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Biomedical subjects

R C Baxter

Publications and source records attributed to R C Baxter.

At least 235 records · Page 13Linked to original sources
PubMed1984

Induction of hepatic receptors for growth hormone (GH) and prolactin by GH infusion is sex independent.

To determine whether induction of rat liver GH and PRL receptors by GH infusion is dependent upon the sex of the animal or whether or not the pituitary is intact, rat GH (rGH) or rat PRL (rPRL) was infused at approximately 200 micrograms/day for 7 days into male and female, intact and hypophysectomized rats, and the binding of radioiodinated bovine GH (bGH) and ovine PRL (oPRL) to liver microsomal membranes was measured. In females, bGH binding was reduced by hypophysectomy whether or not membranes were MgCl2 treated to remove endogenous ligand. However, in males, hypophysectomy caused an apparent 3-fold induction of bGH binding sites, which was absent in MgCl2-treated membranes, suggesting that the effect was due to receptor occupancy by endogenous rGH in the intact males. Hypophysectomy also lowered oPRL binding in females but had no effect in males. Infusion of rGH significantly induced binding sites for bGH and oPRL in all treatment groups, independently of sex or the presence of the pituitary, whereas rPRL infusion had no effect on either receptor type except for mild induction of bGH binding in hypophysectomized females. Serum somatomedin-C (SM-C), reduced 95% by hypophysectomy, was restored by rGH, but not rPRL, infusion. However, in intact animals of both sexes, rGH infusion significantly lowered SM-C levels by 30-40%; thus both bGH and oPRL binding in individual pituitary-intact rats were negatively correlated with serum SM-C. In contrast, in rGH-treated hypophysectomized rats, induced GH (but not PRL) binding sites showed significant positive correlation with SM-C levels. These results indicate that the induction of GH and PRL receptors by rGH occurs independently of SM-C generation, but suggest that newly induced GH receptors in GH-treated hypophysectomized rats may be involved in SM-C generation.

Animals↗
PubMed1984

Immunoreactive somatomedin-C/insulin-like growth factor I and its binding protein in human milk.

Milk contains cell growth factors, but somatomedin-C/insulin-like growth factor I has not previously been identified. Acid-ethanol extracts of fresh human milk samples displaced somatomedin-C tracer from the specific somatomedin-C antiserum Tr4 parallel to standard. In samples obtained between days 1 and 9 postpartum, highest levels [17.6 +/- 10.4 (+/- SD) ng/ml; n = 7) were found in day 1 samples; these fell to stable levels of 6-8 ng/ml over the next several days. Sephadex G-200 chromatography of fresh 1- to 2-day-postpartum milk samples revealed immunoreactive peaks of approximately 150,000 and 40,000 mol wt, accounting for about 80% of the immunoreactivity; the remainder corresponded in elution volume to free somatomedin-C. On chromatography of the high molecular weight fractions in 1 M acetic acid, most of the immunoreactivity shifted to the position of somatomedin-C. Milk samples stripped of their endogenous somatomedins by ion exchange chromatography at low pH specifically bound somatomedin-C tracer in proportion to the amount of added protein up to 20% binding. Displacement of tracer by unlabeled somatomedin-C indicated an association constant of about 1 X 10(10) liter/mol. Fresh milk incubated with somatomedin-C tracer and fractionated by neutral gel chromatography bound the tracer predominantly in the 40,000 mol wt region, which was abolished when excess unlabeled peptide was included in the incubation. These results indicate that fresh human milk contains immunoreactive somatomedin-C, of which a significant proportion is not protein bound at neutral pH. Somatomedin-C may account for some of the mitogenic activity of milk reported by others. The role of this activity in neonatal growth is unknown.

Carrier Proteins↗
PubMed1984

Identification of human semen insulin-like growth factor-I/somatomedin-C immunoreactivity and binding protein.

