The effect of glucose on growth hormone (GH)-releasing hormone-mediated GH secretion in man

The effect of glucose on GH-releasing hormone (GHRH)-mediated GH secretion was examined in six normal young men. In two paired experiments, the six men drank a 75-g glucose solution or an equal volume of water 30 min before receiving, iv, 100 micrograms of the 44-amino acid form of human pancreatic GHRH (hGHRH-44). One week later, the same men underwent an identical experimental protocol in a cross-over trial. Basal plasma GH concentrations before hGHRH-44 administration were not statistically different in the two experiments [glucose experiment, 2.1 +/- 0.1 (+/- SE) ng/ml; water experiment, 2.6 +/- 0.6 ng/ml]. The mean peak plasma GH level occurred 30 min after hGHRH-44 administration in both experiments. However, the mean GH response was significantly diminished when the men received glucose (8.12 +/- 1.12 ng/ml) compared to that when they received only water (23.70 +/- 8.46 ng/ml; P less than 0.01). Thus, hyperglycemia may exert an inhibitory effect on the plasma GH response to hGHRH-44.     

Discordant effects of insulin-hypoglycemia on growth hormone (GH)-releasing hormone-stimulated GH and thyrotropin (TSH)-releasing hormone-stimulated TSH secretion

The hypothesis that insulin hypoglycemia-induced GH release is mediated by a decrease in hypothalamic somatostatin (SRIH) secretion was tested by investigating whether insulin administration enhanced the responses of SRIH-sensitive pituitary hormones to hypothalamic hormone stimulation. Eight normal men were given a combined iv injection of GHRH (1 microgram/kg) and TRH (0.3 microgram/kg) on two occasions, on one of which regular insulin (0.1 U/kg, iv) was given 30 min before GHRH-TRH administration. Insulin hypoglycemia augmented the maximal incremental (P less than 0.01) and integrated (P less than 0.025) plasma GH responses to GHRH. In contrast, plasma TSH responses to TRH were diminished by insulin (maximal increment, P less than 0.025; integrated response, P less than 0.05). TRH-stimulated PRL secretion was not altered by prior insulin administration. The enhancement of GH responsiveness to maximal GHRH stimulation indicates mediation by a non-GHRH pathway. However, the discordant decrease in TSH responsiveness to TRH argues against a reduction in hypothalamic SRIH secretion as a mechanism for the action of insulin.     

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.     

The effect of free fatty acids on growth hormone (GH)-releasing hormone-mediated GH secretion in man

The effect of FFA on GH-releasing hormone (GHRH)-mediated secretion of GH was examined in six normal young men. Three of the men were infused with 250 ml of a lipid-heparin solution at 1.67 ml/min for 150 min, and the other three were given an equivalent volume of saline in the same manner. Thirty minutes after the start of infusion, 100 micrograms GHRH (the 44-amino acid form) were injected iv, and plasma GH and FFA were measured. One week later, the same men participated in an identical experiment, but the ones who had received lipid-heparin previously were given saline and vice versa. In both experiments, plasma FFA increased to 2.25 +/- 0.16 meq/liter (mean +/- SEM) 60 min after the start of lipid-heparin infusion, whereas FFA levels did not change significantly in the saline-treated group. Mean plasma GH levels reached peak concentrations in both groups 30 min after GHRH treatment. However, the peak GH response when lipid-heparin was given was significantly diminished (8.4 +/- 1.7 ng/ml), compared with the peak response when saline was given (28.9 +/- 7.1 ng/ml). These data suggest that plasma FFA elevations induced by lipid-heparin infusion inhibit GH secretion induced by GHRH.     

Impaired inhibitory effects of somatostatin on growth hormone (GH)-releasing hormone stimulation of GH secretion after short term infusion

