMed1986

Glycosylation defects alter insulin but not insulin-like growth factor I binding to Chinese hamster ovary cells.

Insulin binding to two Chinese hamster ovary cell lines with well-defined defects in their glycosylation pathway has been characterized and compared to insulin-like growth factor I (IGF-I) binding in the same cell lines. Insulin competition curves indicate that B4-2-1 cells, which transfer co-translationally to proteins an endoglycosidase H insensitive, truncated lipid-linked oligosaccharide, bind insulin with higher than normal affinity. Lec 1 cells, which fail to process oligosaccharide side chains to complex types, bind with a reduced affinity. The potencies of chicken and guinea pig insulins are appropriate for an insulin receptor in the control (WTB) and both mutant cell lines, whereas rat IGF-II is 3 times more potent than expected in the Lec 1 cells and human IGF-I is less potent than anticipated. Insulin bound to Lec 1 cells dissociates more quickly upon dilution than does insulin bound to either WTB or B4-2-1 cells. The Lec 1 insulin receptor is insensitive to pH change, whereas the other lines show the usual optimum of 8. 125I-IGF-I binds well to all three cell lines and is equally pH-sensitive in all three. Serum from a patient with circulating autoantibodies to the insulin receptor competes for insulin but not IGF-I binding, whereas alpha IR3, a monoclonal antibody directed toward the human IGF-I receptor inhibits IGF-I but not insulin binding. Cross-linking of either 125I-insulin or 125I-IGF-I reveals a typical alpha-subunit in the WTB and B4-2-1 cells but a band with faster mobility in the Lec 1 cells. Insulin (10(-8) M) stimulates autophosphorylation of a beta-subunit in all three lines, but again the Lec 1 subunit demonstrates an anomalous mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. These data demonstrate the differential effect of glycosylation on two closely related receptor molecules.

Animals↗
PubMed1986

Growth hormone-dependent insulin-like growth factor (IGF) binding protein from human plasma differs from other human IGF binding proteins.

A growth hormone-dependent binding protein for insulin-like growth factors (IGF-I and IGF-II) has been isolated from human plasma. Analyzed on SDS gels, the preparation contained a major protein band of 53 kDa, and a minor band of 47 kDa. After transfer to nitrocellulose, both species bound iodinated IGF-I, and could be detected using an antibody raised against the purified preparation. In contrast, an IGF binding protein purified from human amniotic fluid bound IGF-I but was not detectable immunologically. The amino acid comparison of the plasma binding protein preparation was different from that reported for amniotic fluid and HEP G2 hepatoma proteins, and the unique amino-terminal sequence, Gly-Ala-Ser-Ser-Ala-Gly-Leu-Gly-Pro-Val-, was different from that of the amniotic fluid and hepatoma proteins. This study indicates that the growth hormone-dependent IGF binding protein of human plasma is structurally and immunologically distinct from other IGF binding proteins.

Amino Acid Sequence↗
PubMed1986

Insulin-like growth factor-binding protein from human plasma. Purification and characterization.

Insulin-like growth factor (IGF)-binding protein (BP) has been purified from Cohn fraction IV of human plasma by acidification, ion exchange to remove endogenous ligands, and affinity chromatography on agarose-IGF-II. The pure protein appeared as a single peak by high performance reverse-phase and gel permeation chromatography (molecular mass, 45-50 kDa), but on sodium dodecyl sulfate-polyacrylamide gel electrophoresis gave a major band at 53 kDa and a minor band at 47 kDa, unreduced, or 43 and 40 kDa, respectively, reduced. The two bands stained for both protein and carbohydrate. After storage at 2 degrees C for 5 months at pH 3, two additional bands, at 26 and 22 kDa on unreduced gels, were also present. Autoradiography after affinity labeling with IGF-I or IGF-II tracer revealed a single labeled band of 61 kDa. BP, quantitated using a specific radioimmunoassay, was retained by agarose-immobilized IGF-I, IGF-II, concanavalin A, and wheat germ lectin, but not Helix pomatia lectin. Competitive binding curves using pure BP and human IGF-I and IGF-II as both labeled and unlabeled ligands indicated association constants of 2-3 X 10(10) liters/mol for both peptides, with a slightly higher affinity for IGF-II than IGF-I, and 0.9 binding sites for either peptide per 53-kDa protein. The exact relationship of this acid-stable IGF BP to the 150-kDa complex from which it is derived remains to be determined.

