Suppression of growth hormone (GH) secretion by a selective GH-releasing hormone (GHRH) antagonist. Direct evidence for involvement of endogenous GHRH in the generation of GH pulses.

To study the potential involvement of growth hormone-releasing hormone (GHRH) in the generation of growth hormone (GH) pulses in humans we have used a competitive antagonist to the GHRH receptor, (N-Ac-Tyr1,D-Arg2)GHRH(1-29)NH2(GHRH-Ant). Six healthy young men were given a bolus injection of GHRH-Ant 400 micrograms/kg body wt or vehicle at 2200 h and nocturnal GH concentrations were assessed by every 10-min blood sampling until 0800 h. Integrated total and pulsatile GH secretion were suppressed during GHRH-Ant treatment by 40 +/- 6 (SE) % and 75 +/- 5%, respectively. GHRH-Ant suppressed maximum (7.6 +/- 2.2 vs 1.8 +/- 0.5 micrograms/liter; P < 0.001) and mean (3.3 +/- 1.0 vs 1.1 +/- 0.2 micrograms/liter; P = 0.02) GH pulse amplitudes. There was no change in integrated nonpulsatile GH levels, pulse frequency, or interpulse GH concentration. GHRH-Ant 400 micrograms/kg also suppressed the GH responses to intravenous boluses of GHRH 0.33 micrograms/kg given 1, 6, 12, and 24 h later by 95, 81, 59, and 4%, respectively. In five healthy men, the responses to 10-fold larger GHRH boluses (3.3 micrograms/kg) were suppressed by 82 and 0%, 1 and 6 h after GHRH-Ant 400 micrograms/kg, respectively. These studies provide the first direct evidence that endogenous GHRH participates in the generation of spontaneous GH pulses in humans.

Dexamethasone alters responses of pituitary gonadotropin-releasing hormone (GnRH) receptors, gonadotropin subunit messenger ribonucleic acids, and gonadotropins to pulsatile GnRH in male rats.

Dexamethasone (Dex), when administered in high doses, has been shown to suppress spontaneous and GnRH-induced gonadotropin secretion, but the level and the mechanism(s) of this effect are unknown. We administered Dex to castrate testosterone-replaced male rats to determine if gonadotropin gene expression is affected and whether Dex differentially influences GnRH-modulated parameters of gonadotrope function: induction of GnRH receptors (GnRH-R) and gonadotropin synthesis and secretion. GnRH was given iv at 25 ng/pulse at 8, 30, and 120 min intervals for 48 h. Rapid GnRH injection frequency preferentially increased alpha and LH-beta messenger RNA (mRNA) responses to GnRH as well as LH secretion. Slower GnRH injection frequencies were required to increase levels of GnRH-R, FSH-beta mRNA, and FSH secretion. Dex selectively inhibited the serum LH, alpha, and LH-beta mRNA responses to GnRH, but not the serum FSH or FSH-beta mRNA responses. Additionally, it augmented the GnRH-induced increase in GnRH-R. We conclude: 1) induction of GnRH-R, gonadotropin synthesis, and secretion require different modes of GnRH stimulation; 2) Dex acts directly on the gonadotrope to differentially modulate GnRH-induced increases in GnRH-R levels, gonadotropin gene expression, and gonadotropin secretion; and 3) GnRH effects upon induction of GnRH-R, LH, and FSH synthesis and secretion are likely to be mediated via different cellular pathways.

Regulation of pulsatile growth hormone secretion by fasting in normal subjects and patients with acromegaly.

