J. Endocrinol. Invest. 14: 265-268, 1991
Sex differences in growth hormone response to growth hormone-releasing hormone P. Benito*, L. Avila*, M.S. Corpas*, J.A. Jimenez*, L. Cacicedo**, and F. Sanchez Franco** *Endocrine Section, Department of Internal Medicine, Reina Sofia Hospital, Cordoba, and **Endocrine Division, Ram6n y Cajal Hospital, Madrid, Spain ABSTRACT. The aim of the present study was to ascertain whether sex differences exist in GH response to GHRH, and the influence that menstrual cycle have on this response. A GHRH test was performed on nine healthy men and ten women on days one and twelve of the menstrual cycle. Basal GH levels, (mean ± SE) (7.98 ± 3.09 ng/ml in women and 0.13 ± 0.07 ng/ml in men, p < 0.05) as well as maximal GH response (40.17 ± 11.96 ng/ml in women and 9.63 ± 2.32 ng/ml in males, p < 0.01)
were significantly higher in women than in men. In spite of a significant increase in estradiol levels during the menstrual cycle (75.88 ± 2.48 pg/ml on day one and 198.40 ± 28.65 pg/ml on day twelve, p < 0.01) neither basal plasma GH levels, nor GH response to GHRH were significantly different. In conclusion, these results confirm that GH response to GHRH is higher in women than in men, and that this difference is not modified during the first phase of the menstrual cycle.
INTRODUCTION
these stimuli in women than in men, suggests that estradiol plays a more important role that testosterone on the regulation of GH secretion. In fact, the reversion of the effect of testosterone on GH secretion after estradiol receptor blockade with tamoxifen (17), and the absence of the effect of oxandrolone (a nonaromatizable androgen) (13) on GH secretion, suggests that testosterone needs to be aromatized to estradiol before it can act (1, 18), This study was conducted in an effort to clarify both issues, sex differences in GH response to GHRH, and the influence that the physiological elevation of the endogenous estradiol levels during the first phase of the menstrual cycle, have on this response in women.
The influence of sex on pituitary GH responsiveness to GHRH in humans, has not yet been well established (1). In some studies, no significant differences were observed in GH secretion in response to GHRH stimulus between the sexes (2, 3), whereas others have shown a greater response in men (4) or in women (5, 6). Likewise, differences in the GH secretory pattern in both sexes, as well as the main gonadal steroid that influences GH secretion in humans, are not clearly established. Although the integrated GH concentration (IGHC), but not GH response to GHRH (7), increases with puberty in both sexes (8, 9), the steroid responsible of it, still remains under discussion (9, 10); thus some studies have demonstrated a clear correlation between IGHC and estradiol levels (10), but others have failed to support it (11, 12). Furthermore, the administration of exogenous testosterone to men (13) or oral contraceptive steroids to women (14) both seem to increase GH secretion. Additionally, estradiol as well as testosterone administration are effective in increasing the GH peak after classical stimuli (15, 16), but the higher GH response to
SUBJECTS AND METHODS Nineteen healthy volunteers (9 men and 10 women) aged from 23 to 29 years were studied, the women on days one and twelve of the menstrual cycle. Written informed consent from all subjects was obtained. After an overnight fast, an indwelling catheter was placed in an antebrachial vein for blood withdrawal. The study was done in recumbency, and none of the subjects was on medication. A standard GHRH test (GRF 1-29 NH2 Serono, at a dose of 1 iJ,g/kg iv) was performed at 09:00. Blood samples for GH were obtained at -15, 0, 15, 30, 45, 60, and 90 minutes after GHRH injection. Basal serum estradiol, progesterone, FSH, LH and prolactin were determined in women on both days.
Key-words. Growth hormone, growth hormone-releasing hormone, GHRH test, menstrual cycle. Correspondence: P. Benito L6pez, Secci6n de Endocrinologia, Departamento de Medicina Interna, Hospital Reina Sofia, Avda Menendez Pidal sin. 14004, C6rdoba, Espana. Received May 14,1990; accepted December 15, 1990.
265
P. Benito, L. Avila, M.S. Carpas, et a/.
Table 1. - Growth hormone response to GHRH (mean ± SE) in men (MJ, women on day one (W1) and women on day twelve (W12) of the menstrual cycle.