Chromatography, Affinity↗
PubMed1986

Comparative binding of bovine, human and rat insulin-like growth factors to membrane receptors and to antibodies against human insulin-like growth factor-1.

The immunological properties of human, bovine and rat insulin-like growth factors (IGF) and insulin were compared in competitive binding studies with Tr10 and NPA polyclonal antisera raised in rabbits against human IGF-1. Bovine IGF-1 was 11-19% as effective as human IGF-1 in competing for binding with 125I-labelled human IGF-1, whereas IGF-2 reacted poorly and insulin did not compete. Similar competitive binding curves were obtained with the mouse monoclonal anti-(human IGF-1) antibody 3D1, except that bovine IGF-1 showed a severalfold greater affinity for the monoclonal antibody than for either polyclonal antiserum. Membranes isolated from human placenta, sheep placenta and foetal-human liver were used as sources of cellular receptors. In human placental membranes, most of the binding of IGF-1 tracers could be attributed to a type-1 receptor, because insulin inhibited up to 65% of tracer binding. The other two tissues apparently contain only type-2 receptors, as evidenced by the very low potency of bovine or human IGF-1 in competing for binding with IGF-2 tracers and the absence of any competition by insulin. In competition for binding with labelled bovine or human IGF-1 to human placental membranes, bovine IGF-1 had a similar potency to human IGF-1, whereas bovine IGF-1 was more potent in binding studies with tissues rich in type-2 receptors. Rat IGF-2 was considerably less effective than human IGF-2 in competition for receptors on any of the membrane preparations.

Animals↗
PubMed1986

Adrenal involvement in the diabetes-induced loss of growth hormone and prolactin receptors in the livers of female rats.

Streptozotocin-induced diabetes causes a decrease in growth hormone and prolactin receptors in the livers of female rats, and in the serum concentration of somatomedin-C/insulin-like growth factor-I, concomitantly with an increase in the serum testosterone levels. In this study, a possible role for adrenal androgens in the loss of receptors was examined. Rats were adrenalectomised bilaterally 3 days after the induction of diabetes with streptozotocin (100 mg/kg intravenously), and livers were removed 3 days later. Adrenalectomy had no effect on binding of ovine prolactin or bovine growth hormone to liver microsomal membranes from non-diabetic rats, but in diabetic rats it entirely abolished the 56% decrease in prolactin binding and significantly reversed the 66% decrease in growth hormone binding and the parallel fall in serum levels of somatomedin-C/insulin-like growth factor-I (p less than 0.05). Adrenalectomy also prevented the diabetes-induced rise in serum testosterone. Daily injection of testosterone to normal and diabetic rats for 12 days significantly reduced both prolactin and growth hormone binding (p less than 0.001), with the effect of diabetes being additive upon the testosterone effect. Implantation of testosterone-filled silastic capsules at the time of adrenalectomy (i.e. for 3 days) did not prevent the adrenalectomy-induced restoration of both growth hormone and prolactin receptors. The resulting high serum testosterone level did not reduce binding to growth hormone receptors in control rats over the 3 day period, and caused no further decrease in diabetic rats. However, binding to prolactin receptors was reduced by 47% in control animals with no further loss in diabetic animals (p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenalectomy↗
PubMed1986

Measurement of insulin-like growth factor-II by radioreceptor assay using ovine placental membranes.

