The effects of human proinsulin on glucose turnover and intermediary metabolism in insulin-dependent-diabetes mellitus.

Abstract

We have compared the action of human proinsulin and insulin on glucose turnover, intermediary carbohydrate, and lipid metabolism in insulin-dependent-diabetic (IDDM) subjects. Six, young, weight-matched (23 +/- 2 kg-2) IDDM subjects underwent separate hyperinsulinemic euglycemic clamps. Three, low dose, iv infusions of both insulin and proinsulin were used to construct dose response curves. The proinsulin infusions were chosen to give steady state levels approximately or equal to 20-fold higher on a molar basis than insulin, based on previous findings that proinsulin has only 5-10% the biological potency of insulin. Hepatic glucose production, measured using [6'6'2H2]glucose, was suppressed equally by proinsulin and insulin at the three dose levels; (I1) 2.8 +/- 0.7 (P1) 3.3 +/- 0.6, (I2) 2.3 +/- 0.9 (P2) 3.3 +/- 1.1, (I3) -2.0 +/- 1.7 (P3) -1.1 +/- 0.6 mumol/kg min-1. Percentage elevation of glucose disposal was significantly increased during the insulin infusions compared to proinsulin; (I1) 132 +/- 12 (P1) 78 +/- 4 p < 0.01; (I2) 157 +/- 18 (P2) 104 +/- 14; P < 0.05; (I3) 242 +/- 23 (P3) 159 +/- 24 p = 0.02. Dose response curve analysis demonstrated that proinsulin stimulated glucose disposal approximately or equal to 3.7% whereas suppression of HGP was congruent to 5.7% compared to insulin. Proinsulin had a significantly weaker effect than insulin, at the lowest infusion dose, in percent suppression of plasma nonesterified fatty acids (I1 34 +/- 4, P1 14 +/- 15%; P < 0.05), blood glycerol (I1 47 +/- 4, P1 30 +/- 3%; P < 0.01) and 3-hydroxybutyrate levels (I1 81 +/- 7, P1 42 +/- 17%; P < 0.05). Proinsulin caused significant net reductions in blood lactate levels compared to insulin at each infusion dose; (P1) -130 +/- 34, (I1) -32 +/- 30 mumol/L (P < 0.05) (P2) -139 +/- 76 (I2) +8 +/- 65 mumol/L (P < 0.05) (P3) 48 +/- 60 (I3) 230 +/- 64 mumol/L (P < 0.05). We conclude that in IDDM: 1) proinsulin has a preferential effect on the liver compared to muscle, in terms of glucose handling; 2) proinsulin may have a different effect on lactate metabolism compared to insulin; 3) proinsulin at the lowest dose resulted in an inability to suppress lipolysis and ketogenesis; 4) glucose turnover can be underestimated using [6'6'2H2]glucose.