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.     

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.     

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.     

Increased responsiveness to the hyperglycemic, hyperglucagonemic and hyperinsulinemic effects of circulating norepinephrine in ob/ob mice

OBJECTIVE: Several studies have implicated increased sympathetic tone as a contributing factor to the hyperglycemia and hyperglucagonemia of ob/ob mice. However, the responsiveness of plasma glucose, insulin and glucagon to circulating norepinephrine (NE) in ob/ob vs normal lean mice has never been described. Therefore, the present study investigated the effect of a 15 min intravenous NE infusion (1 pmol/min/g) on plasma glucose, insulin and glucagon in anesthetized lean, ob/ob, ob/ob-concurrent yohimbine (alpha(2) antagonist) treated, and ob/ob-chronically sympatholytic dopamine agonist treated (for 14 days prior to infusion) mice. In an effort to gain insight into a possible relation between norepinephrine, hyperglucagonemia and hyperinsulinemia in ob/ob mice, this study also examined the isolated islet responses to NE and glucagon in lean, ob/ob and ob/ob-sympatholytic dopamine agonist treated mice. RESULTS: Basal humoral values of glucose, insulin and glucagon were all elevated in ob/ob vs lean mice (by 63, 1900 and 63%, respectively, P<0.01). However, NE infusion further increased levels of glucose, insulin and glucagon in ob/ob (by 80, 90 and 60%, respectively, P<0.05) but not in lean mice (between group difference for all parameters P<0.05). Acute concurrent yohimbine treatment as well as chronic prior sympatholytic dopamine agonist treatment (bromocriptine plus SKF38393) simultaneously strongly aborgated or abolished all these humoral hypersensitivity responses to intravenous NE in ob/ob mice (P<0.05). Clamping the plasma glucose level in untreated ob/ob mice at a high level (30 mM) established by NE infusion did not significantly alter the plasma insulin level, suggesting that some other influence of NE was responsible for this insulin effect. Direct NE administration at 1 microM to islets from lean and ob/ob mice inhibited 15 mM glucose-stimulated insulin secretion in both groups, but at 0.1 microM it was inhibitory only in islets from ob/ob mice. However, glucagon (10 nM) increased 15 mM glucose-stimulated insulin secretion in ob/ob (by 170%, P<0.05) but not lean mice (between group difference P<0.05). CONCLUSION: These findings suggest that hypersensitivity to circulating NE may potentiate hyperglycemia and hyperglucagonemia in ob/ob mice, and the subsequent hyperglucagonemia coupled with increased islet beta-cell insulin secretory responsiveness to glucagon in ob/ob mice may support hyperinsulinemia, thus explaining the increased plasma insulin level response to intravenous NE in these animals. These findings further support a role for increased peripheral noradrenergic activities in the development and maintenance of the hyperglycemic, hyperglucagonemic and hyperinsulinemic state, characteristic of type 2 diabetes.     

In vitro studies on insulin secretion in the genetically obese mouse

Summary Pancreatic insulin content and insulin secretion, from the pancreas of obese mice fed ad lib (ob/ob), obese mice maintained on a restricted diet (ob/ob RD) and lean mice has been studied using incubated pieces of pancreas in vitro and in a perifusion system. The ob/ob mice pancreas contained approximately twice as much insulin as the lean mice pancreas, whereas the ob/ob RD mice had a normal insulin content. Increasing the glucose concentration had a marked and prolonged stimulating effect on insulin secretion in the pancreas of the ob/ob and ob/ob RD mouse, but not in the lean mouse. Leucine stimulated insulin secretion in all three groups of animals both in the absence and presence of glucose in a biphasic manner; in the presence of glucose the insulin secreted from the pancreas of the ob/ob and ob/ob RD mice was abnormally high. Arginine stimulated insulin secretion from the lean mouse pancreas in the absence of glucose, whereas in the obese mouse pancreas stimulation was observed only in the presence of glucose, and the effect increased with increasing glucose concentration. The large amounts of insulin which can be secreted by the pancreas of the ob/ob and ob/ob RD mice under stimulation suggest that an abnormal response of the pancreas to biological stimuli of insulin secretion could be a primary defect in these animals.     

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.     

