Partitioning glucose distribution/transport, disposal, and endogenous production during IVGTT

Abstract

We have separated the effect of insulin on glucose distribution/transport, glucose disposal, and endogenous production (EGP) during an intravenous glucose tolerance test (IVGTT) by use of a dual-tracer dilution methodology. Six healthy lean male subjects (age 33 ± 3 yr, body mass index 22.7 ± 0.6 kg/m2) underwent a 4-h IVGTT (0.3 g/kg glucose enriched with 3–6% d-[U-13C]glucose and 5–10% 3- O-methyl-d-glucose) preceded by a 2-h investigation under basal conditions (5 mg/kg ofd-[U-13C]glucose and 8 mg/kg of 3- O-methyl-d-glucose). A new model described the kinetics of the two glucose tracers and native glucose with the use of a two-compartment structure for glucose and a one-compartment structure for insulin effects. Insulin sensitivities of distribution/transport, disposal, and EGP were similar (11.5 ± 3.8 vs. 10.4 ± 3.9 vs. 11.1 ± 2.7 × 10−2ml · kg−1 · min−1 per mU/l; P = nonsignificant, ANOVA). When expressed in terms of ability to lower glucose concentration, stimulation of disposal and stimulation of distribution/transport accounted each independently for 25 and 30%, respectively, of the overall effect. Suppression of EGP was more effective ( P < 0.01, ANOVA) and accounted for 50% of the overall effect. EGP was suppressed by 70% (52–82%) (95% confidence interval relative to basal) within 60 min of the IVGTT; glucose distribution/transport was least responsive to insulin and was maximally activated by 62% (34–96%) above basal at 80 min compared with maximum 279% (116–565%) activation of glucose disposal at 20 min. The deactivation of glucose distribution/transport was slower than that of glucose disposal and EGP ( P < 0.02) with half-times of 207 (84–510), 12 (7–22), and 29 (16–54) min, respectively. The minimal-model insulin sensitivity was tightly correlated with and linearly related to sensitivity of EGP ( r = 0.96, P < 0.005) and correlated positively but nonsignificantly with distribution/transport sensitivity ( r = 0.73, P = 0.10) and disposal sensitivity ( r = 0.55, P = 0.26). We conclude that, in healthy subjects during an IVGTT, the two peripheral insulin effects account jointly for approximately one-half of the overall insulin-stimulated glucose lowering, each effect contributing equally. Suppression of EGP matches the effect in the periphery.