Human seminal plasma (SP) samples have been tested for insulin-like growth factor-I/somatomedin-C (IGF-I/SM-C) immunoreactivity. Gel chromatography of SP at neutral pH indicated that all immunoreactive IGF-I/SM-C was present at a higher molecular weight than the free peptide from plasma, while in 1 M acetic acid, a major peak of immunoreactivity was seen at an apparently lower molecular weight than the free peptide, together with a peak with molecular weight about 40 000. This latter peak contained a binding protein which, on Scatchard analysis, was shown to have a single class of binding site, Ka = 1.2 X 10(10) L/mol. The low molecular weight peak material was displaced from monoclonal and polyclonal IGF-I/SM-C antibodies parallel to standard preparations. Its elution on gel chromatography later than plasma IGF-I/SM-C was shown by re-chromatography to be due to physical retardation on the column, rather than a lower molecular weight. As acid-ethanol extraction of SP samples failed to remove binding activity, samples from patients were all subjected to acid gel chromatography before assay. The mean IGF-I/SM-C content in 5 normal men was 20.7 +/- 3.7 (SD) ng/ml while that for 4 azoospermic subjects was 9.2 +/- 3.7 ng/ml. A hyposomatotrophic subject with normal sperm density and low serum IGF-I/SM-C had an SP IGF-I/SM-C value in the normal range. This study indicates that human semen contains protein-bound immunoreactive IGF-I/SM-C which is at least in part of testicular origin, and apparently not growth hormone-dependent.

Adult↗
PubMed1983

Reciprocal modulation of insulin and insulin-like growth factor-I receptor affinity by calcium.

In contrast to its stimulation of insulin binding to human placental membranes, calcium inhibited the binding of insulin-like growth factor-I. The effects on receptors for both peptides were half-maximal at 2 mM calcium, and were entirely due to alterations in high affinity binding sites for the respective ligands. Calcium decreased the affinity of insulin-like growth factor-I sites, while stimulating the expression of high affinity insulin sites. Competition by each peptide at the receptor for the other peptide was enhanced by calcium. Modulation by calcium might provide a mechanism to amplify functional differences between the two structurally similar receptors.

Binding, Competitive↗
PubMed1983

Pulsatility of immunoreactive somatomedin-C in chronically cannulated rats.

Pituitary GH secretion is pulsatile in man and the rat, but evidence of pulsatility in the GH-dependent somatomedins (insulin-like growth factors) has not been described. In this study serum immunoreactive somatomedin-C periodicity was examined in 10 chronically cannulated unstressed rats. Blood samples were taken at 15-min intervals over 6 h, and serum rat GH and somatomedin-C measured by RIA. For somatomedin-C assay samples were first extracted into acid-ethanol to dissociate protein-bound peptide. Serum GH levels indicated episodic secretion, with a frequency of 2.85 +/- 0.24 h; some secretory episodes were polyphasic. The mean frequency of all GH spikes reaching 400 ng/ml or greater was 1.99 +/- 0.87 h. Somatomedin-C levels showed fluctuations over an average 2-fold concentration range, 0.60 +/- 0.20 to 1.21 +/- 0.29 U/ml (mean, 0.86 +/- 0.18 U/ml), with peaks occurring 1-1.5 h after most GH secretory peaks. The somatomedin-C peak frequency was 1.93 +/- 0.47 h. Summed GH values from 0-5 h were significantly correlated with summed somatomedin-C values from 1-6 h (r = 0.861, P = 0.0007), suggesting a 1-h lag between GH pulses and the following rise in somatomedin-C. Somatomedin-binding protein showed no regular fluctuations. This study indicates that serum somatomedin-C levels in unstressed rats show periodicity which may be directly related to pulsatile GH secretion.

Animals↗
PubMed1982

Induction of somatogenic receptors in livers of hypersomatotropic rats.

Male and female Wistar-Furth rats bearing the pituitary tumor MtT/W15 had serum GH and PRL levels several hundred-fold higher, and immunoreactive somatomedin levels 3-fold higher, than those of controls. The presence of tumor appeared to have no effect on specific binding of radioiodinated bovine GH to liver microsomal membranes in male animals, and to decrease specific binding by 78% in females. However, after desaturating receptors by treatment with 3.2 M MgCl2 for 5 min, bovine GH binding to membranes from tumor-bearing males was 3-fold higher than in normal males, while in tumor-bearing females binding was more than twice as high as in normal females. Competitive binding curves showed the induced receptors to be specific for growth hormones, and thus somatogenic in nature. This study indicates that a high degree of occupancy of GH receptors in hypersomatotropic rat liver does not cause their down-regulation. The mechanism of receptor induction is unknown.

Animals↗
PubMed1982

Monoclonal antibody against human somatomedin-C/insulin-like growth factor-I.

