Human growth hormone and somatomedin C suppress the spontaneous release of growth hormone in unanesthetized rats

To investigate the feedback control of GH secretion, we examined the effects of human GH (hGH) and somatomedin C (SmC) on spontaneous GH secretory surges in unanesthetized, freely moving rats. Under pentobarbital anesthesia a right atrial catheter and an intracerebroventricular cannula were placed 7-10 days before the experimentation. For iv studies, hGH (0.3 U/ml.h) was infused for 6 h after an iv loading dose (0.3 U) at the beginning of the experiments. For intraventricular injections, hGH (0.1 U/10 microliter) or SmC (500 ng/10 microliter) were injected into the lateral ventricle 2 h before the experiments. The equivalent dose of crystalline BSA diluted in the same vehicle solutions was administered to the same rat as a control on a separate day. Venous blood samples were collected every 20 min for 6 h. Intravenous and intraventricular administration of crystalline BSA did not affect the typical rat GH (rGH) surges which appeared about every 3 h and reached peak values of more than 300 ng/ml. The iv infusion of hGH significantly inhibited the amplitude of rGH surges compared to controls (planimetric areas under the secretory profile 752 +/- 172 vs. 1921 +/- 183, P less than 0.01, n = 6). rGH secretion was similarly inhibited by intraventricular hGH (701 +/- 127 vs. 2208 + 225, P less than 0.01, n = 6) and by intraventricular SmC (537 +/- 70 vs. 1503 +/- 114, P less than 0.01, n = 6). These findings suggest that both GH and SmC are active in the feedback regulation of rGH secretion.     

Comparison of growth and somatomedin C responses following growth hormone treatment in children with small-for-date short stature, significant idiopathic short stature and hypopituitarism

Somatomedin-C (Sm-C) and growth hormone (GH) levels were determined before, during and after human growth hormone (hGH) treatment in 18 children with small-for-date short stature ( SDSS ), 7 children with significant idiopathic short stature ( SISS ) and 14 children with hypopituitarism. Data on the acute effects of hGH on Sm-C were compared to growth responses after 6 to 9 months therapy. Eleven of the 25 non-hypopituitary patients with normal basal and stimulated serum GH levels and normal basal Sm-C levels increased their rates of growth more than 3.0 cm/year. This compared with 11 of the 14 children with hypopituitarism who increased their rates of growth by at least 3.0 cm/year when treated with GH. Neither the basal somatomedin levels nor the GH-stimulated somatomedin levels correlated well with subsequent growth in the non-hypopituitary patients. These studies indicate that GH therapy may be effective in treating short stature in children without demonstrable GH deficiency.     

Evidence that selenium induces growth retardation through reduced growth hormone and somatomedin C production

Growth retardation has long been known to be a major characteristic in selenium-intoxicated animals. As selenium is known to accumulate in the anterior pituitary, especially in the secretory granules of the somatotroph, we have investigated the GH secretion after GH-releasing factor 40 stimulation and the somatomedin C secretion in young male rats exposed to 15 mg sodium selenite/liter drinking water. The immediate output to 900 +/- 120 ng/ml GH in control animals was reduced to 200 +/- 69.4 ng/ml in selenium-treated animals. The somatomedin C level was reduced from 720 +/- 16 ng/ml in control to 119 +/- 17 ng/ml in selenium-treated animals. Both differences were highly significant. These findings suggest that growth retardation in selenium-treated rats could be mediated by reduced GH and somatomedin C production.     

Growth hormone, somatomedin levels and growth regulation in Turner's syndrome

In a total of 56 children and adolescents with Turner's syndrome (41 with karyotype 45,X) basal serum levels of somatomedin bioactivity, Sm-C/IGF-I (RIA), IGF II (RIA), GH response to arginine and GHRH (GRF(1-29)NH2), and spontaneous GH secretion during 5.5 h of deep sleep were determined in a cross-sectional manner. GH responses to GRF and arginine as well as IGF-II levels were found to be in the normal range. Levels of somatomedin bioactivity were higher than normal before a bone age of 10 years, in the low-normal range thereafter, and below normal in some patients. Levels of Sm-C/IGF-I were found normal before and low-normal after a bone age of ten years. There was a trend towards increasing Sm-C/IGF-I levels with age. In contrast to the normal pattern, spontaneous sleep-related GH secretion was declining with age and did not show the puberty-associated rise. These findings suggest a normally functioning growth hormone-somatomedin axis in Turner's syndrome with alterations of its functioning level occurring secondarily as a result of absent gonadal activation. In single patients abnormally low growth hormone and/or somatomedin secretion may be present.     