We examined the effect of prior exposure to somatostatin (SRIH) on its inhibition of GH and TSH responses to GHRH and TRH stimulation to determine whether SRIH desensitization has physiological significance in man. Six men received GHRH (1 microgram/kg, iv) and TRH (0.3 microgram/kg, iv) 20 min after starting a saline or SRIH (5.5 ng/kg/min, iv) infusion and again 6 h later. Hormone responses were quantified by measuring the area under the curve, corrected for GH concentration at injection time. Similar results were obtained when GH responses were quantified by measuring the hormone secretory rate using the program Detect. Plasma GH and TSH responses to the two GHRH and TRH injections during saline were similar. However, the effects of prior exposure to SRIH were hormone specific. SRIH blunted GH responses to GHRH at 20 min (1609 +/- 286 micrograms/L.min vs. 451 +/- 224), but did not significantly inhibit the responses 6 h later (1422 +/- 410 micrograms/L.min vs. 1000 +/- 302). In contrast, SRIH inhibition of TSH responses to the two TRH injections was similar (first, 946 +/- 201 micrograms/L.min vs. 700 +/- 148; second, 813 +/- 175 micrograms/L.min vs. 562 +/- 66). We next used these results to study whether the previously reported attenuation of GH responses to repeated GHRH stimulation at 2-h intervals is mediated by SRIH. Eight men received GHRH (1 microgram/kg, iv) 380 min after starting a saline or SRIH (5.5 ng/kg/min, iv) infusion or 90 min after starting a primed (5 mg, iv) infusion of propranolol (80 micrograms/min, iv) and again 2 h later. As in the first protocol, GH responses to GHRH were not inhibited when preceded by a 6-h SRIH infusion. However, the 6-h SRIH infusion resulted in a partial restoration of plasma GH responses to the second GHRH injection (saline infusion: first, 1429 +/- 342 micrograms/L.min; second, 254 +/- 75; SRIH infusion: first, 1042 +/- 247 micrograms/L.min; second, 468 +/- 105). beta-Blockade by propranolol resulted in enhanced GH responses to GHRH, but did not prevent the attenuation of GH responses to the second GHRH injection (first, 1937 +/- 366 micrograms/L.min; second, 614 +/- 99). The desensitization to SRIH inhibition of GH responses to GHRH after a 6-h SRIH infusion provides evidence of physiological consequences of SRIH receptor down-regulation. The impaired GH responses to repeated GHRH stimulation are mediated at least in part by enhanced SRIH secretion, which appears independent of a beta-adrenergic mechanism.     

Physiological role of growth hormone (GH)-releasing factor and somatostatin in the dynamics of GH secretion in adult male rat

To investigate the physiological role of GRF and somatostatin (SRIF) in GH secretion in adult male rats, we prepared in vitro models using the perifusion system of cultured rat anterior pituitary cells exposed to various combinations of human GRF-(1-44)NH2 (hGRF) and SRIF. We studied the following three models on GRF secretion: 1) pulsatile GRF secreted at 1-h intervals, 2) pulsatile GRF secreted at 3-h intervals, and 3) GRF continuously secreted. When 5-min pulses of 20 nM GRF were delivered at 1-h intervals, the responses to GRF gradually declined. The addition of continuous 20 nM SRIF with short pauses prevented this attenuated response and produced high peaks of GH at 3-h intervals. When 5-min pulses of 20 nM GRF were delivered at 3-h intervals, three high peaks of GH were observed regardless of the addition of SRIF. Pretreatment with GRF pulses enhanced the peaks of GH during SRIF pauses. When 20 nM GRF was continuously delivered, a rapid attenuated response to GRF was observed. Although the addition of continuous 20 nM SRIF with short pauses produced three small peaks of GH, these results were caused by the post-SRIF rebound release. These observations suggest that preexposure to prolonged SRIF can prevent an attenuated response to repeated GRF pulses, and that pretreatment with GRF pulses enhances post-SRIF rebound release.     

Effect of withdrawal of somatostatin plus growth hormone (GH)-releasing hormone as a stimulus of GH secretion in Cushing's syndrome

