Measurements of insulin activities in pancreas and serum of mice with spontaneous ("Obese" and "New Zealand Obese") and induced (Goldthioglucose) obesity and hyperglycemia, with considerations on the pathogenesis of the spontaneous syndrome

Pathophysiology and genetics of obesity and diabetes in the New Zealand obese mouse: a model of the human metabolic syndrome

The New Zealand Obese (NZO) mouse is one of the most thoroughly investigated polygenic models for the human metabolic syndrome and type 2 diabetes. It presents the main characteristics of the disease complex, including early-onset obesity, insulin resistance, dyslipidemia, and hypertension. As a consequence of this syndrome, a combination of lipotoxicity and glucotoxicity produces beta-cell failure and apoptosis resulting in hypoinsulinemia and diabetic hyperglycemia. With NZO as a breeding partner, several adipogenic and diabetogenic gene variants have been identified by hypothesis-free positional cloning (Tbc1d1, Zfp69) or by combining genetic screens and candidate gene approaches (Pctp, Abcg1, Nmur2, Lepr). This chapter summarizes the present knowledge of the NZO strain and describes its pathophysiology as well as the known underlying genetic defects.

Modulation of islet ATP content by inhibition or stimulation of the Na(+)/K(+) pump

High (30 mM) K(+), known to cause beta-cell membrane depolarisation, significantly decreased the islet total ATP content, supporting the view that beta-cell membrane depolarisation can activate the ATP-consuming Na(+)/K(+) pump. Ouabain (1 mM) did not change the islet ATP content after 5-15 min of incubation in the absence or presence of 3 mM glucose but reduced it after 30 min, and in the presence of 20 mM glucose, the reduction by ouabain occurred already after 15 min. Incubation of islets with ouabain for 60 min decreased the islet ATP content in the presence of 3, 10 or 20 mM glucose or 30 mM K(+). Also, the islet glucose oxidation rate was decreased by ouabain. When K(+) deficiency was used to inhibit the Na(+)/K(+) pump, no change in ATP content was observed irrespective of glucose concentration, although K(+) deficiency caused a slight inhibition of the glucose oxidation rate. Diazoxide reduced the islet glucose oxidation rate and increased the islet ATP content in the presence of 20 mM glucose. There may exist a feedback mechanism decreasing the flow of glucose metabolism in response to reduced ATP consumption by the Na(+)/K(+) pump.

Relationships between the Na(+)/K(+) pump and ATP and ADP content in mouse pancreatic islets: effects of meglitinide and glibenclamide

We have previously demonstrated that both D-glucose and glibenclamide stimulate the Na(+)/K(+) pump and suggested that this may be part of the membrane repolarization process, following the primary depolarization by these agents. The aim of this study was to investigate whether the non-sulphonylurea meglitinide (HB 699) exerts similar effects as glibenclamide or glucose on the islet Na(+)/K(+) pump and if effects of meglitinide or glibenclamide on this pump activity is paralleled by changes in islet ATP content and/or ATP/ADP ratio. The acyl-amino-alkyl benzoic acid derivative, meglitinide, stimulated the islet ouabain-sensitive portion of (86)Rb(+) influx (Na(+)/K(+) pump) by 53%, while the ouabain-resistant portion was inhibited by 70%. The stimulatory effect was not additive to that caused by D-glucose, suggesting that both agents may activate the Na(+)/K(+) pump via the same mechanism. Glibenclamide (10 microM) decreased the islet ATP and ADP content as well as the ATP/ADP ratio at 0 mM glucose. These effects were no longer observed at 10 mM glucose. Meglitinide (10 or 50 microM) lowered the islet ATP and ADP content at 0 mM glucose without affecting the ATP/ADP ratio. At 10 mM glucose, however, 10 microM of the drug reduced the islet ATP content but not the ATP/ADP ratio, while 50 microM of the drug, besides lowering the ATP content, also reduced the ATP/ADP ratio. Diazoxide (0.5 mM) increased the islet ATP content in the absence of glucose, an effect not seen in the presence of 10 mM glucose. The rate of glucose oxidation at 1, 10 or 20 mM of the sugar was not affected by glibenclamide (0.1 - 10 microM) and at 10 or 20 mM of the sugar not affected by meglitinide (1 - 100 microM). These results suggest that glibenclamide and meglitinide lower the islet ATP level by indirectly activating the beta-cell Na(+)/K(+) pump, which is a major consumer of ATP in the islets, while diazoxide increases the ATP level due to inhibition of the pump.

Vagotomy in young obese hyperglycemic mice: effects on syndrome development and islet proliferation

Obese hyperglycemic mice have large pancreatic islets and high levels of serum insulin and blood glucose. Vagotomy was performed on 3-wk-old animals to investigate the role of gut cholinergic innervation in young Umea ob/ob mice. After vagotomy, obesity and hyperglycemia are dissociated. Weight increase in obese vagotomized mice was lower than in sham-operated controls during the 1st wk postoperatively but not thereafter. Blood glucose was lower up to 5 mo after vagotomy, but vagotomized mice showed reduced glucose tolerance. Islet cell proliferation rate was reduced 2 and 3 wk but not 5 mo after vagotomy. After 5 mo, islet volume was smaller in vagotomized mice. Serum insulin levels were the same in vagotomized animals as in sham-operated controls. The effects of reduced cholinergic innervation are probably caused both by direct effects of denervation and by lowered metabolic demand.

