Recent data suggest that circulating retinol binding protein-4 (RBP4) is involved in the pathogenesis of insulin resistance in rodents and humans. Moreover, protein C inhibitor (PCI) which specifically binds retinoic acid was found to be increased in myocardial infarction survivors who are also insulin-resistant.
Therefore, we investigated the association of insulin resistance with plasma retinol binding factors (RBP4 and PCI active antigen) in nondiabetic humans with high (IS; n=20, f/m=14/6, age: 47.2±1.9 years, BMI: 26±1 kg/m2) and low (IR; n=20, f/m=14/6, age:45.5±1.7 years, BMI:28±1 kg/m2) insulin-stimulated glucose-disposal (M), measured by 2-h hyperinsulinemic-(40 mUmin-1m-2)-isoglycemic clamp-tests.
M (80120 min) was higher in IS (10.9±0.6 mgmin-1kg-1) than in IR (4.0±0.2; P<1012). Fasting plasma RBP4 concentrations were comparable in IS (4.4±0.3 mg/dl) and IR (4.6±0.3). Fasting plasma PCI active antigen was similar in both groups (IS: 106.6±15.6%; IR: 95.3±4.0%). Plasma RBP4 and PCI were not significantly related to M.
In conclusion, our data demonstrate that healthy, nondiabetic, insulin-resistant humans do not show altered plasma retinol binding factors, such as RBP4 and PCI. Both do not significantly correlate with insulin sensitivity. Thus, our findings do not support the hypothesis of insulin sensitivity modulation by proteins involved in retinol transport.