Statistical analysis was performed using ANOVA, Student's ttest for paired data, Bartlett's test for homogeneity of variances and Pearson's coefficient of correlation test. Results are expressed as mean ± SEM. Statistical significance was established at p < 0.05.
GH (ng/ml)
M
W1
-15.-
0.51 ± 0.39
9.87 ± 403 '
6.45 ± 1.89
0.-
0.13± 0.07
7.89± 3.09 '
7.76 ± 2.91
15.-
7.40± 2.14
40.17± 119 ' .2
37.34 ± 10.37
30.-
9.63 ± 2.322
35.18± 9.53 '
35.41 ± 9.09
45.-
9.33± 2.25
28.52± 6.74 '
21.87 ± 5.59
60.-
7.45± 1.51
22.62 ± 5.23 '
19.80 ± 4.84
90.-
3.97 ± 1.30
11.15± 397 '
11.42 ± 3.29
Time (min.)
'p < 0.05 vs males;
W12
RESULTS
Table 1 and Figure 1 show the GH response to GHRH in men and women. Basal GH levels were higher in women (7.98 ± 3.09 ng/ml) than in men (0.13 ± 0.07 ng/ml) (p < 0.05). Maximal GH response was obtained at 30 minutes in men (9.63 ± 2.32 ng/ml) (p < 0.01) and at 15 minutes in women (40.17 ± 11.96 ng/ml) (p < 0.01). The maximum net increment of GH after GHRH was significantly higher in women (36.07 ± 9.66 ng/ml) than in men (11.04 ± 2.36 ng/ml) (p < 0.03). The variance in GH response to GHRH was similar in both sexes. In spite of a significant increase on estradiol levels from day one (75.88 ± 2.48 pg/ml) to day twelve (198.40 ± 28.65 pg/ml) (p < 0.01), basal GH levels (7.89 ± 3.09 ng/ml on day one, vs 7.76 ± 2.91 ng/ml on day twelve) as well as maximum GH peaks (40.17 ± 11.96 ng/ml on day one versus 37.34 ± 10.37 ng/ml on day twelve) were not different. There was no correlation between plasma estradi01 levels and the GH increase after GHRH stimulation. Progesterone, prolactin, FSH and LH were similar on both days.
2p < 0.01 vs basal
Serum was immediately separated and stored at 20 C until assayed. GH was measured by double antibody RIA in aliquots duplicate in the same assay using an antibody supplied by Serono (Italy), and second antibody precipitation. The intraassay coefficient of variation was 7.8% and the lower limit of assay detection was 0.24 ng/ml. Estradiol was determined in the same assay by Fluorometric Immunoassay (Delfia®, Pharmacia). The sensitivity of the assay was 13.6 pg/ml and the intraassay coefficient of variation was 5%. Progesterone, FSH, LH and prolactin were also determined by Fluorometric Inmunoassay (Delfia®, Pharmacia). 60
•• *
50
o-w
1",1 .
40
~I
e---M
*
~I
20
•• 10
,/' -15
o
/'
'"
T . _ ... ---~-
15
..1-----1
~b
.- --
---- ... --. .. T
..
30
45
60
266
----.- i
-.
90
Fig. 1 - Growth hormone (mean ± SE) response to GHRH in men (e) and women on day one of the menstrual cycle (0). *p < 005 vs men. **p < 0.01 vs basal.
