C Dieguez and FF Casanueva
E Carro, LM Seoane, R Senaris, FF Casanueva and C Dieguez
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.
JE Caminos, LM Seoane, SA Tovar, FF Casanueva and C Dieguez
OBJECTIVE: To assess whether some of the alterations in energy homeostasis present in thyroid function disorders and GH deficiency could be mediated by ghrelin. DESIGN: To assess the influence of thyroid status on ghrelin, adult male Sprague-Dawley rats were treated with vehicle (euthyroid), amino-triazole (hypothyroid) or l-thyroxine (hyperthyroid). The influence of GH on ghrelin was assessed in wild-type (control) and GH-deficient (dwarf) Lewis rats. Evaluation of gastric ghrelin mRNA expression in the stomach was carried out by Northern blot. Circulating levels of ghrelin were measured by radioimmunoassay. RESULTS: Hypothyroidism resulted in an increase in gastric ghrelin mRNA levels (euthyroid: 100+/-3.2% vs hypothyroid: 127.3+/-6.5%; P<0.01), being decreased in hyperthyroid rats (70+/-5.4%; P<0.01). In keeping with these results, circulating plasma ghrelin levels were increased in hypothyroid (euthyroid: 124+/-11 pg/ml vs hypothyroid: 262+/-39 pg/ml; P<0.01) and decreased in hyperthyroid rats (75+/-6 pg/ml; P<0.01). Using an experimental model of GH deficiency, namely the dwarf rat, we found a decrease in gastric ghrelin mRNA levels (controls: 100+/-6% vs dwarf: 66+/-5.5%; P<0.01) and circulating plasma ghrelin levels (controls: 124+/-12 pg/ml vs dwarf: 81+/-7 pg/ml; P<0.01). CONCLUSION: This study provides the first evidence that ghrelin gene expression is influenced by thyroid hormones and GH status and provides further evidence that ghrelin may play an important role in the alteration of energy homeostasis and body weight present in these pathophysiological states.
M Tena-Sempere, L Pinilla, LC Gonzalez, J Navarro, C Dieguez, FF Casanueva and E Aguilar
The obese gene (ob) product, leptin, has recently emerged as a key element in body weight homeostasis, neuroendocrine function and fertility. Identification of biologically active, readily synthesized fragments of the leptin molecule has drawn considerable attention, as they may provide a powerful tool for detailed characterization of the biological actions of leptin in different experimental settings. Recently, a fragment of mouse leptin protein comprising amino acids 116-130, termed leptin(116-130) amide, was shown to mimic the effects of the native molecule in terms of body weight gain and food intake, and to elicit LH and prolactin (PRL) secretion in vivo. As a continuation of our previous experimental work, the present study reports on the effects of leptin(116-130) amide on basal and stimulated testosterone secretion by adult rat testis in vitro. In addition, a comparison of the effects of human recombinant leptin and leptin(116-130) amide at the pituitary level on the patterns of LH, FSH, PRL and GH secretion is presented. As reported previously by our group, human recombinant leptin(10(-9)-10(-7)M) significantly inhibited both basal and human chorionic gonadotrophin (hCG)-stimulated testosterone secretion in vitro. Similarly, incubation of testicular tissue in the presence of increasing concentrations of leptin(116-130) amide (10(-9)-10(-5)M) resulted in a dose-dependent inhibition of basal and hCG-stimulated testosterone secretion; a reduction that was significant from a dose of 10(-7)M upwards. In addition, leptin(116-130) amide, at all doses tested (10(-9)-10(-5)M), significantly decreased LH and FSH secretion by incubated hemi-pituitaries from adult male rats. In contrast, in the same experimental protocol, recombinant leptin(10(-9)-10(-7)M) was ineffective in modulating LH and FSH release. Finally, neither recombinant leptin nor leptin(116-130) amide were able to change basal PRL and GH secretion in vitro. Our results confirm the ability of leptin, acting at the testicular level, to inhibit testosterone secretion, and map the effect to a domain of the leptin molecule that lies between amino acid residues 116 and 130. In addition, we provide evidence for a direct inhibitory action of leptin(116-130) amide on pituitary LH and FSH secretion, a phenomenon not observed for the native leptin molecule, in the adult male rat.
