Growth Hormone Pulse Research Articles

Pulsatility of growth hormone (GH) signalling in liver cells: role of the JAK-STAT5b pathway in GH action.

The intracellular signalling molecule and transcriptional activator STAT5b is a key mediator of the effects of intermittent plasma growth hormone (GH) pulses on the male-specific pattern of liver gene expression and pubertal body growth rates in rodents. Experiments with Stat5b gene-knockout mice have revealed that these GH-regulated, male-specific phenotypes are a direct consequence of GH pulse-dependent STAT5b activation and that loss of function of STAT5b cannot be compensated for by the closely related signalling molecule STAT5a. Physiological plasma GH pulses are required to obtain the high levels of activated STAT5b seen in the livers of males, and down-regulation of the GH receptor (GHR)-JAK-STAT5b pathway in hepatocytes exposed to GH in a near-continuous fashion underlies the low level of liver STAT5b activity that is characteristic of adult female rats. Termination of nuclear STAT5b signalling occurs at the conclusion of a plasma GH pulse, with STAT5b deactivation catalysed by a tyrosine phosphatase. In males, termination of the intracellular signalling stimulated by a plasma GH pulse is proposed to be additionally facilitated by GH-STAT5b-inducible SOCS-CIS proteins, which block the further activation of STAT5b by binding to and inhibiting the action of the GHR-JAK2 complex via multiple mechanisms. In this manner, the liver cell is rendered temporarily unresponsive to further GH-signalling events. SOCS-CIS proteins synthesized in liver cells stimulated continuously with GH may also contribute to the apparent down-regulation of STAT5b signalling that is observed in the female rat liver.

Food deprivation and feeding of broiler chickens is associated with rapid and interdependent changes in the somatotrophic and thyrotrophic axes

1. In several experiments, hormonal changes in the somatotrophic axis, growth hormone (GH) sensitivity to a GH-secretagogue, thyroid hormones and their metabolising enzymes and plasma glucose levels were measured in relation to food deprivation and reinitiation after a single daily meal in 4- to 5-week-old male broiler chickens. 2. Floor-reared male broiler chickens were fed ad libitum or were restricted to a daily food intake of 40 or 45 g per d from the age of 2 weeks onwards. The daily food allowance was consumed in 0.5 h. 3. Food deprivation increased plasma GH concentrations but decreased GH-dependent variables such as plasma insulin-like growth factor-I and 3,3',5-triiodothyronine (T 3 ) concentrations. Hepatic inner ring deiodinating type III activity was markedly elevated, presumably as a consequence of low hepatic GH receptor numbers, and is thought to be the causal mechanism for the low plasma T 3 concentrations. Food intake reversed these variables in a time-related manner. 4. GH pulsatility characteristics, as calculated by deconvolution analysis, revealed profound changes between food restricted and ad libitum fed animals. Chickens deprived of food for about 23.5 h were characterised by an enhanced pulsatile GH release as reflected in the higher GH secretory burst amplitude, GH mass per burst, GH production rate and GH pulse frequency . These variables returned very quickly to normal values after refeeding. 5. In summary, these experiments taken together demonstrate very clearly the interdependent and timerelated changes of the somatotrophic and thyroid axes upon a single meal in previously food-deprived broiler chickens.

Growth hormone pulse-activated STAT5 signalling: a unique regulatory mechanism governing sexual dimorphism of liver gene expression.

Growth hormone (GH) exerts sexually dimorphic effects on liver gene transcription that are regulated by the temporal pattern of pituitary GH release; this release is intermittent in male rats and nearly continuous in females. Comparisons of liver nuclear protein tyrosine phosphorylation in male and female rats have led to the discovery that the liver transcription factor STAT5b is tyrosine phosphorylated in male but not female rats in response to GH pulses. Intermittent plasma GH pulses trigger a rapid and repeated tyrosine phosphorylation and nuclear translocation of liver STAT5b in intact male rats, while the more continuous pattern of GH exposure down-regulates the STAT5b signalling pathway in female rat liver. The central importance of STAT5b for the physiological effects of GH pulses has been verified using a mouse gene knockout model. STAT5b gene disruption leads to a major loss of multiple sexually differentiated responses associated with the sexually dimorphic pattern of pituitary GH secretion. Male-characteristic body growth rates and male-specific liver gene expression are decreased to wild-type female levels in STAT5b-/- males, while female-predominant liver gene products are increased in males to near female levels. STAT5b is thus a liver-expressed, latent cytoplasmic transcription factor that undergoes repeated tyrosine phosphorylation and nuclear translocation in response to intermittent plasma GH stimulation, and is a key intracellular mediator of the stimulatory effects of GH pulses on male-specific liver gene transcription. Other studies indicate, however, that STAT5a and STAT5b are both required for constitutive expression in female, but not male liver, of certain GH-regulated CYP enzymes. GH activation of both STAT5 proteins, which in turn form distinct homodimeric and heterodimeric DNA-binding complexes, is thus an important determinant of the sex-dependent and gene-specific effects that GH has on the liver.