In acromegaly, GH hypersecretion occurs despite elevated insulin-like growth factor-I (IGF-I) levels, implying defective IGF-I feedback. To study the possible mechanisms of defective IGF-I negative feedback in acromegaly, we assessed parameters of pulsatile GH secretion during fasting-induced decrease in plasma IGF-I. Seven patients with active acromegaly and six normal controls were fasted for 6 days and GH secretory profiles were obtained by frequent (every 10 min) blood sampling for 24 h and analyzed by Cluster. Fasting resulted in similar decreases in IGF-I, body weight, and blood glucose levels, and increases in free fatty acid and beta-hydroxybutyrate in all subjects. Normal subjects showed increases in 24-h total and pulsatile GH production, GH pulse frequency, maximal pulse amplitude, interpulse and nadir levels, implying suppression of hypothalamic somatostatin secretion and increase in GH-releasing hormone (GHRH) pulse frequency. In acromegalic patients, GH (and, by inference, GHRH) pulse frequency was unchanged. Three patients had increases in GH production, interpulse, and nadir levels similar to the normals while the other four had no change or paradoxical decreases in these parameters. Percentage change in GH production was highly correlated with percentage change in interpulse and nadir levels in both normals and patients. Mean GH response to GHRH (0.33 micrograms/kg iv) did not change significantly in any group as a result of fasting. Our data suggest that in healthy humans IGF-I negative feedback on GH secretion involves suppression of GHRH pulse frequency. GH (and, by inference, GHRH) pulse frequency is resistant to decrease in IGF-I in acromegaly, suggesting that lowered sensitivity of GHRH neurons to IGF-I may be the mechanism of high GH pulse frequency in this disease.

Persistence of rapid growth hormone (GH) pulsatility after successful removal of GH-producing pituitary tumors.

GH concentration profiles in patients with acromegaly are characterized by rapid GH pulsatility and high interpulse GH concentrations. Animal and human studies have shown that GH pulses are consequent upon periodic discharges of hypothalamic GHRH, whereas interpulse GH levels might reflect tonic secretion of hypothalamic SRIH. Thus, the pattern of GH secretion in acromegaly may conceivably be attributed to high GHRH pulse frequency and/or SRIH deficiency. If this assumption is correct, removal of a GH-producing tumor should be followed by a persistently high GH pulse frequency and a high recurrence rate. We have studied pulsatile GH secretion in 12 patients with acromegaly before and after apparently complete removal of their pituitary tumors. Despite normalization of GH secretion after surgery, the disease recurred in 3 patients within 3 yr. The other 9 patients had normal insulin-like growth factor-I and basal and dynamic GH concentrations for 24 +/- 4 months postsurgery. Parameters of GH secretion in this group (pre- and postsurgery) were compared to sex-, age-, and body mass index-matched controls. Plasma GH concentrations in the postoperative and control series were analyzed by a chemiluminescent assay with a sensitivity of 0.01 micrograms/L. Removal of the somatotroph tumor led to normalization of mean and interpulse (but not the nadir) GH levels, pulse amplitude, and responses to GHRH. However, GH pulse frequency (14.2 +/- 1.2 vs. 11.8 +/- 0.9 pulses/24 h) did not change and was significantly (P < 0.001) higher than the control value (8.7 +/- 0.9 pulses/24 h). Thus, SRIH secretion in acromegaly is not inherently deficient, and high interpulse GH levels reflect the mass of tumorous somatotrophs. The persistence of rapid GH pulsatility in apparently "cured" patients with acromegaly suggests that abnormally rapid GHRH pulsatility may be an inherent component of the disease process.

Acute growth hormone (GH) response to GH-releasing hexapeptide in humans is independent of endogenous GH-releasing hormone.

The synthetic GH-releasing hexapeptide (GHRP: His-DTrp-Ala-Trp-DPhe-Lys-NH2) releases GH in man by an undetermined mechanism. To investigate whether acute GH response to GHRP is mediated by endogenous GHRH, we examined the effect of GHRP on GH release during pituitary desensitization to GHRH induced by short-term GHRH infusion. In five healthy men on six occasions, we infused saline (sal) or 1 microgram/kg.h GHRH-44 for 6 h. After 4 h, a bolus of sal, GHRH-44 1 microgram/kg body weight, or GHRP 1 microgram/kg body weight was given iv. GH concentration, measured by RIA, was analyzed by mean area under the curve (AUC) of GH released over the 2 h immediately after bolus injection. Infusion of GHRH had a biphasic effect on GH release; plasma GH increased to 12.7 +/- 3.3 micrograms/L within the first hour, with subsequent decrease to 2.9 +/- 0.3 micrograms/L during the last 2 h of infusion. GH AUC (hours 4-6 of infusion) microgram/L.2 h [table: see text] GH response to bolus GHRH was abolished by GHRH infusion, whereas GH response to GHRP persisted under the same conditions. Thus, we conclude that acute GH response to GHRP in humans is not mediated by endogenous GHRH.

