GH-releasing peptide-2 does not stimulate arginine vasopressin secretion in healthy men

Clinical usefulness of the growth hormone-releasing peptide-2 test for hypothalamic-pituitary disorder

Purpose

Growth hormone deficiency (GHD) develops early in patients with hypothalamic-pituitary disorder and is frequently accompanied by other anterior pituitary hormone deficiencies including secondary adrenal insufficiency (AI). A growth hormone-releasing peptide-2 (GHRP2), which is wildly used for the diagnosis of patients with GHD, has been considered to induce not only growth hormone (GH) release but also ACTH release. However, its clinical usefulness in hypothalamic-pituitary disorder is unclear.

Methods

The GHRP2 test, a cosyntropin stimulation test, corticotropin-releasing hormone (CRH) tests and/or insulin tolerance tests (ITTs) were performed on 36 patients having hypothalamic-pituitary disorder.

Results

Twenty-two (61%) had severe GHD, and 3 (8%) had moderate GHD by GHRP2. There was no difference in baseline ACTH and cortisol between non-GHD, moderate GHD and severe GHD participants. However, a cosyntropin stimulation test and subsequent CRH tests and/or ITTs revealed that 17 (47%) had secondary AI and 16/17 (94%) cases of secondary AI were concomitant with severe GHD. ROC curve analysis demonstrated that the ACTH response in the GHRP2 test was useful for screening pituitary-AI, with a cut-off value of 1.55-fold (83% sensitivity and 88% specificity). Notably, the combination of ACTH response and the peak cortisol level in the GHRP2 test using each cut-off value (1.55-fold and 10 µg/dl, respectively) showed high specificity (100%) with high accuracy (0.94) for diagnosis of pituitary-AI.

Conclusion

We recommend measuring ACTH as well as GH during the GHRP2 test to avoid overlooking and delays in diagnosis of secondary AI that frequently accompanies GHD.

Evaluation of growth hormone-releasing peptide-2 for diagnosis of thyrotropin-producing pituitary adenomas

Thyrotropin (TSH)-producing adenomas are a rare cause of hyperthyroidism and are a type of functional pituitary adenoma. The diagnosis of TSH-producing adenoma is a challenging problem in clinical endocrinology. Since growth hormone-releasing peptide-2 (GHRP-2) fails to induce TSH secretion in normal subjects, the effect of GHRP-2 on TSH levels was therefore examined in patients with TSH-producing adenomas. A total of 5 patients (4 women and 1 man) referred to our departments for further evaluation of pituitary hormones were followed-up using the GHRP-2, TSH-releasing hormone (TRH), octreotide, and bromocriptine tests to examine and evaluate TSH secretory dynamics in TSH-producing adenomas. Of 5 patients, 2 (40%) showed such a significant response, defined as a >50% increase in serum TSH level above baseline in the GHRP-2 test. Additionally, 1 patient showed a 48% increase in serum TSH level. In 1 patient whose adenoma was completely removed, basal serum concentrations of TSH were sufficiently suppressed after the operation, and serum TSH levels failed to increase in response to GHRP-2 administration. In 4 patients (80%), a poor response of serum TSH levels was observed in the TRH test. In 2 out of 5 patients (40%), serum TSH levels were significantly decreased following octreotide administration. No patient demonstrated a significant response to the bromocriptine test. In addition to TRH test, the GHRP-2 test as a potential diagnostic tool for TSH-producing pituitary adenomas.

Investigation of the clinical significance of the growth hormone-releasing peptide-2 test for the diagnosis of secondary adrenal failure

The aim of this study was to evaluate the ability of the growth hormone-releasing peptide-2 (GHRP-2) test to clinically diagnose hypothalamo-pituitary-adrenal (HPA) axis failure. We performed an insulin tolerance test (ITT), CRH stimulation test, and GHRP-2 test on 47 patients suspected of having a hypothalamo-pituitary disorder. Patients with pituitary disorders had significantly lower ACTH responses to the GHRP-2 test compared to patients with hypothalamic disorders and the control group. In contrast, peak cortisol levels in response to the GHRP-2 test were significantly lower in both hypothalamic and pituitary disorder cases compared with the control group. Assignment of a cut-off value of 11.6 μg/dL for the peak serum cortisol level demonstrated that the GHRP-2 test was able to predict secondary hypoadrenalism with 88.9% specificity and 89.7% sensitivity. The responses of ACTH and cortisol to the GHRP-2 test had no correlation to the CRH test, suggesting the involvement of a different mechanism of ACTH secretion. These results indicate that the GHRP-2 test may induce ACTH secretion from the pituitary gland through direct stimulation. Although the GHRP-2 test does not have the same predictive value as the insulin tolerance test (ITT), it has similar diagnostic potential as the CRH stimulation test for evaluating HPA axis failure.

