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Arginine abolishes the inhibitory effect of glucose on the growth hormone response to growth hormone-releasing hormone in man
Arginine
Abolishes Response
the Inhibitory Effect of GIucose on the Growth Hormone to Growth Hormone-Releasing Hormone in Man
E. Ghigo, C. Miola, G. Aimaretti, F. Valente, M. Procopio, E. Arvat, W. Yin-Zhang, and F. Camanni Acute hyperglycemia inhibits the growth hormone (GH) response to several stimuli including growth hormone-releasing hormone (GHRH), likely acting by stimulation of endogenous somatostatin release. The aim of our study was to verify whether arginine ([Arg] 30 g intravenously [IV] in 30 minutes), a well-known GH secretagogue likely acting via inhibition of hypothalamic somatostatin release, counteracts the inhibitory effect of oral glucose (OG) administration (100 mg orally) on the GH response to GHRH (1 pg/kg IV bolus) in seven normal subjects (aged 20 to 30 years). The GH response to GHRH (peak, 11.6 f 1.8 pg/L) was inhibited by previous OG load (peak, 7.4 + 0.8 pg/L; P < .02 Y GHRH alone) and potentiated by Arg coadministration (peak, 36.2 2 8.8 pg/L; P < .03 Y GHRH alone). The potentiating effect of Arg on the GHRH-induced GH increase was unaffected by previous OG load (peak, 30.4 -C 6.9 pg/L). In conclusion, our results show that Arg abolishes the inhibitory effect of OG administration on the GHRH-induced GH response in man. These data, although indirect, suggest that both acute hyperglycemia and Arg act at the hypothalamic level, stimulating and inhibiting, respectively, the release of somatostatin. Copyright 0 1992 by W.B. Saunders Company
I
T IS WIDELY ACCEPTED that acute hyperglycemia inhibits GH secretion in man. In fact, it has been shown that hyperglycemia reduces both physiologically elevated basal growth hormone (GH) levels and the somatotrophic responsiveness to various stimuli,r-4 including growth hormone-releasing hormone (GHRH).?-I0 In accordance with the hypothesis that hyperglycemia influences GH secretion at the hypothalamic level,1t the enhancement of cholinergic activity by pyridostigmine, a cholinesterase inhibitor12 likely acting by inhibition of somatostatin release,*“-i5 has been reported to reverse the inhibitory effect of oral glucose (OG) administration on GHRH-induced GH increase in man.8-iu Recently, it has also been shown that arginine (Arg), a well-known GH secretagogue,lh potentiates the GHRH-induced GH increase, likely acting via inhibition of hypothalamic somatostatin release.17-2? Based on the foregoing data, our aim was to verify whether Arg also counteracts the inhibitory effect of OG administration on the GH response to GHRH. SUBJECTS AND METHODS Seven normal male subjects (aged 20 to 30 years) gave informed consent to the study. All subjects were within 10% of ideal body weight, and none had familial history of diabetes mellitus. All subjects underwent the four following tests at least 3 days apart and in random order: (1) GHRH (GHRH29, GEREF, Serono. Italy; 1 kg/kg intravenous [IV] bolus at 0 minutes): (2) OG (100 g orally in cold water 45 minutes before GHRH administration) + GHRH: (3) Arg (Arg hydrochloride, Pierre], Italy; 30 g in 100 mL infused IV over 30 minutes from 0 to 30 minutes) + GHRH; and (4) OG + Arg + GHRH. All tests were begun between 8:30 AM and 9:00 AM after an overnight fast and 30 minutes after cannulation of cubital veins
From the Division of Endocrinology, Depantnent of Clinical Pathophysioto6y, Universityof Turin, Italy. Supported by grants of the Minister0 dell’ Universita e Ricerca Scientifica e Tecnologica. Address reprint requests to F. Camanni, MD, Divisione di Endocrinologia, Ospedale Molinette, C.so Dogliotti, 14, 10126 Totino, Italia. Copyright 0 1992 by W.B. Saunders Company 0026-0495/92/4109-0012$03.OOiO 1000
kept patent by slow infusion of 0.9% saline. Blood samples taken at -45, -30. -15.0, 1.5, 30.45,60, 75. and 90 minutes. Serum GH was measured assay (HGH-CTK IRMA,
were
in duplicate by immunoradiometric Sorin. Italy). All samples from an
individual subject were analyzed together. The sensitivity of the assay was 0.15 kg/L. The interassay and intraassay coefficients of variation
were
between
2.3%
and
5.5%
and
I.YC+ and
3.9%.
