Hepatic Growth Hormone Receptor Research Articles

IGF-I and IGF-binding protein-3 in plasma of GH-deficient rats

The majority of IGF-I circulates in a large (150 kDa) ternary complex with IGF-binding protein-3 (IGFBP-3) and a non-IGF-binding acid-labile subunit. The secretion of ternary complex into the circulation from liver has been considered to be GH-dependent; however, recent data indicate that GH does not directly regulate hepatic IGFBP-3 synthesis. To examine the role of insulin in regulating plasma IGFBP-3 levels, postpubertal male GH-deficient (dw/dw) rats were treated every 8 h with injections (s.c.) of 0.9% saline, 20 micrograms insulin/day, 200 micrograms hIGF-I/day, or 20 micrograms insulin/day plus 200 micrograms hIGF-I/day, for 10 days with the animals being killed 2-3 h after the final injection. Hypoglycaemia was not observed in any of the treatment groups. hIGF-I treatment increased longitudinal growth and weight gain (P < 0.05), while insulin treatment had no effect. Plasma IGF-I levels were increased in groups treated with hIGF-I (P < 0.05), while insulin treatment resulted in a reduction (P < 0.05): saline = 267.1 +/- 15.6 (ng/ml +/- S.E.M.), insulin = 219.3 +/- 17.5, hIGF-I = 391.7 +/- 17.6, insulin plus hIGF-I = 357.5 +/- 31.8. Hepatic IGF-I mRNA expression was increased in insulin-treated dw/dw rats in comparison with hIGF-I-treated animals (P < 0.05) but not in comparison with saline control or the combined treatment groups. Plasma levels of intact IGFBP-3, measured by ligand blot analysis, were increased in all treatment groups compared with saline (P < 0.05): saline = 100.0 +/- 9.4% (% of saline +/- S.E.M.), insulin = 149.9 +/- 17.5%, hIGF-I = 191.4 +/- 17.3%, insulin plus hIGF-I = 205.4 +/- 15.3%. The levels of the 28/32 kDa IGFBPs and IGFBP-4 in plasma were increased by hIGF-I treatment (P < 0.05) but not by insulin treatment. Hepatic specific 125I-bovine GH binding was not significantly different in any of the treatment groups. This study provides the first evidence in nondiabetic animals that insulin regulates hepatic IGF-I mRNA expression, plasma IGF-I and plasma IGFBP-3 levels in the GH-deficient state without changes in hepatic GH receptors. The divergent response of plasma IGF-I and IGFBP-3 levels to insulin treatment in the present study may indicate an effect of insulin on the clearance of IGF-I from the circulation.

Growth hormone binding protein and free growth hormone in chronic renal failure.

Chronic renal failure in children is associated with growth failure. While the pathogenesis of uremic growth failure is multifactorial, an abnormal growth hormone/ insulin-like growth factor (GH-IGF) axis is an important contributory element. Patients with uremia exhibit insensitivity to the action of GH, as exemplified by high plasma GH levels, low IGF-I activity, and poor somatic growth. This insensitivity can be overcome by supraphysiological doses of exogenous GH. Plasma GH binding protein (GHBP, the circulating ectodomain of the GH receptor) levels are decreased in patients with renal failure, as are hepatic GH receptor levels in animal models. Since GHBP levels are thought to reflect GH receptor levels in tissues, it is likely that the uremic GH insensitivity in humans is mediated by a decreased number of GH receptors. Another implication of the low plasma GHBP is a disproportionate elevation of free plasma GH (the biologically active moiety) relative to total GH, lending additional support to the concept of GH insensitivity in uremia. GH kinetics are altered in renal failure because of: (1) inability to excrete GH and (2) changes in the bound fraction of GH in the circulation.

The ontogeny of hepatic growth hormone receptor and insulin-like growth factor I gene expression in the sheep fetus during late gestation: developmental regulation by cortisol.