Comparison of the effects of perchlorate and Bay K 8644 on the dynamics of cytoplasmic Ca2+ concentration and insulin secretion in mouse beta-cells

Non-inbred ob/ob mice were used to study the dynamics of cytoplasmic Ca2+ concentration ([Ca2+]i) in isolated pancreatic beta-cells using microfluorimetry with fura 2/AM as probe, and the dynamics of insulin secretion in isolated pancreatic islets. D-Glucose (20 mM) caused a transient peak increase in [CA2+]i which changed to either an oscillating or a flat, elevated phase. The lag-time before the first peak increase in [Ca2+]i was markedly shortened by 12 mM ClO4- and the glucose-stimulated level of [Ca2+]i after the first peak was clearly elevated by the anion. ClO4- did not change the basal [Ca2+]i at 3 mM glucose. Extracellular Ca2+ deficiency abolished the effect of high glucose and ClO4- on [Ca2+]i. This suggests that ClO4- acts as an amplifier of transmembrane Ca2+ inflow. The L-type Ca2+ channel agonist, Bay K 8644 (0.01-1.0 microM), strictly reproduced all the effects of perchlorate on the glucose-stimulated beta-cell [Ca2+]i. Both phases of insulin release (20 mM glucose) were markedly enhanced by ClO4- (12 mM) or Bay K 8644 (1.0 microM). The lag-time for glucose-stimulated insulin release was shortened by both agents. Taken together, these data strengthen the idea that perchlorate amplifies the glucose-stimulation of [Ca2+]i and insulin release by directly modifying the function of the L-type Ca2+ channel. This effect can induce both a more prompt onset of and an amplified level of beta-cell secretory activity.

Development of insulin secretory function in young obese hyperglycemic mice (Umeå ob/ob)

The obese-hyperglycemic syndrome is well characterized in adult mice. However, little is known about islet morphology and function at an early age when obese mice islets start to proliferate. We have now studied islet morphology and functional development in obese-hyperglycemic mice (Umeå ob/ob) and their lean littermates at ages < or = 38 days. The weight of obese mice began to increase more than that of the lean littermates at days 8 to 12. At day 18, clinical diagnosis of the ob/ob syndrome could be made with 100% certainty. Islets from obese mice started to show enhanced growth rate during week 4, coinciding with the time of onset of hyperglycemia. 3H-thymidine labeling index is enhanced in ob/ob mice from day 22. Insulin secretion in islets from mice aged 18 to 21 days was the same in obese and lean mice from the same litter. At days 30 to 33, second-phase release and islet insulin content were decreased in islets from obese animals, but were restored after an overnight fast. It is likely that the hyperglycemia rather than increased insulin demand triggers increased beta-cell growth.

The susceptibility of spontaneously diabetic mice to cadmium-metallothionein nephrotoxicity

Cadmium metallothionein (CdMT) was injected subcutaneously into obese hyperglycaemic Umeå ob/ob mice or their lean litter mates (normal mice) at doses of 0, 0.1 and 0.4 mg Cd/kg. Proteinuria and calciuria were induced in both types of mice, but in the ob/ob mice this condition developed at a lower dose of CdMT (0.1 mg Cd/kg) than in the normal mice (0.4 mg Cd/kg). These results show, therefore, that Umeå ob/ob mice are particularly susceptible to CdMT-induced nephrotoxicity. The mechanism underlying this phenomenon needs to be further investigated. After the administration of CdMT, a dose-related increase in glycosuria was observed in both types of mice, in spite of decreased levels of serum insulin and glucose. It is suggested that such glycosuria induced by CdMT could be one of the signs of cadmium nephrotoxicity. The results of the present study thus indicate that metabolic changes like those in diabetes may increase susceptibility to cadmium-induced renal tubular damage.

Furosemide treatment causes age-dependent glucose intolerance and islet damage in obese-hyperglycaemic mice

The effects of furosemide on fasting serum glucose, glucose tolerance and pancreatic islet morphology were studied in ob/ob mice of two age groups, 3 months and 8 months. A single dose of furosemide (200 mg/kg body weight) induced acute hyperglycaemia in the young (3 months) as well as the old (8 months) ob/ob mice. Two days after the furosemide injection the glucose tolerance was markedly impaired in older animals, whereas it was normal in younger animals. Glucose tolerance in old mice varied markedly between individuals and showed two patterns. Thus, in one group of 8 months old mice, fasting serum glucose was elevated and glucose tolerance was very poor, whereas in the other group it was at least as good as in the saline-injected controls. Histological analysis showed normal islet morphology in furosemide-treatment young mice but an inflammatory reaction in islets from furosemide-injected old animals. A significant correlation between the degree of islet abnormality and glucose tolerance was observed. The data suggest that susceptibility to develop furosemide-induced long-term glucose intolerance is associated with the development of the obese-hyperglycaemic syndrome rather than being linked to the inheritance of the ob/ob genome as such.

