Basal insulin hypersecretion in insulin-resistant Zucker diabetic and Zucker fatty rats: role of enhanced fuel metabolism.

The biochemical mechanisms responsible for basal hyperinsulinemia in insulin-resistant states have not been fully defined. We therefore studied pancreatic beta-cell function in vitro to characterize the relative importance of fuel metabolism or secretion via a constitutive pathway in the maintenance of high basal insulin secretion in Zucker diabetic fatty (ZDF) and Zucker fatty (ZF) rats. Insulin secretion from ZF (10+/-1.8 v 5+/-0.6 pmol/ng DNA/h) and ZDF (30+/-4 v 7+/-0.8 pmol/ng DNA/h) islets at 2.8 mmol/L glucose was two to four times greater than secretion from islets of lean littermate control rats. In response to a decreasing glucose concentration (from 12 to 0 mmol/L), a paradoxical increase in insulin secretion was observed in perfused ZDF rat pancreas. Insulin secretion at 2.8 mmol/L glucose was suppressed approximately 70% to 80% in islets from ZDF and ZF rats following exposure to diazoxide, a K+-adenosine triphosphate (K(ATP)) channel opener that inhibits membrane depolarization, or rotenone and oligomycin, agents that inhibit ATP production, or by incubation at 23 degrees C. Inhibition of glycolysis with mannoheptulose, 2-deoxyglucose, and iodoacetate or fatty acid oxidation with a carnitine palmitoyltransferase I inhibitor also significantly inhibited basal insulin secretion in islets of ZDF and ZF rats but not their lean littermates. Furthermore, the glycolytic flux at 2.8 mmol/L glucose was significantly higher in ZDF islets versus ZDF lean littermate (ZLC) islets (2.2+/-0.1 v 3.7+/-0.3 pmol/ng DNA/2 h, P < .01) and was suppressed by mannoheptulose. In ZDF and ZF islets, high basal insulin secretion was maintained despite a 50% reduction in the rate of proinsulin/insulin biosynthesis at 2.8 mmol/L glucose. The rate of proinsulin to insulin conversion and the ratio of proinsulin to insulin secretion by islets of ZDF rats were similar to the values in the lean littermates. Thus, basal hypersecretion in these two insulin-resistant models appears to be related to enhanced fuel metabolism rather than the contribution of a constitutive pathway of secretion.     

Insulin response of cultured islets from diabetic and nondiabetic BB rats.

This study examines the insulin response of pancreatic islets isolated from diabetic BB rats (BBD), nondiabetic BB rats (BBN), and Wistar rats to in vitro stimulation. After a 48-hour culture period, insulin release in response to glucose (17.8 mmol/L) either alone, with glucose-dependent insulinotropic polypeptide (GIP) +/- somatostatin (SS), or with Arg +/- SS was measured. A static incubation system was used. Insulin secretion from islets cultured in 4.4 mmol/L glucose (basal) did not differ between BBN and BBD rats (0.50% +/- 0.08%, 0.67% +/- 0.25% of total islet cell content [TCC], respectively). High glucose concentrations (17.8 mmol/L) stimulated a modest increase in insulin release from BBD and BBN islets (1.8% +/- 0.48% and 2.1% +/- 0.19% TCC, respectively). The addition of GIP (1 nmol/L) enhanced glucose-stimulated insulin secretion from BBN rat islets (2.9% +/- 0.42% TCC), but had no effect on BBD islets (2.04% +/- 0.57% TCC). Somatostatin (1 mumol/L) completely reversed the glucose- and/or GIP-stimulated insulin secretion from both BBN and BBD rat islets to basal levels (0.42% +/- 0.043%, 0.42% +/- 0.09% TCC, respectively). Arg (1 mmol/L) enhanced glucose-stimulated insulin secretion in both groups, although the greatest response was elicited from BBD rat islets (8.4-fold v 3.2-fold). Experiments comparing BB rats with Wistar rats demonstrated significant differences in the glucose-stimulated (17.8 mmol/L) insulin response of the islets. Islets taken from BBN and BBD were less responsive to glucose than those from Wistar rats. However, islets from BBD rats were hyperresponsive to Arg when compared with islets from Wistar rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Changes in islet capillary angioarchitecture coincide with impaired B-cell function but not with insulin resistance in male Otsuka-Long-Evans-Tokushima fatty rats: dimorphism of the diabetic phenotype at an advanced age

