Somatostatin inhibition of growth hormone secretion by somatotropes from male, female, and androgen receptor-deficient rats: evidence for differing sensitivities

To investigate the role of somatostatin (SRIF) in regulating sexually dimorphic GH secretion, we used a reverse hemolytic plaque assay and acutely dispersed somatotropes from age-matched normal male, normal female, and androgen receptor-deficient, testicular feminized (Tfm) rats. Hemolytic plaques were developed after a 90-min incubation in the presence of GH antiserum, 10 nM GH-releasing hormone (GHRH), and the following concentrations of SRIF: 0, 0.001, 0.003, 0.01, 0.03, 0.1, 0.3, 1, 3, 10, 30, and 100 nM. Additional studies were performed with 0 or 100 nM SRIF in the absence of GHRH. The absolute number of somatotropes (x10(6); mean +/- SEM) recovered from the pituitaries of Tfm rats (1.73 +/- 0.18) was significantly greater than that from the males (1.11 +/- 0.13; P = 0.01); the number from female rats (1.30 +/- 0.15) was not different from that of either male or Tfm animals. GHRH-stimulated GH secretion, as estimated by the mean GH plaque area (micron2 x 10(4); mean +/- SEM) in the absence of SRIF, was greater for somatotropes from male rats (3.36 +/- 0.41) than that for either Tfm (2.27 +/- 0.32; P = 0.02) or female (1.78 +/- 0.24; P = 0.001) rats; values for the latter two groups did not differ. However, mean GH plaque areas for each group during maximal SRIF inhibition in either the presence or absence of GHRH were indistinguishable from each other and from mean plaque areas obtained under basal conditions. As demonstrated by a lesser EC50 value (0.04 +/- 0.02 nM; mean +/- SEM), somatotropes from female rats were more sensitive to the inhibitory effect of SRIF than were those from either male (EC50 = 1.82 +/- 0.45; P = 0.0001) or Tfm (EC50 = 0.74 +/- 0.22, P = 0.0001) rats; values for the latter two groups were indistinguishable. These observed differences suggest that gender and/or the gonadal hormone environment may be important determinants of the inhibitory effects of SRIF on GH secretion by the somatotrope. While these gender-associated differences may represent effects of the gonadal hormones directly on the somatotrope, they could reflect modulation of the secretion of hypothalamic SRIF and/or GHRH by the prevailing gonadal hormone environment. Such gender-related differences may contribute to the overall sex-dependent patterns of GH secretion in the intact animal.     

Gonadotropin-releasing hormone-stimulated luteinizing hormone secretion by perifused pituitary cells from normal, gonadectomized, and testicular feminized rats

To elucidate further the manner in which gonadal steroids influence the secretion of LH, we examined the effects of gonadectomy and the absence of functional androgen receptors on GnRH-induced LH release from dispersed rat anterior pituitary cells. Intact and gonadectomized (GNX) normal rats and androgen-resistant, testicular feminized (Tfm) animals from the King x Holtzman strain (a mutant strain that possesses defective androgen receptors) were used. Dispersed pituitary cells were perifused with Medium 199 during a 4-h equilibration period and then subjected to eight 2.5-min pulses of GnRH introduced at 30-min intervals at concentrations ranging from 0.03-100 nM. Basal LH secretion by cells from intact male and female rats was indistinguishable (P = 0.79) and was substantially lower (P less than 0.0001) than that by cells from GNX male and female animals. Basal LH secretion by cells from Tfm rats was significantly higher (P less than 0.01) than that by cells from intact animals, but lower (P less than 0.005) than that by cells from GNX animals. In response to GnRH, perifused pituitary cells from animals representing all experimental groups demonstrated concentration-dependent LH release. Pituitary cells from intact female rats showed an overall greater (P less than 0.05) response to GnRH than cells from intact male rats. Pituitary cells from Tfm rats demonstrated a greater GnRH-stimulated LH mean response than cells from intact male (P less than 0.0001) or intact female (P less than 0.0001) rats. Gonadectomy of male rats resulted in an overall GnRH-stimulated LH release similar to that exhibited by cells from gonadectomized female rats (P = 0.61). Cells from Tfm animals released more LH in response to GnRH than those from gonadectomized male and female rats (P less than 0.001). These data demonstrate that the release of LH in response to GnRH by pituitary cells from intact male rats (i.e. in the presence of androgen and functional androgen receptors) is less than that seen by cells from intact females rats. Since circulating levels of testosterone and estradiol are known to be elevated in the testicular feminized rat, the heightened GnRH-stimulated LH release by cells from such animals may reflect either the long term lack of androgenic influence and/or the combined effects of androgen resistance and elevated levels of circulating estrogens.     

