Assessment of proinsulin's effects on intermediary metabolism using the forearm technique in normal man

Abstract

We have compared the effects of human proinsulin and insulin on forearm metabolism. Seven normal, non-obese subjects were infused with 386 pmol/kg per hour of proinsulin and 180 pmol/kg per hour of insulin using the euglycaemic clamp technique. Glucose appearance and utilization rates were quantified using a primed continuous infusion of [6′,6′-²H₂]glucose. Mean blood glucose was 4.1±0.1 and 4.1±0.2 mmol/l during proinsulin and insulin infusions respectively. Basal insulin concentrations increased from 0.02±0.01 to 0.25±0.03 nmol/l. The proinsulin infusion was chosen to give steady-state levels approximately 20-fold higher on a molar basis than those of insulin, based on previous findings that proinsulin has only 5% the biological potency of insulin. Basal proinsulin concentrations increased from 0.003 to 5.4±0.3 nmol/l. Hepatic glucose production was suppressed similarly during the last hour of each hormone infusion: 0.07±0.16 (proinsulin, P), and 0.01±0.13 (insulin, I) mg/kg per minute. Glucose disposal, however, was significantly increased during the final hour of the insulin infusion: 4.7±0.4 (I) and 3.4±0.2 (P) mg/kg per minute (P=0.025). Net forearm glucose uptake (FGU) increased by a greater amount during insulin compared with proinsulin infusion: 1.44±0.02 (I) and 0.71±0.01 (P) μmol/100 ml forearm per minute (P<0.02). There was a small but significant net drop in arterialized blood lactate and pyruvate concentrations during proinsulin compared with insulin infusion: lactate −43±29 (P) and +63±35 (I) μmol/l (P<0.01); pyruvate −8±3 (P) and +6±2 (I) μmol/l (P<0.02). Arterialized blood alanine concentrations were similar during both series of hormone infusions. Forearm production and arterialized concentrations of glycerol were suppressed by equal amounts during the last hour of each hormone infusion. Despite greater FGU during insulin infusion, forearm production of lactate, pyruvate and alanine were similar during the last hour of each glucose clamp. These results indicate that in overnight fasted normal man: (1) proinsulin may have a preferential effect on the liver compared with muscle in terms of glucose handling; (2) proinsulin is less effective in stimulating FGU than is insulin; (3) from calculation of carbon flux across the forearm, proportionally less glucose was oxidized or stored during infusion of proinsulin compared with insulin; (4) proinsulin has similar effects on forearm lipolysis compared with insulin; (5) proinsulin may have a differential effect on splanchnic lactate metabolism compared with insulin.