respectively. The GH secretory responses were expressed either as absolute values (kg/L) or as areas under the curve ((AX] &L/h) calculated by trapezoidal integration. Plasma glucose (mg/ 100 mL) was analyzed by a glucose oxidase method (Beckman Glucose
Analyzer
2, Beckman
Instruments,
Brea, CA).
Results were expressed as means t SEM. Statistical the data was performed using Wilcoxon’s signed-rank paired
Student’s
analysis of test and a
t test where appropriate.
RESULTS
Serum GH concentrations after the four treatments are shown in Fig 1, and plasma glucose levels arc reported in Table 1. In tests no. 2 and no. 4, glucose administration 45 minutes before GHRH injection induced an increase in plasma glucose levels at -15 and 0 minutes that was significantly higher (P < ,002) than glucose levels in the control tests. Moreover, plasma glucose levels were not significantly different in treatments no. 2 and no. 4 at all times. Mean basal serum GH levels were similar in all tests. The administration of GHRH induced a clear-cut increase in GH values (peak, 11.6 2 1.8 pg/L; AUC, 479.5 2 83.9 kg/L/h). When preceded by OG administration, the GHRH-induced GH response was significantly inhibited (peak. 7.4 + 0.8 ug/L; AUC, 260.8 ? 38.3 kg/L/h; P < .02 r’ GHRH alone). The GHRH-induced GH increase was potentiated by coadministration with Arg (peak, 36.2 2 8.8 p,g/L; AUC, 1460.4 ? 359.9 ug/Lih; P < .03 and < .02 v GHRH alone or preceded by glucose, respectively). The potentiating effect of Arg on the GHRH-induced GH increase was unaffected by previous OG load (peak, 30.4 2 6.4 kg/L; AUC, 1206.9 + 273.97 ug/L/h). A transient facial flushing was observed in four subjects after GHRH administration. No side effects were obsewcd after both Arg and OG administration. Metabohsm, Vol 41, No 9 (Septemberl,
1992: pp 1000-1003
EFFECTS
OF ARGININE
AND
GLUCOSE
1001
ON GH RELEASE
50
40
10
Fig 1. Mean f SEM GH curves (A) and AUCs (B) after GHRH alone (0.m) or combined with glucose (CE3). Arg (ALI), or glucose and Arg (*,a) in seven normal subjects.