Growth hormone secretion by individual somatotropes of the testicular feminized rat

To investigate the cellular mechanisms underlying the unique GH secretory apparatus of the androgen-resistant testicular feminized (Tfm) rat we employed a reverse hemolytic plaque assay to assess GH secretion by individual cells from normal male, normal female, and Tfm rats. Acutely dispersed pituitary cells were incubated for 90 min with GH anti-serum in the presence of medium alone, 0.01, 0.1, 1, 10, or 100 nM GHRH, or 3 microM forskolin after which hemolytic plaques were developed over an additional 30 min. Body weights of the Tfm rats [318 +/- 7 g (mean +/- SEM)] were intermediate between intact males (372 +/- 18 g) and females (218 +/- 7 g). The total number of cells recovered from dispersion of Tfm rat pituitaries [3.20 +/- 0.42 X 10(6) (mean +/- SEM)] was greater than that from males (1.43 +/- 0.12 X 10(6); P = 0.001), but not distinguishable from that from females (2.31 +/- 0.30 X 10(6); P = 0.06). However, the absolute population of recovered somatotropes from the Tfm animals (1.24 +/- 0.22 X 10(6) exceeded both male (0.56 +/- 0.10 X 10(6); P = 0.002) and female (0.80 +/- 0.14 X 10(6); P = 0.046) values. Mean basal and maximal GH plaque areas were greater for cells from male rats than for those from either female or Tfm rats (P less than 0.05) regardless of whether GHRH or forskolin was used as the secretagogue. Plaque areas from female and Tfm cells were indistinguishable under all study conditions. These data suggest that a deficiency of androgen receptors prevents establishment of the greater GH secretory capacity of individual somatotropes characteristic of the adult male rat. This androgen receptor-dependent modulation of GH secretory capacity appears to occur at a step distal to the GHRH receptor. The data also suggest that an increase in the absolute population of somatotropes is an additional consequence of androgen receptor deficiency. This combination of individual somatotropes, each possessing a GH secretory capacity similar to that of cells from normal females, but present in greater absolute numbers, may explain the intermediate values found during previous studies of the Tfm rat GH axis which were based on assessment of large mixed populations of pituitary cells.     

Pyridostigmine does not reverse dexamethasone-induced growth hormone inhibition

Glucocorticoids inhibit the growth hormone (GH) response to a variety of stimuli, including GH-releasing hormone (GHRH) in vivo, but they increase GHRH-stimulated GH secretion when added, in vitro, to animal and human pituitary cells. This discrepancy has led to the hypothesis that glucocorticoids act in vivo by increasing somatostatin secretion from the hypothalamus. To examine this hypothesis, we used a cholinergic drug, pyridostigmine (PD), which reduces hypothalamic somatostatin secretion. Eight normal volunteers were studied. They underwent four tests: (1) GHRH test; (2) Dex + GHRH (GHRH test after treatment the night before, with dexamethasone (Dex)); (3) PD + GHRH; (4) Dex + PD + GHRH. Dex significantly inhibited the GH response to GHRH expressed as area under the GH/time curve (AUC, microgram/1/min) (mean +/- SEM = 895.2 +/- 196.6 vs 1970.9 +/- 600.1, P less than 0.05). PD significantly increased the AUC of GH secretion in PD + GHRH compared with GHRH alone (3541.2 +/- 571.3 vs 1970.9 +/- 600.1, P less than 0.01) but by no means restored completely the normal GH response to GHRH, when given to Dex-pretreated subjects. Furthermore, the mean AUC of Dex + PD + GHRH was significantly lower than that of PD + GHRH (1621.7 +/- 500.6 vs 3541.2 +/- 571.3, P less than 0.01), demonstrating that Dex continues to exert its inhibitory effect on GH secretion in the presence of PD. These results suggest that glucocorticoid-induced GH inhibition does not act solely through an increase in hypothalamic somatostatin secretion.     

