Fasting and glucose effects on pituitary leptin expression: is leptin a local signal for nutrient status?

Leptin, a potent anorexigenic hormone, is found in the anterior pituitary (AP). The aim of this study was to determine whether and how pituitary leptin-bearing cells are regulated by nutritional status. Male rats showed 64% reductions in pituitary leptin mRNA 24 hr after fasting, accompanied by significant (30-50%) reductions in growth hormone (GH), prolactin, and luteinizing hormone (LH), and 70-80% reductions in target cells for gonadotropin-releasing hormone or growth hormone-releasing hormone. There was a 2-fold increase in corticotropes. Subsets (22%) of pituitary cells coexpressed leptin and GH, and <5% coexpressed leptin and LH, prolactin, thyroid-stimulating hormone, or adrenocorticotropic hormone. Fasting resulted in significant (55-75%) losses in cells with leptin proteins or mRNA, and GH or LH. To determine whether restoration of serum glucose could rescue leptin, LH, and GH, additional fasted rats were given 10% glucose water for 24 hr. Restoring serum glucose in fasted rats resulted in pituitary cell populations with normal levels of leptin and GH and LH cells. Similarly, LH and GH cells were restored in vitro after populations from fasted rats were treated for as little as 1 hr in 10-100 pg/ml leptin. These correlative changes in pituitary leptin, LH, and GH, coupled with leptin's rapid restoration of GH and LH in vitro, suggest that pituitary leptin may signal nutritional changes. Collectively, the findings suggest that pituitary leptin expression could be coupled to glucose sensors like glucokinase to facilitate rapid responses by the neuroendocrine system to nutritional cues.     

Acromegaly in a patient with normal pituitary gland and somatotropic adenoma located in the sphenoid sinus

Ectopic acromegaly is a very rare clinical entity occurring in less than 1% of acromegalic patients. In most cases it is caused by GHRH or rarely GH-secreting neoplasms. Even rarer are ectopic pituitary adenomas located in the sphenoid sinus or nasopharynx that originate from pituitary remnants in the craniopharyngeal duct. This dissertation presents the difficulties in visualizing GH-secreting adenoma located in the sphenoid sinus. A 55-year-old man had somatic features of acromegaly for several years. MRI imaging revealed a slightly asymmetric pituitary gland (14 yen 4 mm) without focal lesions. Simultaneously, a spherical mass, 10 mm in diameter, corresponding with ectopic microadenoma was demonstrated on the upper wall of the sphenoid sinus. The serum GH level was 4.3 mg/l, IGF-1 = 615 mg/l, and a lack of GH suppression with oral glucose was proven. After preliminary treatment with a long-acting somatostatin analogue, transsphenoidal pituitary tumour removal was performed. Histopathological, electron microscopical and immunohistochemical analysis revealed densely granulated somatotropic pituitary adenoma: GH(+), PRL(-), ACTH(-), TSH(-), FSH(-), LH(-), MIB1 < 1%, SSTR3(+) and SSTR5(+). Post-surgical evaluation showed normal pituitary MRI scans, GH and IGF-1 levels 0.18 mug/l and 140 mg/l, respectively, as well as normal GH suppression with oral glucose. The careful analysis of possible pituitary embryonic malformations points out their significance for proper localization of extrapituitary adenomas.     

Plasma Prolactin Activity in Inappropriate Lactation

Using a bioassay dependent on the development of a lactogenic response in rabbit mammary tissue cultured in vitro, prolactin distinct from immunoreactive growth hormone has been found in the plasma of patients of both sexes with inappropriate lactation with and without evidence of pituitary tumours. It has also been found in one patient with primary hypothyroidism and galactorrhoea, and in another during chlorpromazine therapy, but not in nine patients with gynaecomastia without galactorrhoea. Plasma prolactin levels were examined in seven patients during oral glucose tolerance tests: no change occurred in the four patients with pituitary tumours, but the levels were suppressed in the three patients with normal pituitary fossae. Prolactin appears to be a distinct pituitary hormone in man, as in animals, and also to be aetiologically related to states of inappropriate lactation.     

