Endogenous Growth Hormone Research Articles

Short-term estradiol supplementation augments growth hormone (GH) secretory responsiveness to dose-varying GH-releasing peptide infusions in healthy postmenopausal women.

Estrogen is a prominent stimulus to GH secretion throughout the human life span, albeit via neuroendocrine mechanisms that are incompletely defined. Here, we test the hypothesis that estradiol replacement in postmenopausal women enhances the responsiveness of the hypothalamo-pituitary unit to the GH-releasing effect of GH-releasing peptide-2 (GHRP-2). GHRP-2 is a potent and selective synthetic hexapeptide capable of activating an endogenous GHRP receptor/effector pathway, for which a (3)Ser-octanoylated 28-amino acid ligand was cloned recently. To examine this postulate, we studied 10 healthy estrogen-withdrawn postmenopausal women, who were given oral placebo or estrogen supplementation [1 mg micronized 17 beta-estradiol (E(2)) twice daily for 7-15 days] in a patient-blinded, prospective, randomized, and within-subject cross-over design. The GH-releasing actions of five semilogarithmically increasing doses of GHRP-2 (absolute range, 0.03-3 microg/kg by bolus iv infusion) vs. saline were evaluated by frequent blood sampling on separate days in the morning while fasting. Serum GH concentrations were determined in blood sampled every 10 min using an ultrasensitive chemiluminescence assay and analyzed by multiparameter deconvolution to calculate the summed mass of GH secreted during the 2-h interval after bolus GHRP-2 infusion. Logarithmically transformed secretory responses were compared across the different dosages of infused GHRP-2 by two-way repeated measures ANOVA. Estradiol replacement increased the global mean (+/-SEM) serum E(2) concentration from 15 +/- 0.8 to 470 +/- 17 pg/mL (55 +/- 2.9 to 1725 +/- 62 pmol/L; P = 0.004) and lowered insulin-like growth factor I levels by approximately 27% (P = 0.087). Administration of E(2) elevated the geometric mean basal (saline-infused) GH secretory burst mass by 2.1-fold (95% confidence interval, 1.4- to 3.1-fold) compared with placebo ingestion (geometric mean ratios; P < 0.001). E(2) exposure enhanced the efficacy of the highest GHRP-2 dose tested (3 microg/kg) by 2.1-fold (1.3- to 3.3-fold; P = 0.010). Compared with the effect of placebo and saline, E(2) combined with the highest dose of GHRP-2 stimulated GH secretory burst mass by a total of 31-fold (24- to 41-fold; P < 0.001). Random coefficient regression analysis of the relationship between the logarithm of GHRP-2 dose and GH secretory burst mass revealed that E(2) significantly augmented the amount of GH secreted per unit GHRP-2 dose (E(2), 16.6 +/- 1.8 slope units; placebo, 10.1 +/- 1.4 slope units; P = 0.03). Although the global mean endogenous GH half-life did not differ between the E(2) and placebo sessions (E(2), 18 +/- 0.6 min; placebo, 17 +/- 0.5 min), GH half-life varied directly with dose of GHRP-2 (and, hence, the mean serum GH concentration) in both the E(2) and placebo sessions (test of zero slope hypothesis, P = 0.0018). The deconvolved GH secretory burst peaked within 8-13 min of the bolus iv injection of GHRP-2, and this latency was not altered by E(2). Based on a mixed effects analysis of covariance model, GHRP-2 dose and E(2), but not the plasma insulin-like growth factor I concentration, determined the magnitude of the GH secretory response (P < 0.001). We conclude that short-term oral E(2) repletion in postmenopausal women selectively augments GH secretory pulse mass, enhances the steepness of the GHRP-2 dose-GH secretory response relationship (greater sensitivity), and heightens the maximal GH secretory response to the highest dose of GHRP-2 tested (greater efficacy). These data point to a facilitative interaction between E(2) and the GHRP receptor/effector pathway in driving the mass of GH secreted per burst.

