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Biomedical subjects

M E Hadley

Publications and source records attributed to M E Hadley.

At least 109 records · Page 6Linked to original sources
PubMed1982

[half-Cys4,half-Cys10]-alpha-Melanocyte-stimulating hormone: a cyclic alpha-melanotropin exhibiting superagonist biological activity.

alpha-Melanocyte-stimulating hormone (alpha-melanotropin; alpha-MSH) is a linear tridecapeptide (Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH2) that reversibly darkens amphibian skins by stimulating melanomsome (pigment granule) dispersion within melanophores. By using a number of in vitro melanocyte assays, we have examined the conformational requirements for alpha-MSH activity. Synthesis of [half-Cys4,half-Cys10]-alpha-MSH, a cyclic, conformationally restricted, "isosteric" analogue of alpha-MSH, provided a melanotropin with a potency greater than 10,000 times that of the native hormone in stimulating frog (Rana pipiens) skin darkening. The cyclic analogue also showed substantially prolonged activity relative to the native hormone. [half-Cys4,half-Cys10]-alpha-MSH was approximately 30 times more potent than alpha-MSH in stimulating lizard (Anolis carolinensis) skin melanophores in vitro. By using a cell-free Cloudman S-91 mouse melanoma plasma membrane preparation, we found the cyclic analogue to be approximately 3 times as potent as the native hormone in stimulating adenylate cyclase activity. These results provide insight into the conformational requirements for biological activity of alpha-MSH, and the comparative conformational requirements of alpha-MSH at a number of pigment cell receptors.

Adenylyl Cyclases↗
PubMed1981

Calcium-dependent prolonged effects on melanophores of [4-norleucine, 7-D-phenylalanine]-alpha-melanotropin.

A single injection of the melanotropin analog [4-norleucine, 7-D-phenylalanine]-alpha-melanotropin into frogs (Rana pipiens) caused near maximum darkening of the skins of the frogs for at least 6 weeks. Injections of the natural hormone alpha-melanotropin or of the analog [Nle4]-alpha-melanotropin also caused darkening, but this effect lasted only a few days. Morphological examination of the skins of frogs injected with [Nle4, D-Phe7]-alpha-melanotropin revealed that both dermal and epidermal melanophores were dispersed during the entire 6-week period. In vitro [Nle4, D-Phe7]-alpha-melanotropin also causes prolonged darkening of the skin of the lizard Anolis carolinensis. In the absence of the melanotropin, skins previously darkened with the analog could be lightened by removal of calcium from the incubation medium but could then be redarkened by adding calcium. The cycle could be repeated indefinitely without addition of melanotropin. These results demonstrate the role of calcium in receptor signal transduction and the prolonged biological effects of [Nle4, D-Phe7]-alpha-melanotropin long after its removal from the assay medium.

Animals↗
PubMed1981

Biological actions of melanocyte-stimulating hormone.

Melanocyte-stimulating hormone (alpha-melanotropin, MSH) may function in a number of diverse physiological roles. MSH stimulates (1) rapid translocation of melanosomes (melanin granules) in dermal melanophores to effect rapid colour change and (2) melanogenesis in normal and abnormal (melanoma) epidermal melanocytes. Both actions involve (1) initial binding of the peptide on the melanocyte membrane, (2) transduction of signal to adenylate cyclase, and (3) increased cytosolic levels of cyclic AMP. Efforts to prepare radioiodinated MSH and analogues for radioreceptor studies using melanoma membranes and intact cells reveal that conventional iodination procedures inactivate the hormone because of oxidative and iodination effects on specific structural components of the peptide. These effects can be circumvented by the use of synthetically tailored MSH analogues. Transduction of signal from receptor to adenylate cyclase requires calcium, but prostaglandin or beta-adrenoceptor stimulation of melanophores does not. The nucleotide and metal ion requirements for mouse melanoma adenylate cyclase activity have been characterized. There is both a transcriptional and translational requirement for MSH stimulation of tyrosinase activity and melanin production in melanoma cells. Melanosome translocation within melanophores is enhanced in the absence of extracellular calcium. A model for the MSH control of melanosome movements suggests a bifunctional, but compartmentalized, role for calcium in the action of MSH.

Adenylyl Cyclases↗
PubMed1981

Racemization effects on melanocyte-stimulating hormones and related peptides.

