Diane T. Finegood

Defects in Insulin Secretion and Action in Women With a History of Gestational Diabetes

Gestational diabetes mellitus (GDM) is associated with defects in insulin secretion and insulin action, and women with a history of GDM carry a high risk for the development of non-insulin-dependent diabetes mellitus (NIDDM). Assessment of subjects with a history of GDM who are currently normoglycemic should help elucidate some of the underlying defects in insulin secretion or action in the evolution of NIDDM. We have studied 14 women with normal oral glucose tolerance who had a history of GDM. They were compared with a group of control subjects who were matched for both body mass index (BMI) and waist-to-hip ratio (WHR). All subjects underwent tests for the determination of oral glucose tolerance, ultradian oscillations in insulin secretion during a 28-h glucose infusion, insulin secretion in response to intravenous glucose, glucose disappearance after intravenous glucose (Kg), and insulin sensitivity (SI) as measured by the Bergman minimal model method. The BMI in the post-GDM women was similar to that in the control subjects (24.9 ± 1.2 vs. 25.4 ± 1.4 kg/m2, respectively), as was the WHR ratio (0.80 ± 0.01 vs. 0.76 ± 0.01, respectively). The post-GDM women were slightly older (35.2 ± 0.9 vs. 32.1 ± 1.4 years, P = 0.04). The fasting plasma glucose levels were significantly higher in the post-GDM group than in the control group (4.9 ± 0.1 vs. 4.4 ± 0.1 mmol/l, respectively, P < 0.001) and remained higher at each of the subsequent determinations during the oral glucose tolerance test, although none had a result indicative of either diabetes or impaired glucose tolerance. All measures of ultradian insulin secretory oscillations in post-GDM subjects were indistinguishable from those in the control subjects. The first-phase insulin release to intravenous glucose was lower in the post-GDM group. SI was also impaired in the post-GDM group compared with the control subjects (4.6 ± 0.5 vs. 6.8 ± 1.0·10−4·min−1· ³U−1·ml, respectively, P < 0.05). Kg was reduced in the post-GDM women compared with the control subjects (1.3 ± 0.1 vs. 2.7 ± 0.4%, P < 0.01). When the subjects were divided according to their BMI, lean post-GDM subjects (<24.2, n = 8) were more insulin resistant than the lean control subjects: SI 5.3 ± 0.6 vs. 8.8 ± 1 · 1·10−4 min −1· ³U−1·ml, P = 0.02, whereas obese post-GDM (>24.2 kg/m2, n = 6) and control subjects had a lower SI than the lean subjects, but they were not different from each other (3.6 ± 0.7 vs. 4.2 ± 1.2· 10−4· min−1 · ³U−1 · ml, respectively, P = 0.67). The acute insulin responses to glucose (0–10 min) within these groups showed that the lean post-GDM group had a significantly lower insulin response compared with control subjects (1,205 ± 179 vs. 2,404 ± 416 pmol· 1−1 min, respectively, P = 0.007), whereas the obese groups had similar responses (2,777 ± 1,112 vs. 3,114 ± 847 pmol ·1−·min, post-GDM vs. control subjects, P = 0.8). We have found defects in insulin secretion and action in post-GDM subjects who are at high risk for the development of NIDDM at a time that oral glucose tolerance is normal. These defects are present in the absence of obesity. Ultradian insulin secretory oscillations during constant glucose infusion are normal in these post-GDM subjects predisposed to NIDDM. We conclude that defects in both insulin secretion and insulin action are present before the development of hyperglycemia in women with a history of GDM.

Insulin Secretory Function in Relation to Transplanted Islet Mass in STZ-Induced Diabetic Rats

In vivo insulin secretion was quantified as the AIRg or AIRa in islet-transplanted rats. Male Wistar-Furth rats previously made diabetic by STZ administration (55 mg/kg) were transplanted with 500,1000, 2000, or 3000 islets infused into the portal vein (n = 12–14 per group) and were compared with sham-treated controls (CN, n = 16). At 4–5 wk posttransplantation, no significant differences were noted in the FPG or fasting plasma insulin of the experimental groups (P > 0.05). Body weight, however, was 10% less (P < 0.05) in rats receiving 500 islets than in controls, indicating an effect of β-cell deficiency on growth rates. To determine the relationship between islet mass and insulin secretion, we measured AIRg after a 0.3 g/kg glucose bolus in fasted conscious animals. A significant correlation was observed between the AIRg and islet number (r = 0.61, P = 0.0001), and both 500- and 1000-islet groups could be differentiated from controls by ANOVA (500: 8%; 1000: 12% of controls; P < 0.05). During a glycemic potentiation protocol, AIRa was measured at basal and elevated blood glucose (∼16 mM). At neither basal nor elevated blood glucose was AIRa correlated with islet number (basal r = 0.0622, P = 0.7834; elevated r = 0.3133, P = 0.1667). None of the groups could be differentiated by ANOVA (elevated 500: 37%; 1000, 68% of controls; P > 0.05). Although this study illustrates that AIRa may be better preserved in islet-transplanted rats, AIRg is the better correlate of islet number. This study is thefirst to demonstrate that the acute insulin secretory response to glucose is proportional to the number of transplanted islets. This model of graded insulin secretory response allows for predetermination of a wide range of insulin secretory function in animals with fasting normoglycemia.

