Genetic disorders in the growth hormone - insulin-like growth factor-I axis

In the last few years, our knowledge of genetically determined causes of short stature has greatly increased by reports of challenging patients, who offered the opportunity to study genes that play a role in growth. Since the first paper that showed the etiology of Laron syndrome [Godowski PJ, et al: Proc Natl Acad Sci USA 1989;86:8083-8087], many mutations in the growth hormone (GH) receptor have been identified. Recently, new mutations or deletions have been found in several components of the GH-insulin-like growth factor-I (IGF-I) axis: a homozygous mutation of the GH1 gene, resulting in a bio-inactive GH; mutations in the STAT5b gene, which plays a major role in the GH signal transduction; a homozygous missense mutation in the IGF-I gene; heterozygous mutations in the IGF-I receptor gene and a homozygous deletion of the acid-labile subunit gene. In this mini review, we describe the clinical and biochemical features of these genetic defects. Genetic analysis has become essential in the diagnostic workup of a patient with short stature. However, regarding the time consuming nature of molecular analysis, it is important to carefully select the patient for specific genetic evaluation. To help in this selection process, we developed flowcharts, based on the recently described patients, that can be used as guidelines in the diagnostic process of patients with severe short stature of unknown origin.     

Intranasal administration of His-D-Trp-Ala-Trp-D-Phe-LysNH2 (growth hormone releasing peptide) increased plasma growth hormone and insulin-like growth factor-I levels in normal men

The effects of intranasal and iv administration of His-D-Trp-Ala-Trp-D-Phe-LysNH2 (GHRP) on plasma GH, PRL, LH, FSH, TSH, cortisol, insulin, IGF-I as well as GHRH-like immunoreactivity (LI) levels were examined in 6 healthy male subjects. An iv bolus injection of GHRP(1 micrograms/kg BW) caused a remarkable increase in plasma GH levels with a mean (+/- SE) peak of 54.9 +/- 4.2-micrograms/L. In addition an intranasal administration of GHRP resulted in a significant, dose-related increase in plasma GH with peaks of 39.6 +/- 15.3 micrograms/L at a dose of 30 micrograms/kg BW, 14.1 +/- 5.0 micrograms/L at 15 micrograms/kg BW and 7.5 +/- 5.7 micrograms/L at 5 microgram/kg BW. Plasma PRL and cortisol levels were slightly but significantly increased after iv administration of GHRP, whereas GHRP totally failed to affect plasma TSH, LH, FSH, insulin, blood sugar and GHRH-LI levels. Seven consecutive, intranasal administrations of 15 micrograms/kg BW GHRP every 8h were well tolerated in all subjects examined. During this treatment, GH responsiveness to GHRP was not attenuated by desensitization and plasma IGF-I was increased from 94.5 +/- 5.8 micrograms/L before GHRP to 125.8 +/- 6.0 micrograms/L after repeated GHRP administration. These findings indicate that intranasal administration of GHRP stimulates GH secretion and consequently enhances IGF-I production in normal subjects. If GHRP is demonstrated to be beneficial in the treatment of some patients with GH deficiency, the intranasal route of administration may be more useful than the painful injection because a prolonged period is required for the treatment.     

Effects of homologous ghrelins on the growth hormone/insulin-like growth factor-I axis in the tilapia, Oreochromis mossambicus

Ghrelin is a gut-brain peptide synthesized mainly in the oxyntic mucosal cells of the stomach, and has potent growth hormone (GH)-releasing and orexigenic activities. Recently, two forms of ghrelin, ghrelin-C8 and -C10, were identified in the Mozambique tilapia (Oreochromis mossambicus). The present study describes in vitro and in vivo effects of these endogenous ghrelins on the GH/insulin-like growth factor-I (IGF-I) axis. Ghrelin-C8 (100 nM) stimulated GH release from primary cultures of pituitary cells after 4 and 8 h of incubation, whereas no effect was seen on prolactin (PRL) release. Stimulatory effects of ghrelin-C8 and -C10 (100 nM) on GH release during 6 h of incubation were blocked by pre-incubation with GHS receptor antagonist, [D-Lys(3)]-GHRP-6 (10 microM). Intraperitoneal injection of ghrelin-C8 (1 ng/g body weight) and -C10 (0.1 and 1 ng/g body weight) significantly increased plasma GH levels after 5 h. Significant increases were observed also in hepatic expression of IGF-I and GH receptor (GHR) mRNA following injections of both forms of ghrelin (0.1 and 1 ng/g body weight), although there was no effect on plasma levels of IGF-I. In the next experiment, both forms of ghrelin (1 ng/g body weight) significantly increased plasma IGF-I levels 10 h after the injection. No significant effect of either ghrelin was observed on plasma PRL levels. Both forms of GHS receptor (GHSR-1a and -1b) were found in the pituitary, clearly indicating that tilapia ghrelins stimulate primarily GH release through the GHS receptor. Stimulation of hepatic expression of IGF-I and GHR suggests metabolic roles of ghrelin in tilapia.     

