1976 Volume 19 Issue 1 Pages 22-29
Responses of serum C-peptide immunoreactivity (CPR) and immunoreactive insulin (IRI) during 50g oral glucose tolerance tests (OGTT) were compared among healthy subjects, obese subjects, adult-onset insulin-treated diabetics and juvenile-onset insulin-treated diabetics.
The radioimmunoassay method of CPR was investigated by the double antibody technique of Kaneko. During 50g OGTT, the fasting serum CPR in healthy subjects was 2.67±0. llng/m/(741±31×10-12M) and reached a peak, 7.07±0.24ng/ml (1963±7-10-12M), at 60 min. The serum IRI peaked at 30 min. The molar C-peptide concentration was higher than insulin. The fasting molar CPR: IRI ratio was 6.2, and following glucose, the ratio appeared to decline. The nadir of the ratio occured after thirty minutes with a gradual return toward the fasting level by the end of the test. From the previous in vitro studies by others, that insulin and C-peptide are secreted in equimolar concentration, these findings suggest that the metabolic organ of insulin is different from that of C-peptide.
The serum CPR in obese subjects was significantly higher than in healthy subjects at 0 min., 90 min. and 180 min., and molar CPR: IRI ratios before and after glucose loading were similar to those in the healthy subjects. The significant correlation between CPR and IRI could be found in the healthy and obese subjects (r=0.7983). These results confirmed that the insulin secretory capacity of the pancreatic β-cell can be assessed by CPR response.
In the adult-onset insulin-treated diabetics, IRI could not be measured because of the presence of circulating insulin antibodies. The CPR in these diabetics showed a small response, reaching a peak at 90 min. In contrast, the CPR levels in juvenil-onset insulin-treated diabetics who recovered from diabetic ketoacidosis remained lower than those in the adult-onset insulin-treated diabetics at all times. From these results, it seems that the insulin secretory capacity is retained in most of the insulin-treated diabetics. The C-peptide radioimmunoassay seems to be useful in evaluating the clinical course and therapeutic procedures in the treatment of diabetics.