OBJECTIVE: Somatostatin (SS) may not merely be inhibitory to GH secretion but, under appropriate temporal conditions, may act in a paradoxically positive manner to sensitize somatotroph responsiveness to GHRH. SS infusion withdrawal (SSIW) produces a rebound GH rise in humans and increases GHRH-induced GH release. Theoretically SSIW leaves the somatotroph cell in a situation of low endogenous SS. In Cushing's syndrome, GH secretion appears blunted to all stimuli. The mechanisms by which glucocorticoids impair GH secretion in Cushing's syndrome are unknown. There are no data evaluating GH responsiveness to SSIW plus GHRH in Cushing's syndrome patients. The aim of the present study was to evaluate the GH response to SSIW plus GHRH in a group of Cushing's syndrome patients, in order to further understand the deranged GH secretory mechanisms in Cushing's syndrome. PATIENTS AND MEASUREMENTS: Eight female patients with Cushing's syndrome were studied. As a control group, eight normal subjects of similar age and sex were studied. Three tests were done. On one day, SS intravenous (i.v.) infusion (500 micro g for 0-90 min) was performed followed by placebo i.v. bolus at min 90 after SS withdrawal (SSIW). On another day, SS i.v. infusion (500 micro g for 0-90 min) was performed followed by GHRH (100 micro g) i.v. bolus at min 90 after SS withdrawal. On a third day, slow infusion of 150 mmol/l NaCl administration was performed followed by GHRH (100 micro g) i.v. bolus at min 90 after the start of the infusion. Blood samples were taken at appropriate intervals for determination of GH. RESULTS: GHRH-induced GH secretion in normal subjects showed a mean peak of 15.4 +/- 2.1 micro g/l (conversion factor: 1 micro g/l = 1.2 mUI/l). Normal control subjects had a mean peak of 3.3 +/- 1.6 micro g/l after SSIW-induced GH secretion. When GHRH was administered after SSIW there was increased GH secretion with a mean peak of 23.7 +/- 4.2 micro g/l significantly greater than the response after SSIW alone (P < 0.05) and GHRH alone (P < 0.05). The patients with Cushing's syndrome had a blunted GH response after GHRH administration with a mean peak of 1.4 +/- 0.4. After SSIW, Cushing's syndrome patients had a mean peak of 1.0 +/- 0.5 micro g/l. When GHRH was administered after SSIW there was a similar GH response with a mean peak of 1.7 +/- 0.6 micro g/l, not statistically different than the response after SSIW alone (P = ns) and GHRH alone (P = ns). When we compare the response of normal subjects and Cushing's syndrome patients, after SSIW plus GHRH, it was decreased in Cushing's syndrome patients (P < 0.05), with a mean GH peak of 23.7 +/- 4.2 micro g/l and 1.7 +/- 0.6 micro g/l for normal subjects and Cushing's syndrome patients, respectively. CONCLUSIONS: This study has demonstrated a significantly blunted peak GH response to somatostatin infusion withdrawal plus GHRH in Cushing's syndrome patients. In this theoretical situation of decreased somatostatinergic tone there is persistence of GH hyposecretion in Cushing's syndrome, suggesting the existence of a pituitary defect responsible for the decreased GH secretion in Cushing's syndrome.     

Role of dopamine in the regulation of growth hormone secretion: dopamine and bromocriptine augment growth hormone (GH)-releasing hormone-stimulated GH secretion in normal man

The role of the dopaminergic system and its interaction with GH-releasing hormone (GHRH) in the regulation of GH secretion was investigated in normal men in two complementary studies. The men were given continuous iv infusions of 0.15 M saline (5 h), dopamine (4 micrograms/kg X min; 1 h), GHRH (2 ng/kg X min; 2 h), and GHRH (2 ng/kg X min; 2 h) plus dopamine (4 micrograms/kg X min; 1 h) on four separate occasions, and serum GH responses were measured. In a second study, on separate days, placebo or bromocriptine (2.5 mg/dose) was administered, and GH and PRL responses to a single iv GHRH dose were measured. A continuous infusion of dopamine and GHRH on separate days stimulated GH secretion in all subjects. The mean integrated GH secretion was 13.2 +/- 3.1 (+/- SEM) ng/mL X h during the dopamine infusion and 14.7 +/- 4.6 during GHRH, compared with 1.7 +/- 0.4 during the saline infusion. The combination of GHRH and dopamine resulted in the greatest stimulation of GH secretion (29.8 +/- 5.7 ng/ml X h; P less than 0.05 vs. 3 other study days). The oral dopamine agonist bromocriptine also augmented GHRH-stimulated GH secretion. Integrated GH secretion after a single iv injection of GHRH following two doses of bromocriptine was 160 +/- 29.5 ng/ml X h compared with 81.3 +/- 22.2 after placebo (P = 0.04). We suggest that these findings are compatible with the hypothesis that dopamine inhibits hypothalamic somatostatin secretion, which then allows for a greater stimulatory effect of GHRH.     

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.     