Effect of 2-deoxy-D-glucose on gonadotropins,prolactin and serum glucose concentrations in the mare

In a variety of species, glucoprivation results in the suppression of the reproductive axis. Two experiments were performed to test the hypothesis that blockade of glucose metabolism via administration of the glucose inhibitor 2-deoxy-D-glucose (2DG) to mares would cause a modification in gonadotropin and prolactin secretion. Long-term ovariectomized mares (Experiment 1, n=4) or ovary-intact mares during the follicular phase of a synchronized estrous cycle (Experiment 2, n=4 per dose) were treated with 2DG. The dose of 2DG used in Experiment 1 was 100mg 2DG/kg BW, but because severe behavioral responses occurred, lower doses (50, 25, and 12.5mg 2DG/kg BW) were used for Experiment 2. In addition to the effects of 2DG, the pituitary responsiveness after glucoprivation was determined by an injection of gonadotropin-releasing hormone (100 microg) 6h post-treatment. In both experiments, treatment with 2DG was unaccompanied by changes in gonadotropin secretion or pituitary responsiveness. Mares treated with 100 mg 2DG/kg BW exhibited a significant increase in prolactin and mares treated with 100mg 2DG or 50mg 2DG/kg BW exhibited a significant increase in serum glucose concentrations, suggesting that glucoprivation was detected at these doses. Lower doses of 2DG did not cause significant alterations in prolactin or glucose levels. These results indicate that 2DG inhibits glucose utilization, but short-term glucoprivation via this metabolic inhibitor does not alter gonadotropin secretion in the mare. This lack of response to glucoprivation may reflect species differences in the response to glucoprivation or may be due to metabolic responses to the inhibition of glucose availability.     

Alteration of glucose transport and diauxic growth in 5-thio-D-glucose-resistant mutants of Azotobacter vinelandii.

Spontaneous mutants of Azotobacter vinelandii defective for glucose utilization were selected as resistant to 5-thio-D-glucose. Mutant strains AM2, AM38, and AM39 exhibited longer generation times than the wild type when grown on glucose. Mutant strain AM2 also exhibited an altered Km and Vmax for glucose uptake. During acetate-glucose diauxie, glucose utilization in the 5-thio-D-glucose-resistant mutants was subject to severe inhibition by acetate. These mutants did not exhibit the normal glucose phase of diauxie. Transport studies during diauxie indicated that glucose uptake was not induced in mutant strain AM2. However, increasing the glucose concentration from 25 to 200 mM relieved the severe acetate inhibition, and under these conditions the mutant strain AM2 exhibited normal diauxie. Revertants of mutant strain AM2 exhibited normal glucose and diauxie growth. The results are discussed in terms of a model for acetate regulation of glucose utilization in A. vinelandii.     

Aged PROP1 deficient dwarf mice maintain ACTH production

Humans with PROP1 mutations have multiple pituitary hormone deficiencies (MPHD) that typically advance from growth insufficiency diagnosed in infancy to include more severe growth hormone (GH) deficiency and progressive reduction in other anterior pituitary hormones, eventually including adrenocorticotropic hormone (ACTH) deficiency and hypocortisolism. Congenital deficiencies of GH, prolactin, and thyroid stimulating hormone have been reported in the Prop1(null) (Prop1(-/-)) and the Ames dwarf (Prop1(df/df)) mouse models, but corticotroph and pituitary adrenal axis function have not been thoroughly investigated. Here we report that the C57BL6 background sensitizes mutants to a wasting phenotype that causes approximately one third to die precipitously between weaning and adulthood, while remaining homozygotes live with no signs of illness. The wasting phenotype is associated with severe hypoglycemia. Circulating ACTH and corticosterone levels are elevated in juvenile and aged Prop1 mutants, indicating activation of the pituitary-adrenal axis. Despite this, young adult Prop1 deficient mice are capable of responding to restraint stress with further elevation of ACTH and corticosterone. Low blood glucose, an expected side effect of GH deficiency, is likely responsible for the elevated corticosterone level. These studies suggest that the mouse model differs from the human patients who display progressive hormone loss and hypocortisolism.     