Sex influence on GH response to GHRH
DISCUSSION
ACKNOWLEDGMENTS
Sex differences in GH response to GHRH are yet uncleared. While some investigators have reported higher responsiveness in men than in women (4), others have shown no difference (2) or even higher response in women (5, 6). Moreover, it has been suggested that testosterone may either increase or not modify (17, 19) GH responsiveness to GHRH when administered previous to this stimulus. These discrepancies may be explained by the difference in testosterone doses used. This study presents evidence that GH response to GHRH in women is higher than in men, supporting the hypothesis that estradiol could increase the sensitivity of the somatotrope cells to GHRH. Discrepancies with other studies can not be explained by the age of subjects, the dose of GHRH used, or the hour at which the test was performed, since they are similar. The great variability in GH response to GHRH (2) could explain these different results. This variability has been attributed to the phase of GH endogenous secretion at the beginning of the test. According to this hypothesis, Devesa et. al suggest that GH response to GHRH would be higher in the secretory than in the refractory phase (5), in such a way that the greater percentage of women (80%) than men (20%) being in secretory phase in his study, might explain the higher GH response to GHRH in women found there. In our study, 90% of the women and 10% of the men were in secretory phase similarly to the study of Devesa, but the magnitude of the response of GH to GHRH does not correlate with the phase at the beginning of the test. As has been previously described (2, 17), in our study GH response to GHRH was similar throughout the menstrual cycle, as opposed to the hypothesis that endogenous estradiol levels are responsible for the difference in GH response to GHRH in both sexes. One possible explanation is that estradiol may potentiate somatotrope cell responsiveness to GHRH in an "on-off" mode, in such a way that on the first day of the menstrual cycle, estradiol levels are sufficient to increase GH response to GHRH. Another possibility is that estradiol acts in women by conditioning a different maturation in the GH axis during puberty, the subsequent response to GHRH being independent of the endogenous estradiol levels at the beginning of the test. The latter hypothesis may explain the absence of a significant change in the GH response to GHRH in hypoestrogenic women (20) or following the administration of a GnRH agonist (17).
We acknowledge with thanks the assistance of Mrs R. Adam in the execution of this study
REFERENCES 1. Tresguerres JAF., Lima L., Arce V., Devesa J. Controversies in the GH response to GHRH in man. In: Casanueva F., Dieguez C. (Eds.), Recent advances in basic and clinical neuroendocrinology. Excerpta Medica International Congress Series 864, Amsterdam, 1989, p 255.
2. Gelato M.C., Pescovitz O.H., Cassorla F., Loriaux D.L., Merriam G.R. Dose response relationships for the effect of growth hormone releasing factor (1-44) NH2 in young adult men and women. J. Clin. Endocrinol. Metab. 59: 197, 1984. 3. Williams T., Berelowitz M., Joffe S.N., Thorner M.O., Rivier J., Vale W., Frohman LA. Impaired growth hormone responses to growth hormone releasing factor in obesity. A pituitary defect reversed with weight reduction. N. Engl. J. Med. 311: 1403,1984. 4. Smals A.E.M., Pieters G.F.M., Smals A.G.H., Benraad T.J., Van Laarhoven J., Kloppenborg P.W.C. Sex difference in human growth hormone (GH) response to intravenous human pancreatic GHReleasing hormone administration in young adults. J. Clin. Endocrinol. Metab. 62: 336,1986. 5. Devesa J., Lima L., Lois N., Fraga C., Lechuga M.J., Arce V., Tresguerres JAF. Reasons for the variability in GH response to GHRH challenge: The endogenous HypothalamicSomatotroph rhythm (HSR). Clin. Endocrinol (Oxf.) 30: 367,1989. 6. Lang I., Schernthaner G., Pietschmann P., Kurz R., Stephenson J., Tempi H. Effects of sex and age on growth hormone response to growth hormone-releasing hormone in healthy individuals. J. Clin. Endocrinol. Metab. 65: 535,1987. 7. Gelato M., Malozowski S., Caruso-Nicoletti M., Levine J, Pescovitz 0., Rose S., Loriaux D., Cassorla F., Merriam G. Growth hormone (GH) responses to GH-Releasing hormone during pubertal development in normal boys and girls: Comparison to idiopatic short stature and GH deficiency. J. Clin. Endocrinol. Metab 63: 174,1986. 8. Miller J.D., Tannenbaum G.S., Colle E., Guyda H.J. Daytime pulsatile growth hormone secretion during childhood and adolescence. J. Clin. Endocrinol. Metab. 55: 989,1982. 9. Zadik Z., Chalew S.A., Me Carter R.J., Meistas M.,
Kowarsky A.A. The influence of age on the twenty-four hour inte-
267
P. Benito, L. Avila, M.5. Corpas, et al.
10.
11.
12.
13.
