Elevated ratio of summed serum proinsulin to insulin response after oral glucose load in type 2 diabetes decreases following sulfonylurea treatment

Relationship between proinsulin-to-insulin ratio and advanced glycation endproducts in Japanese type 2 diabetic subjects

OBJECTIVE

Type 2 diabetes (T2DM) is characterized by increased proinsulin-to-insulin ratio (P/I ratio), increased glycation and oxidative stress, and beta-cell dysfunction. Previous reports implicated that increased P/I ratio, glycation and oxidative stress constitute markers of beta-cell dysfunction in T2DM. However, its clinical relevance remains to be elucidated. Therefore, in the present study we investigated the relationship between the P/I ratio, glycation and oxidative stress markers in patients with T2DM, using newly developed intact chemiluminescent immunoassay for proinsulin.

METHODS

Fasting intact proinsulin, insulin, advanced glycation endproducts (AGEs), pentosidine, lipid peroxide and urine 8-isoprostane as well as other metabolic parameters were measured in 64 T2DM subjects.

RESULTS

Using univariate analysis, P/I ratio showed significant positive correlations with plasma glucose (r=0.465), HbA1c (r=0.434) and AGEs (r=0.282), and significant negative correlations with insulin (r=-0.330) and HOMA-beta (r=-0.520) even after adjustment for age, sex, duration of diabetes, family history of diabetes, use of sulfonylureas, smoking and body mass index. Additionally, stepwise multiple regression analysis revealed that HOMA-beta, HbA1c and AGEs were independently and significantly correlated with P/I ratio.

CONCLUSION

These findings suggest that not only hyperglycemia per se but also glycation is involved in beta-cell dysfunction in T2DM subjects.

Effects and serum levels of glibenclamide and its active metabolites in patients with type 2 diabetes

OBJECTIVE

To study the effects and serum levels of glibenclamide (Gb) and its active metabolites in patients on chronic Gb medication on different daily doses.

MATERIAL AND METHODS

Fifty patients with type 2 diabetes on regular Gb therapy (1.75-14.0 mg daily). Blood samples were taken immediately before and 90 min after regular Gb intake. A standardized breakfast was served 30 min after drug intake. Serum insulin and proinsulin levels were determined by ELISA methods without cross-reactivities. Serum drug levels were determined by HPLC. Fischer's R to Z-test (correlation coefficients) and paired Student t-tests were used when comparing values within the entire group and unpaired non-parametric Mann-Whitney tests were used when comparing high and low dose levels. A p-value < 0.05 was considered significant.

RESULTS

There were significant correlations between daily Gb dose, on the one hand, and, on the other, HbAlc (r = 0.55), Delta-insulin (r = - 0.59) and Delta-proinsulin (r = - 0.52) levels. Significant correlations between Gb therapy duration and insulin (r = - 0.40) and proinsulin (r = - 0.34) secretion and between Gb dose and ratio proinsulin/insulin (RPI) at both time points (r = 0.32 and 0.30) were also found. The RPI was lower after Gb intake. In patients on > or = 10.5 mg steady state serum metabolite levels (Ml and Ml + M2) were higher (29(0-120) and 33 (0-120) ng/ml) than those of Gb itself (18(0-64) ng/ml). A great inter-subject variability in Gb levels at both time points was seen.

CONCLUSIONS

Our results indicate that, in patients on chronic medication, Gb is capable of stimulating both insulin and proinsulin secretion; the effect on insulin release is relatively greater. The effect was more pronounced in patients on a low Gb dose, either because of less impaired beta-cells in those receiving low doses, or due to reduced sulphonylurea sensitivity in those on high dosage (down-regulation). In patients on a daily dose of 10.5 mg or more, serum metabolite levels of clinical relevance were demonstrated; the metabolites may contribute to hypoglycaemic events.

Characteristics of pancreatic beta-cell secretion in Type 2 diabetic patients treated with gliclazide and glibenclamide

The aim of the present cross-over study was to compare the beta-cell response to gliclazide and glibenclamide administered orally during and following a hyperglycaemic clamp in sulphonylurea treated Type 2 diabetes. Nine patients (6 males), aged 61.4 (S.D. 6.9) years with a body mass index of 27.5 (3.1) kg m(-2) and HbA(1c) at baseline of 7.2 (0.9)% were included. Eight healthy control subjects underwent the same tests. Patients received 80-240 mg gliclazide or 5-15 mg glibenclamide for 6 weeks. Thirty minutes after administration of 160 mg of gliclazide or 10 mg of glibenclamide a 1-h hyperglycaemic clamp (11.0 mmol l(-1)) was begun, and followed by a 3.5-h observation period. Nadir blood glucose levels were 4.2 (1.0), 4.3 (1.2) and 3.4 (1.0) mmol l(-1) for glibenclamide gliclazide and controls, respectively (both P=0.07 vs. controls). Glucose levels decreased slowly and linearly in people with diabetes and reached nadirs after 204 (8) and 198 (18) min, respectively, after cessation of glucose infusion, while in controls, glucose levels declined steeply to a nadir at 98 (47) min (P<0.003 vs. both drugs). No first phase insulin secretion peak was observed in people with diabetes. Insulin levels remained elevated during the 3-h observation period in SU treated patients but, in control subjects decreased to baseline values within 2 h of the clamp. The proinsulin to C-peptide ratio increased during the observation period. In conclusion, the effects of glibenclamide and gliclazide on insulin secretion are very similar in patients with Type 2 diabetes who are in moderate glycaemic control, with a slow rise to lower amplitude, poor responsiveness to falling glucose levels, and raised proinsulin to C-peptide ratio.