DNA replication in transplanted and endogenous pancreatic islets of obese-hyperglycemic mice at different stages of the syndrome

It was the aim of the present study to examine, at different stages of the obese-hyperglycemic syndrome, rates of islet-cell DNA replication in endogenous pancreatic and grafted islets of such mice. For this purpose obese-hyperglycemic mice were given an islet transplant prepared from lean mice under the kidney capsule. Two weeks later the animals were given an IP injection of 3H-thymidine one hour before being killed. Autoradiography of pancreas and kidney sections indicated the highest labeling index (LI) for the endogenous islet cells in the youngest obese mice (6 weeks old), which was more than fivefold higher than that of lean, normoglycemic controls. The LI, however, decreased extensively with age. The transplanted islets had LI, which were higher and constant during the most intense period of the syndrome, but again there was a decrease in the oldest mice. By isolating and transplanting islets of the oldest obese mice (greater than 12 months) into younger obese mice, it was possible to revive the high cell replicatory activity of the ob/ob islets. Starvation for three days was found to markedly decrease the rate of islet cell DNA replication. Adrenalectomy of obese-hyperglycemic mice resulted in a decrease of the serum glucose and insulin concentrations; concomitantly, there was a decrease of LI of both transplanted and endogenous islets. Thus, it seems as if the attenuation of the hyperglycemia is most probably responsible for the decline of the islet cell replication with increasing age in obese-hyperglycemic mice.

Evidence that an L-fucose-containing component in the beta-cell plasma membrane is involved in the regulation of glucose-induced insulin release

The effect of the L-fucose-selective lectin Ulex Europeus I (UEA I), a blocker of the Na+, K+, Cl- co-transport system in the kidney, was tested on insulin secretion from isolated beta-cell-rich pancreatic islets. UEA I at doses from 50 to 100 micrograms ml-1 significantly reduced the glucose-induced (20 mmol l-1) insulin release whereas the basal (3 mmol l-1) release was unaffected. The inhibitory effect of 100 micrograms ml l-1 UEA I was completely abolished by 10 mmol l-1 L-fucose. The data suggest that an L-fucose-containing structure in the beta-cell plasma membrane participates in the regulation of glucose-induced insulin release. This structure may be similar to the L-fucose-containing glycoprotein in the kidney tubules that is believed to be the Na+, K+, Cl- cotransporter.

Furosemide and Ca2+ affect 86Rb+ efflux from pancreatic beta-cells by different mechanisms

The interaction between furosemide, calcium and D-glucose on the 86Rb+ efflux from beta-cell-rich mouse pancreatic islets was investigated in a perifusion system with high temporal resolution. Raising the glucose concentration from 4 to 20 mM induced an initial decrease in 86Rb+ efflux, which was followed by a steep increase and then a secondary decrease. Removal of extracellular calcium increased the 86Rb+ efflux at 4 mM D-glucose but reduced it at 20 mM. The initial biphasic changes in 86Rb+ efflux induced by 20 mM D-glucose were inhibited by calcium deficiency. Furosemide (100 microM) reduced the 86Rb+ efflux rate both at 4 and 20 mM D-glucose and the magnitudes appeared to be similar at either glucose concentration. Furosemide (100 microM) reduced the glucose-induced (10 mM) 45Ca+ uptake but did not affect the basal (3 mM D-glucose) 45Ca+ uptake. However, the ability of furosemide (100 microM) to reduce the 86Rb+ efflux at a high glucose concentration (20 mM) was independent of extracellular calcium. The inhibitory effects of furosemide and calcium deficiency on the 86Rb+ efflux rate appeared to be additive. It is concluded that the effect of furosemide on 86Rb+ efflux is not secondary to reduced calcium uptake and that the effects of furosemide and calcium deficiency are mediated by different mechanisms. The effect of furosemide is compatible with inhibition of loop diuretic-sensitive co-transport of Na+, K+ and Cl- and the effect of calcium deficiency with reduced activity of calcium-regulated potassium channels.

Evidence for co-transport of sodium, potassium and chloride in mouse pancreatic islets

1. The presence of a loop diuretic-sensitive co-transport system for Na+, K+ and Cl- was tested in isolated pancreatic islets. 2. Substitution of Cl- with the impermeant anion isethionate or addition of frusemide both reduced the ouabain-resistant islets uptake of 86Rb+ (K+ marker) without affecting the ouabain-sensitive uptake or equilibrium content of 86Rb+. The effects of Cl- substitution and frusemide were overlapping. 3. D-Glucose reduced the ouabain-resistant islets uptake of 86Rb+. This effect was additive to the effect of Cl- substitution or frusemide. 4. Substitution of Cl- with isethionate or addition of frusemide both reduced the efflux of 86Rb+ from the islets. These effects were additive to the reduction of 86Rb+ efflux induced by D-glucose. 5. Substitution of K+ or Na+ with choline reduced the equilibrium content of 36Cl- in the pancreatic islets. 6. These data are compatible with the operation in the pancreatic beta-cells of a loop diuretic-sensitive co-transport system for Na+, K+ and Cl-, that may serve as an inwardly directed Cl- pump.