The Otsuka-Long-Evans-Tokushima fatty (OLETF) rat is a genetic model of spontaneous development of non-insulin-dependent diabetes mellitus (NIDDM) established as an inbred strain after 20 generations of selective breeding. Although they are thought to be genetically homogeneous, they show a dimorphism regarding the diabetic phenotype at an advanced age, with one remaining obese and modestly diabetic while the other becomes lean and overtly diabetic. To clarify the causes for this divergence, we examined the physical, biochemical, and histopathological features in rats at 50 weeks of age, including an analysis of islet angioarchitecture. Sixty-one of 85 male OLETF rats lost weight, while the remainder remained obese. Mean nonfasting plasma glucose in the lean group was 21.8+/-4.6 mmol/L, significantly higher versus the obese group (10.5+/-1.4 mmol/L) and the age-matched control Long-Evans-Tokushima-Otsuka (LETO) group (7.1+/-0.6 mmol/L). Morphological studies of the pancreas from the lean group showed enlarged multilobulated fibrotic islets with a paucity of B cells, whereas islets from the obese group appeared slightly enlarged and showed a relative abundance of B cells. The fine capillaries that form a network in the islets were extremely sparse in the lean group, resulting in a defective glomerular-like configuration, whereas those from the obese group were dense, forming a nearly typical glomerular-like configuration. Increased plasma insulin responses to oral and intravenous (i.v.) glucose and i.v. glucagon loads were nearly absent in the lean group, while they were evident in the obese group, although to a lesser extent compared with the LETO group. Mean insulin secretory output from the perfused pancreas in response to 11.1 mmol/L glucose in the lean group (3.5+/-2.2 pmol/20 min) was significantly lower versus the obese group (8.8+/-6.5 pmol/20 min) and LETO group (22.0+/-10.8 pmol/20 min). Similarly, pancreatic insulin content was significantly lower in the lean group (9.3+/-6.1 microg) versus the others (26.1+/-17.3 microg for obese and 41.1+/-24.8 microg for LETO). In vivo insulin-stimulated glucose uptake measured by a euglycemic clamp technique was significantly higher in the lean group compared with the obese group. These results demonstrate that the dimorphism regarding the diabetic phenotype in male OLETF rats at 50 weeks of age was due to differences in the number of islet B cells, which could be the result of a variation in the capacity for B-cell proliferation among male OLETF rats.     

Adaptation in enzyme (metabolic) pathways to obesity, carbohydrate diet and to the occurrence of NIDDM in male and female SHR/N-cp rats.

Twenty-four male (12 obese and 12 lean) and 21 female (11 obese and 10 lean) SHR/N-cp rats were fed a diet containing either 54% sucrose or starch for periods of 3-4 months. Rats were killed after a 14-16 h fast and liver enzyme activities were determined in both sex groups. Liver glucose-6-phosphatase (G6Pase), fructose 1,6-bisphosphatase (FBPase), phosphoenolpyruvate carboxykinase (PEPCK), glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6PGDH), malic enzyme (ME), phosphofructokinase (PFK), glucokinase (GK), aspartate aminotransferase (AST) and alanine aminotransferase (ALT) levels (per total liver capacity) were significantly affected by phenotype (obese > lean). Arginase and ornithine transcarbamylase levels were analysed only in male rats and were found to be elevated in obese rats as compared to lean littermates. Some of the above changes in enzyme levels were exaggerated by sucrose feeding but not the changes in FBPase, PEPCK, ME and GK (in both sexes) plus AST, arginase and arginine synthase activities in male rats and ALT levels in female rats. Results from SHR/N-cp rats published in this paper were compared to results obtained from LA/N-cp rats published previously. Comparison of the non-diabetic obese LA/N-cp with the diabetic obese SHR/N-cp male shows a greater excess in lipogenic capacity of the liver in the LA/N-cp male rat. The SHR/N-cp obese female also shows a greater liver lipogenic capacity as compared with the obese male SHR/N-cp rat. The results suggest that an adaptation of excessive lipogenesis in the liver of obese rats may be an anti-diabetogenic adaptation resulting in increased glucose conversion to lipids, thus reducing blood glucose levels.     

