Pulsatile growth hormone secretion in patients with acromegaly and normal men: the effects of growth hormone-releasing hormone infusion

Twenty-four GH secretory patterns were studied before and during continuous infusions of GHRH in six patients with active acromegaly and in six normal adult men. GH release was episodic in both groups. Control subjects showed a normal diurnal variation in GH release, with the majority of GH released at night (2200-0800 h); mean levels were 1.5 +/- 0.4 (SE) ng/mL (day) and 4.2 +/- 0.8 ng/mL (night). Acromegalics had no diurnal variation in GH; levels were 45.3 +/- 13.7 ng/mL (day) and 39.8 +/- 12.2 ng/mL (night). Acromegalics demonstrated an increased frequency of GH pulses compared to normals (11.8 +/- 0.8 vs. 2.2 +/- 0.3/24 h). During continuous 24-h infusions of GHRH, the normal subjects continued to show a diurnal variation in GH release, but GH pulse frequency increased to a rate (11.7 +/- 1.4 pulses/24 h) very similar to that of the patients with acromegaly. In contrast, GHRH infusion did not alter the GH pulse frequency in the acromegalics. GHRH increased the mean levels of GH in both groups (patients 80.2 +/- 20.3 vs. 41.0 +/- 12.1 ng/mL, x +/- SE. P less than 0.05; controls 10.2 +/- 2.0 vs. 3.33 +/- 0.5 ng/mL, P less than 0.01). Some of the patients with acromegaly showed a progressive decline in GH levels during the infusion period, suggesting desensitization or exhaustion of releaseable stores; however, GH levels remained above basal values in all patients. After the 24-h GHRH infusions, the GH response to a bolus of GHRH was diminished in the normal subjects (2.1 +/- 0.9 vs. 16.8 +/- 5 ng/mL, x +/- SE; P less than 0.01) but not in the acromegalic patients (30.2 +/- 8.9 vs. 35.5 +/- 12.5 ng/mL; NS). These results indicate that GH release is episodic under basal conditions and during continuous GHRH infusion in both acromegalic and normal subjects, indicating the importance of other modulators of GH release, such as somatostatin, which may remain pulsatile even in acromegaly.     

Radioreceptor-inactive growth hormone associated with stimulated secretion in normal subjects.

We have previously reported systematic discrepancies between radioreceptor (RRA) and radioimmunoassay (RIA) measurements of growth hormone (hGH) in acromegalic patients. Due to limitations in RRA sensitivity, such comparisons could not be made in normal subjects. RRA methodology has now been adapted to allow detection of hGH at normal circulating levels. Since variations in Na+, K+, Ca++, and Mg++, incubation at 37 C and 4 C, and delayed tracer addition failed to improve assay sensitivity, specimen size was increased to 300 mul and incubation volume to 1.5 ml, while holding the quantity of added receptor constant. Best assay sensitivity, in room temperature incubations in 25 mM Tris for 16 h at pH 7.6 and 10 mM Ca++, was 0.66 +/- 0.30 ng hGH per ml serum. Under these conditions, 200 mug hepatic receptor protein bount 15.8 +/- 0.83% of added 125I-hGH, and 8.72 +/- 0.85% of bound tracer was displaced by 0.25 ng added unlabeled hGH. Nonspecific depression of binding by serum did not impair assay sensitivity with most receptor preparations. The basal hGH measured by RIA (antiserum 68-416) in a group of normal short children was 1.97 ng/ml, similar to the RRA result, 1.89 ng/ml (P = NS). Comparative measurements were also made in selected samples of sufficient volume during the 1 1/2 h following administration of hGH secretagogues (insulin, arginine, L-dopa). In these samples, the RIA value was 9.34 +/- 0.68 and the RRA value 6.29 +/- 0.62 ng/ml (P less than 0.01); the RIA/RRA was 1.77 +/- 0.18. Thus, no significant measurement discrepancy was found in basal samples from normal subjects, in contrast to previous findings in acromegalics. The appearance of such a discrepancy within 90 min after stimulation of hGH might be due to RIA/RRA discordance in secreted molecular subspecies, or might arise from peripheral hGH metabolism.     