Involvement of hypothalamic growth hormone (GH)-releasing factor in GH secretion induced by intracerebroventricular injection of somatostatin in rats

Intracerebroventricular (icv) injection of somatostatin-14 (SRIF-14, 5 micrograms/rat) caused an increase in plasma GH in urethane-anesthetized rats and in conscious freely moving rats. Antiserum specific for rat GH-releasing factor (GRF) (0.5 ml/rat, iv) blunted GH release induced by SRIF-14 in these animals. The antiserum also suppressed spontaneous GH surges in conscious rats. In contrast, GH release induced by prostaglandin E2 (5 micrograms/100 g BW, iv) was not affected by the antiserum. SRIF-14 (5 micrograms/rat, icv) also raised plasma GH levels in conscious rats during the constant iv infusion of SRIF-14 (55 ng/55 microliter X min) which suppressed spontaneous GH secretion. Neither plasma TSH levels nor TSH release induced by a TRH analog were affected by icv injection of SRIF-14. These results suggest that the central stimulating effect of SRIF-14 on rat GH secretion is mediated, at least in part, by hypothalamic GRF and not due to a direct action on the pituitary.     

Atropine blockade of growth hormone (GH)-releasing hormone-induced GH secretion in man is not exerted at pituitary level

The role of acetylcholine (Ach) in the regulation of human GH secretion was assessed using atropine, which selectively blocks cholinergic muscarinic receptors. Paired tests were performed in seven normal subjects using GH-releasing hormone (GHRH) 1-44 (1 microgram/kg iv), with and without atropine pretreatment (1 mg im). The GHRH 1-44-induced GH secretory peak [20.7 +/- 4.5 (SEM) ng/ml] was completely blocked by atropine administration (2.3 +/- 0.6 ng/ml) (P less than 0.01). To determine whether this atropine blockade was at the pituitary level, a series of in vitro studies were conducted using monolayer cultures of cells from bovine anterior pituitary glands. GHRH 1-44 (10(-8) M) stimulated bovine GH release (11.1 +/- 1.5 micrograms/ml) as compared to control values (5.1 +/- 0.4 microgram/ml) (P less than 0.01). This response was not altered by 10(-6) M atropine (14.9 +/- 0.9 microgram/ml). Similar results were obtained with GHRH, 10(-9) M, with or without atropine, 10(-7) M. Addition of 10(-6) M Ach to the incubation medium significantly increased bovine GH release (12.7 +/- 1.2 microgram/ml) and the effect of 10(-6) M Ach and 10(-8) M GHRH was additive (20.9 +/- 2.1 micrograms/ml) (P less than 0.01). Similar results were obtained with Ach, 10(-5) M, and GHRH, 10(-9) M. Atropine or eserine alone did not alter basal GH secretion, and atropine blocked Ach-stimulating activity. In conclusion, atropine blockade of GHRH-induced GH secretion appears to be exerted at a site other than pituitary.     

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.     

Oral administration of growth hormone (GH)-releasing peptide stimulates GH secretion in normal men.

Intravenous infusions of the synthetic hexapeptide GH-releasing peptide (His-DTrp-Ala-Trp-DPhe-Lys-NH2; GHRP) specifically stimulate GH release in man. To determine whether orally administered GHRP stimulates GH secretion, 10 normal men received oral doses of placebo, 30, 100, and 300 micrograms/kg GHRP, and an iv injection of 1.0 micrograms/kg GHRP at weekly intervals in a single blind, randomized design. Serum GH concentrations were measured in blood samples obtained at 5-min intervals for 1 h (0700-0800 h) before and 4 h (0800-1200 h) after each dose. Mean (+/- SE) peak GH concentrations were 4.0 +/- 1.5, 5.2 +/- 1.6, 9.2 +/- 3.3, 18 +/- 3.7, and 26 +/- 5.6 micrograms/L for placebo; 30, 100, and 300 micrograms/kg oral GHRP; and 1 micrograms/kg iv GHRP, respectively; mean 4-h (0800-1200 h) integrated GH concentrations were 312 +/- 109, 406 +/- 159, 698 +/- 284, 1264 +/- 303, and 1443 +/- 298 min.micrograms/L, respectively. To analyze changes in the pulsatile pattern and amount of GH secretion after the administration of GHRP, a waveform-independent deconvolution method was used to estimate GH secretion rates. Variable increases in GH secretion after placebo and GHRP treatments were observed. Despite this variability, weighted least squares linear regression revealed that increasing doses of oral GHRP progressively stimulated GH secretion (P less than 0.005); similar relationships were observed for the peak GH concentration and 4-h integrated GH concentrations. The GH responses to oral GHRP (300 micrograms/kg) and iv GHRP (1 microgram/kg) were significantly greater than that to placebo (P less than 0.05) and were comparable in magnitude. Pairwise comparisons revealed that increases in GH concentrations and secretion rates after the 30 and 100 micrograms/kg oral doses of GHRP were not significantly different from those after placebo. The increase in GH secretion after GHRP treatment was accounted for entirely by an increase in the amplitude of GH secretory events, as no significant increase in the number of GH secretory pulses was observed. The onset and duration of action of GHRP were analyzed by a proportional hazards general linear regression model. Intravenous GHRP had a more rapid onset of action than all doses of oral GHRP (P less than 0.02). Increasing doses of oral GHRP resulted in earlier GH responses (P = 0.006). However, the duration of the GH response was similar for iv GHRP and all doses of oral GHRP, averaging 120-150 min.(ABSTRACT TRUNCATED AT 400 WORDS)     