Aerobic Glycolysis by the Pituitary Gland In Vivo

Arteriovenous differences of glucose, lactate, and pyruvate were measured across the pituitary glands of overnight-fasted female pigs to determine whether net uptake of glucose occurred, and if so, whether it was oxidized or converted to lactate. Arteriovenous differences were also measured across a portion of the cerebral cortex as a control. Cerebral cortex oxidized 84% of the glucose taken up, but this was not true in the pituitary gland, where glucose was almost completely converted to lactate. The arteriovenous difference across the pituitary gland was 0.61 mumol/ml for glucose while the venoarterial differences of lactate and pyruvate were 1.0 and 0.07 mumol/ml, respectively. The results indicated that little net oxidation of glucose either by the Krebs cycle or the pentose phosphate pathway occurs in the pituitary gland in vivo. Estimates of the amount of energy released from aerobic glycolysis indicate that, unless the requirements of the neurohypophysis are very different from those in the rest of brain, very little energy could be derived from the metabolism of glucose.     

Hormone release from thyrotrophs and lactotrophs during hypo- and hyperglycemia

To test whether changes in carbohydrate metabolism influence anterior pituitary function, iv TRH tests (25 micrograms TRH) were carried out on three different occasions in 6 normal subjects. On one of these occasions TRH was administered during normoglycemia (blood glucose level 4.5 mmol/l - on the other, during hyperglycemia (10 mmol/l) - and on the third, during hypoglycemia (3 mmol/l). Hypoglycemia reduced the TRH-elicited TSH response significantly (19 +/- 6%), but failed to affect the corresponding PRL response. Hyperglycemia left both the TSH and PRL responses to TRH unaffected. These results imply that thyrotrophs and lactotrophs react differently to changes in carbohydrate metabolism. Thyrotrophs - in contrast to lactotrophs - seem to require a certain minimal glucose delivery to function normally. Glucose excess does not change the reactivity of these pituitary cells significantly.     

PDK1 deficiency in POMC-expressing cells reveals FOXO1-dependent and -independent pathways in control of energy homeostasis and stress response

Insulin- and leptin-stimulated phosphatidylinositol-3 kinase (PI3K) activation has been demonstrated to play a critical role in central control of energy homeostasis. To delineate the importance of pathways downstream of PI3K specifically in pro-opiomelanocortin (POMC) cell regulation, we have generated mice with selective inactivation of 3-phosphoinositide-dependent protein kinase 1 (PDK1) in POMC-expressing cells (PDK1(DeltaPOMC) mice). PDK1(DeltaPOMC) mice initially display hyperphagia, increased body weight, and impaired glucose metabolism caused by reduced hypothalamic POMC expression. On the other hand, PDK1(DeltaPOMC) mice exhibit progressive, severe hypocortisolism caused by loss of POMC-expressing corticotrophs in the pituitary. Expression of a dominant-negative mutant of FOXO1 specifically in POMC cells is sufficient to ameliorate positive energy balance in PDK1(DeltaPOMC) mice but cannot restore regular pituitary function. These results reveal important but differential roles for PDK1 signaling in hypothalamic and pituitary POMC cells in the control of energy homeostasis and stress response.     