Heat-alkali treatment of synthetic alpha- and beta-melanocyte-stimulating hormones (MSH), known to cause racemization of amino acids within the peptides, results in prolongation of the darkening (melanophore dispersion) effect of these hormones on frog and lizard skins in vitro. Skins remain darkened for hours or even days if supramaximal concentrations of the racemized hormones are used. This response can be partially reversed by melatonin or noradrenaline. Heat-alkali treatment of alpha-MSH at either 60 or 97 degrees C results in a retardation of the response of the skins to the racemized peptides. In contrast, the response of frog skins to heat-alkali-treated beta-MSH is immediately enhanced and potentiated. Heat-alkali treatment also prolongs and potentiates the activity of synthetic [des-acetyl]-alpha-MSH (in contrast to the retardation effect on the natural acetylated peptide). These data suggest a role for the N-acetyl group in the retardation phenomenon. The activity of synthetic [2-D-tyrosine]-alpha-MSh is much lower than that of alpha-MSH itself, indicating that heat-alkali treatment of the hormone may produce either potentiation or partial inactivation of the peptide, depending on the site of racemization.

Animals↗
PubMed1980

Synthesis and structure-function studies of melanocyte stimulating hormone analogues modified in the 2 and 4(7) positions: comparison of activities on frog skin melanophores and melanoma adenylate cyclase.

The synthesis and purification of several analogues of the melanotropins with amino acid substitutions at the tyrosine-2 and methionine-4(7) positions are reported. The compounds synthesized included [4-norleucine]-alpha-MSH, [7-norleucine]-beta p-MSH, [2-3',5'-diiodotyrosine]-alpha-MSH, [2-D-tyrosine]-alpha-MSH, and [2-phenylalanine,4-norleucine]-alpha-MSH. The biological activities of these derivatives were measured and compared on normal melanocytes (frog skins) and on transformed melanocytes (mouse melanoma adenylate cyclase), over the entire dose-response range. All compounds tested were full agonists in both assay systems but varied considerably in potency. The relative potencies in the frog skin assay (alpha-MSH = 1.0) were as follows: [Nle7]-beta p-MSH (5.2) > [Nle4]-alpha-MSH (2.3) > alpha-MSH (1.0) > [Phe2,Nle4]-alpha-MSH (0.80) > beta p-MSH (0.55) > [I2-Tyr2]-alpha-MSH (0.12) > [D-Tyr2]-alpha-MSH (0.04). The relative potencies in the melanoma adenylate cyclase system were [Nle7]-beta p-MSH (4.2) > beta p-MSH (2.2) > [Nle4]-alpha-MSH (2.0) > alpha-MSH (1.0) approximately equal to [Phe2,Nle4]-alpha-MSH (0.9) > [I2-Tyr2]-alpha-MSH (0.40) > [D-Tyr2]-alpha-MSH (0.20). There appears to be some differences in structural specificity at the melanotropin receptors of the two cell systems.

Adenylyl Cyclases↗
PubMed1980

4-Norleucine, 7-D-phenylalanine-alpha-melanocyte-stimulating hormone: a highly potent alpha-melanotropin with ultralong biological activity.

alpha-Melanocyte-stimulating hormone (alpha-MSH) reversibly darkens frog skins by stimulating melanosome movement (dispersion) within melanophores. Heat-alkali treatment of alpha-MSH results in prolonged biological activity of the hormone. Quantitative gas chromatographic analysis of the hydrolyzed heat-alkali-treated peptide revealed partial racemization particularly at the 4(methionine) and 7(phenylalanine) positions. [Nle4]-alpha-MSH, a synthetic analogue of alpha-MSH, reversibly darkens frog skins and also exhibits prolonged activity after heat-alkali treatment. Synthesis of [Nle4, D-Phe7]-alpha-MSH provided an analogue with prolonged biological activity identical to that observed with heat-alkali-treated alpha-MSH or [Nle4]-alpha-MSH. [Nle4, D-Phe7]-alpha-MSH was resistant to enzymatic degradation by serum enzymes. In addition, this peptide exhibited dramatically increased biological activity as determined by frog skin bioassay, activation of mouse melanoma adenylate cyclase, and stimulation of mouse melanoma cell tyrosinase activity. This Nle4, D-Phe7 synthetic analogue of alpha-MSH is a very porent melanotropin, 26 times as potent as alpha-MSH in the adenylate cyclase assay. The resistance of the peptide to enzymatic degradation and its extraordinarily potent and prolonged biological activity should make this analogue of alpha-MSH an important molecular probe for studying the melanotropin receptors of both normal and abnormal (melanoma) melanocytes.

Animals↗
PubMed1980

Synthesis, pharmacological, conformational, and dynamic studies of the potent hormone antagonists [1-penicillamine, 4-threonine]-oxytocin and [1-penicillamine, 2-phenylalanine, 4-threonine]-oxytocin. Conformational and dynamic considerations in the design of antagonists.