Effect of adrenalectomy on the development of a pancreatic islet lesion in fa/fa rats

Summary Adrenalectomy prevents development of obesity and hyperinsulinaemia in obese (fa/fa) Zucker rats, thereby implicating the hypothalamopituitary-adrenal axis in the pathogenesis of obesity. In this study glucose-induced insulin secretion and glucokinase activity were investigated in isolated islets from adrenalectomized and control obese and lean female rats. Islets from control fa/fa rats were more sensitive to glucose with a half-maximal effective concentration (EC₅₀) of 6.1±2.0 mmol · l^–1 compared with 10.6±2.7 mmol · l^–1 for adrenalectomized fa/fa rat islets. Adrenalectomy did not alter the islet sensitivity to glucose in the lean rats (EC₅₀ of 9.4±1.5 mmol · l^–1 and 9.3±2.0 mmol · l^–1 for adrenalectomized and control lean rats respectively). Mannoheptulose did not inhibit insulin secretion from control obese rats; however at concentrations of 1.0 mmol · l^–1 or more it significantly inhibited glucose-induced insulin secretion in adrenalectomized obese and lean, and control lean rat islets (p<0.05). In adrenalectomized fa/fa islets the glucokinase K_m was increased twofold compared with the control fa/fa rats (9.5±1.5 mmol · l^–1 vs 5.0±1.5 mmol · l^–1, respectively), but there was no significant change in glucokinase K_m in the lean rat islets after adrenalectomy. Mannoheptulose (10 mmol · l^–1) caused a significant reduction in glucose phosphorylation in disrupted islets of adrenalectomized fa/fa and lean, and of control lean rats, but not of control fa/fa rats. These data demonstrate that development of abnormal regulation of glycolysis in pancreatic islet beta cells of fa/ fa rats, as indicated by the insulin response to mannoheptulose and glucokinase activity, is dependent on an intact hypothalamo-pituitary-adrenal axis.Abbreviations ADX Adrenalectomy/adrenalectomized - CRH corticotrophin releasing hormone - DMEM Dulbecco's modified Eagle's medium - EC₅₀ half-maximal effective concentration - HPA hypothalamo-pituitary-adrenal - MH mannoheptulose - Hepes 4-(2-hydroxyethyl)-1-piperazineethane sulphonic acid     

Atherogenesis in two strains of obese rats. The fatty Zucker and LA/N-corpulent

Two strains of obese rats, the fatty Zucker and the LA/N-corpulent have been compared at 6 months age for the presence of vascular and myocardial disease. Both strains, when obese, exhibit a VLDL hyperlipidemia with elevated triglycerides and moderate elevations of plasma cholesterol concentrations compared to the lean rats of the same strain. The hyperlipidemia is more modest in the fatty Zucker than the corpulent LA/N, and the serum lipid concentrations of the lean Zucker are lower than those of the lean LA/N. Apolipoprotein concentrations were similar and elevated in the two obese genotypes compared to the lean genotypes which were also similar to each other. Male and female obese animals of both strains exhibited hyperinsulinemia under fasting conditions and after oral glucose, with obese male LA/N rats exhibiting the most severe hyperinsulinemia. Glucose tolerance was impaired in obese LA/N animals but was normal in lean rats of both strains and fatty Zucker rats of both sexes. The glucose intolerance observed in obese LA/N animals was more severe in the male than in the female rats. Unlike the corpulent rat, which develops atherosclerotic lesions, the fatty Zucker shows no evidence of advanced vascular lesions on scanning electron microscopy. The fatty Zucker also does not develop the myocardial lesions that are frequent in the male corpulent LA/N rat. It is suggested that the initiation of the atherogenic process is dependent upon elevated insulin levels or transient hyperglycemia. Development of the advanced lesions appears to require the presence of hyperlipidemia.     