Furosemide-induced glucose intolerance in mice is associated with reduced insulin secretion

The effect of furosemide on carbohydrate metabolism was studied in ob/ob mice. Intraperitoneal injection of a single dose of furosemide (200 mg/kg body weight) into fasted mice resulted in acute hyperglycaemia and two days after such a single dose, the mice showed fasting hyperglycaemia and glucose intolerance. Pancreatic islets from mice that had been injected with furosemide (200 mg/kg body weight) two days prior to the in vitro experiments showed increased basal (3 mmol/1 D-glucose) and decreased glucose-stimulated (20 mmol/1) insulin release. Islets from furosemide- or saline-injected animals showed no difference in islet insulin content. The results show that furosemide has both acute and long-term effects on carbohydrate metabolism in ob/ob mice. It is suggested that this, at least in part, is due to an effect on the pancreatic beta-cells.

Effect of perchlorate on calcium uptake and insulin secretion in mouse pancreatic islets

Microdissected beta-cell-rich pancreatic islets of non-inbred ob/ob mice were used in studies of how perchlorate (CIO4-) affects stimulus-secretion coupling in beta-cells. CIO4- at 16 mM potentiated D-glucose-induced insulin release, without inducing secretion at non-stimulatory glucose concentrations. The potentiation mainly applied to the first phase of stimulated insulin release. In the presence of 20 mM-glucose, the half-maximum effect of CIO4- was reached at 5.5 mM and maximum effect at 12 mM of the anion. The potentiation was reversible and inhibitable by D-mannoheptulose (20 mM) or Ca2+ deficiency. CIO4- at 1-8 mM did not affect glucose oxidation. The effects on secretion were paralleled by a potentiation of glucose-induced 45Ca2+ influx during 3 min. K+-induced insulin secretion and 45Ca2+ uptake were potentiated by 8-16 mM-CIO4-. The spontaneous inactivation of K+-induced (20.9 mM-K+) insulin release was delayed by 8 mM-CIO4-. The anion potentiated the 45Ca2+ uptake induced by glibenclamide, which is known to depolarize the beta-cell. Insulin release was not affected by 1-10 mM-trichloroacetate. It is suggested that CIO4- stimulates the beta-cell by affecting the gating of voltage-controlled Ca2+ channels.

Aromatic-L-amino-acid decarboxylase activity in mouse pancreatic islets

Aromatic-L-amino-acid decarboxylase activity has been measured in intact or homogenised pancreatic islets of ob/ob mice (Umeå ob/ob). The method used involves the trapping and measuring of the 14CO2 released from L-[1-14C]dihydroxyphenylalanine (L-dopa). Islets showed a decarboxylase activity which was dependent on pyridoxal phosphate and inhibitable by 0.1 mM benserazide or 0.1 mM alpha-monofluoromethyldopa. Maximum activity in intact islets was about 330 mmol/kg dry islet per h with an apparent Km of 3.3 mM. Islet homogenates had a Vmax of about 120 mmol/kg per h with a Km of 0.3 mM. L-5-Hydroxytryptophan, m-tyrosine and o-tyrosine interfered with the decarboxylation of L-dopa in a way that suggested a high activity also towards those substrates. L-Phenylalanine, L-tyrosine and D-glucose had no effect. At 0.05 mM L-dopa islet homogenates showed a much higher activity than homogenates of liver, kidney, or spleen. Islet uptake of L-[3H]dopa was well in excess of the decarboxylation rate and thus probably not rate-limiting. It is concluded that mouse pancreatic islets have a high activity of aromatic-L-amino-acid decarboxylase. This is in accordance with previous suggestions of a stimulatory effect of this enzyme on insulin secretion.

Effect of intracellular alkalinization on pancreatic islet calcium uptake and insulin secretion

Microdissected beta-cell-rich pancreatic islets of ob/ob mice were used in studies of the relationship between intracellular pH (pHi) and 45Ca2+ uptake and insulin release. Stepwise increases in extracellular pH (pHo) from 6.80 to 8.00 resulted in a parallel, although less pronounced, elevation of pHi from 7.24 to 7.69. Experimental conditions that alkalinize the islet cell interior, i.e. addition of 5 mM-NH4+, sudden withdrawal of extracellular bicarbonate buffer or increase in pHo, induced insulin secretion in the absence of other types of secretory stimulation (1 mM-D-glucose). Intracellular acidification by lowering pHo below 7.40 or sudden addition of bicarbonate buffer did not induce insulin secretion. The removal of extracellular bicarbonate buffer, increase in pHo from 7.40 to 8.00, or the addition of 5 mM-L-5-hydroxytryptophan or 5 mM-NH4+, which all alkalinize the islet cells and induce insulin secretion, also increased the La3+-non-displaceable 45Ca2+ uptake in the presence of 1 mM-D-glucose. The results suggest that intracellular alkalinization in beta-cells can trigger insulin secretion. Taken together with the fact that D-glucose increases pHi in the islet cells, the results also point to the possibility that alkalinization may be a link in the stimulus-secretion coupling sequence in beta-cells.

