Basal, postprandial (2 h after breakfast), and glucagonstimulated plasma C-peptide concentrations were determined in a group of 36 adult diabetic patients. Basal and postprandial C-peptide values were measured on consecutive days to estimate the degree of variation of C-peptide secretion. In a subgroup of 15 diabetic patients treated chronically with diet and oral hypoglycemic agents (sulfonylureas or a combination of sulfonylureas and metformin), we studied whether administration of sulfonylureas immediately before breakfast had any effect on postprandial C-peptide values. Absolute differences between two consecutive fasting C-peptide concentrations in insulin-requiring patients were <0.1 nM in all but 1 patient, in whom the difference was 0.18 nM. In subjects treated with oral hypoglycemic agents the median difference was 0.12 nM (range 0-0.38 nM). Absolute differences between two consecutive postprandial C-peptide concentrations were all <0.1 nM in insulin-requiring patients. No significant difference was found between postprandial C-peptide concentrations with or without preceding administration of oral hypoglycemic agents (medians 1.35 and 1.30 nM, respectively). Glucagon-stimulated C-peptide concentrations were somewhat higher than the postprandial values. However, equal discrimination between insulin-requiring and non-insulin-requiring diabetic patients was found by measuring postprandial or glucagon-stimulated C-peptide concentrations.

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