Long-term effects of a high glucose concentration in vitro on the oxidative metabolism and insulin production of isolated rat pancreatic islets

The present study was performed to clarify whether exposure in tissue culture of pancreatic islet B cells to high glucose concentrations will lead to glucose insensitivity and/or toxicity. For this purpose, isolated rat islets were maintained in tissue culture for up to 7 days in the presence of either 5.6, 11, or 56 mmol/L glucose and subsequently analyzed with regard to oxidative metabolism, insulin release, islet content of insulin, and insulin mRNA. Islets maintained at 56 mmol/L glucose showed a decreased insulin content, but no changes in insulin mRNA content when compared with control islets (cultured at 11 mmol/L glucose). In short-term incubations of the high-glucose cultured islets, the rate of insulin release at 1.67 mmol/L glucose was enhanced and could not be further stimulated by a 16.7-mmol/L glucose challenge. However, the insulin release at 16.7 mmol/L was decreased when compared with islets cultured at 11 mmol/L glucose. Islets cultured at 56 mmol/L glucose showed an increased oxygen uptake when incubated at 1.67 mmol/L glucose with no further stimulation at 16.7 mmol/L glucose. These islets also showed increased rates of glucose oxidation at incubation with 1.67 mmol/L glucose, but similar rates of oxidation at 16.7 mmol/L glucose as compared with islets cultured in 11 mmol/L glucose. Islets cultured at 5.6 mmol/L glucose showed decreased insulin release when incubated at either 1.67 mmol/L or 16.7 mmol/L glucose. The rates of glucose oxidation of these islets were also decreased at 16.7 mmol/L glucose when compared with the controls, whereas the oxygen uptake was decreased only during incubation at 1.67 mmol/L glucose. There was also a decreased content of insulin mRNA in these islets.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mechanism linking insulin resistance to defective thermogenesis in brown adipose tissue of obese diabetic SHR/N-cp rats.

We previously reported that the decreased sensitivity of brown adipose tissue (BAT) from obese Zucker rats to the calorigenic effects of norepinephrine is associated with a marked resistance to insulin, and we suggested that this defect may explain, at least in part, the increased energy gain efficiency of fa/fa rats. To test whether insulin resistance and/or diabetes leads to a reduced BAT thermogenesis in other genetic models of obesity, we compared BAT metabolic properties of obese Zucker rats with that of obese-nondiabetic LA/N-cp and obese-diabetic SHR/N-cp rats. It was found that the responsiveness and sensitivity of isolated brown adipocytes to the calorigenic effects of norepinephrine (10-100 mM) were markedly reduced in SHR/N-cp rats as compared to their lean controls (the Vmax was decreased by 3-4 times and the EC50 value was doubled). In the same cells, there was a similar decrease in the respiratory effects of dibutyryl cAMP (DBcAMP), revealing the presence of a major post-receptor defect. Remarkably, total cytochrome oxidase activity (an index of cell mitochondrial content) was also decreased by 3-4 times in SHR/N-cp rats, suggesting that a reduced BAT mitochondrial content is responsible for the defective thermogenesis. Similarly to Zucker rats, adipocytes isolated from SHR/N-cp rats were resistant to the metabolic effects of insulin (glucose transport and antithermogenesis). Cells from obese Zucker rats were also desensitized to the metabolic effects of norepinephrine and insulin but their thermogenic capacity was not reduced. In contrast, all the above parameters were normal in obese-nondiabetic LA/N-cp rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of pertussis toxin on islet insulin secretion in obese (fa/fa) Zucker rats.

We used isolated islets of lean and obese Zucker rats to determine whether inhibitory pathways mediated by pertussis toxin-sensitive guanyl nucleotide-binding (Gi) proteins contribute to hyperinsulinemia in obese rats. Epinephrine (10(-4) M) and somatostatin (10(-7) M) inhibited insulin secretion by +/- 75% in lean and fa/fa rats. Overnight culture of islets with pertussis toxin (300 ng/ml) enhanced insulin release more in lean (+/- 120%) than obese (+/- 60%) rats. In lean rats incubation of pertussis toxin-treated islets with epinephrine resulted in lower immunoreactive insulin release (p = 0.0005) than pertussis toxin-treated islets without epinephrine. However, in obese rats pertussis toxin treatment reversed this inhibition. Pertussis toxin completely reversed inhibition by somatostatin in both phenotypes. Galanin had no effect on insulin secretion. Cellular cAMP content was similar in lean and obese rats. Inhibitory hormones had no effect on cAMP production. We conclude that islets of obese rats respond normally to inhibitors of insulin release. Reversal of somatostatin-induced inhibition by pertussis toxin indicates normal function of Gi in obese rats. A subtle difference in sensitivity to pertussis toxin between lean and obese islets was noted.     