A new radioreceptor assay for insulin-like growth factor-II (IGF-II), using receptors on ovine placental membranes, is described. Half-maximal displacement of specifically bound radioiodinated human IGF-II tracer was seen at 1.0 ng/tube of unlabelled IGF-II. The cross-reactivity of IGF-I was 1%, and insulin was entirely without effect. Measured on serum samples from 100 healthy adults, the mean IGF-II concentration (+/- SD) was 576 +/- 160 ng/ml. Identical mean values were seen for all adult age groups up to 65 years. The mean value for 10 acromegalic adults was 583 +/- 155 ng/ml, and for 9 GH-deficient subjects, 161 +/- 26 ng/ml (P less than 0.001 compared to normals). Of eight patients with chronic renal failure, none had an IGF-II level less than 2SD above the normal mean. No significant effect of renal dialysis was seen. In groups of patients with gastric, breast, lung, testicular, oat cell, ovarian, colonic and prostatic carcinoma, Hodgkin's lymphoma, non-Hodgkin's lymphoma, sarcoma and teratoma (5-12 patients per group), mean IGF-II levels were in the lower part of the normal range. Thus this study does not provide evidence supporting a role for excessive IGF-II production in the growth of any of these tumour types.

Acromegaly↗
PubMed1986

Radioimmunoassay of growth hormone-dependent insulinlike growth factor binding protein in human plasma.

A specific radioimmunoassay has been established for a growth hormone-dependent insulinlike growth factor (IGF) binding protein (BP) from human plasma. Although the assay was directed against a 53-kD, acid-stable BP subunit, the main immunoreactive BP in the circulation had an apparent molecular mass of approximately 125 kD. Only higher primate species showed cross-reactivity, and IGF-I, IGF-II, and other peptides were without effect. Circulating BP levels in healthy subjects rose threefold from early childhood to puberty. In 65 adults aged 18 to 65, the mean level (+/- SD) was 6.12 +/- 1.43 micrograms/ml, and declined with age. Strong growth hormone-dependence of BP was also seen; there was a 2.2-fold increase in active acromegaly and a 50-80% reduction in growth hormone deficiency. Poorly controlled diabetic subjects had BP levels 40% below normal, whereas in renal failure and third-term pregnancy a mild elevation was seen. Measurement of BP may provide a useful adjunct to IGF assays in growth disorders.

Adolescent↗
PubMed1986

Production of insulin-like growth factor I and its binding protein in rat hepatocytes cultured from diabetic and insulin-treated diabetic rats.

This study examines the effect of experimental diabetes on the release of rat insulin-like growth factor I (rIGF-I) and its binding protein (IGF-BP) by adult rat hepatocytes in primary culture. Rats treated with streptozotocin (75 mg/kg) had decreased serum rIGF-I values of 0.37 +/- 0.04 U/ml compared to 1.06 +/- 0.04 in age-matched untreated rats (1 U = 770 ng human IGF-I). Concomitant decreases in hepatocyte production rates for rIGF-I (15% of the rate in cells from normal rats) and IGF-BP (30% of normal) were also observed for hepatocytes isolated from diabetic rats. Insulin replacement therapy (1.2 U/day) for 3-4 days normalized serum rIGF-I levels (0.92 +/- 0.07 U/ml) and increased rIGF-I production by isolated hepatocytes to 67% the rate of normal cells and IGF-BP production to 70% normal. Treatment of streptozotocin-treated rats with rGH (150 micrograms/day) in vivo for 7 days failed to increase serum rIGF-I levels or hepatocyte production of rIGF-I. Insulin in vitro (3 X 10(-7) M) increased rIGF-I release by hepatocytes from nondiabetic rats, but had no effect on cells from diabetic animals, suggesting that factors other than insulin are required to maintain rIGF-I synthesis in diabetes. Serum rIGF-I levels showed a strong correlation with hepatocyte rIGF-I production in the animals used in this study. However, calculation of circulating rIGF-I half-life based on these values showed a 2-fold higher half-life in diabetic rats (7.91 +/- 1.58 h) and rGH-treated diabetic rats (7.52 +/- 1.25 h) than in nondiabetic (2.99 +/- 0.35 h) and insulin-treated diabetic animals (3.85 +/- 0.36 h). This suggests that the rate of clearance of circulating rIGF-I may be slower in diabetic animals.

