Endocrine and metabolic effects of physiologic r-metHuLeptin administration during acute caloric deprivation in normal-weight women

Leptin is a nutritionally regulated hormone that may modulate neuroendocrine function during caloric deficit. We hypothesized that administration of low-dose leptin would prevent changes in neuroendocrine function resulting from short-term caloric restriction. We administered physiologic doses of r-metHuLeptin [(0.05 mg/kg sc daily or identical placebo in divided doses (0800, 1400, 2000, and 0200 h)] to 17 healthy, normal-weight, reproductive-aged women during a 4-d fast. Leptin levels were lower in the placebo-treated group during fasting (3.3 +/- 0.2 vs. 9.6 +/- 1.0 ng/ml, P < 0.001, placebo vs. leptin-treated at end of study). Fat mass decreased more in the leptin than the placebo-treated group (-0.6 +/- 0.1 vs. -0.2 +/- 0.1 kg, P = 0.03). Both overnight LH area (38.9 +/- 21.5 vs. 1.2 +/- 11.1 microIU/ml.min, P = 0.05) and LH peak width increased (15.8 +/- 7.1 vs. -2.3 +/- 6.7 min, P = 0.06) and LH pulsatility decreased (-2.0 +/- 0.9 vs. 1.0 +/- 0.8 peaks/12 h, P = 0.03) more in the leptin vs. placebo group. LH pulse regularity was higher in the leptin-treated group (P = 0.02). Twenty-four-hour mean TSH decreased more in the placebo than the leptin-treated group, respectively (-1.06 +/- 0.27 vs. -0.32 +/- 0.18 microIU/ml, P = 0.03). No differences in 24-h mean GH, cortisol, IGF binding protein-1, and IGF-I were observed between the groups. Hunger was inversely related to leptin levels in the subjects randomized to leptin (r = -0.76, P = 0.03) but not placebo (r = -0.18, P = 0.70) at the end of the study. Diminished hunger was seen among subjects achieving the highest leptin levels. Our data provide new evidence of the important role of physiologic leptin regulation in the neuroendocrine response to acute caloric deprivation.     

Neuroendocrine consequences of fasting in adult male macaques: effects of recombinant rhesus macaque leptin infusion

Fasting inhibits the gonadotropic axis and stimulates the corticotropic and somatotropic axes. Since leptin is a product of fat cells that has been implicated in the control of both reproduction and metabolism, we hypothesized that the decrease in leptin observed during fasting was responsible for these effects on reproductive and metabolic hormones. Recombinant rhesus leptin (rrhLep) produced in our laboratory was infused (100 microgram/h) into fasted adult male rhesus macaques (6-9 kg) beginning at midnight after the first missed meal and continuing until the end of the study. Bioactive luteinizing hormone (LH), testosterone, cortisol and growth hormone (GH) were measured in plasma from samples collected at 15-min intervals for the last 15 h (42-57 h) of the fast. We analyzed pulsatile LH and GH secretion by deconvolution analysis and the orderliness of pulsatile LH and GH release by the approximate entropy (ApEn) statistic. There was no difference in LH pulse frequency between control and fasted groups, but there was a significant decrease in the mean concentration of LH released (7.6 +/- 1.4 ng/ml control vs. 2.7 +/- 0.65 ng/ml fasted) that was not relieved with rrhLep infusions (2.8 +/- 0.83 ng/ml). Model-free Cluster analysis confirmed these inferences and also indicated that the peak height was lower in the fasted (4.6 +/- 1.0 ng/ml) and the fasted + rrhLep (2.85 +/- 1.0 ng/ml) groups compared to controls (16. 3 +/- 1.4 ng/ml). Testosterone levels reflected those of LH. Fasting resulted in an increase in GH secretory pulse frequency (5.3 +/- 0. 95 pulses/15 h control vs. 12.8 +/- 1.4 pulses/15 h fasted) and this increase was not affected by rrhLep infusion (12.5 +/- 1.4 pulses/15 h). In addition, fasting also increased the ApEn (decreased the orderliness) of pulsatile GH secretion, and this characteristic was not relieved with rrhLep infusions. Cortisol levels in fasted animals were 2- to 3-fold higher than those observed in control studies, and this increase was particularly pronounced at the time when the animals expected their first meal of the day. The increase in circulating cortisol observed in fasted animals was not affected by rrhLep infusion. Glucose levels at the end of the sampling period were 80 mg/dl in controls, 48 mg/dl in fasted animals and 58 mg/dl in the fasted + rrhLep group. Circulating leptin levels averaged 1.2 +/- 0.37 ng/ml in control animals, 0.7 +/- 0.2 ng/ml in fasted animals and 10.1 +/- 5.6 ng/ml in fasted animals infused with rrhLep. These studies suggest that intravenous replacement with homologous leptin does not reverse the acute changes in GH, LH and cortisol secretion observed with fasting in the adult male macaque.     

Effect of restricted feeding on the relationship between hypophysial portal concentrations of growth hormone (GH)-releasing factor and somatostatin, and jugular concentrations of GH in ovariectomized ewes.