Inhibition by prednisone of growth hormone (GH) response to GH-releasing hormone in normal men

Glucocorticoids increase GHRH-stimulated GH secretion when added in vitro to cultured monkey, rat, and human pituitary cells and when injected in vivo into anesthetized rats. Yet, in man glucocorticoids inhibit linear growth and GH secretion. To clarify this apparent disparity and to determine if glucocorticoid stimulation can augment GH release in man after direct pituitary stimulation with GHRH, we administered 1 microgram/kg GHRH dosage to seven normal men before and after a 4-day course of prednisone (20 mg, orally, three times daily). The second GHRH test was done 12 h after the last dose of prednisone was given. Prednisone significantly inhibited the mean maximal increase in serum GH after GHRH treatment [20.7 +/- 4.5 (+/- SE) vs. 6.3 +/- 2.4 micrograms/L; P less than 0.01] as well as the GH value obtained by summing and averaging the individual means of the 15, 30, 45, 60, 75, and 90 min serum GH concentrations (11.1 +/- 1.2 vs. 4.3 +/- 0.9 micrograms/L; P less than 0.05). The mean serum insulin-like growth factor I and plasma glucose concentrations were not significantly altered by prednisone administration. These results together with previous in vitro findings imply that glucocorticoid-induced inhibition of GH secretion in man does not occur at the level of the pituitary gland, but, rather, at the hypothalamus or above.     

Effects of sex and age on growth hormone response to growth hormone-releasing hormone in healthy individuals

The influence of age and sex on human GH secretion is controversial. In previous studies, serum GH responses to arginine and insulin-induced hypoglycemia were significantly higher in pre- and postovulatory women than in men. In contrast, recent studies suggest that GH responsiveness to GHRH is higher in normal young men than in age-matched women. To clarify the question of sex and age influence on GHRH-(1-44)-stimulated GH secretion, we studied 116 normal women and men (with body mass indexes of 18-25 and 19-26, respectively) between the ages of 18 and 95 yr. The peak serum GH increments after GHRH administration were significantly higher in premenopausal women than in age-matched men (P less than 0.003 for the age group 18-30 yr and P less than 0.03 for the age group 30-50 yr, as assessed by analysis of variance). The responses were not different in postmenopausal women and age-matched men. Multiple regression analysis revealed a significant negative correlation between GHRH-induced GH responses (integrated area under the curve) and age in both women (P less than 0.002) and men (P less than 0.001). In addition, we determined basal serum testosterone, estradiol, cortisol, and PRL levels in all subjects. Multivariate regression analysis of the GH responses to GHRH administration revealed a significant positive correlation (P less than 0.01) between serum estradiol and both GH increase and the area under the GH response curve. No correlation was found between GHRH-stimulated GH secretion and basal serum cortisol, testosterone, or PRL concentrations. Our data clearly demonstrate a marked influence of both sex and age on GHRH-stimulated GH secretion. We found a higher GH increase in premenopausal women compared with age matched-men and an age-dependent decrease in GHRH-stimulated GH secretion in both sexes. Furthermore, in women a significant influence of estradiol on GH secretion after GHRH administration could be demonstrated.     

Pulsatile secretion of growth hormone (GH) persists during continuous stimulation by CJC-1295, a long-acting GH-releasing hormone analog

CONTEXT: Pulsatile GH secretion is considered important for many of the hormone's physiological effects. Short-term GHRH infusions enhance GH pulsatility and increase IGF-I, but the short GHRH half-life limits its therapeutic use. A synthetic GHRH analog (CJC-1295) that binds permanently to endogenous albumin after injection (half-life = 8 d) stimulates GH and IGF-I secretion in several animal species and in normal human subjects and enhances growth in rats. OBJECTIVE: Our objective was to assess GH pulsatility after a single injection of CJC-1295 and determine which GH secretion parameters correlated to the increase in IGF-I production. METHODS: GH pulsatility was assessed by 20-min blood sampling during an overnight 12-h period in healthy 20- to 40-yr-old men before and 1 wk after injection of either 60 or 90 microg/kg CJC-1295. RESULTS: GH secretion was increased after CJC-1295 administration with preserved pulsatility. The frequency and magnitude of GH secretory pulses were unaltered. However, basal (trough) GH levels were markedly increased (7.5-fold; P < 0.0001) and contributed to an overall increase in GH secretion (mean GH levels, 46%; P < 0.01) and IGF-I levels (45%; P < 0.001). No significant differences were observed between the responses to the two drug doses. The IGF-I increases did not correlate with any parameters of GH secretion. CONCLUSIONS: CJC-1295 increased trough and mean GH secretion and IGF-I production with preserved GH pulsatility. The marked enhancement of trough GH levels by continuous GHRH stimulation implicates the importance of this effect on increasing IGF-I. Long-acting GHRH preparations may have clinical utility in patients with intact pituitary GH secretory capability.     