A monoclonal IgG3 antibody produced by mouse hybridoma shows high specificity for human somatomedin-C (SM-C) and insulin-like growth factor-I (IGF-I). The apparent Ka for SM-C is 1.7 x 10(10) M-1. At 1:2000 final dilution, culture medium from antibody-producing cells binds 35-40% of radioiodinated SM and IGF-I tracers, while similar binding is seen with ascites fluid from tumor-bearing mice at 1:200,000 dilution. Rat SM, multiplication stimulating activity, IGF-I C-peptide and human insulin show little or no crossreactivity, while IGF-II has 7%, and IGF-I 70%, of the potency of SM-C. Of a variety of species tested, acid-ethanol extracted guinea pig serum has the greatest immunoreactivity, and extracts of rabbit, rat and mouse serum have the lowest activity. Human plasma extracts give displacement curves parallel to purified SM, and vary in potency depending upon the GH status of the donor. These results suggest that this antibody will be of value for SM-C/IGF-I radioimmunoassay.

Acromegaly↗
PubMed1982

Purification of tracer for somatomedin C radioimmunoassay by hydrophobic interaction chromatography.

We have purified tracer for use in the somatomedin C radioimmunoassay by hydrophobic interaction chromatography. Material showing greatest immunoreactivity binds to Octyl Sepharose CL-4B (Pharmacia) in a buffer mixture consisting of 130 mL of acetonitrile and 870 mL of 0.1 mol/L NH4HCO3, pH 7.8, but is eluted by increasing the acetonitrile content to 180 mL/L. As compared with tracer purified by binding to specific antiserum in liquid phase, precipitating the complex with second antibody, and then dissociating by gel chromatography at acid pH, this tracer shows equal immunoreactivity against specific somatomedin C antiserum. Either preparation allows excellent discrimination between extracts of normal, acromegalic, and hypopituitary plasma samples; thus either is suitable for use in the somatomedin C radioimmunoassay. Tracer purification by hydrophobic interaction chromatography is rapid and inexpensive. It may be useful in preparing highly immunoreactive tracers for other peptide radioimmunoassays.

Chromatography↗
PubMed1982

Radioimmunoassay for somatomedin C: comparison with radioreceptor assay in patients with growth-hormone disorders, hypothyroidism, and renal failure.

We raised an antiserum (Tr4) in rabbits against a basic somatomedin C-like peptide preparation. Using high-immunoreactivity somatomedin C tracer, we compared the performance of radioimmunoassays in which we used the Tr4 antiserum and a well-characterized somatomedin C antiserum distributed by the National Pituitary Agency (NPA) with that of the human placental-membrane somatomedin radioreceptor assay (RRA). In their cross reactivity toward various somatomedin-like and unrelated peptides, the two radioimmunoassay methods were almost identical, although NPA antiserum, with about fourfold higher titer than Tr4 antiserum, showed a slightly greater sensitivity for most peptides tested. Radioimmunoassay of acid-ethanol-extracted plasma samples from normal persons and acromegalic, hypopituitary, hypothyroid, and renal-failure patients revealed no analytical differences between the antisera (for 122 samples, r = 0.979 between methods). Somatomedin values for acromegalic and hypopituitary samples showed no overlap with normals. Values for hypothyroid and pre-dialysis renal-failure samples were significantly lower than normal. By comparison, the RRA showed greater cross reactivity toward some somatomedin-like peptides and gave significantly lower values than radioimmunoassay for acromegalic and hypothyroid plasma extracts, and significantly higher value for hypopituitary and renal-failure samples. We conclude that the radioimmunoassay methods clearly are of greater diagnostic value than RRA for clinical somatomedin measurement.

Acromegaly↗
PubMed1981

The effect of fasting on liver receptors for prolactin and growth hormone.

The effects of 3 day fasting on liver prolactin and growth hormone receptors have been investigated in male and female rats. Fasting caused a significant fall in serum immunoreactive insulin (67% decrease), while receptor-reactive somatomedin fell 82% when measured in whole serum and by 72% when measured in serum fractions following gel chromatography at low pH. Tracer ovine prolactin binding to liver microsomal membranes was reduced by 55% on fasting in females, but unchanged in males. Tracer bovine growth hormone binding fell significantly in both sexes. Analysis of competitive binding curves showed the decrease binding to be due to a loss of prolactin receptors in females, and of high affinity (but not low affinity) growth hormone receptors in males and females. Significant correlations were seen between serum insulin and tracer prolactin (females) and growth hormone (males and females) binding to liver membranes. Correlations between serum insulin and liver high affinity growth hormone binding sites were particularly significant (r = 0.899 in females, r = 0.910 in males). It is proposed that the hypoinsulinemia of fasting causes a loss of high affinity growth hormone receptors in the liver, which could result in a relative hepatic resistance to growth hormone and a decreased hepatic generation of somatomedin.