0 I
-1s
0
15
30
45
Time
lmin)
DISCUSSION
Present data demonstrate that Arg abolishes the inhibitory effect of OG administration on the GHRH-induced GH response in man. For many years, the inhibitory effect of acute hyperglycemia on GH release under different circumstances has been clearly shown. In fact, a glucose load inhibits both physiologically elevated basal GH levels and the somatotrophic responsiveness to some stimuli,1-4 with the exceptions being sleep, pyrogens, stress, and, interestingly, Arg.23-25More recently, it has been shown that acute hyperglycemia blocks even the GH response to GHRH.S-lo This inhibitory effect was shown after both OGs.6,8,9and IV’O glucose load and when elevated glucose levels were maintained with the glucose clamp technique.’ At present, the precise mechanism by which the negative influence of hyperglycemia on GH secretion takes place in man remains unclear. In vitro basal and GHRH-stimulated GH release from rat anterior pituitary cells is not influenced by different glucose concentrations in the culture medium.26 On the other hand, other studies using perifused rat hypothalamic fragments have shown an inverse relationship between glucose concentrations and somatostatin release. In fact, somatostatin release from the rat hypothalamus is stimulated in the presence of glucopenia, whereas increased
80
75
so
medium glucose levels inhibit somatostatin release.27-29 Accordingly, in the rat in vivo, it has been shown that hypoglycemia inhibits GH secretion, while hyperglycemia has little effect on GH release and fails to affect the GH response to GHRH.30.31 These data indicate that the influence of glucose in the contrgl of GH secretion in the rat is different from that in man. Thus, based on the animal model, it is very difficult to explain findings in humans. To our knowledge, no studies in vitro were performed in man, and only indirect studies are available. The clear inhibiting effect of hyperglycemia on the GHRH-induced GH increase5-10 negates the possibility that glucose could act by inhibiting endogenous GHRH release. On the other hand, according to the hypothesis of’ a somatostatin-mediated effect of hyperglycemia, it has been recently shown that the enhancement of cholinergic activity by pyridostigmine, a cholinesterase inhibitor12 likely acting by inhibition of somatostatin release,13-I5 reverses the inhibitory effect of OG administration on GHRH-induced GH response in man.8-10 Based on these findings, an inhibitory effect of hyperglycemia directly on the somatotroph cells seems unlikely. The effect of pyridostigmine is concurrent with our present data showing that the potentiating effect of Arg on the GHRH-induced GH response overrides the inhibitory
Table 1. Plasma Glucose Levels (mean 2 SEMI After GHRH Alone or Combined Wiih Glucose, Arg, or Glucose and Arg in Seven Normal Subjects Time (min) Test
GHRH
-45
-15
0
15
77.1 r 1.9
77.4 2 2.2
77.3 r 2.0
78.7 -t 2.7
30
77.9 k 2.6
45
60
75
90
79.0 + 2.9
78.7 + 2.7
79.1 zk 2.2
79.4 + 2.1
OG + GHRH
78.0 * 1.4
131.6 It 10.4
131.0 + 11.1
118.6 2 10.0
110.0
+ 7.8
99.6 f 6.4
94.0 ? 7.9
96.6 k 7.6
99.3 + 7.8
Arg + GHRH
80.9 z 2.9
82.4 k 2.7
81.3 k 2.9
90.9 2 4.7
86.3
+ 4.1
72.0 2 4.3
65.1
t 4.9
71.5 A 4.4
77.9 + 4.1
OG + Arg + GHRH
79.0 5 2.9
130.7 k 8.1
125.6 + 11.1
100.6 2 7.2
81.9 -c 6.0
66.1 k 6.0
JO.9 t 6.0
75.7 k 7.5
115.3
2 8.7
1002
GHIGO ET AL
effect of a glucose load. There is now evidence in man, although indirect, for a somatostatin-suppressing effect of Arg at the hypothalamic level. In fact, Arg clearly potentiates the GH response to the maximal GHRH dose in man.“-*” Moreover, Arg induces a clear GH increase and reinstates the blunted GH response to GHRH after a prior GHRH administration.2U~21 The potentiating effect of Arg on the GHRH-induced GH increase strictly overlaps with that of pyridostigmine, while Arg fails to potentiate pyridostigmine-induced GH secretion.lxJ’ Again, favoring a somatostatin-mediated mechanism, ornithine, the active form of Arg, does not modify plasma GHRH concentrations in humans.‘? Moreover, in vitro, Arg fails to influence both basal and GHRH-induced GH release from rat while in vivo. this amino acid has a anterior pituitary,” stimulating effect on GH secretion both in rats and in
sheep.33,3J Nevertheless, a direct effect of Arg on somatotroph cells cannot be ruled out in man. Taking into account that in man indirect studies are the best available at this time, it could be hypothesized that hyperglycemia and Arg likely act by stimulating and inhibiting, respectively, the activity of somatostatinergic neurons at the hypothalamic level. As the stimulatory effect of Arg on GH secretion counteracts the inhibitory effect of acute hyperglycemia, it could be argued that the inhibitory influence release
of this amino overrides
acid
on hypothalamic
the stimulatory
effect
somatostatin
of glucose.