Association between premature mortality and hypopituitarism

Four retrospective studies have reported premature mortality in patients with hypopituitarism with standard mortality ratios (SMRs) varying between 1.20 and 2.17. Patients with hypopituitarism have complex endocrine deficiencies, and the mechanisms underpinning any excess mortality are unknown. Furthermore, the suggestion has emerged that endogenous growth-hormone deficiency might account for any excess mortality. We aimed to clarify these issues by doing a large prospective study of total and specific-cause mortality in patients with hypopituitarism. We followed up 1014 UK patients (514 men, 500 women) with hypopituitarism from January, 1992, to January, 2000. 573 (57%) patients had non-functioning adenomas, 118 (12%) craniopharyngiomas, and 93 (9%) prolactinomas. SMRs were calculated as the ratio of observed deaths to the number of deaths in an age-matched and sex-matched UK population. The number of observed deaths was 181 compared with the 96.7 expected (SMR 1.87 [99% CI 1.62-2.16], p<0.0001). Univariate analysis indicated that mortality was higher in women (2.29 [1.86-2.82]) than men (1.57 [1.28-1.93], p=0.002), in younger patients, in patients with an underlying diagnosis of craniopharyngioma (9.28 [5.84-14.75] vs 1.61 [1.30-1.99], p<0.0001), and in the 353 patients treated with radiotherapy (2.32 [1.71-3.14] vs 1.66 [1.30-2.13], p=0.004). Excess mortality was attributed to cardiovascular (1.82 [1.30-2.54], p<0.0001), respiratory (2.66 [1.72-4.11], p<0.0001), and cerebrovascular (2.44 [1.58-4.18], p<0.0001) causes. There was no effect of hormonal deficiency on mortality, except for gonadotropin deficiency, which, if untreated was associated with excess mortality (untreated 2.97 [2.13-4.13] vs treated 1.42 [0.97-2.07], p<0.0001). Multiple regression analyses identified age at diagnosis, sex, a diagnosis of craniopharyngioma, and untreated gonadotropin deficiency as independent significant factors affecting mortality. Patients with hypopituitarism have excess mortality, predominantly from vascular and respiratory disease. Age at diagnosis, female sex, and above all, craniopharyngioma were significant independent risk factors. Specific endocrine-axis deficiency, with the exception of untreated gonadotropin deficiency, does not seem to have a role.

GH mRNA levels are elevated by forskolin but not GH releasing hormone in GHRH receptor-expressing MtT/S somatotroph cell line

The MtT/S somatotroph cell line should be a growth hormone-releasing hormone (GHRH)-responsive model system for the study of physiological control of growth hormone (GH) transcription because GH secretion from these cells is stimulated by GHRH. To examine the GH transcriptional activity of these cells, endogenous GH mRNA levels were measured using a ribonuclease protection assay following treatment under a variety of hormonal conditions. While omission of serum led to reduction of GH mRNA to 22% of control levels by 2 days and to 8% by 5 days ( P<0.05 for both), GH mRNA levels were maintained at control values in serum-free medium containing 5 nM dexamethasone and 30 pM triiodothyronine (TDM). However, the addition of 10 nM GHRH under any treatment condition did not significantly alter GH mRNA levels. Characterization of the MtT/S cells showed that GHRH-receptor (GHRH-R) mRNA was detectable by reverse transcription-polymerase chain reaction (RT-PCR) amplification. Measurement of extracellular cAMP showed that the MtT/S cells have basal levels of ≥20 nmol/10 6 cells per h in both serum-containing and serum-free media, and that GHRH had no effect on cAMP levels, suggesting constitutive activation. To rule out the possibility of autocrine stimulation by GHRH produced endogenously, GHRH mRNA was not detectable in MtT/S cells using RT-PCR amplification. The stimulatory G-protein α subunit, mutations of which are known to activate adenylate cyclase constitutively in acromegaly, was sequenced but found not to differ from normal pituitary in the regions most commonly mutated. Finally, treatment with 10 μM forskolin, to directly activate adenylate cyclase, increased GH mRNA to 140% of controls in TDM, and to 163% in serum-free medium after 2 days, and to 166% in TDM-treated cells and 174% in serum-free culture after 5 days (all P<0.05). Taken together, these data indicate that although MtT/S cells express the GHRH-R, GHRH cannot stimulate adenylate cyclase to increase GH transcription due to constitutive elevation of cAMP levels, by a means that may be similar to that in cases of acromegaly not caused by oncogenic gsp mutations.