The effects of cortisol on hepatic GH receptor and insulin-like growth factor-I (IGF-I) gene expression were investigated in sheep fetuses during late gestation and after experimental manipulation of plasma cortisol levels by fetal adrenalectomy and exogenous infusion of cortisol. Hepatic GH receptor and IGF-I messenger RNA (mRNA) levels increased with increasing gestational age in parallel with the normal rise in fetal cortisol levels toward term (145 +/- 2 days). These increases in mRNA abundance toward term were prevented when the prepartum cortisol surge was abolished by fetal adrenalectomy and were stimulated prematurely in fetuses younger than 130 days by exogenous infusion of cortisol. Both the class 1 and class 2 transcripts of the IGF-I gene were increased when cortisol levels were elevated either endogenously or exogenously. However, there were no significant changes in fetal plasma IGF-I levels either with increasing gestational age or in response to experimental manipulation of the fetal cortisol level. When the data from all the fetuses were combined irrespective of treatment or gestational age, there were significant positive correlations between the log plasma cortisol concentration in utero and the abundance of GH receptor and IGF-I mRNA in the fetal liver. There was also a significant inverse relationship between log plasma cortisol and the ratio of class 1 to class 2 transcript abundance in the fetal liver. These findings show that cortisol is a physiological regulator of hepatic GH receptor and IGF-I gene expression in fetal sheep during late gestation and indicate that it preferentially increases the class 2 transcript of the IGF-I gene. The prepartum cortisol surge therefore appears to have an important maturational role in initiating the perinatal switch from the fetal to adult modes of somatotrophic regulation.

The effects of octreotide on GH receptor and IGF-I expression in the GH-deficient rat

Somatostatin has been suggested to influence the somatotrophic axis outside the central nervous system, in reducing GH-induced IGF-I mRNA and IGF-I generation. This study aimed to determine whether such effects were mediated via the GH receptor (GHR). GH-deficient dwarf rats aged 45-47 days (n = 8 per group) received twice daily subcutaneous injections of octreotide (1 mg/kg) (group O), saline (group S), octreotide (1 mg/kg) plus bovine GH (0.25 mg/kg) (group OG), or bovine GH (0.25 mg/kg) plus saline (group G) for 10 days. Octreotide-treated animals had less weight gain compared with saline-treated animals, but not when GH cotreated (group OG vs G). Octreotide had an overall effect on decreasing length gain (P < 0.01). Serum IGF-I (ng/ml) was reduced by octreotide (group O 171 +/- 11, group S 239 +/- 20, P < 0.01; group OG 283 +/- 30, group G 362 +/- 10, P < 0.001), as was serum insulin (P < 0.001). A significant decrease in hepatic and muscle IGF-I mRNA expression was found as expected, yet this was not associated with decreased hepatic GHR expression. Rather, an increase in hepatic 125I-bovine GH specific binding was observed (P < 0.001) and, in GH-cotreated animals (OG), hepatic GHR and GH binding protein (GHBP) mRNA expression were also increased by octreotide by approximately 40%. In muscle, octreotide was associated with an approximately 30% decrease in GHBP mRNA and no effect on GHR mRNA. This study suggests that the suppressive effects of octreotide on IGF-I metabolism, at least in liver, are not mediated via down-regulation of GHR expression, but more likely by direct effects on IGF-I expression.

The ontogeny of hepatic growth hormone receptor and insulin-like growth factor I gene expression in the sheep fetus during late gestation: developmental regulation by cortisol

The ontogeny of hepatic growth hormone receptor and insulin-like growth factor I gene expression in the sheep fetus during late gestation: developmental regulation by cortisol

Perinatal ontogeny of porcine growth hormone receptor gene expression is modulated by thyroid status.