ACKNOWLEDGMENT The authors wish to thank Serono, Division, Italy, for providing GHRHS
Italy. and Pierre1 Hormone
and arginine hydrochloride,
respectively.
REFERENCES 1. Roth J, Glick SM, Yalow RS, et al: Secretion of human growth hormone: Physiological and experimental modification. Metabolism 12577-579, 1963 2. Lawrence AM, Goldfine ID, Kirsteins L: Growth hormone dynamics in acromegaly. J Clin Endocrinol31:239-245, 1970 3. Hansen AP: The effect of intravenous glucose infusion on the exercise-induced serum growth hormone rise in normals and juvenile diabetics. Stand J Clin Lab Invest 28:195-205. 1971 4. Mims RB, Scott CL, Modebe OM, et al: Prevention of L-dopa-induced growth hormone stimulation by hyperglycemia. J Clin Endocrinol Metab 37:660-663. 1973 5. Davies RR, Turner S, Johnston DG: Oral glucose inhibits growth hormone secretion induced by human pancreatic growth hormone factor 1-44 in normal man. Clin Endocrinol 21:477-481, 1984 6. Masuda A, Shibasaki T, Nakahara M, et al: The effect of glucose on growth hormone-releasing hormone-mediated GH secretion in man. J Clin Endocrinol Metab 60:523-526, 1985 7. Sharp PS, Foley K, Chanal P, et al: The effect of plasma glucose on the growth hormone response to human pancreatic growth hormone releasing factor in normal subjects. Clin Endocrinol20:497-501, 1984 8. Balzano S, Loche S, Murtas ML, et al: Potentiation of cholinergic tone counteracts the suppressive effect of oral glucose administration on the GH response to GHRH in man. Horm Metab Res 21:52-53, 1989 9. Penalva A, Burguera B, Casabiell X, et al: Activation of cholinergic neurotransmission bypyridostigmine reverses the inhibitory effect of hyperglycaemia on GHRH-induced GH secretion in man. Does acute hyperglycaemia act through hypothalamic release of somatostatin? Neuroendocrinology 49:551-554. 1989 10. Delitala G. Tomasi PA, Palermo M, et al: Interaction of glucose and pyridostigmine on the secretion of growth hormone induced by GH-releasing hormone. J Endocrinol Invest 13:653656,199O 11. Muller EE, Nistic6 G: Brain Messengers and the Pituitary. San Diego, CA, Academic, 1989 12. Taylor P: Anticholinesterase agents, in Goodman Gilman A, Goodman LS, Rall TW, et al (eds): The Pharmacological Basis of Therapeutics. New York, NY, Macmillan, 1985, pp 110-129 13. Richardson SB, Hollander CS, D’Elettra RD, et al: Acetylcholine inhibits the release of somatostatin from rat hypothalamus in vitro. Endocrinology 107:122-126, 1980 14. Locatelli V. Torsello A, Redaelli M, et al: Cholinergic agonist and antagonist drugs modulate the growth hormone
response to growth hormone releasing hormone in the rat: Evidence for mediation by somatostatin. J Endocrinol 111:271-278, 1986 15. Torsello A, Panzeri G, Cermenati P. et al: Involvement of the somatostatin and cholinergic system in the mechanism of growth hormone autofeedback regulation in the rat. J Endocrinol 117273-281. 1988 16. Parker ML, Hammonds JM, Daughaday WH: The arginine provocative test: An aid in the diagnosis of hyposomatotropinism. J Clin Endocrinol Metab 27: 1129-l 134. 1967 17. Alba-Roth J, Albrecth Miiller AO. Schopohl J, et al: Arginine stimulates growth hormone secretion by suppressing endogenous somatostatin secretion. J Clin Endocrinol Metab 67:1186-1189.1988 18. Ghigo E. Bellone J, Mazza E, et al: Arginine potentiates the pyridostigmine-induced GH secretion in normal short children. Further evidence for a somatostatin suppressing effect of arginine. Clin Endocrinol (Oxf) 32:763-767-1990 19. Ghigo E. Goffi S, Nicolosi M, et al: GH responsiveness to combined administration of arginine and GHRH does not vary with age in man. J Clin Endocrinol Metab 71:1481-1485, 1990 20. Ghigo E, Arvat E, Valente F, et al: Arginine reinstates the somatotrope responsiveness to intermittent growth hormonereleasing hormone administration in normal adults. Neuroendocrinology 54:291-294, 1991 21. Page MD, Dieguez C. Valcavi R, et al: GH responses to arginine and L-dopa alone and after GHRH pretreatment. Clin Endocrinol (Oxf) 28:551-558. 