Abnormalities of pancreatic exocrine function in obesity: studies in the obese mouse

Insulin is known to play a specific role in the biosynthesis of pancreatic amylase. In the insulin resistant adult C57 BL/6J--ob/ob mouse there is a reduction of pancreatic amylase content. The differences of enzyme content could not be explained by differences of food intake between obese and lean mice, but are more likely to be the consequence of insulin resistance at the level of the exocrine pancreas. By contrast, greater pancreatic content of amylase and lipase seen in young obese mice (less than 2-months old) was associated with the greater food intake of these mice with respect to lean controls.

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

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.

The regulation of hepatic stearoyl-coenzyme A desaturase in obese-hyperglycaemic (ob/ob) mice by food intake and the fatty acid composition of the diet

1. The effects of food intake and the fatty acid composition of the diet on the hepatic stearoyl-CoA desaturase activity of obese-hyperglycaemic (ob/ob) mice were investigated. 2. Obese mice fed on a commercial mouse diet, ad libitum, had 6.5-fold more activity per liver cell than had lean mice. 3. On a diet containing 14% corn oil the activity was 65% less in obese mice and 62% less in lean mice compared with animals fed on the commercial diet. 4. Feeding with 14% saturated fat in the diet doubled the activity in lean mice compared with those on the commercial diet, but had no effect on the activity in obese mice. 5. Obese mice fed on the corn-oil diet contained a higher proportion of linoleic acid in the liver lipids than did lean mice fed on the commercial diet, but the acyl-CoA desaturase activity was 125% higher than in the lean mice. 6. Limiting the food intake of obese mice by pair-feeding with lean mice decreased their acyl-CoA desaturase activity when the animals were fed on the saturated-fat diet, but the activity remained 75% higher than in lean mice, whereas in obese mice pair-fed on the corn-oil diet the activity was the same as in lean mice. 7. During starvation the acyl-CoA desaturase activity in livers from obese mice decreased more slowly and proportionately less than in livers from lean mice. 8. It is concluded that increased substrate supply as a result of hyperphagia and not low concentration of linoleic acid is the main factor causing high acyl-CoA desaturase activity in obese mice.

CuZn-superoxide dismutase, Mn-superoxide dismutase, catalase and glutathione peroxidase in pancreatic islets and other tissues in the mouse

Exogenous superoxide dismutase, catalase and scavengers of the hydroxyl radical protect pancreatic-islet cells against the toxic actions of alloxan in vitro [Grankvist et al. (1979) Biochem. J. 182, 17--25]. To test whether the extraordinary sensitivity of islet cells to alloxan is due to a deficiency of endogenous enzymes protecting against oxygen-reduction products, we assayed CuZn-superoxide dismutase, Mn-superoxide dismutase, catalase and glutathione peroxidase in mouse islets and other tissues. To correct for any blood contamination, haemoglobin was also measured in the tissue samples. Pancreatic islets were found to belong to tissues with relatively little activity of the protective enzymes. However, the deviation from other tissues in this respect is probably not large enough to explain the especially great susceptibility of islet cells to alloxan.

The relationship between the hyperplastic pancreatic islet and insulin insensitivity in obesity

In summary, the present review provides evidence in support of the proposition that pancreatic islet cell hyperplasia precedes the development of insulin insensitivity in the obese mouse and, it is likely, that similar events occur in obese humans. Moreover, the hyperplastic pancreatic islet appears to be responsible for the development of insulin insensitivity, since suppression of the hyperplastic islet, by either alloxan or streptozotocin administration to the obese mouse, results in amelioration of insulin insensitivity in vivo. Since no change occurred in the degree of obesity or in adipocyte cell size or number, it is evident that insulin sensitivity is independent of obesity per se. Hence, although obesity and insulin insensitivity frequently co-exist, insulin insensitivity is independent of obesity and is due rather to the presence of pancreatic islet cell hyperplasia. Light and electron microscopy of the hyperplastic pancreatic islets of the obese mouse reveal increased numbers of A- B- and D-cells. Islet suppression with alloxan or streptozotocin results in the selective reduction of B-cells with preservation of A- and D-cells. Therefore, restoration of insulin sensitivity in the obese mouse following pancreatic islet cell suppression appears to be directly related to suppression of B-cell hypersecretion. Biochemical studies of muscle and adipose tissues from the obese mouse reveal profound insulin unresponsiveness without clear cut improvement in vitro following pancreatic islet cell suppression and restoration of insulin sensitivity in vivo. These data are consistent with a relatively modest reduction in the number of available insulin receptors upon these tissues in relation to the marked insulin resistance and imply an impairment of insulin action beyond the insulin receptor interaction [either transport or intracellular action(s)] as the major site(s) of insulin resistance in the muscle and adipose tissues of obese mice. Conversely a reduction of insulin receptors upon hepatocytes of obese mice and their improvement following a reduction of B-cell hypersecretion support the proposition that the number of available insulin receptors may be the major site for the regulation of insulin action upon that tissue. Finally, evidence is presented which suggests that an inability of insulin to limit hepatic gluconeogenesis may be the predominant cause of insulin insensitivity in the obese mouse.

