Depletion of hypothalamic growth hormone-releasing hormone by neonatal monosodium glutamate treatment reveals an inhibitory effect of betamethasone on growth hormone secretion in adult rats

Rats were treated with monosodium glutamate (MSG), 4 mg/g on alternate days for the first 10 days of life, to induce lesions of the arcuate nucleus and destroy the majority of growth hormone-releasing hormone (GHRH) neurones. At 10 weeks of age, control (n = 42) and MSG-treated (n = 36) male rats were used to test the effect of glucocorticoids on growth hormone (GH) secretion. Each treatment group was divided into six study groups to determine the effect of betamethasone (BM), administered either 3 or 20 h prior to sacrifice, alone and in combination with hypoglycaemia (insulin 0.1 U/100 g). BM treatment of male rats was without effect on plasma GH levels in control animals. In contrast, glucocorticoid administered either 3 h before sacrifice or the previous evening significantly reduced circulating GH (p less than 0.001) in MSG-treated animals. The difference in plasma GH response to BM pretreatment in control rats and those with lesions of the arcuate nucleus indicates a hypothalamic action of glucocorticoids, presumably on somatostatin and GHRH neurones. In control animals the effects appear to be counterbalancing, but following destruction of GHRH neurones an uncompensated inhibitory influence was observed. Male MSG-treated rats had lower body weight (-25%) and reduced hypothalamic GHRH (-89%) and pituitary GH content (-69%) compared to male controls. Female rats which had undergone the same neonatal MSG treatment (n = 40) when sacrificed 1 week after their male counterparts showed similar reductions in body weight (-15%), hypothalamic GHRH (-74%), and pituitary GH (-67%).(ABSTRACT TRUNCATED AT 250 WORDS)     

Signalling elements in the ultradian rhythm of circulating growth hormone regulating expression of sex-dependent forms of hepatic cytochrome P450

Neonatal male rats were treated with monosodium glutamate (MSG) at either 2 or 4 mg/g BW on alternate days during the first 10 days of life. As adults, the 4 mg MSG-treated rats displayed the obesity, growth retardation, and reduced pituitary weights that typify this syndrome. These animals had no detectable plasma GH as determined from serial blood samples taken every 20 min for 8 consecutive h. Associated with this loss of circulating GH was an induction of the female-specific hepatic cytochrome P450 2d (gene IIC12) and the disappearance of the male-specific form of cytochrome P450 2c (gene IIC11). The catalytic activities of cytochrome P450 2c (i.e. androgen 16 alpha- and 2 alpha-hydroxylase), sex-dependent hexobarbital hydroxylase and total cytochrome P450 were similarly feminized. Rats exposed to the 2-mg dose of MSG were also obese, but their growth rates and pituitary sizes were not as severely affected as in the 4 mg MSG-treated rats. Circulating GH in these lower dosed males was secreted in a pulsatile pattern similar to that in normal males, except that the pulse amplitude was reduced as much as 90%. In spite of this profound decline in GH peak heights in the 2 mg MSG-treated males, liver metabolism was characteristically masculine. That is, female-specific cytochrome P450 2d remained undetectable, while the male forms of cytochrome P450 2c, 2a (gene IIIA2), and RLM2 (gene IIA2), their respective catalytic steroid hydroxylase activities, and associated sex-dependent drug-metabolizing enzymes were expressed at the level of or greater than that in normal males. Thus, while an ultradian pulse of circulating GH is necessary for the characteristically masculine profile of sex-specific forms of hepatic cytochrome P450, neither the amplitude of the secretory peaks nor their total GH content is critical, and these can be greatly reduced from the normal male levels.     