1988 22. Evain Brion D, Donadieu M, Liapi C. et al: Plasma GHRH levels in children: Physiologically and pharmacologically induced variation. Horm Res 24:116-l 18, 1986 23. Glick SM. Roth J, Yalow RS, et al: The regulation of growth hormone secretion. Recent Prog Horm Res 21:241-283. 1965 24. Parker DC, Rossman LG: Human growth hormone release in sleep, nonsuppression by acute hyperglycemia. J Clin Endocrinol 32165-69, 1971 25. Merimee TJ, Rabinowiz D. Fineberg SE: Arginine initiated release of human growth hormone, factors modifying the response in normal man. N Engl J Med 280:1434-1438, 1969 26. Page MD, Kopperschaar PF, Edwards CA. et al: Additive effects of growth hormone releasing factor and insulin hypoglycaemia on growth hormone release in man. Clin Endocrinol 26:589594.1987 27. Berelowitz M, Dudlak D, Frohman LA: Release of somatostatin-like immunoreactivity from incubated rat hypothalamus and
EFFECTS OF ARGININE AND GLUCOSE ON GH RELEASE
cerebral cortex. Effects of glucose and glucoregulatory hormones. J Clin Invest 69:1293-1301,1982 28. Langyel A-NJ, Grossman A, Nieuwenhuyzen-Kruseman AC, et al: Glucose modulation of somatostatin and LHRH release from rat hypothalamic fragments in vitro. Neuroendocrinology 39:31-38,1984 29. Baes M, Vale WW: Characterization of the glucosedependent release of growth hormone-releasing factor and somatostatin from superfused rat hypothalami. Neuroendocrinology 51:202201,199o 30. Tannenbaum GS, Martin JB, Colle E: Ultradian growth hormone rhythm in the rat: Effects of feeding, hyperglycemia, and insulin-induced hypoglycemia. Endocrinology 99:720-727. 1976
31. Imaki T, Shibasaki T, Masuda A, et al: The effects of glucose and free fatty acids on growth hormone-releasing factor-mediated GH secretion in rats. Endocrinology 118:2390-2395, 1986 32. Kelijman M, Frohman LA: The role of the cholinergic pathway in growth hormone feedback. J Clin Endocrinol Metab 72:1081-1087,199l 33. Franchimont P, Campistron G, Creuzet MH, et al: Effects of arginine aspartate and its components on growth hormone, insulin and glucagon secretion in the rat. Arch Int Pharmacodyn Ther 267:161-168,1984 34. Kuhara T, Ikeda S, Ohneda A. et al: Effects of intravenous infusion of 17 amino acids on the secretion of GH, glucagon, and insulin in sheep. Am J Physiol260:E21-E26, 1991
Abolishes Response
the Inhibitory Effect of GIucose on the Growth Hormone to Growth Hormone-Releasing Hormone in Man
E. Ghigo, C. Miola, G. Aimaretti, F. Valente, M. Procopio, E. Arvat, W. Yin-Zhang, and F. Camanni Acute hyperglycemia inhibits the growth hormone (GH) response to several stimuli including growth hormone-releasing hormone (GHRH), likely acting by stimulation of endogenous somatostatin release. The aim of our study was to verify whether arginine ([Arg] 30 g intravenously [IV] in 30 minutes), a well-known GH secretagogue likely acting via inhibition of hypothalamic somatostatin release, counteracts the inhibitory effect of oral glucose (OG) administration (100 mg orally) on the GH response to GHRH (1 pg/kg IV bolus) in seven normal subjects (aged 20 to 30 years). The GH response