Clinical Endocrinology (1998) 49, 659–664

659q 1998 Blackwell Science Ltd

The effect of body composition on hexarelin-inducedgrowth hormone release in normal elderly subjects

A. Rahim, P. O’Neill and S.M. ShaletDepartment of Endocrinology, Christie Hospital,Manchester, UK

(Received 10 February 1998; returned for revision 23 March1998; finally revised 15 April 1998; accepted 7 June 1998)

Summary

OBJECTIVE Growth hormone (GH) release is influ-enced by several factors including age, gender, phy-sical exercise, nutritional status, sex steroids andbody composition. The relationship with body com-position is complex. Obesity is accompanied bysuppression of spontaneous and stimulated GHrelease. As increasing body fat reduces stimulatedGH secretion following a standard provocative test,the potential clinical uses of GH-releasing peptides(GHRPs), therapeutically or diagnostically, may bedependent on the relationship between body fat andGHRP-stimulated GH release. We have thereforeassessed the effect of body composition andgender on the GH releasing capacity of hexarelin.DESIGN A single bolus of subcutaneous hexarelin ata dose of 1·5 mg per kg of body weight was adminis-tered at time 0. Blood samples were taken at – 10, 0,10, 20, 30, 40, 50, 60, 90, 120, 170 and 180 min.SUBJECTS Twenty-one (eight male) healthy elderlysubjects with a median (range) age of 68 (60–81)years and BMI of 26 (19–30) kg/m 2 were studied.METHODS Dual-energy X-ray absorptiometry (DEXA)was used to assess body composition.RESULTS Peak GH response correlated negativelywith fat mass, BMI, percentage body fat, and weight[r ¼¹ 0·72, P ¼ 0·0001; r ¼ ¹ 0·56, P ¼ 0·009; r ¼ ¹ 0·63,P ¼ 0·002 and r ¼ ¹ 0·48, P ¼ 0·029, respectively,]. AUC

GH correlated negatively with fat mass, BMI and per-centage fat mass [ r ¼ ¹ 0·58, P ¼ 0·006; r ¼ ¹ 0·51,P ¼ 0·019 and r ¼ ¹ 0·66, P ¼ 0·001 respectively].Using multiple linear regression, fat mass was themost useful predictor for both peak GH response[R2 ¼ 0·61, P<0·0001] and AUC GH [R2 ¼ 0·38,P ¼ 0·003]. Gender was not a significant variable.

CONCLUSIONS Increasing total fat mass results in ablunted GH response following subcutaneous hexar-elin. Total fat mass appears to be a useful predictor ofpeak GH response even in normal individuals as noneof the subjects in the present study was morbidlyobese. This indicates that there is a continuum ofeffect of fat mass on hexarelin-stimulated GH release.Any impact of gender on the GH response to hexarelinis almost certainly indirect and mediated via differ-ences in body composition.This observation will havean impact on the potential diagnostic and therapeuticuses of hexarelin and related GH secretagogues.

Growth hormone (GH) secretion is regulated by a complexarray of central and peripheral mechanisms with the maincentral control of GH release dependent on the interactionbetween somatostatin and growth hormone releasing hormone(GHRH). The recent development of growth hormone releasingpeptides (GHRPs) (Bowerset al., 1991; Bercuet al., 1992;Renneret al., 1994), and isolation of the GHRP receptor (Ponget al., 1996) have demonstrated however, that this third factoralso affects GH release by influencing GHRH and SS. The GHreleasing capacity of these centrally acting hormones is itselfinfluenced by several factors including age, gender, physicalexercise, nutritional status, sex steroids and body composition(Finkelstein et al., 1972; Rudman, 1985; Hoet al., 1987;Iranmaneshet al., 1991; Weltmanet al., 1994). SpontaneousGH release decreases with increasing age, reduced physicalactivity and is higher in fertile female subjects compared withmales. The relationship with body composition is complex.Obesity is accompanied by suppression of spontaneous(Veldhuiset al., 1991; Iranmaneshet al., 1991) and stimulated(Vahl et al., 1996) GH release. Furthermore obesity may occuras a consequence of GH deficiency in adults (Salomonet al.,1989; De Boeret al., 1992) with GH deficient individualstending to be centrally obese compared with normal adults.Even in healthy clinically nonobese adults, abdominal adiposityappears to be a negative determinant of stimulated GH releasesuggesting a continuum in the relationship between GH releaseand adiposity (Vahlet al., 1996).