Rigorous running increases growth hormone and insulin-like growth factor-I without altering ghrelin

It has been suggested that ghrelin may play a role in growth hormone (GH) responses to exercise. The present study was designed to determine whether ghrelin, GH, insulin-like growth factor-I (IGF-I), and IGF-binding protein-3 (IGFBP-3) were altered by a progressively intense running protocol. Six well-trained male volunteers completed a progressively intense intermittent exercise trial on a treadmill that included four exercise intensities: 60%, 75%, 90%, and 100% of Vo2max. Blood samples were collected before exercise, after each exercise intensity, and at 15 and 30 mins following the exercise protocol. Subjects also completed a separate control trial at the same time of day that excluded exercise. GH changed significantly over time, and GH area under the curve (AUC) was significantly higher in the exercise trial than the control trial. Area under the curve IGF-I levels for the exercise trial were significantly higher than the control trial. There was no difference in the ghrelin and IGFBP-3 responses to the exercise and control trials. Pearson correlation coefficients revealed significant relationships between ghrelin and both IGF-I and IGFBP-3; however, no relationship between ghrelin and GH was found. In conclusion, intense running produces increases in total IGF-I concentrations, which differs from findings in previous studies using less rigorous running protocols and less frequent blood sampling regimens. Moreover, running exercise that produces substantial increases in GH does not affect peripheral ghrelin levels; however, significant relationships between ghrelin and both IGF-I and IGFBP-3 exist during intense intermittent running and recovery, which warrants further investigation.     

Physical and functional interaction of growth hormone and insulin-like growth factor-I signaling elements

GH and IGF-I are critical regulators of growth and metabolism. GH interacts with the GH receptor (GHR), a cytokine superfamily receptor, to activate the cytoplasmic tyrosine kinase, Janus kinase 2 (JAK2), and initiate intracellular signaling cascades. IGF-I, produced in part in response to GH, binds to the heterotetrameric IGF-I receptor (IGF-IR), which is an intrinsic tyrosine kinase growth factor receptor that triggers proliferation, antiapoptosis, and other biological actions. Previous in vitro and overexpression studies have suggested that JAKs may interact with IGF-IR and that IGF-I stimulation may activate JAKs. In this study, we explore interactions between GHR-JAK2 and IGF-IR signaling pathway elements utilizing the GH and IGF-I-responsive 3T3-F442A and 3T3-L1 preadipocyte cell lines, which endogenously express both the GHR and IGF-IR. We find that GH induces formation of a complex that includes GHR, JAK2, and IGF-IR in these preadipocytes. The assembly of this complex in intact cells is rapid, GH concentration dependent, and can be prevented by a GH antagonist, G120K. However, it is not inhibited by the kinase inhibitor, staurosporine, which markedly inhibits GHR tyrosine phosphorylation. Moreover, complex formation does not appear dependent on GH-induced activation of the ERK or phosphatidylinositol 3-kinase signaling pathways or on the tyrosine phosphorylation of GHR, JAK2, or IGF-IR. These results suggest that GH-induced formation of the GHR-JAK2-IGF-IR complex is governed instead by GH-dependent conformational change(s) in the GHR and/or JAK2. We further demonstrate that GH and IGF-I can synergize in acute aspects of signaling and that IGF-I enhances GH-induced assembly of conformationally active GHRs. These findings suggest the existence of previously unappreciated relationships between these two hormones.     

