4.3. Oral Glucose Tolerance Test and Insulin Response, Insulin Tolerance Test and Insulin Resistance

The oral glucose tolerance (OGTT) test was performed on rats that had been fasted 4 h, before a glucose anhydrous solution (2 g/kg BW) was administered orally, glucose was measured from the vein of the tail. Then, the glucose loading was administered, and glucose plasmatic concentration was determined at 30, 60, 90 and 120 min from the vein of the tail. The glucose concentration was measured by a commercial kit (Spinreact, Spain) and an automatized analyzer A-15 (BioSystem, Jalisco, Mexico). At the same time (0, 30, 60, 90 and 120 min), plasmatic insulin was quantified by a commercial kit (Diagnostica International Company, Jalisco, Mexico), with the resulting antibody-antigen complex assessed at 415 nm in a Stat Fax 2600 plate reader (WinerLab Group, Rosário, Argentina). Insulin concentrations were obtained from a standard curve ranging from 0 to 200 µUI/mL.

Five days after the oral glucose tolerance test. On rats that had been fasted 4 h, an insulin tolerance test (ITT) was performed. The animals were intraperitoneally challenged with a dose of 0.75  U/kg BW of human insulin (Humulin 70/30; Lilly, Indianapolis, IN, USA). This insulin is a combination of human insulin isophane suspension and human insulin (rDNA), which combines intermediate-acting insulin with the more rapid onset of action of regular human insulin. The pharmacologic effect begins at approximately 50 min (range: 30 to 90 min); thus, it is ideal insulin for realizing an ITT. Blood samples were drawn from the tail vein at different time points (0, 15, 30, 60 and 90 min), and glucose levels were determined as described previously.

Finally, the insulin resistance was evaluated using the homeostasis model assessment insulin resistance (HOMA-IR). The mathematical model was realized according to the report by [82].