As corticotroph adenomas are usually very small compared to other hormone-secreting adenomas, it is extremely difficult to study these tumours in vitro. The presence of GHS-R mRNA has been shown in some, but not all, corticotroph adenomas. Similarly, a response in terms of ACTH release and/or intracellular calcium concentration has been found in one but not in another preliminary report. The effect observed on ACTH release was actually greater than that seen after CRH stimulation on cells derived from the same adenoma. Although normal rat pituitary cells do not respond to GHS stimulation with ACTH release, this strong positive effect on human corticotroph adenoma cells is in accordance with in vivo data of ACTH responses in patients with Cushing's disease, and with data describing the presence and indeed over-expression of GHS-R in corticotroph adenomas. Some non-functioning pituitary adenomas express GHS-R, while others do not. In a preliminary study elevation of intracellular calcium levels has been observed in response to GHRP-6 by non-functioning adenomas in culture. The use of in vitro cell culture has led to a considerable increase in the understanding of the intracellular mechanisms of the action of GHSs on human pituitary cells. Specifically, the studies indicate that the PI-PKC/Ca transduction system plays a pivotal role in GH secretory control, perhaps involving a natural counterpart to GHSs. Additionally, the studies have raised the possibility that elevated GH secretion in acromegaly may be due to defects in this second messenger system, at least in some cases, while in lactotroph and corticotroph adenomas the expression of GHS-R may influence the hormone response to specific stimuli.
In spite of the large number of experimental and clinical studies with different GHS analogues, the exact mode of their GH-releasing action has yet to be fully established. A number of studies have suggested a possible direct activation of GHRH-secreting neurons in the arcuate nucleus: increased electrical activity and c-fos expression has been shown after the administration of GHSs; GHRH antisera attenuates the GH-releasing effect of GHSs; and increased hypophysial portal blood GHRH has been shown following GHS administration in sheep. Recently, the presence of GHS-R mRNA has been shown to co-localize to GHRH cells. Others have been unable to confirm the elevated GHRH levels in hypophysial portal blood samples, but have reported increased frequency of GHRH release after GHS administration. In the rat, but not in the guinea pig, very high doses of intracerebroventricular GHRP-6 produced a paradoxical decrease in circulating GH levels. GH secretagogues not only stimulate GH release but also stimulate prolactin and activate the hypothalamo-pituitary- adrenal (HPA) axis in both animal and human studies, an effect which is not inhibited by somatostatin (SS). Since GHSs do not stimulate ACTH release directly from pituitary cell cultures, it is probable that GHSs affect the HPA via either one of the two major ACTH stimulators in the hypothalamus, corticotrophin-releasing hormone (CRH) and/or arginine vasopressin (AVP). In our in vitro hypothalamic incubation system, none of the GHRP analogues in the dose range of 10-10 M had any effect on basal GHRH release at 20, 40, 60 or 80 min incubation. However, at 10 M a strong inhibitory effect was shown on both basal and potassium-chloride-stimulated GHRH release using GHRP-6, hexarelin and L-692,585. Somatostatin release was not inhibited by GHSs; on the contrary, a small, unexpected increase in somatostatin secretion was observed. Interestingly, an increase in somatostatin release was also observed by two other groups: in a rat fetal hypothalamic incubation system stimulation with 10 M GHRP-6 significantly increased the release of somatostatin, confirming our results in the adult rat. In a similar system to ours, significant somatostatin release was observed after stimulation with high concentrations of GHRP-6. The release of AVP was strongly stimulated by GHSs; however, we could not detect any change in CRH levels (data not shown).
The effects of active GHS analogues were not parallelled by using an inactive non-peptide analogue, L-692,428, ruling out a non-specific effect of high drug concentrations. Recently, neuropeptide Y has been implicated as a mediator of the effects of GHSs. It has been shown that NPY stimulates SS release and there have been suggestions that it might inhibit GHRH; however, this has never been shown directly. We found that NPY inhibited GHRH release from the hypothalamus in vitro. NPY also augmented potassium-induced AVP release, suggesting that it might be a possible mediator of the effects of GHSs in the hypothalamus. The effect of NPY would stimulate the HPA axis, in accordance with in vivo data, but the effect on the GH axis is opposite to what would be expected if it was the mediator of the principal effects of GHSs. However, certain in vivo studies show compatible results with NPY mediating the effects of GHSs. In a recent study it has been shown that hypophysial portal plasma concentrations of GHRH in sheep did not show a coincident release of GHRH after the intravenous bolus administration of GHRP-6; infusion of GHRP~6 caused no change in GHRH pulse amplitude but a 50 percent rise in pulse frequency, suggesting an effect on the frequency of the pulsatile discharge of GHRH. Furthermore, the results of experiments of intracerebroventricular administration of GH- releasing peptides on OH levels have failed to show consistent results. While in guinea pigs the expected rise was shown, in rats a paradoxical decrease in circulating GH levels was observed. The dose administered via the intracerebroventricular catheter in the latter study corresponds to a very high concentration of GHRP-6 in solution (2.3 x 10 M).
Our finding, that the release of GHRH is inhibited by the presence of similarly high doses of GHS analogues, is compatible with these results in the rat, while species-specific differences might explain the conflicting data in guinea pig and rat studies. The regulation of the GH axis in the rat is different from that of other species; for example, stress or hypoglycaemia causes GH inhibition in rats, while in guinea pigs or humans these tests cause stimulation of the GH axis. Thus, certain effects observed in rat experiments cannot be readily extrapolated to other species. Other studies indicated that GHSs inhibit the effect of SS. Our results suggest that certain of the endocrine effects of GHSs may be mediated by NPY, but it remains unclear as to whether the major GH-releasing effect in the hypothalamus involves direct activation of GHRH. Bowers and colleagues have long suggested the effects of GHSs on GH could not be explained purely in terms of the modulation of GHRH and SS, but required the presence of an unknown endogenous factor to fully explain their complex effects. Our results are compatible with this speculation. In summary, synthetic GH secretagogues act on a specific receptor which is present in normal and abnormal pituitary tissue and in the hypothalamus. They act via the protein kinase C phosphatidyl inositol Ca pathway in somatotrophs and augment the effect of GHRH on cAMP release.
The effect in the hypothalamus is controversial: somatostatin release is slightly stimulated by high doses of GHSs from the hypothalamus as has now been shown by three separate groups. There are a number of arguments suggesting a direct stimulation of GHRH release; however, we were not able to show this in rat hypothalamic tissue culture. In contrast, we found an opposite effect, and the apparent GH inhibitory effect of intracerebroventricular GHRP-6 is in Une with our findings. Clearly, the complex hypothalamic mechanism of effect of GHSs needs further investigation.
Our website is not responsible for the information contained by this article. Articleinput.com is a free articles resource thus practically any visitor can submit an article. However if you notice any copyrighted material, please contact us and we will remove the article(s) in discussion right away.
Note: This article was sent to us by: Walter N. at 12162009
1. Observations of in vitro hormonal studies
© 2009 ArticleInput.com.