Effects of neonatal administration of monosodium glutamate on somatotrophs and growth hormone secretion in prepubertal male and female rats

Female and male rats were injected with monosodium L-glutamate (MSG; 4 mg/g BW) or 0.9% saline as neonates and then decapitated on days 35 and 40 of life, respectively. Trunk blood was collected for RIA of serum GH. Anterior pituitary glands (APGs) were bisected. One half was assayed for GH. The other half was placed in culture medium to study the basal GH release rate. Pituitary sections from additional rats were stained for GH, and morphometric analyses were performed on the GH cells. Treatment with MSG lowered serum GH levels and gland GH content in female but not in male rats. MSG did not alter the gland GH concentration or the basal GH release rate whether expressed per mg APG or per entire gland in either sex. The mean cross-sectional area of GH cells was reduced in either sex of MSG-treated rats. The numerical density of GH cells and the percentage of GH cells in APGs were similar in saline- and MSG-treated rats of either sex. The volume density of GH cells was lower in MSG-treated male rats only. The results suggest that in prepubertal rats which had been given MSG as neonates there is a sex difference in mean serum GH concentration and APG GH content, GH cell size is reduced in both sexes, and the individual GH cells contain normal amounts of GH in spite of their smaller size.     

Evidence that reduced growth hormone secretion observed in monosodium glutamate-treated rats is the result of a deficiency in growth hormone-releasing factor

The present experiments were designed to examine various aspects of GH secretion in adult male rats given monosodium glutamate (MSG; 4 mg/g BW, sc) during the neonatal period. MSG-treated animals sustained lesions localized to the hypothalamic arcuate nuclei (ARC) and had reduced nasal-anal lengths and body weights. Anterior pituitary (AP) weights were decreased, but AP concentrations of GH and PRL were not significantly altered. Analysis of pulsatile GH secretion showed depressed GH pulses and prolonged GH trough periods. Mean 5-h plasma GH levels were reduced, whereas PRL levels were not affected. Morphine sulfate (MS) at doses of 0.01, 0.1, 1.0, and 3.0 mg/kg induced a prompt rise in GH during the 45 min after drug administration in controls. MSG-treated animals showed a significant rise in GH only with 1.0 and 3.0 mg/kg MS. A significant elevation in PRL was found in both control and MSG-treated animals after 1.0 and 3.0 mg/kg MS. The pentobarbital-induced rise in GH was also blunted in MSG-treated animals. MSG-treated animals which were administered antisomatostatin serum showed elevated GH trough and mean GH levels, with no apparent effect on GH peak levels. In view of the mechanisms by which MS and pentobarbital act to increase GH secretion, the present data suggest that the GH regulatory deficit observed in MSG-treated rats is due to a relative loss of GH-releasing factor secondary to ARC damage.     

Sexual dimorphism of growth hormone (GH) regulation in humans: endogenous GH-releasing hormone maintains basal GH in women but not in men

GH secretory patterns in humans are sexually dimorphic in terms of pulse regularity, amplitude of the diurnal rhythm, and magnitude of basal (trough) secretion. The neuroendocrine mechanisms of gender-specific GH regulation in humans are currently unknown, but the interpulse GH levels are generally assumed to be controlled by somatostatin. In rats, however, administration of antiserum to GHRH lowers GH interpulse levels in females but not males. In this study, using a competitive antagonist to GHRH in humans, we investigated whether endogenous GHRH has differential, gender-specific effects on the interpulse GH levels. Six healthy men and five healthy women (20-28 yr old) who were nonobese, did not smoke, and were on no medications known to influence GH secretion were studied. Each served as his or her own control during an infusion of GHRH antagonist or saline for a 27-h period. A control bolus of GHRH was given near the end of the infusion. In both sexes during GHRH antagonist infusion, mean GH, pulse amplitude, and GH response to GHRH decreased significantly, whereas pulse frequency remained unchanged. However, during the GHRH antagonist infusion, trough GH did not significantly change in men (P = 0.54) but significantly decreased in women (P = 0.008). Deconvolution analysis confirmed the lack of a significant change in basal secretion in men (P = 0.81) as opposed to women (P = 0.006). We conclude that sexual dimorphism in the neuroendocrine regulation of GH secretion in humans involves a differential role of endogenous GHRH in maintaining baseline GH.     

Sustained improvement in growth velocity and recovery from suboptimal growth hormone (GH) secretion after treatment with human pituitary GH-releasing hormone-(1-44)-NH2.

