CONTRIBUIÇÃO DA FUNÇÃO DA CÉLULA BETA PANCREÁTICA NA MANUTENÇÃO DA NORMOGLICEMIA EM RATOS OBESOS – MSG

GENERAL AIM: To investigate functional readjustments that guara ntee normoglycemia during the development of MSG obesity. Theoretical bases: Equilibrium in body metabolism requires a perfect harmony among the activities triggered by central and endocrine nervo us system. Homeostasis’s key functions strictly ensure glucose levels in the plasma which is warranted by the adequate release of insulin. Necessary functional adaptations occur thr oughout development and sustain control so that the metabolism’s deterioration could be pos tponed or avoided. Non-adjustments to insulin control, or rather, a glycemic condition, a re proper to obesity. Current obesity epidemic is followed by an inevitable rise in non-d ependent insulin diabetes or Type 2 diabetes. Close coupling between obesity and diabet es is a result of the control of insulin secretion by beta cells. There is an increase in in sulin release by beta cells during the onset of obesity proportional to adiposity mass and to pe ripheral resistance to insulin. When metabolic and functional adjustments by beta cells are sufficient to sustain demand and peripheral resistance is overcome, control of gluco se plasma levels is guaranteed and kept at a new adjustment level. Monosodium glutamate (MS G) neonatal treatment induces obesity, hyperinsulinemia and peripheral resistance to insulin in rats. MSG animals’ pancreatic islets show modified responses to glucos e and to autonomous neurotransmitters. Although MSG-caused obesity has all the factors tha t predispose the subject to Type 2 diabetes, the abnormality does not occur in these a nimals. Procedure. Obesity: Male Wistar rats were subcutaneously injected wit h MSG (4g/kg body weight) during the first five days of life. Control s (CON) received an equimolar saline solution. Animals were weaned after 21 days and mai ntained under light control (12h lightdark) and temperature (21 ±2°C) conditions. Development studies were undertaken with 30-, 60and 90-day-old animals. Obesity was assessed by Lee Index (LI), body weight (g)1/3/nasal-anal length (cm)x1000) and weight of periepi dydimal and retroperitoneal fats. Tolerance glucose (inGTT) and insulin (ITT) test : A silicon catheter was introduced in the right jugular vein of animals of the two groups and age; after 12h fasting they underwent glucose (1g/kg) or insulin (1U/kg) infusion. Blood samples, removed during ivGTT and ITT, were deployed for glycemia (oxydase-peroxydase ) and insulin (radioimmunoassay) dosages. ivGTT was also undertaken with acetylcholi ne (27nM/kg). Analyses of areas under curve (AUC), obtained during ivGTT, were used to calculate insulinogenic index (∆1/∆G) and measure peripheral resistance to insulin. Ra tio of glucose disappearance (K itt/min) after insulin injection was evaluated. Pancreatic islets : pancreatic islets were isolated by collagen method and incubated for 60min w th glucose (2.8-28.8mM) or glucose 8.3mM with different acetylcholine concentr ations (0.001 – 3000 μM). Glucose dependent and independent pathways on K ATP were studied in a medium composed of 16.7mM glucose and/or diazoxide (250 μM) and high potassium solution (30mM). The participation of acetylcholine through subtype M 2 muscarinic receptor was tested in a medium composed of 8.3mM glucose with acetylcholine (10μM) plus antagonistic M2 muscarinic metoctramine – MTT – (1, 5 and 10 μM). Transplantation of islets : 600 isolated islets of lean and fat rats were transplanted in di abetic rats through the hepatic vein. Glycemia was monitored during 4 days after transpla ntation. After 12h fasting the transplanted animals were killed and blood samples were taken for glycemia and insulinemia dosages. Retroperitoneal fat was remove d and weighed. Statistical analysis : Data were given as mean ± mean standard error. Student’s t test was used to compare the two groups, while variance analysis (ANOVA) compare d more than two groups; *p<0.05 was significant. Results. On average LI was 9% higher in MSG animals when c ompared to CONs of the same age bracket. Development influenced LI of the two groups similarly, approximately 5% less in 60-day-old animals when compared to 30and 90-day-old ones. Periepididimal and retroperitoneal fats were, on an average 100% h igher in 30-, 60-, and 90-day-old MSG rats when compared to CONs of the same age bracket. Although development raised fat contents in both groups, it was higher in MSG anima ls. While fasting glycemia was practically unaffected by MSG treatment or by devel opment, plasma levels of insulin varied for the two factors. 60and 90-day-old anim als had approximately 6 and 3 times more insulin when compared to respective CONs. Deve lopment raised 4 fold fasting insulin of 30-day-old to 90-day-old CON rats. In MSG ones t he effect increased progressively 6 and 12 fold respectively in 60and 90-day-old rats when compared to 30-day-old ones. Although glucose and insulin AUC obtained during iv GTT were higher in MSG rats by 47 and 160% respectively when compared to those of CON s in the three age brackets under analysis, parameters were the same in the two group s during development. Whereas glucose AUC did not change, insulin AUC increased 4 fold in CONs within 30 to 90 days period and 10 fold in MSGs during the same period. Insulin resistance was 78 and 112% higher in 60and 90-day-old MSG rats when compared to CONs of the same age. This is corroborated by the least K itt, 36% in adult MSG rats. Pancreatic islets had a ty pical dose curve to glucose and to acetylcholine in the three ag s under analysis for the two groups. However, islets of MSG rats released approximately 2 fold more insulin and high concentrations of glucose (8.3 – 16.7mM), even thou g there were no differences in response to low glucose concentrations (2.8 – 5.6mM ). After 60 days sensitivity to glucose of MSG rats’ pancreatic islets was 12% higher when compared to that in CONs of the same age. Secretory response of pancreatic islets to glu cose was higher for low concentrations in young animals and for high concentrations in adult animals. In spite of the above difference, glucose sensitivity was similar in CONs at all ages . Sensitiveness in MSG animals was 13% less in 60-day-old animals when compared to tha t in 30and 90-day-old ones. Islets of CON and MSG animals had an 85% reduction in respons e to glucose when diazoxide was present. However, effects of high concentrations of K/diazoxide were present only in CON rats. Although development does not affect sensitiv eness to acetylcholine in islets of both groups, an approximately 85% decrease has been repo rted in MSG rats as maximum response to acetylcholine at the three ages evaluat ed when compared to their respective CONs. Adult rats’ islets had a 50% decrease in thei r maximum response to acetylcholine when compared to that in young ones. Only in MSG ra ts’ islets did antagonist MTT potentialize acetylcholine effect to about 120%. Tr ansplantation of pancreatic islets was sufficient to normalize fasting glycemia in diabeti c animals regardless of the islets’ origin.