Abstract
Five days of treadmill training in rats leads to increased muscle size and running time. This was used to examine the effect of exercise on circulating insulin-like growth factor I [IGF-I; radioimmunoassay (RIA)], local muscle (hindlimb) IGF-I (by RIA), and muscle IGF-I mRNA (by ribonuclease protection assay). Eight-week-old female Sprague-Dawley rats were divided into three groups: control (n = 10); single-exercise test (n = 10), untrained but with one maximal exercise test at the end of the study; and training (n = 16), trained for 5 days and one maximal exercise test on day 6. There were no differences among the groups with respect to circulating IGF-I. Muscle IGF-I protein in trained rats (4.2 +/- 1.5 ng/g of muscle tissue) was significantly greater than both control (0.27 +/- 0.1 ng/g) and single-exercise test (0.62 +/- 0.19 ng/g, P < 0.05 by analysis of variance). There was no difference among the groups in IGF-I mRNA gene expression. These data suggest that there is an early, marked, local muscle increase in IGF-I protein in response to exercise. This increase, however, may not be related to increased muscle IGF-I gene expression. Moreover, the IGF-I response was probably local in nature since it was not matched by any increase in circulating IGF-I.
Highlights
Title Increase in muscle insulin-like growth factor-I (IGF-I) protein but not IGF-I mRNA after 5 days of endurance training in young rats
Post hoc analysis showed that muscle IGF-I was significantly greater (P Ͻ 0.05) in the trained rats compared with both the control and single-exercise test rats (Fig. 2)
We had hypothesized that a relatively brief period of exercise training in young rats would lead to increases in both muscle IGF-I gene expression and protein
Summary
Title Increase in muscle IGF-I protein but not IGF-I mRNA after 5 days of endurance training in young rats. There was no difference among the groups in IGF-I mRNA gene expression These data suggest that there is an early, marked, local muscle increase in IGF-I protein in response to exercise. In an attempt to gain insight into the early growth factor responses to training, we recently developed a 5-day treadmill training protocol that led to increased hindlimb muscle weight in young (late pubertal) female rats [11]. Using this model, we hypothesized that the 5 days of training would be accompanied by significant increases in gene expression and protein levels of IGF-I in the trained hindlimb
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