As increasing body fat reduces spontaneous and stimulatedGH secretion following a standard provocative test, thepotential clinical uses of GHRPs, therapeutically or diagnos-tically, may to some degree be dependent on the relationshipbetween body fat and GHRP-stimulated GH release. Ghigo

Correspondence: Professor S. M. Shalet, Department ofEndocrinology, Christie Hospital NHS Trust, Wilmslow RoadManchester M20 4BX, UK.

et al. (1994) administered subcutaneous hexarelin to normalsubjects and found that female subjects released slightly lessGH in response to subcutaneous hexarelin compared with malesubjects. The reason for this was unclear and thought to beassociated with the sex-related difference in subcutaneous fatdistribution. To explore the relationship between hexarelin-stimulated GH release and body composition further, we haveassessed the GH response to a subcutaneous injection ofhexarelin in normal elderly subjects, in whom body composi-tion has been measured by DEXA.

Subjects and methods

Subjects

Twenty-one (eight male) healthy elderly subjects with a median(range) age of 68 (60–81) years and BMI of 26 (19–30) kg/m2

were recruited. Five subjects were taking regular medication noneof which interfered with GH release. All were nonsmokers andhad no significant medical problems as determined by medicalhistory, physical examination and screening laboratory analyses.

The study was approved by South Manchester MedicalResearch Ethics Committee and written informed consent wasobtained from each subject. The procedure at the study visit wasas follows; at 0730–0800 h, after an overnight fast, anintravenous cannula was inserted into a vein in the antecubitalfossa. Hexarelin at a dose of 1·5mg per kg body weight wasadministered at time 0 as a subcutaneous injection and bloodwas taken at – 10, 0, 10, 20, 30, 40, 50, 60, 90, 120, 170 and180 min. Only water was allowed during the study visit period.Serum was separated from each sample and stored at¹ 708C, tobe analysed subsequently for GH concentration. The bodycomposition of each subject was then determined using DEXA.

Hexarelin

Hexarelin (His/D-2-Methyl-Trp/Ala/Trp/D-Phe/lys-NH2)(Pharmacia-Upjohn Ltd, Stockholm) was given as a subcuta-neous injection. Hexarelin was supplied as a sterile lyophilizedwhitish powder for subcutaneous injection. Each vial contained100mg hexarelin and 20 mg mannitol. Each vial was recon-stituted in 1 ml of physiological sodium chloride solution. Thestudy product was reconstituted immediately prior to admin-istration and the unconstituted vials were stored between 2 and88C at all times. The composition per ml after reconstitutionwas hexarelin 100mg, mannitol 20 mg and sodium chloride9 mg.

Assays

hGH Growth hormone was assayed using a DELFIA-hGH

assay commercial kit from Wallac Oy, Finland. The DELFIA-hGH assay is a solid phase (microtitreplate), two-sitefluouroimmunometric assay based on the direct sandwichtechnique in which two monoclonal antibodies are directedagainst two separate antigenic determinants on the hGHmolecule. Standards, controls and patient sample containinghGH react simultaneously with immobilized antibodies andeuropium-labelled antibodies directed against a differentantigenic site. Enhancement solution dissociates europiumions from the labelled antibody into solution where they formhighly flourescent chelates with components of the enhance-ment solution. The flourescence from each sample is propor-tional to the concentration of hGH in the sample.

The minimum detectable concentration was 0·03 mU/l. At68 mU/l the intra-assay and interassay coefficients of variation(CV) were 1% and 2·9%, respectively, at 1·1 mU/l the intra-assay and interassay CV was 1·6% and at 0·26 mU/l theinterassay CV was 5·3%. The assay has negligible cross-reactivity against human prolactin, TSH, FSH, and 20 kd GH(<0·001%) and human LH (<0·1%).

