Stimulation of growth hormone release in vivo and in vitro by alpha-MSH

Functions of alpha-melanotropin and other opiomelanocortin peptides in labour, intrauterine growth and brain development

In a number of animals and in humans, factors from the fetal hypothalamus function in intrauterine growth, in labour and in brain development. Peptides of the opiomelanocortin family are produced by the pituitary, brain and placenta and are probably involved in these developmental processes. In the rat, alpha-MSH stimulates fetal growth, protein synthesis, wound healing and liver regeneration and it reduces periosteal bone resorption. In chick embryos, alpha-MSH restores the corticosteroid-induced growth retardation. Thus alpha-MSH seems to possess general trophic properties. The fetal brain in humans is involved in timing the moment of birth. This process is probably mediated by peptides of the opiomelanocortin family as suggested from observations in anencephaly and other congenital brain anomalies and from the influence of corticosteroids or ACTH on labour. The high percentage of premature deliveries in heroin addicts is worth examining endocrinologically, in this respect. The exact nature of the peptides and mechanisms involved in labour is not yet known. Some peptides of the opiomelanocortin family induce an acceleration of brain development. Neonatal treatment of rats with alpha-MSH alters their later behaviour while ACTH fragments accelerate the onset of eye-opening. Opiates and methadone inhibit brain development, and neonatal administration of beta-endorphin or naloxone causes permanent insensibility to temperature stimuli. The interrelated nature of the fetal pituitary, brain and placenta does not, at present, allow us to pin down which of these structures is primarily involved in the regulation of intrauterine growth, labour and brain development.

Evidence that corticotropin-releasing hormone acts as a growth hormone-releasing factor in a primitive teleost, the European eel (Anguilla anguilla)

The inhibitory control of growth hormone (GH) release by somatostatin (SRIH) has been conserved throughout vertebrate evolution. In contrast, the neuropeptides involved in the stimulatory control of GH vary according to species and/or physiological situations. We investigated the direct pituitary regulation of GH release in a primitive teleost, the European eel (Anguilla anguilla L.) at the juvenile stage. Short-term serum-free primary cultures of dispersed pituitary cells were used, and GH release was measured by an homologous radioimmunoassay. Whereas growth hormone-releasing hormone (GHRH), gonadotropin-releasing hormone (GnRH), thyrotropin-releasing hormone (TRH), neuropeptide Y (NPY) and cholecystokinin (CCK) failed to induce any change in GH release, corticotropin-releasing hormone (CRH) dose-dependently stimulated GH release with a significant effect at 1 nM and a maximal effect (> or =400% of controls at 24 h) at 100 nM. In agreement with our previous studies, PACAP also stimulated GH release but its maximal effect was lower than that of CRH. Proopiomelanocortin (POMC)-peptides, corticotropin (ACTH), melanotropin (alpha-MSH), beta-endorphin) had no effect on GH release, at any dose tested (0.1-1000 nM), indicating that the stimulatory effect of CRH on GH release by somatotrophs was not mediated by CRH-induced release of POMC-peptides from corticotrophs and melanotrophs. The CRH antagonist, alpha-helical CRH(9-41), significantly inhibited the stimulatory effect of CRH on GH release, suggesting the implication of specific CRH receptors related to mammalian ones. The stimulatory effect of CRH on GH release was reduced after 24 h of incubation, indicating a desensitization. In contrast, no desensitization to the inhibitory effect of SRIH was observed. SRIH inhibited CRH action in a dose-dependent manner. The effect of SRIH was overriding, 1 nM SRIH being able to abolish the effect of 1000 nM CRH. In conclusion, in the eel, CRH stimulates GH release directly at the pituitary cell level. GH and cortisol secretions could interact in controlling several physiological functions such as metabolism and ion exchange. This study suggests that CRH may have played an important early role in vertebrates co-ordinating the activation of various endocrine axes involved in metamorphosis, osmoregulation, stress and fasting. The stimulatory role of CRH on GH release may have been partially conserved during evolution, as it is found in some human physio-pathological situations such as stress, fasting and depression.