Chemical diabetes in the adult rat as the spontaneous evolution of neonatal diabetes

Injection of streptozotocin in newborn rats induced a severe diabetic syndrome on day 4 after birth, with acute hyperglycaemia and glycosuria. Over the next 3 weeks spontaneous recovery occurred as attested by normal basal blood glucose and plasma insulin levels. Recovery was, however, incomplete in the adult since a definite impairment in insulin release and glucose disposal was observed. This state was characterized by the following features: 1) a 72% decrease in pancreatic insulin stores without change in pancreatic glucagon stores; 2) a slight but consistent elevation of blood glucose in the fasted and fed basal states and especially of blood glucose 90 min after an IV glucose load (2 g/kg) performed under pentobarbitone anaesthesia; 3) a considerable decline in the glucose-induced insulin release with a decrease in the maximal response. Both early and late phases of insulin release were impaired, as indicated by in vivo glucose infusion experiments. Basal plasma glucagon levels were normal. Over a period of 12 months with a normal laboratory diet no aggravation of the chemical diabetic state was observed. This new experimental syndrome is a potentially interesting model for the study of the influence of environmental factors on the development of overt diabetes.

Somatostatin in the pancreas and hypothalamus of obese mice

The pancreatic content of somatostatin, insulin, and glucagon and the hypothalamic content of somatostatin were examined in ob/ob mice at various ages and in goldthioglucose-obese mice. The total pancreatic content of somatostatin was increased in ob/ob mice compared to controls: 92 ng vs 75 ng (a 22% increase) at 2 months of age; 208 ng vs 131 ng (a 60% increase) at 6 months of age; and 184 ng vs 118 ng (a 60% increase) at 8 months of age. The total pancreatic content of glucagon in ob/ob mice was already enhanced by 70% over controls at 2 months of age (301 ng vs 173 ng) and did not increase further at later stages, whereas that of insulin progressively rose with age. In goldthioglucose-obese mice the pancreatic content of insulin was also increased but to a lesser extent than in ob/ob mice; the pancreatic levels of somatostatin and glucagon were unaltered. In both ob/ob mice (regardless of age) and goldthioglucose-obese mice, there was no significant change in the hypothalamic content of somatostatin compared with that of lean controls.

Somatotrophic diabetes: insulin release responses to arginine and glucagon in dogs

Growth hormone injected daily in 6 dogs for 6 days caused a 20-fold elevation in fasting serum immunoreactive insulin (IRI) without appreciable change in serum glucose in 1 day. In the somatotrophic diabetes that occurred after 2 days, the hyperinsulinaemia was maintained and the serum IRI/glucose (I/G) ratio declined from the early high level but remained elevated. During this treatment, in response to glucose infusion, the rise in serum IRI above the initially high fasting level was 16 times the normal. In response to glucagon, the rise in IRI was twice the normal and the rise in glucose was more prolonged, resulting in a decline in the I/G ratio. In response to arginine infusion, the rise in serum IRI was 8 times the normal and the rise in the I/G ratio was twice normal. Following a meal, the rise in serum IRI was 8 times the normal. Thus, with growth hormone treatment the insulin secretory responses to these stimulating factors were magnified over the already elevated fasting level of secretion. The insulin content of the pancreas was reduced to less than 10% of normal by growth hormone treatment for 6 days, due apparently to elevation of the rate of secretion over the rae of formation of insulin.

Laboratory animals exhibiting obesity and diabetes syndromes

Spontaneous hyperglycemia, hyperinsulinemia and obesity are common features for at least one period of the lifetime in some strains of mice. Both genetic and environmental factors are involved in the pathogenesis of the diabetes-like syndrome, making these strains excellent models for studies in both obesity and diabetes-like states. The metabolic peculiarities can be due to a dominant gene, as for the yellow obese, or a single recessive gene, as in the obese and the diabetes mouse; or they can be of polygenic origin, as for the KK and the NZO mouse. However, the severity of the metabolic disorder is due to the interaction of the mutant genes iwth modifiers in the bat genes themselves. Studies on the pathophysiology and biochemistry of these animals have revealed interstrain differences, different patterns of development of the metabolic disorder, and different degrees of severity of the diabetes-like syndrome. Although the primary causes of the syndrome remain unclear in some strains, an involvement of hypothalamic feeding centers has been implicated.

Effects of food restriction and mutation on central catecholamine levels in genetically obese mice

Norepinephrine (NE) levels in the hypothalamus and telencephalon of genetically obese mice with the OBOB and DBDB mutations are significantly higher than those of their lean littermates. Obese mice with the viable yellow mutation failed to show this increase. Restricting the diets of OBOB animals to prevent excessive weight gain does not affect NE levels in the hypothalamus, telencephalon or brainstem.

