Influence of the pituitary gland on insulin secretion in the genetically obese (ob/ob) mouse.

The influence of the pituitary gland of lean and genetically obese (ob/ob) mice on insulin secretion from microdissected pancreatic islets of lean and ob/ob mice has been studied by perifusing the pituitaries of these animals in series with the isolated islets and measuring insulin secretion at 5-min intervals over a period of 60 min. It has been shown that the pituitary perifusate of both lean and obese mice stimulate insulin secretion from lean mouse islets but not from obese mouse islets. The maximum stimulation occurs in the first 10 min and with the lean mouse pituitaries returns to the basal level in about 20 min, whereas with the obese mouse pituitaries insulin secretion is about double that from the control islets even after 40 min. A concentration of pure porcine ACTH equivalent to about three times the amount released from the pituitary gland under the experimental conditions used, caused only a small stimulation of insulin release. Possible interpretations of these findings and further lines of investigation are discussed.     

Pancreatic islets of obese hyperglycemic mice (ob/ob).

The development of the obesity-associated hyperglycemic syndrome in ob/ob mice, genetically determined, was observed over time by a combined functional and structural study of pancreatic islets. Islet areas increased with advancing age in ob/ob mice from 2 times at 1 month to 30 times at 6 months of age the size of lean mouse islets. Islet areas apparently increased more than pancreatic insulin content in ob/ob mice. Glucose and insulin tolerance tests were performed to study in vivo responses to glucose and insulin, respectively, in 1-, 3-, and 6-month-old mice. With ob/ob mice, glucose tolerance tests revealed more elevation of plasma glucose than in lean mice, the lean mice revealing more elevated plasma insulin than the obese mice. In insulin tolerance tests, lean mice presented marked hypoglycemia, whereas ob/ob mice revealed slightly higher plasma glucose at 1 month of age but three to four times higher amounts than that of lean mice at 6 months of age. Thus, increasing insulin resistance in ob/ob mice older than 3 months is associated with progressively increasing islet area, which contains proportionally less insulin than that of lean mouse pancreas. The data suggest that insulin resistance in ob/ob mice progressively develops up to 6 months of age and that marked islet hyperplasia is likely in response to sustained hyperglycemia, leading to hyperinsulinemia and eventual marked obesity.     

Modulation of basal insulin secretion in the obese,hyperglycemic mouse

The modulation of basal insulin secretion in vivo in the obese hyperglycemic, hyperinsulinemic $\text{ob}\text{ob}$ mouse was investigated by recording the plasma insulin and glucose levels during 2 hr after administration of various insulin secretory modifiers assumed to interfere with different regulatory mechanisms for insulin release. Normal, lean NMRI mice served as controls. Glucose sensitive mechanisms were tested by studying the effects of mannoheptulose. It was observed that mannoheptulose induced a marked hyperglycemia of comparable magnitude in both obese and lean mice. However, it exerted a more profound and sustained inhibitory effect on insulin secretion in the obese mouse than in the lean control animals, suppressing the insulinogenic index by 85%–90% in the obese animals from 30 min and onwards. Autonomic regulation was studied by means of receptor blockade. β-Adrenoceptor blockade by l-propranolol transiently inhibited basal insulin secretion in both obese and lean animals by a maximum of about 40%, the depression in obese animals being somewhat more sustained. Further, in both types of mice, α-adrenoceptor blockade by phentolamine induced a sustained elevation of plasma insulin levels to about 200% over control values. Thus no impressive abnormality in the adrenergic regulation of insulin secretion in the obese mouse was detected. Cholinergic blockade by methylatropine was found to induce a pronounced and sustained reduction of plasma insulin levels of about 60% in the obese mouse whereas the lean mice exhibited only a slight and transitory depression. The islet hormone, pancreatic polypeptide (PP) was observed to induce a slight short-lasting depression, about 35%, of basal insulin levels in both obese and lean mice concomitant with a slight reduction in plasma glucose levels. Vinblastine, in a dose known to induce a 75% reduction of the amount of microtubules in the insulin cells, was found to depress basal insulin secretion in the obese mouse by about 60% from 30 min and onwards, whereas only a slight slow-onset depression of about 25% was noted in the lean mouse. A slight hyperglycemia was noted in lean but not in obese mice. It is concluded that the basal hypersecretion of insulin in the obese mouse is of multifactorial origin. Fully established it probably is largely governed by an increased sensitivity and reactivity to glucose stimulation and to an enhanced responsiveness to a normal and/or increased vagal activity.     

