Antiserum to rat growth hormone (GH)-releasing factor suppresses but does not abolish antisomatostatin-induced GH release in the rat

Injection of SRIF antiserum (oA-SRIF) increases serum GH and TSH levels in urethane-anesthetized rats. These responses require an intact hypothalamus with endogenous GH-releasing factor (GHRF) or TRH, respectively. We examined whether pretreatment of rats with an antiserum against rat GHRF (oA-rGHRF) would abolish the GH response to oA-SRIF, since anti-TRH serum has been shown to abolish TSH response to oA-SRIF. Prior injection of oA-rGHRF reduced oA-SRIF-induced GH response in a dose-related manner in a dose up to 1 ml antiserum, but failed to produce any further suppression at higher doses. The maximum suppression of the GH response was approximately 50%. oA-rGHRF also suppresses basal GH levels significantly. On the other hand, oA-rGHRF completely abolished the GH-releasing activity of 1 microgram synthetic rGHRF when incubated for 30 min at room temperature before injection. The results suggest two conclusions: 1) 43-residue rGHRF is a physiological regulator of both basal and stimulated GH release; 2) failure of oA-rGHRF to completely abolish the GH response to oA-SRIF suggests the presence of other physiological GHRFs in the rat.     

Growth hormone (GH)-releasing factor stimulates hypothalamic somatostatin release: an inhibitory feedback effect on GH secretion

GH-releasing factor (GRF) is a hypothalamic peptide that stimulates the secretion of pituitary GH. The possibility of feedback effects of GRF within the central nervous system was studied in conscious freely moving male rats with indwelling iv and intracerebroventricular (icv) cannulae. Animals were injected icv or iv with 10 ng-10 micrograms human (h) GRF(1-40)-OH (hGRF-40) or GRF(1-44)-NH2 (hGRF-44), and blood samples were obtained every 10-20 min from 1000-1400 h. GH secretion was pulsatile, with major secretory peaks at around 1200 h in most control animals. When 10 ng hGRF-40 were injected icv at 1100 h, immediately before the expected onset of the spontaneous GH secretory burst, GH secretion was suppressed during the following 2-h period. An iv injection of 10 ng hGRF-40 was without effect. In contrast, when 1 microgram hGRF-40 was injected icv or iv, plasma GH levels peaked at 20 and 10 min, respectively, and returned toward baseline shortly thereafter. The spontaneous GH secretory pulse after 1 microgram hGRF-40 (icv or iv) was suppressed in proportion to the magnitude of the GH secretory response to GRF (r = 0.78, p less than 0.01), and the prolongation of the interval between the injection of GRF and the subsequent spontaneous GH surge was directly related to the GH response to GRF (r = 0.85, p less than 0.001). The icv or iv injection of a larger dose of either hGRF-40 or hGRF-44 (10 micrograms) at 1100 h also resulted in marked and comparable increases in plasma GH levels, with peaks at 20 min (icv) and 10 min (iv) after injection. No changes in behavior or plasma glucose were observed up to 3 h after icv injection of any of the doses of hGRF-40 or of hGRF-44. The suppressive effect of centrally administered hGRF-40 (10 ng) on GH secretion was blocked by the iv administration of a specific antisomatostatin serum immediately before the injection of hGRF. These results demonstrate a dual action of GRF on spontaneous GH secretion and indicate the presence of an inhibitory feedback system within the central nervous system for the regulation of GH secretion which is mediated by hypothalamic somatostatin.     

Growth hormone (GH)-releasing peptide stimulates GH release in normal men and acts synergistically with GH-releasing hormone

The acute GH release stimulated by the synthetic hexapeptide, His-DTrp-Ala-Trp-DPhe-Lys-NH2 [GH releasing peptide (GHRP)], was determined in 18 normal men and compared with the effects of GH-releasing hormone, GHRH-(1-44)-NH2. Specificity of effect was assessed by measurement of serum PRL, LH, TSH, and cortisol. GHRP was administered at doses of 0.1, 0.3, and 1.0 microgram/kg by iv bolus. GHRH at a dose of 1.0 microgram/kg was administered alone and together with various does of GHRP. No adverse clinical effects of laboratory abnormalities were observed in response to GHRP. A side-effect of mild facial flushing of 1- to 3-min duration occurred in 16 of the 18 subjects who received GHRH-(1-44)-NH2. Mean (+/- SEM) peak serum GH levels after injection of placebo and 0.1, 0.3, and 1.0 microgram/kg GHRP were 1.2 +/- 0.3, 7.6 +/- 2.5, 16.5 +/- 4.1, and 68.7 +/- 15.5 micrograms/L, respectively. The submaximal dosages of 0.1 and 0.3 microgram/kg GHRP plus 1 microgram/kg GHRH stimulated GH release synergistically. Serum PRL and cortisol levels rose about 2-fold above basal levels only at the 1 microgram/kg dose of GHRP, and there were no changes in serum LH and TSH over the first hour after administration of the peptide(s). GHRP is a potent secretagogue of GH in normal men. Since GHRP and GHRH together stimulate GH release synergistically, these results suggest that GHRP and GHRH act independently. This supports our hypothesis that the GH-releasing activity of GHRP reflects a new physiological system in need of further characterization in animals and man.     