14.
grated concentration of growth hormone in normal individuals. J. Clin. Endocrinol. Metab. 60.· 513, 1985. Ho K.Y., Evans S., Blizard R.M., Veldhuis J.D., Merriam G.R., Samojlik E., Furlanetto R., Rogol AD., Kaiser D.L., Thorner M.O. Effects of sex and age on the 24 hour profile of growth hormone secretion in man: importance of endogenous estradiol concentrations. J. Clin. Endocrinol. Metab. 64: 51,1987. Spiliotis B.E., August G.P., Hung W., Sonis W., Mendelson W., Bercu B.B. Growth hormone neurosecretory dysfuction: a treatable cause of short stature. JAMA 251: 2233,1984. Thompson A.G., Rodriguez A., Kowarsky A, Migeon G.J., Blizzard R.M. Integrated concentrations of growth hormone correlated with plasma testosterone and bone age in preadolescent and adolescent males. J. Clin. Endocrinol. Metab. 35: 344, 1972. Link R., Blizzard R.M., Evans W.S., Kaiser D.L., Parker MW., Rogol AD. The effect of androgens on the pulsatile release and the twenty-four-hour mean concentration of growth hormone in peri pubertal males. J. Clin. Endocrinol. Metab. 62: 159,1986. Thompson R.G., Rodriguez A, Kowarsky A, Blizzard A.M. Growth hormone metabolic clearance rates, integrated concentrations and production rates in normal adults and the effects of prednisone. J. Clin. Invest. 51: 3193,1972.
15. Merimee T.J., Fineberg S.E. Studies of the sex based variation of human growth hormone secretion. J. Clin. Endocrinol. Metab. 33: 896, 1971. 16. Martin L.G., Grossman M.S., Connor T.B., Levitsky L.L., Clark JW., Camitta FD. Effect of androgen on growth hormone secretion and growth in boys with short stature. Acta Endocrinol (Copenh.) 91: 201, 1979. 17. Lima L., Arce V., Lois N., Fraga C., Lechuga M.J., Tresguerres J.F., Devesa J. Growth hormone responsiveness to GHRH in normal adults is not affected by short term gonadal blockade. Acta Endocrinol. (Copenh.) 120: 31, 1989. 18. Bourgignon J., Linear growth as a function of age at onset of puberty and sex steroid dosage. Therapeutic implications. Endocr. Rev. 9: 467, 1988. 19. Avila L., Benito P., Corpas M.S., Quesada J.M., Castro M. Influencia de la testosterona sobre la secreci6n de hormona de crecimiento. Estudio preliminar. Endocrinologfa 36. 233, 1989. 20. Casanueva F.F., Borras C.G., Burguera B., Lima L. , Muruais C., Tresguerres JAF., Devesa J. Growth hormone and prolactin secretion after growth hormone releasing hormone administration in anorexia nervosa patients, normal controls and Tamoxifen pretreated volunteers. Clin Endocrinol. (Oxf.) 27: 517,1987.
268
Sex differences in growth hormone response to growth hormone-releasing hormone P. Benito*, L. Avila*, M.S. Corpas*, J.A. Jimenez*, L. Cacicedo**, and F. Sanchez Franco** *Endocrine Section, Department of Internal Medicine, Reina Sofia Hospital, Cordoba, and **Endocrine Division, Ram6n y Cajal Hospital, Madrid, Spain ABSTRACT. The aim of the present study was to ascertain whether sex differences exist in GH response to GHRH, and the influence that menstrual cycle have on this response. A GHRH test was performed on nine healthy men and ten women on days one and twelve of the menstrual cycle. Basal GH levels, (mean ± SE) (7.98 ± 3.09 ng/ml in women and 0.13 ± 0.07 ng/ml in men, p < 0.05) as well as maximal GH response (40.17 ± 11.96 ng/ml in women and 9.63 ± 2.32 ng/ml in males, p < 0.01)
were significantly higher in women than in men. In spite of a significant increase in estradiol levels during the menstrual cycle (75.88 ± 2.48 pg/ml on day one and 198.40 ± 28.65 pg/ml on day twelve, p < 0.01) neither basal plasma GH levels, nor GH response to GHRH were significantly different. In conclusion, these results confirm that GH response to GHRH is higher in women than in men, and that this difference is not modified during the first phase of the menstrual cycle.