Acute regulation of insulin release by the pituitary gland in relation to hyperinsulinaemia and obesity.

The pituitary glands from mice rendered obese by gold thioglucose treatment and by dietary manipulation, and pituitary glands from lean mice after a high food intake or a glucose load, were shown to stimulate insulin secretion from isolated pancreatic islets. The insulin releasing activity of pituitary glands from obese (ob/ob) mice was reduced by fasting for 24 and 48 h. Results obtained with pituitary glands from ob/ob and from lean ob/+ and +/+ mice suggest that the insulin releasing property manifests a gene dosage effect. Pituitary glands from 3-week-old (young) ob/ob mice stimulated insulin secretion to the same extent as pituitary glands from 3-month-old (adult) ob/ob mice. The pancreatic islets of young ob/ob mice were shown to be somewhat more responsive to stimulation by the pituitary factor than were lean ob/+ or +/+ islets from this age group. The concept that high insulin level, partly under pituitary control, and high caloric intake may be interlinked and may, in combination, be a major factor in producing obesity is discussed. Furthermore, it is suggested that the pituitary insulin releasing factor may play a role in the early development of obesity in the animal models studied.     

Effect of adrenalectomy on the metabolism of glucose in obese (C57 B1/6J ob/ob) mice

The genetically obese mouse, C57 B1/6J ob/ob, has been suggested as an appropriate model for the study of obesity associated with diabetes mellitus. Employing glucose ¹⁴C(μl) as a tracer, the data presented here indicate that obese mice are able to clear glucose from the blood compartment at the same rate as their lean littermates. This was demonstrated with or without an associated cold glucose load. The abnormal glucose tolerance curves observed in the obese animals may be a result of secretion of glucose into the blood. Removal of the adrenal glands from the obese mice and their lean littermate does not impair their ability to clear a glucose load from the vascular compartment. The capacity for endogenous glucose secretion of ob/ob mice is severely curtailed by adrenalectomy, in that the glucose tolerance curves of these adrenalectomized animals become similar to those of sham-operated lean littermates. Thus, it appears that a considerable component of the hyperglycemia in ob/ob mice reflects major adrenal involvement that is activated by stress, ie, ether anesthesia and blood sampling. The hyperglycemia in ob/ob mice may reflect glucocorticoid-dependent gluconeogenesis.     

Obese gene expression: reduction by fasting and stimulation by insulin and glucose in lean mice, and persistent elevation in acquired (diet-induced) and genetic (yellow agouti) obesity.

Mutations in the obese (ob) gene lead to obesity. This gene has been recently cloned, but the factors regulating its expression have not been elucidated. To address the regulation of the ob gene with regard to body weight and nutritional factors, Northern blot analysis was used to assess ob mRNA in adipose tissue from mice [lean, obese due to diet, or genetically (yellow agouti) obese] under different nutritional conditions. ob mRNA was elevated in both forms of obesity, compared to lean controls, correlated with elevations in plasma insulin and body weight, but not plasma glucose. In lean C57BL/6J mice, but not in mice with diet-induced obesity, ob mRNA decreased after a 48-hr fast. Similarly, in lean C57BL/6J controls, but not in obese yellow mice, i.p. glucose injection significantly increased ob mRNA. For up to 30 min after glucose injection, ob mRNA in lean mice significantly correlated with plasma glucose, but not with plasma insulin. In a separate study with only lean mice, ob mRNA was inhibited >90% by fasting, and elevated approximately 2-fold 30 min after i.p. injection of either glucose or insulin. These results suggest that in lean animals glucose and insulin enhance ob gene expression. In contrast to our results in lean mice, in obese animals ob mRNA is elevated and relatively insensitive to nutritional state, possibly due to chronic exposure to elevated plasma insulin and/or glucose.     