Amniotic fluid reactivity detected by somatomedin C radioreceptor assay: correlation with growth hormone, prolactin and fetal renal maturation.

Amniotic fluid was assayed by a radioreceptor assay utilizing 125I-somatomedin C and placental membranes. Growth hormone and prolactin levels were measured by radioimmunoassay at different gestational ages (8-41 weeks). Somatomedin receptor activity, growth hormone, and prolactin reached high levels during early gestation (8-26 weeks) displaying different patterns of appearance which reflect fetal serum levels of these hormones. After 26 weeks gestation all these hormones decreased in concentration. This decrease showed a strong correlation with fetal renal maturation as measured by amniotic fluid creatinine levels.     

Growth hormone restores normal growth in selenium-treated rats without increase in circulating somatomedin C

Selenium intake (5.0 ppm) induces growth retardation, accumulation of selenium in somatotrophs, lack of growth hormone response to GHRH and an 80 per cent reduction in serum somatomedin C in infant rats. In addition, it induces a slight reduction in serum albumin and occasionally slight central liver necrosis. In order to determine the role of insufficient growth hormone production, the influence of exogenous growth hormone was studied during selenium intake in groups of rats during 25 to 46 days of age (post-weaning). A dosage of human growth hormone (100 micrograms twice daily) was chosen, this being sufficient to restore normal growth rate and normal serum somatomedin C in hypophysectomized rats of similar age. The weight gain in selenium-treated (3.3 ppm) rats was 46.0 +/- 11.7 g (SD)/21 days, whereas in selenium rats given growth hormone it was 66.6 +/- 8.9 g (P = 0.01), which was similar to the gain in control rats 72.3 +/- 6.9 g. The latter two were less than the weight gain in control rats given growth hormone: 91.5 +/- 11.5 g (P = 0.01 and P less than 0.01). Serum somatomedin C in untreated rats was 151 +/- 66 (SD) micrograms/l (25 days) increasing to 532 +/- 91 micrograms/l (34 days) and 482 +/- 64 micrograms/l (46 days). It did not increase above these levels in control rats given growth hormone. In selenium-treated rats, no increase occurred during growth hormone administration 138 +/- 148 micrograms/l (34 days) and 185 +/- 84 micrograms/l (46 days) (P less than 0.001 and P less than 0.001 vs untreated controls).(ABSTRACT TRUNCATED AT 250 WORDS)     

The Brattleboro rat: normal growth hormone secretion, decreased hepatic growth hormone receptors and low plasma somatomedin activity

Three-month-old male Brattleboro rats with hereditary diabetes insipidus (DI) present a growth defect; Brattleboro rats were studied together with age-matched Long-Evans (LE) rats. Pituitary growth hormone (GH) content was comparable in both groups of rats. Pulsatile GH release and mean 6 h GH plasma levels did not appear significantly different in chronically catheterized DI and control animals. In parallel with the growth defect, the plasma somatomedin bioactivity was significantly lower in DI than in LE rats. The specific binding of [125I]iodo-hGH to liver microsomal membranes of DI rats was 59.7% that of controls. The number of the GH binding sites rather than the affinity of the binding was decreased. The specific binding of [125I]iodo-insulin was oppositely affected by the DI state: it was 1.5 times higher in liver membranes of DI rats than in membranes of LE rats. These findings make a non-specific effect of the DI state on liver membrane proteins unlikely. The Brattleboro rats present a growth failure without reduction of their GH secretion. The decreased number of the hepatic GH receptors and the subsequent low plasma somatomedin activity could explain the growth retardation of the DI rats.     

Acute and chronic estrogen effects upon serum somatomedin activity, growth hormone, and prolactin in man.