to GHRH (peak, 11.6 f 1.8 pg/L) was inhibited by previous OG load (peak, 7.4 + 0.8 pg/L; P < .02 Y GHRH alone) and potentiated by Arg coadministration (peak, 36.2 2 8.8 pg/L; P < .03 Y GHRH alone). The potentiating effect of Arg on the GHRH-induced GH increase was unaffected by previous OG load (peak, 30.4 -C 6.9 pg/L). In conclusion, our results show that Arg abolishes the inhibitory effect of OG administration on the GHRH-induced GH response in man. These data, although indirect, suggest that both acute hyperglycemia and Arg act at the hypothalamic level, stimulating and inhibiting, respectively, the release of somatostatin. Copyright 0 1992 by W.B. Saunders Company
I
T IS WIDELY ACCEPTED that acute hyperglycemia inhibits GH secretion in man. In fact, it has been shown that hyperglycemia reduces both physiologically elevated basal growth hormone (GH) levels and the somatotrophic responsiveness to various stimuli,r-4 including growth hormone-releasing hormone (GHRH).?-I0 In accordance with the hypothesis that hyperglycemia influences GH secretion at the hypothalamic level,1t the enhancement of cholinergic activity by pyridostigmine, a cholinesterase inhibitor12 likely acting by inhibition of somatostatin release,*“-i5 has been reported to reverse the inhibitory effect of oral glucose (OG) administration on GHRH-induced GH increase in man.8-iu Recently, it has also been shown that arginine (Arg), a well-known GH secretagogue,lh potentiates the GHRH-induced GH increase, likely acting via inhibition of hypothalamic somatostatin release.17-2? Based on the foregoing data, our aim was to verify whether Arg also counteracts the inhibitory effect of OG administration on the GH response to GHRH. SUBJECTS AND METHODS Seven normal male subjects (aged 20 to 30 years) gave informed consent to the study. All subjects were within 10% of ideal body weight, and none had familial history of diabetes mellitus. All subjects underwent the four following tests at least 3 days apart and in random order: (1) GHRH (GHRH29, GEREF, Serono. Italy; 1 kg/kg intravenous [IV] bolus at 0 minutes): (2) OG (100 g orally in cold water 45 minutes before GHRH administration) + GHRH: (3) Arg (Arg hydrochloride, Pierre], Italy; 30 g in 100 mL infused IV over 30 minutes from 0 to 30 minutes) + GHRH; and (4) OG + Arg + GHRH. All tests were begun between 8:30 AM and 9:00 AM after an overnight fast and 30 minutes after cannulation of cubital veins
From the Division of Endocrinology, Depantnent of Clinical Pathophysioto6y, Universityof Turin, Italy. Supported by grants of the Minister0 dell’ Universita e Ricerca Scientifica e Tecnologica. Address reprint requests to F. Camanni, MD, Divisione di Endocrinologia, Ospedale Molinette, C.so Dogliotti, 14, 10126 Totino, Italia. Copyright 0 1992 by W.B. Saunders Company 0026-0495/92/4109-0012$03.OOiO 1000
kept patent by slow infusion of 0.9% saline. Blood samples taken at -45, -30. -15.0, 1.5, 30.45,60, 75. and 90 minutes. Serum GH was measured assay (HGH-CTK IRMA,
were
in duplicate by immunoradiometric Sorin. Italy). All samples from an
individual subject were analyzed together. The sensitivity of the assay was 0.15 kg/L. The interassay and intraassay coefficients of variation
were
between
2.3%
and
5.5%
and
I.YC+ and
3.9%.
respectively. The GH secretory responses were expressed either as absolute values (kg/L) or as areas under the curve ((AX] &L/h) calculated by trapezoidal integration. Plasma glucose (mg/ 100 mL) was analyzed by a glucose oxidase method (Beckman Glucose
Analyzer
2, Beckman
Instruments,
Brea, CA).