L Pinilla, LM Seoane, L Gonzalez, E Carro, E Aguilar, FF Casanueva and C Dieguez
The aim of this study was to investigate the regulation of serum leptin levels by gender and gonadal steroid milieu. Thus, we measured serum leptin levels by radioimmunoassay in (a) intact male and female rats, (b) female rats at different stages of the estrous cycle and (c) ovariectomized or orchidectomized rats. Gonadectomized groups were or were not implanted with silastic capsules (10 or 30 mm in length, 1.519mm internal diameter; 3.06 mm external diameter) containing estradiol or testosterone and decapitated two weeks later. We found (i) intact female rats weighing 50 g, 250 g and 300 g exhibited higher serum leptin concentrations than intact male rats of similar body weight; (ii) leptin concentrations were not affected by the phase of the estrous cycle; (iii) two weeks after gonadectomy serum leptin concentrations increased in both male (from 4.47+/-1.87 to 8.76+/-1.24 ng/ml) and female (from 1.97+/-0.46 to 5.29+/-0.51 ng/ml) rats. The ovariectomy-induced increase in serum leptin levels was not dependent, at least completely, on changes in body weight since it could be observed when comparisons were made between ovariectomized rats and intact rats in estrus matched for body weight. In contrast the effect of orchidectomy on serum leptin levels appears to be dependent on changes in body weight since it was no longer observed when comparisons were made with a group of intact male rats matched for body weight. In conclusion, these results suggest that serum leptin concentrations are controlled by gonadal function either directly or as a consequence of changes in body weight.
C Menendez, R Baldelli, M Lage, X Casabiell, V Pinero, J Solar, C Dieguez and FF Casanueva
OBJECTIVE: Leptin is an adipocyte-secreted hormone acting as a signal to the central nervous system, where it regulates energy homeostasis and neuroendocrine processes. Leptin plasma levels are mainly regulated by the percentage of body fat, but are also controlled by several metabolic and nutritional variables. Data regarding leptin secretion suggest that it is gender regulated, and higher levels are present in women than men; however, the biological basis for this sex-related difference is unknown. To clarify those points, a systematic study with tissue cultures from human omental adipose tissue was performed. DESIGN AND METHODS: Surgically obtained samples from 137 patients (68 women, 69 men) were evaluated. The assay was standardized in periods of 24 h ending at 96 h. Each adipose tissue sample from a single donor was incubated in triplicate and leptin results expressed as the mean of the integrated secretion into the medium (nanograms of leptin/g tissue per time). RESULTS: Tissue adipose cultures showed a steady leptin secretion throughout the 96 h studied, with the peak of secretory activity reached at 48 h; afterwards, the in vitro secretion reached a plateau state. Spontaneous leptin secretion in the 24 h and 48 h period, as well as the area under the curve analyzed in the 0-48 h period, showed a gender-based difference that was significantly (P<0. 05) higher in women than in men. When data of spontaneous leptin secretion were correlated with the body mass index (BMI) of the donors, no correlation was found. This suggests that in vivo leptin levels are dependent on the total amount of fat of the individual, but independent of the leptin secretory rate by the adipose tissue of the donor. CONCLUSIONS: Leptin secretion from omental adipose tissue in vitro is: (i) significantly higher in samples from women than in samples from men; and (ii) not correlated with the BMI, showing that in vitro leptin secretion is not related to the adiposity of the donor.