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.     

Interactions among epinephrine, thyrotropin (TSH)-releasing hormone, dopamine, and somatostatin in the control of TSH secretion in vitro

Epinephrine and TRH independently release TSH from rat anterior pituitary cells in primary monolayer culture (ED50, 11 and 5 nM, respectively; maximum responses, 80% and 110%, respectively). The effects of these compounds together are additive, even at concentrations at which each is maximally effective alone. Dopamine inhibited basal and epinephrine-stimulated TSH secretion by 25 +/- 5% (+/-SE; ED50, 50 +/- 9 nM in each case). Somatostatin was effective against epinephrine-stimulated, but not basal, TSH secretion (80 +/- 4% inhibition; ED50, 1 +/- 3 nM). The data show that epinephrine is a potential regulator of TSH secretion by its own action and via its interactions with TRH, dopamine, and somatostatin.     

The effect of glucose and free fatty acids on growth hormone (GH)-releasing factor-mediated GH secretion in rats

We have examined the effect of glucose and FFA on GH-releasing factor (GHRF)-mediated GH secretion in rats under pentobarbital anesthesia. Hyperglycemia did not affect GH secretion induced by administration of 20, 100, and 200 ng GHRF/100 g body weight. In contrast, GH response to 50 ng GHRF/100 g body weight in lipid heparin-treated rats, which showed high plasma FFA levels, was significantly suppressed compared with the control group (plasma peak GH: control, 1526 +/- 263 ng/ml; lipid-heparin group, 377 +/- 69 ng/ml P less than 0.05, mean +/- SEM). This suppressive effect of FFA on GH secretion was abolished by pretreatment with antisomatostatin serum (ASS) (GH level at 4 min after GHRF administration: ASS-saline group, 1606 +/- 210 ng/ml; ASS-lipid-heparin group, 1531 +/- 174 ng/ml; mean +/- SEM). These results suggest that hyperglycemia does not change the GH response to GHRF and that elevation of plasma FFA suppresses GHRF-induced GH secretion by the stimulation of somatostatin secretion in rats.     

Half-time of endogenous growth hormone (GH) disappearance in normal man after stimulation of GH secretion by GH-releasing hormone and suppression with somatostatin

The half-life (t1/2) of disappearance of endogenous GH from serum was studied using physiological effectors to stimulate and then suppress GH release. GH secretion was stimulated by a single iv injection of GHRH, followed 45 min later by an iv bolus dose and then a 2.5-h infusion of somatostatin (SRIH) to suppress further release. The in vivo t1/2 of GH in seven men was calculated from serum GH concentrations measured at frequent intervals after beginning the SRIH infusion. The mean t1/2 of endogenous GH was 18.9 +/- 0.8 (+/- SE) min by monoexponential analysis and 3.5 +/- 0.7 and 20.7 +/- 0.7 min by biexponential fitting. In these normal men, the decline in GH concentrations after GHRH and SRIH administration was similar to that after the administration of GHRH alone, which yielded a t1/2 of 20.3 +/- 1.9 min. We conclude that the physiological kinetics of endogenous GH removal/disappearance can be estimated in vivo in man using GHRH with or without SRIH infusion.     

