Is Lipotoxicity presents in the early stages of an experimental model of autoimmune diabetes? Further studies in the multiple low dose of streptozotocin model.

An increased availability of plasma free fatty acids (FFA) seems to play a role in the early stages of experimental type 1 diabetes mellitus induced in C57BL/6J mice by multiple low doses of streptozotoxin (mld-STZ). We analyzed the temporal changes of: (1) plasma and skeletal muscle lipids and their relationship with glucose metabolism; (2) triglyceride (Tg) concentration in isolated islets; (3) intraperitoneal glucose tolerance test; and (4) insulin secretion patterns when the three mutually interactive glucose signaling pathways were activated. Animals were killed by cervical dislocation at days 4, 6, 7, 8, 9 and 12 post first injection of mld-STZ. Compared with control mice, we observed: (1) at day 6, a significant increase of plasma FFA and both muscle and islet Tg content and a significant decrease of muscle pyruvate dehydrogenase activity. These parameters further deteriorated with time. (2) plasma Tg, glucose and insulin levels and glucose tolerance test were significantly different only after day 8. (3) an increase in both phases of the glucose plus palmitate-stimulated insulin secretion was observed at day 4. This effect progressively decreased since day 7 up to day 9. Moreover, an inhibitory action of cerulenin over glucose plus palmitate-stimulated insulin secretion was observed between days 6 and 9. Taken together these results suggest that early alteration in carbohydrate and lipid metabolism could represent a "metabolic window" which would develop between days 6 and 8. Afterwards, subsequent immunological alterations, apoptosis and necrosis induced the destruction of ß cells and would mask the results mentioned above.

The association of acetyl-l-carnitine and nicotinamide remits the experimental diabetes in mice by multiple low-dose streptozotocin.

OBJECTIVES: We studied the effect of acetyl-l-carnitine plus nicotinamide (AC + N) on murine diabetes mellitus induced by multiple low doses of streptozotocin. METHODS: Male C57BL/6J inbred mice were injected intraperitoneally with citrate buffer or streptozotocin (40 mg/kg) for 5 consecutive days, followed by injections of saline solution or AC + N (50 + 25 mg/kg) from days 6 to 110. Four groups were studied: normal control mice (C), treated normal control mice (TC), diabetic mice (D), and treated diabetic mice (TD). TD group was divided into 2 at day 86; treatment was suspended in one group (TDs) and continued in the other until day 110. RESULTS: Weight, plasma glucose, plasma insulin, cellular immune aggression, glucose-stimulated insulin secretion from perifused pancreatic slices, and pancreas histology were studied in each experimental group. Diabetic mice treated with AC + N showed improvements in weight, plasma glucose, and plasma insulin levels without mortality, reaching control values at day 110. Cellular immune aggression and insulin release from pancreatic slices perfusions improved without reaching control values. Histology showed that insulin-immunostained area, the index of insulin immunostained beta cells and beta-cell size, was normalized at the end of the study. CONCLUSIONS: The treatment with AC + N induced remission of autoimmune type 1 diabetes in mice produced by multiple low doses of streptozotocin.

Early manifestations in multiple-low-dose streptozotocin-induced diabetes in mice.

OBJECTIVE: Administration of multiple low doses of streptozotocin (mld-SZ) to mice results in the development of autoimmune diabetes. Hyperglycemia does not develop until a few days after the last injection. In this study, we explored immune-related alterations found in the very early stages of this diabetic syndrome and the capacity of mononuclear spleen cells (MSs) from mld-SZ mice to impair insulin secretion. METHODS: Mice injected with mld-SZ were used as an animal model of type 1 diabetes. MSs were isolated from control and mld-SZ mice at days 4, 6, 9, 12, and 16 after the first injection of the diabetogenic drug. MSs were transferred to normal syngeneic recipients or were cocultured with dispersed rat islet cells as an in vitro insulin secretion study. RESULTS: MSs from mld-SZ mice were able to diminish insulin secretion when transferred to normal syngeneic recipients and presented anti-beta-cell immune aggression when cocultured with dispersed rat islet cells as early as day 4 after mld-SZ administration. This capacity persisted throughout the experimental period. As early as 6 days after mld-SZ, islets showed insulitis followed by cell death with progressive severity. Hyperglycemia and diminished insulin secretion from perifused pancreatic islets only appeared at day 9 after mld-SZ. CONCLUSIONS: This study suggests that transferred or cocultured MSs from mld-SZ mice exert a functional immune aggression against beta cells at a very early stage, before donor mice develop impaired insulin secretion and hyperglycemia.

