Growth hormone (GH) responsiveness to GHRH in normal adults is not affected by short-term gonadal blockade

Chronic orchidectomy does not influence the sensitivity of the pituitary somatotropes to varying doses of GHRH administered intravenously to the adult male rhesus monkey

In the present study, the pituitary growth hormone (GH) response to graded doses of GH-releasing hormone (GHRH) was determined in intact (n = 3) and chronically orchidectomized (n = 3) adult rhesus monkeys (Mucaca mulatta). GHRH in doses of 0, 6.25, 12.5 and 25 microg/kg BW was infused through a teflon cannula implanted in the saphenous vein. Blood samples were collected 60 min before and 90 min after the injection of the neurohormone at 15 min intervals. All bleedings were carried out under ketamine hydrochloride anesthesia. The plasma levels of GH were determined by using AutoDELFIA time-resolved flouroimmunoassay, whereas plasma levels of testosterone and estradiol were determined using specific radioimmunoassay systems. The GH responses to GHRH were not significantly different between intact and chronically orchidectomized monkeys at any of the dose levels tested (p > 0.05). The administration of GHRH resulted in a significant (p < 0.05) stimulation of GH secretion at all the doses tested and in both the groups studied. In both intact and orchidectomized animals, the greatest response was observed at 6.25 microg/kg and no further increase was noted with the higher doses of GHRH. In conclusion, the present study suggests that chronic orchidectomy does not influence the sensitivity of the pituitary somatotropes to GHRH stimulation implying that the responsiveness of the pituitary somatotropes to GHRH is independent of testicular steroid modulation.

GH/IGF-I axis in azoospermia in primary and secondary hypogonadism: a study before and during replacement therapy

The growth hormone/insulin-like growth factor-I (GH/IGF-I) axis was studied in 15 azoospermic patients and in 10 control men. Eight patients were affected by hypergonadotrophic hypogonadism and 7 by hypogonadotrophic hypogonadism. All were studied before and during replacement therapy with testosterone and gonadotrophin, respectively, using the alpha 2 adrenergic agonist, clonidine (clonidine test). The data demonstrate no differences in basal levels for IGF-I and for the GH response to clonidine in azoospermic patients, affected by primary and secondary hypogonadism, before and during replacement therapy when compared with control fertile men. In contrast to some studies which describe a reduced GH response in azoospermia and oligozoospermia, we conclude that basal serum levels of IGF-I and the GH response to clonidine are not impaired in azoospermic patients affected by primary hypogonadism before and after the restoration of normal androgenization, and in azoospermic patients affected by secondary hypogonadism, both before and after restoration of spermatogenesis.

Pathophysiology of the neuroregulation of growth hormone secretion in experimental animals and the human

During the last decade, the GH axis has become the compelling focus of remarkably active and broad-ranging basic and clinical research. Molecular and genetic models, the discovery of human GHRH and its receptor, the cloning of the GHRP receptor, and the clinical availability of recombinant GH and IGF-I have allowed surprisingly rapid advances in our knowledge of the neuroregulation of the GH-IGF-I axis in many pathophysiological contexts. The complexity of the GHRH/somatostatin-GH-IGF-I axis thus commends itself to more formalized modeling (154, 155), since the multivalent feedback-control activities are difficult to assimilate fully on an intuitive scale. Understanding the dynamic neuroendocrine mechanisms that direct the pulsatile secretion of this fundamental growth-promoting and metabolic hormone remains a critical goal, the realization of which is challenged by the exponentially accumulating matrix of experimental and clinical data in this arena. To the above end, we review here the pathophysiology of the GHRH somatostatin-GH-IGF-I feedback axis consisting of corresponding key neurotransmitters, neuromodulators, and metabolic effectors, and their cloned receptors and signaling pathways. We propose that this system is best viewed as a multivalent feedback network that is exquisitely sensitive to an array of neuroregulators and environmental stressors and genetic restraints. Feedback and feedforward mechanisms acting within the intact somatotropic axis mediate homeostatic control throughout the human lifetime and are disrupted in disease. Novel effectors of the GH axis, such as GHRPs, also offer promise as investigative probes and possible therapeutic agents. Further understanding of the mechanisms of GH neuroregulation will likely allow development of progressively more specific molecular and clinical tools for the diagnosis and treatment of various conditions in which GH secretion is regulated abnormally. Thus, we predict that unexpected and enriching insights in the domain of the neuroendocrine pathophysiology of the GH axis are likely be achieved in the succeeding decades of basic and clinical research.