LM Seoane, S Tovar, R Baldelli, E Arvat, E Ghigo, FF Casanueva and C Dieguez
Ghrelin is a growth hormone-releasing acylated peptide from stomach. The purified peptide consist of 28 amino acids in which the serine 3 residue is n-octanoylated. Ghrelin has been reported to increase in vitro GH secretion as well as in vivo plasma GH levels in pentobarbital anaesthetized rats. The aim of this work was to characterize the stimulatory effect of Ghrelin on in vivo GH secretion in freely-moving rats. Furthermore, we compare the effect of Ghrelin with GHRH. In addition to vehicle, we administered different doses of Ghrelin (3 nmol/Kg, 12 nmol/Kg and 60 nmol/Kg); GHRH (3 nmol/Kg and 12 nmol/kg). Plasma GH levels were measured in blood samples taken at 5, 10, 15, 20, 30 and 45 min after their administration as an i.v. bolus at 0 min. Administration of Ghrelin led to an increase in plasma GH levels at all time-points tested (5, 10, 15, 20 and 30 min, P<0.01; and 45 min, P<0.05) in comparison to control untreated rats. A maximal stimulatory effect on plasma GH was observed following administration of 12 nmol/Kg of Ghrelin, the effect being similar to the one obtained with 60 nmol/Kg in terms of both AUC and mean peak GH levels. At the dose of 3 nmol/Kg GHRH and Ghrelin exhibited a similar stimulatory effect in term of both, AUC and mean peak GH levels. However following administration of a dose of 12 nmol/Kg, the effect of Ghrelin was much greater than the same dose of GHRH in terms of both AUC and mean peak GH levels. In summary, this study provides the first evidences that Ghrelin exert a marked stimulatory effect in plasma GH levels in freely-moving rats and provides further evidences that Ghrelin may play an important role in the physiological control of GH secretion.
F Cordido, P Alvarez-Castro, ML Isidro, FF Casanueva and C Dieguez
OBJECTIVE: It has been gradually realized that GH may have important physiological functions in adult humans. The biochemical diagnosis of adult GHD is established by provocative testing of GH secretion. The insulin-tolerance test (ITT) is the best validated. The ITT has been challenged because of its low degree of reproducibility and lack of normal range, and is contra-indicated in common clinical situations. Furthermore, in severely obese subjects the response to the ITT frequently overlaps with those found in non-obese adult patients with GHD. DESIGN: The aim of the present study was to evaluate the diagnostic capability of four different stimuli of GH secretion: ITT, GHRH, GHRH plus acipimox (GHRH+Ac), and GHRH plus GHRP-6 (GHRH+GHRP-6), in two pathophysiological situations: hypopituitarism and obesity, and normal subjects. METHODS: Eight adults with hypopituitarism (four female, four male) aged 41-62 Years (48.8+/-1.4 Years), ten obese normal patients (five female, five male) aged 38-62 Years (48.1+/-2.5 Years), with a body mass index of 34.2+/-1.2 kg/m(2), and ten normal subjects (five female, five male) aged 33-62 Years (48.1+/-2.8 Years) were studied. Four tests were performed on each patient or normal subject: An ITT (0.1 U/kg, 0.15 U/kg for obese, i.v., 0 min), GHRH (100 microg, i.v., 0 min), GHRH (100 microg, i.v., 0 min) preceded by acipimox (250 mg, orally, at -270 min and -60 min) (GHRH+Ac); and GHRH (100 microg, i.v., 0 min) plus GHRP-6 (100 microg, i.v., 0 min) (GHRH+GHRP-6). Serum GH was measured by radioimmunoassay. Statistical analyses were performed by Wilcoxon rank sum and by Mann-Whitney tests. RESULTS: After the ITT the mean peak GH secretion was 1.5+/-0.3 microg/l for hypopituitary, 10.1+/-1.7 microg/l (P<0.05 vs hypopituitary) for obese and 17.8+/-2.0 microg/l (P<0.05 vs hypopituitary) for normal. GHRH-induced GH secretion was 2+/-0.7 microg/l for hypopituitary, 3.9+/-1.2 microg/l (P=NS vs hypopituitary) for obese and 22.2+/-3.8 microg/l (P<0.05 vs hypopituitary) for normal. After GHRH+Ac, mean peak GH secretion was 3.3+/-1.4 microg/l for hypopituitary, 14.2+/-2.7 microg/l (P<0.05 vs hypopituitary) for obese and 35.1+/-5.2 microg/l (P<0.05 vs hypopituitary) for normal. GHRH+GHRP-6 induced mean peak GH secretion of 4.1+/-0.9 microg/l for hypopituitary, 38.5+/-6.5 microg/l (P<0.05 vs hypopituitary) for obese and 68.1+/-5.5 microg/l (P<0.05 vs hypopituitary) for normal subjects. Individually considered, after ITT, GHRH or GHRH+Ac, the maximal response in hypopituitary patients was lower than the minimal response in normal but higher than the minimal response in obese subjects. In contrast, after GHRH+GHRP-6 the maximal response in hypopituitary patients was lower than the minimal response in normal and obese subjects. CONCLUSIONS: This study suggests that, in this group of patients, although both acipimox and GHRP-6 partially reverse the functional hyposomamotropism of obesity after GHRH, but are unable to reverse the organic hyposomatotropism of hypopituitarism, the combined test GHRH+GHRP-6 most accurately distinguishes both situations, without the side effects of ITT.