These studies characterized the secretion of GH-releasing factor (GRF) and somatostatin (SRIF) into the hypophysial portal circulation in ewes after long term restricted feeding. In addition, we examined the temporal relationship between the concentrations of these two hypothalamic peptides in portal blood and the concentration of GH in jugular blood. Six sheep were fed 1000 g hay/day (normal feeding) and 6 sheep were fed 400-600 g hay/day (restricted feeding). This resulted in a wt loss of 35% in restricted animals compared with 6% in control animals after 20 weeks. Fluctuations in portal levels of GRF indicated a pulsatile pattern of secretion with approximately 60% of pulses coincident with, or immediately preceding, a GH pulse. Similarly, 65% of GH pulses were associated with GRF pulses. Restricted feeding increased (P less than 0.01) mean ( +/- SEM) plasma GH levels (9.8 +/- 1.4 vs. 2.9 +/- 0.6 ng/ml) and mean GH pulse amplitude (7.9 +/- 1.8 vs. 2.8 +/- 0.3 ng/ml) but did not affect mean GH pulse frequency (6.0 +/- 1.1 vs. 5.7 +/- 1.1 pulses/8 h). The level of feeding had no effect on mean portal concentration of GRF (restricted: 5.5 +/- 0.8, normal: 6.6 +/- 1.4 pg/ml), GRF pulse amplitude (14.7 +/- 2.3 vs. 13.5 +/- 0.7 pg/ml), or GRF pulse frequency (5.3 +/- 1.1 vs. 6.7 +/- 0.9 pulses/8 h). Portal concentrations of SRIF in sheep on a restricted diet were half (P less than 0.01) those of sheep fed a normal diet (10.2 +/- 2.3 vs. 19.6 +/- 1.6 pg/ml). Pulses of SRIF were not significantly associated with changes in GH or GRF concentrations. These data indicate a functional role for hypothalamic GRF in initiating GH pulses. Furthermore, the increase in GH secretion in underfed sheep was most probably due to a decrease in the release of SRIF into hypophysial portal blood. Restricted feeding had no affect on GRF secretion, but because of the reduced exposure of the pituitary gland to SRIF, it is possible that responsiveness to GRF is enhanced.     

Evidence for decreased luteinizing hormone-releasing hormone pulse frequency in men with selective elevations of follicle-stimulating hormone

To examine the hypothesis that the frequency of endogenous pulsatile LHRH stimulation controls the relative secretion of FSH and LH from the pituitary, we studied men with elevated FSH levels and normal LH levels to determine whether they have an altered frequency of pulsatile LHRH secretion compared to normal men. Because peripheral blood measurements of LHRH do not reflect the pulsatile characteristics of hypothalamic LHRH secretion, and it is generally accepted that the pulse frequency of LH secretion is an index of the frequency of endogenous LHRH pulsation, we used LH pulse frequency as the indicator of LHRH pulse frequency. Frequent blood sampling was performed to characterize LH pulse patterns in five men with selective elevations of FSH and seven age-matched normal men. Beginning at 0800-0930 h, blood samples were obtained every 10 min for 24 h through an indwelling iv catheter. Serum LH and FSH levels were measured by RIA in each sample, and the pattern of LH secretion was determined. Testosterone (T), estradiol, sex hormone-binding globulin, and free T were measured in a pooled serum sample from each man. Men with selective elevations of FSH had fewer LH pulses per 24 h (mean +/- SEM, 10.6 +/- 0.5) than the control group (12.9 +/- 0.6; P less than 0.01). There was no statistically significant difference in LH pulse amplitude (23 +/- 4 vs. 17 +/- 3 ng/ml). There were no statistically significant differences in T (4.9 +/- 0.5 vs. 6.1 +/- 0.5 ng/ml), estradiol (23 +/- 7 vs. 31 +/- 5 pg/ml), sex hormone-binding globulin (7.7 +/- 1.4 vs. 7.7 +/- 1.2 ng bound dihydrotestosterone/ml), or free T (0.16 +/- 0.02 vs. 0.23 +/- 0.04 ng/ml) in these men vs. normal subjects. We conclude that 1) compared to normal men, men with selectively elevated FSH levels have decreased LH pulse frequency, which suggests decreased LHRH pulse frequency; and 2) the relative secretion rates of LH and FSH by the pituitary may be regulated by the frequency of pulsatile LHRH secretion from the hypothalamus.     

Endogenous opioid suppression of luteinizing hormone pulse frequency and amplitude in the ewe: hypothalamic sites of action.

Evidence suggests that endogenous opioid peptides (EOP) inhibit pulsatile luteinizing hormone (LH) secretion during both the luteal and follicular phases of the ovine estrous cycle. Further data from sheep and other species indicate that the hypothalamus is the primary site of action for this EOP inhibition. The purpose of the following experiments was to determine which areas of the hypothalamus are involved in the EOP inhibition of pulsatile LH secretion. Regularly cycling ewes (n = 10) were stereotaxically implanted with guide tubes into the preoptic area (POA) and medial basal hypothalamus (MBH). Implants containing the EOP antagonist WIN 44,441-3 (WIN) were placed into each of these areas. Blood samples were collected at 12-min intervals for 3 h before and during WIN administration in the luteal phase and for 4 h before and during WIN administration in the follicular phase of the estrous cycle. During the luteal phase, WIN implants in either area increased (p less than 0.01) LH pulse frequency (POA 1.4 +/- 0.3/3 h before vs. 3.1 +/- 0.4/3 h during; MBH 1.1 +/- 0.2/3 h before vs. 2.8 +/- 0.5/3 h during). There was no effect on LH pulse amplitude. In contrast, during the follicular phase, WIN implants selectively increased (p less than 0.01) LH pulse frequency when implanted in the POA (3.2 +/- 0.4/4 h before vs. 5.2 +/- 0.6/4 h during) while increasing (p less than 0.05) only LH pulse amplitude when placed in the MBH (0.7 +/- 0.2 ng/ml before vs. 1.4 +/- 0.3 ng/ml during).(ABSTRACT TRUNCATED AT 250 WORDS)     

Leptin(116-130) stimulates prolactin and luteinizing hormone secretion in fasted adult male rats.