Body composition

Body composition was determined using DEXA. All the scanswere performed using the Hologic QDR-4500 A scanner(Hologic Inc, Massachusettes, USA). The scanner employs afan beam dual X-ray absorptiometry method. The coefficientsof variation for the determination of fat mass and fat free masswere 1·0%.

Statistics

Results are expressed as the median (range). Comparisonbetween male and female subjects were made using the Mann–Whitney U-test. The Spearman rank correlation was used todetermine relationships between parameters. The Spearmanrank coefficient is expressed as r. AP-value of less than 0·05was considered statistically significant.

Multiple linear regression analysis was performed to assessthe influence of age, sex, BMI, percentage and total fat massand percentage and total lean mass on predicting peak GHresponse and AUCGH. Both dependent variables, i.e. AUCGH

and peak GH response, were transformed with quarter roots forregression purposes. The coefficient of determination isexpressed as R2. A P-value of less than 0·05 was consideredstatistically significant.

Results

The median (range) age, weight, BMI, total and percentage fatmass, total and percentage lean mass and peak GH response for

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male subjects, female subjects and the two groups combined areshown in Table 1.

Using the Mann–WhitneyU-test, no significant differencewas noted in male subjects compared with females in BMI, fatmass, peak GH response and AUCGH (P¼ 0·8, P¼ 0·21,P¼ 0·5 andP¼ 0·19, respectively). The percentage body fat,not unexpectedly, was significantly greater in females(P¼ 0·003). For the purposes of statistical analysis, the twogroups were combined.

Peak GH response correlated negatively with fat mass (Fig.1), BMI, percentage body fat and weight [r ¼ ¹ 0·72,P¼ 0·0001;r ¼ ¹ 0·56,P¼ 0·009;r ¼ ¹ 0·63,P¼ 0·002 andr ¼ ¹ 0·55 P¼ 0·009 respectively]. Peak GH response corre-lated positively with percentage lean tissue [r ¼ 0·62,P¼ 0·03].Correlation with AUCGH revealed similar results except forweight which did not correlate significantly [r ¼ ¹ 0·17,P¼ 0·51].

Using multiple linear regression with the variables sex, age,BMI, total lean mass, total fat mass and weight, fat mass wasthe best predictor for both peak GH response [R2¼ 0·61,P< 0·0001] and AUCGH [R2¼ 0·29,P¼ 0·012]. Gender wasnot a significant variable in the peak GH response.

In male subjects there was a significant negative correlationbetween the peak GH response and fat mass (Fig. 1), percentagebody fat and BMI [r ¼ ¹ 0·74,P¼ 0·037r ¼ ¹ 0·74,P¼ 0·037andr ¼ ¹ 0·78,P¼ 0·023 respectively]. There was a significantpositive correlation between peak GH response and percentagelean tissue [r ¼ 0·8, P¼ 0·015]. There was no significantcorrelation with total lean tissue [r ¼ ¹ 0·4,P¼ 0·32] or totalbody weight [r ¼¹ 0·62,P¼ 0·1].

In male subjects AUCGH correlated negatively with weight,percentage body fat, fat mass and BMI [r ¼ ¹ 0·71,P¼ 0·047;r ¼ ¹ 0·81, P¼ 0·015; r ¼ ¹ 0·81, P¼ 0·015 andr ¼ ¹ 0·74,P¼ 0·035].

In female subjects there was a significant negative correla-tion between the peak GH response and weight, fat mass (Fig.1) and percentage body fat [r ¼ ¹ 0·79,P¼ 0·001;r ¼¹ 0·7,P¼ 0·008 andr ¼ ¹ 0·62,P¼ 0·0024 respectively]. There wasa significant positive correlation between peak GH responseand percentage lean tissue [r ¼ 0·63,P¼ 0·021] but there wasno significant correlation with total lean tissue [r ¼¹ 0·4,P¼ 0·167]. Correlation with BMI was not significant[r ¼ ¹ 0·53,P¼ 0·067]. In female subjects, AUCGH did notcorrelate significantly with any variable.

