Modification of chemically induced diabetes in rats by vitamin E. Supplementation minimizes and depletion enhances development of diabetes

Administration of the antioxidant vitamin E to rats, prior to administration of either streptozotocin or alloxan, provided protection against the diabetogenic effect of both these agents. This was demonstrated by their response to a glucose load, their pancreatic insulin content and light microscopy findings. In addition, rats whose antioxidant state was depleted, by being maintained on a vitamin E and selenium-deficient diet, demonstrated increased diabetogenic susceptibility to normally nondiabetogenic doses of streptozotocin. These findings provide indirect support for the suggestion that the chemical agents streptozotocin and alloxan may exert their diabetogenic effect by acting as oxidants or free radical producers.     

Insulin and glucose as modulators of the amino acid-induced glucagon release in the isolated pancreas of alloxan and streptozotocin diabetic rats.

The hyperglucagonemia that occurs in vivo in animals made diabetic with alloxan or streptozotocin is not suppressed by high glucose but is suppressed by exogenous insulin. These observations together with other studies suggested that insulin-dependent glucose transport and metabolism by the alpha-cells serves as the primary mechanism controlling glucagon secretion. This hypothesis was tested in the present investigation. The possible interactions between glucose, insulin, and a mixture of 20 amino acids at physiological proportions were examined in the isolated-perfusin diabetic rats. Release of insulin and glucagon were used as indicators of theta-cell and alpha-cell function. According to rigid criteria the diabetic animals entering the study were severely diabetic. It was found that in vitro: (a) basal glucagon release (measured in the absence of an alpha-cell stimulus or inhibitor) was extremely low, even lower (i.e. 10%) than the basal rates seen in controls; (b) the alpha-cells of alloxanized- and streptozotocin-treated rats responded with a biphasic glucagon release to stimulation by an amino acid mixture; (c) this alpha-cell response was reduced after both streptozotocin and alloxan; (d) glucose at 5 mM was a potent inhibitor of amino acid-induced glucagon secretion in both types of experimental diabetes; (e) in alloxan diabetes alpha-cell stimulation by amino acids can be curbed by exogenous insulin, whereas glucagon secretion by the perfused pancreas of streptoxotocin diabetic rats appeared to be resistant to insulin action. The data indicate that the modulation of glucagon secretion by glucose in vitro is indipendent of insulin and that other unknown factors extrinsic to the pancreatic islets are responsible for the hyperglucagonemia observed in vivo.     

血清游离脂肪酸与胰岛β细胞功能及胰岛素抵抗

目的观察糖代谢异常不同阶段患者空腹及口服75g葡萄糖后血清游离脂肪酸（free fatty acid,FFA）的变化,探讨FFA随糖代谢异常进展的变化趋势及其与胰岛β细胞功能、胰岛素抵抗的关系。方法 2003年5月-2005年4月对123例受试者通过口服葡萄糖耐量试验（OGTT）分为正常糖耐量组（NGT）20例,糖调节受损组（IGR）30例,初诊2型糖尿病组（NDM）41例,已诊2型糖尿病组（TDM）32例。测定其空腹及口服75g葡萄糖后的FFA、血糖、胰岛素及血脂的水平。结果①空腹FFA、OGTTFFA曲线下面积在IGR组、NDM组、TDM组均较NGT组明显增高,有统计学意义（P〈0.05）;各组OGTT2hFFA数值无差异,但随着2型糖尿病病情发展有逐渐增高的趋势。②空腹FFA、OGTT2hFFA与血糖、血脂均有相关性,且与胰岛β细胞功能有一定的负相关,OGTT2hFFA与胰岛素抵抗指数有一定的正相关。结论 FFA对胰岛β细胞功能及胰岛素抵抗均有影响。     

Insulin release and cyclic AMP accumulation in response to glucose in pancreatic islets of fed and starved rats.