On the role of the peptide galanin in regulation of growth hormone secretion.

The effects of the peptide galanin on growth hormone secretion were studied in vitro using cultured rat and human anterior pituitary cells, and in vivo by iv administration of galanin in both rats and humans. Galanin in concentrations from 10 nmol/l to 1 mumol/l did not alter basal GH release, but slightly inhibited GHRH-stimulated GH release from cultured rat anterior pituitary cells. Galanin (1 mumol/l) did not significantly change basal or GHRH-stimulated GH secretion from cultured human anterior pituitary cells. In contrast, iv injection of 1 microgram (300 pmol) galanin to rats induced an increase in plasma GH that was reproducible at repetitive injections. The galanin-induced GH release in rats was of a lower magnitude than the increase in plasma GH after iv injections of GHRH, and was seen with a 5-15 min delay in comparison to iv administered GHRH. In man, iv infusions of galanin (40 pmol.kg-1.min-1.(40 min)) also caused a significant increase in plasma GH, but it occurred 25-30 min after the beginning of the infusion. These results suggest an indirect action of galanin on GH release in both rats and humans, i.e. galanin does not directly affect the somatotropes. In agreement with a central action, no binding sites for galanin could be demonstrated in the rat anterior pituitary by autoradiography. Since galanin did not affect somatostatin release from fragments of rat mediobasal hypothalamus, the stimulatory effects of galanin on GH release are most likely mediated via a stimulatory effect on GHRH neurons.     

Growth hormone (GH) response to GH-releasing hormone in children with subnormal integrated concentrations of GH

We determined the GH responses to human GH-releasing hormone-40 (GHRH) in poorly growing children who had either normal or deficient GH secretion, as measured by pharmacological stimulation and integrated concentration of GH (IC-GH). Ten patients had both normal pharmacologically stimulated GH and IC-GH (GH-normal), 15 patients had normal pharmacologically stimulated GH but deficient IC-GH [GH neurosecretory dysfunction (GHND)], and the remaining 7 patients had both subnormal stimulated GH and IC-GH [GH deficiency (GHD)]. The mean peak plasma GH response to GHRH was 11.7 +/- 8.5 (+/- SD) ng/ml in GHD patients, significantly lower than the responses of both the GHND (49.2 +/- 39.2 ng/ml; P less than 0.0001) and GH-normal (51.8 +/- 44 ng/ml; P less than 0.0001) groups. The range of peak GH responses to GHRH in GHD patients overlapped the lower end of the range of responses in the GHND and GH-normal patients. Three GH-normal and eight GHND patients had greatly enhanced GH responses to GHRH (greater than 50 ng/ml); no GHD patients had a response over 24.2 ng/ml. There was no difference between the GH responses of male and female patients within groups to GHRH. There was a significant correlation between the log of the peak GH response to GHRH and the log of the maximal GH response to standard pharmacological stimuli (r = 0.51; P less than 0.005). Because of the variability of GH responses to GHRH encountered among the patients, the response to GHRH cannot be used as a test for identifying patients with inadequate spontaneous GH secretion. The IC-GH is the only method that can identify children with GHND.     

Studies on the response of growth hormone (GH) secretion to GH-releasing hormone, thyrotropin-releasing hormone, gonadotropin-releasing hormone, and somatostatin in acromegaly

