Clonidine treatment for short stature

Increased Secretion of Endogenous GH after Treatment with an Intranasal GH-releasing Peptide-2 Spray Does Not Promote Growth in Short Children with GH Deficiency

We investigated whether treatment with an intranasal GH-releasing peptide (GHRP)-2 spray, which acts as a potent GH secretagogue that stimulates endogenous GH secretion, promotes growth in patients with GH deficiency (GHD). This study involved 126 prepubertal short children (81 males, 45 females) with a height SD score of –2 SD or less, who had been diagnosed as having GHD based on GH stimulation tests, and in whom the serum GH concentrations increased up to 9 ng/ml after preliminary administration of an intranasal GHRP-2 spray. The subjects included in this study were divided into 3 groups by use of a double-blind method; that is 44 were placed into the placebo group (P group: 30 males, 14 females), 41 were placed into the GHRP-2 low dose group (L group: 25 males, 16 females), and 41 were placed into the GHRP-2 high dose group (H group: 26 males, 15 females). Those with a body wt of less than 20 kg were administered a placebo (P group), 50 μg of GHRP-2 (L group) or 100 μg of GHRP-2 (H group), and those with a body wt of 20 kg or more were administered a placebo (P group), 100 µg of GHRP-2 (L group) or 200 µg of GHRP-2 (H group) twice daily (morning and evening) for 48 continuous wk. Age and height SD scores at baseline were not significantly different among the three groups: 7.5 yr old and –2.26 SD in the P group, 7.3 yr old and –2.38 SD in the L group, and 7.5 yr old and –2.27 SD in the H group. Of the 126 subjects, 44, 40 and 40 subjects in the P, L and H groups, respectively, completed the 48 continuous wk of treatment. The changes in the mean height SD scores (mean growth rate) after 48 wk of treatment in the P, L and H groups were 0.07 SD, 0.03 SD, and 0.02 SD, respectively, and thus no significant differences was observed among the 3 groups. Also no significant changes in blood IGF-I levels at baseline or after 48 wk of treatment were observed among the 3 groups. This study revealed that in patients with GHD, an increase in endogenous GH secretion as a result of treatment with GHRP-2 does not promote growth. It is speculated that the area under the curve of serum GH concentration by GHRP-2 spray is too small to produce biological effects. In conclusion, it was demonstrated that growth cannot be promoted by a transient increase in endogenous GH secretion.

Neuroendocrine control of growth hormone secretion

The secretion of growth hormone (GH) is regulated through a complex neuroendocrine control system, especially by the functional interplay of two hypothalamic hypophysiotropic hormones, GH-releasing hormone (GHRH) and somatostatin (SS), exerting stimulatory and inhibitory influences, respectively, on the somatotrope. The two hypothalamic neurohormones are subject to modulation by a host of neurotransmitters, especially the noradrenergic and cholinergic ones and other hypothalamic neuropeptides, and are the final mediators of metabolic, endocrine, neural, and immune influences for the secretion of GH. Since the identification of the GHRH peptide, recombinant DNA procedures have been used to characterize the corresponding cDNA and to clone GHRH receptor isoforms in rodent and human pituitaries. Parallel to research into the effects of SS and its analogs on endocrine and exocrine secretions, investigations into their mechanism of action have led to the discovery of five separate SS receptor genes encoding a family of G protein-coupled SS receptors, which are widely expressed in the pituitary, brain, and the periphery, and to the synthesis of analogs with subtype specificity. Better understanding of the function of GHRH, SS, and their receptors and, hence, of neural regulation of GH secretion in health and disease has been achieved with the discovery of a new class of fairly specific, orally active, small peptides and their congeners, the GH-releasing peptides, acting on specific, ubiquitous seven-transmembrane domain receptors, whose natural ligands are not yet known.

