beta-cell tropin, a peptide of the pituitary pars intermedia which stimulates insulin release

Effects of different green light intensities on the growth performance and endocrine properties of barfin flounder Verasper moseri

We previously reported that the somatic growth of barfin flounder, Verasper moseri, was effectively stimulated by the green light compared to the blue and red lights. Herein, we report the effects of different green light intensities on the growth and endocrine system of the fish. Fish were reared in a dark room with light from a light-emitting diode (LED) at a peak wavelength of 518nm under controlled photoperiod (10.5:13.5h, light:dark cycle; 06:00-16:30, light) with three levels of photon flux density (PFD)-2 (low), 7 (medium), or 21 (high) μmol·m^-2·s^-1 at the water surface. The average water temperature was 10.2°C, and the fish were fed until satiety. The fish reared under high PFD of green light showed the highest specific growth rates, followed by the medium PFD group. Under high PFD, the fish showed the highest amount of melanin-concentrating hormone mRNA in their brains and insulin in plasma, while the lowest amount of growth hormone was observed in their pituitary glands. These results suggest that the green light stimulated the growth of barfin flounders in a light intensity-dependent manner in association with their central and peripheral endocrine systems. However, when the fish were reared in an ordinary room where they received both ambient and green LED lights, the fish under LED and ambient light grew faster than those under ambient light only (control). Moreover, no difference was observed in the specific growth rate of the fish reared under the three different green LED light intensities, suggesting that the growth was equally stimulated by the green light within a certain range of intensities under ambient light.

Beta-cell-tropin is associated with short-term stimulation of food intake in chicks

Beta-cell-tropin, a peptide derived from adrenocorticotropic hormone, is an insulin secretagogue. When centrally injected, it increases food intake in rats, but its appetite-associated effects have not been reported in any other species. Thus, the present study was designed to evaluate the effects of central beta-cell-tropin on appetite-associated parameters in an alternative vertebrate model, the chick. Central injection of 2 or 4 nmol beta-cell-tropin increased food intake for 60 min. Whole hypothalamus was collected at 60 min post-injection, and real-time PCR performed to measure mRNA abundance of agouti-related peptide, corticotropin releasing factor, galanin, melanin concentrating hormone, neuropeptide Y, orexin, prohormone convertase 2, pro-opiomelanocortin, peroxisome proliferator-activated receptor γ, urotensin 2, and visfatin, not one of which were affected by beta-cell-tropin treatment. Results demonstrate that beta-cell-tropin is associated with short-term stimulation of food intake.

PP/PYY cells from endocrine pancreas of the scincid lizard Eumeces inexpectatus synthesize ACTH- and alpha-MSH-like molecules

The endocrine pancreas of the scincid lizard Eumeces inexpectatus secretes four major hormones, insulin, glucagon, somatostatin, and pancreatic polypeptide (PP); in addition, other peptides and neuropeptides, often colocalized in one of the principal cell types (A, B, D, and PP), were detected by light and ultrastructural immunocytochemistry. In particular, the pancreas is rich in peptide tyrosine tyrosine (PYY), ACTH, and alpha-MSH immunoreactivity. When single- and double-immunolabeled serial sections were compared for immunostaining for PP, PYY, ACTH, and alpha-MSH, there was broad coincidence with PP, termed PP/PYY, cells in view of the extensive colocalization of these two peptides. Furthermore, ultrastructural morphometric studies revealed similar secretory granules for PP immunoreactive (ir) and ACTH ir cells, while the endocrine cells express pro-opiomelanocortin (POMC) mRNA, indicating an active, extrapituitary synthesis of the POMC-derived peptides in these cells. In conclusion, the presence of POMC-derived peptides in the endocrine pancreatic cells suggests that they may regulate insulin secretion.