Treatment with a growth hormone secretagogue in a model of developing heart failure: effects on ventricular and myocyte function.

Exogenous administration of growth hormone (GH) and subsequently increased production of insulin-like growth factor-1 can influence left ventricular (LV) myocardial growth and geometry in the setting of congestive heart failure (CHF). This study determined the effects of an orally active GH secretagogue (GHS) treatment that causes a release of endogenous GH on LV function and myocyte contractility in a model of developing CHF. Pigs were randomly assigned to the following treatment groups: (1) chronic rapid pacing at 240 bpm for 3 weeks (n=11); (2) chronic rapid pacing and GHS (CP-424,391 at 10 mg x kg(-1) x d(-1), n=9); and (3) sham controls (n=8). In the untreated pacing CHF group, LV fractional shortening was reduced (21+/-2% versus 47+/-2%) and peak wall stress increased (364+/-21 versus 141+/-5 g/cm(2)) from normal control values (P:<0.05). In the GHS group, LV fractional shortening was higher (29+/-2%) and LV peak wall stress lower (187+/-126 g/cm(2)) than untreated CHF values (P:<0.05). With GHS treatment, the ratio of LV mass to body weight increased by 44% from untreated values. Steady-state myocyte velocity of shortening was reduced with pacing CHF compared with controls (38+/-1 versus 78+/-1 microm/s, P:<0.05) and was increased from pacing CHF values with GHS treatment (55+/-7 microm/s, P:<0.05). The improved LV pump function that occurred with GHS treatment in this model of CHF was most likely a result of favorable effects on LV myocardial remodeling and contractile processes. On the basis of these results, further studies are warranted to determine the potential role of GH secretagogues in the treatment of CHF.

Synthetic growth hormone secretagogues control growth hormone secretion in the chicken at pituitary and hypothalamic levels.

In the chicken growth hormone (GH) secretion is predominantly controlled by two hormones, thyrotropin-releasing hormone (TRH) and somatostatin (SRIH), respectively stimulating and inhibiting GH release. In view of the hypothesis of a novel GH secretagogue (GHS) in mammals, this specific species was used to further assess the exact function of two nonpeptidyl GHSs-L-692,429 and L-163,255. Both synthetic products stimulate GH secretion directly at the level of the pituitary as shown in in vitro perifusion studies. Plasma GH levels increase within 10-15 min after a single challenge of L-692,429 or L-163,255. A SRIH pretreatment dimishes this GH response. Both GH-releasing peptide mimetics decrease hypothalamic TRH concentrations, whereas SRIH levels are not affected. The novel GHS may therefore control GH secretion both at the level of the pituitary and the hypothalamus. The present article shows that nonpeptidyl mimetics also control GH secretion in nonmammalian species suggesting that the endogenous hormone may be a conserved GH stimulator in several vertebrates. The GH response to GHS in birds may be regulated both directly at the level of the pituitary and by releasing another endogenous GH stimulator (TRH) from the hypothalamus.

Effect of growth hormone on intestinal Na+/glucose cotransporter activity.