Animals↗
PubMed1981

Changes in rat liver prolactin binding sites in diabetes are sex dependent.

The effect of streptozotocin-induced diabetes (100 mg/kg) on lactogenic binding sites, measured by iodinated ovine prolactin (PRL) binding, has been studied in liver microsomal membranes from males and female rats. In females, specific binding was reduced in diabetes from 13% to 4.5% of total tracer, while in males specific binding increased from 0.5% to 2.5%. Similar results were obtained using iodinated human growth hormone as tracer, through overall binding was higher. Scatchard plots of binding curves in females showed that changes in binding were due to changes in receptor concentration, while affinity remained unchanged at 2 X 10(9) M-1. In diabetes, serum PRL and estradiol levels fell by 60% in males but showed no significant change in females, and could therefore not account for receptor changes. In contrast, mean testosterone levels fell in diabetic males from 9.0 to 3.9 nM, and rose in diabetic females from 2.1 to 5.8 nM. Estrogen treatment of male rats caused a marked induction of binding in nondiabetic animals, and a change from the male to the female response to diabetes. Testosterone treatment of nondiabetic females suppressed binding, although not to the male levels, and diabetes caused further suppression. These results are consistent with a role for testosterone in regulating PRL receptors in experimental diabetes, but suggest that other hormonal influences are also involved.

Animals↗
PubMed1980

Association between serum insulin, serum somatomedin and liver receptors for human growth hormone in streptozotocin diabetes.

Streptozotocin-induced diabetes in the female rat caused a decrease in the serum level of somatomedin (Sm), measured by radioreceptor assay. The decrease was reversed by insulin therapy. In diabetes of varying severity, serum insulin and Sm levels showed highly significant association up to the insulin concentration (18 microU/ml) corresponding to normal serum Sm (1 U/ml). Similarly, the hepatic binding of human growth hormone (hGH) showed highly significant association with serum Sm levels up to the degree of binding (7% of tracer) corresponding to normal serum Sm. Binding of hGH to normal liver was about 12% of tracer. These results suggest that insulin might regulate serum Sm via its effect on liver lactogenic receptors, and that about half of these receptors are "spare", or in excess of those required to maintain normal serum Sm levels.

Animals↗
PubMed1980

Somatogenic receptors of rat liver: regulation by insulin.

Somatogenic (i.e. GH) receptors have been studied on liver microsomal membranes from male and female rats. Tracer bovine GH was displaced from its binding sites by GHs of various species, but was displaced only weakly by PRLs. Specific bovine GH binding was 3.5-fold higher to female rat liver membranes than to membranes from males. Streptozotocin-induced diabetes significantly reduced binding, by 80% in females and 50% in males, while insulin therapy to normalize weight gain reversed the decrease in binding. Competitive binding curves were consistent with two independent classes of binding site: low affinity sites with K equal to 0.5 nm-1 in both sexes, and high affinity sites with K equal to 12.1 nm-1 in males and 21.4 nm-1 in females (P less than 0.001). The addition of excess ovine PRL caused a substantial loss of high affinity binding with little loss in the low affinity region, suggesting a weak somatogenic role for ovine PRL. In diabetic animals, low affinity sites were unchanged from normal, while high affinity sites were decreased in number, with no change in affinity, and restored on insulin therapy. Serum immunoreactive rat GH levels were the same in normal and diabetic, male and female animals. These studies suggest that the apparent hepatic resistance to GH seen in diabetes when liver somatomedin release is low despite normal serum GH might be explained by the loss of GH receptors in this condition.

Animals↗
PubMed1980

Receptors for insulin and insulin-like molecules.

An increasing body of evidence supports the regulation of hormone action by changes in receptor concentration and affinity. Down and up regulation of insulin and somatomedin receptors by changes in hormone concentration explains the alterations in receptor numbers in obesity or diabetes. However, the changes in receptor affinity that have been described have no known mechanism. In most cases, they occur in vivo, but changes can be produced in vitro by ketone bodies and calcium ions. Calcium ions have also been implicated in insulin action, thus strengthening this relationship. Evaluation of the control of receptor affinity must take into account the effects of metabolites such as glucose and minerals, as well as multiple other factors. The relationship between binding and the biological effect of insulin has to be resolved before the full understanding of insulin action can be reached. The controversial areas of insulin receptor research are becoming clearer, but much more information is required before these questions can be resolved.

Adrenocorticotropic Hormone↗