Human urinary C-peptide immunoreactivity (CPR) was measured by a double-antibody radioimmunoassay using synthetic human C-peptide. Urine CPR was stable as long as one year, if stored frozen. Urine CPR was eluted by gel filtration as a single peak in the region that corresponded to 125I-labeled C-peptide. Even in a patient with insulin antibody whose serum contained a CPR fraction with higher molecular weight, urine CPR was eluted in its expected region.
Normal subjects excreted 81 ± 36 μg. CPR per day. Urine CPR excretion was very low in juvenile-onset diabetics, variable in adult-onset diabetics and liver disease patients, and increased in patients on corticosteroid treatment. Diurnal changes and a 50-gm. glucose tolerance test revealed that urine CPR excretion rate increased in parallel with plasma CPR levels. The responses of diabetic patients were smaller and more sluggish. Urine CPR excretion tended to be decreased in patients with renal diseases and extremely low in those on regular hemodialysis. CPR was positively correlated with serum creatinine. Urine CPR was not increased in patients with marked β2-microglobulin excretion.
Highly significant correlations were observed between 24-hour urine CPR values and fasting plasma CPR or summed CPR after glucose load. Thus, urine CPR seems to provide a good means to assess B-cell function, and it is particularly suitable to monitor B-cell function continuously or when multiple blood samplings are not practical. Continuous measurement of daily urine CPR in diabetic patients on glibenclamide treatment revealed that improvement of blood sugar control could occur without preceding increases in total daily B-cell secretion.