Animals↗
PubMed1986

Serum insulin-like growth factor I levels in adult diabetic patients: the effect of age.

Despite reports of reduced serum insulin-like growth factor (IGF) levels in experimentally diabetic animals, human diabetic patients have been reported to have decreased, normal, or even elevated levels. This study was a cross-sectional examination of the effect of age on immunoreactive IGF-I levels in adult patients with insulin-dependent or noninsulin-dependent diabetes mellitus (IDDM and NIDDM) attending a diabetes out-patient clinic. The patients and normal subjects studied were divided into the age ranges 21-30, 31-40, 41-50, 51-60, and over 60 yr. For all ages combined, the mean IGF-I level (+/- SD) was 0.84 +/- 0.26 U/ml (202 +/- 62 ng/ml) in 133 normal subjects, significantly reduced to 0.41 +/- 0.17 U/ml in 121 IDDM patients, and 0.49 +/- 0.19 U/ml in 46 NIDDM patients (both P less than 0.001). In both groups there was a marked decline in IGF-I with increasing age (P less than 0.01). Except for NIDDM patients aged 21-30 yr (only two patients), IGF-I levels in both IDDM and NIDDM patients were significantly lower in every age range than those in age-matched normal subjects, but did not differ between the two diabetic groups. Glycosylated hemoglobin levels correlated inversely with IGF-I levels only in younger patients with IDDM (r = -0.486; P less than 0.05 for patients aged 21-40 yr). We conclude that factors common to IDDM and NIDDM, perhaps related to relative nutritional deficiency at the cellular level, cause a reduction in serum IGF-I levels, and that this reduction occurs independently of age-related changes in IGF-I.

Adult↗
PubMed1985

Production of insulin-like growth factor I and its binding protein by adult rat hepatocytes in primary culture.

This study examines the production and characteristics of insulin-like growth factor I (IGF I) and its binding protein (BP) from adult rat hepatocytes in primary culture. IGF I and BP in samples of conditioned medium were separated by automated high pressure gel permeation chromatography at pH 2.8, with total recovery of both species. Measured by a specific RIA, with about 2% IGF II cross-reactivity, the rate of production of IGF I [compared with a human IGF I (hIGF I) standard] was 4.39 +/- 0.57 pmol/mg cell protein X 24 h; measured by a radioligand assay using radioiodinated hIGF I as tracer, BP was produced at a rate of 2.97 +/- 0.54 pmol binding sites/mg cell protein X 24 h. High pressure permeation chromatography studies indicated approximate mol wts of 7,500 for hepatocyte IGF I and 50,000 for BP. The pI of hepatocyte IGF I was 8.8, and of the predominant BP species, 6.8. Rat serum treated in the same way as hepatocyte medium showed IGF I and BP activities at identical pI values to those from hepatocytes but, in addition, contained BP activity at more acidic pI values, apparently not produced directly by the liver. Competitive binding studies using acid-chromatographed hepatocyte or serum BP, and hIGF I and IGF II as labeled and unlabeled ligands, also suggested that BP activity in serum was heterogeneous, while BP from hepatocytes appeared to be a single species with similar affinities for IGF I and IGF II. It is concluded that adult rat hepatocytes produce immunoreactive IGF I, pI 8.8, and BP, pI 6.8, which are released into the circulation, but that additional, more acidic, BP species which are also found in serum could result from extrahepatic production or processing.

Animals↗
PubMed1985

Rat hepatocyte insulin-like growth factor I and binding protein: effect of growth hormone in vitro and in vivo.