The plasma GH response to GH-releasing hormone (GHRH), TRH, or GnRH administration was examined in 25 acromegalic patients. Plasma GH levels increased in 21 patients after GHRH, in 19 after TRH, and in 4 after GnRH. The four GHRH nonresponders had had acromegaly longer than had the GHRH responders. No specific combination of GH responsiveness to these 3 releasing hormones was found among the patients. Infusion of 1 mg GHRH for 150 min gradually increased plasma GH levels, with some fluctuations, from the beginning to the end of infusion in normal subjects and in 7 patients who were GHRH responders, but a bolus injection of 100 micrograms GHRH at the end of the infusion did not further elevate plasma GH levels. These results suggest that desensitization to GHRH occurred in the normal subjects and acromegalic patients. However, in 5 acromegalic patients who responded to both GHRH and TRH, a bolus injection of 500 micrograms TRH given at the end of the 150-min infusion of 1 mg GHRH evoked a further plasma GH rise. In 5 normal subjects and 2 patients who were responders to GHRH but not TRH, a bolus injection of 500 micrograms TRH did not cause plasma GH elevation at the end of 150-min infusion of 1 mg GHRH. These results imply that TRH and GnRH stimulate GH secretion from the adenoma cells in vivo through receptors different from those for GHRH. In vitro studies using cultured pituitary adenoma cells from 2 patients revealed that the responses of GH secretion to GHRH were similar to those in vivo. These data, therefore, suggest that the responsiveness of GH secretion to stimuli is determined by the specificity of the receptors on adenoma cells. The action of somatostatin-28 was more potent than that of somatostatin-14 in the suppression of GH secretion from adenoma cells.     

EFFECTS OF INTRANASAL CALCITONIN ADMINISTRATION ON PITUITARY GH RESPONSE TO hGHRH (1–29)NH2 IN NORMAL ADULT SUBJECTS

Studies in man demonstrated that intramuscular salmon (s) calcitonin (CT) administration blunted pituitary GH response to hypothalamic stimuli such as arginine infusion and insulin-induced hypoglycaemia. However, the mechanisms underlying this inhibiting action of CT are still unclear. The aim of our study was to investigate the effects of intranasal (i.n.) and i.m. sCT administration on GH secretion elicited by direct pituitary stimulation in man with human GH-releasing hormone (GHRH) (1-29)NH2. Seven healthy non-obese volunteers (five men, two women; mean age +/- SDM 25 +/- 2) underwent a bolus intravenous injection of GHRH, 100 micrograms, alone or associated with sCT, administered either i.n., 300 IU, or i.m., 100 IU. Our data demonstrate a significant decrease in GH secretion after GHRH when either i.n. or i.m. sCT is administered. GH peak (mean +/- SDM); GHRH alone 22.9 +/- 2.5 vs GHRH plus i.n. sCT, 8.9 +/- 1.5 micrograms/l, P less than 0.001; or vs GHRH plus i.m. sCT 12.3 +/- 2.5 micrograms/l, P less than 0.001. Area under the curve of GH secretion: GHRH alone 1211 +/- 196 vs GHRH plus i.n. sCT 551 +/- 116 micrograms 120 min/l. P less than 0.001; or vs GHRH plus i.m. sCT 700 +/- 167 micrograms 120 min/l, P less than 0.001. We conclude that sCT is able to inhibit GHRH-stimulated GH secretion in man.     

Semiquantification of hypothalamic GH-releasing hormone output in women: evidence for sexual dimorphism in the mechanism of the somatopause.

The neuroendocrine mechanisms underlying the decline of GH with aging (somatopause) are uncertain. We recently found that the age-dependent diminution of the hypothalamic GH-releasing hormone (GHRH) output contributes to the somatopause in men. As the regulatory mechanisms of GH secretion are sexually dimorphic, we assessed the suppressibility of spontaneous and GHRH-stimulated GH secretion by graded doses of a specific competitive GHRH receptor antagonist in nine young (20-27 yr old) and eight elderly (65-77 yr old) healthy nonobese women to semiquantify hypothalamic GHRH output. Nocturnal mean GH was lower in elderly women (2.2 +/- 0.4 vs. 0.9 +/- 0.2 microg/liter; P = 0.01). Graded boluses of GHRH-44 resulted in similar GH responses in both populations (P = 0.28). Graded infusions of GHRH antagonist inhibited in a dose-dependent manner the GH responses to GHRH in both groups (P = 0.0001-0.04). The dose-inhibition curve for the lowest GHRH bolus dose was shifted to the left compared with the highest one (P = 0.04). However, the dose-inhibition curves for spontaneous GH secretion were not different in young and elderly women (P = 0.50). Thus, the female somatopause is not associated with a measurable decrease in hypothalamic GHRH output. When the dose-inhibition curves for young men and young women were compared, the latter was shifted to the left (P = 0.009), suggesting that the somatotropic system in women operates with less GHRH. We conclude that the contribution of endogenous GHRH to the maintenance of GH secretion and the neuroendocrine mechanisms of somatopause in humans are sexually dimorphic.     