The ontogeny of growth hormone receptors (GHR) represents a critical stage in growth and metabolism. We have investigated the perinatal ontogeny of hepatic and skeletal muscle GHR gene expression in piglets, and its modulation by GH and thyroid hormones. Test piglets were rendered hypothyroid in late gestation by feeding the sow a high-glucosinolate rapeseed meal. Plasma and tissue samples were obtained from test and control piglets at various ages between 80 days of fetal life (80F) and 2 days postnatally. Plasma hormone levels were determined by radioimmunoassay and GHR mRNA by RNase protection assays. In controls, plasma thyroxine (T4) and 3,5,3'-triiodothyronine (T3) levels increased between 80F and birth and the early postnatal period was characterized by a marked surge in plasma T3. Test piglets were hypothyroid at 110F with total T4, total T3 and free T3 levels being reduced by 28, 53 and 33% respectively. By contrast, the postnatal increase in T3 was more marked in test than in control animals. Plasma GH levels decreased over the perinatal period and there was no effect of treatment. Hepatic GHR mRNA was at the lower limit of detection at 80F but by 110F was expressed in both groups of animals. However, fetal hypothyroidism at 110F resulted in a marked 70% decrease in hepatic GHR mRNA (p < 0.01). The higher postnatal rise in T3 in test piglets was accompanied by a recovery of hepatic GHR mRNA levels. By contrast with liver, skeletal muscle (longissimus dorsi) expressed high levels of GHR mRNA at 80F and hypothyroidism induced a 68% increase in GHR mRNA (p < 0.001). The present results suggest that thyroid hormones may modulate the perinatal ontogeny of GHR gene expression, in addition to other hormonal factors, and that this modulation is tissue-specific.

Growth, growth hormone (GH)-binding protein, and GH receptors are differentially regulated by peak and trough components of the GH secretory pattern in the rat.

Body growth, GH secretory pattern, hepatic GH receptor (GHR), and plasma GH-binding protein (GHBP) levels are all sexually dimorphic in the rat. Male rats grow faster than females, and in GH-deficient animals, GH therapy is more effective when given in a pulsatile pattern rather than a continuous infusion. This contrasts with GHBP and hepatic GHR levels, which are lower in males than in females and raised by continuous but not pulsatile GH therapy. One possible explanation is that growth is primarily regulated by GH pulses, whereas GHR and GHBP are regulated mostly by the trough levels (which are lower in males than in females). To test this hypothesis directly, GH-deficient dwarf rats were given patterned iv infusions of hGH in which the relative contributions of the peak and trough components of the GH pattern were systematically varied, independently of dose, and their effects on weight and length gain, plasma GHBP, and hepatic GHR binding were measured. We found that the dose-response curves for GH given by pulsatile vs. continuous infusion were significantly nonparallel, and that growth was primarily stimulated by the pulsatile component of a mixed GH infusion pattern; doubling the GH dose by adding a continuous (c) infusion to a series of pulses (p) neither enhanced nor inhibited weight gain (36 micrograms hGH/day pulses (36p) vs. 36 micrograms hGH/day pulses + 36 micrograms hGH/day continuously (36p + 36c):0.9 +/- 0.2 g/day vs. 1.1 +/- 0.2 g/day), whereas doubling the GH dose by adding a pulsatile component significantly enhanced growth (72p:2.1 +/- 0.2 g/day, P < 0.01). Conversely, hepatic GHR and plasma GHBP levels were highly sensitive to the continuous element of the mixed infusion pattern and were totally unaffected by varying the pulsatile component over a wide range of doses. These results strongly suggest that growth and hepatic GHR/plasma GHBP respond differentially to the peak and trough components of the GH secretion pattern in the rat.

Responses of young energy-restricted sheep to chronically administered insulin-like growth factor I (IGF-I): evidence that IGF-I suppresses the hepatic growth hormone receptor

Responses of young energy-restricted sheep to chronically administered insulin-like growth factor I (IGF-I): evidence that IGF-I suppresses the hepatic growth hormone receptor

Responses of young energy-restricted sheep to chronically administered insulin-like growth factor I (IGF-I): evidence that IGF-I suppresses the hepatic growth hormone receptor.