Growth hormone (GH) secretion in primary adrenal insufficiency: effects of cortisol withdrawal and patterned replacement on GH pulsatility and circadian rhythmicity.

We studied the effects of cortisol withdrawal and patterned replacement upon spontaneous GH secretion and circadian rhythmicity in 7 patients with Addison's disease. Hydrocortisone was administered in physiological daily total dosages, and all resulting plasma cortisol values were 2-15 micrograms/dl. It was given in 3 pulsatile modes: simulating "physiological" rhythm, "reverse" diurnal rhythmicity and "continuous" pulsatility. All modes of cortisol administration increased mean 24 h, GH pulse amplitude and interpulse GH levels. During saline infusions circadian GH rhythmicity was preserved, with GH being at its highest between 2400-0400 h. Administration of hydrocortisone in any mode did not modify circadian GH rhythmicity. We conclude: Cortisol replacement in physiological daily doses increases GH output in patients with Addison's disease by augmenting GH pulse amplitude and interpulse levels. This is likely due to the attenuation of hypothalamic somatostatin (SRIF) secretion by physiologic levels of cortisol. By inference, it implies that cortisol deficiency leads to diminution of GH output with low GH pulse amplitude, likely as a result of an augmented hypothalamic somatostatin secretion. However, circadian rhythmicity of GH secretion is glucocorticoid-independent.

STAT5b-deficient Mice Are Growth Hormone Pulse-resistant: ROLE OF STAT5b IN SEX-SPECIFIC LIVER P450 EXPRESSION

The signal transducer and transcriptional activator STAT5b is required to maintain the adult male pattern of liver gene expression and whole body pubertal growth rates, as demonstrated by the loss of these growth hormone (GH) pulse-dependent responses in mice with a targeted disruption of the STAT5b gene. The present study investigates whether these phenotypes of STAT5b-deficient mice result from impaired intracellular GH signaling associated with a loss of GH pulse responsiveness, as contrasted with a feminization of the pituitary GH secretory profile leading to the observed feminization of body growth and liver gene expression. Pulsatile GH replacement in hypophysectomized mice stimulated body weight gain in wild-type but not in STAT5b-deficient mice. Expression of the male-specific liver P450 enzyme CYP2D9, which is reduced to female levels in hypophysectomized male mice, was restored to male levels by GH pulse replacement in wild-type but not in STAT5b-deficient mice. Similarly, a female-specific liver CYP2B P450 enzyme that was up-regulated to female levels following hypophysectomy of males was suppressed to normal basal male levels by GH pulses only in wild-type hypophysectomized mice. Finally, urinary excretion of the male-specific, GH pulse-induced major urinary protein was restored to normal male levels following pulsatile GH treatment only in the case of wild-type hypophysectomized mice. STAT5b-deficient mice are thus GH pulse-resistant, supporting the proposed role of STAT5b as a key intracellular mediator of the stimulatory effects of plasma GH pulses on the male pattern of liver gene expression.

Suppression of the nocturnal rise in growth hormone reduces subsequent lipolysis in subcutaneous adipose tissue

The aim of this study was to examine the effect of the nocturnal rise in growth hormone (GH) concentration on lipolysis in adipose tissue the following morning. Eight healthy subjects were studied on two occasions (control vs. suppression of GH secretion) and six were studied on a third occasion (control vs. replacement of GH). Lipolysis in the whole body was assessed by measurement of systemic glycerol turnover. Lipid metabolism in the subcutaneous adipose tissue of the anterior abdominal wall was studied by measurement of arterio-venous differences. Suppression of the nocturnal rise in GH did not affect systemic glycerol turnover. However, in subcutaneous abdominal adipose tissue it led to a significant reduction in the veno-arterial differences in nonesterified fatty acid (NEFA, P = 0.041) and glycerol (P = 0. 014) concentrations, reflecting a reduction in intracellular lipolysis (P = 0.011). Although arterialized plasma triacylglycerol (TG) concentrations were reduced in the absence of the nocturnal GH pulse, the extraction of TG in subcutaneous abdominal adipose tissue remained unchanged. We conclude that the normal nocturnal rise in plasma GH concentration leads to site-specific regulation of lipolysis in adipose tissue on the following day, with preferential fat mobilization from central depots.