Dynamics of glycemic normalization following transplantation of incremental islet masses in streptozotocin-diabetic rats

We examined the dynamics of glycemic normalization following intraportal infusion of an incremental number of islets of Langerhans in male Wistar-Furth rats. Non-fasted plasma glucose, 24-hr urine volume, and body weight were determined weekly during three weeks of streptozotocin-induced diabetes and for 5 weeks following transplantation of 250-3000 freshly isolated islets. At one week following transplantation, urine volume was inversely proportional to the mass of islets transplanted, but by 5 weeks posttransplantation urine volume was near-normal except in rats receiving only 250 islets. On the basis of the mean data, the nonfasted plasma glucose fell linearly at a rate of 66 mg/dl per week in rats receiving 500-1000 islets, with normoglycemia (147 +/- 9 mg/dl) being obtained 5 weeks posttransplantation. Examination of the individual time courses for nonfasted plasma glucose revealed a different pattern of glycemic normalization, which consisted of sustained hyperglycemia followed by a rapid fall in the plasma glucose level. During the week prior to normalization glucose fell at a rate of 170 mg/dl per week and normoglycemia was obtained from 1 to 5 weeks following transplantation. Examination of the frequency distribution of nonfasted glucose levels suggested a threshold of 300 mg/dl for glycemic normalization. We conclude that the dynamics of glycemic normalization following transplantation of a suboptimal islet mass include sustained hyperglycemia of variable duration, followed by a rapid fall in the nonfasted plasma glucose level. The contributions of changes in insulin secretion and insulin action underlying this dynamic behavior remain to be determined.

Markedly Reduced β-Cell Function Does Not Result in Insulin Resistance in Islet Autografted Dogs

Autotransplantation of islets of Langerhans has resulted in long-term normoglycemia in pancreatectomized dogs. This canine model is useful in evaluating both the progress of islet transplantation and the effect of a reduced islet mass upon the determinants of glucose tolerance: i.e., insulin secretion, insulin sensitivity, and glucose effectiveness. To determine the effect of a reduced islet mass on these factors, we measured the acuteinsulin response to arginine (AIRa) and glucose (AIRg), the slope of glycemic potentiation of AIRa (SP), insulin sensitivity (S1), and glucose effectiveness (SG) in control (CN), diabetic (DM), and pancreatectomized dogs rendered normoglycemic with transplanted autografts of islets of Langerhans (TX). Normal fasting plasma glucose (FPG) (TX 4.7 ± 0.2 mM; CN 4.9 ± 0.1 mM; P > 0.05) was maintained despite a markedly reduced insulin secretion in TX (AIRa 24%, AIRg 15%, and SP 11% of CN). All measures of insulin secretion were significantly correlated (SP vs. AIRg r = 0.80, P < 0.0001; AIRa vs. AIRg r = 0.92, P < 0.0001) across all animals, but none of the measures of secretion were significantly correlated with either the number of islets transplanted or time posttransplant (P > 0.10). Insulin sensitivity was normal in islet autografted dogs (TX: 136 ±12 min−1/(nmol/ml); CN: 101 ±11 min−1/(nmol/ml), P > 0.05) but SG was reduced (TX:±1.93 ± 0.28 × 100 min−1; CN: 3.53 ± 0.35 × 100 min−1 P < 0.05), as determined by the minimal-model method. In diabetic animals (FPG = 16.1 ±1.3 mM), insulin secretion was negligible by all measures (P > 0.05), and was associated with insulin resistance (S1 = 28 ± 8 min−1/(nmol/ml)) and reduced SG (1.72 ±0.11 × 100 min−1). These studies indicate that across a range of insulin secretion in dogs, thesecretagogues arginine and glucose provide similar estimates of β-cell function. This markedly reduced β-cell function does not result in insulin resistance when fasting normoglycemia is maintained, but is associated with a decrease in glucose action at basal insulin.

Glycogen synthase polymorphism, insulin resistance and hypertension

The A(2) allele of the human glycogen synthase gene may be associated with hypertension in diabetic and non-diabetic Finnish subjects. The prevalence of the A(2) allele was investigated in 64 non-diabetic hypertensive subjects with borderline hypertension or established hypertension. Ambulatory blood pressure was performed on all subjects. Insulin sensitivity index (S(I)) was determined in subjects with borderline hypertension. The DNA fragment containing the XBaI restriction site was amplified by the polymerase chain reaction, digested by the XBaI enzyme and compared by gel electrophoresis with a positive control from Finland. Mean age +/- SD for age, S(I) and ambulatory blood pressure were respectively: 39 +/- 10 yrs, 60 +/- 30 min(-1)(nmol/mL) and 132 +/- 7/ 83 +/- 6 mmHg. Sixteen of the subjects were insulin resistant as determined by S(I) <70.0 and they had significantly higher BP and BMI than insulin sensitive subjects. The A(2) allele of the glycogen synthase was not detected in any subject. This suggests that the relation between the XBaI polymorphism of the glycogen synthase gene, insulin resistance and elevated blood pressure may be restricted to a limited and genetically uniform Finnish population.