INTRODUCTION
these stimuli in women than in men, suggests that estradiol plays a more important role that testosterone on the regulation of GH secretion. In fact, the reversion of the effect of testosterone on GH secretion after estradiol receptor blockade with tamoxifen (17), and the absence of the effect of oxandrolone (a nonaromatizable androgen) (13) on GH secretion, suggests that testosterone needs to be aromatized to estradiol before it can act (1, 18), This study was conducted in an effort to clarify both issues, sex differences in GH response to GHRH, and the influence that the physiological elevation of the endogenous estradiol levels during the first phase of the menstrual cycle, have on this response in women.
The influence of sex on pituitary GH responsiveness to GHRH in humans, has not yet been well established (1). In some studies, no significant differences were observed in GH secretion in response to GHRH stimulus between the sexes (2, 3), whereas others have shown a greater response in men (4) or in women (5, 6). Likewise, differences in the GH secretory pattern in both sexes, as well as the main gonadal steroid that influences GH secretion in humans, are not clearly established. Although the integrated GH concentration (IGHC), but not GH response to GHRH (7), increases with puberty in both sexes (8, 9), the steroid responsible of it, still remains under discussion (9, 10); thus some studies have demonstrated a clear correlation between IGHC and estradiol levels (10), but others have failed to support it (11, 12). Furthermore, the administration of exogenous testosterone to men (13) or oral contraceptive steroids to women (14) both seem to increase GH secretion. Additionally, estradiol as well as testosterone administration are effective in increasing the GH peak after classical stimuli (15, 16), but the higher GH response to
SUBJECTS AND METHODS Nineteen healthy volunteers (9 men and 10 women) aged from 23 to 29 years were studied, the women on days one and twelve of the menstrual cycle. Written informed consent from all subjects was obtained. After an overnight fast, an indwelling catheter was placed in an antebrachial vein for blood withdrawal. The study was done in recumbency, and none of the subjects was on medication. A standard GHRH test (GRF 1-29 NH2 Serono, at a dose of 1 iJ,g/kg iv) was performed at 09:00. Blood samples for GH were obtained at -15, 0, 15, 30, 45, 60, and 90 minutes after GHRH injection. Basal serum estradiol, progesterone, FSH, LH and prolactin were determined in women on both days.
Key-words. Growth hormone, growth hormone-releasing hormone, GHRH test, menstrual cycle. Correspondence: P. Benito L6pez, Secci6n de Endocrinologia, Departamento de Medicina Interna, Hospital Reina Sofia, Avda Menendez Pidal sin. 14004, C6rdoba, Espana. Received May 14,1990; accepted December 15, 1990.
265
P. Benito, L. Avila, M.S. Carpas, et a/.
Table 1. - Growth hormone response to GHRH (mean ± SE) in men (MJ, women on day one (W1) and women on day twelve (W12) of the menstrual cycle.
Statistical analysis was performed using ANOVA, Student's ttest for paired data, Bartlett's test for homogeneity of variances and Pearson's coefficient of correlation test. Results are expressed as mean ± SEM. Statistical significance was established at p < 0.05.
GH (ng/ml)
M
W1
-15.-
0.51 ± 0.39
9.87 ± 403 '
6.45 ± 1.89
0.-
0.13± 0.07
7.89± 3.09 '
7.76 ± 2.91
15.-
7.40± 2.14
40.17± 119 ' .2
37.34 ± 10.37
30.-
9.63 ± 2.322
35.18± 9.53 '
35.41 ± 9.09
45.-
9.33± 2.25
28.52± 6.74 '
21.87 ± 5.59
60.-
7.45± 1.51
22.62 ± 5.23 '
19.80 ± 4.84
90.-
3.97 ± 1.30
11.15± 397 '
11.42 ± 3.29
Time (min.)