Sexually dimorphic expression of the growth hormone-releasing hormone gene is not mediated by circulating gonadal hormones in the adult rat

The sexual dimorphism characterizing GH secretion in the rat is thought to be related to differences in the hypothalamic synthesis and release of the GH-regulating peptides, GH-releasing hormone (GHRH), and somatostatin. Therefore, the influence of gender and sex steroid hormones on hypothalamic expression of the GHRH gene in adult rats were examined. GHRH messenger RNA (mRNA) levels were measured in individual rat hypothalami by Northern hybridization analysis using a 32P-labeled complementary DNA encoding rat GHRH. Destruction of hypothalamic GHRH neurons by neonatal treatment with monosodium glutamate caused similar 3-fold reductions in the levels of GHRH mRNA in adult male and female animals. In three separate experiments, hypothalamic GHRH mRNA concentrations in male rats were 2- to 3-fold greater than in randomly cycling females (four or five rats per group; P less than 0.01). In spite of the greater abundance of GHRH mRNA abundance in the male rat hypothalamus, circulating gonadal steroids lacked the ability to modulate GHRH gene expression in adult animals, since neither gonadectomy nor pharmacological sex steroid replacement changed GHRH mRNA levels in the hypothalamus of male and female adult rats. Furthermore, GHRH mRNA concentrations in female rats were similar during the proestrus, estrus, and diestrus phase of the estrous cycle. Also, GH inhibited hypothalamic GHRH gene expression in a sex-specific manner. Exposure to high levels of GH secreted by the MtTW15 tumor for 4 weeks reduced GHRH mRNA concentrations 7-fold in male rats (P less than 0.001) but only 2-fold in females (P less than 0.05). These studies demonstrate that GHRH gene expression in the rat hypothalamus is sexually dimorphic. Basal mRNA levels are greater in male rats, and expression in male hypothalami is more sensitive to feedback inhibition by GH than expression in females. There is no evidence for regulation of GHRH mRNA levels by either testosterone or estrogen in adult rats. These gender differences in GHRH gene expression likely contribute to the generation of a sex-specific pattern of GH secretion.     

Dichotomic action of glucocorticoids on growth hormone secretion

The mechanism of apparently discrepant actions of glucocorticoids (GC) on GH secretion, in vivo suppression and in vitro potentiation, was studied in rats. Dexamethasone (Dex), at the concentration of 50 nmol/l, potentiated basal and GHRH-stimulated GH release from monolayer culture of normal rat pituitary cells in 48 h. On the other hand, in vivo administration of Dex, 165 micrograms daily for 3 days, consistently suppressed serum GH levels in female rats. In these rats, the hypothalamic content of immunoreactive (IR) SRIH was significantly increased, whereas that of IR-GHRH was significantly decreased in comparison with the untreated rats. Bioassayable GH-releasing activity was also lower in Dex-treated rats. These findings indicate that the suppressing effect of GC on GH release in vivo is, at least partially, due to the increase in hypothalamic SRIH release and probably also to the decrease in GHRH release, and these effects surpass the potentiating effect of GC on GH release at the pituitary level, resulting in a net inhibitory effect in vivo.     

Comparison of the sensitivity of growth hormone secretion to somatostatin in vivo and in vitro in acromegaly

Somatostatin (SRIH) sensitivity in acromegaly was evaluated in vivo by comparing the inhibition of GHRH (1 microgram/kg, iv)-stimulated GH secretion in eight acromegalic and six normal subjects. A SRIH infusion (50 micrograms/h) that inhibited the mean plasma GH response to GHRH by 74 +/- 5% (+/- SE) in normal subjects had no significant effect in the acromegalic patients. However, when two acromegalic patients in whom SRIH had no suppressive effect were excluded from the analysis, the effect of SRIH in the other six (82 +/- 7%) was comparable to that in the normal subjects. Within the acromegalic group, the percent suppression of basal and GHRH-stimulated GH secretion was inversely correlated with both basal plasma GH (r = -0.751; P = 0.03 and r = -0.727; P = 0.04, respectively) and insulin-like growth factor I (r = -0.800; P = 0.02 and r = -0.727; P = 0.04, respectively) concentrations. The in vitro sensitivity to SRIH was studied in pituitary adenomas from five of the acromegalic patients in 3- to 4-day monolayer cultures of dispersed cells. The SRIH IC50 values were lowest in the tumors (8.6-44 pmol/L) from the three patients who had in vivo SRIH sensitivity (suppression of GHRH-stimulated GH secretion) comparable to that in the normal subjects. The IC50 values were higher in the tumors (150 and 21,000 pmol/L) from the two patients that were least responsive to SRIH in vivo. These results indicate that there is considerable variability of SRIH sensitivity in patients with acromegaly. Although the role of this defect in the pathogenesis of acromegaly is uncertain, it may be an important determinant in the degree of elevation of plasma GH levels.     