We have shown previously that chronic administration (8 weeks) of insulin-like growth factor-I (IGF-I) has little growth-promoting effect in well fed sheep. The aim of this study was to investigate the anabolic effects of IGF-I in energy-restricted conditions in which circulating concentrations of IGF-I in control animals were expected to be low. Young castrate male sheep were offered chaffed lucerne at a rate equivalent to 110% maintenance and were treated by sc injection three times per day for either 8 or 12 weeks with recombinant human IGF-I (150 micrograms/kg live wt x day) or saline in a 2 x 2 factorial design (eight animals per cell). IGF-I treatment significantly increased plasma IGF-I concentrations, but reduced plasma concentrations of IGF-II, GH, urea, and creatinine. Treatment with IGF-I also decreased (P < 0.1) GH secretion in response to a GRF load, but significantly (P < 0.05) increased the nonesterified fatty acid response to an epinephrine load. The reduction in circulating GH levels was accompanied by a suppression of [125I]oGH binding to hepatic microsomal membranes. This effect, if apparent in other tissues, may act as a feedback mechanism to limit the local synthesis of IGF-I and could explain why IGF-I treatment had little effect on the growth rate of the sheep, although it did increase nitrogen digestibility of the feed consumed and decreased the fat content of the hind leg. It also differentially promoted the growth of the spleen, thymus, and mandibular salivary gland and increased blood counts of eosinophils. It is concluded that IGF-I does not have marked effects on growth rate or body composition in sheep fed a near-maintenance diet. Possible reasons include the associated suppression of GH secretion and action, which limits the ability of treated animals to repartition absorbed nutrients.

Growth hormone (GH) status regulates GH receptor and GH binding protein mRNA in a tissue- and transcript-specific manner but has no effect on insulin-like growth factor-I receptor mRNA in the rat

The effect of growth hormone-deficiency (GHD) and treatment with recombinant bovine GH (bGH) or human IGF-I (hIGF-I) for 10 days on the expression of GH receptor (GHR). GH binding protein (GHBP) and of insulin-like growth factor-I receptor (IGF-IR) mRNA was examined using dw dw and normal Lewis rats. Hepatic GHR and GHBP mRNA expression was significantly lower in dw dw rats in comparison to Lewis rats ( P < 0.01) while specific 125I-bGH binding to hepatic microsomal membranes was significantly higher ( P < 0.01), suggesting a reduction in hepatic GHR turnover with GHD. Treatment with bGH reduced hepatic specific 125I-bGH binding in dw dw rats, but had no effect in Lewis rats. Treatment with hIGF-I increased hepatic specific 125I-bGH binding in Lewis rats. Hepatic GHR and GHBP mRNA expression was not changed by bGH or hIGF-I treatment, suggesting that differences in hepatic specific 125I-bGH binding may be due to posttranscriptional mechanisms. GHBP mRNA expression was higher in kidney, heart, and muscle of dw dw rats in comparison to Lewis rats ( P < 0.01), while GHR mRNA abundance was not changed. Treatment of dw dw rats with hIGF-I or bGH resulted in a coordinate reduction of GHR and GHBP mRNAs in kidney ( P < 0.01). IGF-IR mRNA was not detected in liver and despite reduced plasma IGF-I levels and IGF-I mRNA expression IGF-IR mRNA abundance was not changed in nonhepatic tissues by GHD. Our data suggest that changes in plasma IGF-I levels and local IGF-I mRNA do not influence IGF-IR mRNA expression, while GHR and GHBP mRNA expression in different rat tissues are regulated independently. The increased nonhepatic GHBP mRNA expression with GHD suggests that nonhepatic GHBP may have an important physiological function distinct from that of GHBP in liver or in plasma.

Recombinant Human Growth Hormone for Children With Renal Failure

The pathogenesis of growth retardation in children with chronic renal failure (CRF) is clearly multifactorial. A major breakthrough in the understanding of the pathogenesis of uremic growth failure was achieved only recently by a more detailed analysis of the growth hormone (GH)/insulinlike growth factor (IGF) axis. Uremia is characterized by an insensitivity to the somatotropic action of GH. The mechanisms that account for this insensitivity include reduced hepatic GH receptor expression, decreased production of IGF-I, and inhibition of IGF bioactivity by increased binding of IGFs to their specific binding proteins. Recombinant human growth hormone (rhGH) in supraphysiological doses is able to overcome the partial GH resistance and to stimulate longitudinal growth under both experimental and clinical conditions. One possible mechanism of action of rhGH in uremia is the restoration of circulating IGF bioactivity, which results from the differential regulatory effect of rhGH on circulating IGF-I and IGFBP-3 concentrations. RhGH has proven to be an effective, safe, and well-tolerated new treatment modality for growth-retarded children at all stages of CRF. There is strong evidence that final height will increase in these children. Other than a modest chronic stimulation of insulin secretion, no frequent side effects have been observed; in particular, no acceleration in loss of residual renal function has been seen in children treated before the onset of end-stage renal failure. In children after transplantation, rhGH is also effective, but the potential risk of interference with graft function is not yet sufficiently defined.