The dose as well as the time kinetics of insulin and adenosine-3', 5' -monophosphate (cyclic AMP) responses to glucose were compared in pancreatic islets of fed and starved rats. There was a preferential impairment of the early phase of glucose-induced insulin release in perifused islets of rats starved for 16 and 48 h. Similarly, the accumulation of 3H cyclic AMP in islets prelabeled with 3H-2-adenine was less in islets of 48 h starved than fed rats, during the first 10-min of stimulation with 26.7 mM glucose in the presence of 0.1 mM of the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine, whereas at 30 and 60 min 3H cyclic AMP responses to glucose were similar in fed and starved islets. Also, in 10-min incubations with glucose 3.3, 6.7, 10.0, 13.3, and 26.7 mM without and with 0.1 mM and 1.0 mM 3-isobutyl-1-methylxanthine, insulin release correlated strongly with the accumulation of 3H cyclic AMP in the islets of fed as well as starved rats. The thresholds for glucose-induced insulin and 3H cyclic AMP responses were higher and the maximal responses were lower in starved than fed islets. Preincubation of islets of 48-h starved rats with 16.7 mM glucose for 60 min corrected the impaired insulin and 3H cyclic AMP responses to glucose. Starvation-induced impairment of insulin secretory responses to glucose, and their restoration by preincubation with glucose in vitro, may represent acute regulatory effects of glucose on the adenylate cyclase-cyclic AMP system in the pancreatic beta cell.     

The effect of fasting, diet, and actinomycin D on insulin secretion in the rat

The present studies were performed to elucidate the mechanisms responsible for the impairment of glucose-stimulated insulin secretion observed in fasting. Rats fasted for 48 hr displayed marked impairment in their insulin secretory response to both oral and intravenous glucose. Glucose-stimulated insulin secretion was restored within 24 hr by refeeding; actinomycin D given before refeeding blocked the expected return of normal glucose-stimulated insulin secretion despite adequate food intake. Fasted rats refed a diet devoid of carbohydrate failed to display a return of normal insulin secretory responsiveness to oral glucose in contrast to rats fed isocalorically a high carbohydrate diet. Differences in insulin secretion in fed, fasted, and fasted-refed rats could not be attributed to changes in pancreatic insulin content. There was no significant difference in the insulin secretory response to aminophylline of fed, fasted, or fasted-refed rats. The intermittent pulsing of fasted rats with hyperglycemic episodes by the injection of small amounts of glucose (500 mg) intraperitoneally every 8 hr ameliorated the impairment of glucose-stimulated insulin secretion characteristic of the fasting state. These results suggest that the impairment of glucose-stimulated insulin secretion during fasting and its restoration by refeeding are regulated by changes in a glucose-inducible enzyme system in the pancreatic beta cell.     

The effect of alloxan diabetes on experimental cholesterol atherosclerosis in the rabbit. IV. The effect of insulin therapy on the inhibition of atherosclerosis in the alloxan-diabetic rabbit

Experiments were undertaken to ascertain whether the previously demonstrated inhibition of the development of experimental aortic atherosclerosis in alloxan-diabetic rabbits fed cholesterol was due to the injection of alloxan per se or to the existence of the diabetic state produced by alloxan. It was established that, by treating the diabetic state with insulin, the diabetic state could be ameliorated and the inhibitory effect obviated. It was therefore concluded that the inhibitory phenomenon was not due to the injection of alloxan per se but that it was associated with one or more factors that characterize the alloxan diabetic state in the rabbit and that are reversible by insulin therapy. In the course of the experiment it was demonstrated that the inhibitory effect was apparent in cholesterol-fed diabetic rabbits whether or not their diet was supplemented with vegetable oil. The previously reported metabolic abnormalities of the diabetic animals were confirmed. It was established that suitable treatment of the cholesterol-fed diabetic animals with insulin would bring all the metabolic aberrations, including those of the serum lipids, into reasonably close correspondence with those observed in non-diabetic rabbits fed cholesterol.     