Effect of withdrawal of somatostatin and growth hormone (GH)-releasing factor on GH release in vitro

The secretion of GH, in vivo, is pulsatile. We have proposed that the timing of the episodic bursts of GH secretion is set by somatostatin (SRIF) withdrawal, while the magnitude of the bursts is set by the amount of GH-releasing factor (GRF) impinging on the somatotrophs, before and during SRIF withdrawal. We have now used an in vitro model of perifused rat pars distalis cells to further examine the interaction between GRF and SRIF on the magnitude of the burst of GH release that follows SRIF withdrawal. We first characterized the GH response, with time, to constant perifusion with GRF. The initial burst, followed by a rapid decrease in GH release induced by constant perifusion is due to a loss of GRF bioactivity in the perifusion medium and not to a decreasing responsiveness of the somatotrophs. This was followed by studies on the interaction between GRF and SRIF. The burst of GH release after cessation of perifusion with SRIF (10(-9) M) plus GRF (10(-10) M) can be blocked by the administration of SRIF during the burst. Also, the magnitude of the burst is proportional to the concentration of GRF preceding the withdrawal of SRIF. It is likely that similar relations apply in vivo, where SRIF withdrawal sets the timing and duration of the episodic burst of GH release, while GRF sets the magnitude.     

Variations of plasma growth hormone (GH)-releasing factor levels during GH stimulation tests in children

GH-releasing factor (GHRH) was measured by RIA in the plasma of 22 constitutionally short children given an ornithine infusion or an oral dose of L-dopa. After an overnight fast and 1 h of rest, plasma GHRH levels were 49.7 +/- 7.3 pg/ml (+/- SEM). In 5 children, L-dopa induced an increase in mean GH levels from 1.8 to 12 ng/ml at 60 min. Mean plasma GHRH levels increased from 47 pg/ml to a peak of 96 pg/ml at 15 min (P less than 0.02). In 4 other children, no increase in either GH or GHRH occurred after L-dopa treatment. In these 9 children, a significant correlation was found between the peak GH and GHRH concentrations (r = 0.841; P less than 0.001). On the contrary, ornithine-induced GH release was not preceded by a GHRH rise, but was followed by a GHRH decrease, from 51 to 27 pg/ml (P less than 0.02). We conclude that the 2 tests stimulate GH release in different ways, and that GH levels may be involved in the feedback control of GHRH secretion.     

Human growth hormone (GH)-releasing factor stimulates and somatostatin inhibits the release of rat GH variants

Two-dimensional polyacrylamide gel electrophoresis (2D PAGE) was used for the analysis of proteins secreted by male rat pituitary cells in monolayer culture in the presence of 10 nM human GH-releasing factor (hGRF) or 30 nM somatostatin (SRIF) or in the absence of these factors. More than 300 medium proteins were reproducibly detected either by fluorographic autoradiography or by silver staining. Immunoreactivity of each protein was examined after 2D PAGE followed by Western blotting and immunostaining with affinity-purified antirat GH (rGH) antibody. While there was a cluster of immunoreactive spots in the GH dimer range (40,000-50,000 mol wt), at least 16 medium proteins of mol wt 22,000 or less were also stained. Among these 16 proteins the release of 15 was stimulated and the release of 14 was inhibited by hGRF and SRIF, respectively. On the other hand, there were 3 proteins of approximate mol wt 16,000 whose secretion was regulated in a coordinate manner as rGH by the hypothalamic factors but which did not cross-react with anti-rGH antibodies. The increase or decrease in the radioactivity of each protein spot obtained from media after pituitary cells were incubated with [35S]methionine and hypothalamic factors was analyzed statistically. A pulse-chase study suggested that at least 7 of the hormonally regulated proteins, including rGH, were synthesized very rapidly. Finally, the 2D PAGE analysis of cell-free translation products of messenger RNA derived from male rat anterior pituitaries revealed the presence of about 40 rGH-immunoreactive proteins which included pre-GH. These data suggest that there are multiple forms of rGH-variants or rGH-related proteins. The biological significance(s) of all the rGH immunoreactive proteins and of the GRF- and SRIF-regulated pituitary proteins remains unclear. It is evident that a number of these proteins are synthesized and released rapidly by pituitary cells in culture. Furthermore, the presence of multiple genes for these rGH-related proteins is suggested by the large family of immunoreactive gene products identified after cell-free translation of messenger RNA derived from the pituitary.     