Growth hormone insulin-like growth factor I axis in insulin-dependent diabetes mellitus.

Growth hormone (GH) hypersecretion and relative insulin-like growth factor I (IGF-I) deficiency have been implicated in the development of insulin resistance, poor metabolic control, and impaired growth during puberty in insulin-dependent diabetes mellitus (IDDM). Portal levels of insulin are critical for the integrity of the hepatic GH receptor and suppression of the inhibitory IGF-binding protein I. Increasing insulin doses during puberty will result in adequate portal levels of insulin and thus restore IGF-I levels and IGF bioactivity, but at the risk of nocturnal hypoglycaemia and weight gain. Restoration of normal circulating levels of IGF-I using the recombinant peptide will lead to reductions in GH levels and improved insulin sensitivity. Given as a daily subcutaneous injection, low-dose recombinant human IGF-I (40 micrograms/kg/day) could prove to be a useful adjunct to standard insulin therapy during puberty in IDDM. The reduced and more stable insulin requirement might reduce the risks of nocturnal hypoglycaemia and weight gain. However, whereas intensified insulin therapy will reduce the risk of microangiopathic complications, the benefits of combination therapy have not been proven. The role of recombinant human IGF-I in the treatment of IDDM needs to be tested by long-term controlled trials.

Distinct sexual dimorphism in the effect of hypothyroidism on the expression of the growth hormone receptor and growth hormone-binding protein gene in rat liver.

Impairment of growth is a hallmark of hypothyroidism in animals. The ability of the thyroid hormone-thyroid hormone receptor complex to regulate gene transcription may be relevant to the growth impairment associated with hypothyroidism. To study the role of thyroid hormone in the expression of the GH receptor (GHR) and GH-binding protein (GHBP) gene, we examined the serum and liver tissue of female and male hypothyroid (thyroidectomized), thyroxine-treated thyroidectomized and euthyroid control rats. Compared to the control and to the thyroxine-treated group, the hypothyroid rats had significantly lower serum levels of thyroxine, increased levels of TSH, and decreased rates of weight gain. GHR and GHBP mRNA levels in liver were estimated by ribonuclease protection assays. In female rats, the levels of hepatic GHR and GHBP mRNA were increased in the hypothyroid group compared to euthyroid controls (p < 0.001 for GHR and p < 0.05 for GHBP). In contrast, in males the hypothyroid state was associated with decreased levels of GHR (p < 0.001) and GHBP (p < 0.001) mRNA levels compared to euthyroid controls. In both females and males, administration of thyroxine for a period of 2 weeks to the thyroidectomized rats prevented these changes in GHR and GHBP mRNA levels in liver. The differences observed between females and males could not be attributed to differences in the circulating levels of GH at sacrifice (female vs. male. 9.9 +/- 1.3 vs. 13.9 +/- 6.5 ng/ml). We conclude that (1) thyroid hormone affects the transcription of the GHR/GHBP gene; (2) there is a distinct sexual dimorphism in the effect of hypothyroidism on the expression of the GHR/GHBP gene, and (3) this effect is reversible following amelioration of the hypothyroid state. We speculate that regulation of expression of the GHR/GHBP gene by thyroid hormones involves multiple thyroid response elements that have opposite effects depending on the status of other factors such as sex hormones.

Growth hormone (GH) receptor and GH-binding protein deficiency in the growth failure of potassium-depleted rats.

