Impact Of Gastric Inhibitory Polypeptide On Islet Function And Glucose Excursion

Background:Type2diabetes mellitus (T2DM) is a kind of metabolic disease characterized by chronic hyperglycemia due to defective insulin secretion and/or action. With further study people found that, T2DM progressed with the progressive deterioration of islet function, including β-cell insulin secretion defects and increased a cell Glucagon secretion, the so-called "dual hormone doctrine". In recent years, incretin and the relationship among incretin,. insulin and glucagon has become the research hotspot.With the depth research of type2diabetes, in1964Elriek found that in the case of the same blood glucose concentration, oral glucose-stimulated insulin secretion was significantly more than the insulin secretion caused by intravenous glucose infusion, so brought up with the "incretin effect ". The two key hormones of glucose mediated incretin effect are gastric inhibitory polypeptide (GIP) or glucose-dependent insulinotropic polypeptide (GIP) and Glucagon like peptide-1(GLP-1). GIP is a kind of peptide responsed from the duodenum and upper jejunum K-cell after the intake of nutrients. In addition to promoting insulin and glucagon secretion in the form of glucose-dependent, GIP can also promote beta cell proliferation, differentiation, regeneration, inhibition of β-cell apoptosis and increase utilization of glucose uptake by the peripheral muscle tissue. In healthy human beings, the incretin effect accounts for about50-70%of the total postprandial insulin secretion. GIP was reported to be the major incretin in physiological condition.Some study found that incretin effect diminished in people with human-induced insulin resistance and further aggravated when glucose tolerance declined. The insulin secretion stimulated by incretin after oral glucose load was less than20%in type2diabetes.There are two explanations for the reduced incretin effect in patients with type2diabetes:1) reduced postprandial GLP-1secretion;2) GIP resistance. There is still controversy on GIP secretion level in diabetes stage, it was reported to be increased, unchanged and reduced. The decline of incretin effect may be secondary to the disorder of glucose metabolism itself, the aggravation of which in turn damaged the incretin effect further, inducing higher blood glucose level.Hyperglycemia is closely related to diabetic microvascular and macrovascular complications,it damages the body organs mainly through chronic persistent hyperglycemia and glucose fluctuations.Some study found higher risk of chronic complications of the greater blood glucose fluctuations in patients with the same average blood glucose levels. As the gold standard for long-term monitoring of blood glucose, glycosylated hemoglobin (HbAlc) reflects the average blood glucose values within2-3months, but does not reflect the amplitude and frequency of glycemic fluctuations. Continuous glucose montoring system (CGMS) provided a powerful tool to study the glycemic fluctuations and played an important role in the understanding of blood glucose information and guiding therapy.Mean amplitude of glycemic excursions (MAGE) calculated from CGMS is reliable parameter for assessing glucose fluctuations.The parameter excludes fluctuations smaller than a standard deviation and weakens the interference of unknown factors, can reflect glycemic fluctuations in a more objective way. MAGE has been recognized as the gold standard for assessment of glycemic fluctuations when in2006Monnier et al found that there was a significant positive correlation between MAGE and oxidative stress in type2diabetes.GIP was reported to be the major incretin in physiological condition.At present there is still controversy on GIP secretion levels after meals or glucose load in type2diabetes individuals. Researches about the impact of GIP on insulin, glucagon secretion and blood glucose fluctuations are scarce. In this study, subjects with different glucose tolerance were conducted with oral glucose tolerance test (OGTT) during CGMS period.The dynamic changes of plasma glucose, insulin, glucagon and total GIP were detected in order to observe incretin secretion during OGTT in people with different glucose tolerance, analyze effects of GIP on insulin and glucagon secretion and glycemic fluctuations.Objective1.To observe the changs of GIP secretion during OGTT in different glucose tolerance individuals;2.To explore the effect of GIP on insulin