No acute effects of smoking and nicotine nasal spray on lipolysis measured by subcutaneous microdialysis

Smoking is associated with insulin resistance, dyslipidaemia and markers of the insulin resistance syndrome. This study investigated adipose tissue lipolysis in situ by subcutaneous microdialysis twice in 10 healthy, male smokers after smoking four cigarettes over 2 h and after the administration of an equal amount of nicotine given as nasal spray (NNS). Glucose and insulin levels, in situ lipolysis and adipose tissue blood flow were studied in the post-absorptive state and after a 75-g oral glucose tolerance test (OGTT). Post-absorptively, acute smoking and NNS increased neither subcutaneous adipose tissue glycerol production nor plasma free fatty acid (FFA) or glycerol levels. After the OGTT, plasma insulin and lactate levels were significantly higher after smoking, whereas FFA levels were higher after NNS. Normal smoking or the administration of a normal dose of NNS caused only minor metabolic changes. Thus, it does not seem likely that increased lipolysis is an important contributor to the dyslipidaemia seen in smokers.     

Diurnal fluctuations in triglyceride, free fatty acids, and insulin during sucrose consumption and insulin infusion in man

Serial changes in circulating triglyceride, free fatty acids (FFA), insulin, and glucose have been measured in human subjects fed sucrose as the sole source of calories for 2- or 3-day periods. The sucrose was given either during the day with overnight fasting (19 subjects) or as continual 3-hour meals during the day and night (seven subjects). Insulin was infused overnight in five additional subjects on the day-feeding regimen to determine the effect on triglyceride concentration.The concentration of triglyceride increased during the study in all subjects, but there was a clear diurnal pattern in the response which was present even in the continual feeding studies. The rise in triglyceride occurred mainly overnight, and during the day there was frequently a fall in the concentration. The overnight increase was significantly less when insulin was infused. There were also diurnal fluctuations in FFA and insulin in both daytime and continual feeding regimens. The plasma FFA, like triglyceride, rose during the night and fell during the day while the insulin rose during the day and fell overnight.Separate statistical analysis of the daytime and overnight changes revealed that the changes in triglyceride were significantly but negatively correlated with changes in insulin during both periods. The changes in triglyceride and FFA were positively correlated during the day but not significantly related during the night. The data show that when sucrose is eaten for 2 or 3 days, there is a general increase in triglyceride concentration upon which are superimposed major diurnal fluctuations in the concentrations of triglyceride, insulin, and FFA. It is suggested that the highly significant inverse relationship between changes in triglyceride and insulin may be mediated through an effect of insulin on triglyceride removal.     

Effects of selective (beta-1 and beta-2) and nonselective beta adrenoceptor antagonists on the cardiovascular and metabolic responses to isoproterenol: comparison with ICI 141,292

Six groups of four dogs were studied under pentobarbital anesthesia. Plasma levels of free fatty acids (FFA), glucose, insulin and lactate were measured after four challenges with isoproterenol (0.125 micrograms/kg/min i.v. for 10 min). In control animals, infusion of isoproterenol elevated plasma FFA, glucose, insulin and lactate, raised heart rate and lowered diastolic blood pressure. Propranolol lowered resting heart rate and blocked all the isoproterenol-induced responses. Atenolol (beta-1-selective antagonist) slowed resting heart rate and blocked FFA and heart rate responses. ICI 118,551 (beta-2-selective antagonist) blocked the glucose, insulin and lactate responses and the fall in diastolic blood pressure. Selectivity was lost at higher doses. ICI 141,292 (potent beta-1-selective antagonist and modest partial agonist) exhibited beta-1-selective blockade against both cardiovascular (heart rate) and metabolic (FFA) receptors. Partial agonist activity was also beta-1-selective, but there was no evidence of agonist activity at the metabolic receptors.     

Metabolic and hormonal effects of exercise in mild streptozotocin diabetes.

Metabolic and hormonal changes produced by muscular exercise were studied in rats with mild streptozotocin diabetes. In both diabetic and control animals the mean blood glucose increased and liver glycogen decreased during exercise. The postexercise blood glucose level correlated with the liver glycogen content in diabetic rats. A decrease of blood glucose occurred in some diabetic animals, and these also showed a decrease of liver glycogen to very low levels during exercise. During exercise the rise of plasma glycerol was higher in diabetic than in control animals, but changes in circulating free fatty acids, acetone bodies, insulin, corticosterone, lactate, and acid-base equilibrium were similar in the two groups. The results indicate that the only abnormal metabolic response to exercise in rats with mild diabetes is a slight increase of lipid mobilization.     