Growth hormones and sex steroid interactions at puberty

Puberty is a period of dynamic changes mediated by GH and the gonadal steroid hormones. Although these substances exert important independent effects, their interaction is vital to normal pubertal growth and development. This is supported by observations of blunted growth and diminished levels of GH and IGF-1 during adolescence in individuals with panhypopituitarism in whom adequate replacement with both hormones had not been achieved. The independent roles of androgens and estrogens in mediating the rise in GH secretion at puberty have been studied in individuals with complete androgen insensitivity and through the administration of nonaromatizable androgens and the use of selective androgen- or estrogen-receptor blockade. The preponderance of evidence from studies of nonaromatizable (pure) androgens suggests that GH secretion is not enhanced as it is under the influence of testosterone. In addition, studies have shown increased GH secretion following androgen-receptor blockade and diminished GH release after estrogen-receptor blockade. Together these studies suggest a facilitory role of estrogen receptor-mediated processes on GH secretion and IGF-1 production. If androgens influence the GH/IGF-1 axis, it is most likely by an inhibitory mechanism. Observations of delayed skeletal maturation and deficient bone mineralization in individuals with estrogen receptor defects or mutations of the aromatase gene demonstrate the essential role of estrogen in promoting normal bone maturation, the accrual and maintenance of BMD, and control of the rate of bone turnover.

Gonadal steroid hormone regulation of the somatotropic axis during puberty in humans Mechanisms of androgen and estrogen action

The adolescent growth spurt is associated with a sex steroid hormone-dependent rise in GH production; both androgens and estrogens are implicated as positive regulators of the somatotropic axis during puberty. The issue is complicated by the fact that testosterone may act both directly via the androgen receptor and indirectly, after its aromatization to 17beta-estradiol, through the estrogen receptor. Recently, a number of investigators have studied the effects of the administration of androgen and estrogen receptor antagonists, as well as nonaromatizable androgens, on GH secretion. These reports suggest that estrogen receptor-dependent processes play a facilitatory role in the pubertyassociated rise in GH secretion. If androgen receptor-mediated events are involved in the control of the somatotropic axis, their role is likely inhibitory. A hypothalamic site of action of the sex steroids is postulated.

GH secretion status in myotonic dystrophy

Frequent endocrine alterations and abnormal growth hormone (GH) secretion have been reported in myotonic dystrophy (MD). To evaluate GH secretion status in MD, GH response to 100 micrograms of growth hormone releasing hormone (GHRH) with or without pyridostigmine pretreatment and its relation with insulin-induced hypoglycemia was investigated in MD patients and compared with normal controls. The mean peak plasma GH response to GHRH was 27.8 +/- 19.2 micrograms/l normal subjects and 11.4 +/- 8.7 micrograms/l in MD patients. In five of seven patients GH reached a mean peak of 12.6 +/- 4.2 micrograms/l after insulin-induced hypoglycemia, compared with 5 +/- 2.8 micrograms/l after GHRH. Conversely, in two patients GH reached a peak of 16.1 and 32 micrograms/l after GHRH, and only 2.5 and 5.3, respectively, after hypoglycemia. Pretreatment with pyridostigmine in nine patients tested potentiated GHRH-induced GH release with a peak of 17.6 +/- 12.5 micrograms/l, compared with 10.05 +/- 6.7 micrograms/l after GHRH alone; IGF-I levels were normal in all patients.

Dependency of growth hormone (GH) stimulation following releasing hormones on the spontaneous 24-hour GH secretion in healthy male and female subjects

The purpose of this investigation was to evaluate whether in healthy subjects the GH response following stimulation with releasing hormones is dependent on the spontaneous GH secretion within 24 hr prior to the stimulation test. In 18 male and 9 female healthy subjects (21-59 years) GH was measured every 15 min over 26 hr. Twenty-four hours after the beginning of blood sampling, a GH stimulation test was performed by using a combined releasing hormone test. Sleep was recorded in three consecutive nights. A positive correlation was found between the AUCs of the 24-hr GH secretion and the AUCs of GH stimulation, which could not be explained by an age effect only. This study demonstrates that subjects with comparatively high amounts of GH secreted within 24 hr also show good GH secretory responses when immediately after the 24-hr sampling period a stimulation test is undertaken. Therefore, a low GH response to stimulation cannot be explained by feedback effects of high GH amounts secreted during the 24 hr before the test or by empty pituitary GH storages.

Sex differences in growth hormone response to growth hormone-releasing hormone

The aim of the present study was to ascertain whether sex differences exist in GH response to GHRH, and the influence that menstrual cycle have on this response. A GHRH test was performed on nine healthy men and ten women on days one and twelve of the menstrual cycle. Basal GH levels, (mean +/- SE) (7.98 +/- 3.09 ng/ml in women and 0.13 +/- 0.07 ng/ml in men, p less than 0.05) as well as maximal GH response (40.17 +/- 11.96 ng/ml in women and 9.63 +/- 2.32 ng/ml in males, p less than 0.01) were significantly higher in women than in men. In spite of a significant increase in estradiol levels during the menstrual cycle (75.88 +/- 2.48 pg/ml on day one and 198.40 +/- 28.65 pg/ml on day twelve, p less than 0.01) neither basal plasma GH levels, nor GH response to GHRH were significantly different. In conclusion, these results confirm that GH response to GHRH is higher in women than in men, and that this difference is not modified during the first phase of the menstrual cycle.