The solid phase synthesis of [1-penicillamine, 4-threonine]-oxytocin and [1-penicillamine, 2-phenylalanine, 4-threonine]-oxytocin is reported. The two compounds have no in vitro milk ejecting activity and no in vivo or in vitro oxytocic activity, but both are potent antagonists in these three assay systems. In the in vitro oxytocic assay, [1-penicillamine, 4-threonine]- and [1-penicillamine, 2-phenylalanine, 4-threonine]-oxytocin have pA2 values of 7.55 +/- 0.04 and 7.67 +/- 0.02, respectively, and both inhibit the uterine contractile response to oxytocin in nonpregnant and pregnant rats. [1-Penicillamine, 2-phenylalanine, 4-threonine]-oxytocin has a weak antipressor activity and at high doses, consistently caused a weak and transient fall in blood pressure in the rat. Carbon-13 nuclear magnetic resonance chemical shift parameters and spin-lattice relaxation times (T1) indicate that these two new oxytocin antagonists have very similar conformation and dynamic properties to oxytocin inhibitors which have previously been examined. These results are discussed in terms of conformational and dynamic models of oxytocin antagonism at the uterus. It is suggested that conformational restrictions at the 2- and 4-positions of penicillamine-1 analogues of oxytocin are important to antagonist activity and potency.

Amino Acid Sequence↗
PubMed1980

Synthesis of alpha- and beta-melanocyte stimulating hormones.

The solid phase synthesis of alpha-melanocyte stimulating hormone (alpha-MSH) using the benzhydrylamine resin and a number of recently described side chain protecting groups is given. The carboxamide terminal peptide was obtained directly by treatment of the peptide resin with liquid hydrogen fluoride at 0 degrees in the presence of carbonium ion scavengers. The solid phase synthesis of the C-terminal carboxylate hormone, porcine beta-melanocyte stimulating hormone (beta p-MSH), using a 1% cross-linked Merrifield resin and the same side chain protecting groups as in the alpha-MSH synthesis also is presented. Purification of both peptides was carried out by conventional chromatographic techniques. Both hormones were fully active and beta p-MSH was slightly more potent than previously reported in the literature.

Animals↗
PubMed1979

[1-Penicillamine,2-leucine]oxytocin. Synthesis and pharmacological and conformational studies of a potent peptide hormone inhibitor.

[1-Penicillamine,2-leucine]oxytocin was synthesized by the solid-phase method of peptide synthesis and purified by partition chromatography on Sephadex G-25, followed by gel filtration. The peptide was found to be a very potent competitive inhibitor of oxytocin in the oxytocic assay with a pA2 of 7.14 and an inhibitor of oxytocin in the milk-ejecting assay. The compound showed no agonist activity in either of these assays, and its inhibitory activity at the uterus was of prolonged duration. The 13C nuclear magnetic resonance spectral properties and the 13C T1 (spin-lattice) relaxation times of [Pen1,Leu2]oxytocin were determined, and the results were compared with previous studies of [Pen1]oxytocin, a related competitive inhibitor, and oxytocin, the native hormone agonist. These studies indicated that the hormone inhibitors [Pen1,Leu2]oxytocin and [Pen1]oxytocin have similar conformational and dynamic properties which are different than those of the agonist, oxytocin.

Animals↗
PubMed1979

Failure to demonstrate antigonadotrophic activities of arginine vasotocin in the mouse.

Arginine vasotocin (AVT) has previously been proposed to be a pineal hormone which reduces reproductive competency in mammals. The effects of AVT on the growth of immature reproductive organs of the mouse have been studied using blind study techniques. Evidence is presented which fails to support the proposed role of AVT as a mammalian antigonadotrophin.

Animals↗
PubMed1978

Secretion of melanophore-stimulating hormone (MSH) in long-term cultures of pituitary neurointermediate lobes.

Neurointermediate lobes from pituitaries of the frog, Rana berlandieri forreri (Rana pipiens, sensu lato), were maintained in organ culture in media with and without serum for up to six months. The cultured tissues were examined periodically by light microscopy and transmission electron microscopy and by bioassay of the melanophore-stimulating hormone (MSH) secreted and present in the culture media. Light-microscopic observations revealed a high degree of preservation of the pars intermedia at four weeks with isolated areas of some glands maintaining histological integrity for the entire six months. Similarly, at the ultrastructural level the cells appeared morphologically intact and to be actively synthesizing and secreting hormone. Bioassays showed the glands to be continuously secreting MSH; however, larger yields of hormone were obtained in media lacking serum. No significant ultrastructural differences between cells grown in the presence or absence of serum were detected. The difference in concentration of MSH between the two groups therefore apparently results from enzymatic degradation of the hormone by the serum. Organ culture of the vertebrate neurointermediate lobe may provide a unique method for the production of large quantities of MSH and for the study of other melanotropic and opiate peptides as they may be synthesized and secreted by the pars intermedia.

Animals↗