A novel, noninvasive transdermal fluid sampling methodology: IGF-I measurement following exercise.

This study tested the hypothesis that transdermal fluid (TDF) provides a more sensitive and accurate measure of exercise-induced increases in insulin-like growth factor-I (IGF-I) than serum, and that these increases are detectable proximal, but not distal, to the exercising muscle. A novel, noninvasive methodology was used to collect TDF, followed by sampling of total IGF-I (tIGF-I) and free IGF-I (fIGF-I) in TDF and serum following an acute bout of exercise. Experiment 1: eight men (23 ± 3 yrs, 79 ± 7 kg) underwent two conditions (resting and 60 min of cycling exercise at 60% Vo(2)(peak)) in which serum and forearm TDF were collected for comparison. There were no significant changes in tIGF-I or fIGF-I in TDF obtained from the forearm or from serum following exercise (P > 0.05); however, the proportion of fIGF-I to tIGF-I in TDF was approximately fourfold greater than that of serum (P ≤ 0.05). These data suggest that changes in TDF IGF-I are not evident when TDF is sampled distal from the working tissue. To determine whether exercise-induced increases in local IGF-I could be detected when TDF was sampled directly over the active muscle group, we performed a second experiment. Experiment 2: fourteen subjects (22 ± 4 yr, 68 ± 11 kg) underwent an acute plyometric exercise condition consisting of 10 sets of 10 plyometric jumps with 2-min rest between sets. We observed a significant increase in TDF tIGF-I following exercise (P ≤ 0.05) but no change in serum tIGF-I (P > 0.05). Overall, these data suggest that TDF may provide a noninvasive means of monitoring acute exercise-induced changes in local IGF-I when sampled in proximity to exercising muscles. Moreover, our finding that the proportion of free to tIGF-I was greater in TDF than in serum suggests that changes in local IGF-I may be captured more readily using this system.

PI3K p110 alpha and p110 beta have differential effects on Akt activation and protection against oxidative stress-induced apoptosis in myoblasts.

Catalytic subunits of phosphoinositide-3-kinase (PI3K) play a critical role in growth factor signaling and survival by phosphorylating inositol lipids. We found that PI3K Class-IA p110 alpha and p110 beta have distinct functions in myoblasts. Inhibition of p110 alpha reduced insulin-like growth factor-I (IGF-I)-stimulated Akt activity and prevented IGF-I-mediated survival in H(2)O(2)-treated cells; in contrast, siRNA knockdown of p110 beta increased IGF-I-stimulated Akt activity. However, inhibition of p110 beta catalytic activity did not increase IGF-I-stimulated Akt activity, suggesting a role for p110 beta protein interactions rather than decreased generation of phosphoinositides in this effect. Increased Akt activity in p110 beta-deficient myoblasts was associated with diminished extracellular signal-regulated kinase (ERK) activation as well as ERK-dependent IRS-1 636/639 phosphorylation, findings we show to be independent of p110 beta catalytic function, but associated with insulin-like growth factor-I receptor (IGF-IR) endocytosis. We also report that IGF-I protects myoblasts from H(2)O(2)-induced apoptosis through a mechanism that requires p110 alpha, but may be independent of Akt or ERK under conditions of Akt and ERK inhibition. These observations suggest that both p110 alpha and p110 beta are essential for growth and metabolism in myoblasts. Overall, our results provide new evidence for the roles of p110 isoforms in promoting cellular proliferation and homeostasis, IGF-IR internalization, and in opposing apoptosis.