Discussion

Morbid obesity is accompanied by suppression of spontaneous(Iranmaneshet al., 1991; Veldhuiset al., 1991) GH release.Veldhuiset al. (1991) observed mean 24 h GH levels in obesemen with a BMI range of 41–58 kg/m2, to be a quarter that of anage-matched control group, BMI 23–33 kg/m2. The lower

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Table 1 Median (range) values for subjects by gender and combined.

Male Female Male and female

Median Range Median Range Median Range

Age (years) 68 67–76 67 60–81 68 60–81Weight (kg) 76·8 62–90 64·1 50–78 71·2 50–90BMI (kg/m2) 28·9 22–30 26·2 19–30 26 19–30Fat mass (kg) 18·2 10·2–26·2 27·4 11·2–32·6 19·4 10·2–32·6Lean mass (kg) 59·9 51·1–64·7 41·9 33·7–46·8 45·2 33·7–64·7Peak GH response (mu/l) 44·3 19·1–73·6 38·2 9·8–81·5 40·6 9·8–81·5% fat mass 22·6 16·4–29 39·7 21·3–44·5 28·6 16·4–44·5% lean mass 78 71·7–83·2 63·5 56·5–80·0 72 56·5–83·2

90

80

70

60

50

40

30

20

10

00 5 10 15 20 25 30 35

Total fat mass (kg)

Peak G

H r

esp

on

se (

mU

/l)

Fig. 1 The relationship between total fat mass and peak GHresponse in male (B) and female (X) subjects.

endogenous GH levels in the obese were due partly to increasedGH removal rates from the circulation but also a 3-folddecrease in GH secretory burst frequency in obese men. Asspontaneous GH secretory bursts are due to hypothalamicallyderived GHRH stimulation of somatotrophs during periodswhen somatostatin tone is reduced, the alteration in secretoryburst frequency led Veldhuiset al. (1991) to conclude thatobesity results in decreased GHRH stimulation and/or exces-sive somatostatin inhibitory tone. Furthermore, adiposity evenwhen not associated with morbid obesity, affects spontaneousGH release. In subjects with near normal weight, relativeobesity, as determined by BMI, has been shown to correlatenegatively with circulating endogenous GH levels (Iranmaneshet al., 1991; Weltmanet al., 1994; Vahl et al., 1996). Bothreduced GH production, as a consequence of reduced GHsecretory burst amplitude, and increased GH clearance(Iranmaneshet al., 1991) account for the reduced circulatingGH levels with each unit increase in BMI, at a given age,reducing daily GH production by 6% (Iranmaneshet al., 1991).Substantial weight loss may restore both spontaneous (Wil-liams et al., 1984; Rasmussenet al., 1995) and stimulated(Rasmussenet al., 1995) GH secretion.

Although the effects of obesity on stimulated GH releaseusing standard provocative agents have been well documented,there are few data regarding the influence of body fat onstimulated GH release using GHRPs. Cordidoet al. (1993))demonstrated that GHRP-6 is capable of causing GH release inobese subjects and Micicet al. (1993) reported reduced GHrelease in response to GHRP-6 in obese subjects compared withnormal controls. In line with these latter observations hexarelinis capable of stimulating an attenuated GH response in obeseadult subjects (Grottoliet al., 1996) and obese children (Locheet al., 1995). Using standard provocative agents in clinicallynonobese healthy adults, Vahlet al. (1996) observed a negativecorrelation between stimulated GH release and relativeadiposity particularly in the abdominal region. There are nodata on the influence of adiposity on GHRP-stimulated GHrelease in normal, nonobese subjects. We have demonstratedthat adiposity influences hexarelin-stimulated GH release witha reduction in the peak GH response and AUCGH as fat massincreases. Furthermore, we have demonstrated a linear relation-ship between hexarelin-stimulated GH release and fat mass.This is an important observation as it adds support to thefindings by Vahlet al. (1996) that stimulated GH release isinfluenced by relative adiposity and not only morbid obesity.Fat mass was found to be the best predictor of peak GHresponse to hexarelin. The potential use of hexarelin and similaragents may therefore be affected by the body composition ofthe individuals in whom such agents are to be used.

