The kidney is responsible for a considerable part of the clearance of insulin and C-peptide. Two routes are thought to be involved in the renal extraction of insulin and C-peptide from the circulation: (1) glomerular filtration, and (2) uptake by tubular cells from peritubular capillaries. The aim of the present study was to investigate these processes in non-insulin-dependent diabetes mellitus (NIDDM). For this purpose we measured the renal extraction of inulin, insulin, and C-peptide in 12 NIDDM patients and 16 control subjects during elective heart catheterization. In addition, a 24-h urine sample was obtained from all subjects to assess the fractional clearance of the peptides.The total renal extraction of both insulin and C-peptide exceeded the amount that was extracted by filtration, confirming the supposition that both peptides are cleared by an additional mechanism, most probably peritubular uptake. The peritubular uptake of insulin in the NIDDM group was not significantly different from that in the control subjects, whereas the insulin extraction over the legs was significantly lower in NIDDM than in the controls. The peritubular uptake of C-peptide was significantly lower in NIDDM, while the fractional clearance of C-peptide was significantly higher. The latter indicates that the reabsorption of C-peptide from the luminal side of the tubular cell is impaired in diabetes mellitus. It is therefore concluded that urinary C-peptide excretion is not a reliable index for insulin production in NIDDM. Furthermore, the discrepancy between the effect of NIDDM on the peritubular uptake of insulin and C-peptide suggests that peritubular uptake of low-molecular-weight proteins is a selective process.
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