Leptin is a hormone secreted by adipocytes with an important role in the control of feeding behavior and neuroendocrine function. Leptin stimulates in vivo LH secretion in fasted female rats and in vitro PRL secretion. Recent data indicate that leptin(116-130), an active fragment of the native molecule, exerts effects similar to those of the native peptide on body weight and food intake. The present study was carried out to determine whether this fragment is also able to stimulate LH and PRL secretion. Adult male rats fasted for 5 days were injected with saline or leptin(116-130) (15 microgram i.c.v.) and LH and PRL concentrations were measured thereafter at 15-min intervals during a 150-min period. Administration of leptin(116-130) increased the frequency of LH pulses (2.0 +/- 0.26 vs. 1.20 +/- 0. 37/150 min; p 相似文献   

Short-term fasting selectively suppresses leptin pulse mass and 24-hour rhythmic leptin release in healthy midluteal phase women without disturbing leptin pulse frequency or its entropy control (pattern orderliness)

Nutritional signals strongly regulate neuroendocrine axes, such as those subserving release of LH, GH, and TSH, presumptively in part via the adipocyte-derived neuroactive peptide leptin. In turn, leptin release is controlled by both acute (fasting) and long-term (adipose store) nutrient status. Here, we investigate the neuroendocrine impact of short-term (2.5-day) fasting on leptin release in healthy young women studied in the steroid-replete midluteal phase of the normal menstrual cycle. Eight women each underwent 24-h blood sampling at 10-min intervals during a randomly ordered 2.5-day fasting vs. fed session in separate menstrual cycles. Pulsatile leptin release was quantified by model-free Cluster analysis, the orderliness of leptin patterns by the approximate entropy statistic, and nyctohemeral leptin rhythmicity by cosinor analysis. Mean (24-h) serum leptin concentrations fell by 4.6-fold during fasting; namely, from 15.2+/-2.3 to 3.4+/-0.6 microg/L (P = 0.0007). Cluster analysis identified 13.9+/-1.1 and 14.3+/-1.1 leptin peaks per 24 h in the fed and fasting states (P = NS), and unchanging leptin interpeak intervals (89+/-5.4 vs. 92+/-5.3 min). Leptin peak area declined by 4.2-fold (155+/-21 vs. 37+/-7 area units, P = 0.004), due to a reduction in incremental leptin pulse amplitude (4.4+/-0.7 vs. 1.0+/-0.13 microg/L, P = 0.0011). The cosine amplitude and mesor (mean) of the 24-h leptin rhythm decreased by 4-fold, whereas the acrophase (timing of the nyctohemeral leptin peak) remained fixed. The approximate entropy of leptin release was stable, thus indicating preserved orderliness of leptin release patterns in fasting. Cross-correlation analysis revealed both positive (fed) and negative (fasting) leptin-GH relationships, but no leptin-LH correlations. In summary, short-term (2.5-day) fasting profoundly suppresses 24-h serum leptin concentrations and pulsatile leptin release in the sex steroid-sufficient midluteal phase of healthy women via mechanisms that selectively attenuate leptin pulse area and incremental amplitude. In contrast, the pulse-generating, nyctohemeral phase-determining, and entropy-control mechanisms that govern 24-h leptin release are not altered by acute nutrient restriction at this menstrual phase. Leptin-GH (but not leptin-LH) showed nutrient-dependent positive (fed) and negative (fasting) cross-correlations. Whether similar neuroendocrine mechanisms supervise altered leptin signaling during short-term nutrient restriction in men, children, or postmenopausal women is not known.     

Leptin increases in vivo GH responses to GHRH and GH-releasing peptide-6 in food-deprived rats

BACKGROUND: Leptin has recently been shown to have a stimulatory effect on basal GH secretion. However, the mechanisms by which leptin exert this effect are not yet clear. GHRH and GH-releasing peptide (GHRP)-6 are the two most potent GH secretagogues described to date. OBJECTIVE: To determine if leptin could also enhance in vivo GH responses to a maximal dose of GHRH. DESIGN: Leptin (10microg i.c.v.) or vehicle was administered at random before GHRH (10microg/kg i,v.) or GHRP-6 (50microg/kg i.v.), to freely-moving rats with food available ad libitum and to (48h) food-deprived rats. METHODS: Leptin and GH concentrations were measured by radioimmunoassay. Comparison between the different groups was assessed by the Mann-Whitney test. RESULTS: In comparison with fed rats, food-deprived rats showed a marked decrease in GH responses to GHRH as assessed by the area under the curve (5492+/-190ng/ml in fed rats and 1940+/-128ng/ml in fasted rats; P<0.05) and GHRP-6 (3695+/-450 in fed rats and 1432+/-229 in fasted rats; P<0.05). In comparison with its effects in vehicle-treated rats, leptin administered to food-deprived rats markedly increased GH responses to both GHRH (6625+/-613ng/ml; P<0.05) and GHRP-6 (5862+/-441ng/ml; P<0.05). CONCLUSIONS: These data suggest that the blunted GH response to GHRH and GHRP-6 in food-deprived rats is a functional and reversible state, and that the decreased leptin concentrations could be the primary defect responsible for the altered GH secretion in food-deprived rats.     