Decreased basal, noninsulin-stimulated glucose uptake and metabolism by skeletal soleus muscle isolated from obese-hyperglycemic (ob/ob) mice

Insulin resistance of diaphragms of ob/ob mice has been repeatedly demonstrated previously both in vitro and in vivo. In the present study, transport and metabolism of glucose with and without insulin stimulation were compared in a skeletal muscle more likely than diaphragm or heart to be representative of the overall striated muscle mass, i.e. isolated soleus muscle. Compared with soleus muscle from lean controls, unstimulated lactate release in the presence of exogenous glucose was depressed from 16.2 to 12.3 nmol/60 min per mg wet wt in soleus from ob/ob mutants; glycolysis was decreased from 6.6 to 3.7 and [14C]glucose oxidation to 14CO2 from 0.90 to 0.33 nmol glucose/60 min per mg wet wt. Uptake of 2-deoxyglucose (2-DOG), both with and without insulin, was very much less for soleus from ob/ob than from lean mice, at 2-DOG concentrations ranging from 0.1 to 10 mM, and in mice of 6-15 wk. When 2-DOG concentration was 1 mM, its basal uptake was 0.53 nmol/30 min per mg wet wt for soleus of ob/ob as against 0.96 for soleus of lean mice. The absolute increment due to 1 mU/ml insulin was 0.49 in muscle of ob/ob as against 1.21 in that of lean mice. When the resistance to insulin action was decreased by pretreatment in vivo by either streptozotocin injection or fasting, the decreased basal 2-DOG uptake of subsequently isolated soleus muscle was not improved. Inhibition of endogenous oxidation of fatty acids by 2-bromostearate, while greatly increasing 14CO2 production from [14C]glucose, did not affect basal [5-3H]glucose metabolism or 2-DOG uptake. It is suggested that transport and/or phosphorylation of glucose under basal, unstimulated conditions are depressed in soleus muscle of ob/ob mice, whether or not resistance to insulin and hyperinsulinemia are also present. Although the origin of the decreased basal glucose uptake remains unknown it might be related to a similar decrease in basal glucose uptake by ventromedial hypothalamic cells, an event presumably resulting in a tendency to hyperphagia. Decreased basal glucose uptake by soleus muscle of ob/ob mice might explain the hyperglycemia, and hence partly the hyperinsulinemia and excessive fat deposition of those animals.

Resistance to hepatic action of vasopressin in genetically obese (ob/ob) mice

1. Fatty acid synthesis, measured in the perfused liver of genetically obese (ob/ob) mice with 3H2O or [14C]actate, did not show the inhibition by [8-arginine]vasopressin (antidiuretic hormone) that is observed in livers from normal mice. 2. Hepatic glycogen breakdown in obese mice was stimuulated by vasopressin, but not as extensively as in lean mice. 3. If obese mice received a restricted amount of food, then fatty acid synthesis still did not respond to vasopressin, but glycogen breakdown was fully stimulated. 4. Cholesterol synthesis was not inhibited by vasopressin in livers from obese mice. 5. Vasopressin inhibited fatty acid synthesis in intact lean mice, but not in obese animals. 6. These results suggest that genetic obesity could be due to an inborn error within the mechanisms (other than adenylate cyclase) which mediate responses to extracellular effectors.

Effect of high fat vs. high carbohydrate feeding on the development of obesity in weanling ob/ob mice

Mice with hereditary obesity (ob/ob) were fed isocaloric, isoprotein diets with varying proportions of carbohydrate-to-fat from the time of weaning to 18 weeks of age. Body weight, plasma glucose,and plasma insulin were determined at frequent intervals; body lipid and pancreatic insulin were determined at termination. Obesity developed when only carbohydrate was provided as caloric source (and in the presence of essential fatty acid deficiency), but obesity was greatly enhanced when 5% fat was added. Obesity also developed in the absence of dietary carbohydrate, even though hyperinsulinism was significantly reduced by this regimen. It is concluded that 1) both enhanced storage of dietary fat and lipogenesis contribute to obesity, the former more so than the latter; 2) the degree of hyperinsulinism does not determine the degree of obesity; 3) the development of hyperinsulinism does not require stimulation by exogenous oral carbohydrate.

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

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 replacment 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.

Glucagon and insulin from lean rats and genetically obese fatty rats: studies by radioimmunoassay, radioreceptorassay and bioassay

Insulin, proinsulin and glucagon extracted from lean rat pancreases were studied in radioimmunoassay, radioreceptorassay and bioassay systems. Extracted insulin behaved identically to a rat insulin used as a reference standard in radioimmunoassay. On the basis of its immunoreactivity, extracted insulin was slightly less potent (about 70%) than the rat standard insulin in competing with the binding of 125I-insulin to rat liver membranes (radioreceptorassay) and in stimulating glucose oxidation by rat fat cells (bioassay). Extracted glucagon and a pork glucagon used as a reference standard were indistinguishable in two radioimmunoassay systems for glucagon, in competing with the binding of 125I-glucagon to rat liver membranes (radioreceptorassay) and in stimulating adenylate cyclase in rat liver membranes (bioassay). Genetically obese rats (Zucker, "fatty") were compared to their lean littermates with respect to insulin, proinsulin and glucagon extracted from their pancreases. Proinsulin represented the same proportion of total immunoreactive insulin in both types of rats. In the radioimmunoassays, the radioreceptorassays and the bioassays, insulin, proinsulin and glucagon from obese rats were indistinguishable from insulin, proinsulin and glucagon from lean rats. It is concluded that the pancreatic hormones of obese ("fatty") rats possess the same immunoreactivity and biological potency as those of nonobese rats. This excludes the possibility that some alteration in the biological properties of pancreas insulin and/or glucagon of fatty rats could explain the metabolic abnormalities observed in this type of obesity.