Estrogen (E) reduces bioassayable GH-dependent serum somatomedin (SM) activity in acromegalics without affecting plasma growth hormone (GH) levels and inhibits the rise of SM activity normally produced by GH administration in GH-deficient subjects. We have now investigated the effect of E administration on serum SM activity and on plasma GH and prolactin (PRL) in 6 adult male subjects without pituitary pathology. Chronic E administration (ethinyl estradiol 0.5 mg/day for 7 to 70 days) reduced serum SM activity by 40 to 62% in each of 4 subjects (P less than 0.02 to less than 0.001). In 3 of the subjects, basal GH levels increased by 75 to 300% (P less than 0.05 to less than 0.001) and basal PRL levels increased by 90 to 200% (P less than 0.01 to less than 0.001). While iv administration of normal saline did not significantly affect either SM or GH, iv administration of E (bolus injection of 25 mg conjugated estrogens, USP) to 5 subjects resulted in: a) a 46 to 80% decrease in serum SM activity in all subjects, proceeding with an apparent half-life of 2 hours, becoming significant (P less than 0.05) at 2 hours (1 subject) to 3 hours (4 subjects), maximal at 6 hours, and persisting for 12 to 24 hours; b) GH elevation to 3 to 16 times baseline level (P less than 0.01) at 2 to 3 hours in 4 subjects; and c) no significant change of PRL levels in any subject. The mean GH response to iv E was maximal at a time (2 hours) when the mean SM activity had decreased only 20% and subsided well before the nadir of SM activity. The one patient without GH response to chronic or acute E administration may have been affected by absorption of triamcinolone being applied topically during the study. These results demonstrate that in males with normal pituitary function, E reduces serum SM activity, enhances basal GH and PRL secretion, and, upon iv injection, stimulates acute GH release. Although opposite chronic E effects upon GH and SM activity support a putative negative SM-GH feed-back mechanism, iv E administration apparently provokes acute GH release by a different mechanism. The half-life of serum SM activity in the human is probably much shorter than previously estimated.     

Human growth hormone stimulates somatomedin C/insulin-like growth factor I production by the human lymphoid cell line, IM-9

The human lymphoid cell line, IM-9, is known to possess receptors for human growth hormone (hGH), but the only biological response that has been shown to follow binding of this hormone to the cells is receptor down-regulation. We have studied the actions of hGH on production of insulin-like growth factor I (IGF-I) by IM-9 cells. In order to demonstrate effects cells had to be transferred to a serum-free medium in which cell multiplication almost ceased, and cell viability fell to 50-60%. hGH stimulated IGF-I production by up to 400%. The effect was dose-related, but the dose-response curve was bimodal, with peaks of activity at approximately 15 ng/ml and 1000 ng/ml hGH. The effect of hGH was of slow onset, becoming significant only after about 24 h, and approaching a maximum after 2-5 days of treatment. hGH had a much greater stimulatory effect than non-primate growth hormones. The physiological significance of the effect observed is not yet clear, but it is apparent that the IM-9 line is a potentially useful model for study of the actions of growth hormone.     

The effect of hypothyroidism on growth, serum growth hormone, the growth hormone-dependent somatomedin, insulin-like growth factor, and its carrier protein in rats.

To study the possible mechanisms involved in growth retardation associated with hypothyroidism, serum T4, GH, the GH-dependent somatomedin, insulin-like growth factor (IGF), and its carrier protein (CP) were measured in hypothyroid rats and their age-matched controls. Three groups of rats were studied: infant, immature, and adult. Marked hypothyroidism (serum T4, less than 1 microgram/dl) was produced in experimental animals by providing them with drinking water containing 0.05% propylthiouracil. Infant and immature hypothyroid rats weighed markedly less than normal controls and had significantly reduced serum levels of GH, IGF, and CP. Normal adult rats, treated with propylthiouracil for 60 days, also weighed considerably less than control animals and exhibited a significant drop in serum GH, IGF, and CP during this period. The administration of bovine GH to hypothyroid adult rats for 7 days did not restore either IGF or CP levels to normal, indicating that their decrease in serum was, in part, a direct result of hypothyroidism per se. These results indicate that serum levels of GH, IGF, and CP are at least partly under thyroid hormone control. Furthermore, these studies suggest that the growth retardation associated with hypothyroidism may be mediated through somatomedin activity.     

Growth hormone increases ovarian levels of immunoreactive somatomedin C/insulin-like growth factor I in vivo

The hypothesis that GH may affect gonadal function by increasing local levels of the GH-dependent somatomedin C/insulin-like growth factor I (IGF I) was tested. Ovine GH (200 micrograms) was injected into immature, hypophysectomized, estrogen-treated female rats; animals were sacrificed 8 or 12 h later. Renal and ovarian homogenates were acid extracted and chromatographed over Sephadex G-50. Eight h after GH injection, 3.6 to 6.4-fold increases in immunoreactive IGF I (IR-IGF I) levels were observed in either ovarian or renal extracts subjected to acid chromatography. Twelve h after GH treatment, IR-IGF I levels remained elevated, but were lower than after 8 h. In neither case could IR-IGF I levels be accounted for by serum contamination. IR-IGF I eluted with an apparent mol wt near that of synthetic human IGF I in both kidney and ovary. Thus, GH can directly increase ovarian and renal tissue IR-IGF I levels in vivo. Taken with previous observations showing a direct gonadotropin-enhancing effect of IGF I on rat granulosa cells in vitro, our results support the hypothesis that GH may affect ovarian differentiation by inducing the local production or accumulation of IGF I, providing evidence for a novel intraovarian paracrine control mechanism.     

