Abstract
With the use of a commercially available radioimmunoassay kit for blood C-peptide, the assay of urinary C-peptide was conducted. Urine C-peptide immunoreactivity (CPR) could be assayed adequately after dilution of urine to 1: 20-1: 40, as revealed by dilution and recovery tests. Urine CPR was stable for 1-2 days at room temperature, and as long as one year if stored frozen at-20°C. Urine CPR was eluted by gel filtration as a single peak in the region where 125I-labeled C-peptide was eluted. In a patient with an insulin antibody, gel filtration of serum revealed a peak of CPR in the region of higher molecular weight, but urine CPR was eluted in the normal region.
Normal subjects excreted 1.2±0.6 pg CPR per hour in urine at fasting. After oral administration of 50 g glucose, urine CPR increased three-fold, approximately paralleling the change in serum CPR. In diabetic patients, the increase in urine CPR after glucose load was sluggish and small, and in juvenile-type diabetic patients urine CPR response was virtually absent. Normal CPR clearance was 7.1±2.5 ml/min. 24 hour urinary excretion of CPR in norma subjects was 92.2±38.6 μg/kg, 1.52±0.55 μg/kg body weight, and 55.1±18.2 μg/mg creatinine. It was extremely low in juvenile-type diabetics, slightly or moderately decreased in non-obese adult-type diabetics, but variable in obese diabetic patients. Urine CPR was not detected in a patient who underwent total pancreatectomy. Patients on steroid treatment for various blood and other disorders had high fasting serum CPR levels, and excreted excessive amounts of CPR in the urine. In these patients, there were highly significant correlations between 24 hour urine CPR and fasting serum CPR or serum CPR response during the glucose tolerance test. In renal insufficiency, the fasting serum CPR was high, but urinary CPR excretion was decreased. Three insulinoma patients had elevated fasting serum CPR, but their 24 hour urines contained normal amounts of CPR. Despite some limitations in these special cases, urinary CPR can be used as an additional indicator of endocrine pancreatic function.