Modulatory mechanism of ACTH on insulin secretion: effect on cytosolic Ca2+, membrane potential and Ca(2+-ATPase activity

The aim of this work was to get some insight into the mechanism by which ACTH produces its enhancing effect on glucose-induced insulin secretion. For this purpose we have determined: a) the release of insulin by isolated rat islets incubated with 3.3 or 16.6 mM glucose with or without the addition of 500 pg/ml ACTH, together with the changes induced by ACTH on b) cytosolic [Ca2+] of isolated B cells, c) islet plasma membrane Ca(2+)-ATPase activity and d) changes in membrane potential of single mouse islets. ACTH significantly enhanced the release of insulin elicited by either 3.3 or 16.6 mM glucose. This hormone concentration also induced a significant increase in the cytosolic [Ca2+] in isolated B cells. ACTH did not produce B cell membrane depolarization. Conversely, ACTH produced a significant decrease in islet plasma membrane Ca(2+)-ATPase activity. These results suggest that ACTH in concentrations similar to those attained by the endogenous peptide at the islet interstitium exerts its positive modulation on glucose-induced secretion of insulin, at least partly through its increasing effect on cytosolic [Ca2+] of B cells. The latter might be the consequence of the decreasing effect of ACTH on Ca(2+)-ATPase activity rather than to stimulation of voltage-dependent Ca(2+)-channels.

Obesity associated insulin resistance occurs in hypopituitary subjects

The study aimed to assess the role of pituitary hormones in the relationship between insulin resistance and obesity. Twelve male subjects with hypopituitarism secondary to surgery for a non-functioning pituitary adenoma were recruited. Subjects were receiving replacement hormones. Fasting plasma samples were taken for insulin, C-peptide, glucose, and lipids. Body mass index and blood pressure were measured. In hypopituitary subjects on replacement therapy, a positive association between body mass index and fasting plasma insulin levels (r = 0.76, p < 0.004) was similar to that found in normal subjects. In conclusion pituitary function does not have a direct role in determining the relationship between fasting plasma insulin, an index of insulin resistance, and obesity.

Glucose-induced secretion of ACTH-like products by rat pancreatic islets

This work was performed to study the release of proopiomelanocortin (POMC)-derived peptides from isolated pancreatic islets and the effect of ACTH--a member of that peptide family--on insulin secretion. Islets were incubated with 3,3 and 16.6 mM glucose and insulin and ACTH-like products (ACTH-LP) were measured by radioimmunoassay. Glucose stimulated the simultaneous release of insulin and ACTH-LP, the ACTH-LP concentration being higher when assayed with an antibody reacting with the N-terminus of ACTH. However, the increment in this release in the presence of the higher glucose concentration was larger when measured with an antibody against the ACTH mid-portion. Thus, although the islets would release more of a smaller ACTH-LP, 16.6 mM glucose would selectively increase the release of peptides of larger molecular size. Islets incubated with different concentrations of synthetic ACTH (50-500 pg/ml) increased the release of insulin in a dose-dependent manner. These results suggest that the release of endogenous ACTH-LP could contribute to the paracrine regulation of insulin secretion.

The effects of insulin and the pituitary peptide beta-cell tropin on the incorporation of D-3-3H-glucose into lipid in brown adipocytes from lactating and non-lactating rats

Lactating and non-lactating rat brown adipocytes were used to study the dose-dependent stimulation of lipogenesis by Beta-cell tropin (BCT) and insulin. In non-lactating animals BCT increased lipogenesis approximately 2-fold compared to a 3-fold stimulation with insulin; however BCT was effective at a substantially lower molar concentration than insulin. In lactating animals resistance was observed to both BCT and insulin action.

Neuro-humoral control of insulin secretion

Hypothalamic extract from normal rats was shown to contain a principle able to stimulate insulin secretion both in vivo and in vitro. This substance, whose peptidic nature was demonstrated by enzymatic digestion, has a molecular weight of about 1000 Daltons. An active release of this peptide was obtained when incubating hypothalamic fragments under appropriate stimulatory conditions (i.e. KCl 50 mmol l-1 in the incubation medium) (i.e. molecular weight and insulin secretion stimulating activity) similar to the hypothalamic peptide was evidentiated in the plasma of normal rats. Since manipulations of hte hypothalamus (i.e. electrical lesions or stimulations) could respectively decrease or enhance the plasma concentration of this substance, the hypothalamic origin of the insulin secretion stimulating principle present in the plasma was strongly suggested. The possible participation of this hypothalamic peptide to the neural control of insulin secretion is suggested.