Growth hormone (GH) provocative testing frequently does not reflect endogenous GH secretion

GH secretion was studied in 73 children with classical GH deficiency or GH neurosecretory dysfunction (GHND), intrinsic short stature, or normal stature. The GH-deficient group was defined by a peak GH secretory response below 10 ng/ml to all provocative tests (arginine, L-dopa, insulin hypoglycemia, and clonidine). GHND was defined by a mean serum 24-h GH concentration below 3 ng/ml, with a normal response (greater than or equal to 10 ng/ml) to provocative testing. Twenty-one GH-deficient children, 21 children with GHND, and 18 short control children underwent provocative GH testing and a 24-h study with GH sampling every 20 min. A group of 13 normal stature control children also underwent 24-h GH sampling. The mean stimulated peak serum GH level [4.7 +/- 0.6 (+/- SEM) ng/ml] in the GH-deficient group was significantly below that in the GHND (19.5 +/- 1.7 ng/ml) and short control groups (24.0 +/- 3.5 ng/ml; P less than 0.01). The mean 24-h serum GH concentration was reduced in GH-deficient (1.5 +/- 0.2 ng/ml) and GHND (2.0 +/- 0.1 ng/ml) children compared to those in short (5.6 +/- 0.5 ng/ml) and normal stature (5.8 +/- 0.8 ng/ml) control children (P less than 0.01). Peak GH concentrations after provocative testing correlated poorly with 24-h mean concentrations in GH-deficient, GHND, and short control children (r = 0.38, 0.23, and 0.41, respectively; P = NS for all groups). Mean serum GH concentrations from blood sampling intervals of 12 h (day/night; 0800-2000/2000-0800 h, respectively) or even 6 h (day; 0900-1500 h) were statistically different in GHND or GH-deficient groups compared to those in control children; however, there was significantly more overlap for individual children using the 6- and 12-h daytime intervals than for the 24-h data. Plasma somatomedin-C/insulin-like growth factor I correlated with mean 24-h GH concentration endogenous secretion (r = 0.7; P less than 0.001). These data suggest that provocative GH testing frequently does not correlate with endogenous GH secretion.     

Protein kinase C is not essential for growth hormone (GH)-releasing factor-induced GH release from rat somatotrophs

To examine the role of protein kinase-C in the mediation of GH release we used acutely dispersed purified somatotrophs in static incubation and acutely dispersed adenohypophyses in perifusion. In static incubation, activation of protein kinase-C by phorbol 12-myristate 13-acetate (PMA) and 1,2-dioctanoyl-rac-glycerol (diC8) resulted in an increase in GH release and a concurrent concentration-dependent increase in cAMP accumulation. The GH response to diC8 in perifusion was reversible and repeatable. On the other hand, the GH response to PMA was not repeatable. The lack of repeatability is most likely due to the depletion of protein kinase-C by prolonged treatment with PMA. This assumption is strengthened by the observation that 1 h of perifusion with PMA left the somatotrophs refractory to a subsequent application of diC8. When graded pulses of GRF were applied during treatment with PMA, the GH response to GRF was not altered. Somatostatin reduced (in static incubation) or blocked (in perifusion) the release of GH induced by diC8 and PMA, but the accumulation of cAMP was not affected. We conclude that 1) activation of protein kinase-C in normal somatotrophs results in GH release which may not be completely independent of the cAMP pathway; 2) activation of protein kinase-C is not essential for GRF-induced GH release; and 3) SRIF acts at a site distal to or independent of cAMP to inhibit GH release induced by activators of protein kinase-C.     