Sex steroids and the growth hormone/insulin-like growth factor-I axis in adults

In healthy adults insulin-like growth factor (IGF)-I levels do not differ between males and females, whereas spontaneous growth hormone (GH) secretion is approximately twofold higher in females. Untreated GH-deficient (GHD) women exhibit lower IGF-I levels compared with men and the increase in serum IGF-I during GH replacement is also significantly less. These data suggest a resistance to GH in women, which in healthy subjects is compensated for by increased GH secretion. Administration of oral oestrogen in healthy postmenopausal women suppresses hepatic IGF-I production and increases pituitary GH release, and oral oestrogen replacement in women with GHD lowers IGF-I concentrations and increases the amount of GH necessary to achieve IGF-I target levels during treatment. These data clearly suggest that hepatic suppression of IGF-I production by oestrogen subserves the gender difference in GH sensitivity, but it is also likely that sex steroids may interact with the GH/IGF axis at other levels. There is also circumstantial evidence to indicate that testosterone stimulates IGF-I production and it is speculated that a certain threshold level of androgens is essential to ensure hepatic IGF-I production. Whether these data should translate into earlier discontinuation of oestrogen replacement therapy in women with hypopituitarism merits consideration.     

Growth hormone dependence of somatomedin-C/insulin-like growth factor-I and insulin-like growth factor-II messenger ribonucleic acids

The GH dependence of somatomedin-C/insulin-like growth factor I (Sm-C/IGF-I) and insulin like growth factor II (IGF-II) mRNAs was investigated by Northern blot hybridizations of polyadenylated RNAs from liver, pancreas, and brain of normal rats, untreated hypophysectomized rats, and hypophysectomized rats 4 h or 8 h after an ip injection of human GH (hGH). Using a 32P-labeled human Sm-C/IGF-I cDNA as probe, four Sm-C/IGF-I mRNAs of 7.5, 4.7, 1.7, and 1.2 kilobases (kb) were detected in rat liver and pancreas but were not detectable in brain. In both liver and pancreas, the abundance of these Sm-C/IGF-I mRNAs was 8- to 10-fold lower in hypophysectomized rats than in normal rats. Within 4 h after injection of hGH into hypophysectomized animals, the abundance of liver and pancreatic Sm-C/IGF-I mRNAs was restored to normal. A human IGF-II cDNA was used as a probe for rat IGF-II mRNAs which were found to be very low in abundance in rat liver and showed no evidence of regulation by GH status. In pancreas, IGF-II mRNA abundance was below the detection limit of the hybridization procedures. The brain contained two IGF-II mRNAs of 4.7 and 3.9 kb that were 5-fold lower in abundance in hypophysectomized rats than in normal rats. These brain IGF-II mRNAs were not, however, restored to normal abundance at 4 or 8 h after ip hGH injection into hypophysectomized animals. To investigate further, the effect of GH status on abundance of Sm-C/IGF-I and IGF-II mRNAs in rat brain, a second experiment was performed that differed from the first in that hypophysectomized rats were given an injection of hGH into the lateral ventricle (intracerebroventricular injection) and a rat Sm-C/IGF-I genomic probe was used to analyze Sm-C/IGF-I mRNAs. In this experiment, a 7.5 kb Sm-C/IGF-I mRNA was detected in brain polyadenylated RNAs. The abundance of the 7.5 kb mRNA was 4-fold lower in hypophysectomized rats than in normal rats and was increased to 80% of normal within 4 h after icv administration of hGH to hypophysectomized animals. As in the first experiment, the abundance of the 4.7 and 3.9 kb brain IGF-II mRNAs was lower than normal in hypophysectomized rats. Brain IGF-II mRNAs were increased to 50% of normal in hypophysectomized rats given an icv injection of hGH but within 8 h after the injection rather than at 4 h as with Sm-C/IGF-I mRNAs.     

Interleukin-1 inhibits the induction of insulin-like growth factor-I by growth hormone in CWSV-1 hepatocytes

Sepsis results in hepatic "growth hormone (GH) resistance" with reductions in plasma IGF-I despite a two- to fourfold increase in circulating GH. In this study, we examine the effects of IL-1 on GH receptor (GHR) expression, GH signaling (via the JAK/STAT and MAPK pathways), and the induction of gene expression [IGF-I mRNA and serine protease inhibitor (Spi) 2.1] by GH in CWSV-1 hepatocytes. Incubation of cells with IL-1beta (10 ng/ml, 24 h) had no effect on the relative abundance of GHR or signaling proteins JAK2, STAT5b, and ERK1/2 in cell lysates. Baseline phosphorylation of GHR, JAK2, STAT5b, and ERK1/2 was minimal. After GH stimulation, tyrosine phosphorylation of GHR, JAK2, STAT5b, and ERK1/2 increased 2- to 10-fold. However, neither the time course nor the magnitude of GHR, JAK2, and ERK1/2 phosphorylation by GH were significantly altered by IL-1. The GH-induced translocation of STAT5b to the nucleus was not prevented by IL-1. Although phosphorylated STAT5 in nuclear extracts from GH + IL-1 cells was decreased by 24% (vs. controls) 15 min after GH stimulation, this did not result in reduced STAT5-DNA binding activity. Pretreatment with IL-1 did not significantly decrease IGF-I mRNA stability. We conclude that IL-1 only minimally affects the time course of JAK2/STAT5 and MAPK signaling by GH. Therefore, an inhibitory effect of IL-1 on IGF-I and Spi 2.1 mRNA synthesis by GH represents the most likely mechanism for IL-1-mediated GH resistance.     

