Assessment and mathematical modeling of glucose turnover and insulin sensitivity in lean and obese cats

Insulin sensitivity (S_I) of glucose disposal can be quantified with the euglycemic hyperinsulinemic clamp (EHC) with tracer glucose infusion. True steady state is, however, difficult to achieve, and non-steady state analysis of EHC data is preferred. This analysis requires information on glucose kinetics that can be obtained from bolus injection of cold and tracer glucose. The aim of this study was to assess glucose kinetics in cats. Mathematical modeling and non-steady state analysis was applied to assess effects of obesity on glucose turnover, glycolysis/glycogen synthesis, S_I, and inhibition of endogenous glucose production (EGP) in lean cats (L) and obese cats (O). d-[3-³H]-glucose kinetics and ³H-H₂O production were analyzed in 4 L and 4 O with three-compartment modeling. Frequently sampled insulin-modified intravenous glucose tolerance tests (FSIGT) with minimal model analysis were performed in 5 L and 3 O to assess glucose kinetics and S_I. EHC was performed in 10 L and 10 O with primed-constant infusion of ³H-glucose. Data were analyzed with a modified minimal model segregating suppression of EGP by insulin using a non-linear mixed-effects population approach. FSIGT provided estimates of S_I, glucose effectiveness S_G, and distribution volume. EHC provided estimates of S_I, S_G, glycolysis, and suprabasal insulin concentration for 50% EGP inhibition. Obesity appears to affect glucose distribution but not utilization at basal insulin, and reduces S_I estimated by FSIGT and EHC. Differences in S_I between FSIGT and EHC depend on different descriptions of EGP inhibition by insulin. Finally, glucose disposal at basal insulin appears to occur entirely through glycolysis, whereas significant amounts of glucose are sequestrated from oxidation during EHC.