Decreased insulin binding to pancreatic islets of genetically obese (fa/fa) rats

Binding of insulin and insulin secretion were studied in isolated pancreatic islets of homozygous obese fa/fa rats, their lean littermates (Fa/?) and Wistar rats. Despite normoglycemia fa/fa rats exhibit hyperinsulinemia. Glucose-induced insulin secretion from pancreatic islets in vitro was increased by more than 50% in fa/fa rats compared with islets of lean littermates and normal Wistar rats when calculated per microgram islet protein. Exogenous insulin inhibited glucose (16.7 mM)-induced insulin secretion in islets of either of these rats, and maximum inhibition was rather the same (secretion was reduced by 62.3-65.6%). However, the EC50 (half-maximal effective concentration) for inhibition was increased in fa/fa rats being 1.4 +/- 0.1 nM compared with 0.6 +/- 0.2 and 0.5 +/- 0.2 nM in lean littermates and Wistar rats, respectively (P less than 0.05 vs. fa/fa rats). Islets of fa/fa rats found 24% less [125I]insulin (P less than 0.01) than islets of lean littermates and of Wistar rats. Scatchard analysis of data of displacement of [125I]insulin binding by native insulin showed 2 binding sites; a decrease in the number of high affinity insulin binding sites (Bmax) from 4.2 +/- 1.3 and 4.7 +/- 1.6 fmol/mg protein to 2.6 +/- 0.7 fmol/mg protein was calculated when islets of lean littermates and normal Wistar rats were compared to islets of fa/fa rats. The Kd of the high affinity binding site was not changed (0.77 +/- 0.06, 0.78 +/- 0.11 and 0.61 +/- 0.14 nM, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)     

Stimulatory effect of serum from diabetic patients on insulin release from mouse pancreatic islets maintained in tissue culture

Summary Islets of Langerhans from NMRI-mice were kept for one week in tissue culture in medium supplemented with human serum obtained from either normal healthy subjects or newly diagnosed juvenile diabetic patients before insulin treatment. Islets cultured in diabetic serum released more inslin than islets cultured in normal serum, whether tissue culture medium 199 with 5.5–8.3 mmol/1 glucose and 10% serum, or culture medium RPMI1 640 with 11 mmol/1 glucose and 0.5% serum were used. Islets kept for one week in culture with diabetic serum did not show any decrease in DNA content or glucose induced insulin secretion and biosynthesis. It is concluded that serum from newly diagnosed insulin dependent diabetic patients stimulates insulin release from isolated mouse islets kept in tissue culture. The underlying mechanism is unknown.     

Effects of growth hormone-releasing hormone on the secretion of islet hormones and on glucose homeostasis in lean and genetically obese-diabetic (ob/ob) mice and normal rats

The effect of synthetic human growth hormone-releasing hormone(1-40) (hGHRH-40) on the function of the endocrine pancreas and on glucose homeostasis in lean and genetically obese-diabetic (ob/ob) mice and normal rats has been examined. The addition of 1 mumol hGHRH-40/1 to incubated islets from normal lean mice increased insulin release by 90 and 37% at 5.6 and 16.7 mmol glucose/l respectively. Lower concentrations of hGHRH-40 did not affect insulin release. hGHRH-40 (1 mumol/l) increased pancreatic polypeptide release by 50% at 5.6 mmol glucose/l. A range of concentrations of hGHRH-40 (1 nmol/l-1 mumol/l) reduced glucagon release by 42-73% at 5.6 mmol glucose/l, and by 38-70% at 16.7 mmol glucose/l. Somatostatin release was increased (eightfold) by 1 mumol hGHRH-40/1 at 5.6 mmol glucose/1, but at 1 nmol hGHRH-40/1 somatostatin release was reduced (by greater than 50%). At 16.7 mmol glucose/litre 0.01-1 mumol hGHRH-40/1 increased somatostatin release (three- to fourfold), but 1 nmol hGHRH-40/1 produced a reduction of 50%. In vivo, administration of hGHRH-40 (50 micrograms/kg body weight i.p.) to fasted lean and ob/ob mice did not alter basal plasma concentrations of glucose and insulin, or the glucose and insulin responses to a concomitant i.p. glucose challenge. Intravenous injection of hGHRH-40 (20 micrograms/kg body weight) to anaesthetized rats increased plasma concentrations of insulin in the hepatic portal vein. A lower dose of hGHRH-40 (0.2 micrograms/kg) was ineffective, and neither dose of hGHRH-40 altered plasma glucose.(ABSTRACT TRUNCATED AT 250 WORDS)     