The mechanism by which adiposity affects stimulated GHrelease is complex with free fatty acids (FFA) and triglycerides

(TG) possibly playing a role (Tsushimaet al., 1970; Blackardet al., 1971; Muggeoet al., 1975; Imakiet al., 1985). Muggeoet al. (1975) demonstrated altered GH secretion in bothspontaneous and experimental hyperlipidaemia suggesting aninhibitory role for FFA in the regulation of GH secretion.Muggeoet al. (1975) also demonstrated that elevated TG levelscould inhibit the GH response to some stimuli. Further supportfor the role of FFA in GH release comes from the observedincrease in stimulated GH release, to a variety of GHsecretagogues, which occurs with coadministration of acipi-mox, a nicotinic acid analogue that blocks lipolysis and resultsin a reduction of FFA (Leeet al., 1995; Cordidoet al., 1996;Maccarioet al., 1996). The pituitary somatotroph is thought tobe the possible site of action (Alvarezet al., 1991).

Leptin concentrations are highly correlated with percentagefat mass (Maffeiet al., 1995; Considineet al., 1996) and leptinmRNA expression is increased in obesity (Maffeiet al., 1995;Hamiltonet al., 1995; Lonnqvistet al., 1995). Hence it has beenproposed that fat mass is accurately reflected by serum leptinlevels. We have recently demonstrated (Gillet al., 1997) thatthere is a significant negative effect of leptin upon spontaneousGH secretion and postulated that leptin may also provide anadditional signal from fat reserves to inhibit spontaneous GHsecretion. The exact influence of leptin on stimulated GHrelease however, remains to be determined.

Body fat is itself dependent on gender and so it could beargued that gender rather than body fat had influencedhexarelin-stimulated GH release. If gender had influencedthe GH response to hexarelin then it is likely that this would berelated to sex steroids. Although spontaneous and stimulatedGH release by standard provocative agents are influenced bysex steroids, and animal data have demonstrated thatpretreatment with oestrogens enhances the somatotrophresponsiveness to GHRP-6 (Malloet al., 1993), data onGHRP-stimulated GH in man have not been able to confirmthis. No sex-related difference in the GH response tointravenous hexarelin or other GHRPs has been demonstratedin young adults (Penalvaet al., 1993; Ghigoet al., 1994).Furthermore, Arvatet al. (1997) have recently demonstratedthat in elderly female subjects, the GH response before andafter the administration of oestrogen is not significantlydifferent, implying that the role of sex steroids in hexarelin-stimulated GH release is minimal. The female subjects in thepresent study were all postmenopausal and furthermore thepeak GH response and AUCGH were not significantlydifferent between the two sexes. These observations supportour hypothesis that gender influence on body composition,rather than the influence of sex steroids on the neuroregulatorycontrol of GH, is responsible for the differences seen in theGH response to hexarelin. Therefore the influence of genderon the GH response to hexarelin is indirect.

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An alternative interpretation of our findings is that gender-specific fat distribution influences the bioavailability ofsubcutaneously administered hexarelin and thus may havemodified the GH response to hexarelin in our study. Thishypothesis requires testing by pharmacokinetic studies oralternatively assessment of GH response after intravenousadministration of hexarelin.

In summary, we have demonstrated that increased body fat,both total and percentage, result in a blunted peak GH responsefollowing subcutaneous hexarelin. Total fat mass appears to bea useful predictor of peak GH response even in normalindividuals as none of the subjects in the present study wasmorbidly obese. This indicates that there is a continuum ofeffect of fat mass on hexarelin-stimulated GH release. Anyimpact of gender on the GH response to hexarelin is almostcertainly indirect and mediated via differences in bodycomposition.This observation will have an impact on thepotential diagnostic and therapeutic uses of hexarelin andrelated GH secretagogues.

