A MODEL TO MEASURE INSULIN EFFECTS ON GLUCOSE-TRANSPORT AND PHOSPHORYLATION IN MUSCLE - A 3-TRACER STUDY

We studied five healthy subjects with perfused forearm and euglycemic clamp techniques in combination with a three-tracer (D-[C-12]mannitol, not transportable; 3-O-[C-14]methyl-D-glucose, transportable but not metabolizable; D-[3-H-3]glucose, transportable and metabolizable) intr a-arterial pulse injection to assess transmembrane transport and intra cellular phosphorylation of glucose in vivo in human muscle. The washo ut curves of the three tracers were analyzed with a multicompartmental model. A priori identifiability analysis of the tracer model shows th at the rate constants of glucose transport into and out of the cells a nd of glucose phosphorylation are uniquely identifiable. Tracer model parameters were estimated by a nonlinear least-squares parameter estim ation technique. We then solved for the tracee model and estimated bid irectional transmembrane transport glucose fluxes, glucose intracellul ar phosphorylation, extracellular volumes of glucose distribution, and extracellular and intracellular glucose concentrations. Physiological hyperinsulinemia (473 +/- 22 pM) caused 2.7-fold (63.1 +/- 7.2 vs. 23 .4 +/- 6.1 mu mol . min(-1). kg(-1), P < 0.01) and 5.1-fold (42.5 +/- 5.8 vs. 8.4 +/- 2.2 mu mol . min(-1). kg(-1), P < 0.01) increases in t ransmembrane influx and intracellular phosphorylation of glucose, resp ectively. Extracellular distribution volume and concentration of gluco se were unchanged, whereas intracellular distribution volume of glucos e was increased (similar to 2-fold) and intracellular glucose concentr ation was almost halved by hyperinsulinemia. In summary, 1) a multicom partment model of three-tracer kinetic data can quantify transmembrane glucose fluxes and intracellular glucose phosphorylation in human mus cle; and 2) physiological hyperinsulinemia stimulates both transport a nd phosphorylation of glucose and, in doing so, amplifies the role of glucose transport as a rate-determining step of muscle glucose uptake.