The sustained effect of human pancreatic GH-releasing hormone [hpGHRH-(1-44)-NH2] on growth rate and GH secretory patterns was studied in 14 patients (10 males and 4 females; aged 10-16 yr; all Tanner stage I or II). Nine children had inadequate spontaneous GH secretion (ISGHS), while 5 had classic GH deficiency. Seven of 9 patients with ISGHS and 1 of 5 patients with GH deficiency were given 2 sc injections/day of 5 micrograms/kg GHRH for 2-3 months; the others received 5 pulses of GHRH (5 micrograms/kg BW.pulse) for 6 nights a week for 2-13 months, given every 3 h. Six of the nine ISGHS patients increased their growth velocity in response to GHRH therapy. These same six patients maintained an increased growth velocity for up to 24 months after GHRH was discontinued. The remaining three ISGHS patients did not show a significant growth response to GHRH administration. Neither a temporary nor a sustained growth response was correlated with spontaneous overnight GH secretion in these patients. In contrast, three of five classical GH deficiency patients exhibited increased growth velocity while undergoing GHRH therapy, but growth returned to preintervention rates upon discontinuation of treatment. The other two of the five classic GH deficiency patients failed to demonstrate any growth response to GHRH treatment. The increased growth velocity that was sustained for long intervals even after discontinuation of GHRH in ISGHS patients may indicate restoration of normal regulation of the hypothalamic-pituitary GH secretion axis.     

Decreased galanin mRNA levels in growth hormone-releasing hormone neurons after perinatally induced growth retardation

Intrauterine growth retardation (IUGR) is associated with persistent postnatal growth retardation accompanied by dysfunction of the hypothalamic components of the growth hormone (GH) axis. At the adult stage, this is reflected by increased somatostatin (SS) and decreased neuropeptide Y (NPY) mRNA levels, whereas the GH-releasing hormone (GHRH) mRNA levels are normal and the output of GH remains unchanged. To extend our insight into the hypothalamic control of GH secretion in growth retarded rats, we determined galanin (GAL) mRNA levels at the adult stage of perinatally malnourished (i.e. IUGR and early postnatally food restricted) rats. Analyses included comparison of GAL mRNA levels in GHRH neurons in perinatally malnourished adult rats using a semi-quantitative double labeling in situ hybridization technique. We report that IUGR is accompanied by a 60% decrease in GAL mRNA levels in all GHRH neurons in the male IUGR group whereas a tendency towards a decrease was observed in the male early postnatally food restricted (FR) group. These effects became more pronounced when the analysis was restricted to GHRH neurons coexpressing GAL mRNA i.e. decreased GAL mRNA levels were seen in both male and female IUGR rats and in FR males. These data show that GAL mRNA levels in GHRH neurons are persistently decreased after perinatal malnutrition. Taking these results together with our previous data on SS, NPY and GHRH mRNA levels, we can conclude that IUGR leads to a reprogramming of the hypothalamic regulation of GH secretion.     

Growth hormone (GH) secretion, GH-dependent gene expression, and sexually dimorphic body growth in young rats with chronic renal failure

Chronic renal disease results in growth failure in children. This study sought to determine the influences of early renal failure on body growth, growth hormone (GH) secretion, and GH-dependent hepatic gene expression. Neonatal animals were subjected to five-sixth nephrectomy (Nephr) and monitored during growth. Sham-operated male (Sham) and female (Fem) rats served as controls. Whereas Nephr of adult animals causes renal insufficiency, neonatal nephrectomy leads to frank renal failure. In male Nephr compared with Sham animals, GH half-life and GH pulse frequency increased by 1.55- and 1.33-fold, respectively, and GH secretory-burst size decreased by 80%. Approximate entropy analysis quantified more disorderly patterns of GH secretion in Nephr animals, which differed from Sham males, but not from Fem rats. Expression of liver P450 CYP2C11 mRNA, which is dependent upon the male GH pattern, became undetectable, whereas expression of liver P450 CYP2C12 mRNA, which is dependent upon the female GH pattern, increased multifold. Renal failure in young rats abrogates the male pattern of GH pulsatility, abolishes the sexual dimorphism of body weight gain, and induces a female pattern of hepatic gene expression. These data raise the possibility that disruption of pulsatile GH secretion contributes to the growth failure of renal disease.     