Evidence for a limited growth hormone (GH)-releasing hormone (GHRH)-releasable quantity of GH: effects of 6-hour infusions of GHRH on GH secretion in normal man

Human GH-releasing hormone [hGHRH-40 (GHRH)] stimulates GH release in a dose-dependent fashion when administered as single iv bolus doses or as continuous 90-min infusions. However, there has been variability in the GH responses, and it appears that there are waxing and waning effects of GHRH. To address whether these are a result of the dose of GHRH, time, or intermittent changes in sensitivity of the somatotrophs, we administered 6-h infusions of vehicle and different doses of GHRH to six normal men. In addition, an iv bolus injection of GHRH was given after 5.5 h of infusion to evaluate residual GH secretory capacity. The subjects were given infusions of either vehicle or GHRH (1, 3.3, and 10 ng/kg X min), followed by an iv bolus injection of 3.3 micrograms/kg on four separate occasions. GHRH infusions stimulated GH secretion compared to basal secretion. The changes from basal GH secretion (mean +/- SEM) were 2.0 +/- 1.6, 4.6 +/- 1.5, 12.7 +/- 5.1, and 8.2 +/- 1.8 ng/ml X h during the vehicle and GHRH (1, 3.3, and 10 ng/kg X min) infusions, respectively. The changes from basal GH secretion for 2 h after the iv bolus dose (after 5.5 h of infusion) were 33.3 +/- 8.7, 22.4 +/- 3.8, 14.0 +/- 3.6, and 10.5 +/- 2.0 ng/ml X h on the vehicle and GHRH (1, 3.3, and 10 ng/kg X min) infusion days, respectively. The magnitude of the GH response was inversely related to the GHRH infusion dose. The total amount of GH released during the 7.5-h study periods was not different among the vehicle and 3 GHRH infusion days. Thus, it appears that a finite amount of GH is released by GHRH. There was variability in the degree of responsiveness to the continuous infusions of GHRH. Surges of GH release occurred during the GHRH infusions, which may be attributed to intermittent secretion of a GH inhibitor, such a somatostatin.     

Involvement of nitric oxide in growth hormone (GH)-releasing hormone-induced GH secretion in rat pituitary cells.

The involvement of nitric oxide (NO) in human GH-releasing hormone (hGHRH)-induced GH secretion was studied with freshly dissociated male rat pituitary cells. The cells were packed in a column of Bio-Gel-P2 and continuously perifused at 37 C. Hemoglobin (Hb; 10 microM), which is known to strongly bind NO, potentiated 0.01, 0.1, and 1 nM hGHRH-induced GH secretion by 73%, 52%, and 39%, respectively, without affecting the basal secretion of GH. As reported previously, 1-nM or higher concentrations of hGHRH elicit an increase in GH secretion during the application of hGHRH (on-response) and also a transient increase after the cessation of hGHRH application (off-response). It was found that Hb potentiated only the off-response in 1 nM hGHRH-induced GH secretion, and the same concentration of Hb had no effect on 10 nM hGHRH-induced GH secretion. N-Methyl-L-arginine (MeArg; 500 microM), a competitive inhibitor of NO synthase, also potentiated both the on- and off-responses of 1 nM hGHRH-induced GH secretion by 39% without affecting basal GH secretion. Since cAMP is thought to be an intracellular messenger of hGHRH action, the effects of Hb and MeArg on 1 mM (Bu)2AMP-induced GH secretion were examined. Their actions were found to be greater than those in hGHRH-induced GH secretion. Excess K+ (15 and 50 mM)-induced GH secretion, which does not involve cAMP, however, was not affected by either Hb or MeArg. In contrast, 3 mM sodium nitroprusside, which releases NO, suppressed the 1 nM hGHRH-induced off-response by 18%. The same concentration of sodium nitroprusside had no effect on excess K(+)-induced GH secretion. The effect of 8-bromo-cGMP on hGHRH-induced GH secretion was also examined, since NO is thought to exert its action through cGMP by activating guanylate cyclase in neural tissue. The application of 8-bromo-cGMP, however, did not affect 1 nM hGHRH-induced GH secretion. These observations suggest that hGHRH stimulates the synthesis of NO at least partly through cAMP, thereby partially inhibiting hGHRH-induced GH secretion.