Results were expressed as means t SEM. Statistical the data was performed using Wilcoxon’s signed-rank paired
Student’s
analysis of test and a
t test where appropriate.
RESULTS
Serum GH concentrations after the four treatments are shown in Fig 1, and plasma glucose levels arc reported in Table 1. In tests no. 2 and no. 4, glucose administration 45 minutes before GHRH injection induced an increase in plasma glucose levels at -15 and 0 minutes that was significantly higher (P < ,002) than glucose levels in the control tests. Moreover, plasma glucose levels were not significantly different in treatments no. 2 and no. 4 at all times. Mean basal serum GH levels were similar in all tests. The administration of GHRH induced a clear-cut increase in GH values (peak, 11.6 2 1.8 pg/L; AUC, 479.5 2 83.9 kg/L/h). When preceded by OG administration, the GHRH-induced GH response was significantly inhibited (peak. 7.4 + 0.8 ug/L; AUC, 260.8 ? 38.3 kg/L/h; P < .02 r’ GHRH alone). The GHRH-induced GH increase was potentiated by coadministration with Arg (peak, 36.2 2 8.8 p,g/L; AUC, 1460.4 ? 359.9 ug/Lih; P < .03 and < .02 v GHRH alone or preceded by glucose, respectively). The potentiating effect of Arg on the GHRH-induced GH increase was unaffected by previous OG load (peak, 30.4 2 6.4 kg/L; AUC, 1206.9 + 273.97 ug/L/h). A transient facial flushing was observed in four subjects after GHRH administration. No side effects were obsewcd after both Arg and OG administration. Metabohsm, Vol 41, No 9 (Septemberl,
1992: pp 1000-1003
EFFECTS
OF ARGININE
AND
GLUCOSE
1001
ON GH RELEASE
50
40
10
Fig 1. Mean f SEM GH curves (A) and AUCs (B) after GHRH alone (0.m) or combined with glucose (CE3). Arg (ALI), or glucose and Arg (*,a) in seven normal subjects.
0 I
-1s
0
15
30
45
Time
lmin)
DISCUSSION
Present data demonstrate that Arg abolishes the inhibitory effect of OG administration on the GHRH-induced GH response in man. For many years, the inhibitory effect of acute hyperglycemia on GH release under different circumstances has been clearly shown. In fact, a glucose load inhibits both physiologically elevated basal GH levels and the somatotrophic responsiveness to some stimuli,1-4 with the exceptions being sleep, pyrogens, stress, and, interestingly, Arg.23-25More recently, it has been shown that acute hyperglycemia blocks even the GH response to GHRH.S-lo This inhibitory effect was shown after both OGs.6,8,9and IV’O glucose load and when elevated glucose levels were maintained with the glucose clamp technique.’ At present, the precise mechanism by which the negative influence of hyperglycemia on GH secretion takes place in man remains unclear. In vitro basal and GHRH-stimulated GH release from rat anterior pituitary cells is not influenced by different glucose concentrations in the culture medium.26 On the other hand, other studies using perifused rat hypothalamic fragments have shown an inverse relationship between glucose concentrations and somatostatin release. In fact, somatostatin release from the rat hypothalamus is stimulated in the presence of glucopenia, whereas increased
80
75
so