Nocturnal augmentation of growth hormone (GH) secretion is preserved during repetitive bolus administration of GH-releasing hormone: potential involvement of endogenous somatostatin--a clinical research center study.

Pulsatile GH secretion in humans is under the dual and opposing regulation of hypothalamic GHRH and SRIH. GH pulses result from acute GHRH secretory discharges, and their occurrence and amplitude are augmented at night. We hypothesized that normal adults have predictable circadian or ultradian patterns of SRIH secretion and that this rhythm modulates both spontaneous GH secretion and the GH response to GHRH in a similar manner. To test this hypothesis, we compared baseline GH concentration patterns with GH profiles during submaximal iv boluses of GHRH. Every 20 min blood sampling for plasma GH determination over a 24-h period was performed in seven middle-aged men on days 1 and 7. Each subject received a GHRH (0.33 microgram/kg) iv bolus every 2 h on days 2-7, during which the GH responses to the first two boluses were measured. Plasma insulin-like growth factor I (IGF-I) was measured at 0800 h on each day. [The subjects had GH response to every GHRH bolus] and the integrated GH concentration on day 7 was increased 4.3 +/- 0.6-fold over that in the baseline study on day 1. There was no acute or chronic desensitization to GHRH, and the pituitary remained equally responsive to the GHRH boluses despite a 2.6-fold increase in IGF-I by day 7. During this same time period, there was a 1.4-fold increase in IGF-binding protein 3. The subjects showed similar circadian patterns of GH secretion at baseline and during repetitive GHRH boluses, with a maxima on each day occurring during the early night-time hours. These data demonstrate that healthy men remain sensitive to the GH-releasing effects of submaximal bolus doses of GHRH despite significant increases in GH and IGF-I. The similarities between the GH concentration profiles during days 1 and 7 are consistent with the hypothesis that the ultradian pattern of GH secretion in humans is in part a product of hypothalamic GHRH discharges superimposed on more slowly changing SRIH secretion.

Pituitary irradiation is ineffective in normalizing plasma insulin-like growth factor I in patients with acromegaly.

Pituitary irradiation suppresses GH hypersecretion in patients with acromegaly. Within 10 yr after radiotherapy, up to 80% of patients achieve plasma GH levels below 5 micrograms/L. Whether this is sufficient to normalize plasma insulin-like growth factor I (IGF-I) levels, is unknown. We examined the effect of radiotherapy on plasma IGF-I concentrations in patients with acromegaly. We reviewed hospital charts of 140 patients with acromegaly seen in our institution between 1975 and 1996. Data on plasma GH and IGF-I were extracted and tabulated longitudinally together with the information about the concomitant medical therapy. We included data from the patients who received radiotherapy as a part of their treatment and whose IGF-I was monitored for more than 1 yr afterward. To avoid the potential bias, the data for patients who were referred to us for medical therapy, having failed radiation elsewhere, were excluded. A total of 38 datasets were submitted for the final analysis. The average follow-up was 6.8 +/- 0.8 yr (range, 1-19). Only 2 patients achieved age- and sex-adjusted normal IGF-I levels while off medical therapy. Noncured patients had a mean plasma GH level of 4.6 +/- 1.1 micrograms/L but still elevated plasma IGF-I levels (219 +/- 26% of the upper normal limit) at the last follow-up visit. A random GH concentration below 1.5 micrograms/L was associated with a pathologically high plasma IGF-I concentration in 43% of instances. Radiotherapy appears to be ineffective in normalizing plasma IGF-I levels in acromegaly. A multicenter study to reevaluate the future use of this modality in patients with acromegaly is warranted.