Acknowledgements

The authors would like to thank Kate Roberts for her helpduring the study and Mark Dougal for his statistical advice. Wewould also like to thank Christina Herder, (Pharmacia &Upjohn, Stockholm), for her help throughout the study andPharmacia & Upjohn for their support.

References

Alvarez, C.V., Mallo, F., Burguera, B., Cacicedo, L., Dieguez, C. &Casanueva, F.F. (1991) Evidence for a direct pituitary inhibition byfree fatty acids ofin vivo growth hormone responses to growthhormone-releasing hormone in the rat.Neuroendocrinology, 53,185–189.

Arvat, E., Giannotti, L., Broglio, F., Maccagno, B., Bertagna, A.,Deghenghi, R., Camanni, F. & Ghigo, E. (1997) Oestrogenreplacement does not restore the reduced GH-releasing activity ofHexarelin, a synthetic hexapeptide, in post menopausal women.European Journal of Endocrinology, 136,483–487.

Bercu, B.B., Yang, S.W., Masuda, R. & Walker, R.F. (1992) Role ofselected endogenous peptides in growth hormone-releasing hexapep-tide activity: analysis of growth hormone-releasing hormone, thyroidhormone-releasing hormone, and gonadotropin-releasing hormone.Endocrinology, 130,2579–2586.

Blackard, W.G., Hull, E.W. & Lopez, A. (1971) Effect of lipids ongrowth hormone secretion in humans.Journal of Clinical Investiga-tion, 50, 1439–1443.

Bowers, C.Y., Sartor, A.O., Reynolds, G.A. & Badger, T.M. (1991) Onthe actions of the growth hormone-releasing hexapeptide, GHRP.Endocrinology, 128,2027–2035.

Considine, R.V., Sinha, M.K., Heiman, M.L., Kriauciunas, A.,Stephens, T.W., Nyce, M.R., Ohannesian, J.P., Marco, C.C.,McKee, L.J., Bauer, T.L. & Caro, J.F. (1996) Serum immunoreac-tive-leptin concentrations in normal-weight and obese humans.NewEngland Journal of Medicine, 334,292–295.

Cordido, F., Peino, R., Penalva, A., Alvarez, C.V., Casanueva, F.F. &Dieguez, C. (1996) Impaired growth hormone secretion in obesesubjects is partially reversed by acipimox-mediated plasma free fattyacid depression.Journal of Clinical Endocrinology and Metabolism,81, 914–918.

Cordido, F., Penalva, A., Dieguez, C. & Casanueva, F.F. (1993)Massive growth hormone (GH) discharge in obese subjects after thecombined administration of GH-releasing hormone and GHRP-6:evidence for a marked somatotroph secretory capability in obesity.Journal of Clinical Endocrinology and Metabolism, 76, 819–823.

De-Boer, H., Blok, G.J., Voerman, H.J., De Vries, P.M. & van derVeen, E.A. (1992) Body composition in adult growth hormone-deficient men, assessed by anthropometry and bioimpedanceanalysis.Journal of Clinical Endocrinology and Metabolism, 75,833–837.

Finkelstein, J.W., Roffwarg, H.P., Boyar, R.M., Kream, J. & Hellman,L. (1972) Age-related change in the twenty-four-hour spontaneoussecretion of growth hormone.Journal of Clinical Endocrinology andMetabolism, 35, 665–670.

Ghigo, E., Arvat, E., Gianotti, L., Imbimbo, B.P., Lenaerts, V.,Deghenghi, R. & Camanni, F. (1994) Growth hormone-releasingactivity of hexarelin, a new synthetic hexapeptide, after intravenous,subcutaneous, intranasal, and oral administration in man.Journal ofClinical Endocrinology and Metabolism, 78, 693–698.

Gill, M.S., Toogood, A.A., O’Neill, P.A., Adams, J.E., Thorner, M.O.,Shalet, S.M. & Clayton, P.E. (1997) Relationship between growthhormone (GH) status, serum leptin and body composition in healthyand GH deficient elderly subjects.Clinical Endocrinology, 47,161–167.