Free fatty acids suppress growth hormone, but not luteinizing hormone, secretion in sheep

An experiment was conducted to determine the effects of exogenously administered FFA on GH and LH secretion in sheep. Ovariectomized ewes received iv infusions of a mixture of FFA (166 mg/min; n = 5) or 0.9% saline (n = 4) for 10 h. Jugular blood was sampled every 15 min for 14 h, beginning 4 h before initiation of infusion. After 8 h of FFA or saline treatment, each ewe received a pituitary challenge of 10 micrograms GRF and 1 microgram GnRH, administered together as an iv bolus. Lipid infusion increased (P less than 0.01) serum FFA concentrations to levels characteristic of those in fasted sheep [23.0 +/- 0.8 mg/100 ml (mean +/- SE)]. Frequency of GH pulses (P less than 0.01) and the GH response to GRF (P less than 0.0001) were suppressed by FFA treatment. Mean serum GH concentrations increased gradually (P less than 0.01) during the 10-h infusion period in saline-treated but not lipid-treated, ewes. This finding may reflect diurnal changes in somatotrope secretory activity that are blocked by FFA. Mean serum LH concentrations, LH pulse frequency and amplitude, and the LH secretory response to GnRH were unaffected by FFA or saline infusion. In agreement with previous work in sheep and other species, these results provide evidence for an inhibitory effect of FFA on GH release. The exact mechanism responsible for this action, however, remains to be elucidated. Finally, acutely elevated FFA levels do not appear to influence LH secretion in the ovariectomized ewe.     

Regulation of growth hormone and somatomedin-C secretion in postmenopausal women: effect of physiological estrogen replacement

To determine the effects of estrogen deficiency and replacement on GH secretion, we measured the 22-h GH secretory pattern and response to 1 h of light exercise in 16 normal postmenopausal women before and after treatment replacement with ethinyl estradiol (20 micrograms/day for 15 days). To determine whether the changes found were due to pituitary sensitization by estrogen, the response to synthetic GH-releasing hormone (GHRH; 1.0 microgram/kg, iv) was measured. To assess the biological effectiveness of GH in estrogen-treated women, somatomedin-C (Sm-C) responses to GHRH were measured. Pre- and postestrogen GH secretion rates, expressed as mean areas circumscribed by plasma GH values, were as follows: 22-h study, 1.4 +/- 0.1 (+/- SEM) vs. 2.0 +/- 0.3 ng/ml X h (P = 0.04; n = 5); during 1 h of exercise, 2.3 +/- 0.4 vs. 3.2 +/- 0.4 ng/ml X h (P = 0.03; n = 16); after GHRH-(1-40), 6.7 +/- 1.7 vs. 8.5 +/- 1.5 ng/ml X h (P = 0.12; n = 16). There also was a modest but significant increase in resting plasma GH (1.5 +/- 0.2 vs. 2.3 +/- 0.5 ng/ml (P = 0.039). Pre- and postestrogen plasma Sm-C concentrations were 0.56 +/- 0.08 and 0.32 +/- 0.03 U/ml, respectively (P = 0.006; n = 16). Thus, estrogen therapy increased spontaneous and exercise-induced GH secretion in postmenopausal women and reduced Sm-C levels. The mechanisms of GH elevation by estrogen may include both central effects and a negative feedback linkage to reduced plasma Sm activity.     

A reexamination of pulsatile luteinizing hormone secretion in primary testicular failure

The pulsatile pattern of LH secretion was studied in six patients with primary testicular failure and six healthy adult men by drawing peripheral blood samples every 10 min for 12 h. Mean basal LH concentrations were increased approximately 6-fold in hypogonadal men. In these patients mean absolute LH pulse amplitude was 3 times that of the controls [106 +/- 27 (SE) ng/ml vs. 31 +/- 2 ng/ml; P less than 0.01] and the frequency of spontaneous LH secretory episodes was increased 2-fold (between-pulse interval of 53 +/- 3 min vs. 107 +/- 8 min; P less than 0.01). Mean pulse amplitude was directly related to mean LH concentration in hypogonadal men (r = 0.94, P less than 0.01), but not in controls (r = 0.38, P = NS). Within each group there was no relationship between circulating testosterone or mean LH levels and LH pulse frequency. This study demonstrates that the increased circulating levels of LH in hypogonadal men are a consequence of an increase in both LH pulse frequency and pulse amplitude. The prolonged, more frequent sampling protocol used revealed a pattern of LH secretion not evident in previous studies. Finally, these data provide further, albeit indirect, evidence that androgens may regulate the intermittent release of LHRH in man.     