Growth hormone (GH) response to GH-releasing hormone by perifused pituitary cells from male, female, and testicular feminized rats

To determine whether a normal complement of androgen receptors is required to permit full expression of sex-related differences in pituitary GH secretion, we compared the GHRH-stimulated GH secretory responses of continuously perifused anterior pituitary cells from normal male, normal female, and androgen-resistant testicular feminized (Tfm) rats. In each experimental replicate, acutely dispersed pituitary cells were exposed to GHRH (0.03-100 nM) administered as 2.5-min pulses in random order at 30-min intervals. The eluate was collected in 5-min fractions for GH determination by RIA. Basal unstimulated secretion of GH by cells from male rats was greater than that by cells from female (P = 0.007) and Tfm (P = 0.03) rats; basal secretion by the other two groups was similar (P = 0.55). Linear concentration-response relationships between GHRH and GH release were defined for cells from male (P = 0.0002), female (P = 0.0001), and Tfm (P = 0.0002) rats. Overall GHRH-stimulated GH secretion by cells from male rats was greater (P less than 0.0001) than that by cells from female rats. Overall secretion by cells from Tfm rats was less (P less than 0.001) than that by cells from male rats but greater (P less than 0.001) than that by cells from female rats. For all experimental groups, body weight was strongly correlated with both basal (r2 = 0.42; P = 0.001) and GHRH-stimulated (r2 = 0.53; P = 0.0001) GH secretion by the dispersed pituitary cells. These data suggest that a deficiency of androgen receptors results in a diminution of the in vitro GH secretory capability of anterior pituitary cells to a level below that by cells from normal males, but not to the level in normal females. The intermediate position of cells from the Tfm rat may represent a partial masculinization or defeminization within this generally female phenotype.     

Relationship between growth hormone-releasing hormone and somatostatin in the rat: effects of age and sex on content and in-vitro release from hypothalamic explants

Secretion of GH in the rat has been shown to be dependent upon age and sex. Using rat hypothalamic explants in vitro, we have studied the release and hypothalamic content of GH-releasing hormone (GHRH) and somatostatin in male and female Wistar rats at four different ages (10, 30 and 75 days, and 14 months). Basal release of GHRH was not significantly different between male and female rats, but at all ages males released more GHRH in response to stimulation by both 28 and 56 mmol potassium/l than female rats (P less than 0.05). Neither basal nor potassium-stimulated release of GHRH altered with age. In contrast, both basal and potassium-stimulated secretion of somatostatin increased significantly (P less than 0.01) with age, but was the same in the two sexes. Hypothalamic GHRH content, as assessed by the extractable tissue content following incubation, was significantly (P less than 0.01) lower in 10-day-old rats compared with older rats, but remained constant after 30 days of age. Somatostatin content, in contrast, increased progressively with age (P less than 0.01). The hypothalamic content of the two peptides was the same in both sexes. In conclusion, our findings demonstrate that male rats release more GHRH in vitro than female rats, possibly reflecting the increased pulse amplitude of GH seen in males in vivo; the progressive fall in secretion of GH previously reported during ageing appears to parallel the progressive increase in somatostatin release and content seen in our in-vitro system.     

Endogenous testosterone enhances growth hormone (GH)-releasing factor-induced GH secretion in vitro

The influence of endogenous gonadal steroids in male and female rats on basal and growth hormone-releasing factor (GRF)-stimulated GH secretion from perifused anterior pituitaries was studied. After 75 min of perifusion with basal medium, freshly dissected pituitaries were exposed to human GRF(1-44) (10 nmol/l) for 15 min. Neonatal (day 1-2) or prepubertal (day 25) gonadectomy of male rats suppressed baseline GH release (ng/min per mg dry weight) as well as GRF-stimulated GH release by 40-70%. This effect was slightly more pronounced in neonatally gonadectomized animals. In prepubertally gonadectomized male rats, the suppression of GH release was completely reversed by testosterone replacement therapy. In female rats, prepubertal gonadectomy did not affect GH secretion from perfused pituitaries. However, treatment of ovariectomized female rats with oestradiol reduced baseline and GRF-induced GH release to levels lower than those observed in sham-operated or vehicle-treated ovariectomized animals. The data suggest that testicular androgen secretion in adult male rats increases the pituitary GH release in response to GRF in vitro, whereas ovarian oestrogen secretion is of less importance for the GRF responsiveness of female rat pituitaries.     