Growth hormone and insulin-like growth factor-I measurements in high growth (hg) mice

Effects of a recessive gene causing high growth (hg) were studied on two major components of the growth axis in mice. Plasma and pituitary levels of growth hormone and plasma levels of insulin-like growth factor I (IGF-I) were measured in three lines homozygous for hg, each compared with a control line of alike genetic background but wild type for the hg locus (Hg). Line Gh (hghg) and line GH (HgHg) are from a line which had undergone long-term selection for high postweaning weight gain; line Ch (hghg) and line CH (HgHg) were extracted from the second backcross of Gh to C57BL/6J; line L54 (hghg) was from the sixth backcross to C57BL/6J (B6) (HgHg). Pituitary GH levels and plasma IGF-I levels were measured in both sexes at 3, 4.5, 6 and 9 wk of age. Plasma growth hormone was measured in 8- to 12-wk-old males at hourly intervals from 08.00 to 17.00. Body weight in lines homozygous for hg at 6 and 9 wk of age was 10-30% greater than in control lines. The ontogeny of this increased growth depended on genetic background. Pituitary growth hormone content was 52% lower in the two hghg lines measured (lines Ch and Gh) than in control lines at 4.5, 6 and 9 wk. Plasma growth hormone levels were also much lower in hg mice, with values only 20-30% of those in their respective controls. hg lines showed consistently low plasma growth hormone levels throughout the 9 hr sampling period, while control lines expressed the characteristic pulsatile hormone secretion.(ABSTRACT TRUNCATED AT 250 WORDS)     

Growth regulation by insulin-like growth factor-I in fish

Insulin-like growth factor-I (IGF-I) is a mitogenic polypeptide that plays an essential role in the regulation of development and somatic growth of vertebrates, mainly by mediating growth hormone actions. It has clearly been established that the structure of IGF-I and its biological function has been highly conserved among vertebrates. In this paper, we review the recent developments in the molecular, biochemical, and physiological properties of IGF-I in fish.     

Thyroid hormone modulates insulin-like growth factor-I(IGF-I) and IGF-binding protein-3, without mediation by growth hormone, in patients with autoimmune thyroid diseases.

The expression and synthesis of insulin-like growth factor-1 (IGF-I) and IGF-binding protein-3 (IGFBP-3) are regulated by various hormones and nutritional conditions. We evaluated the effects of thyroid hormones on serum levels of IGF-I and IGFBP-3 levels in patients with autoimmune thyroid diseases including 54 patients with Graves' disease and 17 patients with Hashimoto's thyroiditis, and in 32 healthy age-matched control subjects. Patients were subdivided into hyperthyroid, euthyroid and hypothyroid groups that were untreated, or were treated with methylmercaptoimidazole (MMI) or L-thyroxine (L-T4). Serum levels of growth hormone (GH), IGF-I and IGFBP-3 were determined by radioimmunoassay. Serum GH levels did not differ significantly between the hyperthyroid and the age-matched euthyroid patients with Graves' disease. The serum levels of IGF-I and IGFBP-3 showed a significant positive correlation in the patients (R=0.616, P<0.001). The levels of both IGF-I and IFGBP-3 were significantly higher in the hyperthyroid patients with Graves' disease or in those with Hashimoto's thyroiditis induced by excess L-T4 administration than in control subjects. Patients with hypothyroid Graves' disease induced by the excess administration of MMI showed significantly lower IGFBP-3 levels as compared to those in healthy controls (P<0.05). Levels of IGFBP-3, but not IGF-I levels, showed a significant positive correlation with the levels of free T4 and free T3. In Graves' disease, levels of TPOAb, but not of TRAb, showed a significant positive correlation with IGFBP-3. We conclude that in patients with autoimmune thyroid diseases, thyroid hormone modulates the synthesis and/or the secretion of IGF-I and IGFBP-3, and this function is not mediated by GH.     