Dynamic changes of growth hormone-binding protein concentrations in normal men and patients with acromegaly: effects of short-term fasting.

Plasma concentrations of growth hormone (GH) and GH-binding protein (GHBP) were measured at hourly intervals in five healthy men and five patients with acromegaly during the fed state and after a 5-day fast. GHBP concentrations (both total and complexed with endogenous GH) were analyzed by the ligand-mediated immunofunctional assay (LIFA). Total GHBP was similar in both groups during the fed state (104.4 +/- 5.2 and 101.6 +/- 10.3 pmol/L), did not exhibit a diurnal rhythm, and was unchanged by fasting (91.9 +/- 5.4 and 109.9 +/- 10.5 pmol/L, respectively). However, the GHBP/GH complex concentration was significantly higher in acromegalics than in controls (41.0 +/- 2.8 v 18.0 +/- 2.2 pmol/L, respectively; P < .05), closely followed diurnal GH rhythm in normals, and was significantly correlated with mean 24-hour GH concentrations (r = .86, P < .01). We conclude that plasma concentrations of GHBP are stable throughout the day and are unchanged either by short-term calorie deprivation or by chronic exposure to high levels of endogenous GH. In contrast, GHBP/GH complex concentrations are altered both acutely and chronically by ambient GH.

Fasting-induced changes in thyroid hormone economy in normal and acromegalic subjects.

To study whether high growth hormone (GH) milieu may counteract fasting-induced changes in thyroid hormone economy, we measured basal and TRH-stimulated TSH concentrations as well as thyroxine (total, TT4 and free, FT4), total triiodothyronine (TT3) and total reverse T3 (TrT3) before and after a 6-day fast in 6 healthy men and in 8 patients with acromegaly. Baseline values for all parameters were similar in both groups. Fasting induced similar increases in TT4 and TrT3 concentrations and a similar decline in TT3 concentrations in both groups. The TT3/TT4 and TrT3/TT4 ratios changed identically in both groups. We conclude that high GH is incapable of altering fasting-induced changes in thyroid hormone economy.

Ontogeny of GH mRNA and GH secretion in male and female rats: regulation by GH-releasing hormone.

Growth hormone-releasing hormone (GHRH) has been shown in vitro to increase proliferation of pituitary somatotrophs, to increase transcription of the GH gene, to promote accumulation of GH mRNA, and to stimulate GH release. The in vivo involvement of hypothalamic GHRH in regulating GH mRNA content had never been clearly documented. We studied pituitary GH mRNA and GH contents and serum concentrations of GH and insulin-like growth factor I (IGF-I) in rats of both sexes during pubertal growth spurt and investigated the effects of GHRH deficiency (brought about by neonatal administration of monosodium glutamate, MSG) and exogenous GHRH administration on these parameters. In both sexes, GH mRNA content increased three- to fourfold between 4 and 12 wk of life and declined thereafter toward 33 wk of life. This was accompanied by virtually parallel changes in pituitary GH content and in serum IGF-I. Neonatal MSG abolished the pubertal increases in GH mRNA, pituitary GH, and serum IGF-I and severely impaired growth rate. Exogenous GHRH (25 micrograms/kg sc every 8 h for 7 days) given to intact animals between 6 and 7 wk of life significantly augmented pituitary GH mRNA content but was less effective in MSG-treated rats. We conclude that 1) pubertal growth spurt in both sexes is associated with rising pituitary GH mRNA content; 2) GHRH deficiency abolishes the puberty-associated increase in GH synthesis and secretion and attenuates somatic growth rate; and 3) exogenous GHRH augments GH mRNA content. Thus puberty-associated augmentation of GHRH secretion is an important mechanism of somatic growth.