Peripheral Insulin Parallels Changes in Insulin Secretion More Closely Than C-Peptide After Bolus Intravenous Glucose Administration*

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Abstract
The changes in peripheral serum insulin and plsma 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 adminstration. 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 pmpl/L at 3 min and only began declining 45 min after glucose administration, reaching fasting levels 1991 .+-. 15 min after the stimulus. The mean insulin secretory rate increased from 69.8 .+-. 19.9 to a peak of 1412 .+-. 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 dring the basal period (2.505 .+-. 0.365 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.

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