This study examines the regulation of insulin-like growth factor I (IGF I) and binding protein (BP) release by adult rat hepatocytes in primary culture. Increasing the density of plating of cells had a marked positive effect on the IGF I production rate per mg cell protein, with the highest rate, approximately 4 pmol/mg cell protein X 24 h, seen at densities of 1 X 10(5) cells/cm2 or higher. Cycloheximide (15 micrograms/ml) inhibited both IGF I and BP production by more than 90%, while actinomycin D (0.1 microgram/ml) caused less marked, but still significant, inhibition. Over the insulin range 30 pM-300 nM there was a 45% increase in IGF I production, but no effect on BP production. Bovine GH stimulated production of both peptides, significant effects (up to 50% stimulation) being seen at concentrations from 20-500 ng/ml. Cells from hypophysectomized rats, with serum IGF I and GH levels reduced more than 90% from normal, had IGF I and BP production rates only 7% of normal, measured after 48 h in culture. In vivo replacement with rat GH, 150 micrograms/day for 5 days by osmotic minipump, significantly restored the production of both peptides by hepatocytes. Cells isolated from rats bearing the GH-secreting tumor, MtT/W15, had IGF I and BP production rates approximately twice as high as normal. A significant stimulatory effect of bovine GH (200 ng/ml) in vitro was seen on cells from normal and hypophysectomized, GH-replaced rats, but not from unreplaced hypophysectomized, or tumor-bearing rats. The results in hypophysectomized, animals are consistent with known changes in hepatic GH receptors, while the lack of GH responsiveness in tumor-bearing rats indicates a persistence of maximal stimulation in culture. The reflection of in vivo GH status by hepatocytes cultured for 48 h suggests that messenger RNA turnover for IGF I and BP must be slow. The close parallel in the regulation of the two peptides under most conditions might be indicative of coordinated synthesis.

Animals↗
PubMed1985

Measurement of growth hormone and prolactin receptor turnover in rat liver.

To study the rate of disappearance of GH and PRL receptors in the livers of rats treated with cycloheximide, a technique has been devised for multiple sampling from the liver of each anesthetized rat. In rats treated with cycloheximide (1 or 5 mg/kg, iv), binding sites for both bovine GH and ovine PRL disappeared following first order kinetics over the 2-h sampling period. The half-time for the GH receptor was 30-40 min, equivalent to a rate constant of approximately 0.02 min-1. The half-time for the PRL receptor was 40-50 min, equivalent to a rate constant of approximately 0.015 min-1. At 0.1 mg/kg cycloheximide, slower disappearance rates were seen for both receptors, and the GH receptor showed a partial recovery. Over the same period, binding sites for insulin were unaltered at any cycloheximide dose. Assuming cycloheximide acts simply to inhibit new receptor synthesis, these rates represent the turnover time for GH and PRL receptors in rat liver.

Animals↗
PubMed1985

Antibody against acid-stable insulin-like growth factor binding protein detects 150,000 mol wt growth hormone-dependent complex in human plasma.

In order to measure the 150,000 mol wt binding complex for insulin-like growth factors in human plasma without the need for acidification to dissociate endogenous ligands, we have purified an acid-stable plasma binding protein (BP) and raised a specific rabbit antiserum against it. When used in RIA, with a covalent complex of binding protein and radioiodinated insulin-like growth factor-I as tracer, pure BP and human plasma samples gave parallel displacement curves. Incubation of plasma with pure insulin-like growth factor-I or -II (5 micrograms/ml) had no effect on the immunoreactivity of the samples. Compared to purified BP standard, the immunoreactive BP content of plasma from normal adults was 9.20 +/- 1.59 micrograms/ml (mean +/- SD, n = 10), from acromegalic subjects, 22.5 +/- 3.63 micrograms/ml (n = 10), and from GH-deficient subjects, 4.10 +/- 1.59 micrograms/ml (n = 7). Although this antibody reacted with a protein of approximately 60,000 mol wt in acidified plasma, gel chromatography of plasma at pH 7 indicated that a GH-dependent protein of mol wt 150,000 was measured almost exclusively in unacidified samples. Direct binding of insulin-like growth factor-II, however, showed the major peak of binding activity in normal plasma in the 40-45,000 mol wt region, whereas in acromegalic plasma most ligand binding was in the 150,000 mol wt region. It is concluded that there is essentially no free immunoreactive acid-stable BP in native plasma, that the unoccupied binding sites of 40-45,000 mol wt, detectable by ligand binding, are present on an immunologically distinct protein, and that the immunoreactive 150,000 mol wt binding complex in native plasma is strongly GH-dependent.