Provocation of a paradoxical growth hormone response to corticotropin-releasing hormone by pretreatment with metoclopramide in patients with acromegaly and normal subjects

Two of 7 patients with acromegaly and one of 7 normal subjects exhibited a paradoxical rise in growth hormone (GH) to human corticotropin-releasing hormone (CRH) when pretreated with metoclopramide, although CRH alone did not induce an increase in GH. In one of these two patients with acromegaly, the GH increase to metoclopramide alone also reached the criteria of a paradoxical response. These two acromegalic patients showed a GH increase to metoclopramide pretreatment before and up to two months after surgery. In another acromegalic patient, whose GH level remained high 5 months after surgery, metoclopramide induced an increase in GH level, while in a patient who had an above-normal GH level 18 months after surgery, the resumption of physiological GH secretion after surgery was evidenced by a postoperative absence of a GH response to metoclopramide. It is suggested from these results that the GH response to metoclopramide and the metoclopramide-provoked GH response to CRH in patients with acromegaly result from the secretion of GH from nonadenomatous cells of the pituitary.     

Circulating growth hormone releasing factor concentrations in normal subjects and patients with acromegaly.

A highly specific and sensitive radioimmunoassay was developed for measuring circulating growth hormone releasing factor (GRF) in human plasma. Before measuring immunoreactive GRF plasma samples were extracted on to Vycor glass. Immunoreactive GRF concentrations in plasma samples from 37 fasting normal subjects ranged from less than 10 to 60 ng/l (mean 21 ng/l). Fasting concentrations in 76 out of 80 acromegalic subjects were within the normal range, but the remaining four patients had values of 92 to 25 000 ng/l. Of these, only the patient with the highest concentration had evidence of ectopic GRF secretion from a disseminated carcinoid tumour. Two of the others had longstanding pituitary tumours, and the fourth patient had a pituitary growth hormone (GH) secreting tumour proved by its removal and subsequent remission of acromegaly. There was no correlation between serum GH and plasma immunoreactive GRF concentrations, irrespective of whether the patients were untreated or had been given radiotherapy or dopamine agonists. The assay should help elucidate the physiological role(s) of GRF and may also prove useful in differentiating between pituitary and hypothalamic defects in patients with acromegaly.     

Effects of growth hormone and insulin-like growth factor-I on development of in vitro derived bovine embryos

The objectives of this study were to determine whether the addition of growth hormone (GH) to maturation medium and GH or insulin-like growth factor-I (IGF-I) to culture medium affects development of cultured bovine embryos. We matured groups of 10 cumulus-oocyte complexes (COCs) in serum-free TCM-199 medium containing FSH and estradiol with or without 100 ng/ml GH. After fertilization, we transferred groups of 10 putative zygotes to 25 microl drops of a modified KSOM medium containing the following treatments: non-specific IgG (a control antibody, 10 microg/ml); GH (100 ng/ml) + IgG (10 microg/ml, GH/IgG); IGF-I (100 ng/ml) + IgG (10 microg/ml, IGF/IgG); antibody to IGF-I (10 microg/ml, anti-IGF); GH (100 ng/ml) + anti-IGF (10 microg/ml GH/anti-IGF); IGF-I (100 ng/ml) + anti-IGF (10 microg/ml, IGF/anti-IGF); no further additions (control). We repeated the experiment six times. Adding GH to the maturation medium increased cleavage rates at Day 3 compared to control (87.3 +/- 1.2% > 83.9 +/- 1.2%; P < 0.05) but had no effects on blastocyst development at Day 8. At Day 8, blastocyst development was greater (P < 0.01) for GH/IgG (24.8 +/- 2.5%) and IGF/IgG (33.7 +/- 2.5%) than for IgG (16.1 +/- 2.1%) and greater for IGF/IgG than for GH/IgG (P < 0.02). Blastocyst development at Day 8 did not differ between anti-IGF (20.4 +/- 1.8%) and GH/anti-IGF (24.1 +/- 1.9%) or IGF/anti-IGF (17.7 +/- 1.9%), but it was greater for GH/anti-IGF than for IGF/anti-IGF (P < 0.05). The Day 8 blastocysts of GH/IgG and IGF-I/IgG groups had a higher (P < 0.01) number of cells than the IgG group. The addition of anti-IGF-I eliminated the effects of IGF-I on cell number but did not alter GH effects. In conclusion, both GH and IGF-I stimulate embryonic development in cattle and GH effects may likely involve IGF-I-independent mechanisms.     