Evidence for temporal coupling of growth hormone, prolactin, LH and FSH pulsatility overnight during normal puberty

The patterns of secretion of GH, LH, FSH and prolactin were determined over a single night (20.00-08.00 h; 15-min sampling) in 34 normal subjects (17 male, 17 female, aged 9.1-20.9 years). Plasma GH was measured by an immunoradiometric assay and LH, FSH and prolactin by radioimmunoassay in all samples. Data were analysed by Fourier transformation and cross-correlation after stationarization. The highest mean GH levels were noted in girls at Tanner stage 2/3 and in boys at stages 4/5. Prolactin levels were highest in girls at stage 4/5 and in boys at stage 2/3. LH and FSH showed a progressive rise by puberty stage in both sexes. The dominant pulse periodicities of GH and prolactin were 150-180 min in girls and 180 min in boys. LH and FSH pulse periodicity was around 90 min in early puberty and 180 min in later puberty in both sexes. LH and prolactin pulses showed a phase relationship with GH with a lag of 30-75 min (r = 0.32; P less than 0.001) and 30 min (r = 0.47; P less than 0.0001) respectively. Generally, LH and prolactin pulses were in phase (r = 0.42; P less than 0.0001) and there was a highly significant correlation (r = 0.64; P less than 0.0001) between FSH and LH pulsatility. Whereas mean overnight concentrations and pulse periodicity of the principal pituitary hormones varied between the sexes during early puberty, by the end of puberty a dominant pulse periodicity of around 150-180 min was established and there was remarkable temporal coupling of pulsatility.     

Effects of hypothyroidism, tri-iodothyronine and glucocorticoids on growth hormone responses to growth hormone-releasing hormone and His-D-Trp-Ala-Trp-D-Phe-Lys-NH2

The aim of this study was to investigate the role of thyroid hormones and glucocorticoids on GH secretion. Secretion of GH in response to GH-releasing hormone (GHRH) (5 micrograms/kg) was markedly (P less than 0.001) decreased in hypothyroid rats in vivo (peak GH responses to GHRH, 635 +/- 88 micrograms/l in euthyroid rats vs 46 +/- 15 micrograms/l in hypothyroid rats). Following treatment with tri-iodothyronine (T3; 20 micrograms/day s.c. daily for 2 weeks) or cortisol (100 micrograms/day s.c. for 2 weeks) or T3 plus cortisol, a marked (P less than 0.01) increase in GH responses to GHRH was observed in hypothyroid rats (peak GH responses, 326 +/- 29 micrograms/l after T3 vs 133 +/- 19 micrograms/l after cortisol vs 283 +/- 35 micrograms/l after cortisol plus T3). In contrast, none of these treatments modified GH responses to GHRH in euthyroid animals. Hypothyroidism was also associated with impaired GH responses to the GH secretagogue, His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 (GHRP-6). Secretion of GH in response to GHRP-6 in vivo was reduced (P less than 0.01) in hypothyroid rats (peak GH responses, 508 +/- 177 micrograms/l in euthyroid rats vs 203 +/- 15 micrograms/l in hypothyroid rats). In-vitro studies carried out using monolayer cultures of rat anterior pituitary cells derived from euthyroid and hypothyroid rats showed a marked impairment of somatotroph responsiveness to both GHRP-6 and somatostatin in cultures derived from hypothyroid rats. In summary, our data suggest that thyroid hormones and glucocorticoids influence GH secretion by modulating somatotroph responsiveness to different GH secretagogues.     