Growth hormone (GH) has been used alone or as part of a defined regimen in the treatment of patients with short bowel syndrome; however its mode of action remains unclear. Growth hormone has been shown to increase amino acid, water, and electrolyte absorption from the small intestine. The acute effect of growth hormone on intestinal sugar transport has not been described previously. Mucosal preparations of rat jejunum were mounted in the Ussing chamber. Growth hormone (2 x 10(-6) M or 8 x 10(-6) M) or vehicle was added to the serosal chamber 1, 3, or 5 hours later. Twenty or 40 minutes after growth hormone addition, 30 mmol/L 3-O-methylglucose was added to both chambers, and the change in short-circuit current (deltaIsc) was recorded. In separate experiments, tissues were pretreated with phloridzin, an inhibitor of Na+/glucose cotransport, before the addition of 3-O-methylglucose. In the final set of experiments, kinetic studies were performed. GH did not induce any alterations in baseline electrical parameters. Only tissues left in the chambers for 5 hours, but not 1 or 3 hours, before GH treatment displayed a greater 3-O-methylglucose-induced deltaIsc than controls (p < .05). The increase in Isc induced by 3-O-methylglucose was 100% phloridzin-inhibitable. Kinetic analysis showed that growth hormone administration is associated with an increase in Na+/glucose cotransporter maximal velocity (Vmax) but no significant change in carrier affinity for substrate (Km). Growth hormone increases intestinal sugar transport, but only in tissue that has not been exposed to endogenous GH for over 3 hours.

Determination of Insulin-Like Growth Factor I in Children: Normal Values and Clinical Use

Circulating insulin-like growth factor I (IGF-I) levels are stable throughout the day and correlate with pulsatile endogenous growth hormone (GH) secretion. A single measurement of IGF-I may, therefore, provide information on the GH secretory status of an individual. The clinical use of IGF-I determination in the evaluation of short stature in children suspected of GH deficiency will be discussed. Previous methodological problems have now mostly been overcome, and IGF-I can be determined in serum from healthy children, although concentrations exhibit large inter-individual variations relating to age, gender and pubertal maturation. These variations must be taken into account before IGF-I can be evaluated in a clinical situation, and before it can be stated whether a certain value is too low for a given age. The diagnostic sensitivities of IGF-I reported in relation to the outcome of GH provocative testing are variable (47–100%). This variability probably results from the use of different IGF-I assays, IGF-I reference ranges, and different GH testing procedures. When proper IGF-I assays and reference ranges are used, however, IGF-I determination results in valid diagnostic information regarding children exhibiting short stature. Thus, an IGF-I value should always be evaluated together with auxological information and GH testing results before a decision is taken on whether or not to start GH therapy.

Estimation of growth hormone secretion rate: impact of kinetic assumptions intrinsic to the analytical approach.

We compared four common mathematical techniques to determine daily endogenous growth hormone (GH) secretion rates from diurnal plasma GH concentration profiles in 24 women (16 upper- or lower-body obese and 8 normal-weight individuals). Two forms of deconvolution analysis and two techniques based on a priori determined GH clearance estimates were employed. Deconvolution analyses revealed significant differences in the 24-h GH secretion rate between normal-weight and upper-body obese women, whereas the other two techniques did not. Moreover, deconvolution analyses predicted that the reduction in mean plasma GH concentrations in upper-body obese women was accounted for by impaired GH secretion, whereas the other methods suggested that obesity increases GH metabolic clearance. Thus we infer that disparate conclusions concerning GH secretion can be drawn from the same primary data set. The different inferences likely reflect dissimilar kinetic assumptions and the particular limitations intrinsic to each analytical approach. Accordingly, we urge caution in the facile comparison of calculated GH secretion data in humans, especially when kinetic and secretion measurements are performed under different conditions. The most appropriate way to determine the GH secretion rate in humans must be balanced by the exact intent of the experiment and the acceptability of different assumptions in that context.

Central Effect of Ghrelin, an Endogenous Growth Hormone Secretagogue, on Hypothalamic Peptide Gene Expression