Potassium (K+) deficiency is associated with growth retardation in both man and experimental animals. Growth hormone (GH) administration to such animals prevents, to some extent, weight loss and selective muscle atrophy, but does not affect tail and tibia length even with supraphysiological doses. The present study was undertaken to investigate the possible effect of K+ deficiency on the hepatic GH receptor and GH-binding protein (BP). Young female Wistar rats were maintained on K(+)-deficient fodder and distilled water, and compared with pair-fed and ad-libitum-fed control groups. After 15 days GH-BP and electrolytes were measured in sera, GH receptors were studied in liver membranes by 125I-labeled human GH binding and muscles were weighed and saved for electrolyte measurements. K(+)-deficient rats showed complete growth arrest compared with an intermediate weight gain of the pair-fed group. Serum K+ was very low, at 1.5 +/- 0.1 mmol/l, compared with the mean value of 5.3 mmol/l of control animals. Somatogenic and lactogenic receptors in liver membranes and serum GH-BP levels were significantly (P < 0.05) lower in K+ deficiency, as compared with their pair-fed controls. Liver GH receptors correlated significantly (P < 0.05) with serum GH-BP levels. The growth variables correlated positively with both hepatic somatogenic and lactogenic receptors and serum GH-BP levels, with correlation coefficients that were highest against serum GH-BP and lowest against liver lactogenic receptors. Serum and muscle K+ correlated significantly (P < 0.05) with both liver GH receptors and serum GH-BP, with correlation coefficients that were higher against serum GH-BP.(ABSTRACT TRUNCATED AT 250 WORDS)

Upregulation of GH receptor and GH binding protein during pregnancy in the GH deficient rat

During pregnancy there are dramatic changes in the endocrine and metabolic status of the mother: growth hormone (GH) is an important regulator of growth and development. A proportion of GH is bound by specific GH binding proteins (GHBP) that closely resemble the GH receptor (GHR). In the rodent both GHBP and the GHR are considered to be GH dependent, and consequently during pregnancy the increase in serum GH is associated with an increase in GHBP. To examine whether an increase in maternal GH is obligatory for elevation of maternal GHBP or GHR during pregnancy, we used a unique GH-deficient (GHD) strain of rats, to avoid the methodological complications of hypophysectomy and assessed serum GH, GHBP and hepatic GHR binding during the course of pregnancy. In GH normal rats, serum GH concentrations increased twofold and GHBP levels increased threefold; there was no change in hepatic GHR binding. In GHD rats, serum GH concentrations were low and did not increase during pregnancy. Nonetheless, levels of both serum GHBP and hepatic GHR binding increased to that measured in normal rats. Thus, an increase in maternal GH concentration is not required for the gestational upregulation of maternal GHBP or hepatic GHR binding, suggesting that other hormones may be essential in modulating the GH axis during pregnancy.

Ontogenic study of insulin-like growth factor-1 and growth hormone receptor mRNA expression in porcine liver and skeletal muscle

AbstractThe growth hormone-insulin-like growth factor (GH-IGF) axis is involved in the control of postnatal growth in all animals studied to date. At birth, blood levels of IGF-1 are low, although levels of GH are high. Thus a switching occurs during the postnatal period, which involves an increase in liver GH-receptors (GHR). This study investigates the changes in serum IGF-1 and IGF-1 and GHR mRNA in liver and skeletal muscle of growing pigs. The period from birth to 20 days of age showed a gradual increase in both IGF-1 and GHR mRNA expression by the liver, thus accounting for the gradual increase in serum IGF-1. The IGF-1 mRNA expressed was found to be predominantly class 1 transcripts with very few class 2 transcripts. There was a plateau of liver IGF-1 mRNA levels after 20 days and up to 140 days, even though the expression of GHR mRNA continued to increase. The levels of expression of the two genes were found to correlate in the first 20-day period (r = 0·76,P&lt; 0·005), but not in the later period (r = 0·44,P&gt; 0·05).Levels of expression of both genes in the LD muscle were highly variable, with no age related changes being observed. However, a significant negative correlation of expression of the two genes was observed (R = 0·60,P&lt; 0·001), with the correlation still being significant in the two age periods.The data are consistent with a likely initial dependence for postnatal growth upon GH-stimulated IGF-1 mRNA expression by the liver, up to 20 days of age. After this time, hepatic expression of IGF-1 mRNA reaches a plateau, even though the serum IGF-1 and hepatic GHR mRNA levels continue to rise. A possible explanation for this could be an increase in either turn-over or stability of IGF-1 mRNA, giving rise to increased levels of peptide with no apparent change in mRNA level. Another possibility is that an increased expression/production of the IGF-binding proteins leads to an increase in the half-life of the peptide in the circulation.

