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Renal Metabolism of C-Peptide in Man^*

Istituto di Clinica Medica Generate and Cattedra di Endocrinologia (R.L., A.G.), University of Parma 43100 Parma
Istituto Scieritifico di Medicina Interna, Cattedre di Nefrologia Medica and di Patologia Medica (G.D., G.G., C.R.), University of Genoa 16132 Genoa, Italy

Address requests for reprints to: Dr. Ivana Zavaroni, Istituto di Clinica Medica Gerierale, via Gramsci 14, 43100 Parma, Italy.

Renal metabolism of C-peptide was studied in nine nohdiabetic nonobese patients with normal renal function by the arterial-venous difference technique before and after the oral administration of ah amino acid mixture simulating an animal protein meal. In the basal state, the kidney removed 25.7 ± 7.5% (±SD) of the arterial plasma C-peptide. Renal uptake was approximately 7-fold greater than urinary excretion, and thus, more than 85% of the amount extracted was metabolized by the kidney. Renal C-peptide clearance was very high and approximated the glomerular filtration rate, whereas urinary C-peptide clearance was only 14% of its renal clearance.

Shortly after amino acid ingestion, arterial C-peptide levels increased by 107%, and C-peptide renal fractional extraction, uptake, and net metabolism also increased markedly (67%, 278%, and 328%, respectively); urinary clearance and excretion did not change. Renal clearance became 2-fold greater than the glomerular filtration rate, indicating that in this phase the kidney removed substantial amounts of C-peptide from peritu-bular blood as well as by filtration. Both renal uptake and urinary excretion of C-peptide were related to its arterial levels (P < 0.001 and P < 0.05, respectively), but renal uptake increased much more than urinary excretion for each increment in arterial C-peptide levels.

These results indicate that renal C-peptide metabolism is considerable in the postabsorptive state and is even more marked during the postprandial period. The kidney, therefore, plays a key role in both the regulation of circulating plasma levels and the metabolic clearance of C-peptide.

^* Part of this paper appeared in abstracts presented at the 3rd Milan International Meeting on Diabetes, Milan, Italy, March 22–23, 1985, and at the 12th IDF Meeting, Madrid, Spain, September 22–28, 1985. This wofk was supported by grants from the Ministerp della Pubblica Istruzione (Assegnazione per la Ricerca Scientifica "40%") and the Consiglio Nazioriale delle Ricerche (Progetto Finalizzato di Medicina Preventiva, Sp 4, obiettivo 47b).

Received October 17, 1986.

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