Administration of a Nonpeptidyl Growth Hormone Secretagogue, L‐163,255, Changes Somatostatin Pattern, But Has No Effect on Patterns of Growth Hormone‐Releasing Factor in the Hypophyseal‐Portal Circulation of the Conscious Pig

Abstract.The activity of the growth hormone secretagog, L‐163,255, on growth hormone (GH), growth hormone‐releasing factor (GRF), and somatostatin (SRIF) levels was evaluated in a porcine model of hypophyseal portal blood (HPB) collection. Young, castrated pigs had HPB and jugular blood collected for ≈ 300 min. The blood collection was divided into discrete periods: baseline (BL) ≈ 180 min; GH response period (RSP) ≈ 90 min; and positive control period following a GRF bolus, 30 min. RSP was divided into a dominant response period (DOM) and a tail (TL). The spontaneous relationship between HPB GRF and SRIF and peripheral GH during BL has been reported (Proc Soc Exp Biol Med 217:188–196, 1998). The apex of the GH pulse resulting from L‐163,255 administration was nonrandomly associated (P < 0.05) with descending periods of SRIF troughs. Frequency and amplitude of GRF and SRIF pulses, and frequency and depth of SRIF troughs were not different between BL and the beginning of DOM (the 20–30 min of GH increase). GH AUC was significantly greater (P < 0.05) for DOM compared to BL and TL, and for TL compared to BL. GRF AUC tended to be greater (P < 0.1) for RSP compared to BL, but the majority of the increase was in the TL period. There were no significant differences in the SRIF AUCs between the sampling periods. Furthermore, in a separate experiment, fos activity (a marker of neuronal activation) in the hypothalamus of pigs was examined after either L‐163,255 (1x or 4x), isotonic saline (control), or hypertonic saline (positive control) administration. There were no differences in fos activity in the GRF, SRIF, or CRH immunopositive neurons between L‐163,255 treatment and control. The pituitaries of the L‐163,255‐treated pigs showed marked fos activation compared to the controls. In conclusion, L‐163,255 in pigs has its primary effect at the level of the anterior pituitary.

Administration of a nonpeptidyl growth hormone secretagogue, L-163, 255, changes somatostatin pattern, but has no effect on patterns of growth hormone-releasing factor in the hypophyseal-portal circulation of the conscious pig.

The activity of the growth hormone secretagog, L-163,255, on growth hormone (GH), growth hormone-releasing factor (GRF), and somatostatin (SRIF) levels was evaluated in a porcine model of hypophyseal portal blood (HPB) collection. Young, castrated pigs had HPB and jugular blood collected for approximately 300 min. The blood collection was divided into discrete periods: baseline (BL) approximately 180 min; GH response period (RSP) approximately 90 min; and positive control period following a GRF bolus, 30 min. RSP was divided into a dominant response period (DOM) and a tail (TL). The spontaneous relationship between HPB GRF and SRIF and peripheral GH during BL has been reported (Proc Soc Exp Biol Med 217:188-196, 1998). The apex of the GH pulse resulting from L-163,255 administration was nonrandomly associated (P < 0.05) with descending periods of SRIF troughs. Frequency and amplitude of GRF and SRIF pulses, and frequency and depth of SRIF troughs were not different between BL and the beginning of DOM (the 20-30 min of GH increase). GH AUC was significantly greater (P < 0.05) for DOM compared to BL and TL, and for TL compared to BL. GRF AUC tended to be greater (P < 0.1) for RSP compared to BL, but the majority of the increase was in the TL period. There were no significant differences in the SRIF AUCs between the sampling periods. Furthermore, in a separate experiment, fos activity (a marker of neuronal activation) in the hypothalamus of pigs was examined after either L-163,255 (1x or 4x), isotonic saline (control), or hypertonic saline (positive control) administration. There were no differences in fos activity in the GRF, SRIF, or CRH immunopositive neurons between L-163,255 treatment and control. The pituitaries of the L-163,255-treated pigs showed marked fos activation compared to the controls. In conclusion, L-163,255 in pigs has its primary effect at the level of the anterior pituitary.

Tripartite Neuroendocrine Activation of the Human Growth Hormone (GH) Axis in Women by Continuous 24-Hour GH-Releasing Peptide Infusion: Pulsatile, Entropic, and Nyctohemeral Mechanisms1