Sexually dimorphic expression of the growth hormone-releasing hormone gene is not mediated by circulating gonadal hormones in the adult rat

The sexual dimorphism characterizing GH secretion in the rat is thought to be related to differences in the hypothalamic synthesis and release of the GH-regulating peptides, GH-releasing hormone (GHRH), and somatostatin. Therefore, the influence of gender and sex steroid hormones on hypothalamic expression of the GHRH gene in adult rats were examined. GHRH messenger RNA (mRNA) levels were measured in individual rat hypothalami by Northern hybridization analysis using a 32P-labeled complementary DNA encoding rat GHRH. Destruction of hypothalamic GHRH neurons by neonatal treatment with monosodium glutamate caused similar 3-fold reductions in the levels of GHRH mRNA in adult male and female animals. In three separate experiments, hypothalamic GHRH mRNA concentrations in male rats were 2- to 3-fold greater than in randomly cycling females (four or five rats per group; P less than 0.01). In spite of the greater abundance of GHRH mRNA abundance in the male rat hypothalamus, circulating gonadal steroids lacked the ability to modulate GHRH gene expression in adult animals, since neither gonadectomy nor pharmacological sex steroid replacement changed GHRH mRNA levels in the hypothalamus of male and female adult rats. Furthermore, GHRH mRNA concentrations in female rats were similar during the proestrus, estrus, and diestrus phase of the estrous cycle. Also, GH inhibited hypothalamic GHRH gene expression in a sex-specific manner. Exposure to high levels of GH secreted by the MtTW15 tumor for 4 weeks reduced GHRH mRNA concentrations 7-fold in male rats (P less than 0.001) but only 2-fold in females (P less than 0.05). These studies demonstrate that GHRH gene expression in the rat hypothalamus is sexually dimorphic. Basal mRNA levels are greater in male rats, and expression in male hypothalami is more sensitive to feedback inhibition by GH than expression in females. There is no evidence for regulation of GHRH mRNA levels by either testosterone or estrogen in adult rats. These gender differences in GHRH gene expression likely contribute to the generation of a sex-specific pattern of GH secretion.     

Developmental and sex-specific effects of low dose neonatal monosodium glutamate administration on mediobasal hypothalamic chemistry

Neonatal administration of monosodium glutamate (MSG) to rodents results in severe damage to the arcuate nucleus of the hypothalamus (AN). MSG-induced AN damage produces profound alterations in hypothalamic neurotransmitters and anterior pituitary function. Reproductive function is also severely compromised in both male and female MSG-treated rats. The present study investigated the developmental sequelae of MSG-induced alterations in hypothalamic monoamine metabolism as well as other aspects of MSG toxicity. Female rats given 4 mg/g of MSG on postnatal days 2 and 4 did not exhibit any significant alterations in hypothalamic monoamine metabolism on postnatal days 21 or 30, however, postpubertal MSG-treated females had significantly reduced levels of mediobasal hypothalamic (MBH) dopamine and DOPAC. In contrast, male MSG-treated rats had slight reductions in hypothalamic and MBH dopamine levels but these reductions were not statistically significant. Male MSG-treated rats did exhibit significant reductions in hypothalamic DOPAC on postnatal day 30 and MBH homovanillic acid (HVA) levels on day 100. Acetylcholine levels were also measured in the MBH and pituitary of adult male MSG-treated rats and found to be unaltered. The developmental profile of hypothalamic monoamines and their metabolites and MSG-induced alterations in dopamine and DOPAC levels in the MBH of female rats are discussed in relation to the neurochemical mechanisms involved in triggering puberty.     

Ghrelin improves growth hormone responses to growth hormone-releasing hormone in a streptozotocin-diabetic model of delayed onset

GH secretion is markedly altered in diabetes mellitus (DM) in both rats and humans, albeit in opposite directions. In the rat, diabetes suppresses pulsatile GH secretion, especially high amplitude pulses, and decreases GH responses to secretagogue, depending inversely on severity of metabolic alteration. In the present study, we wanted to address the GH responses to GHRH and low doses of ghrelin in a streptozotocin (STZ) model of diabetes characterized by the delayed onset of the metabolic alterations. We have shown that the administration of high doses of STZ (90 mg/kg in 0.01 M solution of chloride-sodium, ip) to five-day-old pups (n5-STZ) can induce the appearance of a characteristic diabetic syndrome in adult age, the diabetic triad, with elevated plasma glucose levels: polyuria, polydipsia, hyperphagia, and reduced body weight gain. At the age of 3 months, in these n5-STZ male and female rats the GH responses to GHRH (1 microg/kg) and GHRH combined with ghrelin (1+3 microg/kg) had diminished both in punctual times and in the area under the curve (AUC). However, the combined administration of GHRH and ghrelin, being the more potent stimulus, elicited a synergistic GH response. Thus, male and female rats with delayed onset diabetes displayed an altered GH response to GHRH, although the combined administration of GHRH and ghrelin was able to restore the GH secretion with a synergistic effect.     

