Exercise and thiazolidinedione therapy normalize insulin action in the obese Zucker fatty rat

Thiazolidinediones and exercise are both known to improve insulin action independently. Therefore, we determined whether combined therapy could normalize insulin action in the Zucker fatty (ZF) rat. Rats were fed troglitazone as a 0.2% food admixture over a 3-week exercise training period (treadmill running 5 days/week, 20 m/min, 0% grade, 60 min/day). Subsequent to drug and/or exercise therapy, animals were chronically cannulated in the carotid artery (sampling) and jugular vein (infusion). After a 4-day recovery from surgery, animals were exposed to a hyperinsulinemic (40 mU x kg(-1) x min(-1)) euglycemic clamp (8.5 +/- 0.12 mmol/l; P = 0.45 between groups). Independently, exercise (n = 7) and troglitazone (n = 7) improved the glucose disposal rate 20% (P = 0.04) and 76% (P = 0.001), respectively, when compared with untreated ZF controls (n = 11). In combination, exercise and troglitazone therapy (n = 6) produced significant increments in the following: tracer-determined glucose disposal rate (combined therapy, 52.4 +/- 2.9 mg x kg(-1) x min(-1), vs. untreated ZF, 25.8 +/- 0.8 mg x kg(-1) x min(-1); P = 0.0001), total GLUT4 protein (twofold increase; P = 0.001), insulin receptor substrate (IRS)-1 protein (fourfold increase; P = 0.0001), and Akt phosphorylation (2.9-fold increase; P = 0.002). In conclusion, 1) exercise and troglitazone therapy each improved insulin action in the ZF rat, whereas the combination of the two led to complete normalization of insulin sensitivity, and 2) combination treatment also resulted in normalization of GLUT4 total protein, IRS-1 protein, and Akt phosphorylation compared with lean littermates.