and glucagon secretion;3.To explore the effect of GIP on glycemic fluctuations;Materials and methods1. SubjectsIn this study,48volunteers were recruited in outpatient department of Endocrinology&Metabolism of NangFang Hospital, from March,2010to November,2010. Inclusion criteria:1) more than20years old,venous fasting plasma glucose is normal or elevated at the first time, the range is3.9-11.0mmol/L.2) Not suffering from a variety of serious acute and chronic complications, and no history of infection and ketosis in nearly3months.3) ALT and AST are less than2times the upper limit of normal,renal function is normal.4) Systolic blood pressure (SBP) is less than or equal to140mmHg,diastolic blood pressure (DBP) is less than or equal to90mmHg. Exclusion criteria:1) Type1diabetes mellitus (TIDM) patients or impaired glucose regulation (IGR) and T2DM patients who have used hypoglycemic drugs.2) Secondary diabetes or drug-induced glucose abnormal patients.3)Severe lipid metabolic disorder patients.4)Pregnant and lactating women.5) A history of gastric resection or severe gastrointestinal disorders.6) Other diseases that may affect glucose metabolism.2. GroupsGroups baseded on results of OGTT during the experiment and the diabetes diagnostic criteria of WHO in1999. Subjects with FPG<6.1mmol/L and/or OGTT2h PG<7.8mmol/L were assigned in NGT group; Subjects with FPG from6.1mmol/L to7.0mmol/L and/or OGTT2h PG from7.8mmol/L to11.1mmol/L were assigned in IGR group; Subjects with FPG≥7.0mmol/L and/or OGTT2h PG≥11.1mmol/L were assigned in T2DM group. NGT group included16subjects with8males and8females; IGR group included13subjects with7males and6females; newly diagnosed T2DM group included19patients with10males and9females.3. ProcessTrained all subjects before the experiment so as to familiar with the general use of CGMS and common blood glucose meter.Specialists installed the CGMS and placed the probe in abdominal subcutaneous fat layer uniformly, and the CGMS was conducted for72h continuously.Subjects were accepted75g-OGTT in the morning of the3rd CGMS day.Blood pressure,height,weight,circumference of waist and hip were measured at8:00AM. Collected forearm venous blood at0,30,60,90, and120min during OGTT.Plasma glucose was detected by glucose oxidase method, plasma total GIP and glucagon were detected by enzyme-linked immunosorbent assay,plasma insulin was assayed with radioimmunoassay.4. Evaluation parameters4.1Early phase insulin secretion was assessed by ΔI30/ΔG30. AI30/AG30=(30min INS-Omin INS)/(30min PG-Omin PG)4.2Area under the curve (AUC) was calculated by approximately trapezoidal formula.120minAUC=(Omin value+120min value)/2+30min value+60min value+90min value4.3lately phase insulin secretion was assessed by (AUC130-120/AUCG30-120),and calculated by approximately trapezoidal formula: AUC130-120/AUCG30-120=[(i30+i60)x30/2+(I60+I90)×30/2+(I9o+I120)x30/2]/[(G3o+G60)×30/2+(G60+G90)×30/2+(G90+G120)×30/2]4.4Homeostasis model insulin resistance index (HOMA-IR):(HOMA-IR)=I0×G0/22.54.5insulin:glucagon ratio after glucose loading:120min-AUCINS/120min-AUCGLUCAGON4.6Glucose excursion was assessed by MAGE, and the value used for statistical analysis was the average values of the first day and the second day.5. Statistical analysisStatistical analysis were conducted with the SPSS13.0for windows.All data were presented as mean±standard deviation.Differences between groups of which data belonged completely random design were analyzed by using One-way analysis of variance (ANOVA),and multiple Comparison were analyzed by LSD method when P value is less than0.05.Welch method was used when equal variances not assumed,and multiple Comparison were analyzed by Dunnett’s T3method when P value is less than0.05. The Comparison among the same indicator’s data from multiple time points were analyzed by single Repeated Measures analysis of variance.The comparison between two variables that are from the same indicator but different time points was analyzed by pairwise Comparison. The correlation analysis between two variables used Spearman correlation method.Statistical significance was accepted at a value of P<0.05.Results1.General informationAll the subjects successfully completed the experiment. The high density lipoprotein cholesterol (HDL-C) levels declined in the T2DM group compared with the NGT group and the IGR group (P<0.001vs. NGT group, P<0.05vs. IGR group). The gender, age, body mass index (BMI), waist hip ratio (WHR), systolic blood