Plasma growth hormone (GH)-releasing hormone levels in patients with lung carcinoma

OBJECTIVE: The aim was to investigate the serum levels of growth hormone releasing hormone and GH in patients with lung carcinoma. DESIGN After an overnight fast a plasma sample was collected for determination of growth hormone releasing hormone and GH. PATIENTS: The investigation was performed in 28 patients with non small cell lung carcinoma, in 44 patients with small cell lung carcinoma, and 10 patients with non malignant lung disease. A group of 37 normal subjects served as control. MEASUREMENTS: Growth hormone releasing hormone and GH were determined by radioimmunoassay. RESULTS: Patients with small cell lung carcinoma showed higher plasma growth hormone releasing hormone levels (49 +/- 9.4 ng/l) than control subjects (16.3 +/- 2.1 ng/l; P less than 0.05), patients with non small cell lung carcinoma (23.9 +/- 8.8 ng/l; P less than 0.05), and patients with non malignant lung disease (12.7 +/- 5.5; P less than 0.05). Basal GH level was lower than 5 micrograms/l in all patients except five patients with small cell lung carcinoma and one patient with non small cell lung carcinoma. CONCLUSIONS: The higher plasma growth hormone releasing hormone levels in patients with small cell lung carcinoma compared to normal controls and patients with non small cell lung carcinoma and patients with non malignant lung disease, confirm the frequent neuroendocrine activity of this particular tumour.     

Growth hormone (GH)-releasing factor differentially activates cyclic adenosine 3',5'-monophosphate- and inositol phosphate-dependent pathways to stimulate GH release in two porcine somatotrope subpopulations

Somatotropes comprise two morphologically and functionally distinct subpopulations of low (LD) and high (HD) density cells. We recently reported that GRF induces different patterns of increase in the cytosolic free Ca2+ concentration in single porcine LD and HD somatotropes, which for LD cells required not only Ca2+ influx but also intracellular Ca2+ mobilization. This suggested that GRF may activate multiple signaling pathways in pig LD and HD somatotropes to stimulate GH secretion. To address this question, we first assessed the direct GRF effect on second messenger activation in cultures of LD and HD cells by measuring cAMP levels and [3H]myo-inositol incorporation. Secondly, to determine the relative importance of cAMP- and inositol phosphate (IP)-dependent pathways, and of intra- and extracellular Ca2+, GRF-induced GH release from cultured LD and HD somatotropes was measured in the presence of specific blockers. GRF increased cAMP levels in both subpopulations, whereas it only augmented IP turnover in LD cells. Accordingly, adenylate cyclase inhibition by MDL-12,330A abolished GRF-stimulated GH release in both subpopulations, whereas phospholipase C inhibition by U-73122 only reduced this effect partially in LD cells. Likewise, blockade of Ca2+ influx with Cl2Co reduced GRF-stimulated GH secretion in both LD and HD somatotropes, whereas depletion of thapsigargin-sensitive intracellular Ca2+ stores only decreased the secretory response to GRF in LD cells. These results demonstrate that GRF specifically and differentially activates multiple signaling pathways in two somatotrope subpopulations to stimulate GH release. Thus, although the prevailing signaling cascade employed by GRF in both subpopulations is adenylate cyclase/cAMP/extracellular Ca2+, the peptide also requires activation of the phospholipase C/IP/intracellular Ca2+ pathway to exert its full effect in porcine LD somatotropes.     

Effect of somatostatin withdrawal and growth hormone (GH)-releasing factor on GH release in vitro: amount available for release after disinhibition

The secretion of GH is strikingly episodic. We have suggested that the timing of the episodic bursts of GH secretion is set by somatostatin (SRIF) withdrawal, whereas the magnitude of the bursts is determined by the amount of GH-releasing factor (GRF) impinging on the somatotrophs before and during SRIF withdrawal. We have now used an in vitro model of perifused rat pars distalis cells to examine the interaction of SRIF and GRF on GH release and, in particular, to examine the effect of GRF on the magnitude of the burst of GH release that follows SRIF withdrawal. After 30 min of perifusion with SRIF (10(-9) M), there follows an immediate but small burst of GH release. The burst of GH release following concurrent perifusion with SRIF plus GRF (10(-10) M) is increased, with a 7.5- to 9.5-fold increase in the peak secretion rate. When GRF is maintained after the withdrawal of SRIF, the peak secretion rate is not different from that seen after simple withdrawal of both SRIF and GRF, but the duration of the burst is increased. These data demonstrate that the presence of GRF during SRIF perifusion, while not altering basal release, does strikingly increase the post-SRIF release of GH. We propose that a similar relation applies in vivo, where SRIF withdrawal sets the timing of the episodic bursts of GH release, whereas GRF determines the magnitude.     