Ghrelin, an endogenous ligand for the growth hormone secretagogue receptor (GHS-R), was originally purified from the rat stomach. Like the synthetic GHSs, ghrelin specifically releases GH following intravenous administration. Also consistent with the central actions of GHSs, ghrelin-immunoreactive cells were shown to be located in the hypothalamic arcuate nucleus as well as the stomach. However, the central actions of ghrelin have not been elucidated. Here, we used radioactive in situ hybridization histochemistry to examine the effects of central administration of rat ghrelin on neuropeptide genes that are expressed in hypothalamic neurons that were previously shown to express GHS-R. We found that central administration of ghrelin increased both agouti-related protein (AGRP) mRNA levels (245.8 ± 28.3% of the saline-treated controls; p < 0.01) in the hypothalamus and food intake (5.7 ± 0.9 g ghrelin vs. 1.9 ± 0.5 g saline; p < 0.05). On the other hand, 1 μg of rat ghrelin central administration did not alter the episodic GH release of freely moving adult male rats. Thus, ghrelin has an alternative role in stimulating food intake via an increase of AGRP rather than the release of GH from the pituitary.

Central effect of ghrelin, an endogenous growth hormone secretagogue, on hypothalamic peptide gene expression.

Ghrelin, an endogenous ligand for the growth hormone secretagogue receptor (GHS-R), was originally purified from the rat stomach. Like the synthetic GHSs, ghrelin specifically releases GH following intravenous administration. Also consistent with the central actions of GHSs, ghrelin-immunoreactive cells were shown to be located in the hypothalamic arcuate nucleus as well as the stomach. However, the central actions of ghrelin have not been elucidated. Here, we used radioactive in situ hybridization histochemistry to examine the effects of central administration of rat ghrelin on neuropeptide genes that are expressed in hypothalamic neurons that were previously shown to express GHS-R. We found that central administration of ghrelin increased both agouti-related protein (AGRP) mRNA levels (245.8 +/- 28.3% of the saline-treated controls; p < 0.01) in the hypothalamus and food intake (5.7 +/- 0.9 g ghrelin vs. 1.9 +/- 0.5 g saline; p < 0.05). On the other hand, 1 microg of rat ghrelin central administration did not alter the episodic GH release of freely moving adult male rats. Thus, ghrelin has an alternative role in stimulating food intake via an increase of AGRP rather than the release of GH from the pituitary.

Growth Hormone Abuse in Athletes

Growth hormone (GH) is being used by elite athletes to enhance performance. The rationale appears to come from studies in adults with GH deficiency, where GH replacement has been shown to alter body composition so that lean tis sue mass is increased and fat mass is decreased, to enhance aerobic and resistance performance, to increase cardiac output, to enhance thermal adaptation during exertion, to increase collagen syn thesis, and to modify intermediary metabolism. Documentation of enhanced performance in athletes, however, is currently lacking. Detection of exogenous GH abuse presents a number of unique problems, in that endogenous GH is secreted in a pulsatile fashion and is stimulated by acute exercise. Also, recombinant 22 kDa GH is currently indistinguishable from the endogenous hormone. An international collaborative study group has been established to develop a means of differentiating supra-physiological doses of exogenous GH from endogenous GH secretion in athletes. Current testing strategies center around markers of GH action (the insulin-like growth factor system and markers of bone and collagen turnover), and molecular isoforms of GH. Although additional validation work is required, a test to detect GH abusers can be implemented in the near future.

Molecular evolution of growth hormone and receptor in the guinea-pig, a mammal unresponsive to growth hormone.

Growth in the guinea-pig is completely unresponsive to endogenous or exogenous growth hormone, despite the fact that the guinea-pig produces normal to high levels of growth hormone and receptor. In primates and artiodactyls, growth hormone exhibits accelerated rates of evolution that appear to be correlated with changes in function. Surprisingly, both guinea-pig growth hormone and receptor exhibit slow rates of evolution similar to those seen in other mammals, implying that both proteins are as functionally conserved in the guinea-pig as in other mammals or that any loss or relaxation of functional constraint was very recent. However, the guinea-pig growth hormone and receptor both exhibit a single amino acid replacement at a site known to have functional significance. Nevertheless, it is unclear whether the aberrant nature of the guinea-pig growth hormone-growth hormone receptor axis is due to these replacements or whether it is due to a defect in post-receptor signalling.