Brain growth-promoting activity in human serum: relationship to growth hormone and somatomedin.

Brain growth-promoting activity and somatomedin activity were assessed in serum collected from patients with pituitary disorders of GH secretion. Brain growth-promoting activity correlated with somatomedin activity in serum, and both rose in response to GH administration.     

Synthetic somatomedin C: comparison with natural hormone isolated from human plasma.

The biological and immunological properties of a chemically synthesized preparation of somatomedin C (Sm-C) were compared with those of the natural product isolated from human plasma. The two preparations produced identical curves in the radioimmunoassay and radioreceptor assay for Sm-C and in the radioreceptor assay for insulin. They were identical in their ability to stimulate DNA synthesis in confluent BALB/c 3T3 cells previously exposed to platelet-derived growth factor, and the biological activities of both preparations were completely neutralized by a monoclonal antibody raised against native Sm-C. These studies demonstrate that the chemically synthesized product is equivalent to the native molecule in all important respects and that it can be used interchangeably with the natural product for any studies that are contemplated.     

Comparison of the delayed actions of growth hormone and somatomedin on adipose tissue metabolism.

Although acute insulin-like effects of growth hormone (GH) on adipose tissue are readily demonstrable in vitro, it has not been possible to reproduce in vitro the delayed (3 h) inhibition of glucose utilization seen after the administration of GH in vivo. To examine the possibility that somatomedin (Sm), the postulated mediator of GH action, might, like GH, have biphasic effects on adipose tissue metabolism and mediate the delayed inhibition of glucose utilization, Sm was prepared by gel filtration from an acid-ethanol extract of normal rat plasma. The partially purified material increased 35SO4 incorporation into costal cartilage of hypophysectomized rats and produced in adipose tissue an acute stimulation of glucose oxidation that was not suppressed by insulin anti-serum. Unlike GH, whose insulin-like effects disappear in 3--4 hours, Sm-stimulated glucose oxidation remained linear for the entire 4 hour incubation period. Although acute stimulation of tissues with GH rendered them refractory to renewed insulin-like stimulation by GH, no such refractoriness to the action of Sm was seen. Furthermore, acute stimulation with Sm failed to render tissues refractory either to itself or to GH. Finally, Sm failed to reproduce the delayed lipolytic effects produced by GH in conjunction with glucocorticoids. These results make it highly unlikely that Sm alone accounts for the delayed metabolic effects of GH.     

Compensatory growth of the lung following partial pneumonectomy

In a variety of species, partial resection of the lung initiates rapid compensatory growth of the remaining tissue adequate to restore normal total lung mass. Increases in tissue content of protein, RNA, and DNA in proportion to dry lung weight suggest hyperplastic growth of the tissue, rather than cellular hypertrophy. A general acceleration of cell division is supported further by the results of quantitative morphometric studies, which indicate that both cellular and functional characteristics of the peripheral lung, including alveolar and capillary volumes and thickness and surface area of the blood-gas barrier, are maintained when compensatory growth is complete. The rate and nature of the growth response are subject to hormonal modulation, particularly by adrenal steroids and growth hormone. Little is known, however, regarding the specific actions of these agents or of additional factors that may be primary regulators of the initiation and cessation of accelerated compensatory growth. Definition of such regulatory mechanisms is of critical importance in understanding normal growth and development of the lung and the response of the lung to injury, as well as in future efforts to manipulate growth and/or repair of the tissue.     

Isolation of a somatomedin from plasma of rats bearing growth hormone secreting tumors.