Effect of antiserum to rat growth hormone (GH)-releasing factor on physiological GH secretion in the female rat

To further study the physiological role of GH-releasing factor (GRF), we examined the effect of antiserum to rat hypothalamic GRF on spontaneous GH secretion in the normal female rat. Two groups of six conscious female rats were passively immunized with either nonimmune rabbit serum (NRS) or antirat GRF serum via a chronic indwelling atrial catheter. The secretory profiles of GH were observed by collecting blood samples at 15-min intervals for 1 h before and 4 h after administration. The NRS-treated rats showed a characteristic female pattern of spontaneous GH secretion. GH pulses were of low amplitude (mean +/- SEM, 26.8 +/- 2.4 ng/ml) and occurred irregularly at a frequency of 4.2 +/- 0.2/5 h, while interpeak through levels of GH were relatively high, with nadir values of 8.6 +/- 0.7 ng/ml. Synthetic rat GRF, given iv at a dose of 1 microgram/kg BW after the last blood sampling, stimulated GH release to a peak level of 153 +/- 37 ng/ml in the control rats. Administration of GRF antiserum caused a profound suppression of both pulse and trough components of GH secretion. This effect occurred rapidly, within 15 min after injection of antiserum, and GH secretion decreased uniformly to very low levels (3.4 +/- 0.1 ng/ml), with little or no fluctuation throughout the observation period. GRF antiserum also abolished the synthetic rat GRF-induced GH release, indicating sufficient potency of immunoneutralization. These results demonstrate that both GH pulses and troughs are dependent upon hypothalamic GRF in normal female rats, thereby substantiating earlier observations in male rats which demonstrated the physiological role of GRF in GH secretion.     

Arginine normalizes the growth hormone (GH) response to GH-releasing hormone in adult patients receiving chronic daily immunosuppressive glucocorticoid therapy.

Glucocorticoids are thought to inhibit GH secretion through an enhancement of endogenous somatostatin tone. The aim of our study was to evaluate the effect of arginine, a secretagogue that increases GH secretion acting at the hypothalamic level, probably by decreasing somatostatin tone, on GH-releasing hormone (GHRH)-induced GH secretion in three male and five female adult patients with nonendocrine disease who were receiving daily immunosuppressive glucocorticoid therapy. Six normal subjects (four males and two females) served as controls. GHRH-induced GH secretion was evaluated after 30-min iv infusion of saline (100 mL) or arginine (30 g) in 100 mL saline. After saline administration, steroid-treated patients showed a blunted GH response to GHRH (GH peak, 8.7 +/- 2.4 micrograms/L) compared to that of normal subjects (GH peak, 23.8 +/- 3.9 micrograms/L). The GH responses to GHRH increased (P less than 0.05) after pretreatment with arginine compared to saline pretreatment in both normal subjects (GH peak, 36.6 +/- 4.0 micrograms/L) and steroid-treated patients (GH peak, 28.4 +/- 5.5 micrograms/L). The GH responses to GHRH plus arginine were not significantly different in steroid-treated and normal subjects. Thus, arginine is able to normalize the GH response to GHRH in patients receiving chronic glucocorticoid treatment. Our data are evidence that the stimulatory action of arginine and the inhibitory action of glucocorticoids on GH secretion are mediated by opposite effects on hypothalamic somatostatin tone.     

Long-term failure of compensatory growth in rats following acute neonatal passive immunization against growth hormone-releasing hormone.