medium glucose levels inhibit somatostatin release.27-29 Accordingly, in the rat in vivo, it has been shown that hypoglycemia inhibits GH secretion, while hyperglycemia has little effect on GH release and fails to affect the GH response to GHRH.30.31 These data indicate that the influence of glucose in the contrgl of GH secretion in the rat is different from that in man. Thus, based on the animal model, it is very difficult to explain findings in humans. To our knowledge, no studies in vitro were performed in man, and only indirect studies are available. The clear inhibiting effect of hyperglycemia on the GHRH-induced GH increase5-10 negates the possibility that glucose could act by inhibiting endogenous GHRH release. On the other hand, according to the hypothesis of’ a somatostatin-mediated effect of hyperglycemia, it has been recently shown that the enhancement of cholinergic activity by pyridostigmine, a cholinesterase inhibitor12 likely acting by inhibition of somatostatin release,13-I5 reverses the inhibitory effect of OG administration on GHRH-induced GH response in man.8-10 Based on these findings, an inhibitory effect of hyperglycemia directly on the somatotroph cells seems unlikely. The effect of pyridostigmine is concurrent with our present data showing that the potentiating effect of Arg on the GHRH-induced GH response overrides the inhibitory
Table 1. Plasma Glucose Levels (mean 2 SEMI After GHRH Alone or Combined Wiih Glucose, Arg, or Glucose and Arg in Seven Normal Subjects Time (min) Test
GHRH
-45
-15
0
15
77.1 r 1.9
77.4 2 2.2
77.3 r 2.0
78.7 -t 2.7
30
77.9 k 2.6
45
60
75
90
79.0 + 2.9
78.7 + 2.7
79.1 zk 2.2
79.4 + 2.1
OG + GHRH
78.0 * 1.4
131.6 It 10.4
131.0 + 11.1
118.6 2 10.0
110.0
+ 7.8
99.6 f 6.4
94.0 ? 7.9
96.6 k 7.6
99.3 + 7.8
Arg + GHRH
80.9 z 2.9
82.4 k 2.7
81.3 k 2.9
90.9 2 4.7
86.3
+ 4.1
72.0 2 4.3
65.1
t 4.9
71.5 A 4.4
77.9 + 4.1
OG + Arg + GHRH
79.0 5 2.9
130.7 k 8.1
125.6 + 11.1
100.6 2 7.2
81.9 -c 6.0
66.1 k 6.0
JO.9 t 6.0
75.7 k 7.5
115.3
2 8.7
1002
GHIGO ET AL
effect of a glucose load. There is now evidence in man, although indirect, for a somatostatin-suppressing effect of Arg at the hypothalamic level. In fact, Arg clearly potentiates the GH response to the maximal GHRH dose in man.“-*” Moreover, Arg induces a clear GH increase and reinstates the blunted GH response to GHRH after a prior GHRH administration.2U~21 The potentiating effect of Arg on the GHRH-induced GH increase strictly overlaps with that of pyridostigmine, while Arg fails to potentiate pyridostigmine-induced GH secretion.lxJ’ Again, favoring a somatostatin-mediated mechanism, ornithine, the active form of Arg, does not modify plasma GHRH concentrations in humans.‘? Moreover, in vitro, Arg fails to influence both basal and GHRH-induced GH release from rat while in vivo. this amino acid has a anterior pituitary,” stimulating effect on GH secretion both in rats and in
sheep.33,3J Nevertheless, a direct effect of Arg on somatotroph cells cannot be ruled out in man. Taking into account that in man indirect studies are the best available at this time, it could be hypothesized that hyperglycemia and Arg likely act by stimulating and inhibiting, respectively, the activity of somatostatinergic neurons at the hypothalamic level. As the stimulatory effect of Arg on GH secretion counteracts the inhibitory effect of acute hyperglycemia, it could be argued that the inhibitory influence release
of this amino overrides
acid
on hypothalamic
the stimulatory
effect
somatostatin
of glucose.
ACKNOWLEDGMENT The authors wish to thank Serono, Division, Italy, for providing GHRHS
Italy. and Pierre1 Hormone
and arginine hydrochloride,
respectively.
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