C-peptide metabolism and the liver

Insulin production rate following glucose ingestion estimated by splanchnic C-peptide output in normal man

Insulin production rate has been estimated in healthy male volunteers (n = 16), and evaluated with respect to splanchnic glucose exchange. Insulin production rate was calculated from splanchnic immunoreactive C-peptide output. C-peptide secretion was estimated by the hepatic venous catheter technique both in the basal state and for 2 h following ingestion of various glucose loads (0, 12.5, 25, 50, 75, and 100 g). The results demonstrate a basal insulin production rate of 0.017 +/- 0.002 U/min (mean +/- SEM) or 2.04 U/2 h. Values rose in a dose dependent manner from 2.6 +/- 1.1 U/2 h after ingestion of 12.5 g of glucose to 10.8 +/- 1.1 U/2 h following a glucose load of 100 g. Insulin retention by the liver was estimated at 0.012 +/- 0.001 U/min in the basal state, and ranged from 47-85% (70 +/- 2%) of production following an oral glucose load. It was also demonstrated 1) that the relative splanchnic glucose output was inversely related to the amount of ingested glucose, and reached a minimum when glucose in excess of 50 g was ingested; and 2) that hepatic glucose retention was directly proportional to insulin production rate (r = 0.83; p less than 0.001; n = 15). It is suggested that the adaptive capacity of the splanchnic bed to retain glucose depending on the amount of ingested glucose guarantees that splanchnic glucose output fluctuates in healthy man only within a narrow range.

Evaluation of insulin secretory capacity in diabetes mellitus

The capacity of the pancreatic beta cells to secrete insulin can be evaluated in vivo by measurement either of circulating immunoreactive insulin or of C peptide, a by-product of insulin synthesis. Evaluation of serum levels is complicated, however, by the variable degradation rates and distribution spaces of these peptides. Also, interpretation of peripheral vein concentrations of insulin, and perhaps C peptide, is more difficult because of hepatic catabolism. Quantitation of these peptides in the urine may provide an integrated measure of insulin secretion. In insulin treated diabetic patients who develop circulating insulin antibodies, beta cell secretory capacity may be assessed by C peptide measurement, or by techniques that allow separate determinations of "free" and "total" insulin. A variety of stimulatory tests may be used to investigate insulin secretory capacity.

Plasma C-peptide in uraemic patients

The kidney has been suggested as the main organ for the degradation of C-peptide. This hypothesis was tested in subjects with normal fasting blood glucose concentration and varying degrees of renal failure. Forty-nine subjects with endogenous creatinine clearance ranging from 0--25 ml/min were studied. The basal steady state concentrations of C-peptide (CP) and the immunoreactivity of insulin (IRI) were determined in plasma from fasting patients. The average IRI was similar to that found in normal subjects while a higher CP was found in all patients but two. The average CP in the nephrectomized patients was six times higher than the mean CP in normal subjects (0.35 pmol/ml). There was a significant inverse correlation between clearance and CP (r = 0.51, P less than 0.001) with the highest CP in nephrectomized patients. It is concluded that the increased CP in renal failure, and especially the markedly increased CP in the nephrectomized group supports the hypothesis of the kidney being the organ mainly responsible for the degradation of C-peptide also in man.