Glipizide increases plasma insulin but not C-peptide level after a standardized breakfast in type 2 diabetic patients

Summary Peripheral blood glucose, plasma insulin and C-peptide levels were investigated after giving a standardized breakfast (500 kcal, 60g carbohydrates) to 10 nonobese Type 2 diabetic patients previously treated by diet alone. Each patient received at random, at 1 week intervals, either 5 mg glipizide (meal + glipizide) or a placebo (meal alone) 30 min before breakfast. Basal values of blood glucose, plasma insulin and C-peptide were similar on both occasions. After meal + glipizide, the blood glucose increase was sharply limited whereas the rise in plasma insulin was steeper and reached twice as high a level. In contrast, the rise in plasma C-peptide was similar in both conditions. Consequently, the areas under the curves (0–300 min) showed a marked reduction in blood glucose after meal + glipizide (2289±149 versus 3101±169 mmol·min/l; 2p<0.001), associated with a significant increase in plasma insulin (14219±3261 versus 7591±1173 µU·min/ml; 2p<0.025) but no significant change in plasma C-peptide (342±45 versus 326±34 pmol·min/ml; N.S.). The insulin/C-peptide molar ratio was thus significantly increased after meal + glipizide (0.41±0.06 versus 0.23±0.04 at the 60th min; 2p<0.02). The dissociation between the responses of insulin and C-peptide suggests that a single dose of 5 mg glipizide in Type 2 diabetic subjects may enhance availability of peripheral insulin by extrapancreatic mechanism(s). This phenomenon may result in a higher circulating level of the hormone and therefore represent a further mode of action of sulphonylureas. Finally, the usual concept that peripheral insulin levels reflect true insulin secretion may be misleading in studies dealing with sulphonylureas.