Biosynthesis of growth hormone in the rat anterior pituitary gland control of biosynthesis in vitro by glucose

We have studied the effect of altering the concentration of glucose (in the incubation medium) on the ability of rat pituitary lobes, incubated in vitro, to incorporate [³H]leucine into total protein and growth hormone. Increasing the glucose concentration caused an increase in the rate of total protein synthesis, but a decrease in growth hormone synthesis.Phloridzin and 2-deoxyglucose both lowered the rate of growth hormone synthesis; they were more effective in media containing 2.8 mM glucose than in 28 mM glucose. Various alternative sugars were substituted for D-glucose in the medium; they had differing effects on the synthesis of total pituitary protein and growth hormone. None of these other sugars was metabolised as effectively as glucose at physiological concentrations, though all could be utilised to some extent.The physiological role of the effect of glucose on growth hormone synthesis is discussed, particularly with reference to what is known about the control of insulin synthesis by glucose. It is proposed that glucose interacts with two control systems in the pituitary. One is concerned with growth hormone synthesis and the other influences the rate of synthesis of other (unidentified) proteins.     

Secretion of glucagon and insulin in hypophysectomized rats: effect of tolbutamide.

Normal and hypophysectomized (hypox) rats, fed ad libitum, received intraperitoneal injections of tolbutamide (75 mg/kg/day) or of saline for 6 weeks. 24 h after the last injection, blood samples were taken for glucose, insulin and glucagon determinations. In normal rats, tolbutamide treatment did not alter serum glucose, insulin and glucagon, although it suppressed the secretion of insulin and glucagon by the pancreatic islets. In hypox rats, tolbutamide decreased serum glucose and insulin, elevated serum glucagon and stimulated the secretion of glucagon, but not that of insulin by the pancreatic islets. In addition, tolbutamide treatment increased the glucagon response to arginine in normal, but not in hypox rats. The serum glucose response to arginine was decreased by tolbutamide treatment and by hypophysectomy and, thus, appeared independent of the glucagon rise or preexisting glucagon level. We conclude that tolbutamide treatment decreased the secretion of glucagon and insulin in normal rats and stimulated that of glucagon in hypox rats, perhaps because of the low levels of insulin in the serum and in the pancreas of the latter. Our results are compatible with the hypothesis that the pancreatic action of tolbutamide is influenced by the pituitary.     

Neurally mediated gastric mucosal damage in hypophysectomized rats.

The role of the pituitary hormones in the development of neurally mediated gastric mucosal damage was examined in both normal and hypophysectomized urethane-anaesthetized male Sprague-Dawley rats. Gastric mucosal damage was elicited either by electrical stimulation of intact vagal nerves or by electrical stimulation in the paraventricular nucleus. Macroscopic damage was scored following the stimulation period and samples of the stomach were fixed for histological assessment. Damage scores were assigned based on a 0 (normal) to 3 (severe) scale. Control experiments in which the vagi were not stimulated did not result in any significant gastric damage in either normal (0.56) or sham surgery (0.14) animals, whereas hypophysectomized animals were observed to have significant damage (1.44, p < 0.05). Stimulation of the vagi in hypophysectomized animals resulted in damage that was not significantly different compared with the hypophysectomized control animals (1.25, p > 0.05). In normal animals, stimulation of vagal nerves resulted in mean damage scores of 2.00, values that were not significantly different from those observed in hypophysectomized animals (1.25, p > 0.05). Similarly, stimulation in the paraventricular nucleus of hypophysectomized animals resulted in gastric lesions (2.00) that were not significantly different from those observed in normal animals (1.91, p > 0.05). These data suggest that such neurally mediated gastric damage does not depend upon neurosecretory projections to the pituitary gland, but that the maintenance of an intact gastric mucosa under normal conditions requires the presence of pituitary hormones.     

Glucose-induced toxicity in insulin-producing pituitary cells that coexpress GLUT2 and glucokinase. Implications for metabolic engineering.