Despite the discovery of potent GH-releasing peptides (GHRPs) more than 15 yr ago and the recent cloning of human, rat, and pig GHRP receptors in the hypothalamus and pituitary gland, the neuroregulatory mechanisms of action of GHRP agonists on the human hypothalamo-somatotroph unit are not well delineated. To gain such clinical insights, we evaluated the ultradian (pulsatile), entropic (pattern orderliness), and nyctohemeral GH secretory responses during continuous 24-h i.v. infusion of saline vs. the most potent clinically available hexapeptide, GHRP-2 (1 microg/kg x h) in estrogen-unreplaced (mean serum estradiol, 12 +/- 2.4 pg/mL) postmenopausal women (n = 7) in a paired, randomized design. Blood was sampled every 10 min for 24 h during infusions and was assayed by ultrasensitive GH chemiluminescence assay. Pulsatile GH secretion was quantitated by deconvolution analysis, orderliness of GH release patterns by the approximate entropy statistic, and 24-h GH rhythmicity by cosinor analysis. Statistical analysis revealed that GHRP-2 elicited a 7.7-fold increase in (24-h) mean serum (+/-SEM) GH concentrations, viz. from 0.32 +/- 0.042 (saline) to 2.4 +/- 0.34 microg/L (GHRP-2; P = 0.0006). This occurred via markedly stimulated pulsatile GH release, namely a 7.1-fold augmentation of GH secretory burst mass: 0.87 +/- 0.18 (control) vs. 6.3 +/- 1.3 microg/L (GHRP-2; P = 0.0038). Enhanced GH pulse mass reflected a commensurate 10-fold (P = 0.023) rise in GH secretory burst amplitude [maximal GH secretory rate (micrograms per L/min) attained within a secretory pulse] with no prolongation in event duration. GH burst frequency, interpulse interval, and calculated GH half-life were all invariant of GHRP-2 treatment. Concurrently, as detected in the ultrasensitive GH assay, GHRP-2 augmented deconvolution-estimated interpulse (basal) GH secretion by 4.5-fold (P = 0.025). The approximate entropy of 24-h serum GH concentration profiles rose significantly during GHRP-2 infusion; i.e. from 0.592 +/- 0.073 (saline) to 0.824 +/- 0.074 (GHRP-2; P = 0.0011), signifying more irregular or disorderly GH release patterns during secretagogue stimulation. Cosinor analysis of 24-h GH rhythms disclosed a significantly earlier (daytime) acrophase at 2138 h (+/- 140 min) during GHRP-2 stimulation vs. 0457 h (+/-42 min) during saline infusion (P = 0.013). Concomitantly, the cosinor amplitude rose 6-fold (P = 0.018), and the mesor (cosine mean) rose 5-fold (P = 0.003). Fasting (0800 h) plasma insulin-like growth factor (IGF-I) concentrations rose by -11 +/- 12 microg/L during saline infusion and by 102 +/- 18 microg/L during GHRP-2 infusion (P = 0.0036). GHRP-2 infusion did not modify (24-h pooled) serum LH, FSH, or TSH concentrations and minimally increased serum (pooled) daily PRL (6.8 +/- 0.83 vs. 12 +/- 1.2 microg/L; P < 0.05) and cortisol (5.3 +/- 0.59 to 7.0 +/- 0.74; P < 0.05) concentrations. In summary, 24-h constant iv GHRP-2 infusion in the gonadoprival female neurophysiologically activates the GH-IGF-I axis by potentiating GH secretory burst mass and amplitude by 7- to 10-fold and augmenting the basal (nonpulsatile) GH secretion by 4.5-fold. GHRP-2 action is highly selective, as it does not alter GH secretory burst frequency, interpulse interval, event duration, or GH half-life. GHRP-2 effectively elevates IGF-I concentrations, unleashes greater disorderliness of GH release patterns, and heightens the 24-h rhythmicity of GH secretion. These tripartite features of GHRP-2's action in estrogen-withdrawn (postmenopausal) women also characterize normal human puberty and/or sex steroid regulation of the GH-IGF-I axis. However, how or whether GHRP-2 interacts further with sex hormone modulation of GH neurosecretory control in older women and men is not yet known.

Effect of cysteamine hydrochloride on secretion of growth hormone in male swine

The objective of this study was to determine the effect of cysteamine hydrochloride (CSH) on growth hormone (GH) secretion in male swine. Twelve Poland China x Yorkshire boars, weighing 103.4 ± 3.0 kg and fitted with indwelling jugular vein catheters, were individually penned in an environmentally controlled room. Boars received I.V. Injections of either 0, 25, 50, or 75 mg CSH/kg body weight (BW) at h 0 (n = 3/treatment). Blood samples were collected every 15 min from h 0 to h 4. Serum concentrations of GH were determined by radioimmunoassay. There was an effect of treatment (P < .05) on mean GH concentrations. Mean GH concentrations (ng/ml) were 1.97 ± .46, 2.24 ± .59, .91 ± .06, and .62 ± .08 for boars receiving 0, 25, 50, and 75 mg CSH/kg BW, respectively. The dose of CSH-mean GH response had a linear (P < .01) component. Cysteamine hydrochloride at the 75 mg/kg B W dose decreased mean GH concentrations (P < .05) compared to the 0 and 25 mg/kg BW groups. The frequency and amplitude of GH pulses were similar (P > .1) among treatments. Overall, GH pulse amplitude was 2.35 ± .58 ng/ml and GH pulse frequency was .75 ± .07 pulses/h. Results from this experiment indicate that CSH suppresses circulating GH concentrations in a dose dependent fashion in boars.

Study of growth hormone secretion and action in growth-retarded children with juvenile chronic arthritis (JCA).