Homeostatic joint amplification of pulsatile and 24-hour rhythmic cortisol secretion by fasting stress in midluteal phase women: concurrent disruption of cortisol-growth hormone, cortisol-luteinizing hormone, and cortisol-leptin synchrony

Short-term fasting as a metabolic stress evokes prominent homeostatic reactions of the reproductive, corticotropic, thyrotropic, somatotropic, and leptinergic axes in men and women. Although reproductive adaptations to fasting are incompletely studied in the female, nutrient deprivation can have major neuroendocrine consequences in the follicular phase. Unexpectedly, a recent clinical study revealed relatively preserved sex steroid and gonadotropin secretion during short-term caloric restriction in the midluteal phase of the menstrual cycle. This observation suggested that female stress-adaptive responses might be muted in this sex steroid-replete milieu. To test this hypothesis, we investigated the impact of fasting on daily cortisol secretion in healthy young women during the midluteal phase of the normal menstrual cycle. Eight volunteers were each studied twice in separate and randomly ordered short-term (2.5-day) fasting and fed sessions. Pulsatile cortisol secretion, 24-h rhythmic cortisol release, and the orderliness of cortisol secretory patterns were quantified. Within-subject statistical comparisons revealed that fasting increased the mean serum cortisol concentration significantly from a baseline value of 8.0+/-0.61 to 12.8+/-0.85 microg/dL (P = 0.0003). (For Systeme International conversion to nanomoles per L, multiply micrograms per dL value by 28.) Pulsatile cortisol secretion rose commensurately, viz. from 101+/-11 to 173+/-16 microg/dL/day (P = 0.0025). Augmented 24-h cortisol production was due to amplification of cortisol secretory burst mass from 8.2+/-1.5 to 12.9+/-2.0 microg/dL (P = 0.017). In contrast, the estimated half-life of endogenous cortisol (104+/-9 min), the calculated duration of underlying cortisol secretory bursts (16+/-7 min) and their mean frequency (14+/-2/day) were not altered by short-term fasting. The quantifiable orderliness of cortisol secretory patterns was also not influenced by caloric restriction. Nutrient deprivation elevated the mean of the 24-h serum cortisol concentration rhythm from 12.4+/-1.3 to 18.4+/-1.9 microg/dL (P = 0.0005), without affecting its diurnal amplitude or timing. Correlation analysis disclosed that fasting reversed the positive relationship between cortisol and LH release evident in the fed state, and abolished the negative association between cortisol and GH as well as between cortisol and leptin observed during nutrient repletion (P < 0.001). Pattern synchrony between cortisol and GH as well as that between cortisol and LH release was also significantly disrupted by fasting stress. In summary, short-term caloric deprivation enhances daily cortisol secretion by 1.7-fold in healthy midluteal phase young women by selectively amplifying cortisol secretory burst mass and elevating the 24-h rhythmic cortisol mean. Augmentation of daily cortisol production occurs without any concomitant changes in cortisol pulse frequency or half-life or any disruption of the timing of the 24-h rhythmicity or orderliness of cortisol release. Fasting degrades the physiological coupling between cortisol and LH, cortisol and GH, and cortisol and leptin secretion otherwise evident in calorie-sufficient women. We conclude that the corticotropic axis in the young adult female is not resistant to the stress-activating effects of short-term nutrient deprivation, but, rather, evinces strong adaptive homeostasis both monohormonally (cortisol) and bihormonally (cortisol paired with GH, LH, and leptin).     

Galanin is a physiological regulator of spontaneous pulsatile secretion of growth hormone in the male rat.

To determine whether galanin (GAL), a 29-amino acid neuropeptide, plays a role in the physiological regulation of the pulsatile secretion of GH and PRL in the male rat, secretory patterns of both hormones were studied in freely moving animals after GAL passive immunoneutralization. Adult male Sprague-Dawley rats were equipped with iv and intracerebroventricular catheters. After 7 days, 3 microliters of a specific GAL antiserum (GAL-AS) or normal rabbit serum (NRS; controls) were infused in the third ventricle of 10 rats, 25 and 1 h before the animals were bled every 15 min for 6 h (1000-1600 h). Plasma GH and PRL concentrations were measured by RIA, and the hormonal secretory patterns were analyzed by the PULSAR program. Control rats, treated with NRS, displayed typical GH secretion, with pulses of high amplitude (167 +/- 27 ng/ml) and low frequency (2.4 +/- 0.2 pulses/6 h), separated by periods of low trough levels (3.8 +/- 0.6 ng/ml). Rats treated with GAL-AS had altered pulsatile GH secretion. Pulse height was markedly reduced (77 +/- 15 ng/ml; P less than 0.01 vs. controls), and peak frequency was higher (3.6 +/- 0.5 pulses/6 h; P less than 0.05), while GH baseline levels and integrated GH secretion over the 6-h sampling period remained unaltered. Injection of rat GH-releasing hormone (1 microgram/rat, iv) caused a similar GH stimulation in both groups of rats, as determined by the peak GH response at 5 min (368 +/- 112 vs. 342 +/- 81 ng/ml) or by the integrated GH response over 1 h (5.13 +/- 1.30 vs. 4.77 +/- 1.15 micrograms.min/ml in NRS- and GAL-AS-treated rats, respectively; P less than 0.05). In contrast to GH, pulsatile secretion of PRL was not affected by the GAL-AS treatment. These results indicate that GAL is a physiological regulator of spontaneous pulsatile secretion of GH, but not PRL, in the male rat. The influence of GAL on GH secretion appears to be exerted within the hypothalamus, mainly by a stimulation of GRF secretion. However, the changes in GH pulse frequency observed after GAL immunoneutralization suggest that GAL might also influence the somatostatin inhibitory tone.     