We have shown that intermediate lobe (IL) pituitary cells can be engineered to produce sufficient amounts of insulin (ins) to cure diabetes in nonobese diabetic mice but, unlike transplanted islets, ILins cells evade immune attack. To confer glucose-sensing capabilities into these cells, they were further modified with recombinant adenoviruses to express high levels of GLUT2 and the beta-cell isoform of glucokinase (GK). Although expression of GLUT2 alone had negligible effects on glucose usage and lactate production, expression of GK alone resulted in approximately 2-fold increase in glycolytic flux within the physiological (3-20 mm) glucose range. GLUT2/GK coexpression further increased glycolytic flux at 20 mm glucose but disproportionately increased flux at 3 mm glucose. Despite enhanced glycolytic fluxes, GLUT2/GK-coexpressing cells showed glucose dose-dependent accumulation of hexose phosphates, depletion of intracellular ATP, and severe apoptotic cell death. These studies demonstrate that glucose-sensing properties can be introduced into non-islet cells by the single expression of GK and that glucose responsiveness can be augmented by the coexpression of GLUT2. However, in the metabolic engineering of surrogate beta cells, it is critical that the levels of the components be closely optimized to ensure their physiological function and to avoid the deleterious consequences of glucose-induced toxicity.     

Placental growth hormone is not suppressed by oral glucose loading in normal human pregnancy.

Placental growth hormone (PGH) progressively replaces pituitary growth hormone in the maternal circulation from mid-gestation onwards in human pregnancy. Our previous investigations have shown that placental growth hormone concentrations correlate well with foetal growth. Despite the apparent correlation between PGH and birthweight, the physiology of its secretion during pregnancy has not been well defined. We investigated the response of maternal serum PGH to oral glucose loading in pregnant women (n = 24) who demonstrated normal glucose tolerance at a mean gestation of 29 weeks. Mean (SEM) fasting PGH concentrations were high (36.9 [6.4] ng/ml). No suppression of PGH was noted at one, two or three hours after a 75 g oral glucose load. Similarly, no changes were noted in growth hormone binding protein or in calculated free PGH over the course of the glucose tolerance test. As expected, insulin concentrations rose sixfold and insulin like growth factor binding protein 1 concentrations fell by 20 % with glucose loading. Correlation analysis showed maternal weight, BMI, fasting serum glucose serum insulin to be significantly correlated with the babies' birthweight. Our results support the proposition that PGH concentrations in maternal serum are not suppressed by oral glucose loading in non-diabetic mothers.     

重型肝炎低血糖的临床分析及防护对策

目的：探讨重型肝炎患者发生低血糖的临床意义,为针对性的防护提供依据。方法：检测20例健康献血员、44例急性肝炎、51例慢性肝炎及30例重型肝炎患者的空腹血糖水平进行比较,患者在发生低血糖症状时检测其血糖水平。结果：重型肝炎组的空腹血糖水平明显低于正常对照组、急性肝炎组及慢性肝炎组,在病程中低血糖发生率明显高于急性肝炎及慢性肝炎患者组,重型肝炎死亡组在病程中低血糖发生率明显高于存活组。结论：重型肝炎患者常发生低血糖,早期识别低血糖征兆,合理饮食,做好健康宣教是预防的关键;检测空腹血糖对判断重型肝炎危重程度和评估预后有重要意义。     

Insuppressibility of plasma glucagon by orally or intravenously administered glucose in diabetes mellitus

In order to study the response of pancreatic alpha cells to the change blood glucose, plasma pancreatic glucagon levels were measured after glucose loading given orally (50g) or intravenously (25g) in twenty-two normal controls and eighty untreated diabetics. Basal plasma pancreatic glucagon levels did not differ significantly in the two groups. However, oral or intravenous glucose administration caused a decrease in plasma pancreatic glucagon in normal subjects but not in diabetics. In "moderate" or "severe" diabetics, plasma pancreatic glucagon tended to increase paradoxically following oral glucose loading. To evaluate the sensitivity of pancreatic alpha cells to glucose, we calculated the index, -sigma delta IRG/sigma delta BS, after oral glucose loading. It was 1.96 +/- 0.57 in normal subjects, and significantly higher than in "mild" (0.11 +/- 0.05), "moderate" (-0.002 +/- 0.06) and "severe" (-0.09 +/- 0.07) diabetics. These results demonstrate the insensitivity of alpha cells to hyperglycemia in patients with diabetes mellitus as compared with normal subjects.