'p < 0.05 vs males;
W12
RESULTS
Table 1 and Figure 1 show the GH response to GHRH in men and women. Basal GH levels were higher in women (7.98 ± 3.09 ng/ml) than in men (0.13 ± 0.07 ng/ml) (p < 0.05). Maximal GH response was obtained at 30 minutes in men (9.63 ± 2.32 ng/ml) (p < 0.01) and at 15 minutes in women (40.17 ± 11.96 ng/ml) (p < 0.01). The maximum net increment of GH after GHRH was significantly higher in women (36.07 ± 9.66 ng/ml) than in men (11.04 ± 2.36 ng/ml) (p < 0.03). The variance in GH response to GHRH was similar in both sexes. In spite of a significant increase on estradiol levels from day one (75.88 ± 2.48 pg/ml) to day twelve (198.40 ± 28.65 pg/ml) (p < 0.01), basal GH levels (7.89 ± 3.09 ng/ml on day one, vs 7.76 ± 2.91 ng/ml on day twelve) as well as maximum GH peaks (40.17 ± 11.96 ng/ml on day one versus 37.34 ± 10.37 ng/ml on day twelve) were not different. There was no correlation between plasma estradi01 levels and the GH increase after GHRH stimulation. Progesterone, prolactin, FSH and LH were similar on both days.
2p < 0.01 vs basal
Serum was immediately separated and stored at 20 C until assayed. GH was measured by double antibody RIA in aliquots duplicate in the same assay using an antibody supplied by Serono (Italy), and second antibody precipitation. The intraassay coefficient of variation was 7.8% and the lower limit of assay detection was 0.24 ng/ml. Estradiol was determined in the same assay by Fluorometric Immunoassay (Delfia®, Pharmacia). The sensitivity of the assay was 13.6 pg/ml and the intraassay coefficient of variation was 5%. Progesterone, FSH, LH and prolactin were also determined by Fluorometric Inmunoassay (Delfia®, Pharmacia). 60
•• *
50
o-w
1",1 .
40
~I
e---M
*
~I
20
•• 10
,/' -15
o
/'
'"
T . _ ... ---~-
15
..1-----1
~b
.- --
---- ... --. .. T
..
30
45
60
266
----.- i
-.
90
Fig. 1 - Growth hormone (mean ± SE) response to GHRH in men (e) and women on day one of the menstrual cycle (0). *p < 005 vs men. **p < 0.01 vs basal.
Sex influence on GH response to GHRH
DISCUSSION
ACKNOWLEDGMENTS
Sex differences in GH response to GHRH are yet uncleared. While some investigators have reported higher responsiveness in men than in women (4), others have shown no difference (2) or even higher response in women (5, 6). Moreover, it has been suggested that testosterone may either increase or not modify (17, 19) GH responsiveness to GHRH when administered previous to this stimulus. These discrepancies may be explained by the difference in testosterone doses used. This study presents evidence that GH response to GHRH in women is higher than in men, supporting the hypothesis that estradiol could increase the sensitivity of the somatotrope cells to GHRH. Discrepancies with other studies can not be explained by the age of subjects, the dose of GHRH used, or the hour at which the test was performed, since they are similar. The great variability in GH response to GHRH (2) could explain these different results. This variability has been attributed to the phase of GH endogenous secretion at the beginning of the test. According to this hypothesis, Devesa et. al suggest that GH response to GHRH would be higher in the secretory than in the refractory phase (5), in such a way that the greater percentage of women (80%) than men (20%) being in secretory phase in his study, might explain the higher GH response to GHRH in women found there. In our study, 90% of the women and 10% of the men were in secretory phase similarly to the study of Devesa, but the magnitude of the response of GH to GHRH does not correlate with the phase at the beginning of the test. As has been previously described (2, 17), in our study GH response to GHRH was similar throughout the menstrual cycle, as opposed to the hypothesis that endogenous estradiol levels are responsible for the difference in GH response to GHRH in both sexes. One possible explanation is that estradiol may potentiate somatotrope cell responsiveness to GHRH in an "on-off" mode, in such a way that on the first day of the menstrual cycle, estradiol levels are sufficient to increase GH response to GHRH. Another possibility is that estradiol acts in women by conditioning a different maturation in the GH axis during puberty, the subsequent response to GHRH being independent of the endogenous estradiol levels at the beginning of the test. The latter hypothesis may explain the absence of a significant change in the GH response to GHRH in hypoestrogenic women (20) or following the administration of a GnRH agonist (17).