pressure (SBP), diastolic blood pressure (DBP), triglycerides (TG), total cholesterol (Tc),and low density lipoprotein cholesterol (LDL-C) among groups were matched (P>0.05)2. The comparison of the main detection indexes2.1GlucoseThe order of120min-PG-AUC was T2DM> IGR> NGT, the differences were statistically significant (P<0.001).The difference between T2DM and IGR group was not significant at the30min time point, while the plasma glucose levels were in order of T2DM> IGR>NGT at the rest fixed time point, the differences were statistically significant (P<0.01). Blood glucose values changed over time,and the difference between time points were statistically significant (P<0.001).2.2InsulinThe120min-INS-AUC of T2DM group was smaller compared with NGT and IGR group, the differences were statistically significant (P<0.05vs. NGT, P<0.01 vs. IGR), The difference of120min-INS-AUC between IGR group and NGT group was not significant.Fasting insulin levels were in order of T2DM>IGR>NGT, but only the difference between the T2DM group and NGT group was statistically significant (P<0.05).The insulin levels of T2DM group were lower than levels of NGT and IGR group at30min and60min after glucose loading and the differences were statistically significant (P<0.01vs. NGT group, P<0.05vs. IGR group respectively).There was no significant difference between IGR group and NGT group. The insulin levels of IGR group were higher than levels of T2DM and NGT group at90min and120min after glucose loading and the differences were statistically significant (P<0.01vs T2DM group, P<0.05vs. NGT group respectively), There was no significant difference between T2DM and NGT group. Plasma insulin values changed over time, and the differences between time points were statistically significant (P<0.001).2.3GlucagonThe order of120min-GLUCAGON-AUC was T2DM>IGR>NGT, the differences were statistically significant (P<0.01, IGR group vs. NGT group; P <0.001, T2DM group vs. IGR group).Fasting glucagon levels were in order of NGT<IGR<T2DM, the differences were statistically significant (P<0.001). The glucagon levels at all time points after glucose loading in the T2DM group were higher compared with IGR and NGT group, the differences were statistically significant (P<0.001). Glucagon levels of IGR group at30min and90min after glucose load were higher than the NGT group, the differences were statistically significant (P<0.001,<0.05vs. NGT group). The differences between glucagon levels of IGR group and NGT group at60min and120min time points were not significant. Plasma glucagon values changed over time and the differences between time points were statistically significant (P<0.001). 2.4Total GIPThe120min-GIP-AUC of the T2DM group was smaller compared with NGT and IGR group, the differences were statistically significant (P<0.01vs. NGT group, P<0.001vs. IGRgroup), the difference of120min-GIP-AUC between NGT and IGR group was not significant.The fasting plasma total GIP levels were not significantly different among groups. Plasma total GIP levels of T2DM group at30min,90min,120min after glucose load were smaller compared with the NGT group, the differences were statistically significant (P<0.01, P<0.05, P<0.01vs.NGT group); total GIP levels of the T2DM group at all the time points after glucose loading were smaller than IGR group, and the differences were statistically significant (P<0.001, P<0.001, P<0.01, P<0.001vs. IGR group). There were no significant differences of total GIP levels between the IGR group and NGT group at each time point after glucose loading. The total GIP levels changed over time,and the differences between time points were statistically significant (P<0.001).3The glycemic characteristics of different glucose tolerance people3.1HbAlcThe order of the HbAlc values was T2DM>IGR> NGT, the differences among groups were significant (P<0.01, IGR vs.NGT;(P<0.01) T2DM vs. IGR).3.2MAGEThe order of the MAGE values was T2DM>IGR> NGT,the differences among groups were significant (P<0.01, IGR vs.NGT; P<0.05, T2DM vs. IGR).4. Comparison of islet function among groups4.1Comparison of Early phase insulin secretion among groupsThe ΔI30/ΔG30of T2DM group was smaller thanThe ΔI30/ΔG30of NGT and IGR group, and the differences were statistically significant (P<0.001vs. NGT group, P<0.05vs. IGR group). There was not significant difference between IGR and NGT group (P>0.05).4.2Comparison of lately phase insulin secretion among groupsThe AUC130-120/AUCG30-120of T2DM