Adrenergic control of the secretion of anterior pituitary hormones

The hypothalamic hypophysiotrophic neurones are densely innervated by adrenergic and noradrenergic nerve terminals. Activation of alpha 1-adrenoceptors located in the brain stimulates the secretion of ACTH, prolactin and TSH. The effects of the alpha 1-adrenoceptors seem to be exerted on hypothalamic neurones that secrete vasopressin, CRH-41 and TRH. These mechanisms are important in the physiological control of the secretion of ACTH and TSH in humans. alpha 2-Adrenoceptors are not involved in the control of secretion of these hormones under basal conditions in humans. However, alpha 2-adrenoceptors exert an inhibitory effect that acts as a negative feedback mechanism, limiting excessive secretion of these hormones. There is no convincing evidence for the involvement of beta-adrenoceptors in the control of the secretion of these three hormones in humans. Studies on cultured anterior pituitary cells suggested that adrenaline and noradrenaline may influence the secretion of ACTH, prolactin and TSH directly at the level of the pituitary. However, these effects are not demonstrable in humans, and are likely to be due to alterations in the pituitary adrenoceptors during culture. In the case of growth hormone, activation of alpha 2-adrenoceptors located in the brain stimulates secretion of this hormone both by increasing the secretion of GHRH and by inhibiting the secretion of somatostatin. Activation of beta-adrenoceptors inhibits the secretion of growth hormone via an increase in the secretion of somatostatin. The effects of the central alpha 2- and beta-adrenoceptors are important in the physiological control of growth hormone secretion in humans. A considerable amount of evidence implicates brain alpha 1-adrenoceptors in the control of secretion of the gonadotrophins in experimental animals, but, despite intensive study, no convincing evidence has been found in humans of reproductive age.

Effects of nightly clonidine administration on growth velocity in short children without growth hormone deficiency: a double-blind, placebo-controlled study

Children with short stature, slowed linear growth velocity, and delayed skeletal maturation may secrete growth hormone (GH) normally in response to provocative stimuli but may also have spontaneous undersecretion of GH. Orally administered clonidine, an alpha 2-adrenergic agonist, is a potent acute stimulator of growth hormone releasing hormone-mediated pituitary GH release. We performed a double-blind, placebo-controlled crossover study of nightly oral clonidine therapy (0.1 mg/m2) in 10 short, slowly growing, non-GH-deficient (stimulated GH level > 15 micrograms/L) prepubertal boys (range, 6.1 to 12.2 years; mean height standard deviation score, -2.3 +/- 0.4). Results of 6 months of clonidine therapy were compared with the results of 6 months of placebo therapy; GH responsiveness was subsequently assessed in 7 of 10 patients. Growth velocity (4.9 +/- 0.6 cm/yr baseline) was not improved by clonidine (4.6 +/- 1.2 cm/yr) or placebo (5.2 +/- 1.2 cm/yr), but it increased (p < 0.001) with GH therapy (8.2 +/- 1.3 cm/yr). Clonidine therapy similarly did not significantly affect plasma levels of insulin-like growth factor I or bone age maturation. Diminution in clonidine-stimulated peak GH levels was not observed after long-term oral clonidine therapy. Thus, in contrast to previous non-placebo-controlled studies, nightly clonidine therapy did not increase growth velocity or plasma insulin-like growth factor I levels. Subsequent acceleration in growth velocity during GH therapy suggests that a proposed increase in clonidine-induced endogenous GH secretion does not result in an effective growth-promoting stimulus.

Evaluation of growth hormone secretion and treatment

The secretion of growth hormone (GH) is regulated by a complex system that includes both neurotransmitters and feedback by hormonal and metabolic substrates. Over the last few years it has been recognized that GH release varies over a wide spectrum from deficient to excessive secretion. The diagnosis of GH deficiency is based on a combination of anthropometric and clinical signs on the one hand and an inadequate stimulated and/or spontaneous GH secretion on the other. There is no distinct boundary between deficient and sufficient GH secretion. The cut-off limit for normal GH release is accordingly relative and has increased over the past decade from 5 to 10 micrograms/l. The effect of GH therapy on growth can be evaluated only after treatment for at least 6 months. There is, therefore, an indisputable need for methods that would reflect growth response soon after the start of treatment. There are several promising biochemical candidates, e.g. the aminoterminal propeptide of type III procollagen, the carboxyterminal propeptide of procollagen I and the bone Gla-protein, which may turn out to be useful early indicators of the growth response to long-term GH therapy.

Clonidine pretreatment modifies the growth hormone secretory pattern induced by short-term continuous GRF infusion in normal man

OBJECTIVE

The aim of this study was to investigate the effect of a single dose of clonidine on the pattern of GH release in response to a 10-hour continuous GRF infusion in normal man.