A longitudinal hormonal profile of the genetically obese mouse

Obese mice (C57BL/6J ob/ob) and their lean littermates were studied at various ages from immediately post weaning until 62 weeks of age, at which mortality increased markedly. Several age-related changes were noted. 1) Plasma glucose levels were elevated in obese mice 5-20 weeks and 62 weeks of age, but were similar to those in the lean mice at 20-60 weeks of age. Plasma insulin levels were elevated in obese mice, and there were no age-related differences. 2) Brain serotonin was elevated in obese mice at all ages and increased with age in both obese and lean animals. 3) Pituitary contents of ACTH and beta-endorphin were elevated in young obese mice and increased further as these mice approached their life expectancy. 4) The ratios of ACTH to beta-endorphin immunoreactivities were similar in obese and lean mice, except in obese mice over 50 weeks of age where this ratio was increased. We conclude that: 1) the obese mouse is characterized by hyperinsulinemia and hyperadrenocorticism throughout its life; 2) the insulin resistance of the obese mouse improves at 20 weeks of age, yet deteriorates as its life expectancy is approached; 3) the obese mouse has an elevated brain serotonin content similar to previously described elevations of the putative neurotransmitters dopamine and norepinephrine in these mice; and 4) as the obese mouse approaches its life expectancy, abnormalities may occur in the synthesis, processing, or secretion of ACTH and/or beta-endorphine.     

Plasma free tryptophan, brain serotonin, and an endocrine profile of the genetically obese hyperglycemic mouse at 4--5 months of age.

Genetically obese hyperglycemic mice (ob/ob) were compared with their nonlittermate lean controls at 4-5 months of age with regard to brain serotonin, pituitary ACTH content, and circulating levels of glucose, glucagon, insulin, TSH, T3, T4, total tryptophan and free tryptophan. Brain serotonin pituitary ACTH content, and plasma insulin, glucose, total tryptophan, and free tryptophan were all significantly higher in obese mice than in the controls. TSH, T3, and T4 were not significantly different in obese mice vs. controls, suggesting that the obese mouse is euthyroid. Fasting improved but failed to normalize the glucose and insulin levels or insulin to glucagon ratios. Since serotonin is an important neurotransmitter with regard to hypothalamic-pituitary function and since its levels in the brain are dependent on the availability of tryptophan, the findings of elevated levels of free tryptophan in the plasma and serotonin in the brain of the obese hyperglycemic mouse may help to explain some of the previously observed abnormalities of pituitary hormone secretion in these animals.     

Insulin secretion and the morphological and metabolic characteristics of pancreatic islets of hyperthyroid ob/ob mice.

Thyroxine treatment induced experimental hyperthyroidism in ob/ob mice, inhibited glucose-induced insulin secretion from the isolated perfused ob/ob mouse pancreas, and reduced total pancreas insulin content. In contrast, glucose-induced insulin release from incubated pancreatic islets and insulin content of pancreatic islets from ob/ob mice isolated by freehand microdissection were not reduced after thyroxine treatment when expressed per microgram dry islet. Histological examination of the ob/ob mouse pancreas revealed islets without degenerative lesions of islet cells. Granularity of beta cells was well preserved. The average number of pancreatic islets was unchanged. However, the beta cell area was significantly decreased in relation to the total pancreatic parenchyma after thyroxine treatment. This implies that insulin release and content per pancreatic islet was half of that of the controls. ATP content of islets was slightly reduced. Glucose oxidation and glucose utilization by islets from treated mice were slightly increased. Thyroxine treatment of the animals did not abolish the stimulation of 45Ca2+ uptake by glucose, but it did suppress the potentiating effect of fasting on the stimulatory effect of glucose on 45Ca2+ uptake. The metabolic characteristics of islets from experimentally hyperthyroid mice are those of all hyperthyroid tissues. The results provide no evidence for the view that the effects of thyroxine treatment may be due to disturbed metabolic function or energy deprivation of pancreatic islets. Inhibition of insulin secretion from the pancreas after thyroxine administration is apparently due to a reduction in pancreas insulin content and a diminished pancreatic islet volume. Reduced pancreatic islet volume represents most probably a reduction of individual islet cell volume.     

