Assessment of factors regulating serum growth hormone binding protein in pigs

Inability of heterologous growth hormone (GH) to regulate GH binding protein in GH-transgenic swine

Transgenic pigs expressing bovine, ovine, or human growth hormone (GH) structural genes fused to mouse metallothionein-I (mMT-bGH), ovine MT (oMT-oGH), or mouse transferrin (mTf-hGH) promoters were used to study the effects of GH on the regulation of serum GH-binding protein (GHBP). In the 14 transgenic pigs studied, circulating concentrations of heterologous GH ranged from 15 to 2,750 ng/mL. Using chromatographic methods, specific binding of GH was detected in serum from normal pigs but was undetectable in serum from all the transgenic pigs used, probably as a result of the high serum concentrations of heterologous GH present in these animals. Thus, to avoid interference of binding by high GH concentrations, serum samples were subjected to immunoblotting using a specific anti-GHBP antibody. A specific 54-kDa band was detected in normal pig serum as well as in sera from mMT-bGH, oMT-oGH, and mTf-hGH pigs. Additionally, sera from transgenic mMT-bGH pigs and their sibling controls were subjected to immunoprecipitation with an anti-GHBP antibody followed by immunoblotting with the same antibody. With this technique, we detected two specific bands of 53 and 45 kDa that could represent different degrees of glycosylation of GHBP. As determined by densitometric analysis the amount of GHBP in transgenic pig sera was similar to that detected in sera of the respective control animals. The amount of circulating GHBP remained unchanged even in oMT-oGH and mTf-hGH pigs that were exposed from birth to circulating concentrations of GH as high as 2,750 ng/mL. Thus, we conclude that heterologous GH do not act as modulators ofthe serum GHBP in pigs.

Pharmacokinetics of radioiodinated growth hormones in the turtle Chrysemys dorbigni

Growth hormone binding proteins (GHBP) have been identified in the blood of many species. The aim of the present work is to study the physiological role of the GHBP in the turtle serum which we recently described. Binding studies were carried out using in vivo pharmacokinetic and chromatographic techniques as well as in vitro methods. When (125)I-GH was injected in physiological concentration into Chrysemys dorbigni turtles, the first step of pharmacokinetics was the binding of a significant fraction of the labeled GH by the GHBPs present in serum. The decay curve followed a three compartments model and gave the equation: Ae(-alphat) + Be(-betat) + Ce(-gammat). The fast compartment with t(1/2) of 14.4 min or 25.2 min, for hGH and bGH represents 30.3% and 18.9% of total radioactivity, respectively, at hypothetical time zero (not experi mental). Chromatographic studies reveal that this rapid compartment represents free GH. The second and third compartments represent complex forms between GH and GHBPs present in the turtle serum, and represent 70% and 80% of total radioactivity for hGH and bGH, respectively. In vitro chromatographic studies showed direct evidence of the presence of GHBPs in the turtle serum. The presence of these GHBPs changed the pharmacokinetics of labeled GH in plasma and the subsequent liver uptake of GH. The labeled hGH or bGH binds to turtle serum in similar proportion, but maximal liver uptake of these hormones are completely different (L/B ratio of 9.2 +/- 0.6 (n = 5) for ( 125)I-hGH and 4.8 +/- 0.3 (n = 7) for (125)I-bGH). The reasons for these differences could be that human GH binds to lactogenic and somatotropic receptors and bovine GH binds only to somatotropic receptors.

Ontogeny of the response to thyroxine (T4) in the porcine fetus: interrelationships between serum T4, serum insulin-like growth factor-1 (IGF-1) and differentiation of skin and several adipose tissues

The temporal aspect of thyroxine (T4) enhanced development of several fetal adipose tissue depots (locations) and skin was examined in the present study. On day 70 of gestation pig fetuses were either hypophysectomized (hypox), hypoxed and implanted with T4 pellets or left intact (control). Subsequently, fetuses were removed for study at either 73, 75, 80, 85 and 90 days of gestation. Serum levels of T4 and insulin-like growth factor I (IGF-I) were significantly elevated by T4 after 3 days of treatment and throughout the study. T4 treatment enhanced development of skin, inner and outer subcutaneous (SQ) adipose tissue and adipocyte lobules around hair follicles (HFL's) but degree of response and response time were tissue or depot dependent. The T4 response time was shortest for skin and HFL's, longest for inner SQ and intermediate for the outer SQ. Considering the entire treatment period, magnitude of the T4 response was much greater for skin and HFL's than for outer and inner SQ tissues. Possibly, these developmental characteristics indicate that T4 directly influences skin and HFL's and indirectly influences inner SQ adipose tissue by increasing production of a locally produced adipogenic growth factor. Furthermore, developmental characteristics (morphological) indicated that putative local growth factor mediation of T4 action may depend on stage of development or maturity of the tissue. IGF-I is clearly the most logical candidate as a putative local mediator of T4, but evidence at the molecular level is needed to prove this suggestion. Regardless, the particular adipose tissue location or depot is an important aspect of hormone responsiveness during the critical or sensitive period of fetal adipose tissue development.

