Amounts Of alpha-MSH Research Articles

Immunoblotting technique to study release of melanophore-stimulating hormone from individual melanotrope cells of the intermediate lobe of Xenopus laevis.

The melanotrope cells in the pars intermedia in the pituitary of Xenopus laevis synthesize and release the melanophore-stimulating hormone (alpha MSH), a small peptide that causes skin darkening during the process of background adaptation. Evidence has been found for a heterogeneity in biosynthetic activity of the melanotrope cells. In the present study two questions were addressed: (1) does the melanotrope cell population also show heterogeneous alpha MSH-release, and (2) can this heterogeneity be changed by extracellular messengers? Since dopamine is known to inhibit alpha MSH-release, this messenger is used to study the regulation of the heterogeneity. To quantify alpha MSH-release from individual cells, a cell blotting procedure has been developed for the binding and relative quantification of the small alpha MSH peptide. The immunoblotting procedure involves binding of the cells to a carrier slide and binding of released alpha MSH to a nitrocellulose filter. After immunostaining, the amount of alpha MSH per cell was quantitated by image analysis. Untreated melanotrope cells reveal a distinct variability in alpha MSH-release, some cells showing low secretory activity, whereas others are strongly secreting, indicating heterogeneity of alpha MSH-release. Dopamine treatment strongly inhibits alpha MSH-release from individual cells, resulting in a clearly less pronounced melanotrope cell heterogeneity. The effect of dopamine appears to be dose-dependent as a low dopamine concentration has only a moderate effect on the alpha MSH-release. It is proposed that dopamine is a physiological regulator of the degree of melanotrope cell heterogeneity in alpha MSH-release.

Developmental changes in the processing of ACTH in the anterior pituitary of the amphibian,Ambystoma tigrinum

Acid extracts of anterior pituitaries obtained from larval, neotene, postmetamorphic young adult, and sexually active adult Ambystoma tigrinum were fractionated by gel filtration and analyzed by RIA. In larval, neotene, and postmetamorphic adults the molar ratio of ACTH(1-39) to alpha-MSH was, respectively, 1.3:1, 1:1.3, and 1:1.3. However, in sexually mature adults the molar ratio of ACTH(1-39) to alpha-MSH was 8:1. These results indicate that prior to and immediately after metamorphosis, both ACTH(1-39) and alpha-MSH are major end products of corticotropic cells in this species. As postmetamorphic animals reach sexual maturity, there is a shift in the processing of ACTH which yields ACTH(1-39) as the major end product of corticotropic cells. In the corticotropic cells of adults only trace amounts of alpha-MSH are produced.

Detection of Adrenocorticotropin-Related and α-Melanocyte-Stimulating Hormone-Related Substances in the Anterior Pituitary of Larval and AdultAmbystoma tigrinum(Class: Amphibia)

Immunohistochemical analysis revealed the colocalization of ACTH-related immunoreactive forms and alpha-MSH-related immunoreactive forms in corticotropic cells of the anterior pituitary of larval Ambystoma tigrinum. Subsequent analysis of acid extracts of anterior pituitaries obtained from larval, neotene, and postmetamorphic adults indicated that the molar ratio of ACTH(1-39) to alpha-MSH was, respectively, 2.5:1, 1:1.5, and 1:1.3. However, in sexually mature adults the molar ratio of ACTH(1-39) to alpha-MSH was 8:1. These results indicate that before and immediately after metamorphosis, both ACTH(1-39) and alpha-MSH are major end products of corticotropic cells in this species. As postmetamorphic animals reach sexual maturity, ACTH(1-39) is a major end product of corticotropic cells and only trace amounts of alpha-MSH are produced. Thus in A. tigrinum the proteolytic processing of ACTH in corticotropic cells is developmentally regulated. The alpha-MSH-related peptide produced by the corticotropic cells appears to be an ACTH(1-13)amide-like form.

Dopaminergic regulation of alpha-melanocyte-stimulating hormone and N-acetyl-beta-endorphin secretion in the fetal lamb.

alpha MSH is present in high concentrations in the intermediate lobe of the fetal pituitary and has been implicated as a regulator of fetal adrenal steroidogenesis and fetal growth. However, there are few data regarding alpha MSH levels in fetal plasma or the control of fetal alpha MSH secretion. We measured alpha MSH immunoactivity in the plasma of chronically catheterized fetal lambs (gestational age, 116-138 days), newborn lambs, and adult sheep both in the baseline state and after dopamine receptor blockade with metoclopramide. The effect of metoclopramide on the release of another proopiomelanocortin-derived peptide, N-acetyl-beta-endorphin (N-acetyl-beta EP), which is synthesized together with alpha MSH in the intermediate lobe, was also studied. Baseline fetal plasma alpha MSH was significantly greater than maternal alpha MSH [35.6 +/- 2.2 (+/- SEM) vs. 10.0 +/- 1.0 pg/ml]. In eight studies in five fetal lambs, alpha MSH rose to a peak level of 121 +/- 23 pg/ml 15 min after metoclopramide administration to the fetus. Simultaneous maternal alpha MSH levels did not change, suggesting that the alpha MSH in fetal plasma was of fetal pituitary origin. Gel filtration of pooled fetal plasma extracts revealed that the alpha MSH immunoactivity eluted in the same position as the alpha MSH standard. Metoclopramide caused the secretion of nearly equimolar amounts of alpha MSH and N-acetyl-beta EP into fetal plasma. In four fetal lambs, basal N-acetyl-beta EP levels of 156 +/- 34 pg/ml rose to 305 +/- 65 pg/ml 15 min after metoclopramide treatment. Metoclopramide also stimulated plasma alpha MSH in newborn and adult sheep. In six newborn lambs, alpha MSH rose from 45.2 +/- 13 to 211 +/- 38 pg/ml 15 min after metoclopramide treatment, whereas in four adult sheep, a basal alpha MSH level of 11.1 +/- 2.2 pg/ml rose to 20.1 +/- 2.7 pg/ml 15 min after metoclopramide. In addition, metoclopramide stimulated fetal and neonatal PRL secretion, but had no effect on plasma vasopressin concentrations or acid-base and blood gas values. These studies indicate that immunoreactive alpha MSH and N-acetyl-beta EP are secreted into ovine fetal plasma and that the secretion of these peptides in the fetus appears to be under tonic dopamine inhibition, as is the case in the adult sheep and newborn lamb.

Distribution, subcellular localization and identity of immunoreactive alpha-melanotropin in the pituitary gland and brain.

In homogenates of adult human or rat hypothalamic tissue, immunoreactive alpha-MSH (alpha-MSHi) is concentrated in synaptosomes, suggesting that it is localized in neurons. This conclusion is supported by immunohistochemical results on the localization of alpha-MSHi in the brain. The function of alpha-MSHi in the hypothalamus is undefined but the amount of alpha-MSH in this tissue increases in young rats and decreases in old rats. There is little alpha-MSHi in pituitary glands from human abortuses before the 15th gestational week. Thereafter the amount of alpha-MSHi in the anterior and neurointermediate lobes increases appreciably. When acetic acid extracts of each lobe were analysed by high-pressure liquid chromatography the alpha-MSHi in extracts of the anterior lobe had a retention time equal to that of desacetyl alpha-MSH; little if any alpha-MSHi had a retention time identical to that of desacetyl alpha-MSH; alpha-MSH, if present in these extracts, appeared to be a minor component. A similar analysis of extracts of adult human hypothalamic tissue yielded results suggesting that the alpha-MSHi in this tissue is also attributable largely to desacetyl alpha-MSH.