Effect of substrate interactions on in vitro fatty acid synthesis in adipose tissue of normal and genetically obese mice

Summary Fatty acid synthesis was measured in vitro in pieces of adipose tissue from lean and obese-hyperglycaemic (ob/ob) mice, using ¹⁴C-glucose or ¹⁴C-lactate and ³H₂O to obtain absolute rates of total fatty acid synthesis. In the presence of lipoproteintriglyceride (2.5 mol/l) metabolic interaction occurred which decreased glucose incorporation into fatty acids by 30% in lean mouse tissue, but not in obese mouse tissue. In the absence of added insulin, the contribution of glucose to total fatty acid synthesis was 69% in obese mouse tissue, significantly lower than the value of 87% obtained in lean mouse tissue. Insulin increased the contribution of glucose to total synthesis in both lean and obese mouse tissues, although the value in obese mouse tissue (83%) remained lower than the value in lean mouse tissue (100%). Lactate was not a major precursor for fatty acid synthesis. When both lactate (2 mmol/l) and glucose (15 mmol/l) were present, the contribution of lactate to total fatty acid synthesis was not increased in obese mouse tissue, suggesting that even in the presence of insulin, about 30% of the carbon was provided by intracellular precursors.     

Insulin releasing effects of adrenocorticotropin (ACTH 1-39) and ACTH fragments (1-24 and 18-39) in lean and genetically obese hyperglycaemic (ob/ob) mice

An excessive insulin releasing effect of adrenocorticotropic hormone (ACTH) and ACTH fragments has been considered as a possible factor contributing to the hyperinsulinaemia of genetically obese hyperglycaemic (ob/ob) mice. To investigate this possibility, plasma glucose and insulin responses of 11- to 14-week-old fed lean and ob/ob mice were examined after intraperitoneal administration of ACTH 1-39, ACTH 1-24 and ACTH 18-39, each at a dose of 25 nmol/mouse (50-115 micrograms/mouse). ACTH 1-39 produced a marked and rapid increase of plasma insulin in both lean and ob/ob mice, the effect being much greater in the ob/ob mutant (maximum increases of 5.5 +/- 1.5 and 46.1 +/- 4.1 ng/ml at 10 min in lean and ob/ob mice respectively, P less than 0.001). In lean mice plasma glucose concentrations showed a protracted decreased (maximum decrease of 3.7 +/- 0.5 mmol/l at 120 min), whereas glucose concentrations were increased (maximum increase of 4.2 +/- 1.3 mmol/l at 60 min) in ob/ob mice. ACTH 1-24 produced qualitatively similar but generally smaller effects than ACTH 1-39, while ACTH 18-39 did not significantly affect glucose and insulin concentrations. In 24 h fasted mice, ACTH 1-39 produced similar but generally smaller effects than in fed mice. The results suggest that the effects of ACTH on glucose and insulin homoeostasis are conferred by the N-terminal 1-24 sequence, and ACTH may exert acute effects which contribute to the hyperinsulinaemia and hyperglycaemia of ob/ob mice.     

Dynamics of insulin hypersecretion by obese Zucker rats

The relationship between obesity and hypersecretion of insulin by the pancreas was studied. We found that pancreata from obese Zucker rats secrete significantly more insulin than do pancreata from lean Zucker rats. At a glucose stimulation of 300 mg/dL, the overall dynamic biphasic insulin secretory profiles from obese and lean rats were similar. Further studies to investigate the glucose-insulin dose response relationship in obese and lean rat pancreata demonstrated insulin hypersecretion by pancreata from obese rats which was particularly pronounced at normoglycemic and hypoglycemic levels (by factors as much as 14-fold). This hypersecretion is so striking as to suggest that in the intact state the obese animal may lack the ability to readily "shut off" its insulin secretion under normoglycemic conditions, whereas lean animals possess such an ability. Under hypoglycemic conditions (75 mg/dL), the hypersecretion is transient and insulin secretion returns to normal basal levels after 30 minutes of perfusion. Thus the degree to which this hypersecretory phenomenon may occur in vivo remains to be established.     