Interruption of hypothalamic growth hormone-releasing hormone (GHRH) secretion by administration of antiserum against GHRH (GHRH-ab) decreases growth hormone (GH) secretion and inhibits growth in rats. The present study was undertaken to investigate whether there is a period of accelerated or catch-up growth following a period of growth arrest induced by GHRH-ab treatment. Neonatal male and female rats were injected daily on days 1-14 of age. Animals received normal rabbit serum (NRS) or GHRH-ab subcutaneously at a dose of 5 microliters/10 g body weight. Body weight, serving as an index of somatic growth, was monitored over the next 3 months. The increase in absolute body weight and growth velocity of GHRH-ab-treated rats, regardless of gender, was lower than the increase of NRS-treated animals. Significant decreases were observed by day 13 of age in the female rats and day 17 in the male rats. The percent differences and absolute difference in weight between the two treatment groups clearly demonstrated that the GHRH-ab-treated rats did not demonstrate any period of catch-up growth. A second group of animals was treated in a similar fashion to evaluate serum GH concentrations at three months of age. Pulsatile GH secretion, as assessed by peak frequency and amplitude, was normal in all of the rats, suggesting that the failure of catch-up growth in the GHRH-ab-treated animals was not due to decreased GH secretion.(ABSTRACT TRUNCATED AT 250 WORDS)     

A paradoxical gender dissociation within the growth hormone/insulin-like growth factor I axis during protracted critical illness

Female gender appears to protect against adverse outcome from prolonged critical illness, a condition characterized by blunted and disorderly GH secretion and impaired anabolism. As a sexual dimorphism in the GH secretory pattern of healthy humans and rodents determines gender differences in metabolism, we here compared GH secretion and responsiveness to GH secretagogues in male and female protracted critically ill patients. GH secretion was quantified by deconvolution analysis and approximate entropy estimates of 9-h nocturnal time series in 9 male and 9 female patients matched for age (mean +/- SD, 67+/-11 and 67+/-15 yr), body mass index, severity and duration of illness, feeding, and medication. Serum concentrations of PRL, TSH, cortisol, and sex steroids were measured concomitantly. Serum levels of GH-binding protein, insulin-like growth factor I (IGF-I), IGF-binding proteins (IGFBPs), and PRL were compared with those of 50 male and 50 female community-living control subjects matched for age and body mass index. In a second study, GH responses to GHRH (1 microg/kg), GH-releasing peptide-2 (GHRP-2; 1 microg/ kg) and GHRH plus GHRP-2 (1 and 1 microg/kg) were examined in comparable, carefully matched male (n = 15) and female (n = 15) patients. Despite identical mean serum GH concentrations, total GH output, GH half-life, and number of GH pulses, critically ill men paradoxically presented with less pulsatile (mean +/- SD pulsatile GH fraction, 39+/-14% vs. 67+/-20%; P = 0.002) and more disorderly (approximate entropy, 0.946+/-0.113 vs. 0.805+/-0.147; P = 0.02) GH secretion than women. Serum IGF-I, IGFBP-3, and acid-labile subunit (ALS) levels were low in patients compared with controls, with male patients revealing lower IGF-I (P = 0.01) and ALS (P = 0.005) concentrations than female patients. Correspondingly, circulating IGF-I and ALS levels correlated positively with pulsatile (but not with nonpulsatile) GH secretion. Circulating levels of GH-binding protein and IGFBP-1, -2, and -6 were higher in patients than controls, without a detectable gender difference. In female patients, PRL levels were 3-fold higher, and TSH and cortisol tended to be higher than levels in males. In both genders, estrogen levels were more than 3-fold higher than normal, and testosterone (2.25+/-1.94 vs. 0.97+/-0.39 nmol/L; P = 0.03) and dehydroepiandrosterone sulfate concentrations were low. In male patients, low testosterone levels were related to reduced GH pulse amplitude (r = 0.91; P = 0.0008). GH responses to GHRH were relatively low and equal in critically ill men and women (7.3+/-9.4 vs. 7.8+/-4.1 microg/L; P = 0.99). GH responses to GHRP-2 in women (93+/-38 microg/L) were supranormal and higher (P<0.0001) than those in men (28+/-16 microg/L). Combining GHRH with GHRP-2 nullified this gender difference (77+/-58 in men vs. 120+/-69 microg/L in women; P = 0.4). In conclusion, a paradoxical gender dissociation within the GH/ IGF-I axis is evident in protracted critical illness, with men showing greater loss of pulsatility and regularity within the GH secretory pattern than women (despite indistinguishable total GH output) and concomitantly lower IGF-I and ALS levels. Less endogenous GHRH action in severely ill men compared with women, possibly due to profound hypoandrogenism, accompanying loss of the putative endogenous GHRP-like ligand action with prolonged stress in both genders may explain these novel findings.     