Sexual dimorphism of pyridostigmine potentiation of growth hormone (GH)-releasing hormone-induced GH release in humans

Sex differences in the neuroregulation of GH secretion are not now known in humans. To investigate whether activation of cholinergic tone by pyridostigmine could cause a sex-related difference in the pituitary responsiveness to GH-releasing hormone (GHRH), we have studied the GH response to GHRH in 16 normal subjects (8 men and 8 women) tested after oral placebo or different doses of pyridostigmine (30, 60, and 120 mg). Each subject presented a normal response after iv administration of 50 micrograms GHRH and placebo. In men each dose of pyridostigmine induced a significant increase in the GH response to GHRH, as assessed by both the maximal GH peak and the area under GH curve. In women, on the contrary, the GH response to GHRH was not potentiated by pretreatment with pyridostigmine at any given dose. Only five female subjects were tested with 120 mg pyridostigmine because of the severe side-effects of the drug at this dosage. Our present data strongly suggest that in humans there is a sex-related difference in the neuroregulation of GH secretion and this is probably expressed through a different cholinergic tone.     

Ceramide enhances growth hormone (GH)-releasing hormone-stimulated cyclic adenosine 3',5'-monophosphate accumulation but inhibits GH release in rat anterior pituitary cells

In this study, the effect of ceramide on GH-releasing hormone (GHRH)-stimulated cAMP accumulation and GH release in rat anterior pituitary cells was investigated. C2-, C6-, and C8-ceramide were found to enhance GHRH-stimulated cAMP accumulation. In contrast, their effects on GHRH-stimulated GH release were inhibitory. Treatment with a glucosylceramide synthase inhibitor produced a similar enhancing effect on cAMP accumulation and an inhibitory effect on GH release. To identify the pathway through which ceramide mediated its effect, it was found that ceramide inhibited GH release stimulated by KCl, BayK 8644, and a GH-releasing peptide, but not that stimulated by ionomycin or an activator of protein kinase C. Direct measurement of intracellular Ca2+ revealed that C2-ceramide inhibited GHRH- and KCl-mediated increases in intracellular Ca2+, suggesting that ceramide probably inhibits GH release through inhibition of the L-type Ca2+ channels. As for its mechanism on cAMP accumulation, the enhancing effect of ceramide on GHRH-stimulated cAMP accumulation was abolished in the presence of a phosphodiesterase inhibitor, isobutylmethylxanthine, suggesting that ceramide enhances the cAMP response through inhibition of its metabolism. Taken together, our results suggest that ceramide plays an important role in the regulation of GHRH-stimulated responses in somatotrophs. By reducing GH secretion while enhancing cAMP accumulation, ceramide may promote the synthesis and storage of GH in rat anterior pituitary cells.     

Human pancreatic growth hormone (GH)-releasing hormone stimulates GH synthesis and release in infant rats. An in vivo study

Administration of human pancreatic GH-releasing factor 1-40 (hpGRF-40) at doses of 1, 10, 20, 100, and 500 ng/100 g BW sc induced in 10-day-old rats a clear-cut rise in plasma GH 15-min post-injection, although the effect was not dose-related and peak GH levels were already present after the lowest GRF dose. In 25-day-old rats, hpGRF induced only a slight rise in plasma GH at the dose of 500 ng/100 g BW sc, whereas it was completely ineffective at the lower doses. In 5-day-old rats, hpGRF (20 ng/100 g BW sc twice daily), administered for 5 days, induced a marked rise in pituitary GH content and plasma GH levels determined 14 h after the last hpGRF injection. In these rats, at the end of treatment, a challenge hpGRF dose (20 ng/100 g BW) induced a rise in plasma GH significantly higher than in infant rats receiving only the challenge hpGRF dose. These data show that: 1) pituitary responsiveness to hpGRF is strikingly higher in infant than in post-weaning rats; 2) in infant rats, subacute administration of hpGRF stimulates GH synthesis and release.     