Thyroid glands: novel sites of growth hormone action.

Thyroid hormones inhibit the synthesis and release of GH in avian species. This may represent a feedback mechanism, since GH enhances the peripheral production of tri-iodothyronine (T3). The possibility that GH may also have direct effects on thyroidal function was therefore investigated. The basal and thyrotrophin-induced release of thyroxine (T4) from incubated chicken thyroid glands was not enhanced, however, in the presence of chicken GH. Contrarily, GH impaired T4 release in a dose-related way. These actions were probably mediated by specific receptors, since binding sites for radiolabelled GH were demonstrated on the plasma membranes of chicken thyroid glands. Expression of the GH receptor gene in these tissues was also demonstrated using a cRNA probe for the rabbit liver GH receptor, which specifically hybridized with RNA moieties of 4.4 kb, 2.7 kb and 1.0 kb. Moreover, reverse transcription of thyroidal RNA and its amplification in the presence of 3'- and 5'-oligonucleotide primers coding for the extracellular or intracellular domains of the GH receptor generated electrophoretically separable fragments of 500 bp and 800 bp respectively, as would be expected from analysis of the hepatic GH receptor cDNA sequence. Digestion of the 500 bp fragment with NcoI or EcoRI also produced moieties of expected size (350 bp and 150 bp or 325 bp and 175 bp respectively), as did BamHI or HaeIII digestion of the 800 bp fragment (yielding fragments of 550 bp and 275 bp or 469 bp and 337 bp respectively). Translation of the GH receptor mRNA was also indicated by the immunocytochemical demonstration of GH receptors in thyroid follicular and parafollicular cells, using a specific polyclonal antibody raised against the chicken GH-binding protein. These results therefore provide evidence, for the first time, of GH receptor gene expression in thyroid tissue and the translation of functional GH receptors in thyroid glands. These results also demonstrate differential effects of GH on the extracellular concentrations of T3 and T4, which may permit subtle regulation within the somatotroph-thyroid axis.

Altered endogenous growth hormone secretory kinetics and diurnal GH-binding protein profiles in adults with chronic liver disease.

Increased serum GH concentrations and GH responses to a variety of stimuli have been reported in patients with chronic liver disease (CLD). We investigated the pulsatile pattern of endogenous GH release and GH-binding protein (GHBP) and insulin-like growth factor-I (IGF-I) diurnal profiles in adults with cirrhosis, in comparison with healthy, matched control subjects. Case-control, cross-sectional. Seven patients with biopsy proven cirrhosis, and sex, age, height, weight and oestrogen status matched controls. Serum immunoreactive GH concentrations in samples collected at 20-minute intervals for 24 hours were analysed using a multi-parameter deconvolution method to simultaneously resolve endogenous GH secretory and disappearance rates. Diurnal patterns of GHBP (specific immunoprecipitation method) and serum IGF-I (RIA after acid-ethanol extraction) were assessed. The mean daily GH secretion rate in patients with CLD was increased (210 +/- 93 vs 100 +/- 55 mU/I/day; P = 0.025), and GH disappearance half-time was prolonged (43 +/- 10 vs 24 +/- 9 min; P = 0.006) compared to controls. Detectable GH secretory bursts were more frequent in patients with CLD (10 +/- 1 vs 6 +/- 3/day; P = 0.038), but of similar mean mass (21 +/- 10 vs 17 +/- 8 mU/I) compared to controls. In patients with CLD, mean serum GHBP was slightly lower (63 +/- 36 vs 71 +/- 14% pooled control; P > 0.1). Fasting serum IGF-I concentrations (after size-exclusion HPLC) were lower in the patients with CLD (13 +/- 5 vs 21 +/- 2 nmol/l; P < 0.0001). Multiple regression analysis showed that GH secretion rate was increased in patients with CLD with higher Child's classifications (R2 = 0.86; P = 0.002) and with lower serum IGF-I concentrations measured after HPLC (R2 = 0.11; P = 0.044). Adults with chronic liver disease have (1) increased total daily GH secretion rates, which appear to be influenced by disease severity and diminished serum IGF-I-mediated negative feedback; (2) markedly impaired endogenous GH clearance, possibly reflecting changes in hepatic GH-receptor status; and (3) GHBP levels which do not correlate with GH kinetics or serum IGF-I concentrations.