Corticosteroid-induced growth hormone secretion in normal and obese subjects

OBJECTIVE: In normal subjects, corticosteroids stimulate growth hormone (GH) secretion at 3 hours. Obesity is associated with blunted GH secretion. In order to clarify both the deranged mechanism of GH secretion in obesity and the corticosteroid mechanism of action we have assessed in normal and obese subjects the effects of dexamethasone, pyridostigmine (a drug capable of suppressing somatostatin release) and GHRH. We also compared in normal subjects the stimulatory effect of three different corticosteroids on plasma GH levels. DESIGN: In both normal and obese subjects the following tests were carried out: placebo; dexamethasone alone (4 mg i.v. at 0 minutes); and dexamethasone plus pyridostigmine (120 mg p.o. at 60 minutes). In normal subjects we also studied the effects of hydrocortisone (100 mg i.v. at 0 minutes) and deflazacort (a corticosteroid that does not cross the blood-brain barrier) (60 mg i.v. at 0 minutes). In obese subjects we also assessed the effect of dexamethasone plus GHRH (100 micrograms i.v. at 150 minutes) on plasma GH levels. PATIENTS: Ten normal subjects and 22 obese subjects were studied. Normal controls were within 10% of their ideal body weight. Obese subjects had a body mass index of 37.1 +/- 1.1 (mean +/- SEM). MEASUREMENTS: Plasma GH levels were measured by radioimmunoassay. RESULTS: Dexamethasone-induced GH secretion in normal subjects (28.6 +/- 7.8 millimicron/l, P less than 0.05). Corticosteroids did not alter GH levels in obese subjects. Pretreatment with pyridostigmine increased dexamethasone-induced GH release in normal subjects (40.8 +/- 6.8 millimicron/l) but this did not achieve statistical significance. Dexamethasone plus pyridostigmine did not alter GH levels in obese subjects (8.0 +/- 1.6 mU/l). In some subjects, dexamethasone pretreatment potentiated GHRH-stimulated GH secretion, while in half the subjects the basal GH levels were not altered. In control subjects, hydrocortisone and deflazacort caused GH release similar to dexamethasone. CONCLUSIONS: Corticosteroids are a new and selective stimulus of GH secretion. They do not cause GH release in obese subjects. Their relative independence from cholinergic control suggest that they act by reducing somatostatin secretion.     

Effects of alcohol on growth hormone secretion in acromegaly.

In 3 normal subjects and in 4 acromegalic patients pretreatment with an alcohol infusion for 4 hours at a constant rate reduced the GH response to Arginine, when comparison was made with pretreatment with saline. The reduction in acromegalics was more marked and sustained than in normals. Though it is likely that the effect of alcohol is exerted on hypothalamic centers, a direct influence on the pituitary cannot be excluded.     

Regulation of pulsatile luteinizing hormone secretion in experimental uremia

Previous studies have shown that male rats with experimental uremia manifest profound suppression of circulating LH and testosterone levels, yet, paradoxically, after castration gonadotropin levels are elevated greatly above those of nonuremic castrate control rats. To investigate further this phenomenon, we characterized pulsatile LH secretion in experimental uremia. Mature orchidectomized male Wistar rats with subtotal nephrectomy demonstrated a 43% reduction of LH pulse frequency, but a 157% increase in pulse amplitude and a 335% increase in mean LH levels compared with sham-operated controls. All pulse parameters were highly correlated with plasma creatinine (r = 0.53-0.75). To determine the mechanism of the increased pulse amplitude, we tested responsiveness of the postcastration uremic pituitary to exogenous GnRH (0.01-10 micrograms/kg) in a Latin square design. Plasma LH response was linearly related to the logarithm of the GnRH dose in uremic and control rats, but was markedly increased in uremic rats. We conclude that the uremia causes decreased LH pulse frequency independent of testicular feedback. Pituitary hypersensitivity to GnRH magnifies LH pulse amplitude and thereby is the major factor causing the paradoxical LH hyperelevation after castration.     

Influence of L-prolyl-L-leucyl-glycine amide on growth hormone secretion in normal and acromegalic subjects.