Growth hormone secretion during pregnancy: altered effects of growth hormone releasing factor and insulin-like growth factor-I in vitro

Growth hormone (GH) secretion is controlled by growth hormone releasing factor (GRF) but changes in the circulating level of this hormone are difficult to measure. Insulin-like growth factor (IGF-I) is a GH-dependent growth factor which significantly but slightly inhibits stimulated GH release in vitro. We have tested the effects of GRF and IGF-I on GH release in pregnancy, a state in which serum concentrations of GH are elevated and levels of IGF-I are lowered. We have found, in a system of acutely dispersed adenohypophysial cells prepared from pregnant (day 21-23) or control cycling female rats, that adenohypophysial cells from pregnant rats have an increased GH release with GRF. In contrast, IGF-I inhibition is similar but slightly smaller. These altered responses may result in elevated serum GH levels during pregnancy.     

Human corticotropin-releasing hormone test in normal subjects and patients with hypothalamic, pituitary or adrenocortical disorders

Human corticotropin-releasing hormone (hCRH) test was performed in 57 normal volunteers and 102 patients with hypothalamic, pituitary and adrenocortical diseases. Intravenous bolus injection of synthetic hCRH, 100 micrograms for adults or 1.5 micrograms/kg for children, increased plasma ACTH and cortisol levels in about 90% of normal subjects. In 47 patients with Cushing's disease, plasma ACTH tended to show an exaggerated response to hCRH and peak ACTH was the most frequent abnormal component among the several reaction parameters. Poor responders among normal subjects and patients with Cushing's disease had significantly higher plasma cortisol levels before CRH administration. Patients with hypothalamic hypopituitarism showed exaggerated response, whereas patients with primary pituitary lesion, isolated ACTH deficiency or adrenal Cushing's syndrome showed no ACTH response. These differences in the response of patients suggest the value of the hCRH test in their differential diagnosis.     

Serum luteinizing hormone, growth hormone and insulin-like growth factor-I after releasing hormone challenge in prepubertal ewe lambs selected for twinning

Two studies evaluated hormonal markers as indicators of the onset of puberty in Debouillet sheep selected for twinning. In Trial 1, 29 ewe lambs (50 +/- 0.5 kg, 159 to 187 d of age) were given 10 microg GnRH (i.v.) on September 15 and blood was collected at 30 min intervals after the injection for 2 h. Additional samples were taken twice weekly and progesterone (P4) was measured. The day that serum P4 was greater than 1 ng/mL for 2 consecutive sampling days was classified as the day of puberty. Average day of puberty was October 12 (average age at puberty was 199 d) and ewes with values less or greater than the average were classified as early or late, respectively. Average weight at GnRH challenge was 50 kg and ewes weighing less or more were classified as light or heavy, respectively. Early ewes weighed more (P = 0.01) and reached puberty sooner (P = 0.01) than late ewes. Heavy lambs reached puberty earlier, weighed more at GnRH challenge, and had more LH area under the curve (AUC, P < 0.05) than light ewes. In Trial 2, we gave 27 ewe lambs (54 +/- 0.9 kg, 173 to 189 d of age) a single i.v. injection of 10 microg GnRH and 10 microg GHRH on September 17. Average day of puberty was October 13, average weight was 54 kg, and average age at puberty was 208 d. Categories were designated as described for Trial 1. Early lambs reached puberty sooner (P = 0.01) and weighed more (P = 0.01) than late lambs, but the puberty groups had similar LH AUC (P = 0.64) and GH AUC (P = 0.75), whereas IGF-I was greater (P = 0.01) in early puberty ewes than in late puberty ewes. Heavy lambs reached puberty earlier (P = 0.06), weighed more (P = 0.01), and tended (P = 0.11) to have more GH AUC than light ewes. No difference was observed in LH AUC or IGF-I between weight groups (P > 0.15). Results suggest that serum LH after GnRH is not a reliable indicator of the onset of puberty in ewe lambs selected for twinning, but heavier ewes tended to produce more GH after a GHRH challenge and reach puberty earlier than lighter ewe lambs.