Endogenous islet uncoupling protein-2 expression and loss of glucose homeostasis in ob/ob mice

We hypothesized that the loss of glucose homeostasis in ob/ob mice is associated with upregulation of islet uncoupling protein-2 (UCP2) expression, leading to impaired glucose-stimulated insulin secretion (GSIS). Changes in glucose homeostasis in lean and ob/ob mice from 5 to 16 weeks were assessed by fasting blood glucose, plasma insulin, oral glucose tolerance, and tissue insulin sensitivity. In vitro GSIS and ATP content were assayed in isolated islets, while UCP2 expression was determined by quantitative real-time PCR and immunoblotting. Short-term reduction of UCP2 expression was achieved through transfection of islets with specific small interfering RNA. Insulin resistance was detected in 5-week-old ob/ob mice, but GSIS and blood glucose levels remained normal. By 8 weeks of age, ob/ob mice displayed fasting hyperglycemia, hyperinsulinemia and glucose intolerance, and also had elevated non-esterified fatty acid concentration in plasma. In vitro, GSIS and ATP generation were impaired in ob/ob islets. Islet UCP2 expression was elevated at 5 and 8 weeks of age. Short-term knockdown of islet UCP2 increased GSIS in islets of lean mice, but had no effect in islets from ob/ob mice. Loss of glucose homeostasis and impairment of insulin secretion from isolated islets at 8 weeks in ob/ob mice is preceded by an increase in UCP2 expression in islets. Moreover, the glucolipotoxic conditions observed are predicted to increase UCP2 activity, contributing to lower islet ATP and GSIS.     

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.     

Disorganization of cytoplasmic Ca<Superscript>2+</Superscript> oscillations and pulsatile insulin secretion in islets from <Emphasis Type="Italic">ob</Emphasis>/<Emphasis Type="Italic">ob </Emphasis>mice

AIMS/HYPOTHESIS: In normal mouse islets, glucose induces synchronous cytoplasmic [Ca(2+)](i) oscillations in beta cells and pulses of insulin secretion. We investigated whether this fine regulation of islet function is preserved in hyperglycaemic and hyperinsulinaemic ob/ obmice. METHODS: Intact islets from ob/ ob mice and their lean littermates were used after overnight culture for measurement of [Ca(2+)](i) and insulin secretion. RESULTS: We observed three types of [Ca(2+)](i) responses during stimulation by 9 to 12 mmol/l of glucose: sustained increase, rapid oscillations and slow (or mixed) oscillations. They occurred in 8, 18 and 74% of lean islets and 9, 0 and 91% of ob/ ob islets, respectively. Subtle desynchronisation of [Ca(2+)](i) oscillations between regions occurred in 11% of lean islets. In ob/ ob islets, desynchronisation was frequent (66-82% depending on conditions) and prominent: oscillations were out of phase in different regions because of distinct periods and shapes. Only small ob/ ob islets were well synchronised, but sizes of synchronised lean and desynchronised ob/ ob islets were markedly overlapped. The occurrence of desynchronisation in clusters of 5 to 50 islet cells from ob/ obmice and not from lean mice further indicates that islet hypertrophy is not the only causal factor. In both types of islets, synchronous [Ca(2+)](i) oscillations were accompanied by oscillations of insulin secretion. In poorly synchronised ob/ ob islets, secretion was irregular but followed the pattern of the global [Ca(2+)](i) changes. CONCLUSIONS/INTERPRETATION: The regularity of glucose-induced [Ca(2+)](i) oscillations is disrupted in islets from ob/ ob mice and this desynchronisation perturbs the pulsatility of insulin secretion. A similar mechanism could contribute to the irregularity of insulin oscillations in Type II (non-insulin-dependent) diabetes mellitus.     

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.     

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.     

Effects of long-term restricted insulin production in obese-hyperglycemic (genotype ob/ob) mice

Summary Primary hypersecretion of insulin has been suggested as one possibility for the genetic fault of ob/ob mice. To test this hypothesis, streptozotocin (SZO) was used to reduce permanently insulin secretion in young lean and obese mice. After establishment of hyperglycaemia and weight reduction in treated obese mice (obese-SZO), daily insulin replacement was begun in some (obese-SZO-Ins). Obese-SZO mice maintained insulin levels and body weights similar to lean controls, though they were shorter and fatter, while food intake and blood sugar levels exceeded lean values. Obese-SZO-Ins mice with reduced islet hyperplasia, but great insulin resistance, gained more weight than obese-SZO mice; had high serum insulin and controlled blood glucose; and exhibited hyperphagia. These results suggest that primary hypersecretion of insulin cannot be the genetic defect, as ob/ob mice are hyperphagic, hyperglycaemic, insulin resistant, and obese even when insulin levels are restricted.     