A Leal-Cerro, E Garcia, R Astorga, FF Casanueva and C Dieguez
Leal-Cerro A, Garcia E, Astorga R, Casanueva FF, Dieguez C. Growth hormone (GH) responses to the combined administration of GH-releasing hormone plus GH-releasing peptide 6 in adults with GH deficiency. Eur J Endocrinol 1995;132:712–5. ISSN 0804–4643
In recent years the health problems of adults with growth hormone deficiency (GHD) and the benefits of GH replacement therapy have received considerable attention. However, the reliability of conventional GH tests in the assessment of pituitary GH reserve in this group of patients is still controversial. In this study, we assessed GH secretion after the combined administration of GH-releasing hormone (GHRH) (1 μg/kg iv) and GH-releasing peptide 6 (GHRP-6, 1 μg/kg iv) in adult patients diagnosed with GHD by conventional GH testing, and correlate this response with insulin-like growth factor I levels. Twenty-one subjects (13 male, 8 female) with long-standing diagnosis of GHD aged 21–54 years were studied. In 13 subjects GH responses to GHRH plus GHRP-6 were markedly reduced (peak GH response <10 mU/I), whereas in the remaining eight the response was greater (range 11–100 mU/l), In conclusion, our data show that combined administration of GHRH plus GHRP-6 elicited a significant increase in plasma GH levels in about 40% of patients diagnosed with GHD by conventional GH testing.
C Dieguez, PO Box 563, 15700 Santiago de Compostela, Spain
R Peino, V Pineiro, O Gualillo, C Menendez, J Brenlla, X Casabiell, C Dieguez and FF Casanueva
OBJECTIVE: Leptin secretion is reduced by low temperatures in experimental animals, and this effect has been explained as an adaptive mechanism to cold environments. This study investigated the in vitro effects of cold exposure on human white adipose tissue. DESIGN: To understand whether the low temperature action is a direct or a mediated effect, leptin secretion was assessed in vitro in human omental adipose tissue incubated at varied temperatures, from 38 donors. As an internal control, the effect of reduced temperatures on in vitro GH secretion by GH3 cells was assessed. METHODS: Measurement of hormones secretion was carried out with an RIA, while human ob gene mRNA expression was assessed with reverse transcription PCR. RESULTS: Compared with the standard temperature of 37 degrees C, leptin secretion by human adipose tissue was significantly (P<0.05) reduced when the incubations were carried out at 34.5 degrees C (41% inhibition), and 32 degrees C (68% inhibition), with no parallel changes in the ob mRNA expression. At these reduced temperatures, glucocorticoid-mediated leptin secretion was well preserved. When the effect of reduced temperatures was assessed on in vitro GH secretion, a superimposable reduction was observed. CONCLUSIONS: These results indicate: (i) that low temperatures reduce leptin secretion by acting directly on the adipose tissue and (ii) that the similar reduction in a hormone unrelated to energy metabolism, such as GH, suggests that the observed reduction is a mechanical perturbation of leptin secretion, which may be devoid of physiological implications.