The stimulated and spontaneous growth hormone (GH) secretion and the response to GH action were assessed in growth-retarded children with juvenile chronic arthritis (JCA), in order to determine the underlying mechanisms of growth retardation in such children. Six children (4 boys and 2 girls aged 10.7-13.8 years) with active JCA of systemic onset were included in the study which involved: (1) anthropometric measurements; (2) assessment of GH responses to insulin-induced hypoglycaemia and clonidine stimulation; (3) assessment of the nocturnal pulsatile GH secretion by measuring GH in blood samples obtained every 20 min from 20.00 to 08.00 h; and (4) the IGF-I generation test. As a control, the latter test was also performed in eight aged-matched children with physiological delay in puberty. Biosynthetic hGH (0.1 IU/kg BW) was administered s. c. for 4 days and blood samples were taken at baseline and the morning after the last GH injection for measurement of IGF-I and IGFBP-3. All six children with JCA were prepubertal and their growth velocity was <3 cm/year. The GH responses to both stimulation tests were normal (peak GH >20 mU/l). Analysis of the pulsatile GH secretion during the night revealed three-to-four GH pulses of normal amplitude (>20 mU/l). IGF-I (26.7+/-4.6 nmol/l, mean+/-SD) and IGFBP-3 (2.1+/-0.2 mg/l) levels were lower in the patients compared with the controls (43.0+/-3.7 nmol/l and 2.8+/-0.2 mg/l, respectively, P<0.01). Following stimulation with exogenous hGH, there was a significant increase in IGF-I and IGFBP-3 levels in the control group (85 and 73%, respectively), but only a small increase in the patients (31 and 14%). It appears that stimulated and spontaneous GH secretion is normal in children with active systemic JCA, but the response to endogenous and exogenous GH with regard to IGF-I and IGFBP-3 production is impaired, indicating a degree of GH insensitivity in such children.

Injection of neuropeptide Y into the third cerebroventricle differentially influences pituitary secretion of luteinizing hormone and growth hormone in ovariectomized cows

Hypothalamic neurons that control the luteinizing hormone (LH) and growth hormone (GH) axes are localized in regions that also express neuropeptide Y (NPY). Increased hypothalamic expression of NPY due to diet restriction has been associated with suppressed secretion of LH and enhanced secretion of GH in numerous species. However, these physiological relationships have not been described in cattle. Thus, two studies were conducted to characterize these relationships using ovariectomized (Experiment 1) or ovariectomized estrogen-implanted (Experiment 2) cows. In Experiment 1, four well-nourished, ovariectomized cows received third cerebroventricular (TCV) injections of 50 and 500 μg of NPY in a split-plot design. Venous blood was collected at 10-min intervals from −4 hr (pre-injection control period) to +4 hr (postinjection treatment period) relative to TCV injection. NPY suppressed (P ⩽ 0.04) tonic secretion of LH irrespective of dose and tended to stimulate (P ⩽ 0.10) an increase in tonic secretion of GH. In Experiment 2, six ovariectomized cows that were well nourished and implanted with estradiol received TCV injections of 0, 50, or 500 μg of NPY in a replicated 3 × 3 Latin Square. Both doses of NPY suppressed (P < 0.06) mean concentration of LH relative to the 0-μg dose. The 50-μg dose of NPY tended (P < 0.10) to increase the amplitude of GH pulses. In conclusion, TCV injection of NPY suppressed pituitary secretion of LH and simultaneously tended to increase pituitary secretion of GH.

Distinctive Roles of STAT5a and STAT5b in Sexual Dimorphism of Hepatic P450 Gene Expression: IMPACT OF Stat5a GENE DISRUPTION

Stat5b gene disruption leads to an apparent growth hormone (GH) pulse insensitivity associated with loss of male-characteristic body growth rates and male-specific liver gene expression (Udy, G. B., Towers, R. P., Snell, R. G., Wilkins, R. J., Park, S. H., Ram, P. A., Waxman, D. J., and Davey, H. W. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 7239-7244). In the present study, disruption of the mouse Stat5a gene, whose coding sequence is approximately 90% identical to the Stat5b gene, resulted in no loss of expression in male mice of several sex-dependent, GH-regulated liver cytochrome P450 (CYP) enzymes. By contrast, the loss of STAT5b feminized the livers of males by decreasing expression of male-specific CYPs (CYP2D9 and testosterone 16alpha-hydroxylase) while increasing to female levels several female-predominant liver CYPs (CYP3A, CYP2B, and testosterone 6beta-hydroxylase). Since STAT5a is thus nonessential for these male GH responses, STAT5b homodimers, but not STAT5a-STAT5b heterodimers, probably mediate the sexually dimorphic effects of male GH pulses on liver CYP expression. In female mice, however, disruption of either Stat5a or Stat5b led to striking decreases in several liver CYP-catalyzed testosterone hydroxylase activities. Stat5a or Stat5b gene disruption also led to the loss of a female-specific, GH-regulated hepatic CYP2B enzyme. STAT5a, which is much less abundant in liver than STAT5b, and STAT5b are therefore both required for constitutive expression in female but not male mouse liver of certain GH-regulated CYP steroid hydroxylases, suggesting that STAT5 protein heterodimerization is an important determinant of the sex-dependent and gene-specific effects that GH has on the liver.

Sexually dimorphic expression of sst1 and sst2 somatostatin receptor subtypes in the arcuate nucleus and anterior pituitary of adult rats.