group was smaller thanThe ΔI30/AG30of NGT and IGR group, and the differences were statistically significant (P<0.001), There was no significant difference between IGR and NGT group (P>0.05).4.3Comparison of HOMA-IR among groupsBoth the HOMA-IR of T2DM and IGR group were bigger than the HOMA-IR of NGT group, the differences were statistically significant (P<0.001, P <0.05respectively),There was no significant difference between T2DM and IGR group (P>0.05).4.4Comparison of AUCINS/AUCGLUCAGON among groupsThe AUCINS/AUCGLUCAGON of T2DM group was smaller than the AUCINS/AUCGLUCAGON of IGR and NGT group, the differences were statistically significant (P<0.001); there was no significant difference between IGR group and NGT group.5. The dynamic changes of plasma glucose, insulin, glucagon, total GIP levels during OGTT in each group5.1NGTBlood glucose level peaked at30min after the glucose load, then declined continuely (P<0.001Omin vs.30min, P=0.00430min vs.120min, P<0.001Omin vs.120min); insulin level continued to rise after glucose load, peaked at60min then rapidly declined (P<0.001Omin vs.60min, P=0.00160min vs.120min, P<0.001Omin vs.120min); glucagon level continued to decline after glucose loading, and reached the lowest point at120min (P<0.001Omin vs30min, P<0.00130min vs.120min, P<0.001Omin vs.120min).The total GIP levels rapidly increased after glucose loading, peaked at30min, then remained at a high level (P<0.001Omin vs.30min, P=0.01130min vs.120min, P<0.001Omin vs.120min). 5.2IGRBlood glucose level peaked at60min after glucose loading,then continuely declined (P<0.001Omin vs.60min, P<0.00160min vs.120min, P<0.001Omin vs.120min); insulin level gradually increased after glucose load and peaked at90min (P <0.001Omin vs90min, p=0.04790min vs.120min, P<0.001Omin vs.120min); glucagon level continuly declined after glucose loading with slightly volatility, and reached the lowest point at120min (P<0.001Omin vs30min, PP<0.00130min vs.120min, P<0.001Omin vs.120min);the total GIP level increased rapidly after glucose load, peaked at60min,and then remained at a high level (P<0.001Omin vs.60min, P=0.00160min vs.120min, P<0.001Omin vs.120min).5.3T2DMBlood glucose level continuly rise after glucose loading, peaked at60min and90min, showing a plateau (P>0.0560min vs.90min), then declined slowly, but remained at a high level (P<0.001Omin vs90min, P=0.00490min vs.120min, P <0.001Omin vs.120min); insulin level gradually increased after glucose loading, peaked at90min (P<0.001Omin vs.90min, P=0.11590min vs.120min, P<0.001Omin vs.120min); glucagon level continuly declined after glucose loading, peaked at60min, but was still below the fasting level, and declined to the lowest point (P <0.001Omin vs.60min, P<0.00160min vs.120min, P<0.001Omin vs.120min);total GIP levels continued to rise after glucose loading, peaked at60min, then declined (P<0.001Omin vs.60min, P<0.00160min vs.120min, P<0.001Omin vs.120min).6. Correlation analysis6.1Correlations between islet function and120min-GIP-AUCThere was negative correlation between120min-GIP-AUC and HOMA-IR (r=-0.339, P=0.018); There was no significant correlation between120min-GIP-AUC and the early phase insulin secretion index (r=0.120, P=0.415); There was positive correlation between120min-GIP-AUC and late-phase insulin secretion index (r=0.300, P=0.038); There was negative correlation between120min-GIP-AUC and120min-GLUCAGON-AUC (r=-0.452, P=0.001);There was positive correlation between120min-GIP-AUC and AUCINS/AUCGLUCAGON(r=0.343, P=0.017).6.2Correlations between MAGE and islet function or120min-GIP-AUCThere was negative correlation between120min-GIP-AUC and MAGE (r=-0.333, P=0.021); There was negative correlation between early phase insulin secretion index and MAGE (r=-0.674, P<0.001); There was negative correlation between late-phase insulin secretion index and MAGE (r=-0.652, P<0.001); There was positive correlation between120min-GLUCAGON-AUC and MAGE (r=0.700, P<0.001); There was negative correlation between AUCINS/AUCGLUCAGON and MAGE (r=-0.567, P<0.001); There was positive correlation between HOMA-IR and MAGE(r=0.475, P=0.001).Conclusion1. There was no significant difference between IGR and NGT group in GIP secretion after oral glucose load,while there was GIP secretion defection in T2DM patients;2. GIP might promote lately phase insulin secretion primarily;3. GIP might regulate glucose fluctuations through stimulating lately phase insulin secretion and affecting insulin:glucagon ratio.