Inhibition by proinsulin of endogenous C-peptide release in healthy man

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Abstract
To determine the role of proinsulin on endogenous insulin release, splanchnic output and arterial concentrations of C-peptide were measured in healthy subjects before and during infusion of human (HPI) and porcine (PPI) proinsulin at increasing rates for 70 min each (HPI, 328 and 656 micrograms X m-2 X h-1; PPI, 54, 134, and 268 micrograms/m-2 X h-1), while euglycemia was maintained by variable glucose infusion. By using this approach splanchnic C-peptide output was reduced by human proinsulin infusion from 143 +/- 16 (mean +/- SE) pmol/min to 111 +/- 18 and 75 +/- 11 pmol/min (P = 0.01). Simultaneously, arterial concentrations of C-peptide decreased from 716 +/- 40 pmol/l by 23 and 32%. Similar inhibition was induced by porcine PPI of splanchnic C-peptide output at an infusion rate of 268 micrograms X m-2 X h-1. Mean metabolic clearance rate was 2.7 and 3.7 ml X kg-1 X min-1 for HPI and PPI, respectively. Splanchnic glucose output was almost completely suppressed by human and porcine proinsulin at maximal infusion rates. This effect preceded both inhibition by proinsulin of splanchnic C-peptide output and stimulation of peripheral glucose utilization. We conclude that human and porcine proinsulin suppress endogenous insulin secretion at pharmacological concentrations. The observed constancy of the metabolic clearance rate of HPI demonstrates that its clearance remains a nonsaturable process up to supraphysiological HPI concentrations, while clearance of PPI appears to be subject to saturation. Furthermore, it appears that splanchnic glucose output responds earlier to proinsulin exposure than suppression of C-peptide release or stimulation of peripheral glucose utilization.