Preservation of dopaminergic and alpha-adrenergic function in children with growth hormone neurosecretory dysfunction

The integrity of dopaminergic and alpha-adrenergic neurotransmitter regulation of GH secretion was examined in children with decreased GH secretion. Children with GH neurosecretory dysfunction (GHND; n = 16) those with classical GH deficiency (n = 9), and short but otherwise normal children (n = 12) underwent 24 h GH studies (blood sampling every 20 min for 24 h) and provocative tests using arginine, insulin hypoglycemia, L-dopa (dopaminergic) and clonidine (alpha-adrenergic), and GH-releasing hormone (GHRH). GHND was defined as children with height in the first percentile or below, growth velocity of 4 cm/yr or less, low plasma somatomedin-C for age, delayed skeletal age by 2 or more yr, peak serum GH responses to any one (or more) provocative test of 10 ng/ml or more, and mean 24-h GH concentration below 3 ng/ml. GHND and GH-deficient children had reduced endogenous GH secretion, expressed as mean serum 24-h GH concentration [1.6 +/- 0.1 (+/- SEM) and 2.1 +/- 0.1 vs. 6.1 +/- 0.5 ng/ml (GH-deficient and GHND vs. normal, respectively); P less than 0.01]. the mean peak serum GH levels after arginine [8.2 +/- 2.0 vs. 20.8 +/- 6.6 ng/ml (GHND vs. normal); P less than 0.05] and insulin [9.3 +/- 1.0 vs. 16.2 +/- 1.7 ng/ml (GHND vs. normal); P less than 0.01) were lower in GHND children. The mean peak responses after L-dopa [13.4 +/- 3.4 vs. 14.6 +/- 4.7 ng/ml (GHND vs. normal); P = NS] and clonidine [19.0 +/- 2.2 vs. 23.3 +/- 3.8 ng/ml (GHND vs. normal); P = NS] were preserved in GHND children. In GH-deficient children, mean peak serum GH concentrations after all four provocative tests were low (arginine, 2.7 +/- 0.8; insulin, 2.6 +/- 0.8; L-dopa, 3.0 +/- 0.9; clonidine, 3.4 +/- 1.0 ng/ml; all P less than 0.01 vs. normal). The mean peak serum GH concentration after GHRH was blunted in GH-deficient children (9.1 +/- 1.7 ng/ml) compared to those in GHND (32.9 +/- 8.5 ng/ml) and normal (43.2 +/- 6.4 ng/ml) children (P less than 0.01). The area under the GH curve after GHRH stimulation was greater for normal than GHND children (P less than 0.05). These data demonstrate preservation of dopaminergic and alpha-adrenergic neurotransmitter pathways in GHND children. They further suggest a defect in the release of pituitary GH secondary to an abnormality in alternative neurotransmitter pathways resulting in decreased GHRH and/or increased somatostatin secretion.     

Short-term fasting suppresses leptin and (conversely) activates disorderly growth hormone secretion in midluteal phase women--a clinical research center study

Short term fasting activates the corticotropic and somatotropic, and suppresses the reproductive, axis in men. Analogous neuroendocrine responses are less well characterized in women. Recently, we identified a negative association between the adipocyte-derived nutritional signaling peptide, leptin, and pulsatile GH secretion in older fed women. In the present study, we investigated the impact of acute nutrient deprivation on pulsatile GH and LH secretion and mean leptin concentrations in eight healthy young women in the sex-steroid replete milieu of the midluteal phase of the normal menstrual cycle. Volunteers underwent 24-h blood sampling during randomly ordered, short term (2.5-day), fasting vs. fed sessions in separate menstrual cycles. Pulsatile GH and LH secretion over 24 h was quantified by deconvolution analysis, nyctohemeral rhythmicity was quantified by cosinor analysis, and the orderliness of the GH or LH release process was quantified by the approximate entropy statistic. By paired statistical analysis, a 2.5-day fast failed to alter mean (pooled) 24-h serum concentrations of LH, progesterone, estradiol, or PRL, but increased cortisol levels more than 1.5-fold (P = 0.0003). Concurrently, mean (pooled) serum leptin concentrations fell by 75% (P = 0.0003), and insulin-like growth factor I (IGF-I; P < 0.05) and insulin decreased significantly (P = 0.0018). In contrast, the daily pulsatile GH secretion rate rose 3-fold (P < 0.001). Amplified daily GH secretion was attributable mechanistically to a 2.3-fold rise in GH secretory burst mass, reflecting an increased GH secretory burst amplitude (P < 0.01). The GH half-life, duration of GH secretory bursts, and GH pulse frequency did not vary during short term fasting. The disorderliness of GH release increased significantly with nutrient restriction (P = 0.005). The mesor and amplitude of the nyctohemeral serum GH concentration rhythm also rose with fasting (P < 0.01), but the timing of maximal serum GH concentrations did not change. Thus, short-term (2.5-day) fasting during the sex steroid-replete midluteal phase of the menstrual cycle in healthy young women profoundly suppresses 24-h serum leptin and insulin (and to a lesser degree, IGF-I) concentrations, augments cortisol release, but fails to alter daily LH, estradiol, or progesterone concentrations. In contrast, the GH axis exhibits strikingly amplified pulsatile secretion, increased nyctohemeral rhythmicity, and marked disorderliness of the release process. We conclude that the somatotropic axis is more evidently vulnerable to short-term nutrient restriction than the reproductive axis in steroidogenically sufficient midluteal phase women. This study invites the question of whether normal (nutritionally replete) GH secretory dynamics can be restored in fasting women by human leptin, insulin, or IGF-I infusions.     