Decreased glucose-induced insulin release and biosynthesis by islets of rats with non-insulin-dependent diabetes: effect of tissue culture

Non-insulin dependent diabetes (NIDD) was obtained in adult rats after a neonatal streptozotocin injection. In the fed state 3- to 5-month-old rats with NIDD exhibited modestly elevated plasma glucose levels (controls, 131 +/- 7 mg/dl; diabetics, 159 +/- 4 mg/dl; P less than 0.01), impaired glucose tolerance, and a very low insulin response after glucose injection. In the present study the secretion and biosynthesis of insulin were measured using isolated islets from these rats. The insulin and DNA content of islets freshly isolated from rats with NIDD were significantly lower (60% and 80%, respectively, P less than 0.001) than in the controls. The insulin content per islet cell from diabetic rats was also significantly reduced (75%) (P less than 0.01) as compared to controls. At a low glucose concentration (2.8 mM) the insulin release from islets of diabetic rats was 60% (P less than 0.05) of that the controls. At a glucose concentration of 16.5 mM it was stimulated 5-fold from the islets of NIDD rats and 7-fold from the islets of control rats. (Pro)insulin biosynthesis, assessed in the same islets by measuring the incorporation of [3H]phenylalanine into immunoprecipitable material, was significantly higher (50%) (P less than 0.01) in islets from NIDD rats when measured at 2.8 mM glucose. Although (pro)insulin biosynthesis in these islets was significantly stimulated by 16.5 mM glucose (5-fold), it was less (P less than 0.05) than in the control islet (9-fold). To determine whether the derangements described above in the islets of the rats with NIDD could be modified by changing the environmental conditions of the B cells, corresponding experiments were performed after a 5-day culture of the islets at 5.5 mM glucose or at 11 mM glucose. The insulin release and the (pro)insulin biosynthesis, either measured in basal or stimulated states, were then found to be similar in the islets of diabetic and control islets after the 5.5 mM glucose culture period. By contrast, after the 11 mM glucose culture period the insulin release and the proinsulin biosynthesis in the islets of diabetic rats were found significantly less stimulated by 16.5 mM glucose than in the control islets. This suggests that islets of rats with NIDD, once removed from the chronic in vivo exposure to diabetic metabolic disorders, can behave as isolated islets of normal rats, at least as far as insulin handling is concerned.(ABSTRACT TRUNCATED AT 400 WORDS)     

Abnormal sensitivity to glucose of human islets cultured in a high glucose medium: partial reversibility after an additional culture in a normal glucose medium

In the experimental animal chronic hyperglycemia alters the islet's sensitivity to glucose. In the present study the glucose sensitivity of human pancreatic islets, isolated and purified, obtained from seven human pancreases using an automated method was evaluated. After a 12-h stabilization period, islets were cultured for 48 h in normal (5.5 mmol/L) or high glucose (16.7 mmol/L) medium. Islets were then perifused to study their insulin response to glucose. Islets cultured in the high glucose medium lost glucose-induced insulin release and, when challenged with an acute fall of glucose concentration in the perifusate, showed a paradoxical insulin release. Insulin release in response to 10 mmol/L L-arginine was preserved in these islets, suggesting a selective reduction of the insulin response to glucose. An additional 48-h culture in 5.5 mmol/L glucose medium partially restored the sensitivity to glucose of the previously unresponsive islets. These findings indicate that short term exposure to high glucose concentrations induces a selective glucose insensitivity of human islets, which can be partially reversed by an additional culture in normal glucose medium.     

Defective early phase insulin release in perifused isolated pancreatic islets of spiny mice (acomys cahirinus)