Mechanism of protection from alloxan diabetes provided by n-butanol.

Pretreatment with n-butanol (10 mmol/kg i.p.) 30 minutes before alloxan (100 mg/kg) protects mice from the permanent hyperglycemic effects (measured at 72 hours) of the diabetogenic agent. This dose of n-butanol caused an elevation of serum glucose at 30 minutes, the time of alloxan administration. Since glucose administration can protect animals from alloxan, the possibility that alcohol-induced hyperglycemia protected mice from alloxan was investigated. Mannoheptulose, an antagonist of glucose action at the pancreatic beta-cell, when given 24 minutes after n-butanol and 6 minutes before alloxan, eliminated the alcohol-induced protection. Fasted mice did not exhibit n-butanol-induced hyperglycemia at 30 minutes and alloxan given at that time produced diabetes. No protection was observed in fed animals when n-butanol was given 5 minutes before alloxan. The high serum levels of butanol and normal serum glucose which were observed at 5 minutes after alcohol administration indicated that the lack of protection was not due to a lack of circulating alcohol but resulted from an absence of hyperglycemia. The results indicate that pretreatment with n-butanol protects mice from alloxan-induced diabetes by the indirect mechanism of producing hyperglycemia at the time of alloxan administration.     

Increased glucose tolerance and reduced adiposity in the absence of fasting hypoglycemia in mice with liver-specific Gs alpha deficiency

The G protein G(s)alpha is essential for hormone-stimulated cAMP generation and is an important metabolic regulator. We investigated the role of liver G(s)-signaling pathways by developing mice with liver-specific G(s)alpha deficiency (LGsKO mice). LGsKO mice had increased liver weight and glycogen content and reduced adiposity, whereas survival, body weight, food intake, and metabolic rates at ambient temperature were unaffected. LGsKO mice had increased glucose tolerance with both increased glucose-stimulated insulin secretion and increased insulin sensitivity in liver and muscle. Fed LGsKO mice were hypoglycemic and hypoinsulinemic, with low expression of hepatic gluconeogenic enzymes and PPARgamma coactivator-1. However, LGsKO mice maintained normal fasting glucose and insulin levels, probably due to prolonged breakdown of glycogen stores and possibly increased extrahepatic gluconeogenesis. Lipid metabolism was unaffected in fed LGsKO mice, but fasted LGsKO mice had increased lipogenic and reduced lipid oxidation gene expression in liver and increased serum triglyceride and FFA levels. LGsKO mice had very high serum glucagon and glucagon-like peptide-1 levels and pancreatic alpha cell hyperplasia, probably secondary to hepatic glucagon resistance and/or chronic hypoglycemia. Our results define novel roles for hepatic G(s)-signaling pathways in glucose and lipid regulation, which may prove useful in designing new therapeutic targets for diabetes and obesity.     

Studies of substrate regulation in fasting: II. Effect of infusion of glucose into the carotid artery upon fasting lipolysis in the baboon

The effect of glucose infusion on rates of lipolysis were studied in a group of chair-trained papio baboons that had been prepared for chronic intravenous and intracarotid infusion. All studies were carried out after a 24 hr period of fasting and when the animals were fully awake. After a control interval of 1 hr, a glucose infusion was begun either intravenously or intra-arterially. The infusion was continued at a constant rate for 2 hr and then changed directly to the alternate route and continued an additional 2 hr. Blood samples were collected at 30-min intervals for glucose, free fatty acid (FFA), glycerol, insulin, and in some studies, growth hormone (GH) determination. When glucose doses less than 0.5 mg/kg per min were used, no change in the products of lipolysis was noted during either venous or carotid administration, and glucose and insulin levels remained stable or fell gradually. With doses of glucose between 0.5 and 0.6 mg/kg per min, a greater fall in both FFA and glycerol was noted during carotid administration. No definite changes in plasma glucose or insulin levels were noted during either infusion period. These changes in lipolysis were noted regardless of the sequence of infusion, and a similar differential suppression of FFA was noted during a 24 hr period of carotid glucose administration. When doses of glucose larger than 0.6 mg/kg per min were used, inhibition of lipolysis was noted during both phases of infusion. No definite change in GH levels was noted during the periods of fasting, and the levels of the hormone did not appear to be related to changes in glucose, insulin, or FFA levels.These data provide additional evidence for the presence in the central nervous system of a glucose-sensitive center which alters lipolytic rates independently of insulin and GH, probably by altering sympathetic tone to adipose tissue.     