The pattern of growth hormone (GH) secretion and rate of somatic growth are markedly sexually dimorphic, but the underlying neuroendocrine mechanisms are far from clear. In the present study, we tested the hypothesis that the sexual dimorphism of GH secretion may be due to gender-related differences in the transduction of somatostatin's actions in brain and/or pituitary. To accomplish this, we compared the distributional pattern and level of expression of two somatostatin receptor subtypes, sst1 and sst2, in the brain and pituitary of adult male and female rats by in-situ hybridization using 35S-labelled antisense riboprobes. In the brain, the hybridization pattern and labelling density of sst1 and sst2 mRNA-expressing cells, as revealed by computer-assisted image analysis, in areas including the cerebral cortex, medial habenula (MHb) and ventromedial hypothalamic nucleus (VMN), were similar in male and female rats. In contrast, there was a marked sex-related difference in sst1 expression in the arcuate nucleus of the hypothalamus; both the number and labelling density of sst1 mRNA-expressing cells were two- to threefold greater in males than in females and this significant increase was homogenous throughout the rostrocaudal extent of the nucleus. No gender-related differences in arcuate sst2 mRNA levels were found. At the level of the anterior pituitary, the labelling density of sst2 mRNA in males was significantly higher than that of females. No sex-related difference in pituitary sst1 mRNA was observed. These results demonstrate a sexual dimorphism in the expression of two somatostatin receptor subtypes, sst1 and sst2, at the level of the arcuate nucleus and anterior pituitary, respectively. Such dimorphism suggests a differential involvement of sst1 and sst2 in GH regulation with respect to gender, and may imply roles for sst2 and sst1 in transducing somatostatin's actions on pituitary somatotrophs and GH-releasing hormone-containing arcuate neurones, respectively, to generate the lower basal and higher GH pulse levels characteristic of the male rat.

Effect of naloxone on intake and growth hormone concentration in lactating and non-lactating ewes

Thirty-two ewes (16 Suffolk and 16 Targhee) were used in two experiments to investigate effects of energy requirements, naloxone, breed, and number of lambs/ewe on feed intake and growth hormone (GH) concentrations. These experiments tested the hypothesis that the setpoint for intake, mediated through endogenous opioid tone, would be greater in lactating ewes than non-lactating ewes. Experiment 1 was conducted during lactation (43 d postpartum) with ewes confined individually with their lambs. Experiment 2 was conducted when ewes were non-pregnant and non-lactating. Naloxone treatments were 0, 0.5, 1.5, or 3 mg of naloxone/kg of ewe BW. After 3.5 h of fasting, ewe DMI was measured every 20 min for 5.5 h and expressed as g/kg of BW. Blood samples were taken at 15 min intervals for 5 h after naloxone was administered. Although level of naloxone and experiment influenced the repeated measures analysis of DMI ( P=0.01), no experiment × level of naloxone interaction was detected ( P=0.86). Increasing level of naloxone had a negative quadratic ( P<0.05) effect on DMI during the first 30 min in Experiments 1 and 2. Total DMI for the 5.5 h collection period was not influenced ( P>0.23) by naloxone level. An experiment by breed interaction was detected for DMI ( P=0.04). In Experiment 1, DMI was greater ( P=0.02) for Targhee than Suffolk ewes but breeds did not differ ( P=0.68) in DMI during Experiment 2. Number and frequency of GH pulses responded quadratically ( P=0.02) to increasing level of naloxone. Number, frequency, and height of GH pulses and mean GH concentration were greater in Experiment 1 than Experiment 2, and greater for Targhee than for Suffolk ewes ( P<0.05). Mean GH concentration ( P<0.05) and GH pulse height ( P<0.10) were greater for ewes with twin lambs than single lambs. An experiment × number of lambs born/ewe interaction was detected ( P=0.05) for mean GH concentration. Mean GH concentration was greater ( P=0.01) in Experiment 1 for ewes rearing twin lambs than single lambs. Mean GH concentration did not differ ( P=0.78) between ewes rearing single and twin lambs in Experiment 2. Although level of naloxone influenced number and frequency of GH peaks, the does of naloxone that would inhibit feeding did not differ between forage-fed ewes at high and low energy requirements.

Growth hormone pulsatility in acromegaly following radiotherapy.