Effect of gonadectomy on growth and GH responsiveness in dwarf rats

Normal rats are sexually dimorphic in their growth and GH secretion. Gonadectomy (Gnx) changes the GH secretory pattern and this could explain differential growth rates in male and female rats. Gonadal steroids may also affect tissue growth directly, or by changing their responsiveness to GH. The effects of Gnx on growth, GH responsiveness, and hepatic GH receptors have now been studied in young (4-7 weeks old) GH-deficient dwarf rats in which the effects of steroid-induced alterations in residual endogenous GH secretion will be much less pronounced. Groups of intact and Gnx dwarf rats (n = 5-7) were infused with recombinant human GH (144 micrograms/day) either continuously or in a pulsatile pattern (every 3 h) for 7 days, whilst control groups received saline infusions. Gains in weight, length and tibial bone growth were measured. Female dwarf rats grew significantly more slowly than male dwarfs. Gnx in male dwarfs inhibited growth significantly, whereas ovariectomy had a lesser stimulatory effect in females. Hepatic lactogenic and somatogenic receptors were higher in females and ovariectomy lowered their values towards male levels. Pulsatile GH infusions were more effective than continuous infusions of the same daily GH dose, but when the different underlying growth rates (measured in saline-infused Gnx animals) were taken into account, the responsiveness to pulses of hGH was not different between males and females or between intact and Gnx animals. We have concluded that Gnx in dwarf rats does affect growth, but the effects are small in comparison with those seen in normal rats. Since pulsatile GH infusions stimulated growth more effectively than continuous GH, irrespective of the gonadal steroid status, and the responsiveness to exogenous GH was not changed by Gnx, the results imply that changes in the GH secretory pattern induced by gonadal steroids may have a larger impact on the growth rate than direct effects at the tissue level, at least in the rat.

Expression and distribution of messenger ribonucleic acids for growth hormone (GH) receptor and GH-binding protein in mice during pregnancy.

The GH-binding protein (GHBP) in rodents consists of a ligand-binding domain, which is identical to the extracellular portion of the GH receptor (GHR), and a hydrophilic carboxyl-terminal domain, in place of the transmembrane and intracellular domains of the GHR. The two proteins are encoded by separate messenger RNAs (mRNAs), which are believed to be derived from a single gene by alternative splicing. In the present study, we report the gestational profiles of mouse GHR (mGHR) and mGHBP mRNAs in adipose tissue, brain, heart, kidney, liver, lung, mammary gland, muscle, ovary, and pituitary and describe the ontogeny of both messages in the liver of late gestational fetuses and newborns. A ribonuclease protection assay was used to simultaneously detect the two transcripts with an antisense RNA probe complementary to the extracellular domain- and hydrophilic tail-encoding regions of the mRNAs. Levels of hepatic GHR and GHBP mRNAs increased with fetal age. In the maternal liver, the abundance of both messages increased during pregnancy, with GHR mRNA levels rising less than GHBP mRNA. Also, the ratio between the two messages in this tissue increased during pregnancy in favor of mGHBP mRNA. In maternal mammary tissue, however, expression levels of both transcripts decreased gradually throughout pregnancy starting on day 8 of gestation and declining further during lactation, reaching a minimum 7-fold reduction on day 6 of lactation relative to nonpregnant values. Although there were no pregnancy-related changes in the remaining tissues we examined, the ratio of the abundance of GHR mRNA to that of GHBP mRNA varied tissue specifically. In the maternal brain, heart, liver, and mammary gland, mGHBP mRNA levels were higher than mGHR mRNA levels. In the maternal muscle and adipose tissue, the abundance of the two mRNA species was comparable. These observations indicate a gestational, developmental, and tissue-specific regulation of the expression of mGHR and mGHBP species.