Grottoli, S., Maccario, M., Procopio, M., Oleandri, S.E., Arvat, E.,Gianotti, L., Deghenghi, R., Camanni, F. & Ghigo, E. (1996)Somatotrope responsiveness to Hexarelin, a synthetic hexapeptide, isrefractory to the inhibitory effect of glucose in obesity.EuropeanJournal of Endocrinology, 135,678–682.

Hamilton, B.S., Paglia, D., Kwan, A.Y.M. & Deitel, M. (1995)Increased obese mRNA expression in omental fat cells frommassively obese humans.Nature Medicine, 1, 953–956.

Ho, K.Y., Evans, W.S., Blizzard, R.M., Veldhuis, J.D., Merriam, G.R.,Samojlik, E., Furlanetto, R., Rogol, A.D., Kaiser, D.L. & Thorner,M.O. (1987) Effects of sex and age on the 24-hour profile of growthhormone secretion in man: importance of endogenous estradiolconcentrations.Journal of Clinical Endocrinology and Metabolism,64, 51–58.

Imaki, T., Shibasaki, T., Shizume, K., Masuda, A., Hotta, M.,Kiyosawa, Y., Jibiki, K., Demura, H., Tsushima, T. & Ling, N.(1985) The effect of free fatty acids on growth hormone (GH) -releasing hormone-mediated GH secretion in man.Journal ofClinical Endocrinology and Metabolism, 60, 290–293.

Iranmanesh, A., Lizarralde, G. & Veldhuis, J.D. (1991) Age and relativeadiposity are specific negative determinants of the frequency andamplitude of growth hormone (GH) secretory bursts and the half-lifeof endogenous GH in healthy men.Journal of Clinical Endocrinol-ogy and Metabolism, 73, 1081–1088.

Lee, E.J., Nam, S.Y., Kim, K.R., Lee, H.C., Cho, J.H., Nam, M.S.,Song, Y.D., Lim, S.K. & Huh, K.B. (1995) Acipimox potentiatesgrowth hormone (GH) response to GH-releasing hormone with orwithout pyridostigmine by lowering serum free fatty acid in normaland obese subjects.Journal of Clinical Endocrinology andMetabolism, 80, 2495–2498.

Loche, S., Cambiaso, P., Carta, D., Setzu, S., Imbimbo, B.P., Borrelli,P., Pintor, C. & Cappa, M. (1995) The growth hormone-releasing

Hexarelin, body composition and GH release 663

q 1998 Blackwell Science Ltd,Clinical Endocrinology, 49, 659–664

activity of hexarelin, a new synthetic hexapeptide, in short normaland obese children and in hypopituitary subjects.Journal of ClinicalEndocrinology and Metabolism, 80, 674–678.

Lonnqvist, F., Arner, P., Nordfors, L. & Schalling, M. (1995)Overexpression of the obese (ob) gene in adipose tissue of humanobese subjects.Nature Medicine, 1, 950–953.

Maccario, M., Procopio, M., Grottoli, S., Oleandri, S.E., Boffano, G.M.,Taliano, M., Camanni, F. & Ghigo, E. (1996) Effects of acipimox, anantilipolytic drug, on the growth hormone (GH) response to GH-releasing hormone alone or combined with arginine in obesity.Metabolism, 45, 342–346.

Maffei, M., Halaas, J., Ravussin, E., Pratley, R.E., Lee, G.H., Zhang, Y.,Fei, H., Kim, S., Lallone, R., Ranganathan, S., Kern, P.A. &Friedman, J.M. (1995) Leptin levels in human and rodent:measurement of plasma leptin and ob RNA in obese and weight-reduced subjects.Nature Medicine, 1, 1155–1161.

Mallo, F., Alvarez, C.V., Benitez, L., Burguera, B., Coya, R.,Casanueva, F.F. & Dieguez, C. (1993) Regulation of His-dTrp-Ala-Trp-dPhe-Lys-NH2 (GHRP-6) -induced GH secretion in the rat.Neuroendocrinology, 57, 247–256.