Acute effects of testosterone infusion and naloxone on luteinizing hormone secretion in normal men.

To evaluate the role of endogenous opioid pathways in the acute suppression of LH secretion by testosterone (T) infusion in men, we studied eight normal healthy volunteers who received a saline infusion, followed 1 week later by a T infusion (960 nmol/h) starting at 1000 h and lasting for 33 h. After 2 h of infusion (both saline and T), four iv boluses of saline were given hourly, and after 26 h of infusion, four hourly iv boluses of naloxone were given. Blood was obtained every 15 min for LH and every 30 min for T. T infusion increased the mean plasma T concentration 2.1-fold (18.7 +/- 2.1 to 39.5 +/- 3.5 nmol/L, saline vs. T infusion, P < 0.01). The mean plasma LH concentration was 7.9 +/- 0.5 IU/L during the saline control study and was decreased to 6.9 +/- 0.6 IU/L by the infusion of T (P < 0.05). LH pulse frequency was similar during both saline and T infusions (0.48 +/- 0.02 vs. 0.43 +/- 0.04 pulses/man.h, saline vs. T infusion). The mean LH pulse amplitude decreased from 4.3 +/- 0.4 IU/L during saline infusion to 3.3 +/- 0.2 IU/L during T infusion (P < 0.05). The administration of naloxone increased the mean plasma LH concentration significantly during saline infusion (7.6 +/- 0.4 to 10.0 +/- 0.9 IU/L, saline vs. naloxone boluses, P < 0.01), but not during T infusion (6.9 +/- 0.6 vs. 7.3 +/- 0.6 IU/L). LH pulse frequency increased significantly after the administration of naloxone during both saline and T infusions (0.54 +/- 0.04 to 0.71 +/- 0.08 pulses/man.h, saline vs. naloxone boluses during saline infusion, and 0.46 +/- 0.08 to 0.60 +/- 0.07 pulses/man.h during T infusion; P < 0.05). LH pulse amplitude was suppressed by T infusion, but administration of naloxone did not reverse this suppression. The mean amplitude of the LH response to exogenous GnRH (250 ng/kg) was decreased by T infusion from 48 +/- 13.5 to 31.2 +/- 8.5 IU/L (P < 0.01). Therefore, in men, the administration of naloxone increases LH pulse frequency during both saline and T infusions, but the acute suppression of LH pulse amplitude seen with T infusion was not reversed by naloxone. This pattern contrasts sharply with the effects of T infusion in pubertal boys, as elucidated by our earlier studies. The negative feedback effects of T on LH secretion are primarily hypothalamic in early pubertal boys and change to pituitary suppression in men.     

The effect of pulsatile administration, continuous infusion, and diurnal variation on the growth hormone (GH) response to GH-releasing hormone in normal men

To examine the relative effectiveness of GH-releasing hormone (GHRH) given either as multiple iv pulses or as a continuous iv infusion, we studied the GH response to a nearly equivalent total dose of GHRH-44 administered by both routes in a group of normal men. Further, in view of the pulsatile nature of GH secretion and its augmentation with sleep, we investigated whether a diurnal difference in GH release was present during chronic pulsatile administration of GHRH during day and night. Seven men received six GHRH pulses (1 microgram/kg, iv) at 2-h intervals during both day (0900-2100 h) and night (2100-0900 h), and four underwent nighttime placebo pulsing. Eight men received a daytime continuous GHRH infusion (0.15 microgram/kg X h for 5 h, followed by 0.75 microgram/kg X h for 5 h) and a separate 10-h placebo infusion. The GH response to a bolus dose of GHRH (1 microgram/kg, iv) was determined after both continuous GHRH and placebo infusions. No significant difference was found in the GH area response (mean +/- SEM) during total day and night GHRH pulsing periods (6095 +/- 1192 vs. 6506 +/- 1483 ng/min X ml; P = NS). GH secretion was blunted after the initial daytime GHRH pulse (P = 0.02), and only two of seven men had a GH increase after the second pulse; responsiveness was restored after the fourth pulse. In contrast, all subjects responded to the second nighttime GHRH pulse. During continuous GHRH infusions, GH secretion was unsustained and pulsatile. The incremental GH response to a single GHRH bolus dose was decreased after GHRH infusion compared to that after placebo (4.4 +/- 1.8 vs. 10.3 +/- 3.4 ng/ml; P less than 0.05). No difference was found in the total GH area response to a nearly equivalent dose of GHRH administered as either multiple pulses or continuous infusion followed by a single GHRH bolus dose. The apparent pulsatile nature of GH secretion during continuous GHRH infusion and the lack of a significant difference in the GH response to a nearly equivalent dose of GHRH administered as either multiple pulses or a continuous infusion suggest that GHRH need not be administered in a pulsatile manner to be an effective therapeutic agent for the stimulation of GH secretion in children with hypothalamic GHRH deficiency.     