Growth hormone (GH)-releasing factor (GRF) pretreatment enhances the GRF-induced GH secretion in rats with the pituitary autotransplanted to the kidney capsule

The long term in vivo effects of the recently characterized human pancreas GH-releasing factor, hpGRF (1-44) were studied in chronically cannulated unrestrained rats. In order to minimize the influence of endogenous hypothalamic GRF and somatostatin on the pituitary, the experiments were carried out in rats with the pituitary autotransplanted to the kidney capsule. The integrated GH release (mean +/- SE) in response to an iv injection of hpGRF (4 micrograms/kg) was markedly enhanced (P less than 0.01) by iv pretreatment with hpGRF every 8 h for 3 days (182 +/- 47 h X ng/ml) as compared to saline-pretreated controls (36 +/- 4 h X ng/ml). TRH pretreatment did not potentiate the effect of hpGRF (47 +/- 9 h X ng/ml). It is concluded that multiple administrations of hpGRF enhance the GH response to a subsequent hpGRF injection in the rat. Moreover, autotransplantation of the pituitary to the kidney capsule may supply a useful in vivo model for further studies on the effects of different modes of GRF administration on GH secretion.     

Long term pulsatile growth hormone (GH)-releasing hormone therapy in children with GH deficiency

We treated seven GH-deficient children with 3-hourly 1 microgram/kg sc pulses of GHRH-(1-44) for 6 months and 2 micrograms/kg.pulse for another 6 months. Four patients had a serum GH response to iv GHRH before treatment, and an additional patient responded to iv GHRH after 1 month of pulsatile sc GHRH administration. The mean cumulative growth velocity increased from a pretreatment mean of 2.7 +/- 0.2 (+/- SE) to 8.4 +/- 2.5 and 5.4 +/- 0.7 cm/yr after 2 months and 1 yr of treatment, respectively. Low dose pulsatile GHRH therapy was effective in promoting growth in five of seven children, with height gain ranging from 4.4-7.5 cm at the end of 1 yr's therapy. Only one of the two patients who did not respond to GHRH had an improvement in linear growth when they were subsequently treated with synthetic GH. The other patient, a 16.5-yr-old pubertal girl who had both satisfactory GH and somatomedin-C responses during GHRH therapy, did not respond to either GHRH or, later, synthetic GH. The pretreatment serum GH response to iv GHRH, the serum somatomedin-C concentrations, and the peak serum GH response during sc GHRH therapy were not reliable predictors of clinical response.     

Growth hormone (GH)-releasing hormone-(1-29) twice daily reverses the decreased GH and insulin-like growth factor-I levels in old men.

Aging is associated with decreased GH and insulin-like growth factor-I (IGF-I) levels and lean body mass, and increased body fat. Recombinant human GH treatment of old men partially reverses body composition changes. Administration of GH-releasing hormone (GHRH) to GH-deficient children and young adults increases GH and IGF-I levels while preserving physiological GH release. We investigated whether GHRH injections restore GH and IGF-I levels in old men to the levels in young men. Healthy young (n = 9; 26.2 +/- 4.1 yr; mean +/- SD) and old (n = 10; 68.0 +/- 6.2 yr) nonobese men underwent baseline blood sampling for measurements of IGF-I and 24-h profiles of GH release, followed by iv bolus GHRH stimulation tests. Old men then took, randomly, both low (0.5 mg) and high (1 mg) dose GHRH-(1-29) sc injections twice daily for 14 days, with an intervening 14-day nontreatment period. The study protocol was repeated on day 14 of each treatment. At baseline, the mean peak duration of spontaneous GH release (P less than 0.005) and IGF-I levels (P less than 0.0001) were lower in the old men. GHRH treatment evoked dose-related increases in all parameters, with significant differences (vs. old basal values) in mean 24-h GH (P less than 0.001), area under peaks (P less than 0.001), peak amplitude (P less than 0.05), and IGF-I (P less than 0.005) only at the high dose. After high dose treatment, there were no significant differences in these parameters between age groups. Peak and integrated responses to iv GHRH stimulation tests did not differ between young and old men either before or during GHRH treatment. Baseline serum levels of both testosterone (P less than 0.01) and phosphate (P less than 0.05) were lower in the older men. Phosphate levels increased (P less than 0.05) during GHRH treatment. GHRH treatment did not affect fasting glucose, urinary C-peptide, blood pressure, or chemistry and hematology profiles. Thus, short term sc administration of GHRH to healthy old men reverses age-related decreases in GH and IGF-I, suggesting that prolonged treatment could improve age-related alterations in body composition.     

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.