Human plasma C-peptide immunoreactivity: its correlation with immunoreactive insulin in diabetes, and chronic liver and renal diseases.

The correlation between plasma C-peptide immunoreactivity (CPR) and immunoreactive insulin (IRI) was investigated during the oral glucose tolerance test in 20 normals, 127 diabetics, and 39 non-diabetics with chronic liver or renal disorders. When all subjects were included, the increment of CPR 30 minutes after glucose load (deltaCPR) correlated well with that of IRI (deltaIRI) (r = 0.66, p less than 0.001), but the return of CPR towards the basal level was delayed as compared with IRI. The positive correlation was also observed between the sum of 6 IRI and that of 6 CPR values during the glucose tolerance test in diabetics and controls (r = 0.53, p less than 0.001). deltaCPR/deltaBS (30 min.) was also well correlated with deltaIRI/deltaBS (30 min.), and was specifically low in diabetics. Insulin-treated maturity-onset diabetics showed low but considerable CPR responses while no CPR responses were observed in insulin-treated juvenile diabetics. In each plasma sample, CPR always exceeded IRI on the molar basis. At fasting CPR/IRI ratio was 15.6 +/- 1.7 (mean +/- SE) in normals and 14.9 +/- 1.3 approximately 16.9 +/- 1.0 in diabetics. In chronic liver diseases IRI response was augmented while CPR response was not different from that of controls, and the molar ratio of CPR/IRI was significantly low (9.5 +/- 1.1). On the contrary, it exceeded that of normals in chronic renal diseases (35.7 +/- 14.9). It is concluded that, first, the plasma CPR response appears to be a valuable indicator of pancreatic B-cell function, and second, it is, nevertheless, modified in chronic liver or renal disorders.