The effects of exercise and food restriction on obesity and diabetes in young ob/ob mice

Hormone levels and body composition were examined in six-week-old C57BL/6J ob/ob mice following 25 d of limited caloric intake, voluntary exercise, or combined treatment. Pair-feeding obese mice to the daily intakes of lean mice reduced body weight gain, skeletal growth and lean body mass. Although weight gain was the same in the two phenotypes, ob/ob mice had fourfold higher rates of fat deposition. When exercise was combined with pair-feeding, skeletal and lean body growth were reduced even further and weight gain was now less than ad libitum-fed lean controls. Carcass fat accretion, however, continued to be two to three times greater. No single treatment reversed the hyperglycemia or elevated hormone production of obese mice, although slightly lower values of glucose, insulin, glucagon and corticosterone were associated with pair-feeding. When diet was combined with exercise, fasting glycemia and glucagonemia were reduced to equal the values of lean mice but insulin and corticosterone levels remained elevated. The present results show that dieted and exercised ob/ob mice continue to exhibit very high rates of fat deposition even though skeletal and lean growth are severely limited. Since fat accretion is maintained under these conditions, it appears that obese mice are not just storing excess calories as fat, but are actively regulating body fat content to levels about 30 percent higher than lean mice.     

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.     

Systemic treatment with sympatholytic dopamine agonists improves aberrant beta-cell hyperplasia and GLUT2, glucokinase, and insulin immunoreactive levels in ob/ob mice.

Sympatholytic dopamine agonist treatment utilizing bromocriptine and SKF38393 (BC/SKF) significantly lowers basal plasma insulin levels and normalizes basal and glucose-induced insulin secretion of the pancreatic beta cell in ob/ob mice. While BC/SKF has no significant effect on pancreatic islet cells directly, drug action is mediated via alterations in the hypothalamic-neuroendocrine axis, which drives metabolic changes in peripheral tissues leading to a marked reduction in hyperglycemia and hyperlipidemia and corrects autonomic control of islet function. To elucidate the nature of the functional response of islets to systemic BC/SKF treatment in ob/ob mice, we investigated the relative changes in the levels of functionally important beta-cell proteins in situ, as well as differences in the beta-cell turnover rate, following a 2-week drug treatment. Isolated islets from treated mice exhibit a 3.5-fold increase in insulin content (P <.01) that correlated with a 51% reduction in basal plasma insulin levels (P <.01) compared with vehicle-treated controls. Using quantitative immunofluorescence microscopy on pancreatic tissue sections, insulin and GLUT2 immunoreactivity of islet beta cells of BC/SKF-treated mice were significantly increased (approximately 2.3-fold and approximately 4.4-fold, respectively; P <.002) to the levels observed in islets of their lean littermates. Glucokinase (GK) immunoreactivity was greatly (75%) reduced in beta cells from ob/ob versus lean mice (P <.0001). A modest increase in GK immunoreactivity in beta cells of drug-treated mice was observed (approximately 1.6-fold; P <.05). Isolated islets from BC/SKF-treated mice exhibit a 42% reduction in DNA content compared with vehicle-treated controls (P <.01) to levels observed in lean mice, but without notable differences in islet size. In situ assays for mitosis and apoptosis, using 5-bromodeoxyuridine (BrdU) and terminal deoxyribotransferase (TdT)-UTP nick end labeling (TUNEL) staining techniques, respectively, were performed in pancreas of these mice to determine if beta cells show a reduction in hyperplasia following BC/SKF treatment. Accordingly, a pronounced decrease in replicating, BrdU-positive beta cells in the drug-treated mice compared with the control group was observed, but without differences in their TUNEL-staining patterns. Collectively, these data suggest that systemic sympatholytic dopaminergic therapy that attenuates hyperglycemia and hyperlipidemia improves islet function in ob/ob mice by improving aberrations in the beta cell's glucose-sensing apparatus, enhancing insulin storage and/or retention, and stabilizing hyperplasia, thus reducing basal insulin levels.     

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.     

Long-term leptin treatment of ob/ob mice improves glucose-induced insulin secretion.

OBJECTIVE: Previous studies have demonstrated that leptin inhibits glucose-stimulated insulin secretion from isolated islets, although a lack of leptin effect on insulin secretion has also been reported. The effect of long term in vivo leptin treatment of insulin secretion has, however, not been established. Therefore, in the present study, we have evaluated the effect of long term in vivo treatment of leptin on glucose-induced insulin secretion in ob/ob mice. METHODS: After 7 days' treatment of leptin (100 microg daily s.c.), insulin release was measured in isolated islets by batch incubation followed by radioimmunoassay. Glucose utilization and oxidation were measured by measuring the formation of (3)H(2)O and (14)CO(2) from [5-(3)H] and [U-(14)C] glucose, respectively. Glucose-6-phosphatase activity was measured by measuring the conversion of (14)C-glucose-6-P to (14)C-glucose. In addition, immunohistochemistry of pancreatic specimens was undertaken for study of expression of insulin, GLUT-2 and hormone-sensitive lipase (HSL). RESULTS: Leptin treatment significantly improved insulin secretion both at 5.5 mM (by 15%; P<0.05) and 16.7 mM (by 85%; P<0.001) glucose, compared to vehicle-treated controls. Furthermore, whereas leptin treatment did not affect islet insulin or DNA contents, a significant decrease in islet triglyceride content and glucose-6-phosphatase activity was observed. Moreover, the immunocytochemical data revealed an increased immunostaining for insulin, GLUT-2 and hormone-sensitive lipase (HSL) in islets from leptin-treated ob/ob mice. CONCLUSION: The results suggest that long-term leptin treatment of ob/ob mice improves glucose-stimulated insulin secretion in parallel with reduced glucose-6-phosphatase activity, increased HSL and decreased triglyceride levels in islets. These perturbations may explain the improvement of glucose-stimulated insulin secretion induced by leptin.     