Neonatal treatment with monosodium glutamate: effects of prolonged growth hormone (GH)-releasing hormone deficiency on pulsatile GH secretion and growth in female rats

Administration of monosodium glutamate (MSG) to neonatal rodents produces permanent lesions of hypothalamic arcuate neurons that secrete GH-releasing hormone (GHRH). The present study was intended to determine the consequences of GHRH deficiency on the pulsatile GH secretory pattern and growth in MSG-treated female rats and to compare these effects with those observed in male littermates. Male and female rats were injected with MSG [4 mg/g body wt (BW), sc] or saline (controls) on days 2, 4, 6, 8, and 10 after birth. Immunoreactive GHRH concentrations were decreased in the hypothalamus (by 60%) and in the median eminence (by 95%) of adult male and female MSG-treated rats. In contrast, somatostatin concentrations were unaffected. BW and linear growth were severely impaired in male MSG-treated rats, but in MSG-lesioned females BW was not different from controls, and the attenuation of longitudinal growth was less severe and the obesity more pronounced than in males. These sex differences occurred despite similar reductions (by 55%) in serum insulin-like growth factor I concentrations in male and female MSG-treated rats. MSG treatment also produced decreases in pituitary wt and GH content (by 60%), independent of sex. Pulsatile GH secretion was studied by serial blood sampling of chronically cannulated, freely moving rats. Plasma GH patterns were analyzed by the PULSAR program. Compared to controls, treatment with MSG led to a marked inhibition (by 90%) of GH secretion in both sexes. Significant reductions in GH pulse amplitude (-95%) and pulse duration (-62%) were observed in males, whereas pulse amplitude (-85%), pulse frequency (-67%), and baseline GH concentrations (-80%) were markedly reduced in females. The GH responses to an iv bolus injection of rat GHRH (1 microgram/rat) was severely blunted in both male and female MSG-treated rats. This study demonstrates that GHRH deficiency in female rats results in a marked inhibition of GH pulses, as in males, but also causes severe and sex-specific reductions in GH basal secretion and pulse frequency. These observations suggest that hypothalamic GHRH secretion in female rats is more continuous than in males and is a determinant of the elevated interpulse secretion of GH. Moreover, body wt and linear growth are less severely affected by arcuate lesions in female animals, compared to males. These sex-related differences in growth rates may result in part from the tendency of female MSG-lesioned rats to become more obese than males, and the development of obesity, in turn, may antagonize the factors that tend to slow linear growth.(ABSTRACT TRUNCATED AT 400 WORDS)     

Arginine stimulates growth hormone secretion by suppressing endogenous somatostatin secretion

To determine how arginine (Arg) stimulates GH secretion, we investigated its interaction with GHRH in vivo and in vitro. Six normal men were studied on four occasions: 1) Arg-TRH, 30 g arginine were administered in 500 mL saline in 30 min, followed by an injection of 200 micrograms TRH; 2) GHRH-Arg-TRH, 100 micrograms GHRH-(1-44) were given iv as a bolus immediately before the Arg infusion, followed by 200 micrograms TRH, iv; 3) GHRH test, 100 micrograms GHRH were given as an iv bolus; and 4) TRH test, 200 micrograms TRH were given iv as a bolus dose. Blood samples were collected at 15-min intervals for 30 min before and 120 min after the start of each infusion. Anterior pituitary cells from rats were coincubated with Arg (3, 6, 15, 30, and 60 mg/mL) and GHRH (0.05, 1, 5, and 10 nmol/L) for a period of 3 h. Rat GH was measured in the medium. After Arg-TRH the mean serum GH concentration increased significantly from 0.6 to 23.3 +/- 7.3 (+/- SE) micrograms/L at 60 min. TRH increased serum TSH and PRL significantly (maximum TSH, 11.1 +/- 1.8 mU/L; maximum PRL, 74.6 +/- 8.4 micrograms/L). After GHRH-Arg-TRH, the maximal serum GH level was significantly higher (72.7 +/- 13.4 micrograms/L) than that after Arg-TRH alone, whereas serum TSH and PRL increased to comparable levels (TSH, 10.2 +/- 3.0 mU/L; PRL, 64.4 +/- 13.6 micrograms/L). GHRH alone increased serum GH to 44.9 +/- 9.8 micrograms/L, significantly less than when GHRH, Arg, and TRH were given. TRH alone increased serum TSH to 6.6 +/- 0.6 mU/L, significantly less than the TSH response to Arg-TRH. The PRL increase after TRH only also was lower (47.2 +/- 6.8 micrograms/L) than the PRL response after Arg-TRH. In vitro Arg had no effect on basal and GHRH-stimulated GH secretion. Our results indicate that Arg administered with GHRH led to higher serum GH levels than did a maximally stimulatory dose of GHRH or Arg alone. The serum TSH response to Arg-TRH also was greater than that to TRH alone. We conclude that the stimulatory effects of Arg are mediated by suppression of endogenous somatostatin secretion.