Beneficial role of l‐arginine in cardiac matrix remodelling in insulin resistant rats

Background The study was performed to determine whether sucrose‐induced insulin resistance could increase the expression of cardiac matrix metalloproteinases (MMPs), indices of matrix remodelling, and whether the addition of 1·25 g die^?1 of L‐arginine (ARG) to a sucrose diet could prevent both the sucrose‐induced metabolic abnormalities and elevated cardiac expression of matrix metalloproteinases in an insulin resistant stage that precedes frank type 2 diabetes. Materials and methods Experiments were performed on 38 male Sprague‐Dawley rats, 16 rats maintained a standard chow diet (ST), 12 rats were switched to a sucrose enriched diet (SU) and 10 rats to a sucrose plus L‐arginine (1·25 g die^?1) enriched diet (SU + ARG) for a period of 8 weeks. After 8 weeks of different diets, an intravenous glucose tolerance test (IVGTT) was performed and samples were drawn for the measurements of insulin, glucose, triglycerides, free fatty acids (FFA), plasma cyclic guanosine‐monophosphate (c‐GMP) and retroperitoneal, omental, epididymal fat pad and heart were dissected and weighed. Results At the end of the study, retroperitoneal fat, heart weight/body weight ratio, fasting plasma glucose, serum insulin, and serum triglyceride levels and integrated insulin area after IVGTT were significantly higher in SU than in SU + ARG and ST. All these parameters were comparable between SU + ARG and ST animals. FFA levels were significantly different among groups, with highest levels in SU and lowest levels in ST. Fasting plasma c‐GMP levels and the integrated c‐GMP area after IVGTT, an index of nitric oxide activity, were significantly lower in SU than in SU + ARG and ST, the result was similar in SU + ARG and in ST MMP‐9 protein expression increased 10·5‐fold, MMP‐2 protein expression increased 2·4‐fold and the expression of tissue inhibitors of metalloproteinase (TIMP‐1) increased 1·7‐fold in SU rats as compared to ST animals. This was accompanied with a significant increase of cardiac triglyceride concentrations. In contrast, cardiac MMP‐9, MMP‐2, and TIMP‐1 protein expressions were not different between SU + ARG and ST animals. Cardiac triglyceride levels were not significantly different between SU + ARG and ST rats. Conclusions SU rats developed insulin resistance and hyperlipidaemia, accompanied with increased fat deposition in the heart and enhanced MMP protein expression. Conversely, ARG supplementation prevents these metabolic abnormalities and restored MMP/TIMP‐1 balance.     

Potentiation of carbon tetrachloride-induced hepatotoxicity in alloxan- or strepto- zotocin-diabetic rats.

Studies were performed to examine the effects of alloxan- or streptozotocin-induced diabetes on carbon tetrachloride (CCl4) liver injury. Male rats were pretreated with single i.v. injections of alloxan monohydrate (40 or 80 mg/kg) or streptozotocin (65 mg/kg). A challenging dose of CCl4 (0.1 ml/kg i.p.) was given to rats 4 days after alloxan pretreatment or 5 days after streptozotocin pretreatment, and the animals were sacrificed 24 hours later. Biochemical and morphologic evidence was obtained to show that pretreatment with the diabetogenic agents markedly enhanced CCl4-induced hepatotoxity. The challenging dose of CCl4 had no effect on the serum glutamic pyruvic transaminase (SGPT) activity in control rats. However, the administration of this dose of CCl4 to rats pretreated with 40 and 80 mg/kg of alloxan as well as to rats pretreated with streptozotocin resulted in 11-, 68-, and 32-fold increases, respectively, in SGPT activity. Hepatic triglyceride concentrations in the diabetic rats were also markedly elevated above control values after CCl4 challenge. Alloxan- or streptozotocin-pretreatment alone did not enhance these biochemical parameters of liver injury. Hepatic glucose-6-phosphatase activity, which increased in the rats given a diabetogenic agent, was lowered as a result of CCl4 injection. Insulin treatment of rats given alloxan (80 mg/kg) markedly protected against CCl4-induced hepatotoxicity. The severity of the morphologic changes in diabetic rats given CCl4 correlated with the biochemical findings.     