Antibodies↗
PubMed1985

Identification of insulin-like growth factor-I and its receptors in the rat testis.

As part of a study of the testicular production and action of insulin-like growth factor-I (IGF-I), adult rat testes were extracted with acidified methanol, yielding an immunoreactive IGF-I fraction corresponding in size to human IGF-I. The mean IGF-I content (+/- SEM) of testes weighing approximately 1.1 g was 51.5 +/- 5.6 ng/testis, and was not due to serum contamination. After a 3-day fast testicular IGF-I decreased by 80%, whereas serum IGF-I levels declined by 90%. Testicular homogenates and isolated Leydig cells were shown to contain specific IGF-I receptors, Ka = 2 X 10(9) M-1, with 10% IGF-II cross-reactivity. The concentration of these receptors was 2 pmol binding sites per testis, or 3.3 fmol per 10(6) Leydig cells. However IGF-I at 250 ng/ml had no effect on basal or hCG-stimulated testosterone production by isolated Leydig cells, measured over 3 h. Although an effect of IGF-I over longer incubation periods cannot be excluded, it is also possible that testicular IGF-I has a mitogenic role, rather than acting on differentiated testicular functions.

Animals↗
PubMed1984

Rat growth hormone (GH) but not prolactin (PRL) induces both GH and PRL receptors in female rat liver.

This study was designed to elucidate which hormone is responsible for the induction of GH and PRL receptors in rat liver. Intact female rats were implanted with osmotic minipumps delivering rat GH (rGH) or ovine GH (oGH) or PRL at various rates from 75 to 800 micrograms/day for 7 days, and binding of radioiodinated bovine GH or ovine PRL (oPRL) tracer was measured on liver microsomal membranes. MgCl2 treatment was used to remove bound hormones from receptors before tracer binding. Infusion of rGH resulted in a significant increase (P less than 0.001) in both GH and PRL binding, the effect being maximal (2.5- to 3-fold for both ligands) at rGH infusion rates from 150 to 400 micrograms/day. Serum rGH levels were elevated 3- to 5-fold in these animals, but somatomedin-C concentrations were not higher than in controls. MgCl2 treatment showed that GH, but not PRL, binding sites in rGH-treated animals were significantly occupied by administered hormone. Analysis of competitive binding curves indicated that receptors for both GH and PRL increased in concentration without changes in binding affinity. In contrast to the rGH effect, oGH infusion from 75 to 400 micrograms/day failed in two experiments to consistently alter either bovine GH or oPRL binding sites. This was not explained by the potency of the preparation at the somatogenic receptor; oGH was in fact more potent than rGH. The effects of rat PRL and oPRL infusion on receptor levels were also assessed. In contrast to previous reports, neither preparation caused induction of either PRL or GH binding sites. oPRL decreased PRL binding by 30-40% when infused between 200 and 400 micrograms/day, whereas rat PRL had a less consistent effect. MgCl2 stripping of membranes suggested that administered PRL preparations did not significantly occupy PRL receptors. GH receptors were unaffected in any PRL-treated group. It is concluded that in intact female rats, rGH regulates the concentration of both GH and PRL receptors. The slight down-regulation of PRL receptors resulting from PRL infusion casts further doubt on the concept that PRL induces its own hepatic receptors.

Animals↗