In order to characterize pancreatic beta cell function in Geneva bred spiny mice (acomys cahirinus), the dynamics of immunoreactive insulin release were examined during perifusion of pancreatic islets isolated from normoglycemic acomys. The initial insulin response of acomys was slow: no clear-cut early (1 to 10 min) peak of insulin release was observed when glucose in the perifusion medium was abruptly raised from 2.8 mM to concentrations as high as 56 mM. This was true for islets of either young, or older more obese acomys. However, after 20 to 30 min of perifusion at the high glucose concentrations, the rate of insulin release from acomysislets became similar to that from islets of rats or mice. By contrast, glucose-induced insulin release responses observed with islets of Wistar-derived rats, Swiss albino mice, and inbred C57BL/6J lean or obese (ob/ob) mice, were clearly biphasic. Tolbutamide 1.5 mM, arginine 16 mM, and theophylline 10 mM were ineffective in stimulating insulin release from acomys islets in the presence of a substimulatory glucose concentration (2.8 mM), whereas these agents were effective in rat islets at the same substimulatory concentration of glucose. On the other hand, when these agents, as well as cyclic AMP 10 mM or cytochalasin B 10 mug/ml were applied in the presence of a stimulating concentration of glucose (16.8 mM), the glucose-stimulated insulin release from acomys islets was increased to the same or to a greater extent than from rat islets. It is suggested that the failure of all the agents tested to stimulate an early rapid phase of insulin release from acomys islets may be secondary to the observed initial insensitivity to glucose, which insensitivity may in turn reflect a selective impairment in the recognition of glucose as an insulinogenic signal in this species.     

Metabolism of high-density lipoprotein in the hyperlipidemic, diabetic SHR/N-corpulent rat

The SHR/N-corpulent rat is a new genetically obese strain that is both hyperlipidemic and diabetic. The high density lipoprotein (HDL) fraction from 12-week-old obese males contained significantly greater amounts of protein (+83%), free (+72%) and esterified (+76%) cholesterol, phospholipid (+94%), and triglyceride (+78%). HDL from obese rats were also enriched in C apolipoproteins (apo C-III0 and apo C-III3) but had similar relative amounts of both apo A-I and apo E compared to HDL from their lean littermates. HDL protein turnover, measured with 125I-labeled HDL, showed that obese rats had a smaller fractional catabolic rate (FCR) than lean rats, but due to their much larger HDL pool size, they had a significantly higher rate of HDL protein catabolism (obese, 1.98 +/- 0.07 mg/whole animal/h v lean, 1.32 +/- 0.05 mg/whole animal/h). Therefore, under steady-state conditions, HDL protein production must also have been increased in the obese animals. To determine whether the increased catabolism of HDL protein was associated with increased catabolism of cholesteryl ester (CE), tissue uptake of HDL CE was measured using the nonhydrolyzable ether analogue [3H]cholesteryl linoleyl ether. After four hours 41.6 +/- 1.6% of the injected dose was cleared from the plasma of lean rats compared with 37.0 +/- 1.1% from the plasma of obese rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Glucagon-like peptide-1 stimulates insulin secretion by a Ca2+-independent mechanism in Zucker diabetic fatty rat islets of Langerhans

This study investigates the mechanisms responsible for glucagon-like peptide-1 (GLP-1)-induced insulin secretion in Zucker diabetic fatty (ZDF) rats and their lean control (ZLC) littermates. Glucose, and 100 nmol/L GLP-1 (7-37 hydroxide) in the presence of stimulatory glucose concentrations, induced insulin secretion in islets from ZLC animals. In contrast, ZDF islets hypersecreted insulin at low glucose (5 mmol/L) and were poorly responsive to 15 mmol/L glucose stimulation, but increased insulin secretion following exposure to GLP-1. The insulin secretory response to 100 nmol/L GLP-1 was reduced by 88% in ZLC islets exposed to exendin 9-39. The intracellular Ca2+ concentration ([Ca2+]i) increased in fura-2-loaded ZLC islets following stimulation with 12 mmol/L glucose alone or GLP-1 in the presence of 12 mmol/L glucose. The increases in [Ca2+]i and insulin secretion in ZLC islets induced by GLP-1 were attenuated by 1 micromol/L nitrendipine. In contrast, neither glucose nor GLP-1 substantially increased [Ca2+]i in ZDF islets. Furthermore, insulin secretory responses to GLP-1 were not significantly inhibited in ZDF islets by nitrendipine. However, the insulin secretory response to GLP-1 in both ZLC and ZDF islets was ablated by cholera toxin. Our findings indicate that in ZLC islets, GLP-1 induces insulin secretion by a mechanism that depends on Ca2+ influx through voltage-dependent Ca2+ channels, whereas in ZDF islets, the action of GLP-1 is mediated by Ca2+-independent signaling pathways.     

