We have developed a highly sensitive C-peptide (CPR) assay and investigated whether any residual B-cell function is present in IDDM patients whose fasting CPR and/or CPR responses to iv glucagon (1 mg) could not be demonstrated by a conventional assay.
Highly sensitive assay-Three ml of plasma sample was extracted with 6ml of 96 %(V/V) ethanol and the extract was evaporated to dryness. The residue was dissolved in 0.3 ml of phosphate buffer and 0.5 ml of trichlorotrifluoroethane was added as a lipid solvent. The aqueous phase was separated by centrifugation and used for radioimmunoassay. The detection limit of this assay was 0.03 ng/ml and the intra-and interassay coefficients of variation were 5.4 and 9.4 %, respectively.
Using this method, significant CPR responses to iv glucagon (Δ CPR 0.19±0.16 ng/ml, mean±SD) were demonstrated in 12 of 21 IDDM patients. The other 9 showed no CPR responses (Δ CPR 0.01±0.01 ng/ml). Hemoglobin A1c and the standard deviation of 10 measurements of fasting plasma glucose (SD_FPG) were significantly higher (p<0.05 and p<0.01, respectively) in the latter group. A significant inverse correlation was also found between incremental CPR areas after iv glucagon and SD_FPG (r=-0.84, p<0.001).
Our sensitive method for CPR assay is very useful for evaluating residual B-cell function in IDDM patients whose plasma CPR levels are below detection limit by the conventional method. It is suggested that the presence of minimal residual B-cell function plays an important role in metabolic stability in IDDM.