The hypothalamic origin of an insulin secretion promoting factor present in the plasma of normal rats

Abstract Hypothalamic and plasma extracts from normal rats both contain a factor whose property is to stimulate insulin secretion. The two factors have many characteristics in common. Both are active in vivo (when administered to normal recipient rats) and in vitro (when infused into perfused rat pancreases). Both are inactivated by HCI hydrolysis. Both elute at the same ratio between elution volume of the peaks and void volume after gel-chromatographies are carried out, first on Sephadex G-50, then on Biogel P-2, indicating that they have a similar molecular weight which ranges between 800 and 1,200 Daltons. When anaesthetized rats are submitted to an acute bilateral electrical stimulation of the lateral hypothalamus, the insulin secretion stimulating activity present in the plasma is enhanced compared to that measured in sham-stimulated control rats. Conversely, the insulin secretion promoting activity is decreased in plasma from rats whose ventral hypothalamus is acutely and electrolytically lesioned. These data indicate that, in normal rats, the hypothalamus contains an insulin secretion promoting factor that is releasable in the plasma.

The effects of structural modifications on the insulin-releasing activity of beta-cell-tropin

The minimal effective concentration of the pituitary insulin secretagogue beta-cell-tropin (beta-CT) on the in vitro perfused pancreas was established and the effects of various modifications of the peptide on its potency were tested: iodination with 127I and acetylation reduced the insulin-releasing activity of beta-cell-tropin, and the C-terminal fragments beta-CT-(2-18), beta-CT-(3-18) and beta-CT-(6-18) were all less potent than the intact molecule; beta-CT-(1-6) was not active and did not inhibit beta-CT-induced insulin release.

Significance of mirex-caused hypoglycemia and hyperlipidemia in rats

Treatment of rats with mirex (40 ppm in diet) caused hypoglycemia, liver enlargement, and inhibition of adrenal corticosteroid-synthesizing enzyme activity. At toxic dosages (20,000 ppm mirex in diet, which has a lethal toxicity-50 [LT-50] of ten days) poisoned female rats showed severe hypoglycemia, fatty liver, adrenal hyperplasia, hypophagia, lipid mobilization, and body weight (bw) loss. A 50 micrograms/kg intraperitoneal (IP) dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in male rats caused similar effects two days posttreatment. Hypoglycemia could be overcome by prednisone (which also inhibited adrenocorticoid-synthesizing enzyme activities) but not by streptozotocin treatment, indicating that hypoglycemia may be related to glucocorticoid deficiency resulting from inhibition of their synthesis and not by direct effects on pancreatic beta-cells. Glucocorticoid deficiency could also cause increased release of adrenocorticoid hormone (ACTH), which may enhance fat mobilization caused by hypophagia.

Fetal alpha-melanocyte-stimulating hormone levels: no correlation with late fetal growth but increased with diabetes mellitus

The pars intermedia of the fetal pituitary produces alpha-melanocyte-stimulating hormone. Previous reports suggest that alpha-melanocyte-stimulating hormone may be a determinant of early fetal growth in animal models. Based on anencephalic fetuses, a similar role was suggested in humans. To examine its relationship to late human fetal growth, alpha-melanocyte-stimulating hormone levels were measured by radioimmunoassay in 185 umbilical cord blood samples from anatomically normal fetuses of 28 to 42 weeks' gestation. With use of stepwise multiple regression analysis, no significant relationship was demonstrated between alpha-melanocyte-stimulating hormone levels and fetal growth. However, significantly higher levels were found in infants of diabetic mothers (p less than 0.05) independent of birth weight, gestational age, or both. No significant relationship with other maternal factors related to fetal growth, labor, and delivery was demonstrated. We conclude that fetal plasma alpha-melanocyte-stimulating hormone levels do not correlate with late human fetal growth. It is speculated that increased alpha-melanocyte-stimulating hormone levels among infants of diabetic mothers might be related to altered neurological maturation.