DESIGN

Plasma GH was analysed in samples withdrawn at 20-minute intervals, from 0900 to 1900 h, according to the following protocols: in a control study, a placebo was given at 1000 h; in other experiments, clonidine (300 micrograms, orally) was given at 1000 h, alone or together with a continuous intravenous infusion of GRF 1-29 (0.3 micrograms/kg/h) starting at this time. In another experiment, the continuous infusion of GRF 1-29 was preceded by placebo administration at 1000 h.

PATIENTS

Eight normal volunteers (four women and four men), aged 19-24 years were studied.

MEASUREMENTS

Plasma GH levels were measured by RIA. Analysis of the pattern of GH secretion was performed using cluster analysis.

RESULTS

Clonidine induced a slight but significant increase in plasma GH values, peaking 60 to 120 minutes later; however, no significant changes were observed in indices of total and pulsatile GH release for the whole sampling period in this study. Continuous GRF administration led to increased episodic GH secretion, by augmenting GH peak amplitude, although peak frequency was not modified. An increase in interpulse GH values was also observed during GRF infusion. Pretreatment with clonidine clearly changed the pattern of GH release during GRF infusion: the amount of GH secreted was significantly higher, the number of GH peaks significantly increased, and almost all the GH was secreted within them.

CONCLUSIONS

These data concord with our previous demonstration that clonidine disrupts the hypothalamic-somatotroph rhythm by inhibiting the hypothalamic release of somatostatin. Given that clonidine pretreatment induced a more physiological episodic pattern of GRF-induced GH release, the possibility of combining clonidine and GRF therapy for short stature in children is envisaged.

Synergistic effect of growth hormone-releasing hormone (GHRH) and clonidine in stimulating GH release in young and old dogs

The effect of acute administration of growth hormone-releasing hormone (GHRH), clonidine (CLO), an alleged GHRH releaser, or GHRH and clonidine given simultaneously was studied in young and old dogs. Simultaneous administration of CLO induced in young dogs an additive effect on GH release and potentiated in old dogs the GHRH-induced GH release, with the GH response being clearly higher than the sum of the GH responses to GHRH or CLO alone. These data suggest that CLO promotes GH release in the dog also by inhibition of somatostatin release.

The interaction between clonidine and growth hormone releasing hormone in the stimulation of growth hormone secretion in man

Six normal adult males were given clonidine and GHRH either separately, or in combination, in random order. The peak serum GH concentrations elicited by clonidine or GHRH were variable but one factor influencing the GH response to GHRH was the GH secretory status in the hour prior to the administration of the GHRH. Peak serum GH concentrations attained were significantly greater when serum GH concentrations were rising (mean 52.9 mU/l, SD 17.2) than if they were falling (mean 27.5 mU/l, SD 13.3) or unchanged/undetectable (mean 20.6 mU/l, SD 9.8) (one-way ANOVA, F = 8.77; P = 0.004). The GH response to clonidine was not influenced by the secretory status in the hour prior to administration of clonidine. Pretreatment with clonidine did not augment the peak serum GH response to GHRH but the direction of response was more predictable than when GHRH was administered separately or repeatedly. Prior treatment with GHRH(1-29)-NH2 led to a marked attenuation of the peak serum GH response to clonidine. These results suggest that the alpha-2 adrenergic agonists probably stimulate GH secretion through pathways other than just GHRH.

Management of constitutional delay in growt and puberty

Constitutional delay in growth and puberty is a variant of normal growth and development that can cause a significant degree of psychological disturbance in otherwise healthy children, and is most often seen in boys of pubertal age. Careful assessment is necessary to rule out other endocrine or nonendocrine diseases. In some patients, therapy with oxandrolone or testosterone may be necessary to advance growth and/or pubertal development and thereby prevent serious psychological disturbance that can persist even into adult life. In the majority, however, reassurance will usually suffice.

Growth hormone secretory profiles: variation on consecutive nights

To evaluate the reproducibility of overnight growth hormone (GH) testing and the effect of daytime administration of levodopa and clonidine on overnight GH secretion, we examined consecutive 12-hour overnight GH profiles of 48 short subjects, ages 5 to 16 years, who had GH stimulation testing with levodopa and clonidine. In six subjects (12%) the overnight pool GH concentration on the second night increased by greater than 100% from the first-night result (night-to-night changes of +1.2 to +5.2 ng/ml). In the remaining subjects, night-to-night changes in pool GH concentrations ranged from -1.2 to +1.8 ng/ml (-60% to +88% changes from the first night value). Night-to-night changes were less than 25% of the first-night value in 17 subjects (35%), 25% to 50% in 18 subjects (38%), and 50% to 100% in 7 subjects (15%). Night-to-night changes in pool GH concentrations correlated with differences in peak nighttime GH concentrations but not with differences in duration of observed sleep. There was no discernible effect of daytime levodopa-clonidine administration on overnight pool GH concentrations. These results demonstrate the potential for night-to-night variation in overnight GH profiles and suggest the need for some means of confirming that overnight GH testing reflects normal physiologic GH secretion. Without such confirmation, the results from a single overnight GH profile should be interpreted with caution.