Pulsatile release of luteinizing hormone and the secretion of ovarian steroids in sheep during anestrus

The peripheral concentration of LH and the secretion rates of estradiol-17beta and androstenedione were measured every 10 min for 4 h during anestrus in six ewes with utero-ovarian autotransplants. The basal concentration of LH was 0.45 +/- 0.06 ng//ml (NIH LH S14; mean +/- SEM), with pulses of LH (6.0 +/- 0.3 ng/ml; N = 5) occurring with an average frequency of one per 5 h. The basal secretion rate of estradiol-17beta was 0.5--1.3 ng/min. Each pulse of LH was followed by a rise in the secretion of estradiol-17beta. Maximum secretion of estradiol-17beta was 5.18 +/- 0.72 ng/min and was reached 25 min after LH discharge. Basal secretion of androstenedione was higher than estradiol-17beta at about 1.5 to 3.4 ng/min, and increased to a maximum 26.8 +/- 6.6 ng/min 25 min after LH discharge. Steroid secretion was not maintained at maximum rates and had returned to basal levels 2 h after LH discharge. The data show that the ovary of the anestrous sheep is capable of secreting steroids following pulses of endogenous LH. These results strongly suggest that as in the luteal phase of the cycle, the quantity of estradiol and androstenedione secreted by the ovary is related to the frequency of episodic pulses of LH.     

Evidence for acyl-ghrelin modulation of growth hormone release in the fed state

CONTEXT: The timing and frequency of GH secretory episodes is regulated by GHRH and somatostatin. This study provides evidence for amplification of these GH pulses by endogenous acyl-ghrelin. DESIGN: Blood was sampled every 10 min for 26.5 h during a fed admission with standardized meals and also during the final 24 h of a 61.5-h fast. GH secretion profiles were derived from deconvolution of 10-min sampling data, and full-length acyl-ghrelin levels were measured using a newly developed two-site sandwich assay. SETTING: The study was conducted at a university hospital general clinical research center. PARTICIPANTS: Participants included eight men with mean (+/- sd) age 24.5 +/- 3.7 yr (body mass index 24 +/- 2.1 kg/m(2)). RESULTS: Correlations were computed between amplitudes of individual GH secretory events and the average acyl-ghrelin concentration in the 60-min interval preceding each GH burst. In the fed state, the peak correlations were positive for all subjects and significantly higher than in the fasting state when acyl-ghrelin levels declined [mean (+/- sem): 0.7 (0.04) vs. 0.29 (0.08), P = 0.017]. In addition, long-term fasting was associated with an increase in the GH secretory pulse mass and amplitude but not frequency [fed vs. fasting pulse mass: 0.22 (0.05) vs. 0.44 (0.06) microg/liter, P = 0.002; amplitude: 5.2 (1.3) vs. 11.8 (1.9) microg/liter/min, P = 0.034; pulses per 24 h: 19.4 (0.5) vs. 22.0 (1.4), P = 0.1]. CONCLUSION: Our data support the hypothesis that under normal conditions in subjects given regular meals endogenous acyl-ghrelin acts to increase the amplitude of GH pulses.     

Chronic sex steroid exposure increases mean plasma growth hormone concentration and pulse amplitude in men with isolated hypogonadotropic hypogonadism

Sex steroid administration can increase the GH response to provocative stimuli, but the relationship of sex steroids to spontaneous GH secretion is still controversial. We sought to characterize the effect of sex steroids on the plasma GH concentration by examining the 24-h pattern of episodic GH secretion in nine previously untreated adult men with isolated hypogonadotropic hypogonadism before and during long term testosterone, gonadotropin, or pulsatile GnRH treatment. After chronic sex steroid exposure, the mean 24-h plasma GH level, mean GH pulse amplitude, and mean area under the curve of pulses were significantly increased compared to pretreatment values [3.2 +/- 1.8 ( +/- SD) vs. 1.8 +/- 1.2 ng/mL (P less than 0.01); 11.4 +/- 7.2 vs. 5.5 +/- 4.4 ng/mL (P less than 0.05); and 720 +/- 547 vs. 316 +/- 371 ng/mL X 20 min (P less than 0.05), respectively], while mean 24-h pre- and posttreatment GH pulse frequencies were indistinguishable (5.7 +/- 2.1 posttreatment vs. 5.0 +/- 3.2 pretreatment; P = NS). The mean posttreatment plasma somatomedin-C level also rose significantly during treatment (1.89 +/- 0.65 vs. 1.28 +/- 0.48 U/mL; P less than 0.01). We conclude that the increase in the mean plasma GH level during chronic sex steroid exposure is due mainly to augmentation of GH pulse amplitude, and that sex steroids probably increase spontaneous GH secretion.