We acknowledge with thanks the assistance of Mrs R. Adam in the execution of this study
REFERENCES 1. Tresguerres JAF., Lima L., Arce V., Devesa J. Controversies in the GH response to GHRH in man. In: Casanueva F., Dieguez C. (Eds.), Recent advances in basic and clinical neuroendocrinology. Excerpta Medica International Congress Series 864, Amsterdam, 1989, p 255.
2. Gelato M.C., Pescovitz O.H., Cassorla F., Loriaux D.L., Merriam G.R. Dose response relationships for the effect of growth hormone releasing factor (1-44) NH2 in young adult men and women. J. Clin. Endocrinol. Metab. 59: 197, 1984. 3. Williams T., Berelowitz M., Joffe S.N., Thorner M.O., Rivier J., Vale W., Frohman LA. Impaired growth hormone responses to growth hormone releasing factor in obesity. A pituitary defect reversed with weight reduction. N. Engl. J. Med. 311: 1403,1984. 4. Smals A.E.M., Pieters G.F.M., Smals A.G.H., Benraad T.J., Van Laarhoven J., Kloppenborg P.W.C. Sex difference in human growth hormone (GH) response to intravenous human pancreatic GHReleasing hormone administration in young adults. J. Clin. Endocrinol. Metab. 62: 336,1986. 5. Devesa J., Lima L., Lois N., Fraga C., Lechuga M.J., Arce V., Tresguerres JAF. Reasons for the variability in GH response to GHRH challenge: The endogenous HypothalamicSomatotroph rhythm (HSR). Clin. Endocrinol (Oxf.) 30: 367,1989. 6. Lang I., Schernthaner G., Pietschmann P., Kurz R., Stephenson J., Tempi H. Effects of sex and age on growth hormone response to growth hormone-releasing hormone in healthy individuals. J. Clin. Endocrinol. Metab. 65: 535,1987. 7. Gelato M., Malozowski S., Caruso-Nicoletti M., Levine J, Pescovitz 0., Rose S., Loriaux D., Cassorla F., Merriam G. Growth hormone (GH) responses to GH-Releasing hormone during pubertal development in normal boys and girls: Comparison to idiopatic short stature and GH deficiency. J. Clin. Endocrinol. Metab 63: 174,1986. 8. Miller J.D., Tannenbaum G.S., Colle E., Guyda H.J. Daytime pulsatile growth hormone secretion during childhood and adolescence. J. Clin. Endocrinol. Metab. 55: 989,1982. 9. Zadik Z., Chalew S.A., Me Carter R.J., Meistas M.,
Kowarsky A.A. The influence of age on the twenty-four hour inte-
267
P. Benito, L. Avila, M.5. Corpas, et al.
10.
11.
12.
13.
14.
grated concentration of growth hormone in normal individuals. J. Clin. Endocrinol. Metab. 60.· 513, 1985. Ho K.Y., Evans S., Blizard R.M., Veldhuis J.D., Merriam G.R., Samojlik E., Furlanetto R., Rogol AD., Kaiser D.L., Thorner M.O. Effects of sex and age on the 24 hour profile of growth hormone secretion in man: importance of endogenous estradiol concentrations. J. Clin. Endocrinol. Metab. 64: 51,1987. Spiliotis B.E., August G.P., Hung W., Sonis W., Mendelson W., Bercu B.B. Growth hormone neurosecretory dysfuction: a treatable cause of short stature. JAMA 251: 2233,1984. Thompson A.G., Rodriguez A., Kowarsky A, Migeon G.J., Blizzard R.M. Integrated concentrations of growth hormone correlated with plasma testosterone and bone age in preadolescent and adolescent males. J. Clin. Endocrinol. Metab. 35: 344, 1972. Link R., Blizzard R.M., Evans W.S., Kaiser D.L., Parker MW., Rogol AD. The effect of androgens on the pulsatile release and the twenty-four-hour mean concentration of growth hormone in peri pubertal males. J. Clin. Endocrinol. Metab. 62: 159,1986. Thompson R.G., Rodriguez A, Kowarsky A, Blizzard A.M. Growth hormone metabolic clearance rates, integrated concentrations and production rates in normal adults and the effects of prednisone. J. Clin. Invest. 51: 3193,1972.
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