Radiotherapy continues to have an important role in the treatment of acromegaly and is particularly effective at halting tumour growth, causing tumour shrinkage and reducing growth hormone (GH) concentrations in the long term. The major disadvantages of radiotherapy include the slow reduction in GH levels and damage to the other hypothalamic-pituitary axes. The 24 hour GH profile in active acromegaly compared with normals, characteristically shows an increased frequency of GH pulses, increased disorderliness (approximate entropy) of GH release, increased mean GH valley nadir, increased non-pulsatile fraction of GH and either similar or increased GH pulse amplitude. Complete surgical excision of a GH secreting adenoma may reverse these abnormalities and reduce circulating insulin-like growth factor-1 (IGF-1) concentrations to normal. However, very few data are available regarding the effects of radiotherapy on GH pulsatility in patients with acromegaly. Radiotherapy rarely leads to normalisation of the pattern of spontaneous GH release and may therefore be associated with an elevated IGF-1 even when 24 hour GH concentrations are comparable to healthy controls. The impact of such a biochemical state on morbidity and mortality in acromegaly is unknown. The continuing effects of radiotherapy may potentially transform an individual from a state of GH excess, to a state of GH deficiency, with as yet undetermined effects. In addition, radiotherapy leads to significant hypothalamic dysfunction, with the possible loss of endogenous somatostatin (SMS) production. This may potentially alter somatostatin (SMS) receptor expression on somatotroph adenomas and alter their responsiveness to subsequent SMS analogue therapy.

Effects of low-dose recombinant human insulin-like growth factor-I on insulin sensitivity, growth hormone and glucagon levels in young adults with insulin-dependent diabetes mellitus

Despite recent interest in the therapeutic potential of recombinant human insulin-like growth factor-I (rhIGF-I) in the treatment of diabetes mellitus, its mechanism of action is still not defined. We have studied the effects of low-dose bolus subcutaneous rhIGF-I (40 μg/kg and 20 μg/kg) on insulin sensitivity, growth hormone (GH) and glucagon levels in seven young adults with insulin-dependent diabetes mellitus (IDDM) using a randomized double-blind placebo-controlled crossover study design. Each was subjected to a euglycemic clamp (5 mmol/L) protocol consisting of a variable-rate insulin infusion clamp (6:00 pm to 8:00 am) followed by a two-dose hyperinsulinemic clamp (insulin infusion of 0.75 mU · kg −1 · min −1 from 8 to 10 am and 1.5 mU · kg −1 · min −1 from 10 am to 12 noon) incorporating [6,6 2H 2]glucose tracer for determination of glucose production/utilization rates. Following rhIGF-I administration, the serum IGF-I level (mean ± SEM) increased (40 μg/kg, 655 ± 90 ng/mL, P < .001; 20 μg/kg, 472 ± 67 ng/mL, P < .001; placebo, 258 ± 51 ng/mL). Dose-related reductions in insulin were observed during the period of steady-state euglycemia (1 am to 8 am) (40 μg/kg, 48 ± 5 pmol/L, P = .01; 20 μg/kg, 58 ± 8 pmol/L, P = .03; placebo, 72 ± 8 pmol/L). The mean overnight GH level (40 μg/kg, 9.1 ± 1.4 mU/L, P = .04; 20 μg/kg, 9.6 ± 2.0 mU/L, P = .12; placebo, 11.3 ± 1.7 mU/L) and GH pulse amplitude (40 μg/kg, 18.8 ± 2.9 mU/L, P = .04; 20 μg/kg, 17.0 ± 3.4 mU/L, P > .05; placebo, 23.0 ± 3.7 mU/L) were also reduced. No differences in glucagon, IGF binding protein-1 (IGFBP-1), acetoacetate, or β-hydroxybutyrate levels were found. During the hyperinsulinemic clamp conditions, no differences in glucose utilization were noted, whereas hepatic glucose production was reduced by rhIGF-I 40 μg/kg ( P = .05). Our data demonstrate that in subjects with IDDM, low-dose subcutaneous rhIGF-I leads to a dose-dependent reduction in the insulin level for euglycemia overnight that parallels the decrease in overnight GH levels, but glucagon and IGFBP-1 levels remain unchanged. The decreases in hepatic glucose production during the hyperinsulinemic clamp study observed the following day are likely related to GH suppression, although a direct effect by rhIGF-I cannot be entirely discounted.

Serum insulin but not leptin is associated with spontaneous and growth hormone (GH)-releasing hormone—stimulated GH secretion in normal volunteers with and without weight loss

