Validation of the intraperitoneal insulin tolerance test for the measurement of insulin sensitivity in mice

Mouse models are helpful in clarifying the pathophysiological mechanisms by which insulin resistance contributes to the development of obesity and diabetes. Thus, the ability to accurately and easily quantify insulin resistance in mice is of interest. The reference standard for measuring insulin sensitivity is the glucose clamp, but simpler methods previously developed in humans are frequently adopted in mouse studies. Here, we focused on the intraperitoneal insulin sensitivity test (IPIST). The domain of validity of the insulin sensitivity index derived from IPIST is unknown since no direct validation against the clamp is available in mice. Aim of this study was to compare the indeces of insulin sensitivity derived from IPIST and the clamp in a group of wild-type mice. Six CB57BL6 12-week old mice underwent the IPIST first, and the euglycemic clamp two weeks later. In the IPIST, after a 30-min stabilization period, a bolus of human insulin (0.5 U/kg) was administered by an intraperitoneal injection at t=0. Blood samples (~5μl) for the determination of glucose concentration were collected at 0, 5, 10, 15, 30, 60, 90, 120 min. The IPIST-based index of insulin sensitivity, SI(IPIST), was calculated as the slope of the regression line between the logarithm of glucose concentration vs. time in the interval 0-15 min (i.e., when the linearity hypothesis was tenable). In the hyperinsulinemic-euglycemic clamp, after a 30-min stabilization period, a primed-continuos infusion of human insulin (2.5 mU/kg/min) was begun at t=0 and maintained constant for 120 min. Concurrently, 20% glucose was infused at a variable rate to maintain glucose at basal concentration. The clamp-based index of insulin sensitivity, ISI(clamp), was calculated as the ratio between the levels of the glucose infusion rate and insulin concentration measured at the end of the clamp. The relationship ISI(IPIST) vs. ISI(clamp) was investigated by linear regression and the coefficient of determination, R2, was evaluated to assess the goodness of the fit. Results were expressed as Mean±SD and the level of statistical significance was 0.05. During IPIST, the glucose decay curve achieved a nadir (69.2±14.4 mg/dl) at 60 min (Figure, left panel). At the end of the clamp, insulin concentration was 50.3±2.0 μU/ml. ISI(IPIST) was 3.36±0.95 10-2 mg/dl per min. ISI(clamp) was 10.41±1.20 10-1 mg/kg min per μU/ml. The association between the two indices was excellent and R2 was 0.96, p=0.001 (Figure, right panel). IPIST is simple and calculation of ISI(IPIST) from glucose data collected during the initial 15 min of the test is straightforward. The findings of this preliminary report are encouraging since ISI(IPIST) resulted extremely well correlated with ISI(clamp). If confirmed in a larger sample of mice, IPIST may constitute a cost-effective and validated approach to measure insulin sensitivity in mice.