Effect of growth hormone secretagogue LY444711 on IGF‐1, growth hormone, and cortisol levels in beagle dogs after one and seven daily oral doses

Growth hormone (GH) release involves interaction of somatostatin and an endogenous GH secretagogue (GHS) on the hypothalamus. GH causes release of IGF-1, which acts by negative feedback to restrain subsequent GH release. GH secretagogues produce increases in cortisol. In this study, we determined if compound LY444711 produces sustained elevation of GH and IGF-1 in beagle dogs without sustained alteration of baseline cortisol secretions after one and seven daily doses. Adult male beagle dogs received oral doses of LY444711 at 1 mg/kg/day, or vehicle (10% hydroxypropyl beta-cyclodextrin). Jugular vein blood was collected periodically after one and seven doses, and plasma levels of IGF-1, GH, and cortisol were determined. LY444711 increased IGF-1 levels by approximately 60% over controls after one and seven daily doses. IGF-1 was elevated within 6 h of dosing on Day 1 and remained elevated 24 h postdose. GH levels (AUC) increased approximately 50-fold above controls following a single dose of LY444711. With repeated dosing, GH levels rose to approximately 8-fold over controls. Regardless of the reduction in GH AUC with repeat dosing, sufficient GH was produced to cause sustained IGF-1 elevation after seven doses. LY444711 produced little or no effect on cortisol AUC level after one or seven doses. These data demonstrate that LY444711 functions as a GH secretagogue in dogs, with associated increases in IGF-1 levels and an absence of meaningful increases in cortisol levels. Drug Dev. Res. 49:260–265, 2000. © 2000 Wiley-Liss, Inc.

Test method: GH

The abuse of recombinant human growth hormone in sports is considered to be a widespread phenomenon, but until today all our knowledge thereof is based on rumor, anonymous surveys or accidental observations by customs officers. Although growth hormone (GH) is listed as a prohibited class E substance by the International Olympic Committee (IOC) no official test for the detection of GH abuse is implemented. Because of the high degree of identity in the amino acid sequence between recombinant and endogenous GH and because of the pulsatile nature of GH secretion, as well as inter-individual variations, it was believed to be undetectable. Recently, new proposals have been made to overcome this problem. On the basis of differential immunoassays it is possible to determine the relative content of GH isoforms in the circulation and thus to detect the application of recombinant GH. Furthermore, changes in GH dependent parameters after the administration of recombinant GH have been shown to be a possible indicator of GH abuse.

Autologous Blood Donation with Erythropoietin in Abdominal Aortic Aneurysm Repair: Special Reference to Blood Pressure and Hormonal Changes

Recombinant erythropoietin (rEPO) is used in autologous blood donation, but its useful ness and side effects have not been evaluated thoroughly. A prospective clinical study was conducted in 20 patients with abdominal aortic aneurysm who underwent surgery following autologous blood predonation with rEPO therapy. The patients were 19 men and one woman ranging in age from 59 to 79 years old. The blood pressure was measured before and after blood withdrawal and rEPO injection, and before and after rEPO injection alone. The levels of endogenous EPO, prolactin, growth hormone, and thyroid hormones were compared before the first blood withdrawal and just before surgery. The volume of blood withdrawn was 800 mL (400 mL at each of two with drawals) within 2 weeks, while 6,000 units of rEPO were injected four times. The blood pressure dropped after blood withdrawal and rEPO injection, but it showed no marked change after rEPO injection alone. The mean value of hemoglobin was decreased from 13.8 to 12.4 g/dL in 2 weeks, and all patients received autologous blood transfusion during surgery. No declamping shock was noticed, and only 2 of the 20 patients needed homologous blood transfusion. The levels of growth hormone and thyroid hormones did not change, but that of prolactin increased following blood predonation with rEPO therapy. No significant change of endogenous EPO was observed.

Dose-response effects of a new growth hormone receptor antagonist (B2036-PEG) on circulating, hepatic and renal expression of the growth hormone/insulin-like growth factor system in adult mice.

