IGF-1 Partially Reproduces Beneficial Effect of Exercise Training on Glucose Tolerance in Normal Rats

Abstract

Exercise transiently elevates the IGF-1 (insulin-like growth factor 1) level, but whether exogenous IGF-1 administration can reproduce exercise training benefit in glycemic control is currently unknown. This study compared the effect of IGF-1 administration and exercise training on glycogen storage, glucose tolerance, and muscle glucose transporter 4 (GLUT4) protein expression in normal rats. Forty rats were weight matched and evenly assigned to the following 4 groups: control (C), exercise trained (E), IGF-1 treated (I), and exercise-trained + IGF-1 (EI). Same volume of saline or IGF-1 (2 μg/kg BW) was injected daily to the rats. Exercise training consisted of daily 90 min of swimming for the first week and gradually increased to 180 min, twice for the third week. Oral glucose tolerance test (OGTT) was performed in all rats under fasted condition. Muscle tissues were removed at the end of the 3-week treatments (3 days after OGTT). The levels of GLUT4 protein and mRNA were determined in red and white portions of the quadriceps muscle (RQ and WQ). Both exercise training and chronic IGF-1 administration increased GLUT4 expression and improved glucose tolerance without an observed additive effect. Exercise training increased glycogen level in RQ and WQ above control level. Despites chronic IGF-1 administration increased muscle GLUT4 expression above control level, glycogen increase was not observed. Our data suggest that IGF-1 can partially reproduce exercise training effect on improving glycemic control.