Vitamin C supplementation decreases insulin glycation and improves glucose homeostasis in obese hyperglycemic (ob/ob) mice

The effects of dietary vitamin C supplementation on glucose homeostasis and insulin glycation were examined in adult lean and obese hyperglycemic (ob/ob) mice. In lean mice, supplementation of the drinking water with vitamin C (25 g/L) for 14 days did not affect food intake, fluid intake, glycated hemoglobin, plasma glucose, or plasma insulin concentrations. Total pancreatic insulin content and the percentage of glycated pancreatic insulin were also similar to control lean mice. In ob/ob mice, vitamin C supplementation caused significant reductions by 26% to 48% in food intake and fluid intake, glycated hemoglobin, plasma glucose, and insulin concentrations compared with untreated control ob/ob mice. The total insulin content and the extent of insulin glycation in the pancreas of ob/ob mice were also significantly decreased by 42% to 45% after vitamin C supplementation. This change was accompanied by a significant 80% decrease in the percentage of glycated insulin in the circulation of vitamin C-supplemented ob/ob mice. These data demonstrate that vitamin C supplementation can decrease insulin glycation and ameliorate aspects of the obesity-diabetes syndrome in ob/ob mice.     

Islet amyloid polypeptide and insulin relationship in a longitudinal study of the genetically obese (ob/ob) mouse

Obese mice (Ume? ob/ob) and their lean litter-mates were investigated from 7 to 52 weeks of age with respect to the plasma concentration of islet amyloid polypeptide (IAPP) and insulin. Plasma levels of IAPP were highly elevated in the ob/ob mice and remained unchanged until age 33 weeks, after which a sudden significant increase occurred at age 40 weeks. The plasma concentration of insulin gradually increased from start to end and reached extremely high levels. In the lean mice, there were no age-related differences in plasma levels of IAPP and insulin, being of the same magnitude as in normal NMRI mice. The plasma IAPP/insulin molar ratio was similar in lean and obese mice until age 14 weeks. At 21 weeks, the ratio in the ob/ob mice had decreased dramatically and remained markedly (sixfold) lower than in the lean mice until the end of the study. The IAPP concentration in the pancreata of 21-week-old ob/ob mice was 25-fold higher than that in the lean mice. Immunohistochemically, a majority of the ob/ob mice displayed enlarged and more numerous pancreatic islets, compared with the lean mice, and the IAPP- and insulin-labeling intensity was equal for all animals. At the electron-microscopic level, there was an increase in the number of IAPP- and insulin-immunoreactive gold particles per whole granule area as well as per core granule area. We conclude that the dramatically increased IAPP levels in severely hyperinsulinemic ob/ob mice may be of importance for the development of insulin resistance. Further, the disproportionate secretion of IAPP and insulin in the adult obese mouse might indicate a disturbed negative feedback effect of IAPP on insulin secretory mechanisms, resulting in very high plasma insulin levels.     

Acute disruption of leptin signaling in vivo leads to increased insulin levels and insulin resistance

Leptin, an adipocyte-derived hormone, plays an essential role in the maintenance of normal body weight and energy expenditure, as well as glucose homeostasis. Indeed, leptin-deficient ob/ob mice are obese with profound hyperinsulinemia, insulin resistance, and often hyperglycemia. Interestingly, low doses of exogenous leptin can reverse the hyperinsulinemia and hyperglycemia in these animals without altering body weight. The hyperinsulinemia in ob/ob mice may result directly from the absence of leptin signaling in pancreatic β-cells and, in turn, contribute to both obesity and insulin resistance. Here, we acutely attenuated endogenous leptin signaling in normal mice with a polyethylene glycol (PEG)ylated mouse leptin antagonist (PEG-MLA) to determine the contribution of leptin signaling in the regulation of glucose homeostasis. PEG-MLA was either injected or continuously administered via osmotic minipumps for several days, and various metabolic parameters were assessed. PEG-MLA-treated mice had increased fasting and glucose-stimulated plasma insulin levels, decreased whole-body insulin sensitivity, elevated hepatic glucose production, and impaired insulin-mediated suppression of hepatic glucose production. Moreover, PEG-MLA treatment resulted in increased food intake and increased respiratory quotient without significantly altering energy expenditure or body composition as assessed by the lean:lipid ratio. Our findings indicate that alterations in insulin sensitivity occur before changes in the lean:lipid ratio and energy expenditure during the acute disruption of endogenous leptin signaling.     

