Variation of endogenous insulin secretion in association with treatment status: assessment by serum C-peptide and modified urinary C-peptide.

The variation of endogenous insulin secretion in association with fasting plasma glucose (FPG) level and the modality of treatment was assessed using serum C-peptide levels before and after breakfast and the corrected value of 24-h urinary C-peptide (24 h-UCP) in inpatients with non-insulin-dependent diabetes mellitus. The corrected value calculated as 24 h-UCP/(urinary C-peptide to creatinine clearance (CCP/CCR) ratio in the fasting state x 10) was correlated with the sum of day-long serum C-peptide levels (r = 0.93) more closely than the measured value of 24 h-UCP (r = 0.79) in 9 patients. In 52 patients treated with diet alone, 38 with sulfonylurea and 28 with insulin, fasting serum C-peptide level did not vary with FPG level, and the increment of serum C-peptide level after breakfast and the corrected value of 24 h-UCP decreased with the rise in FPG level in each treatment. These indexes were the lowest in insulin treatment among the patients with similar FPG levels. In conclusion, 24 h-UCP was demonstrated to be able to reflect day-long endogenous insulin secretion more faithfully after the correction with the CCP/CCR ratio. It was estimated that the insulin response to breakfast and day-long insulin secretion decreased with the rise in FPG level, but basal insulin secretion was maintained over a wide range of FPG levels in each treatment. Endogenous insulin secretion seemed to be somewhat suppressed or rested by exogenous insulin in insulin-treated patients.