Micic, D., Mallo, F., Peino, R., Cordido, F., Leal Cerro, A., GarciaMayor, R.V. & Casanueva, F.F. (1993) Regulation of growthhormone secretion by the growth hormone releasing hexapeptide(GHRP-6).Journal of Pediatric Endocrinology, 6, 283–289.

Muggeo, M., Tiengo, A., Fedele, D. & Crepaldi, G. (1975) Theinfluence of plasma triglycerides on human growth hormoneresponse to arginine and insulin: a study in hyperlipemics andnormal subjects.Hormone Metabolism and Research, 7, 367–374.

Penalva, A., Pombo, M., Carballo, A., Barreiro, J., Casanueva, F.F. &Dieguez, C. (1993) Influence of sex, age and adrenergic pathways onthe growth hormone response to GHRP-6.Clinical Endocrinology,37, 87–91.

Pong, S.S., Chaung, L.Y., Dean, D.C., Nargund, R.P., Patchett, A.A. &Smith, R.G. (1996) Identification of a new G-protein-linked receptorfor growth hormone secretagogues.Molecular Endocrinology, 10,57–61.

Rasmussen, M.H., Hvidberg, A., Juul, A., Main, K.M., Gotfredsen, A.,Skakkebaek, N.E. & Hilsted, J. (1995) Massive weight loss restores

24-hour growth hormone release profiles and serum insulin-likegrowth factor-I levels in obese subjects.Journal of ClinicalEndocrinology and Metabolism, 80, 1407–1415.

Renner, U., Brockmeier, S., Strasburger, C.J., Lange, M., Schopohl, J.,Muller, O.A., von Werder, K. & Stalla,, G.K. (1994) Growthhormone (GH) -releasing peptide stimulation of GH release fromhuman somatotroph adenoma cells: interaction with GH× releasinghormone, thyrotropin-releasing hormone, and octreotide.Journal ofClinical Endocrinology and Metabolism, 78, 1090–1096.

Rudman, D. (1985) Growth hormone, body composition, and aging.Journal of American Geriatric Society, 3, 800–807.

Salomon, F., Cuneo, R.C., Hesp, R. & Sonksen, P.H. (1989) The effectsof treatment with recombinant human growth hormone on bodycomposition and metabolism in adults with growth hormonedeficiency.New England Journal of Medicine, 321,1797–1803.

Tsushima, T., Sakuma, M. & Irie, M. (1970) Effects of changes inplasma free fatty acid levels on secretion of human growth hormone.Endocrinology-Japan, 17, 369–377.

Vahl, N., Jorgensen, J.O., Jurik, A.G. & Christiansen, J.S. (1996)Abdominal adiposity and physical fitness are major determinants ofthe age associated decline in stimulated GH secretion in healthyadults. Journal of Clinical Endocrinology and Metabolism, 81,2209–2215.

Veldhuis, J.D., Iranmanesh, A., Ho, K.K., Waters, M.J., Johnson, M.L.& Lizarralde, G. (1991) Dual defects in pulsatile growth hormonesecretion and clearance subserve the hyposomatotropism of obesityin man.Journal of Clinical Endocrinology and Metabolism, 72,51–59.

Weltman, A., Weltman, J.Y., Hartman, M.L., Abbott, R.D., Rogol,A.D., Evans, W.S. & Veldhuis, J.D. (1994) Relationship betweenage, percentage body fat, fitness, and 24-hour growth hormonerelease in healthy young adults: effects of gender.Journal of ClinicalEndocrinology and Metabolism, 78, 543–548.

Williams, T., Berelowitz, M., Joffe, S.N., Thorner, M.O., Rivier, J.,Vale, W. & Frohman, L.A. (1984) Impaired growth hormoneresponses to growth hormone-releasing factor in obesity. A pituitarydefect reversed with weight reduction.New England Journal ofMedicine, 311,1403–1407.

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