Studies on insulin secretion and the pituitary insulin secretagogue beta-cell-tropin in the sand-rat (Psammomys obesus)

The sand-rat (Psammomys obesus) is an animal model for the study of human maturity onset diabetes which appears to be controlled by caloric intake. In the present investigations, these animals have been studied in relation to the influence of low- and high-energy diets on body weight, plasma insulin and blood glucose levels, and on insulin secretion from the perfused pancreas and the secretion of corticotropin-like intermediate lobe peptide (CLIP, ACTH18-39) and the insulin secretagogue beta-cell-tropin (beta-CT, ACTH22-39) from the pituitary neurointermediate lobe. The sand-rats maintained on the high-energy diet all became obese. Insulin secretion from the perfused pancreas of the obese sand-rat in the presence of 5.6 mM glucose was significantly higher than in the lean controls maintained on low-energy diets. Increasing the glucose concentration to 16.7 mM only produced a small stimulation of insulin secretion in the obese animals, and the difference between the two groups was not significant. Stimulation of insulin secretion by beta-CT was variable, but the obese animals appeared to be more responsive. Pituitary neurointermediate lobes were incubated for 4 h to measure the secretion of the ACTH related peptide. These were separated by gel filtration and the concentrations measured by radioimmunoassay with a CLIP antiserum and a CLIP standard. In all experiments beta-CT was 4-6 per cent of the total CLIP immunoreactive material. In these experiments the obese animals maintained on a high-energy diet were divided into two groups, those with plasma insulin levels less than 500 mu u/ml and those with insulin levels greater than 500 mu u/ml. The latter group had a significantly higher blood glucose level, presumably due to the insulin resistance resulting from the severe hyperinsulinaemia. It was also observed that CLIP-IRM and beta-CT secretion was lower in this group than in the animals maintained on low-energy diets or those on high-energy diets with moderate hyperinsulinaemia. This suggests a possible feedback inhibition by insulin on the secretion of beta-CT.     

Effects of an alpha 2-adrenoceptor antagonist on glucose tolerance in the genetically obese mouse (C57BL/6J ob/ob)

This study examines the effects of a relatively selective alpha 2-adrenoceptor antagonist, 8-(L-piperazinyl)imado-[1,2-alpha] pyrazine (compound A), and the preferential alpha 2-agonist clonidine on blood glucose, glucose tolerance, and plasma insulin levels in the C57BL/6J ob/ob mouse and its lean littermate. While clonidine raised blood glucose levels and impaired glucose tolerance, oral administration of compound A resulted in decreased blood glucose levels, as well as improved glucose tolerance in ob/ob mice. Insulin levels in ob/ob mice treated with clonidine were significantly reduced, while compound A raised insulin levels threefold and blocked the effects of clonidine when co-administered to the same animals. Clonidine-induced hyperglycemia in lean littermates was not accompanied by a decrease in insulin levels, while a small but significant increase in insulin levels was observed by compound A administration. Glycogen synthesis in diaphragm of ob/ob mice was enhanced after oral administration of compound A and was accompanied by an increase in plasma insulin levels. Concomitant treatment with a potent somatostatin analog to inhibit insulin release blocked the effects of the alpha 2-adrenoceptor antagonist, compound A. These observations suggest that the alpha 2-antagonist studied, increased plasma insulin levels with an accompanying reduction in blood glucose and an improvement in glucose tolerance in a genetic model of insulin resistance. Differential sensitivity to alpha 2-agonist in these genetically obese mice, ob/ob, was demonstrated by decreased insulin levels due to clonidine administration.     

