Peripheral insulin parallels changes in insulin secretion more closely than C-peptide after bolus intravenous glucose administration

The changes in peripheral serum insulin and plasma C-peptide levels and in the insulin secretory rate in response to iv glucose (0.5 g/kg BW) administration were studied in seven normal subjects. Insulin secretory rates were calculated according to a two-compartment model of distribution for C-peptide, using individual C-peptide kinetics calculated from iv bolus injections of biosynthetic human C-peptide. The mean plasma glucose level increased from a fasting level of 5.1 +/- 0.1 (+/- SE) to a peak of 24.0 +/- 1.0 mmol/L at 3 min and reached basal levels 101 +/- 6 min after glucose administration. The mean serum insulin value increased from 50 +/- 12 to a peak of 405 +/- 58 pmol/L at 3 min and then declined to fasting levels 139 +/- 14 min after the stimulus. In contrast, the mean plasma C-peptide level increased from 390 +/- 50 to a peak of 1460 +/- 210 pmol/L at 3 min and only began declining 45 min after glucose administration, reaching fasting levels 191 +/- 15 min after the stimulus. The mean insulin secretory rate increased from 69.8 +/- 19.9 to a peak of 1412.7 +/- 159.1 pmol/min at 3 min (15.3 +/- 2.5-fold elevation over baseline) and reached basal levels 135 +/- 12 min after the stimulus. The clearance of endogenous insulin during the basal period (2.505 +/- 0.365 L/min) and that during the 4 h after the stimulus (2.319 +/- 0.230 L/min) were similar. In conclusion, after bolus iv glucose administration: 1) the insulin secretory rate is more closely represented by changes in peripheral serum insulin than in plasma C-peptide levels; and 2) no change in endogenous insulin clearance occurs.