Weight loss in humans is associated with elevated hypothalamic-pituitary growth hormone (GH) secretion. This study evaluates the effects of weight loss on the hypothalamic-pituitary (GH-releasing hormone [GHRH]-GH) axis in 14 normal-weight (body mass index [BMI], 25 ± 1 Kg/m 2) subjects, of whom half had undergone a diet-induced weight loss of 14% ± 2% (mean ± SEM). Insulin-like growth factor-1 (IGF-1), insulin, oral glucose tolerance, leptin, and GH pulse patterns were determined in both groups after weight maintenance for 1 week. Of note, we tested the effects of recent weight loss (3 months) and not a recent dietary intake, since both groups ingested a normal calorie diet for 2 days in the Clinical Research Center (CRC) prestudy. Serum insulin (3.8 ± 0.7 v 9.0 ± 0.9 μU/mL, P < .01) and C-peptide (0.44 ± 0.06 v 0.59 ± 0.04 mg/mL, P < .05) were significantly lower in the weight loss group. Serum leptin was not different. Endogenous GH pulse height (11.9 ± 4.8 v 1.3 ± 0.1 μg/L, P < .05), area per GH pulse ([AUC] 57 ± 28 v 6 ± 1 μg/L, P < .05), and mean GH (3.91 ± 0.76 v 0.85 ± 0.16 μg/L, P < .01) were increased in the weight loss group. The serum insulin level was inversely associated with the mean GH concentration ( r = −.678, P < .01) and GH pulse height ( r = −7.33, P < .01). In addition to spontaneous GH secretion, the GHRH-stimulated GH pulse height (41.8 ± 18.1 v 7.1 ± 1.6 μg/L, P < .05) and AUC (161 ± 35 v 46 ± 13 μg/L/min, P < .05) were also increased in the weight loss group. The insulin concentration was also inversely correlated with the GHRH-stimulated GH pulse height ( r = −.718, P < .01). The leptin concentration was correlated with the BMI ( r = .554, P < .05) and body fat ( r = .744, P < .01), but not with GH secretion. In summary, even though these patients were on a normal calorie diet, a history of recent weight loss in young men and women of normal weight and health can be associated with a significant increase in spontaneous GH pulse height and GHRH-stimulated pulse height. Weight loss was also associated with a reduced serum insulin level. The observed increase in GH secretion may be secondary to the reduction in insulin or alterations of other factors acting at the site of the pituitary.

Growth hormone secretion and pituitary gland weight in suckling lambs from genetically lean and fat sheep

Previous studies have found that weaned lambs from Coopworth sheep selected for low (lean) backfat depth have higher mean plasma growth hormone (GH) levels and heavier pituitary glands than those selected for high (fat) backfat depth. This study examined whether these differences between genotypes occurred in young suckling lambs. Six ewes from each genotype which were suckling a male/female set of twins were kept indoors. Four weeks after birth, ewes and lambs were blood‐sampled for 6 hours at 10 minute intervals and plasma GH levels measured. Lambs were then slaughtered and carcass composition determined. Subcutaneous fat depth was lower in lean than fat lambs at four of the five different sites measured (P < 0.05). Lean lambs and ewes had greater mean and basal plasma GH concentrations and a greater amplitude of GH pulses than fat sheep (P < 0.05). Plasma IGF‐I levels did not differ (P < 0.05) between genotype for either ewes or lambs. Pituitary glands were heavier (P< 0.001) in lean than fat genotype lambs (0.29 versus 0.19, SED = 0.017 g). It is concluded that differences between genotypes in body composition, plasma GH, and pituitary gland weight occur at an early age. An investigation of foetal development patterns may be required to elucidate the relationship between these parameters.

Interrelations Between Sleep and the Somatotropic Axis

In the human as in other mammals, growth hormone (GH) is secreted as a series of pulses. In normal young adults, a major secretory episode occurs shortly after sleep onset, in temporal association with the first period of slow-wave (SW) sleep. In men, approximately 70% of the daily GH output occurs during early sleep throughout adulthood. In women, the contribution of sleep-dependent GH release to the daily output is lower and more variable. Studies involving shifts of the sleep-wake cycle have consistently shown that sleep-wake homeostasis is the primary determinant of the temporal organization of human GH release. Effects of circadian rhythmicity may occasionally be detected. During nocturnal sleep, the sleep-onset GH pulse is caused by a surge of hypothalamic GHRH release which coincides with a circadian-dependent period of relative somatostatin disinhibition. Extensive evidence indicates the existence of a consistent relationship between SW sleep and increased GH secretion and, conversely, between awakenings and decreased GH release. There is a linear relationship between amounts of SW sleep--whether measured by visual scoring or by delta activity--and amounts of concomitant GH secretion, although dissociations may occur, most likely because of variable levels of somatostatin inhibition. Pharmacological stimulation of SW sleep results in increased GH release, and compounds which increase SW sleep may therefore represent a novel class of GH secretagogues. During aging, SW sleep and GH secretion decrease with the same chronology, raising the possibility that the peripheral effects of the hyposomatotropism of the elderly may partially reflect age-related alterations in sleep-wake homeostasis. While the association between sleep and GH release has been well documented, there is also evidence indicating that components of the somatotropic axis are involved in regulating sleep. The studies are most consistent in indicating a role for GHRH in promoting NREM and/or SW sleep via central, rather than peripheral, mechanisms. A role for GH in sleep regulation is less well-documented but seems to involve REM, rather than NREM, sleep. It has been proposed that the stimulation of GH release and the promotion of NREM sleep by GHRH are two separate processes which involve GHRH neurons located in two distinct areas of the hypothalamus. Somatostatinergic control of GH release appears to be weaker during sleep than during wake, suggesting that somatostatinergic tone is lower in the hypothalamic area(s) involved in sleep regulation and sleep-related GH release than in the area controlling daytime GH secretion. While the concept of a dual control of daytime and sleep-related GH secretion remains to be directly demonstrated, it allows for the reconciliation of a number of experimental observations.