Superfused pituitary cell cultures: effects of culture conditions on apparent responsiveness to LHRH stimulation administered as short duration pulses

We wished to study estrous cycle related differences in LH and FSH responsiveness to pulsatile LHRH. Such studies are very difficult to perform in vivo under controlled conditions; therefore, an in vitro superfused anterior pituitary cell culture system was evaluated for its capacity to support differences in estrous stage associated LHRH responsiveness. Three vital culture system parameters were evaluated; these parameters were (1) culture medium composition, (2) duration allowed for cell attachment to microcarrier beads and (3) superfusion flow rate utilized during pulsatile LHRH stimulation. It was found that a culture system which utilized 10% Nu Serum in DMEM (final protein concentration of 1.8 mg/ml; final serum concentration of 2.5%), an attachment time of 48 hrs and a flow rate of 0.125 ml/min most successfully maximized LH responsiveness at the lowest serum concentration. These studies indicated that although one may be able to observe LHRH responsiveness under a wide range of culture conditions, responsiveness may nonetheless be maximized by judicious adjustment of culture conditions.

Comparative biological activities of potent active-site analogues of alpha-melanotropin. Effect of tyrosine substitution at position-4

We have prepared several alpha-melanotropin (alpha-MSH) analogues with tyrosine substituted for methionine at the 4-position and determined their melanotropic activities on the frog (Rana pipiens), lizard (Anolis carolinensis) and S-91 (Cloudman) mouse melanoma adenylate cyclase bioassays. The potencies of Ac-[Tyr4]-alpha-MSH4-10-NH2 and Ac-[Tyr4]-alpha-MSH4-11-NH2 were compared with alpha-MSH and with their corresponding methionine and norleucine substituted analogues. The Tyr-4 analogues were found to be less active than the Nle-4 analogues on both the frog and lizard assays. Ac-[Tyr4]-alpha-MSH4-10-NH2 was found to be less active than Ac-[Tyr4]-alpha-MSH4-11-NH2 on the lizard bioassay, but more active than the longer fragment on the frog skin assay. Ac-[Tyr4]-alpha-MSH4-10-NH2 exhibited extremely prolonged biological activity on frog skin, but not on lizard skin, while the melanotropic activity of Ac-[Tyr4]-alpha-MSH4-11-NH2 was rapidly reversed on both assay systems. The increased potency of Ac-[Tyr4]-alpha-MSH4-10-NH2 over Ac-[Tyr4]-alpha-MSH4-11-NH2 on frog melanocytes may be related to the fact that the shorter 4-10 analogue exhibits prolonged biological activity. Interestingly, it was found that both Tyr-4 analogues were partial agonists on the mouse melanoma adenylate cyclase bioassay, and stimulated the enzyme to only about 50% of the maximal activity of alpha-MSH. We reported previously that replacement of L-Phe-7 by its D-enantiomer in [Nle4]-alpha-MSH and its Nle-4 containing analogues resulted in peptides with increased potency and in some instances prolonged activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Peptides of the neurointermediary lobe of rat pituitary stimulate GH secretion in vitro

alpha-MSH and other fragments of ACTH are potent stimulators of GH release in vivo. The action of such peptides and of extracts of the neurointermediary lobe (NIL) of rat pituitary, a source of endogenous MSH-related peptides, on GH release was investigated in vitro. Peptides with the core sequence of alpha-MSH stimulate GH secretion by primary cultures of rat anterior pituitary cells; however, both the absolute and the relative potencies of these peptides exclude the involvement of melanotropic receptors comparable in specificity to the extrapituitary receptors for these hormones. Extracts of the NIL of rat pituitary stimulate GH release in vitro and the bulk of the releasing activity can be attributed to one (or several) factors with an apparent mass of approx. 10 000 M.W. that can be partially purified by HPLC. The active principle appears to be distinct from both beta-LPH and the human pancreatic GHRF. Thus, while rat NIL contains GH-releasing activity that can be demonstrated in vitro, a direct link to the potent action of MSH-related peptides on GH release in vivo cannot be established, and the action of these peptides in vivo must therefore rely on mechanisms which are not expressed in the in vitro system.