Sex differences in growth hormone (GH) secretion by rats administered GH-releasing hexapeptide

The purpose of this study was to compare GH secretion after the administration of GH-releasing hexapeptide (GHRP-6) in conscious male and female rats. Plasma GH was significantly elevated in female rats (six of six) compared to male rats (three of six) 15 min after administration of a single sc injection of GHRP-6 (0.5 mg/kg). In male rats, GHRP-6 administration was associated with suppression of episodic GH secretion and desensitization to a second injection administered 6 h later, whereas in female rats, GH secretion occurred after both GHRP-6 injections. After 14 consecutive days of administering GHRP-6 twice per day, mean plasma GH concentrations in males decreased from 110 +/- 91 to 2.8 +/- 0.6 ng/ml (P less than 0.05) and in females increased from 170 +/- 53 ng/ml to 361 +/- 81 ng/ml (P less than 0.05). Desensitization to GHRP-6 in conscious male rats was not observed in pentobarbital-anesthetized male rats, suggesting that GHRP-6 administration enhanced somatostatin release in the conscious state. After 14 consecutive days of GHRP-6 administration, the mean pituitary GH concentration in female rats was significantly lower than that in male rats (5.1 +/- 0.2 vs. 12.9 +/- 1.2 micrograms/mg, respectively). Lower pituitary GH concentrations in females correlated with higher GH secretion after GHRP-6 administration. Desensitization to GHRP-6 in male rats is attributed to neurohumoral factors producing their unusual pattern of episodic GH secretion, and the response is probably not typical of other species.     

Growth hormone (GH) secretion in patients with an inactivating defect of the GH-releasing hormone (GHRH) receptor is pulsatile: evidence for a role for non-GHRH inputs into the generation of GH pulses

GH secretion is regulated by the interaction of GHRH and somatostatin and is released in 10-20 pulses in each 24-h cycle. The exact roles in pulse generation played by somatostatin, GHRH, and the recently isolated GH-releasing peptide, Ghrelin, are not fully elucidated. To investigate the GHRH-mediated GH secretion in human, we investigated pulsatile, entropic, and 24-h rhythmic GH secretion in two young adults (male, 24 yr; female, 23 yr) from a Moroccan family with a novel inactivating defect of the GHRH receptor gene. Data were compared with values in age- and gender-matched controls. Plasma GH concentration were measured by a sensitive immunofluorometric assay, with a detection limit of 0.01 mU/L. All plasma GH concentrations in the female patient were measurable; in the male patient 30 of 145 samples were at or below the detection limit. GH secretion was pulsatile, with 21 and 23 secretory episodes/24 h in the male and female patients, respectively. The fraction of basal to total GH secretion was raised in both patients by 0.18 and 0.15, respectively. The total 24-h GH production rate was greatly diminished; in the male patient it was 6.9 mU/L (normal values for his age, 26--63 mU/L), and in the female patient it was 4.2 mU/L (normal values for her age, 96--390 mU/L). The nyctohemeral plasma GH rhythm was preserved (P < 0.001), with normal acrophases (0430 and 0218 h in the male and female, respectively). Approximate entropy was greatly elevated in both subjects (0.82 in the male and 1.17 in the female; upper normal values for age and gender, 0.24 and 0.59, respectively). Intravenous injection of 50 microg GHRH failed to increase the plasma GH concentration in both patients, but 100 microg GH-releasing peptide-2 elicited a definite increase (male patient, 0.13 to 1.74 mU/L; female patient, 0.29 to 0.87 mU/L). Both patients had a partial empty sella on magnetic resonance imaging scanning. In summary, the present studies in two patients with a profound loss of function mutation of the GHRH receptor favor the view that in the human the timing of GH pulses is primarily supervised by intermittent somatostatin withdrawal, and the amplitude of GH pulses is driven by GHRH. In addition, we infer that effectual GHRH input controls the GH cell mass and the orderliness of the secretory process.