Partial purification of an insulin-releasing activity in human serum

An activity that enhances insulin release from perifused rat pancreatic islets has recently been isolated from human serum fractions (molecular weight 1,000-5,000 daltons). To characterize this activity we have studied the insulin-releasing effect of serum subfractions from obese and non-obese children obtained by reversed-phase high-performance liquid chromatography (HPLC). The serum insulin-releasing activity eluted in the HPLC system at 12-13 minutes, which corresponded to the retention time of the tridecapeptide insulin-glucagon liberin isolated from bovine hypothalamus. Insulin-releasing activity was found in serum subfractions from both obese and normal-weight children. The relative insulin-releasing potency of the active subfractions was higher than that of the original total serum fractions, indicating the presence of some substance(s) which inhibit insulin secretion in the total serum fractions. Oral glucose loading increased the relative insulin-releasing activity in the HPLC subfractions from obese children. This study suggests that the insulin secretagogue in human serum might be identical to hypothalamic insulin-glucagon liberin as these substances behave similarly on reversed-phase HPLC and have parallel insulin-releasing properties.

Beta-cell tropin: synthesis and biological activity

The structure of beta-cell tropin, an insulin secretagogue released by the neuro-intermediate lobe of the obese (ob/ob) mouse, has recently been determined as the 22-39 moiety of ACTH. A method for the preparation of this octadecapeptide using mild solid-phase procedures followed by preparative high pressure liquid chromatography is described. The molecular weight of the synthetic peptide has been confirmed by Fast Atom Bombardment mass spectrometry. Synthetic beta-cell tropin is indistinguishable in its chromatographic, antigenic and biological properties from natural beta-cell tropin.

The effect of streptozotocin-induced diabetes on pituitary and hypothalamic endorphin equivalents

This study examines the effect of experimentally induced diabetes mellitus in rats on tissue concentrations of opioid peptides in the neurointermediate lobe (NIL), anterior pituitary (AP) and hypothalamus. Diabetic animals were found to have a marked increase in endorphin equivalents, measured by opiate receptor binding assay, in the NIL whereas no change was observed in beta endorphin-like immunoreactivity (beta ELI) or ACTH measured by RIA. These results may indicate the presence of a feedback mechanism and suggest the possibility that opioid peptides may be physiologically important in the maintenance of glucose homeostasis.

Evidence that the insulin secretagogue, beta-cell-tropin, is ACTH22-39

The pituitary neurointermediate lobe of genetically obese (ob/ob) mice contains a hormone which stimulates insulin release and which cross-reacts with a -COOH-terminal ACTH antiserum, suggesting that it is related to corticotropin-like intermediate lobe peptide (CLIP), the 18-39 fragment of ACTH. The hormone, which we have called beta-cell-tropin, has been shown to be present in the plasma of the ob/ob mouse and to potentiate glucose induced insulin secretion. We have now shown that ACTH22-39 prepared by tryptic digestion of human synthetic CLIP behaves similarly on Biogel chromatography and on reverse-phase HPLC to the naturally occurring beta-cell-tropin. Furthermore, beta-cell-tropin and ACTH22-39 have indistinguishable antigenic and insulin releasing properties.

Corticotrophin-related peptides in the intermediate lobe of the rodent pituitary gland: characterization by high performance liquid chromatography and radioimmunoassay

High performance liquid chromatography (HPLC) and radioimmunoassay have been used to characterize corticotrophin-related peptides in extracts of the intermediate lobe of the rat and mouse pituitary gland. Multiple peaks have been observed, which resemble corticotrophin-like intermediate lobe peptide (CLIP) in that they cross-react with antisera raised against the COOH-terminal region of corticotrophin (ACTH) but not against NH2-terminal directed antisera. These CLIP-like peptides were released from the incubated neurointermediate lobe and their secretion was inhibited in the presence of dopamine. Heterogeneity of peptide species was also observed with antisera raised against alpha-MSH. Multiple peaks of CLIP and alpha-MSH-like material were identified in pituitary extracts from the mouse and levels were elevated in the genetically obese (ob/ob) animal. The nature and possible functions of multiple forms of intermediate lobe peptides are discussed.