Functional interaction of bone morphogenetic protein and growth hormone releasing peptide in adrenocorticotropin regulation by corticotrope cells

Mechanisms by which GHRP stimulates ACTH release in corticotrope cells were investigated using mouse corticotrope AtT20 cells by focusing on the biological activity of BMP-4. GHRP-2 increased ACTH and cAMP secretion by AtT20 cells; however, its effects were less potent than the effects of CRH. BMP-4 suppressed basal ACTH production and POMC transcription, and the inhibition of endogenous BMP receptor signaling led to an increase in ACTH production. Of note, BMP-4 suppressed ACTH production and POMC-promoter activity induced by CRH more efficaciously than that induced by GHRP-2. BMP-4 had no significant effect on cAMP synthesis induced by CRH or GHRP-2. Stimulation with CRH, but not GHRP-2, activated ERK1/2, p38, SAPK/JNK and Akt phosphorylation, in which CRH-induced phosphorylation of ERK and p38 was suppressed by BMP-4. GHRP-2-induced ACTH secretion was not affected by inhibitors of ERK, p38 and Akt pathways, which effectively suppressed CRH-induced ACTH release. Blockage of the cAMP-PKA pathway reversed CRH- as well as GHRP-2-induced ACTH secretion. Furthermore, the inhibition of ERK and p38 significantly reduced cAMP synthesis induced by CRH but not by GHRP-2. Thus, CRH activates ACTH production through ERK and p38 pathways in addition to the cAMP-PKA pathway, which is also activated downstream of MAPK. On the other hand, GHRP-2-induced ACTH production was predominantly linked to the cAMP-PKA pathway. Moreover, CRH and GHRP-2 upregulated BMP receptor signaling, while BMP-4, CRH and GHRP-2 had no significant effect on the expression level of GHSR. In addition, GHRP-2 suppressed the expression of Smad7, which is an inhibitor of the BMP-Smad1/5/8 pathway. Collectively, the results revealed a functional interaction between GHRP-2 and BMP signaling, in which endogenous BMP may act as an autoregulatory system in controlling ACTH production.

Is there an undiscovered neurocircuit for regulating GH secretion? -Pitfalls of GHRP-2 and ITT as GH provocative tests-

GH secretion is mainly regulated at the hypothalamus by a dual interplay between growth hormone releasing hormone (GHRH) and somatostatin, which are modulated by various factors. We examined the regulatory mechanism of GH secretion in an apparently healthy young man without decreased IGF-1 concentration and nocturnal GH secretion, but who showed low responses to insulin tolerance (ITT) and to GHRP-2 tests. The patient also had no GH response to acute aerobic exercise. However, he had normal secretion of pituitary hormone based on hypothalamic releasing hormone tests combined with CRH, GRH as GHRH, LH-RH and TRH. In addition, he had a GH response without paradoxical secretion to TRH stimulation as well as an ACTH response to subcutaneous glucagon stimulation, and AVP secretion responded to 5% hypertonic saline infusion, though it was not adequately stimulated by ITT. MRI showed no structural abnormalities in the hypothalamus-pituitary gland. These findings indicate that this subject may have an undiscovered neurocircuit for regulating GH secretion, as well as other neurohormones, to maintain homeostasis, even though there were low responses of the hormones to ITT and GHRP-2 stimuli, probably via altered secretion of hypothalamic hormones.

Concordant and discordant adrenocorticotropin (ACTH) responses induced by growth hormone-releasing peptide-2 (GHRP-2), corticotropin-releasing hormone (CRH) and insulin-induced hypoglycemia in patients with hypothalamopituitary disorders: evidence for direct ACTH releasing activity of GHRP-2

The insulin-induced hypoglycemia test (insulin tolerance test: ITT) and corticotropin-releasing hormone (CRH) test are used to examine the activities of the hypothalamo-pituitary-adrenal (HPA) axis. Growth hormone-releasing peptide-2 (GHRP-2), a potent GH secretagogue, also stimulates adrenocorticotropin (ACTH) secretion. To evaluate the role of GHRP-2 in assessing the HPA axis, we examined 6 patients with various hypothalamo-pituitary disorders, and measured ACTH and cortisol responses during provocative tests (ITT, CRH, and GHRP-2 test). None of the 6 patients showed any significant ACTH or cortisol responses to ITT, but significant ACTH release was observed during CRH and GHRP-2 tests. These findings suggest GHRP-2 may directly stimulate ACTH secretion in patients with hypothalamo-pituitary disorders.