Melanocyte Release-Inhibiting Peptide (MRIP-I) did not affect circulating levels of ACTH, LH, FSH, TSH,ORL, betaMSH and insulin when iv infused (5.0 mg in 5 min plus 0.4 mg/min for 70-115 min), while it significantly reduced serum GH response to hypoglycemia in normal subjects and lowered serum GH levels in acromegalics. There was no correlation between the fall in serum GH after MRIP and after dopaminergic drugs in acromegaly. These data are compatible with either a direct suppressive action exerted by MRIP-I at pituitary level or an extra-pituitary effect not involving dopaminergic pathways. It can be spec-lated that since labelled MRIP-I accumualtes in the pineal and melatonin blunts GH response to hypoglycemia, the pineal gland might be involved in the MRIP-I-induced suppression of GH secretion.     

Thyrotropin-releasing hormone increases serum levels of growth hormone-releasing hormone and growth hormone in patients with acromegaly.

The effect of intravenous injection of thyrotropin-releasing hormone (TRH) on the plasma concentrations of growth hormone (GH) and growth hormone-releasing hormone (GHRH) was studied in seven patients with acromegaly and in five control subjects. TRH had no effect on plasma GH or GHRH in the five control subjects. A 'paradoxical' increase in plasma GH in response to TRH was observed in four of the seven patients with acromegaly. In these four patients plasma GHRH also increased in response to TRH. No TRH-induced increase in GHRH levels was observed in the other three patients with acromegaly who did not display an increase in GH in response to TRH. The present results imply that GHRH may be involved in the plasma GH response to TRH in patients with acromegaly.     

Tamoxifen attenuates pulsatile growth hormone secretion: mediation in part by somatostatin.

Tamoxifen, a partial competitive antagonist to the estrogen receptor, is a potent inhibitor of the proliferation of experimental mammary carcinoma in the rat and is widely used clinically in the treatment of breast cancer. Blockade of estrogen receptors present on neoplastic cells represents the classic mechanism of action of tamoxifen, but the drug has a variety of other actions that may contribute to its antiproliferative properties. While it is recognized that estrogens play an important role in modulating pulsatile GH release, the effect of antagonists to sex steroid receptors on GH secretory dynamics has not previously been described. In the present study we examined the effect of tamoxifen on pulsatile GH secretion in free-moving adult male and female rats. The drug, when administered in a manner previously shown to be associated with antineoplastic activity, caused a marked suppression of the amplitude of spontaneous GH secretory bursts and significantly reduced mean 6-h plasma GH levels in both sexes compared to those in their respective peanut oil-injected controls. Inhibition of spontaneous GH pulses persisted for up to 7 weeks after tamoxifen administration in both sexes. Immunoneutralization of endogenous somatostatin in tamoxifen-treated male rats completely restored both GH pulse amplitude (121.6 +/- 9.5 vs. 62.5 +/- 13.5 ng/ml in tamoxifen-treated rats given normal sheep serum; P less than 0.02) and mean 6-h plasma GH levels (53.3 +/- 6.6 vs. 17.9 +/- 3.6 ng/ml in normal sheep serum-treated rats; P less than 0.01) to levels observed in our peanut oil-injected controls. These results demonstrate that 1) tamoxifen has potent inhibitory effects on pulsatile GH secretion; and 2) the blunting of GH pulse amplitude by tamoxifen is mediated at least in part by increased release of endogenous somatostatin. These findings motivate further investigation of the clinical significance of tamoxifen-induced suppression of GH secretion in relation to the antineoplastic activity of this commonly used drug.     

Clobazam increases pulsatile luteinizing hormone secretion in normal men

To investigate the effects of the 1,5-benzodiazepine, clobazam, on LH secretion in normal men, LH pulsatile secretion was defined after oral administration of 40 mg of clobazam or a placebo to 6 healthy male volunteers, according to a randomized cross-over design. LH pulse frequency increased significantly from a mean of 3.8 (range 3-5) pulses/8 h after placebo, to a mean of 5 (range 4-7) pulses/8 h (P less than 0.05), after clobazam. Mean LH concentrations and peak amplitudes did not change significantly. These results suggest that clobazam mediates its effects on LH secretion at the hypothalamic level by increasing the frequency of episodic GnRH release.     