The effects of growth hormone (GH) in regulating the expression of the hepatic and renal GH and insulin-like growth factor (IGF) system were studied by administering a novel GH receptor antagonist (GHRA) (B2036-PEG) at different doses (0, 1.25, 2.5, 5 and 10 mg/kg/day) to mice for 7 days. No differences were observed in the groups with respect to body weight, food consumption or blood glucose. However, a dose-dependent decrease was observed in circulating IGF-I levels and in hepatic and renal IGF-I levels at the highest doses. In contrast, in the 5 and 10 mg/kg/day GHRA groups, circulating and hepatic transcriptional IGF binding protein-3 (IGFBP-3) levels were not modified, likely resulting in a significantly decreased IGF-I/IGFBP-3 ratio. Hepatic GH receptor (GHR) and GH binding protein (GHBP) mRNA levels increased significantly in all GHRA dosage groups. Endogenous circulatory GH levels increased significantly in the 2.5 and 5 mg/kg/day GHRA groups. Remarkably, increased circulating IGFBP-4 and hepatic IGFBP-4 mRNA levels were observed in all GHRA administration groups. Renal GHR and GHBP mRNA levels were not modified by GHRA administration at the highest doses. Also, renal IGFBP-3 mRNA levels remained unchanged in most GHRA administration groups, whereas IGFBP-1, -4 and -5 mRNA levels were significantly increased in the 5 and 10 mg/kg/day GHRA administration groups. In conclusion, the effects of a specific GHR blockade on circulating, hepatic and renal GH/IGF axis reported here, may prove useful in the future clinical use of GHRAs.

Effect of growth hormone secretagogue LY444711 on IGF-1, growth hormone, and cortisol levels in beagle dogs after one and seven daily oral doses

Growth hormone (GH) release involves interaction of somatostatin and an endogenous GH secretagogue (GHS) on the hypothalamus. GH causes release of IGF-1, which acts by negative feedback to restrain subsequent GH release. GH secretagogues produce increases in cortisol. In this study, we determined if compound LY444711 produces sustained elevation of GH and IGF-1 in beagle dogs without sustained alteration of baseline cortisol secretions after one and seven daily doses. Adult male beagle dogs received oral doses of LY444711 at 1 mg/kg/day, or vehicle (10% hydroxypropyl beta-cyclodextrin). Jugular vein blood was collected periodically after one and seven doses, and plasma levels of IGF-1, GH, and cortisol were determined. LY444711 increased IGF-1 levels by approximately 60% over controls after one and seven daily doses. IGF-1 was elevated within 6 h of dosing on Day 1 and remained elevated 24 h postdose. GH levels (AUC) increased approximately 50-fold above controls following a single dose of LY444711. With repeated dosing, GH levels rose to approximately 8-fold over controls. Regardless of the reduction in GH AUC with repeat dosing, sufficient GH was produced to cause sustained IGF-1 elevation after seven doses. LY444711 produced little or no effect on cortisol AUC level after one or seven doses. These data demonstrate that LY444711 functions as a GH secretagogue in dogs, with associated increases in IGF-1 levels and an absence of meaningful increases in cortisol levels. Drug Dev. Res. 49:260–265, 2000. © 2000 Wiley-Liss, Inc.

Insulin-like growth factor-1 causes a switch-like reduction of endogenous growth hormone mRNA in rat MtT/S somatotroph cells.