Gastrointestinal apudosis in obese hyperglycaemic mice

Quantitative histological and immunocytochemical studies have been carried out on the endocrine cells of the gastrointestinal tract in genetically obese mice and their heterozygous (lean) litter mates. In the ob/ob mice hyperplasia of most of the endocrine (APUD) cells of the gut was found, a condition which can be described as apudosis. Quantitative histology of silver-stained preparations, using a method which demonstrates the majority of endocrine cells, showed a significant degree of hyperplasia in all regions of the gastrointestinal tract, with statistically significant differences in the upper intestine (p = less than 0.001). Quantitative immunocytochemical studies by image analysis showed a difference in both number and hormone content of Gastric Inhibitory Peptide (GIP) (p = less than 0.001) and Enteroglucagon (EG) cells in obese as compared to lean mice. Differences in the case of Secretin (S), Gastrin (G) and Vasoactive Intestinal Peptide (VIP) cells were not great but in the obese mice both S and G cells were present in larger numbers in the lower intestine whereas in the lean, and in normal mice, they are predominant in the upper intestine. Whether these complex gut endocrine changes are primary, or secondary to the metabolic abnormalities seen in the ob/ob mouse, cannot presently be determined.

Clearing-factor lipase in obese hyperglycaemic mice (ob-ob)

1. Clearing-factor lipase was assayed in acetone-ether-dried powders of heart and epididymal fat-pads of lean and genetically obese mice (ob/ob). In both tissues the enzyme activity in the adult was higher in the obese mice. 2. In heart the enzyme activity was unchanged from 8 to 48 weeks of age in lean mice, but in obese mice it increased between 8 and 12 weeks of age and remained elevated. 3. Starvation produced changes in the heart clearing-factor lipase activity in obese, but not lean, mice. 4. The clearing-factor lipase activity of epididymal fat-pads decreased rapidly during 24h starvation in both lean and obese mice, but the activity in the obese mice remained higher than that in lean mice. 5. Plasma triglyceride and cholesterol concentrations were determined in both lean and obese mice. Triglyceride concentrations were not greatly different, but the obese mice were hypercholesterolaemic. Plasma cholesterol concentrations were not correlated with changes in clearing-factor lipase activity.

Effects of hypo- and hyper-thyroidism on liver composition, blood glucose, ketone bodies and insulin in the male rat

1. Thyroidectomized rats injected daily with 0, 0.1, 2 or 25mug of l-thyroxine/100g body wt. were compared with intact controls. In plasma, the protein-bound iodine was decreased in the rats given the 0 or 0.1mug doses and increased in those given the 25mug dose. 2. Blood glucose decreased in those given 2mug and was augmented in those given 25mug, and ketone bodies were the same in all the groups. 3. Plasma insulin was lowest in the rats given the 0 or 0.1mug doses and was highest in those given the 2 or 25mug doses of thyroxine. 4. After 48h starvation, the decrease in blood glucose and increase in ketone bodies observed in all the groups was greatest in the group not supplemented with thyroxine. 5. Plasma insulin concentrations remained at the value for fed animals in the rats given the 25mug dose of thyroxine but decreased in the other groups. 6. In fed animals, concentrations of hepatic DNA P, citrate, total fatty acids and acetyl-CoA were similar in all the groups, and glycogen was low only in the rats given the 25mug dose of thyroxine. 7. After 48h starvation, liver DNA P, total fatty acids and acetyl-CoA increased in all the groups, except in the rats given the 25mug dose, where both total fatty acids and acetyl-CoA remained at the value for fed animals. Liver citrate did not change in the groups given the 0 or 25mug doses of thyroxine, but decreased in the other groups. 8. The results are discussed in relation to the regulation of intermediary metabolism in hypo- and hyper-thyroidism.

Carbohydrate metabolism of the isolated perfused liver of normal and genetically obese--hyperglycaemic (ob-ob) mice

1. A technique for perfusion of the mouse liver has been developed, and aspects of carbohydrate metabolism have been investigated in the perfused liver of normal and genetically obese mice, homozygous for the recessive gene ob. 2. Rates of gluconeogenesis in perfused mouse liver were faster than those reported for slices of mouse liver, particularly from lactate and pyruvate. 3. The rate of glycogen breakdown to glucose, but not to lactate, was faster in liver from fed obese mice. 4. The capacity for glycogen synthesis from glucose was enhanced in liver from 20h-starved obese mice. 5. The capacity for gluconeogenesis from a number of substrates was not significantly altered in livers from fed or starved obese mice when compared with that of lean mice. 6. These results suggest that the liver contributes to the hyperglycaemia of the obese mice by increased glycogenolysis, and that liver glycogen in obese mice is maintained by synthesis from dietary glucose.