Continuous infusion of growth hormone-releasing factor: effects on pulsatile growth hormone secretion in normal rats

The effects of a continuous infusion of growth hormone-releasing factor (GRF) on pulsatile growth hormone (GH) secretion were studied in conscious, freely moving rats. Male Sprague-Dawley rats were prepared with 2 indwelling venous catheters and were then housed individually for 6-16 days in isolation chambers. After this period, normal saline, 1.5 microgram/h GRF or 15 micrograms/h GRF were continuously infused for 8 or 31 h via one of the catheters. Blood samples were drawn at approximately 20-min intervals via the other catheter during the last 6-7 h of the infusion. Plasma GH concentrations were determined by RIA. Saline-treated animals exhibited the typical pattern of spontaneous GH pulses. The frequency of GH pulses in animals infused with either dose of GRF was not different from that of the saline-infused rats. Likewise, trough GH concentrations were similar in all three treatment groups. However, peak GH concentrations in the rats receiving the 15-micrograms/h GRF infusion were remarkably higher than the concentrations observed in the other two groups, regardless of the duration of the infusion. This resulted in mean GH concentrations being significantly higher (p less than 0.01) in the 15-micrograms/h GRF-infused rats. These results demonstrate that pituitary episodic GH secretion continues even in the presence of a continuous GRF stimulation.     

Sex-related naloxone influence on growth hormone-releasing hormone-induced growth hormone secretion in normal subjects

The effect of opiate-receptor antagonist naloxone on growth hormone (GH) release after growth hormone-releasing hormone (GHRH) 1-44 administration was investigated in ten normal men and 18 normal women during different phases of their menstrual cycle. Naloxone was infused at a rate of 1.6 mg/h in women and 1.6- and 3.2 mg/h in men, starting one hour before GHRH administration (50 micrograms iv as a bolus). On different day sessions, naloxone, GHRH, or saline were administered as controls. Naloxone infusion reduced the GHRH-induced GH release in normal women. The mean % inhibition of peak GH response was 83% during follicular phase, 46.5% during periovulatory phase, and 77.6% during luteal phase. On the contrary, in normal men, both doses of naloxone infusion were ineffective in blunting the GH response to GHRH. Our studies indicate that naloxone infusion was capable of inhibiting GH release induced by direct stimulation with GHRH in normal women, suggesting an opiate-antagonist action at the anterior pituitary level. The absence of such an effect in normal men strongly indicates a sex dependence of naloxone effects and suggests a role of the sexual steroid environment in opioid modulation of pituitary hormone secretion.     

Effects of growth hormone-releasing factor on growth hormone secretion in acromegaly

Twenty-nine patients with acromegaly (8 untreated and 21 previously treated in various ways) and 16 normal men were given iv bolus doses of human pancreatic tumor GH-releasing factor (hpGRF-40). Twenty-five of the 29 patients responded to hpGRF-40 with elevations of plasma GH. The magnitude of the responses varied widely. Responses of untreated patients were generally similar to those of the normal subjects. Previously treated patients had a significantly lower response than normal individuals [change in GH, 7.5 +/- 1.8 vs. 42.0 +/- 11.0 ng/ml (mean +/- SEM); P less than 0.01], and 4 patients who had received radiation therapy failed to respond to hpGRF-40. There was no significant correlation between the magnitude of the response and patients' age, sex, baseline GH levels, GH responsiveness of TRH, or GH suppression after oral glucose administration. Patients studied both pre- and postoperatively were responsive to hpGRF-40 at all times tested, but the magnitude of the response decreased after successful surgical removal of the adenoma. Thus, most patients with treated or untreated acromegaly respond to hpGRF-40, but their responses do not clearly distinguish them from normal subjects. GH-releasing hormone testing is unlikely to replace other endocrine tests available for the diagnosis and evaluation of acromegaly.     

Glucose-dependent insulinotropic polypeptide induced growth hormone secretion in acromegaly

Glucose-dependent insulinotropic polypeptide (GIP), a peptide released from the intestines after meals, is thought to stimulate insulin secretion. GIP receptor cDNA has recently been cloned and its mRNA has been recognized in several organs including the pituitary, but the physiological roles of GIP receptors of the pituitary have yet to be determined. We have demonstrated the possibility that GIP stimulates GH secretions from the pituitary adenoma cells of acromegalics. GIP-stimulated GH responses were studied in four acromegalics. In two acromegalics whose GH showed paradoxical secretion to oral glucose tolerance test (OGTT), GIP infusion (0.6 microg/kg/h) drove GH secretion (13.7 to 68.1, 22.5 to 76.2 ng/ml, respectively). However, in the other two acromegalics whose GH showed no paradoxical response to OGTT, GIP infusion did not induce GH secretion. One of the patients who was studied extensively had a GH that responded to OGTT. This patient's serum GH levels increased after meals while adenomectomy abolished both the paradoxical GH secretions by OGTT and GH responses to the GIP infusion. These data suggested that some somatotroph adenoma cells have an aberrant response to GIP which may go toward explaining paradoxical GH secretions to OGTT in acromegalics.     

