Microdialysis of insulin-like growth factor-I in human muscle

The determination of the insulin like growth factor I (IGF-I) concentration in the interstitial milieu is of outstanding importance to explore its autocrine/paracrine function. We previously reported a method to calibrate microdialysis probes for lactate and glucose (slope method). In the present study, we investigated the ability of our method to determine the concentration of larger molecules, such as IGF-I. We observed in vitro a close linear relationship (r = 0.86, P < 0.0005) between the recoveries of lactate (RecLac) and of IGF-I (RecIGF-I), giving access to the measurement of IGF-I with the same accuracy as the one previously found for lactate and glucose. In seven human volunteers, we calibrated each probe with the slope method: we first determined in vivo for every probe the specific RecLac/loss ethanol relationship and thereafter, using that relationship, we deduced RecLac from the loss ethanol value measured in every dialysate. This allowed calculation of RecIGF-I from the calculated RecLac value and the in vitro RecLac/RecIGF-I relationship, and finally free IGF-I concentration in muscle interstitial fluid. The mean free IGF-I interstitial concentration was 6.8 +/- 3.2 ng/ml while the mean plasma concentration was 0.4 +/- 0.2 ng/ml. This large gradient from interstitium to plasma for free IGF-I could be related to the local action of this growth factor.