Comparison of glucose-induced changes in electrical activity, insulin release, lactate output and potassium permeability between normal and ob/ob mouse islets: effects of cooling

A comparison has been made between the glucose-induced changes in electrical activity, insulin release, lactate output and potassium permeability in normal and ob/ob obese (Norwich strain) mice. The electrical response of the islet membrane to high glucose (22.2 mmol/l) stimulation was different in the two types of mice, generating continuous spike activity in normal but producing bursts of activity in ob/ob mouse islets. The absolute amounts of insulin and lactate produced by ob/ob islets in response to both basal and high glucose concentrations were greater than the absolute amounts produced by normal islets, though the ratio of the amount produced in high glucose concentrations to the amount produced in basal glucose concentrations was not significantly different between normal and ob/ob islets for both parameters. Glucose-induced changes in potassium permeability were smaller in ob ob than in normal mice. Cooling from 37 to 27 degrees C, during steady-state glucose stimulation, reduced both lactate output and insulin release, the temperature coefficients being similar in both types of mice. The effect of temperature reduction on electrical activity was more marked in the islets of ob/ob mice than in those of normal mice; spike frequency was unaffected in normal but reduced in ob/ob mice, whereas spike amplitude was decreased in both. Cooling-induced inhibition of potassium permeability was greater in the islets of ob/ob mice than in those of normal mice.(ABSTRACT TRUNCATED AT 250 WORDS)     

Reproductive hormonal function in the genetically obese (ob/ob) mouse.

Reproductive function is impaired in the genetically obese (C57 B1/6J) ob/ob mouse. Serum LH, FSH, and testosterone concentrations were assessed in male ob/ob and lean littermates from 39 to 78 days of age. The lean animals demonstrated a three-fold rise in serum LH between 39 and 45 days of age that preceded a steep increase in serum testosterone which peaked at age 70 days. The obese animals did not demonstrate this LH rise; serum testosterone levels were low and had a blunted increase with age that paralleled that of normal animals. Serum FSH was lower than normal at all ages in the obese mice. The ventral prostrate and testes were small in the ob/ob mice. The castration of adult animals resulted in increased serum concentrations of both LH and FSH, with higher levels attained in the lean animals. Fifty-four-day-old castrated lean and obese mice were treated with testosterone for 15 days. Measurements of serum LH and FSH after 8 and 15 days of treatment demonstrated a marked sensitivity in the ob/ob animals to feedback inhibition of gonadotropins. This finding suggested persistent immaturity of the hypothalamic-pituitary axis in obese mice. These studies indicate that the hypogonadism of the ob/ob mouse is the result of altered hypothalamic-pituitary function.     

Functional and histological characteristics of skeletal muscle and the effects of leptin in the genetically obese (ob/ob) mouse

BACKGROUND: Skeletal muscle mass in genetically obese (ob/ob) mice displays a reduced mass compared with their normal lean counterpart mice. However, the functional capacity of the available skeletal muscle mass in these animals has not yet been determined. OBJECTIVE: To investigate the properties of skeletal muscle in ob/ob mice and determine the effects of leptin administration on skeletal muscle in these mice. METHODS: Following 4 weeks of i.p. leptin administration (or control treatment) anaesthetized ob/ob and lean mice had their extensor digitorum longus and soleus muscles removed, and standard measures of isometric contractile properties and fatigability were performed. Histochemistry was used to determine fibre type proportions and individual fibre areas of all muscles. RESULTS: Leptin had no effect on the morphology or function of ob/ob skeletal muscle despite reducing body mass in ob/ob mice. Force production was unaltered in obese mice. However, a significant prolongation of contraction and relaxation times were evident. Obese skeletal muscle was also more fatigue resistant. Fibre proportions displayed a more slow type profile in ob/ob skeletal muscle, and in conjunction with previous work a reduced ability to hypertrophy. CONCLUSION: Skeletal muscle from obese mice is morphologically and functionally different from lean mouse skeletal muscle. Obese muscle is very similar to skeletal muscle from aged mice, and the specific contractile properties examined appear to be determined by the fibre make-up of these muscles.