Pancreatic polypeptide and other hormones in pancreas of obese (ob/ob) mice

Summary The insulin, glucagon, somatostatin and pancreatic polypeptide content of acid-ethanol extracts of pancreas from lean and obese (ob/ob) mice of various ages was determined by radioimmunoassay. Rat and mouse pancreatic polypeptide react weakly with antibodies to avian, bovine and canine pancreatic polypeptide, but immunoassay for this peptide was possible using an antibody to the carboxyl-terminal hexapeptide of bovine pancreatic polypeptide and amino-terminal labelled bovine pancreatic polypeptide as tracer. The insulin, glucagon and pancreatic polypeptide content of pancreas from obese mice was greater than that of lean controls. The increase in insulin content uniformly involved ventral, dorsal and splenic lobes. Glucagon content was elevated primarily in the splenic lobe. Pancreatic polypeptide content was most significantly elevated in the splenic lobe where pancreatic polypeptide cells are infrequent in normal lean mice and this was accompanied by increased numbers of pancreatic polypeptide cells per islet in this lobe.     

Effects of streptozotocin on glucose metabolism, insulin response, and adiposity in ob/ob mice.

To determine whether hyperglycemia in the obese hyperglycemic (ob/ob) mouse is related to enhanced activity of the pancreatic beta cell, streptozotocin (175 mg/kg) was injected into lean and ob/ob mice at 8 wk of age. The influence of this injection upon glucose metabolism, adipose cellularity, pancreatic morphology and immunoreactive insulin (IRI) release from isolated pancreatic islets was measured. The plasma glucose levels before and after an oral glucose load were elevated in lean and decreased in ob/ob mice 2 wk after treatment with streptozotocin. By 5 wk after this treatment, a reduced pancreatic islet size, beta cell number and a decreased pancreatic islet IRI release were present in both lean and ob/ob mice. At this time, plasma glucose was still elevated in lean, but depressed in ob/ob mice and the insulin responsiveness in muscle and adipocytes was unchanged. Hyperglycemia abates in the ob/ob mouse as hypersecretion of insulin is diminished, but these observations may not be directly related, since streptozotocin affects key metabolic activities of the livers as well as the pancreatic beta cell. The progression of obesity and status of adipose cellularity are not directly related to hyper-insulinemia, since they are not altered following streptozotocin treatment.     

VGF ablation blocks the development of hyperinsulinemia and hyperglycemia in several mouse models of obesity

Targeted deletion of the gene encoding the neuronal and endocrine secreted peptide precursor called VGF (nonacronymic) produces a lean, hypermetabolic, hyperactive mouse. Because VGF mutant mice are resistant to specific forms of diet-, lesion-, and genetically induced obesity, we investigated the role that this polypeptide plays in glucose homeostasis. We report that VGF mutant mice have increased insulin sensitivity by hyperinsulinemic euglycemic clamp analysis, and by insulin and glucose tolerance testing. Blunted counterregulatory responses in VGF-deficient mice were likely influenced by their significantly lower liver glycogen levels. VGF deficiency lowered circulating glucose and insulin levels in several murine models of obesity that are also susceptible to adult onset diabetes mellitus, including A(y)/a agouti, ob/ob, and MC4R(-)/MC4R(-) mice. Interestingly, ablation of Vgf in ob/ob mice decreased circulating glucose and insulin levels but did not affect adiposity, whereas MC4R(-)/MC4R(-) mice that are additionally deficient in VGF have improved insulin responsiveness at 7-8 wk of age, when lean MC4R(-)/MC4R(-) mice already have impaired insulin tolerance but are not yet obese. VGF mutant mice also resisted developing obesity and hyperglycemia in response to a high-fat/high-carbohydrate diet, and after gold thioglucose treatment, which is toxic to hypothalamic glucose-sensitive neurons. Lastly, circulating adiponectin, an adipose-synthesized protein the levels of which are correlated with improved insulin sensitivity, increased in VGF mutant compared with wild-type mice. Modulation of VGF levels and/or VGF signaling may consequently represent an alternative means to regulate circulating glucose levels and insulin sensitivity.