Hormonal regulation of peripheral blood mononuclear cells in sheep

The presence of growth hormone receptors (GHR) on sheep peripheral blood mononuclear (PBMN) cells was studied in two ways. The first was to directly measure specific GH-binding sites on PBMN cells drawn from lambs from birth to 5 months of age. The second was to measure the effect of GH on resting and interleukin-2 (IL-2)-activated PBMN cells in vitro and also the effect of other hormones such as prolactin (PRL), insulin-like growth factor-1 (IGF-1), and glucocorticoid. The specific binding of [125I]human GH (hGH) was low on PBMN cells but increased (P < 0.05) with age up to 5 months. Interestingly, serum concentrations of GH-binding protein also increased (P < 0.01) with age and were highly correlated (r = 0.89; P < 0.05) with the specific binding of GH to PBMN cells. The addition of ovine GH (oGH) to PBMN cells in vitro resulted in increased (P < 0.01) proliferation (at a dose of 100 ng/ml). Higher doses of oGH (2 micrograms/ml) also increased PBMN cell proliferation. When PBMN cells were previously stimulated with IL-2, the dose of 100 ng/ml) oGH was no longer able to stimulate proliferation. IGF-1 inhibited (P < 0.04) resting PBMN cell proliferation at 100 ng/ml but had no effect on IL-2-activated PBMN cells. Other hormones such as PRL caused a stimulation of PBMN cell proliferation (P < 0.01) at 100 ng/ml, whereas dexamethasone (dex) inhibited PBMN cell proliferation at doses as low as 63 ng/ml. These studies show that GH, PRL, IGF-1, and glucocorticoids are effective in modulating the proliferation of PBMN cells from sheep and therefore suggest that a modulation of immune system function in vitro by these hormones is consistent with a purpose in vivo.

Concentrations of thymulin in unextracted serum from pigs, sheep and cattle as measured by ELISA

These studies were conducted to develop an ELISA for measurement of thymulin concentrations in unextracted blood serum (or plasma) from domestic animal species (pigs, sheep and cows). This assay was quite variable (intraassay C.V. of 13.3 and 6.4% at 12.6 and 50.5 pg/mL and interassay C.V. of 24.2%). Serial dilutions of serum from these species produced inhibition curves parallel to the reference standard, suggesting that there were no substances in serum causing non-specific interference in the assay. In addition, none of the other thymic peptides tested resulted in problematic displacement of thymulin binding to the antiserum. Using this assay, it was found that somatotropin (ST) treatment had no effect on serum thymulin concentrations in either pigs or cows. Chromatographic separation of thymulin activity in sheep serum showed three peaks with approximate MW estimates of 95, 80 and 1 kDa. Serum thymulin concentrations in a sheep injected with thymulin was cleared from blood with a half-life (t1/2) of 10.3 +/- 0.6 min. Serum thymulin concentrations increased between birth and 6 mo old in pigs. These data indicate that a rapid and reliable ELISA has been developed to measure thymulin in blood of these domestic animals. This assay should be of value in the study of thymulin function and factors regulating its secretion.

Factors affecting circulating growth hormone binding protein in chickens

Growth hormone binding protein (GHBP) may be an important factor in the regulation of growth and might provide an indirect, relatively noninvasive means of predicting the status of hepatic growth hormone receptor (GHR) activity. Several factors have been reported to influence growth hormone (GH), GHR, or GHBP. Therefore, these studies were conducted to test how age, sex, nutritional status, and glucocorticoids (cortisone acetate, CA) influence serum concentrations of chicken GHBP. Serum GHBP activity was highest (mean percentage specific binding (%SB) = 12.43 +/- .80) at hatch and decreased linearly (P < .0001) to 5 wk of age (%SB = 1.99 +/- 1.13). There were no sex-related differences in serum GHBP activity from hatch to 5 wk of age (P > .08). Short-term nutrient deprivation (24-h) of 4-wk-old broilers also had a significant effect on serum GHBP activity (P < .0001). Measurement of serum GHBP activity with refeeding (after a 24-h period without feed) restored %SB to normal values. Feeding broilers a low-protein diet (12% CP) did not significantly affect serum GHBP activity when compared with that of broilers fed a commercial broiler diet (23% CP; P > .30). Administration of cortisone (1, 5, and 10 mg/day), every 24 h for 7 days, had no effect, at any dose, on serum GHBP activity at 48 h and 1 wk after the last injection. These results indicate that serum GHBP activity is influenced by factors such as age and feed deprivation. It remains to be determined whether these changes in GHBP are associated with changes in GHR as reported for mammalian species.