Progesterone modulation of pulsatile luteinizing hormone secretion in normal women

Recent studies show that the frequency and amplitude of pulsatile LH secretion change during the normal human menstrual cycle; however, the neuroendocrine mechanisms underlying these changes are poorly understood. To assess the role of progesterone (P) in regulating LH secretion patterns, we treated normal women (n = 5) with im P in oil during the follicular phase of their cycle and compared LH pulse frequency, amplitude, and mean plasma level during treatment to those in normal cycling women. Normal women were studied five times in five menstrual cycles. Each study lasted 24 h, with a sampling interval of 20 min. The cycle phases studied were early follicular (twice), late follicular (LF), midluteal, and LF with P therapy to simulate luteal phase plasma P levels. LH pulse frequency was slower (P less than or equal to 0.001) in the midluteal phase than in either the early follicular phase or LF, and furthermore, P, administered in the normal follicular phase, slowed LH pulse frequency, augmented pulse amplitude, and reduced mean plasma LH levels compared to those in untreated women studied at the same cycle phase (P less than or equal to 0.02). We infer that P secretion by the ovary mediates the change in the LH secretory pattern during the luteal phase of the normal menstrual cycle, and that at least part of this effect is mediated by the central nervous system.     

Leptin regulates pulsatile luteinizing hormone and growth hormone secretion in the sheep

Administration of leptin during reduced nutrition improves reproductive activity in several monogastric species and reverses GH suppression in rodents. Whether leptin is a nutritional signal regulating neuroendocrine control of pituitary function in ruminant species is unclear. The present study examined the control of pulsatile LH and GH secretion in sheep. We determined whether exogenous leptin could prevent either the suppression of pulsatile LH secretion or the enhancement of GH secretion that occur during fasting. Recombinant human met-leptin (rhmet-leptin; 50 microg/kg BW; n = 8) or vehicle (n = 7) was administered s.c. every 8 h during a 78-h fast to estrogen-treated, castrated yearling males. LH and GH were measured in blood samples collected every 15 min for 6 h before fasting and during the last 6 h of fasting. Leptin was measured both by a universal leptin assay and by an assay specific for ovine leptin. During the fast, endogenous plasma leptin fell from 1.49 +/- 0.16 to 1.03 +/- 0.13 ng/ml. The average concentration of rhmet-leptin 8 h after leptin administration was 18.0 ng/ml. During fasting, plasma insulin, glucose, and insulin-like growth factor I levels declined, and nonesterified fatty acid concentrations increased similarly in vehicle-treated and leptin-treated animals. In vehicle-treated animals, LH pulse frequency declined markedly during fasting (5.6 +/- 0.5 vs. 1.1 +/- 0.5 pulses/6 h; fed vs. fasting; P < 0.0001). Leptin treatment prevented the fall in LH pulse frequency (5.0 +/- 0.4 vs. 4.9 +/- 0.4 pulses/6 h; P = 0.6). Neither fasting nor leptin administration altered GH pulse frequency. Fasting produced a modest increase in mean concentrations of circulating GH in control animals (2.4 +/- 0.5 vs. 3.4 +/- 0.6 ng/ml; P = 0.04), whereas there was a much greater increase in GH during leptin treatment (2.7 +/- 0.6 vs. 8.6 +/- 1.6 ng/ml; P = 0.0001). GH pulse amplitudes were also increased by fasting in control (P = 0.04) and leptin-treated sheep (P = 0.007). The finding that exogenous rhmet-leptin regulates LH and GH secretion in sheep indicates that this fat-derived hormone conveys information about nutrition to mechanisms controlling neuroendocrine function in ruminants.