We have purified a protein which has somatomedin-like properties from the serum of Wistar-Furth rats bearing a growth hormone producing pituitary tumor (MStT/W15). Activity was measured by a placental insulin and/or somatomedin C radioreceptor assay (SmC-RRA). The serum was initially filtered through Sephadex G-150 equilibrated with 0.1 M NH4HCO3 and 0.02% NaN3. On the G-150 column, radioreceptor insulin (RRI) and radioreceptor somatomedin C (RRSm-C) activities coincided and appeared predominantly in the 160,000 mol wt range with a minor proportion in the 50,000 mol wt range. The pooled active fractions were boiled for 30 min at pH 5.5. After removing denatured protein by centrifugation, the extract was passed through G-50 Sephadex equilibrated with 1% formic acid and 0.15 M NaCl. Sixty to 90% of the SmC-RRA activity in the effluent appeared in the 9000 mol wt range. This material has an isoelectric focusing range of 8.4--9.6, similar to that described for human somatomedin C. On SDS-urea polyacrylamide gel electrophoresis only one protein band was seen. The isolated peptide (rSm) stimulated sulfate uptake in hypophysectomized rat cartilage. The potency of two preparations was variously assayed from 14.0 to 54.7 units/mg. Rat somatomedin was iodinated and purified by absorption on and elution from placental membranes. Eight to 12% of rat [125I]Sm was specifically bound by human placental membranes. Rat [125I]Sm was displaced by hSmC and rSm and human NSILA-S, partially displaced by procine proinsulin and poorly displaced by rat insulin. In preliminary studies, rat [125I]Sm was displaced from receptors on human placental membranes by sera from pituitary tumor bearing rats greater than normal rat sera greater than hypophysectomized rat sera.     

Effect of somatomedin and growth hormone on bone collagen synthesis in vitro

Although pituitary hormones, particularly growth hormone (GH), are known to influence skeletal growth, there is no evidence for a direct effect of GH or GH-dependent factors (somatomedins) on bone as opposed to cartilage. We have examined the effects of GH, a somatomedin (Sm) preparation, and serum from intact and hypophysectomized rats on bone collagen synthesis in cultures of 21-day fetal rat calvaria. Collagen synthesis and non-collagen protein synthesis were measured by the incorporation of ³H-proline into collagenase-digestible (CDP) and noncollagen protein (NCP). Bovine and rat GH caused a small inhibition in the incorporation of labeled proline into CDP which was not dose related. Sm in doses of 18–540 mU/ml increased the incorporation of proline into CDP up to three-fold after 24 hr in culture. Sm also had a smaller and more variable stimulatory effect on the labeling of NCP. The effects of Sm were maximal after 3 hr of treatment and were maintained for 96 hr. Sm (60 mU/ml) and insulin (10^?8 M) had effects of similar magnitude and were not additive. The addition of 10% serum from hypophysectomized rats stimulated the labeling of both CDP and NCP, but serum from rats with intact pituitaries had a greater effect. Treatment of hypophysectomized rats with thyroxine, corticosterone, and GH. did not increase the bone collagen synthesis stimulating activity of the serum, although GH treatment did increase serum Sm activity by a pig cartilage assay. The results indicate that GH dose not stimulate bone collagen synthesis directly. However, they suggest that the pituitary gland either releases or stimulates the production of factors which stimulate bone collagen synthesis. Sm may be such a factor, but sulfation activity and bone collagen synthesis stimulating activity may be dissociable.     

The effect of growth hormone treatment on somatomedin levels in growth hormone-deficient children

Serum levels of immunoreactive somatomedin (IRSM) and insulin-like activity (ILAs) by receptor assay have been measured in 177 GH-deficient (GHD) children treated with 2 U GH three times weekly for 6-10 months. The overall mean (+/- SD) pretreatment IRSM and ILAs levels were 0.21 +/- 0.30 and 0.39 +/- 0.25 U/ml, respectively. Pretreatment IRSM and ILAs levels were in the normal range for chronological age and sex in 15.8% and 33% of these GHD children. Mean IRSM and ILAs levels increased after 1 month of therapy to 0.36 +/- 0.51 and 0.62 +/- 0.33 U/ml, respectively (P less than .0001). The increase in serum IRSM levels (from 0.2 to 2.0 U/ml) that normally occurs from 1-16 yr of age is also evident in untreated GHD children, albeit to a lesser extent. In addition, the mean increase in IRSM after GH was greater in older patients. However, individual responses varied greatly. An increase in IRSM or ILAs with an increase in height velocity (HV) was observed in the majority of children, but all other combinations occurred, including increased SM with decreased HV, decreased SM with increased HV, and decreased SM with decreased HV. In summary 1) age-dependent factors in GHD children importantly influence basal and treatment SM leels; 2) a basal SM level is not a very sensitive diagnostic test, since a significant proportion of GHD children have SM levels in the normal range; and, 3) SM levels in individual patients may not increase with GH therapy and, thus, cannot be used to predict a clinical response to GH therapy.