Role of lipids in the early developmental stages of experimental immune diabetes induced by multiple low-dose streptozotocin.

The present work examines the role of lipids in the development of the Type 1 diabetes induced by the administration of multiple low doses of streptozotocin (STZ) in C57BL/6J mice. The study was performed before and after the onset of clear hyperglycemia, and the results were as follows. First, 6 days after the first dose of STZ, while plasma glucose and insulin levels remained similar to those observed in the control mice, plasma free fatty acid (FFA) levels were significantly increased (P < 0.05). At that time, a marked increase of triglyceride content in gastronemius muscle was accompanied by a diminished activity of pyruvate dehydrogenase complex, suggesting an impaired glucose oxidation. Furthermore, a decrease of both triglyceride content and lipoprotein lipase activity was observed in the epididymal fat tissue. Second, 12 days after the first injection of STZ, hyperglycemia was accompanied by hypertriglyceridemia, a more pronounced increase of plasma FFA, and a significant (P < 0.05) reduction of insulinemia. At this time, both the adipose tissue and the gastrocnemius muscle showed a further deterioration of all parameters mentioned after 6 days. Moreover, in the gastrocnemius muscle, an impaired nonoxidative pathway of glucose metabolism was observed [significant reduction (P < 0.05) of glycogen mass, glucose-6-phosphate content, and glycogen synthase activities] at this time point. Finally, the data suggest for the first time that, in mice, Type 1 diabetes induced by multiple low doses of STZ and enhanced lipolysis of fat pads leads to an increase in the availability of plasma FFA, which seems to play a role in the early steps of diabetes evolution.

Muscle lipid metabolism and insulin secretion are altered in insulin-resistant rats fed a high sucrose diet.

Feeding rats a sucrose rich diet (SRD) induces hypertriglyceridemia and insulin resistance. The purposes of this study were to determine the time course of changes in lipid and glucose metabolism in the gastrocnemius muscle, both in the basal state and after the euglycemic hyperinsulinemic clamp, in rats fed a SRD for 3, 15 or 30 wk, and to analyze the changes in glucose-stimulated insulin secretion from perifused isolated islets from SRD-fed rats and their relationships to peripheral insulin insensitivity. A control group of rats was fed a control diet (CD) for the same period of time. After 3 wk of consuming the SRD, long-chain acyl CoA (LCACoA) levels in muscle were greater than in rats fed the CD, an early indication of the disturbance of lipid metabolism. Neither glycogen storage nor glucose oxidation were impaired at this time. Moreover, the biphasic patterns of glucose-stimulated insulin secretion showed a marked increase in the first peak, which helped maintain normoglycemia in SRD-fed rats. After 15 or 30 wk of consuming the SRD, triglyceride and LCACoA levels in muscles were greater than in rats fed the CD. Glucose oxidation as well as insulin-stimulated glycogen synthase activity and glycogen storage were lower than in rats fed the CD. Moreover, the altered pattern of insulin secretion further deteriorated. This was accompanied by peripheral insulin resistance and moderate hyperglycemia. Our results indicate that the dyslipemia present in rats chronically fed a SRD may play an important role in the progressive deterioration of insulin secretion and sensitivity in this animal model.

Effect of modified diabetic splenocytes on mice injected with multiple low-dose streptozotocin.

This work reports the effects of a previous injection of mitomycin-modified splenocytes from multiple-low dose streptozotocin-treated mice (mld-sz) on autoimmune diabetes produced by mld-sz. Our work shows that a previous inoculation of modified mononuclear splenocytes from mld-sz mice prevents alterations in glycemia, in insulin secretion (IS) pattern from isolated perifused islets, and in mass of pancreatic islets. Immunohistochemistry showed an alteration in the number of beta, but not of alpha or delta cells. While a mononuclear intra-islet infiltration was observed in mld-sz mice, a predominantly polar or peri-islet infiltration was seen in vaccinated mice. Islet-associated mononuclear cells from mld-sz mice produced diabetes and induced a diminished IS when transferred to normal receptors. Those cells from previously vaccinated mld-sz mice had no effect when injected into normal receptors. In addition, they also inhibited the damage induced in normal receptors by the islet-associated mononuclear cells from mld-sz animals. Cellular death was also prevented by previous vaccination. Our results suggest that vaccination with modified splenocytes from mld-sz mice is capable of shifting the islet cells infiltration pattern from an aggressive one toward a protective one and thus preventing the beta cell destruction observed in mld-sz mice.