Essentiality of circulating fatty acids for glucose-stimulated insulin secretion in the fasted rat.

We asked whether the well known starvation-induced impairment of glucose-stimulated insulin secretion (GSIS) seen in isolated rat pancreas preparations also applies in vivo. Accordingly, fed and 18-24-h-fasted rats were subjected to an intravenous glucose challenge followed by a hyperglycemic clamp protocol, during which the plasma-insulin concentration was measured. Surprisingly, the acute (5 min) insulin response was equally robust in the two groups. However, after infusion of the antilipolytic agent, nicotinic acid, to ensure low levels of plasma FFA before the glucose load, GSIS was essentially ablated in fasted rats, but unaffected in fed animals. Maintenance of a high plasma FFA concentration by coadministration of Intralipid plus heparin to nicotinic acid-treated rats (fed or fasted), or further elevation of the endogenous FFA level in nonnicotinic acid-treated fasted animals by infusion of etomoxir (to block hepatic fatty acid oxidation), resulted in supranormal GSIS. The in vivo findings were reproduced in studies with the perfused pancreas from fed and fasted rats in which GSIS was examined in the absence and presence of palmitate. The results establish that in the rat, the high circulating concentration of FFA that accompanies food deprivation is a sine qua non for efficient GSIS when a fast is terminated. They also serve to underscore the powerful interaction between glucose and fatty acids in normal beta cell function and raise the possibility that imbalances between the two fuels in vivo could have pathological consequences.     

Defective fatty acid uptake modulates insulin responsiveness and metabolic responses to diet in CD36-null mice

Deficiency of the membrane protein FAT/CD36 causes a marked defect in fatty acid uptake by various tissues and is genetically linked to insulin resistance in rats and humans. Here, we examined insulin responsiveness of CD36-/- mice. When fed a diet high in complex carbohydrates and low (5%) in fat, these animals cleared glucose faster than the wild-type. In vivo, uptake of 2-fluorodeoxyglucose by muscle was increased severalfold, and in vitro, insulin responsiveness of glycogenesis by the soleus was enhanced. Null mice had lower glycogen levels in muscle and liver, lower muscle triglyceride levels, and increased liver triglyceride content--all findings consistent with increased insulin-sensitivity. However, when the chow diet was switched to one high in fructose, CD36-/- mice but not wild-type mice developed marked glucose intolerance, hyperinsulinemia, and decreased muscle glucose uptake. High-fat diets impaired glucose tolerance equally in both groups, although CD36 deficiency helped moderate insulin-responsive muscle glucose oxidation. In conclusion, CD36 deficiency enhances insulin responsiveness on a high-starch, low-fat diet. It predisposes to insulin resistance induced by high fructose and partially protects from that induced by high-fat diets. In humans, CD36 deficiency may be an important factor in the metabolic adaptation to diet and in susceptibility to some forms of diet-induced pathology.     

Effects of short-term feeding of a highly palatable diet on vascular reactivity in rats

BACKGROUND: Numerous studies have shown that long-term consumption of high-fat, high-energy diet results in obesity, which in turn, leads to cardiovascular disorders. However, there is little or no data on the acute effects of a highly palatable diet on vascular function. MATERIAL AND METHODS: In this study we aimed to evaluate the changes in metabolic and vascular reactivity in Wistar rats fed a palatable diet for 2 days. RESULTS: Two-days feeding of rats with a palatable diet did not effect body weight, fat-pad mass or gastrocnemius muscles weights. Nor there were any changes in plasma glucose, insulin or leptin levels. However, compared with chow-fed rats, palatable diet-fed rats had significantly raised plasma free fatty acids and triglycerides levels (for both, P < 0.01). Compared with chow-fed animals, vasorelaxation responses to carbamylcholine and sodium nitroprusside were significantly attenuated in palatable diet-fed rats (for both, P < 0.01). However, there were no differences in histamine-induced vasorelaxation between chow-fed and palatable diet-fed rats. CONCLUSION: These data indicates that diet-induced endothelium-dependent and -independent vascular dysfunction occurs long before obesity develops.     

