Urine C-peptide Immunoreactivity: 24 Hour Excretio and Diurnal Variations in Normal and Diabetic Subjects

Abstract

Urine C-peptide immunoreactivity (UCPR) was measured by a double antibody radioimmunoassay method in normal and diabetic subjects who had no gross renal impairment. The total UCPR excreted in 24 hr in healthy subjects during ordinary life was 74.7 ± 26.3μg (mean ± SD). Individual total UCPR values showed a significant correlation with body height, weight index and urinary creatinine (Cr) excretion. Thus, the amount of UCPR per gram of creatinine excreted during 24 hr appears to represent a convenient index for quantifying pancreatic B cell secretory function. The 24-hr urine CPR/Cr was 53.5 ± 13.8μg/g in 20 healthy subjects.
In ketotic diabetics, the mean UCPR and UCPR/Cr were markedly decreased but some variation existed in their residual B cell function. In cases of post-pancreatitic, unstable, insulin-requiring diabetics, UCPR was reduced to undetectable levels, suggesting that B cell function was almost completely destroyed in these patients. On the other hand, urine CPR excretion was variable in adult-type diabetics. It was significantly decreased in non-obese patients. UCPR/Cr was also significantly decreased in insulin-treated, non-ketotic patients. The majority of patients treated on a diet and with sulfonylureas, however, excreted normal or higher than normal amounts of CPR in the urine. There was no correlation between UCPR and the duration of diabetes or the severity of retinopathy.
Meal-dependent diurnal variations were observed in urine CPR excretion rate in healthy subjects. In general, urine CPR changed in parallel with the changes in plasma CPR and insulin. Urine CPR excretion rate was the lowest from bedtime to breakfast (normal value: 1.14μg/hr). It increased promptly after each meal. The ratio of the UCPR excretion rate during daytime to that from bedtime to breakfast was 3.33 in normal subjects, 1.49 in ketotic diabetics, 1.84 in nonketotic diabetics on insulin treatment, 1.86 in non-ketotic diabetics on sulfonylurea drugs, and 2.57 in diabetics undergoing diet treatment. Although the basal CPR excretion rate was higher than normal in some adult-type diabetics, its relative increase during the daytime was not as high as in normal subjects, suggesting a decrease in extra-secretion of insulin relative to its basal secretion.
The results of the present study indicate that urine CPR is a useful index of the endocrine function of the pancreas, and measurements of divided urine samples appear to provide further information on the daily activity of B cells.