Relationship of plasma clonidine to growth hormone concentrations in children and adolescents

The relationships of plasma clonidine to increases in growth hormone, blood pressure response and degree of sedation were studied in nine children with short stature after a 0.15 mg/m2 oral clonidine tolerance test. A direct correlation was found between the peak plasma clonidine concentration and the increase in plasma growth hormone (P = 0.055). The patients experienced varying degrees of blood pressure reduction and sedation that were unrelated to age. No correlation was found between the plasma clonidine concentration and degree of blood pressure reduction or degree of sedation. The half-life of oral clonidine ranged from 1.75 to 2.38 hours, which is significantly less than that for adults.

Acute administration of pyridostigmine and clonidine has an additive stimulatory effect on GH release in normal children

It has been shown in humans that both alpha 2-adrenoceptor activation by clonidine (CLON) and cholinergic enhancement by pyridostigmine (PD) have a clear-cut stimulatory effect on GH release. As this effect is probably mediated by two different mechanisms, i.e. via increased endogenous GHRH for CLON and via inhibition of endogenous somatostatin for PD, in 8 normal children we studied the effect of both single and combined acute oral administration of CLON (150 micrograms/m2) and PD (60 mg). When administered alone, CLON and PD induced a similar GH increase (peak, mean +/- SE: 14.6 +/- 2.4 vs 14.2 +/- 3.1 ng/ml; area under curve, AUC: 376.9 +/- 57.6 vs 390.0 +/- 74.3 ng/ml/h). Combined administration of CLON and PD had an additive effect on GH release (peak: 27.5 +/- 4.5 ng/ml; AUC: 920.8 +/- 153.3 ng/ml/h; p less than 0.005 vs CLON and PD alone). In conclusion, presented data show that: i) CLON and PD have similar GH-releasing effect in normal children; ii) The additive stimulatory effect on GH release exerted by acute combined administration of CLON and PD agrees with the hypothesized different mechanism of action of these two drugs; iii) A therapeutic association of CLON and PD may be envisaged in the treatment of some children of short stature.

Lack of benefit of clonidine treatment for short stature in a double-blind, placebo-controlled trial

Twelve short (more than two standard deviations below the mean height for age), prepubertal children (ten boys, two girls) who had a normal peak growth hormone (GH) response to provocative stimulation with clonidine (more than 10 ng/ml) were enrolled in a double-blind, placebo-controlled, crossover study of the effects of a single, nightly dose of clonidine (0.1 mg/m2 by mouth). The children's mean age was 7.2 years (range 3.6-10.5 years). The results of 6 months of clonidine therapy were compared with those of 6 months of placebo. Clonidine therapy resulted in no significant difference in height standard deviation score, growth velocity, bone age, 24 h integrated GH concentration, peak GH response to clonidine stimulation, levels of insulin-like growth factor 1, or predicted height by the RWT method. In contrast to other studies, this study shows no sustained increases in GH production or in improved growth velocity with long-term administration of a single daily dose of clonidine. Furthermore, this study demonstrates the need for well-designed, placebo-controlled trials in paediatrics.

Alpha-2-adrenergic pathways release growth hormone via a non-GRF-dependent mechanism in normal human subjects

Administration of a supramaximal dose of GRF 1-44 (200 micrograms, i.v.) to normal human volunteers increased GH levels while a further bolus of GRF (200 micrograms i.v.) given 2 hours later failed to increase plasma GH levels. In contrast, alpha-adrenergic receptor agonism with either propranolol-adrenaline infusion or clonidine increased plasma GH levels at a time when GH responses to this supramaximal dose of GRF were absent. This indicates that alpha-adrenergic pathways stimulate GH secretion through a non-GRF-dependent mechanism in normal human subjects.