Diabetogenic action of streptozotocin: relationship of dose to metabolic response

The relationship between the dose of intravenously administered streptozotocin (a N-nitroso derivative of glucosamine) and the diabetogenic response has been explored by use of the following indices of diabetogenic action: serum glucose, urine volume, and glycosuria, ketonuria, serum immunoreactive insulin (IRI), and pancreatic IRI content. Diabetogenic activity could be demonstrated between the doses of 25 and 100 mg/kg, all indices used showing some degree of correlation with the dose administered. Ketonuria was only seen with the largest dose, 100 mg/kg. The most striking and precise correlation was that between the dose and the pancreatic IRI content 24 hr after administration of the drug, and it is suggested that this represents a convenient test system either for both related and unrelated beta cytotoxic compounds or for screening for modifying agents or antidiabetic substances of a novel type. Ability to produce graded depletion of pancreatic IRI storage capacity led to an analysis of the relationship between pancreatic IRI content and deranged carbohydrate metabolism. Abnormal glucose tolerance and insulin response were seen when pancreatic IRI was depleted by about one-third, while fasting hyperglycemia and gross glycosuria occurred when the depletion had reached two-thirds and three-quarters, respectively. The mild yet persistent anomaly produced by the lowest effective streptozotocin dose, 25 mg/kg, exhibits characteristics resembling the state of chemical diabetes in humans and might thus warrant further study as a possible model. Finally, the loss of the diabetogenic action of streptozotocin by pretreatment with nicotinamide was confirmed and was shown to be a function of the relative doses of nicotinamide and streptozotocin and of the interval between injections.     

Protective Role of Superoxide Dismutase against Diabetogenic Drugs

Copper-zinc superoxide dismutase (SOD) is present in relatively high concentrations in the beta-cells of human islets. The activity of the extracted enzyme is partially inhibited upon incubation with the diabetogenic drugs alloxan, streptozotocin, or Vacor. The role of this enzyme in protecting beta-cells against chemically induced diabetes was further investigated.Incubation of intact canine islets with alloxan (0.2 mg/ml) and 4 mM glucose decreased the insulin secretory response by 87% during subsequent exposure to 28 mM glucose. Concomitantly the SOD-specific activity (units of enzyme activity per milligram immunoreactive SOD) decreased 50% in alloxan-exposed islets. When islets were protected from alloxan toxicity by including 28 mM glucose with alloxan, the insulin secretory response and SOD specific activity remained identical to controls. Thus, SOD specific activity correlates with maintenance of beta-cell function.To test the effectiveness of SOD against streptozotocin in vitro, canine islets were incubated 10 min with or without streptozotocin (0.1 mg/ml) with 4 mM glucose; their functional integrity was tested subsequently as the insulin secretory response to 28 mM glucose. Exposure to streptozotocin alone decreased the response by 70%; inclusion of SOD (1.5 mg/ml) before and during exposure to streptozotocin completely prevented this effect. Cyanide-inactivated SOD was not effective.The potential of SOD to prevent streptozotocin-induced diabetes was tested in rats in vivo. SOD injected 10 s or 50 min before streptozotocin prevented or significantly attenuated diabetes. Injection of SOD and streptozotocin simultaneously was much less effective, and cyanide-inactivated SOD was ineffective. No protection was afforded by injection of SOD 12 or 24 h before streptozotocin.Our results support hypotheses that (a) oxygen radicals mediate the beta-cell toxicity of both alloxan and streptozotocin, and (b) beta-cells may be particularly vulnerable to oxygen radical damage.