Ducto-insular proliferation of beta-cells after syngeneic islet transplantation into the spleen of streptozotocin-diabetic Lewis rats

Syngeneic transplantation of cultured and functionally characterized neonatal islets into the spleen of streptozotocin diabetic Lewis rats resulted in long time survival up to 200 d and in plasma glucose levels lower than 9 mmol/L. The daily plasma glucose profile of transplanted rats had shown significantly above that of nondiabetic control rats. Two-hundred d after transplantation, morphologically intact, insulin containing beta-cells were demonstrable in the spleen, thus demonstrating the long-term survival of functioning islet cells. Proliferation of beta-cells was shown in the transplanted islets. In addition, beta-cell clusters were found that derived from pancreatic ductules transplanted together with the isolated islets into the spleen. Mitoses were visible within ductular epithelial cells. The proliferative response of islets after intrasplenic transplantation is probably the result of a long-term stimulation by slightly enhanced plasma glucose values of the transplant acceptors compared to control animals.     

Insulin action in non-insulin-dependent diabetes mellitus: the relationship between hepatic and extrahepatic insulin resistance and obesity

To determine the contribution of obesity to the insulin resistance of non-insulin-dependent diabetes mellitus, insulin dose response curves for suppression of glucose production and stimulation of glucose utilization were generated in lean and obese diabetic patients and compared to those observed in weight-matched nondiabetic subjects. Glucose utilization during 0.4, 1.0, and 10.0 mU/kg x min insulin infusions (producing insulin concentrations ranging from approximately 50 to 2,000 microU/mL) was lower (p less than .02 to .001) in lean and obese diabetic patients compared to weight-matched nondiabetic subjects indicating insulin resistance. Glucose utilization was not correlated with obesity in the diabetic subjects. Suppression of glucose production was impaired (P less than .03 and .001) in both the lean and obese diabetic subjects at physiologic but not supraphysiologic insulin concentrations. We conclude that patients with NIDDM have both hepatic and extrahepatic insulin resistance, the severity of which appears to be independent of the degree of obesity.     

Human islets chronically exposed in vitro to different stimuli become unresponsive to the same stimuli given acutely: evidence supporting specific desensitization rather than beta-cell exhaustion.

The aim of this study was to evaluate the effects of long term in vitro exposure of human pancreatic islets to different secretagogues on their subsequent secretory activity. Therefore, groups of 100 islets were cultured for 48 h in standard tissue culture medium (CMRL 1066) in the presence of 1 of the following: 5.5 mmol/L glucose, 16.7 mmol/L glucose, 5.5 mmol/L glucose plus 10 mmol/L L-arginine, or 5.5 mmol/L glucose plus 100 mumol/L tolbutamide. Insulin levels in the culture medium declined with time under all culture conditions. Islets were then perifused and acutely stimulated with glucose (16.7 mmol/L), L-arginine (10 mmol/L), and tolbutamide (100 mumol/L). Islets cultured in 16.7 mmol/L glucose showed no response to 16.7 mmol/L glucose [net area under the curve (delta AUC), 11% of control], and a reduced response to acute tolbutamide (delta AUC, 35% of control), but responded to L-arginine (delta AUC, 75% of control). Islets cultured in the presence of 10 mmol/L L-arginine had reduced responses to glucose (delta AUC, 11% of control) and tolbutamide (delta AUC, 27% of control), but responded to L-arginine (delta AUC, 75% of control). Islets cultured in tolbutamide did not respond to tolbutamide (delta AUC, 14% of control) and showed a reduced responses to acute glucose (delta AUC, 36% of control) and L-arginine (delta AUC, 24% of control). In a second set of experiments, islets cultured in 5.5 or 16.7 mmol/L glucose showed an insulin response to a supramaximal glucose stimulation (30 mmol/L glucose plus 0.5 mmol/L isobutylmethylxanthine) that was not statistically different. Similarly, islets that were cultured in the presence of 100 mumol/L tolbutamide still responded to 1 mmol/L tolbutamide. In conclusion, all stimuli evaluated in this study, chronically applied, reduced the insulin response to further acute stimulations. The different patterns of unresponsiveness observed together with the finding of a preserved insulin content in the islets after perifusions and a maintained capability to release insulin in response to supramaximal stimulations suggest that after chronic exposure to different stimuli, human islets become selectively desensitized to the same stimuli given acutely and do not become exhausted.