Reduction of mRNA expression from the endogenous GH gene by insulin-like growth factor 1 (IGF-1) in somatotroph-like rat MtT/S cells was measured. GH mRNA levels were reduced by 65 nM IGF-1 treatment in a time-dependent manner over 5 d of culture with a calculated GH mRNA half-life of 50 h, in line with previous values from primary cultures. Inhibition of inositol 3-phosphate kinase by wortmannin or LY-294,002 treatment was ineffective in blocking IGF-1 decreases in GH mRNA, as was inhibition of MAP kinase activity by PD 098059. The inhibition by IGF-1 also did not regulate Pit-1 (GHF-1) mRNA levels, which were constant during 65 nM IGF-1 treatment. MtT/S cells were shown to have both IGF-1 and insulin receptors as detected by Western blotting. There was also shown to be the suggestion of "hybrid" receptors containing different beta chains from each of these related heterotetrameric receptors. Analysis of the effects of IGF-1 and insulin on MtT/S cells showed that each reduced GH mRNA in a dose-dependent manner gave a calculated EC50 of 15.5 nM for IGF-1 and 0.6 nM for insulin, suggesting that the respective receptors for each hormone were activated. However, GH mRNA response to IGF-1 treatment was "ultrasensitive," exhibiting a switch-like effect; below 10 nM IGF-1, there was no decline in GH mRNA, but then maximal reduction occurred at IGF-1 concentrations above 20 nM. The degree of this ultrasensitive effect was calculated from the Hill equation for cooperativity, with a Hill coefficient of -4.1, greater than the classic cooperativity exhibited by hemoglobin binding to oxygen. The ultrasensitive response was specific for IGF-1, as insulin did not display this effect. These results suggest that the response evoked by the IGF-1 receptor could act as a binary molecular switch controlling GH mRNA expression in somatotrophs.

M-mode echocardiographic evaluation of systolic function, LV volume and mass in children on growth hormone therapy.

Since abnormal endogenous growth hormone (GH) secretion in adults is associated with cardiac dysfunction, it is important to ensure that GH therapy in children and adolescents does not cause similar effects. Forty-two growth hormone-deficient children (Group 1) (19 girls, 23 boys) were evaluated. Six girls and seven boys were prepubertal with a mean age of 6.65 yr (range 4.37-9.73 yr). Twenty-nine were pubertal (13 girls, 16 boys), mean age 13.57 yr (range 10.08-16.76 yr). The patients had been on long-term GH therapy for 34.97 +/- 18.78 months with an average weekly dose of 17.61 IU/m2/wk. The mean height SDS was -2.85 +/- 1.22 for boys and -2.5 +/- 0.64 for girls at the onset of therapy, and at the time of examination -1.8 +/- 1.32 for the boys and 1.87 +/- 0.94 for the girls. Thirty-four normal control subjects (Group 2) matched for age, sex and body size were also studied. Left ventricular volume (LV), mass and systolic function [shortening fraction (FS)] were evaluated by two-dimensional guided M-mode echocardiography. Blood pressure was also measured. No differences in blood pressure were observed between patients and controls. There was no correlation of GH dose and duration of therapy with LV measurements. No significant differences were found between Group 1 and Group 2. These observations suggest that long term administration of GH does not produce adverse cardiac effects in GH deficient children. Nevertheless, longer follow-up studies are still needed to confirm the safety of long-term rhGH treatment.

Growth during and after a trial of growth hormone releasing hormone 1-29 in children with idiopathic short stature or growth hormone neurosecretory dysfunction.

The growth promoting effects of once nightly subcutaneous injections of growth hormone releasing hormone (GHRH) 1-29 (30 microg/kg) for 6 months were studied in 16 slowly growing prepubertal children with idiopathic short stature (ISS; Group 1) and 8 similar children with growth hormone neurosecretory dysfunction (GHND; Group 2). Each child underwent endogenous growth hormone evaluation using both pharmacological and physiological testing; each had stimulated values > 10 microg/l and were subsequently placed into one of two groups based on pooled 12-hour overnight GH of < or > or = 3 microg/l. Each patient was followed every three months for one year. There were no significant differences in the two groups throughout the study with the exception of the endogenous GH levels. Both groups responded to GHRH therapy with similar significant increases in their rates of growth. Although a subset of patients (6 of 21) continued to grow at a rate significantly greater than the pre-therapy rate of growth, overall rates of growth were not significantly different from the pre-therapy growth rates 6 months following the discontinuation of GHRH treatment. We conclude that GHRH 1-29, given in the doses provided, leads to similar changes in growth rates in short, slowly growing children who are GH sufficient and those with GHND. Despite prior reports to the contrary, GHND patients do not experience a sustained increased in growth rate upon discontinuation of GHRH.